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Materials Technology

2005;():3-10. doi:10.1115/OMAE2005-67005.

Corrosion control design and management for a new build Floating Production Storage and Offloading installation (FPSO) operating in certain benign regions, such as West Africa, China and Brazil, can provide significantly increased challenges compared to their North Sea counter parts. Even though there are over 100 FPSO’s operating worldwide, designing and implementing a cost optimal Inspection, Maintenance and Repair (IMR) system for a 20 year service still remains a major challenge. Primarily this is due to the fact that there is limited information available to facilitate the corrosion control design for a 20 year continuous service. Therefore, it is difficult to select a cost effective corrosion control design that addresses both the fabrication and operational aspects. This paper describes a probabilistic model for coating degradation and its application for implementing IMR for long year service of FPSO’s. The model can be utilized as a tool for planning of inspection and maintenance of the corrosion protective coatings and also estimate the associate costs. This enables the operator to model various scenarios for future inspection and maintenance work and thereby select cost optimal solutions for the given FPSO requirements. The paper will also demonstrate the proposed model through a realistic case study.

Commentary by Dr. Valentin Fuster
2005;():11-19. doi:10.1115/OMAE2005-67007.

It is now well accepted that welded structures may contain flaws, and that these do not necessarily affect structural integrity or service performance. This is implicitly recognized by most welding fabrication codes that specify weld flaw tolerance, or acceptance, levels based on experience and workmanship practice. However, these levels are somewhat arbitrary and do not provide a quantitative measure of structural integrity, i.e. how “close” a particular structure containing a flaw is to the failure condition. This concept is of special interest in cases in which the pipe is subjected to loads that produce important deformations. In particular the reeling process, used to install offshore lines, produce large cyclic plastic deformation on the pipes. In this work the method to perform a structural reliability analysis (SRA) for a tube subject to reeling is considered in detail. A fracture mechanics based methodology is reviewed and the points that need to be resolved before extending the methods to include reeling are clearly identified. The effect of the strain history on the applied and material fracture mechanics parameters were studied. A theoretical model was developed to describe the crack driving force evolution through strain cycles. A criterion was proposed and corroborated to represent material fracture resistance behavior. An experimental program was carried out. The material analyzed was a X65 - tube 355.4 × 22.2 mm. Monotonic and cyclic fracture mechanic tests were performed on single edge notch in tension (SENT) specimens. The material fracture resistance curve was determined based on the monotonic tests. The cyclic tests were used to determine experimentally the applied fracture mechanic parameters evolution. A very good agreement between predicted and measured CTOD values was obtained for the cases analyzed. A methodology to perform a SRA for tube subjected to reeling is proposed.

Commentary by Dr. Valentin Fuster
2005;():21-26. doi:10.1115/OMAE2005-67026.

Line pipes internally clad by nickel alloy are being considered as steel catenary risers when exploitation of corrosive oils is envisaged and corrosion-fatigue failures are possible. The presence of the Ni alloy as internal clad requires a matching filler metal and, consequently, the girth welds are dissimilar joints. Although these types of welds are widely used in the petroleum industry their fracture toughness are not well documented. Of particular concern is the formation of hard zones in HAZ which are usually associated with low fracture toughness. Joints of API X-60 pipes welded using NiCrMo-3 consumables were CTOD tested at 0°C. Results consistently revealed high fracture toughness values for the weld metal. The HAZ-FL, on the other hand, presented lower values and they were significantly dispersed. Microstructural and fractographic analyses revealed that low values of toughness are associated with local brittle zones (LBZ) mainly IRCGHAZ. The fracture of the specimen that presented the lowest toughness initiated in a partially mixed zone containing hard martensite.

Commentary by Dr. Valentin Fuster
2005;():27-34. doi:10.1115/OMAE2005-67055.

In strain-based design, the overmatch condition in the girth weld portion primarily must be maintained. The pipes may also be required to have a low yield to tensile (Y/T) ratio and a high uniform elongation (U.EL) in the longitudinal direction to achieve a high compressive buckling strain. However, change in the mechanical properties by heating during coating treatment has not been paid attention so much. Furthermore, how much the mechanical properties change is affected by production conditions is unclear. This study aims to clarify firstly the relation between the mechanical properties (Y/T ratio, U.EL etc.) and the microstructure and secondly the change in mechanical properties by thermal coating treatment. The Y/T ratio and U.EL are affected by the volume fraction of ferrite and the secondary phase, which are changed by thermomechanical control processing (TMCP) conditions. For example, use of dual phase microstructure is very effective for decreasing the Y/T ratio and increasing the U.EL as the pipe. On the other hand, yield strength (YS) rises and the U.EL does not change after coating. The increase in the YS after coating is influenced by the microstructure and TMCP conditions. Resultantly, dependence of the Y/T ratio on the microstructure and TMCP conditions is reduced for line pipes after thermal coating treatment.

Commentary by Dr. Valentin Fuster
2005;():35-42. doi:10.1115/OMAE2005-67058.

The purpose of this paper is to predict crack propagation and especially the remaining life after through-the-thickness crack at typical joints in deck and bottom structures of FPSOs. This information can be used in planning inspection, repair and maintenance. The growth of fatigue cracks is studied in typical welded joints through the use of analytical and numerical methods. The simplified analytical model is based on the British Standard 7910 [3] and Dexter’s analytical model [1–2]. Numerical analysis is performed with the finite element method, considering the effect of residual stresses, and using the J-integral approach to determine the stress intensity factor at the crack tip during different stages of crack growth. The first stage is referred the surface crack stage, in which the crack grows from an initial surface crack of a size of about 0.1 mm in depth and 0.2mm-1.0mm in length to the through-thickness crack. The second stage is named the long crack stage, in which the crack grows from an approximately 40–100mm long through-thickness crack to the final critical long crack. The computed stress intensity factors, along with the Paris law, are used to predict the crack propagation at each stage with reasonable accuracy. The effect of welding residual stresses on fatigue behaviour is considered by introducing an effective SIF concept. It is concluded that stable crack propagation behaviour can be conservatively predicted by using relatively simple approaches. These techniques can be used in making rational decisions regarding scheduling of inspections, repairs, and allow a better prediction of the structural reliability in view of fatigue cracks.

Commentary by Dr. Valentin Fuster
2005;():43-48. doi:10.1115/OMAE2005-67080.

The Radial Friction Welding (RFW) is a solid-state welding process in which two long elements of several metallic alloys can be joined, without the occurrence of common problems to the conventional welding processes that include fusion. During friction welding the temperature evolution is directly related with the deformation gradient, and these fields govern the joint properties. In this work, the finite element method was used to solve the full coupled termomechanical problem in order to determine the deformation and the stress fields and the variation of the temperature during RFW process. The simulation of the RFW process permitted to establish the influence of the welding parameters, like rotation and approximation speed, on the joint quality. Furthermore, the knowledge of the temperature gradient and cooling rates allowed the prediction of the resulting microestruture and determination of the level of residual stresses of the joint. To verify the analytical results the determination of the residual stresses was accomplished by the hole drilling method in several points along the perimeter of two welded workpieces.

Commentary by Dr. Valentin Fuster
2005;():49-61. doi:10.1115/OMAE2005-67124.

The development of DQEM solution of structural problems with structures resting on a two-parameter foundation was carried out. The DQEM uses DQ or EDQ to discretize the governing differential equation defined on each element, the transition conditions defined on the inter-element boundary of two adjacent elements and the boundary conditions of the beam. Some EDQ models can be generated by DQ. They are DQ generated EDQ. Numerical results solved by the developed numerical algorithms are presented. The convergence of the developed DQEM analysis models is efficient.

Commentary by Dr. Valentin Fuster
2005;():63-70. doi:10.1115/OMAE2005-67149.

Extensive studies to develop high deformability linepipe have been conducted. In case of linepipes laid at seismic region, higher resistance to buckling against large strain induced by earthquake related ground movements are required. In order to improve the deformability of pipes, two different types of microstructural control technologies were proposed, base on theoretical and analytical studies on the effect of microstructural characteristics on stress-strain behavior. Grade X65 to X100 linepipes with ferrite-bainite microstructure were manufactured by optimizing the microstructural characteristics. Grade X80 linepipe with bainitic microstructure containing dispersed fine M-A constituents particles was also developed by applying new conceptual TMCP process. Deformability of developed linepipes with two different types of microstructure were evaluated by axial compression test, and all the developed linepipes showed superior resistance to buckling comparing with conventional pipes. Tensile properties after thermal coating of developed high deformability pipe was also investigate. It was shown that increase in yield strength by thermal strain aging was minimized and round-house type stress-strain curve was maintained for the linepipe manufactured by new conceptional TMCP process.

Commentary by Dr. Valentin Fuster
2005;():71-82. doi:10.1115/OMAE2005-67209.

Early in the life of the Genesis spar, cracking developed at the welded connections between the riser guide supports and the hull wall plate. The cracking was caused by the movements of the top-tensioned risers within the steel guide frames in the moon pool of the structure. The remedial action taken to minimize the riser movements and its effects on the hull involved the use of novel rubber bumpers, which were installed in lieu of the steel guides. The bumpers around the periphery of the moon pool were fastened to the hull wall via threaded studs that were friction welded to the hull wall plate underwater. This paper describes a testing program specifically designed to qualify the fatigue performance of the stud-plate friction welds. Results verify the use of the F2 S-N curve from British Standard 7608 with a single slope for the design of the friction-welded connections subjected to axial load. It was also found that the fatigue performance of friction welds is sensitive to the stud preload. One unique feature of the fatigue failure mode of the connection, when the load is transferred through the stud into the plate, is that cracking takes place along the semi-circular heat-affected zone (HAZ) of the bond-line between the stud and the plate, and not through the hull plate thickness. As a result, failure of a stud connection does not compromise the structural integrity of the spar hull.

Topics: Fatigue , Friction
Commentary by Dr. Valentin Fuster
2005;():83-92. doi:10.1115/OMAE2005-67232.

Structural components of ships are subjected to corrosion, especially when exposed to sea environment. It is, therefore, essential to seek rational standards for the structural integrity of aging structures without economic penalties with respect to the repair and maintenance costs incurred over the life cycle of the structure. To make proper decisions about costs and safety, knowledge about the effect of corrosion and other degradation phenomena on the strength is of crucial importance. The main objective of the present study is to investigate the strength of pitted stiffened plates under biaxial compression loading. For this purpose, systematic numerical FE investigations of ultimate collapse and post-ultimate responses for the stiffened plates with pit corrosion wastage are undertaken, varying the degree of pit corrosion intensity (DOP), pit depth and the location of the densest pitted zone. The focus here is on local plate failure mode which is desired to be governing failure mode in designing of stiffened plates. Geometric imperfections and residual stresses were included in the models. It is found that the ultimate strength of a stiffened plate can be significantly decreased due to pitting corrosion. The ultimate strength of pitted stiffened plates is governed not only by the level of DOP, but also by the smallest sectional area and the location of densest pitted zone. This improved knowledge of the damage tolerance assessment of stiffened plates can be used to schedule repairs more efficiently, while risk or reliability assessment schemes are normally applied for that purpose.

Commentary by Dr. Valentin Fuster
2005;():93-101. doi:10.1115/OMAE2005-67242.

The present article presents the fatigue crack growth behavior of new high strength steels designated R4S grade. Eight Compact Tension (CT) specimens with 25 mm thickness were subjected to constant amplitude loading while exposed to seawater without and with cathodic protection. The Cathodic Potential (CP) was set to −890 mV and −1100 mV relative to an Ag/AgCl reference cell. Rates in air are included as a reference. The crack growth parameters were determined from a linear relation between da/dN and δK for a log-log scale. The derived figures are given in the table below. The figures for dry air and cathodic protection are valid for δK between 15 and 30 MPam0.5 . Below this range the slope m of the linear relation will change and further investigations have to be carried out for this region. The figures for free corrosion are valid for δK values from 10 to 30 MPam0.5 . The threshold value for δK is close to 5 MPam0.5 in this case. The measured growth rates were compared with the rates for medium strength carbon manganese steels found in rules and regulation, i.e. BS7910. The present growth rates are well within the scatter band given for these steels in air and free corrosion. The present growth rates found in seawater with cathodic protection are however substantially lower than the rates given in BS7910. When a cathodic potential of −1100 mV was applied, crack closure was observed at medium levels of δK. The explanation is the formation of calcareous deposit in the wake of the crack front that gives significantly reduced growth rates and finally leads to crack closure. This finding is a surprise for a high strength steel. The results are promising and should be investigated further. Finally, a linear elastic fracture mechanics model was established to study the fatigue behavior in a stud-less link. The model was used to construct S-N curves that are consistent with experimental fatigue lives and the design curve given in the DNV rules. The present growth parameters were used in conjuction with a crack-like initial flaw with depth in the range from 0.12 to 0.20 mm. The difference found between the growth rates in dry air and in free corrosion are in accordance with tested fatigue lives for these two environments.

Commentary by Dr. Valentin Fuster
2005;():103-112. doi:10.1115/OMAE2005-67254.

This paper reviewed the development of microalloying technology, mainly took place in Japan. This is neither comprehensive survey nor scrutiny on a specific subject in the microalloying technology, but rather a summary of personal development experience in this field. Because too many papers have been already published on improvement of the base metal properties by microalloying and/or TMCP, I intentionally focused on the application side of microalloying to the HAZ and the weld metal. But every phase was surveyed by the common metallurgical effect of microalloying elements: that is, the kinetics of carbo-nitrides. Firstly the reason for the usefulness of microalloying is explained in relation with the TMCP. Secondly precipitation, growth, and dissolution kinetics of microalloying carbo-nitrides and their application to the HAZ property control are reviewed. Lastly, the influence of steel design on the weld metal property is introduced, by exemplifying the SAW material design of line pipes.

Topics: Precipitation
Commentary by Dr. Valentin Fuster
2005;():113-118. doi:10.1115/OMAE2005-67271.

A new model for determining fracture toughness of ductile materials is presented that uses indentation and is based on continuum damage mechanics (CDM) and the concept of critical void volume fraction. Fracture toughness evaluated using the model showed good agreement with standard fracture toughness test results.

Commentary by Dr. Valentin Fuster
2005;():119-128. doi:10.1115/OMAE2005-67276.

Syntactic foam, a composite material made by combining spherical fillers in a polymeric binder, has been used for over thirty years in the offshore oil industry. To date, the applications of this material have fallen into two categories: (1) buoyancy modules or floats to support drilling risers, or (2) thermal insulation for subsea equipment and flowlines. In the first category, the syntactic foam is exposed only to cold water (4° C). In the second category, the insulation may be subjected to temperatures as high as 150° C. The contrast of these two separate applications has led to two distinct classes of materials, each with its own properties and accepted standards and criteria. Now a new category of usage has arisen: Vertical production risers that require buoyant lift, and sometimes some degree of thermal insulation, for long-term service (20–25 years) in “warm” water that may be in the range of 40° C to 65° C. By combining the buoyancy requirement of lowest possible density with the insulation requirement of prolonged hydrothermal stability, this application poses new challenges for syntactic foam development and demands new directions in testing and analysis. Because of the increasingly large size of emerging offshore projects, the potential requirement here is for very large volumes. This paper describes the materials that have been identified as candidates for the new service, and outlines the testing philosophy that is being evolved to test and qualify them with confidence for very long periods of service. Preliminary test data is presented, along with predictions of long-term performance. Lessons learned during the project will have implications for all syntactic materials, and will be useful to any managers and technologists involved in marine engineering.

Commentary by Dr. Valentin Fuster
2005;():129-134. doi:10.1115/OMAE2005-67280.

In this paper, the deformability of line pipe with local metal loss was examined. A full-scale experiment and a finite element (FE) analysis were carried out for line pipe with local metal loss subjected to an axial compressive load. As a result, a good agreement was obtained between the analytical and experimental results. This indicated that the present analytical method was applicable to evaluate the deformability of line pipes with local metal loss subjected to a large ground movement. Parametric studies were then conducted to clarify the relationship between the geometry of the local metal loss and the deformability using the FE analytical method.

Topics: Metals , Stress , Pipes
Commentary by Dr. Valentin Fuster
2005;():135-141. doi:10.1115/OMAE2005-67281.

The continuous indentation technique, because it is fast, precise, and nondestructive, has been widely used to determine such mechanical properties as flow properties, residual stress, fracture properties, viscoelastic properties and hardness of materials and structural units. In particular, continuous indentation by a spherical indenter can provide hardness and flow properties such as yield strength, tensile strength, and work-hardening exponent, using the characteristic that strain from the loaded indenter changes with indentation depth. Since the stress and strain values on the flow curve are defined based on the contact area between the indenter and material in the loaded state, accurate determination of the contact area is essential. Determination of the contact area is closely connected with elastic deflection and plastic pile-up/sink-in behavior. In this study, the pile-up phenomenon is considered as two independent behaviors, elastic deflection and plastic pile-up/sink-in, which can each be described by a formula. The formulas can be obtained from FE simulation with conditions reflecting real indentation tests for materials used for various purposes and with a wide range of material properties. By analyzing indentation morphology from the FE simulation, the two phenomena were quantified as formulas. In particular, plastic pile-up/sink-in behavior was formulated in terms of work-hardening exponent and indentation ratio.

Commentary by Dr. Valentin Fuster
2005;():143-151. doi:10.1115/OMAE2005-67298.

In recent years, higher safety and reliability of steel welded structures have been required as it shows growing concern about environmental problems. To prevent fatigue fracture is one of the most important challenges to improve the safety and reliability. A lot of studies how reduce stress concentration at critical areas have been carried out from the viewpoint of structural design as prevention measures while nothing has been studied from the viewpoint of material because fatigue strength of welded joints converges in limited range regardless of material strength. On the other hand, it was found that an appropriate dual phase microstructure could reduce the fatigue crack growth rate remarkably. The newly developed steel plate with high resistance to fatigue crack growth could extend the fatigue life of structures. The developed steels have already been applied to some ships and vessels, and a new bulk carrier applied the developed steels acquired the notation and descriptive note as the valuable ship with resistance to fatigue fracture by Nippon Kaiji Kyokai for the first time in the world. From further studies, it was found the developed steels had also high resistance to fatigue crack initiation as well as the growth even in welded structure. In this study, it was clarified that the fatigue strength of HAZ, where fatigue crack generally initiates, in the developed steel was higher than that in conventional steel and the stress concentration at toe of weld in the developed steel was smaller than in the conventional steel. It was considered the mechanism of suppression of fatigue crack initiation with FEM analysis and fatigue test. The newly developed steel can effectively extend fatigue fracture life of welded structure from the viewpoint of material.

Commentary by Dr. Valentin Fuster
2005;():153-162. doi:10.1115/OMAE2005-67317.

The development of differential quadrature element method in-plane deflection analysis model of curved nonprismatic beam structures considering the effect of shear deformation was carried out. The DQEM uses the differential quadrature to discretize the governing differential equation defined on each element, the transition conditions defined on the inter-element boundary of two adjacent elements and the boundary conditions of the beam. Numerical results solved by the developed numerical algorithm are presented. The convergence of the developed DQEM analysis model is efficient.

Commentary by Dr. Valentin Fuster
2005;():163-172. doi:10.1115/OMAE2005-67353.

Several mooring chains of an off-loading buoy failed after only 8 months of service. These chains were designed according to conventional fatigue assessment using API RP 2SK T-N curves to a fatigue life or 20 years with a factor of safety equal to 3 on life. Of particular interest is that the mooring chain failure underwent significant mooring chain motions that caused interlink rotations. Although traditionally neglected, these interlink rotations, when combined with significant chain tensions can cause bending stresses in the chain links. In this paper we identify a mechanism, here identified as Out-of-Plane Bending (OPB) that explains the extensive fatigue damage causing the mooring chains of the off-loading buoy to fail. A full scale test frame was constructed that has the capacity of applying inter-link rotation to a pre-tensioned chain. Although the test frame limits the number of links that can be tested together as a chain, a significant amount of testing was performed for the following chain sizes: 1. 81 mm Studded Grade R3S. 2. 107 mm Studdless Grade RQ3. 3. 124 mm Studless Grade R4. 4. 146 mm Studless Grade RQ4. Various pretension levels were used, with instrumentation to extract link angles and chain link stresses. In this paper the OPB mechanism is explained, and the test frame and results are presented. An empirical relationship is found to predict the OPB stresses in the chain links as a function of pretension and inter-link rotation. The OPB stress relationship obtained was applied to the failed mooring chain of the off-loading buoy with reasonable agreement. To comply with Single Buoy Moorings (SBM) requirements addressing publication of internal research, many of the graphs included in this paper have had the stress values removed from the y-axis. However, with SBM’s management approval, some numerical references to stress amplitudes remain in the text. Overall, this limitation does not detract from the study, trends are evident and relevant comparisons can be made.

Topics: Chain , Testing
Commentary by Dr. Valentin Fuster
2005;():173-182. doi:10.1115/OMAE2005-67354.

Several mooring chains of an off-loading buoy failed after only 8 months of service. These chains were designed according to conventional fatigue assessment using API RP 2SK T-N curves to a fatigue life of 20 years with a factor of safety equal to 3 on life. Of particular interest is that the mooring chain failure underwent significant mooring chain motions that caused interlink rotations. Although traditionally neglected, these interlink rotations, when combined with significant chain tensions can cause bending stresses in the chain links. In this paper we identify a mechanism, here identified as Out-of-Plane Bending (OPB) that explains the extensive fatigue damage causing the mooring chains of the off-loading buoy to fail. A previous paper [4] presented experimental results of applying inter-link rotation to a pre-tensioned chain. Various pretension levels were used, with instrumentation to extract link angles and chain link stresses. In this paper, the physics of the OPB mechanism is examined through finite element models of the 124mm chain link tests. The various modes of interlink rotation are examined. The proof loading procedure that the chain undergoes at manufacture is identified as a likely cause for creating a tightly mated surface that is conducive to activating the OPB mechanism. To comply with Single Buoy Moorings (SBM) requirements addressing publication of internal research, many of the graphs included in this paper have had the stress values removed from the y-axis. However, with SBM’s management approval, some numerical references to stress amplitudes remain in the text. Overall, this limitation does not detract from the study, trends are evident and relevant comparisons can be made.

Commentary by Dr. Valentin Fuster
2005;():183-190. doi:10.1115/OMAE2005-67405.

With the new sceneries of oil exploration in ultra deep waters that move forward to 3000 m, it has been putting for the companies that accepted this technological challenge significant border of the techno-scientific knowledge. Therefore, in this cases of ultra deep waters, where the use of the polyester synthetic rope in the Petrobras Floating Production Units, is present, it can appears the need replace. THE NEED TO LOOK FOR A SOLUTION FOR THE RELATIVE PROBLEMS THE CHANGES OF THE POLYESTER ROPE IN THE PRODUCTION PLATFORMS WITHOUT THE CHANGE OF THE EXTENSION OF BOTTOM AND OF ITS FUNDATION (FIXED POINT). THEREFORE, APPEARS THE KS HOOK AND ITS OTIMIZATION UNDER THE OPTICS OF THE CONCEPTS OF THE MECHANICS OF THE FRACTURE AND OF THE TOOLS EXISTENT COMPUTACIONAIS (MEF AND MOORING SOFTWARE). This way it was needed to make available for the Mooring System a connection Device and remote disconnection for permanent use in production and drilling units. This device, already approved for the classifying society, it is the hook KS32/40/50, one of the first open device used in permanent mooring systems in the world. This device presents as fundamental characteristic, the easy installation for at adverse environmental condition in Campos’ Basin, where the sea is Bimodal and Bi-directional (sea in development).

Commentary by Dr. Valentin Fuster
2005;():191-197. doi:10.1115/OMAE2005-67411.

The objective of this work was to evaluate the porosity variation along weld bead and its relation with weld metal mechanical properties and electrode heating during underwater wet welding. Three commercial covered electrodes were used to make bead-on-plate welds. V-groove welds were also made using two electrodes at 50 and 100 meters depth in three different steels. Electrode temperature and weld metal porosity measurements and mechanical tests were performed. The results of temperature measurements indicated that electrode temperature increases during underwater wet welding. Simultaneously, porosity reduces along the BOP and V-groove weld beads. Mechanical tests showed that the mechanical properties are better at the end of welds. Additionally, the load supported by side bend tests samples extracted from 50 meter welds were higher than that measured for the 100 meter welds. Tensile test results also showed similar trend. Charpy test did not reveal any relation between the absorbed energy and porosity reduction along weld bead. However presented the same trend of bend and tensile tests at 50 and 100 m depth.

Commentary by Dr. Valentin Fuster
2005;():199-204. doi:10.1115/OMAE2005-67444.

In the past three decades, Colorado School of Mines researchers have investigated flux-related welding processes for pipeline applications and systematically characterized the fundamental behavior of welding fluxes. They also established the relationships between flux ingredients, weld metal microstructure, and weld joint mechanical properties. These studies clarified for high strength steel welds the importance of the bimodal nature of weld metal inclusions, related to weld metal transformations. As strength and toughness levels of the steels continue to increase, new generations of consumables must be developed. Two novel consumables design concepts are being investigated at the CSM. The first one is based on a duplex microstructure consisted of lath martensite and ferrite, and the second is based on low carbon, high alloy martensite.

Commentary by Dr. Valentin Fuster
2005;():205-210. doi:10.1115/OMAE2005-67449.

Syntactic foams (hollow glass microspheres embedded in an organic matrix) were traditionally dedicated to buoyancy applications and are now used for thermal insulation purpose. The paper focuses on the hygrothermal ageing of such materials with model diepoxy/diamine thermoset matrix having high Tg and controlled architecture revealed by the mechanical and thermal properties and by gravimetric measurements. The ageing of syntactic foams of different volume fractions is studied under the combination of both high pressure / high temperature conditions (up to 300 bar / 100°C) to discuss the factors governing the long-term performance of these materials. Attention is paid to the degradation phenomenon observed on the foam after matrix has reached saturation and associated with markedly increase of mass gain. This study points out how the hydrolytic ageing of glass microspheres may reduce the durability of syntactic foams used in hot wet conditions.

Commentary by Dr. Valentin Fuster
2005;():211-219. doi:10.1115/OMAE2005-67468.

Tubular T-joints were fatigue tested with in-plane bending loading. Six models were tested, three models with R-ratio of 0 and three with R = −1. Hot spot stress was measured for the brace and for the chord using the ECSC linear extrapolation procedure. Fatigue loading was applied in load control, to obtain through thickness cracking at a number of cycles in the range 3 000–64 000 cycles. The data were analysed and compared with the current fatigue design criteria for tubular joints.

Commentary by Dr. Valentin Fuster
2005;():221-229. doi:10.1115/OMAE2005-67474.

Failure of critical titanium parts, including some offshore components, has drawn attention to delayed cracking in Ti-6Al-4V alloys, but, given good design and alloy variant selection, such failures are avoidable. Delayed cracking, or sustained load cracking (SLC), can occur at low to moderate temperature (approximately: −50 to 200°C), depending on the titanium alloy and condition. Appropriate testing methods are required to generate stress intensity threshold values (KISLC ) that can be incorporated into the design of titanium structures and recommendations are needed on the optimum chemistry and microstructure for greatest resistance. In the present work threshold stress intensity factor data (KISLC ) were generated for Ti-6Al-4V alloy sheet, forgings, pipe and weldments using two different rising stress intensity factor test methods. It is concluded that material with a beta-annealed microstructure and low oxygen content (i.e. extra-low interstitial material such as ASTM Grades 23 and 29), has high resistance to SLC and that weld metal and transformed heat-affected zone also perform well, before and after postweld heat treatment, provided interstitial element pick-up during welding is prevented. Purchasing material in a general ‘mill annealed’ condition is not recommended without specifying acceptable microstructures. Further refinement of test method is also recommended for defining KISLC .

Commentary by Dr. Valentin Fuster
2005;():231-236. doi:10.1115/OMAE2005-67488.

With the publication of the ABS Guide for Fatigue Assessment of Offshore Structures (2003) and the Commentary to the Guide for the Fatigue Assessment of Offshore Structures (2004), application of the Fatigue Design Factor (FDF) is highlighted in fatigue assessments of offshore structures. Following review of FDF’s in available Rules/Guides from other authorities, FDF’s applied in the ABS Guide is presented and quantified with a corresponding safety level thus helping the user to relate FDF’s to estimated failure probability levels.

Commentary by Dr. Valentin Fuster
2005;():237-243. doi:10.1115/OMAE2005-67498.

Steel risers for deepwater offshore oil and gas field developments are subject to seawater on the external surfaces, produced fluids on the internal surfaces and to fatigue loading. This paper reviews current knowledge of the corrosion fatigue behaviour of welded stainless steel for risers and presents results of testing of supermartensitic, duplex and superduplex grades in relevant environments.

Commentary by Dr. Valentin Fuster
2005;():245-251. doi:10.1115/OMAE2005-67499.

Steel risers for deepwater offshore oil and gas field developments are subject to seawater on the external surfaces, produced fluids on the internal surfaces and to fatigue loading. This paper reviews current knowledge of the fatigue behaviour of welded carbon-manganese steel for risers in relevant environments. A substantial body of data exists relating to the performance of girth welds in seawater with cathodic protection and consequently recent attention has been turned to establishing the fatigue performance in the internal environment, which may contain water, CO2 , H2 S and chloride and bicarbonate ions.

Commentary by Dr. Valentin Fuster
2005;():253-260. doi:10.1115/OMAE2005-67504.

Following the warning of a flooded bow horizontal brace of a semi-submersible production platform, an inspection diving team was mobilized and cracks were found at both bow and aft K-joints. Analysis of the service life of the platform, together with the results of structural analysis and local strain measurements, concluded that cracking was caused by fatigue initiated at high stress concentration points on the gusset plates inserted in the tubular joints. As a consequence of the fractured plates other cracks were nucleated close to the intersection lines of the braces that compose the K-joints. Based on this analysis different repair possibilities were proposed. To comply with the production goals of the Business Unit it was decided to repair the platform on-site and in production in agreement with the Classification Society. The proposed repair contemplated the installation of two flanges on the gusset plates between the diagonal braces by underwater wet (UWW) welding. Cracks at the gusset plates were also removed by grinding and wet welding. Defects located at the braces are being monitored and repaired by the installation of backing bars, by wet welding, followed by grinding and welding from the inside. To carry out the job two weld procedures and ten welder-divers were qualified.

Topics: Maintenance
Commentary by Dr. Valentin Fuster
2005;():261-267. doi:10.1115/OMAE2005-67506.

Following some experiences of bending stiffeners fatigue failures during full scale tests performed at Flexi France on flexible pipe and stiffener assemblies, Technip decided to launch in 1999 a major research program on fatigue life analysis of bending stiffeners made of Polyurethane material. This fatigue life assessment is now systematically performed by Technip for all new design of flexible riser bending stiffeners. This totally innovative method comprises a number of features as follows: Firstly fatigue behaviour of polyurethane material is described. The theoretical background, based on effective strain intensity factor, is detailed, together with experimental results on laboratory notched samples, solicited under strain control for various strain ratios, to obtain fatigue data. These fatigue data are well fitted by a power law defining the total number of cycles at break as a function of the effective strain intensity factor. The notion of fatigue threshold, below which no propagation is observed, is also demonstrated. Secondly the design used by Technip for its bending stiffeners, and most of all the critical areas regarding fatigue for these massive polyurethane structures are presented. Thirdly the methodology for fatigue life assessment of bending stiffeners in the critical areas defined above is discussed. Calibration of the strain calculation principle is presented versus finite element analysis. Based on all fatigue test results, the size of the equivalent notch to be considered at design stage, in the same critical areas, is discussed. Finally, a comprehensive calibration of the methodology according to full and middle scale test results is presented. The present paper is therefore a step forward in the knowledge of fatigue behaviour of massive polyurethane bending stiffener structures, which are critical items for flexible risers integrity, and widely used in the offshore industry. The confidence in bending stiffeners reliability is greatly enhanced by the introduction of this innovative methodology developed by Technip.

Commentary by Dr. Valentin Fuster
2005;():269-274. doi:10.1115/OMAE2005-67530.

Supermartensitic 13% Cr stainless steels have been in use in offshore satellite flowlines for several years. Since they contain microstructure that is susceptible to hydrogen cracking, the pick up of hydrogen in welding with subsequent transport to critical areas may be very important, also with respect to hydrogen embrittlement when hydrogen is coming from other sources than welding (e.g., cathodic protection). In the present investigation the pick up of hydrogen has been assessed using mechanized TIG welding with superduplex 25% Cr wire. The WM and HAZ hydrogen levels were analyzed. With addition of hydrogen in the shielding gas in multipass welding, the mean WM hydrogen contents were found to be approximately 10 and 6 ppm in the cap layer and root pass, respectively. The corresponding mean HAZ concentration was 3.1 ppm (scatter between 1.3 and 4.8) immediately after welding. Post weld hydrogen diffusion heat treatment showed that hydrogen diffusion was retarded at room temperature, even for 1 month storage. Limited diffusion took place at 90°C, particularly for the cap region. The results indicate that superduplex weld metal with high hydrogen content (6–10ppm) will act as a hydrogen reservoir supplying H to the 13% Cr HAZ as long as 2–3 years after welding. Fitting the data by using the uniaxial diffusion model gave diffusion coefficients in the range of ∼3–5×10−13 m2 /s at room temperature for the superduplex WM. At 90°C a diffusivity of 5.5×10−12 m2 /s for the cap area and 2.5×10−11 m2 /s for the root area were found. For a holding temperature of 150°C, diffusion from the WM was much more significant. The hydrogen WM cap content was reduced from an initial level of 10 ppm down to 2 ppm within 3 months giving a diffusion coefficient of 1.0×10−11 m2 /s. The supermartensitic HAZ samples contained up to 5 ppm hydrogen a short time after welding. This is an important observation, since it may provide sufficient amount of hydrogen in the HAZ to cause cold cracking in the as welded condition. The uniaxial model indicated diffusivities of D = 8.0×10−11 m2 /s at 20°C and D = 2.0×10−10 m2 /s at 90°C in the HAZ.

Commentary by Dr. Valentin Fuster
2005;():275-287. doi:10.1115/OMAE2005-67543.

The acceptability of localized corrosion for pipelines in service is discussed and the methodology for probabilistic assessment of corrosion damage using the 2004 revision of Det Norske Veritas document DNV-RP-F101 is presented with regard to predicting the remaining life of onshore, offshore pipelines and also pressure vessels for the oil and gas industry. Capacity assessment of corrosion defects in pipelines and pressure vessels is a challenge for the industry, and has resulted in several methods and codes in the recent years. The methods include ASME B31G for pipelines and other standards and recommended practices that can be applied for pipelines as well as pressure vessels such as BS7910 Annex G and API 579. In 1999 DNV issued a recommended practice for the assessment of corroded pipelines, DNV-RP-F101, which was developed in co-operation with the pipeline industry. A revision of this document was issued in October 2004. The latest revision provides guidelines for probabilistic assessment of the pipeline reliability and remaining life. Pipeline corrosion is often of localized nature and depends on many factors such as fluid composition and partial pressures, temperature, pH, flow rate and efficiency of corrosion inhibition. These factors may not be easy to quantify with great certainty and a probabilistic approach is particularly justified. An approach is presented in this paper where the actual measured corrosion damage is fitted to an extreme value distribution. The future corrosion rate distribution for internal CO2 corrosion is predicted using the latest de Waard and Milliams model and an inhibitor distribution, to predict the corrosion of each pipeline segment. An Gumbel type extreme value approach is applied to estimate the present condition and the local corrosion flaw distribution that may cause a leak of the pipeline in the future. The future corrosion is estimated using the anticipated future operating conditions of the pipeline to predict the corrosion rate distribution expressed in terms of a Weibull distribution. The paper highlights three cases as examples where the approach has been applied for assessing the probability of failure and reliability during service of two offshore pipelines carrying oil and gas with wet CO2 , and one stainless steel pressure vessel in a process plant occasionally exposed to trace amounts seawater originating from leaking heat exchangers.

Commentary by Dr. Valentin Fuster
2005;():289-296. doi:10.1115/OMAE2005-67544.

The possible sources causing weld metal hydrogen and oxygen pick-up during offshore hyperbaric tie-in TIG welding have been identified and simulated at a hyperbaric chamber pressure of 12 bar, i.e. 110 meter sea depth. The base material was supermartensitic 13% Cr stainless steel. Matching filler wire was used. The weld metal hydrogen and oxygen pick-up from water vapor in the shielding and chamber gases has been investigated by girth welding of pipes. Moist chamber gas seems to have insignificant effect on hydrogen and oxygen pick-up. The largest contribution is from moist shielding gas. Most of the hydrogen content in the supermartensitic welds is diffusible. By applying post-heat, it is possible to reduce the hydrogen content in the weld metal. Microcracks in the cap were observed for welds deposited with 14 mbar water vapor in the shielding gas (117 ppm) and above. Microcracks in the root welds were not observed, which was also confirmed for self restrained cracking tests. During constant load test at 150 MPa simulating hydrostatic testing of pipelines, the microcracks grew, and for the weld deposited with fully moistened shielding gas (175 ppm), a macrocrack appeared in the centre of the capping pass longitudinally to the welding direction. Crack Tip Opening Displacement (CTOD) values based on Single Edge Notched Bend (SENB) testing were all low, and the high constraint of the SENB specimen did not show any effects of hydrogen on fracture toughness. CTOD values based on Single Edge Notched Tension (SENT) testing decreased with increasing shielding gas moisture contents. For specimens without microcracks it was possible to increase the CTOD (SENT) values by post-heat, close to the toughness observed for specimens deposited with dry shielding gas.

Commentary by Dr. Valentin Fuster
2005;():297-306. doi:10.1115/OMAE2005-67549.

The last revision of the DNV recommended practice “Fatigue Analysis of Offshore Steel Structures” is from October 2001. During use of this standard some feed back from designers around the world have been received. Also some new research in the area has been performed in the time interval from it was first developed. It is also realised that the document is being used for fatigue design of some other types of details and structures than was thought of when the document was originally developed. Therefore it was now found convenient to revise the document to incorporate the experience gained and new research and developments made in the area of fatigue of offshore structures the last 7 years since the main content for this recommended practice was developed.

Commentary by Dr. Valentin Fuster
2005;():307-311. doi:10.1115/OMAE2005-67560.

Carbon steels, used in pipelines for the transport of oil and its derivatives, are frequently exposed to fluids. This can result in stress induced corrosion cracking (SCC) and/or hydrogen embrittlement (HE). The present paper evaluates the susceptibility of pipeline steels (API-X70 and API-X80) to SCC and HE, using a slow strain rate test (SSRT) based on the National Association of Corrosion Engineers’ (NACE) norm and a traditional standard NACE test. The (SSRT) method used, employed a sodium thiosulphate solution to evaluate susceptibility to HE, thereby offering a simpler experimental procedure than the standard NACE test. The results confirm the efficacy of the sodium thiosulphate as an H2 S–SCC susceptibility test solution when utilised in SSRT testing. Though no secondary cracks were detected in the materials investigated, both steels were observed to suffer a ductility loss upon exposure to this solution. In NACE type tests, the test pieces were subjected to constant loading at 80% of σy . Fracture did not occur for these samples.

Commentary by Dr. Valentin Fuster
2005;():313-319. doi:10.1115/OMAE2005-67563.

FITNET is a four-year European thematic network with the objective of developing and extending the use of fitness-for-service (FFS) procedures for welded and non-welded metallic structures throughout Europe. It is partly funded by the European Commission within the fifth framework programme and commenced in February 2002. The network currently consists of about 50 organisations from 17 European countries but also includes contributions from organisations in the USA, Japan and Korea. Further information can be found in the FITNET TN website: http://www.eurofitnet.org. The FITNET FFS Procedure is built up in four major analysis modules namely; Fracture, Fatigue, Creep and Corrosion and the procedure is being developed for completion in early 2006 in the form of CEN Document. The aim of this paper is to present the features, main analysis routes and major areas of technical development pertinent to the Fracture Module of the FITNET FFS Procedure. The procedure is based on previous developments carried out within the SINTAP project as well as advances in other standards such as the British Energy R6 rev 4 and the current amendments to the British Standard BS7910. In addition the work from other EU projects has been used to extend the treatment of several problem areas, such as the effect of constraint and the treatment of thin walled structures.

Commentary by Dr. Valentin Fuster
2005;():321-328. doi:10.1115/OMAE2005-67564.

A number of Fitness-for-Service (FFS) procedures (include analytical methods) have been specifically developed and used to address the components of a particular industrial sector. A number of industrial sectors, such as nuclear power, petrochemical, offshore, aerospace or pipeline girth weld applications have established FFS standards in place for the assessment of flaws found in-service. Some methods for design and remaining life assessments of fatigue-loaded structures are still unduly conservative in different loading regimes. Hence, there is still a need to generate a general purpose, unified, comprehensive and updated FFS methodology in Europe by covering four major failure modes (fracture, fatigue, creep and corrosion) in metallic load bearing components with or without welds. As a result, the European Community funded the project FITNET in the form of a Thematic Network (TN) organisation to review the existing FFS procedures and develop an updated, unified and verified European FITNET FFS Procedure to cover structural integrity analysis to avoid failures due to fracture, fatigue, creep and corrosion. FITNET TN is a four year project with the objective of developing and extending the use of FITNET FFS Procedure for welded and non-welded metallic structures throughout Europe. It is partly funded by the European Commission within the fifth framework programme and launched at February 2002. The network currently consists of 50 organisations from 16 European and three non-European countries representing various industrial sectors and academia. Further information can be found in the FITNET TN website: http://www.eurofitnet.org . FITNET Fitness-for-Service analysis of engineering structures aims to provide better design principles, support for fabrication of new components, prevention of service failures due to fracture, fatigue, creep and corrosion damages (no coverage of structural instability due to buckling). FITNET FFS criteria can be used to establish the size limits for defects in various engineering structures and can provide substantial cost savings in operating such structures. The use of the FITNET FFS Procedure involves making an assessment of a component containing a defect to ensure its structural integrity for its intended design life or until its next inspection period. The outcome of the assessment of a component in service is a decision to operate as is, repair, monitor (including re-setting of inspection intervals), or replace. The aim of this paper is to give an overview of the objectives and technical content of the FITNET FFS Procedure currently developed and validated by the European Fitness for Service Network FITNET and hence inform the offshore technical community.

Commentary by Dr. Valentin Fuster
2005;():329-338. doi:10.1115/OMAE2005-67565.

FITNET is a four-year European thematic network with the objective of developing and extending the use of fitness-for-service (FFS) procedures for welded and non-welded metallic structures throughout Europe. It is partly funded by the European Commission within the fifth framework program and it was launched in February 2002. The network currently consists of about 50 organisations from 17 European countries and supported by institutions from USA, Japan and Korea. Further information can be found in the FITNET TN website: http://www.eurofitnet.org. The FITNET FFS Procedure is built up in four major analysis modules namely fracture, fatigue, creep and corrosion. The first official draft is available in early 2006 in the form of an official CEN document. Industrial components are as a rule exposed to fluctuating loads and hence consideration of fatigue damages accumulation or of fatigue crack growth is a critical issue. The aim of this paper is to present the features and the main analysis routes of the FITNET FFE Fatigue Module of the FITNET FFS Procedure to assess the fatigue life of the load carrying metallic components manufactured with and without welds. The paper includes an industrial case from ship structure for the application of two fatigue assessment routes.

Commentary by Dr. Valentin Fuster
2005;():339-343. doi:10.1115/OMAE2005-67566.

A summary is given of the key steps in undertaking an assessment of structural integrity where stress corrosion cracking or corrosion fatigue (collectively environment induced cracking) is the dominant failure mode.

Commentary by Dr. Valentin Fuster
2005;():345-351. doi:10.1115/OMAE2005-67567.

Shallow cracks in components subjected to tension have lower hydrostatic and maximum principal stresses near the crack tip than deep cracks in bending stress fields. These lower stresses near the crack tip lead to increased load carrying capacity compared to that estimated using the fracture toughness obtained from deeply cracked bend specimens. This is referred to as ‘loss of constraint’ and fracture assessment procedures such as R6 contain methods for estimating the increased load carrying capacity under these conditions. More generally, low constraint occurs near blunt notches and at small cracks emanating from such notches. Within the European project FITNET, methods are being developed to address constraint effects both for shallow cracks and for notches and cracks at notches. After an initial review, this paper describes the constraint procedures being developed within FITNET. These are separate procedures for cracks, notches and cracks at notches. The paper then discusses how these separate procedures might, in the future, be combined to produce an overall constraint procedure.

Commentary by Dr. Valentin Fuster
2005;():353-364. doi:10.1115/OMAE2005-67568.

Recent developments of the advanced welding processes such as laser beam welding (LBW), solid state friction stir welding (FSW) and hybrid welding, numbers of advanced structures are being designed and constructed in industries such as aerospace, power generation, oil and gas transmission and transportation. Development of new structural aluminum and magnesium alloys as well as high strength steels provide further possibilities for the welded structures in similar and dissimilar (material-mix) configurations. Consequently, there is an increasing demand for “Fitness-for-Service” (FFS) assessment of those advanced welded structures by considering the specific features of these weld joints (such as narrow weld width, high strength mis-match, etc.). In year 1999, S tructural Int egrity A ssessment P rocedure SINTAP has been developed for analysis of flaws to avoid fracture within the European Commission funded project SINTAP. Recently, the European Community funded project FITNET in the form of a Thematic Network (TN) organisation has started to review the existing FFS procedures and develop an updated, unified and verified European FITNET FFS Procedure to cover structural integrity analysis to avoid failures due to fracture, fatigue, creep and corrosion. This new FFS Procedure has adopted the SINTAP approach for assessing of the welded structures. This paper describes the FITNET FFS weld assessment route and also aims to demonstrate suitability of weld joint assessment route of the FITNET FFS Procedure in prediction of the critical conditions of various advanced welded joints containing flaw. The welded specimens used in this work cover conventional multi-pass welded Inconel-718 turbine blade (T-joint), center cracked wide plates of electron beam welded 13% Cr supermartensitic stainless steel, laser beam welded shipbuilding C-Mn steel and aluminum alloy. The results are showing that the weld strength mismatch analysis option of the FITNET FFS is conservative and degree of conservatism is similar to the analysis options for the homogeneous materials. This provides confidence in the use of the FITNET FFS procedure for assessing of the structural significance of flaws in welded structures.

Commentary by Dr. Valentin Fuster
2005;():365-368. doi:10.1115/OMAE2005-67569.

In an ideal situation, fracture toughness data to be used in structural integrity assessments are generated through the use of appropriate fracture mechanics-based toughness tests. In reality, such data are often not available or cannot be easily obtained due to lack of material or the impracticability of removing material from the actual structure. In such circumstances, and in the absence of appropriate historical data, the use of correlations between Charpy impact energy and fracture toughness can provide the fracture toughness value to be used in the assessment. The FITNET (Fitness-for-Service) procedure, presently being developed in the frame of a European Thematic Network, includes a section which deals with the use of empirical correlations between Charpy and fracture toughness data. This paper will outline the contents of this chapter, along with some examples of application of selected correlations to actual test data.

Commentary by Dr. Valentin Fuster
2005;():369-374. doi:10.1115/OMAE2005-67573.

Pipeline applications that are subject to global plastic strains require specific testing and qualification programs intended to verify the strain capacity of the girth welds. Such strain demands are generally beyond the limits of standard ECA applicability which normally cover demands up to 0.5% strain. Therefore, qualification of welding procedures for high strain environments require significantly more testing than weld procedures intended for stress-based designs. The plastic strain capacity of girth welds is a function of the pipe and weld metal properties, as well as the maximum flaw size allowable in the girth weld. Specific weld metal/heat affected zone properties, based on small scale testing, should be combined with full scale curved wide plate testing of girth welds that include artificial flaws.

Topics: Pipelines
Commentary by Dr. Valentin Fuster

Pipeline Technology

2005;():377-384. doi:10.1115/OMAE2005-67018.

Design implies choice from among alternatives — for each alternative solution the designer should assess the total associated risk. The total risk is a composite of both the likelihood of various levels of hazards, and the uncertainty in the response behaviour of the system. The selection or decision process demands a weighing of the costs, risks, and benefits of the alternatives. Using an example, a probabilistic method is outlined for deciding upon the depth of burial of pipelines in shipping lanes. Producing a well-engineered solution to this problem involves the synthesis of many kinds of information. There is a requirement to analyse statistical information on the types of hazards, their frequencies and their possible growth. There is also generally a choice of measures that can be used to protect the pipeline. The choice of protective measure demands a weighing of costs, risks, and benefits and must also include any characteristic which leads to economic or other losses. In this paper each hazard is classified into five levels based on the amount of damage a hazard could impart on the system (this is termed the “demand levels”). These demand levels, when subjected to the system, result in various damage states. The damage states, which are assumed to be the cost of repair or replacement, are also represented in five discrete forms. These five general states of damage are defined in terms of such loss-related factors as level of repair cost, degree of the structural and non-structural damage and the fraction of people injured or killed.

Topics: Pipelines
Commentary by Dr. Valentin Fuster
2005;():385-393. doi:10.1115/OMAE2005-67039.

Modern micro-alloyed, control-rolled TMCP steels generally have good strength, high toughness, and good weldability. However, these valuable properties come along with certain undesirable features, such as low strain hardening (high yield to tensile ratio), low ductility as measured by uniform elongation (elongation at ultimate tensile strength), and possible heat-affect-zone (HAZ) softening due to reduced hardenability. These undesirable features are particularly detrimental in strain-based design of pipelines. Although the phenomenon of HAZ softening has been known for a long time, the impact of the HAZ softening on the integrity of pipeline girth welds was not well understood. The objective of this work was to understand the impact of HAZ softening on girth weld integrity. Finite element analysis was conducted to investigate the effects of HAZ softening on crack driving force and strain concentration in girth welds under longitudinal tensile loading. The material properties of WM and BM were obtained from an X100 girth weld. The HAZ was modeled as a functionally graded material based on its measured hardness. The models contained surface-breaking defects located at the fusion boundary simulating lack-of-sidewall fusion defects. The analysis results showed that increased CTOD driving force can be expected due to HAZ softening. The extent of increase is positively related to the width and degree of softening of the HAZ. On the other hand, weld strength overmatch reduces the total CTOD driving force. The strain concentration in the softened HAZ circumferentially remote from a surface-breaking defect was small. However, high strain concentration existed over the circumference covering the length of the defect. This concentration was primarily attributable to the existence of the defect and secondarily to the HAZ softening. One significant result from this work was that the relative increase in CTOD driving force and strain concentration due to HAZ softening was independent of defect size. In other words, on a relative basis, HAZ softening was no worse on large defects than on smaller defects. This result should be helpful in rationalizing the effects of HAZ softening for defects of various sizes that exist in field applications. Non-symmetrical crack-tip deformation occurred with softened HAZ. A large proportion of the crack-tip deformation was located in the HAZ. The magnitude of non-symmetric deformation increased with the increase of HAZ width and degree of softening. Even higher degree of non-symmetric deformation occurred with the increase of weld overmatching level. The structural significance of reduced total CTOD driving force and increased un-symmetric deformation at the crack tip due to weld strength overmatch warrants further study. The reduction in total CTOD driving force alone does not necessarily results in a higher level of weld integrity if the “intrinsic” toughness of the HAZ is substantially lower than the weld metal.

Commentary by Dr. Valentin Fuster
2005;():395-401. doi:10.1115/OMAE2005-67043.

The most important problem in pulling pipeline in the directionally drilled hole is overcoming of the soil resistance. This paper presents the 2-D model of contact interaction between the pipeline and soil as well as a numerical method of calculation of the pipeline vertical displacement and drag while pulling. The model is based on dynamic equation of lateral forces. Using this model it is quite easy to take into account nonlinear contact interaction of pipeline and soil in terms of both contact force and friction drag as well as a convective nature of the lateral load propagation at an axial motion of pipeline. Based on the suggested numerical model, a computer program was developed to improve an accuracy of pulling process analysis with moderate computer time needed. The developed program was applied to analyze and select parameters of the Lisichansk-Tikhoretsk oil pipeline in the tunnel under the river of Don. Calculations provided the updating of pulling process parameters and identification of those tunnel intervals, which might be challenging with pipelining.

Commentary by Dr. Valentin Fuster
2005;():403-417. doi:10.1115/OMAE2005-67059.

In this paper the burst tests of seven tubular specimens are presented. In these tests the tubular specimens were loaded with internal pressure only. The specimens were cut from longitudinal welded tubes made of API 5L X80 steel with a nominal outside diameter of 457.2 mm (18 in) and a nominal wall thickness of 7.93 mm (0.312 in). The specimen IDTS 1 is a defect-free pipe. The specimen IDTS 2 contains only one defect, herein called base defect. The base defect is an external flat bottomed defect with uniform width (circumferential dimension). The other five specimens contain groups of interacting defects constituted by the combination of two or more base defects. All the defects were machined using spark erosion. Measurements were carried out in order to determine the actual dimensions of each tubular specimen and its respective groups of defects. Tensile specimens and impact test specimens were tested to determine material properties. The failure pressures measured in the laboratory tests are compared with those predicted by six assessments methods, namely: the ASME B31G method, the RSTRENG 085dL method, the DNV RP-F101 method for single defects, the RPA method, the RSTRENG Effective Area method and the DNV RP-F101 method for interacting defects.

Commentary by Dr. Valentin Fuster
2005;():419-429. doi:10.1115/OMAE2005-67074.

The development of finite element analysis, in terms of simulation power and theoretical model accuracy, enables one to understand and simulate industrial processes more precisely, especially those involving non linear behaviour and analysis. Reeled pipe technology is one of these, and has a lot to gain from this increasing efficiency. In the reel-lay process the pipe is first reeled onto a drum on a vessel for transportation. During offshore installation the pipe is unreeled, straightened and deployed into the sea. During the process, the pipe is fully and cyclically plastified. Plastification modifies the pipe properties, which is not by itself detrimental but should be understood by the designer. Pipe properties are affected in three ways: geometrical shape – reeling and straightening induce some residual ovalisation; mechanical properties – yield stress, hardening slope, isotropy are modified; and fatigue properties. Technip and IFP have studied these property evolutions for many years, both from an experimental and a numerical point of view. The present paper discusses the first two points. A wide experimental programme has been performed. Full scale pipes were reeled and straightened on a bending rig device especially built for that purpose. Pipe ovalisation was monitored through the whole process. Pipe mechanical properties were also fully characterised in the pipe axial, hoop and thickness directions, both in tension and compression, before and after reeling process. Extruded and UOE pipes were tested and characterised. Pipe initial properties are dependent on the manufacturing process but they are modified by the reeling process. Reeling induces some anisotropy that cannot be properly accounted for by usual plasticity models. Finite element simulations with Abaqus software, using the material behaviour of unreeled pipe, underestimate stiffness evolution in the hoop direction and overestimate ovalisation induced by the reeling process. Anisotropy has indeed a great effect on ovalisation that results from an interaction between axial and hoop loading. Hardening is also a key parameter. A new plasticity model has been written in an Abaqus User Material Model, known as UMAT. The new model is based on an anisotropic Hill criterion and special attention is paid to the hardening. This new model reduces by more than two the error on ovality estimation, and gives a realistic prediction of material anisotropy evolution through the process. Although, the tuning of the model coefficients is more complex than for usual models, its use is quite straightforward and does not increase computation time.

Commentary by Dr. Valentin Fuster
2005;():431-435. doi:10.1115/OMAE2005-67082.

The objective of this paper is to summarize current ultra-deepwater (i.e. up to 3,500 meters water depth) pipeline mechanical design methodologies in the Gulf of Mexico (GOM), and the rest of the world. The standard mechanical design for ultra-deepwater pipelines in the GOM is based on API RP 1111. API code also has been used for deepwater projects in west Africa. DNV code OS-F101 was mostly used for deepwater projects in offshore Brazil and Europe. Some pipeline designs in the GOM have started to incorporate parts of the DNV design methodology. A discussion of collapse only design and combined loading (i.e. pressure + bending) design will be presented. The buckle arrestors shall be designed along with the ultra-deepwater pipeline mechanical design. The best design criteria are obtained from physical full-scale collapse testing. The comparison of the physical test data and collapse calculations using the DNV and API codes will be presented. It was found that the conservatism still exists within collapse prediction for ultra-deepwater pipeline using modern design codes such as DNV OS-F101 and API RP 1111. Recommendations of the mechanical design methodologies for the ultra-deepwater pipeline are presented.

Topics: Pipelines , Collapse
Commentary by Dr. Valentin Fuster
2005;():437-446. doi:10.1115/OMAE2005-67102.

Often in heated pipeline projects in the shore approach area the pipeline is buried with the disturbed soil but a partial natural recovering inevitably takes place once the trench is open. This condition may not be enough to ensure the required pipeline soil support for its design life. This peculiar condition of shallow and insufficient soil support may lead to an upheaval buckling. Hence, this paper intends to develop a numerical model to simulate the thermo-mechanical buckling phenomenon of buried heated pipelines in shore approach areas. Furthermore, a sensibility analysis adopting geometrical, physical and operational data is carried out for a particular Petrobras project. The in situ geophysical survey, geotechnical and oceanographic information are also taken into account. The influence of the sediment transport after opening the trench and required backfilling with a heavier soil is also addressed. This paper focuses on the challenges for the design and installation group, due to the burial requirements, in order to find a safe solution for the burying process by trenching and backfilling known methods.

Topics: Pipelines
Commentary by Dr. Valentin Fuster
2005;():447-455. doi:10.1115/OMAE2005-67106.

This paper investigates the higher order geometrical effects on the deflections of a helical armour wire when the pipe is subjected to uniform bending and is away from any end effects from its remaining fittings (end fittings), and the effect on the subsequent bending stresses. Due to its complexity in both geometrical shape and loading conditions, the approaches found in the literature are often to assume either a bent helix or geodesic as its deflected configuration with linearized mathematical expressions for simplification. The bending stresses are then calculated based on the geometrical difference between the assumed deflection and the original helical shape. The effect of the wire cross-section characteristics, for example the width and thickness ratio, over its deflection are often ignored. This paper presents an analytical strain energy model to quantify the influence of the wire width and thickness ratio over its final deflection. The higher order geometrical effects are fully considered in determining the wire deflection by using the exact mathematical expressions, and in the subsequent wire deformation and stress calculations. The paper also discusses briefly the structural coupling behavior between the pipe axial and bending deformations raised by using the exact expressions. The analytical results are validated by finite element simulation of an identical structure. The results are shown in good agreement in both deflection and the bi-normal bending stress. The results also show desirable conservatism in the normal and the total bending stresses. The presented analytical approach is demonstrated as an efficient and conservative way for investigating the behaviour of such a helical wire.

Topics: Wire , Pipes , Armor
Commentary by Dr. Valentin Fuster
2005;():457-461. doi:10.1115/OMAE2005-67109.

Heavy oil production in deepwater is a major challenge to pipeline technology due to high viscosity and associated high pressure drop. In this paper, we propose to apply active electrical heating in multilayered composite pipelines for heavy oil production in deepwater. A thermal-hydraulic analysis is carried out to evaluate the pressure drop reduction and power input requirement. We concluded by comparative numerical studies that the heated sandwich pipeline provides a significant pressure drop reduction and thus a boost of mass flow rate of the oil production for a given available pressure drop between the wellhead and the separator.

Commentary by Dr. Valentin Fuster
2005;():463-478. doi:10.1115/OMAE2005-67147.

Steel pipes are studied considering external pressure, axial compression and bending actions to empty condition, taking into account the initial geometrical imperfections. The main objective is to study the behavior and structural stability of these pipes submitted to combine loads, considering the influence of the geometrical imperfections and to estimate the critical external pressure. Critical pressure of buckling, and modal configurations are evaluated by theoretical methods and numerical approaches such as finite element method (FEM). The numerical results are compared with theoretical results.

Commentary by Dr. Valentin Fuster
2005;():479-488. doi:10.1115/OMAE2005-67182.

The present paper examines the denting deformation of offshore pipelines and tubular members (D/t≤50) subjected to lateral (transverse) quasi-static loading in the presence of uniform external pressure. Particular emphasis is given on pressure effects on the ultimate lateral load of tubes and on their energy absorption capacity. Pipe segments are modeled with shell finite elements, accounting for geometric and material nonlinearities, and give very good predictions compared with test data from non-pressurized pipes. Lateral loading between two rigid plates, a two-dimensional case, is examined first. Three-dimensional case, are also analyzed, where the load is applied either through a pair of opposite wedge-shaped denting tools or a single spherical denting tool. Load-deflection curves for different levels of external pressure are presented, which indicate that pressure has significant influence on pipe response and strength. Finally, simplified analytical models are proposed for the two-dimensional and three-dimensional load configurations, which yield closed-form expressions, compare fairly well with the finite element results and illustrate some important features of pipeline response in a clear and elegant manner.

Commentary by Dr. Valentin Fuster
2005;():489-498. doi:10.1115/OMAE2005-67216.

Sandwich pipes composed of two steel layers separated by a polypropylene annulus can be used for the transport of oil&gas in deepwaters, combining high structural resistance with thermal insulation in order to prevent blockage by paraffin and hydrates. In this work, sandwich pipes with typical inner diameters of those employed in the offshore production are analyzed numerically to evaluate the ultimate strength under external pressure and longitudinal bending as well as the effect of the reeling installation method on the collapse pressure. Numerical models were developed using the commercial finite element software ABAQUS. The validation was based on experimental results. The analyses for combined loading were performed using symmetry conditions and the pipe was reduced to a ring with unitary length. The analysis of bending under a rigid surface was simulated numerically according to the experiments performed using a bending apparatus especially built for full scale tests. Symmetry conditions were employed in order to reduce the analysis to a quarter of a pipe. Mesh sensitivity studies were performed to obtain an adequate mesh refinement in both analyses. The collapse pressure was simulated numerically either for the pre or post reeling process. Bauschinger effect was included by using kinematic hardening plasticity models. The influences of plasticity and out-of-roundness on the collapse pressure have been confirmed.

Commentary by Dr. Valentin Fuster
2005;():499-507. doi:10.1115/OMAE2005-67260.

The Blue Stream pipeline is a gas transmission line delivering natural gas from the Russian grid. across the Black Sea, to Turkey. The submarine portion of this pipeline consists of a pair of 24-inch diameter, 31.8 mm wall thickness, API grade X65 pipelines running almost 400 km along the floor of the Black Sea. Over one half of the submarine pipeline lies at a water depth of more than 2000 metres, with the deepest portion of the line reaching a depth of 2150 metres. First gas was supplied through the lines in February of 2003. There were numerous engineering obstacles facing this technically challenging pipeline undertaking. including a lack of heavy-wall, large diameter pipe experimental data to support the pipeline design. Recognizing this need, PeterGaz commissioned a collapse test program during the preliminary engineering phase of the project to generate these data and to gain a better understanding of pipe behavior under combined loading conditions. Numerous full-scale tests were performed on prototype pipe samples, including external pressure testing, combined external pressure and bend testing, and bend testing. Hundreds of material coupon tests were also performed to characterize material stress strain behavior around the circumference of the pipe, through the wall thickness of the pipe, and before and after UOE manufacturing. Tests were also performed to quantify the strength recovery of thermally treated pipe material resulting from the pipe coating process. This paper presents the results of this experimental work and provides some comparisons to collapse predictions.

Topics: Collapse
Commentary by Dr. Valentin Fuster
2005;():509-519. doi:10.1115/OMAE2005-67261.

The loss of metal in a pipeline due to corrosion usually results in localized pits with various depths and irregular shapes on its external and internal surfaces. The effect of external corrosion defects was studied via a series of small-scale experiments and through a nonlinear numerical model based on the finite element method. After calibrated in view of the experimental results, the model was used to determine the collapse pressure as a function of material and geometric parameters of different pipes and defects.

Commentary by Dr. Valentin Fuster
2005;():521-529. doi:10.1115/OMAE2005-67303.

The currently valid worldwide standards allow for taking into consideration plastic deformations in order to achieve a higher degree of utilization. The maximum plastic strains, which can be allowed for steel pipes subjected to internal pressure and additional loads, are particularly interesting. In this paper results of investigations on the elasto-plastic bearing behavior of steel pipelines subjected to internal pressure and bending are presented. Four-point bending tests on eight steel pipes were carried out in order to make the buckling analysis in the elasto-plastic range possible. Finite-element-models were checked by test results for the application on buried pipelines. Taking into account bedding conditions of the pipeline in the soil was made possible. Furthermore, an analytical method based on the differential equation for beams with longitudinal tensile force and variable bending stiffness was developed. It is suitable to determine the elasto-plastic bearing capacity for internal pressure and bending. The collapse due to plastic shell buckling is considered by a limit criterion based on critical strains.

Commentary by Dr. Valentin Fuster
2005;():531-538. doi:10.1115/OMAE2005-67321.

Bend stiffeners are polymeric structures employed to ensure a smooth and safe transition in the upper connection of risers and umbilical cables, protecting them against accumulation of fatigue damage and excessive curvatures. Recent failures have stimulated a better understanding of the mechanical response in order to increase the reliability in design and analysis of bend stiffeners. This work presents a mathematical formulation that represents the system riser/bend stiffener considering geometric non linearity and polyurethane with viscoelastic behaviour, an inherent characteristic to polymers. The following assumptions are considered: cross-sections remain plane after deformation, large deflections are accepted but it is a small strain bending problem, the self-weight and external forces are disregarded and the material is assumed with linear viscoelastic behaviour. The curves that represent the viscoelastic response of the material have been raised by means of creep tests, whose specimens were cut from actual bend stiffeners. The time dependent data obtained in the experimental tests were well approximated by a third order Prony series which describes the creep function. The set of four first order non linear ordinary differential equations results from geometrical compatibility, equilibrium of forces and moments and linear viscoelastic constitutive relations. The numerical solution of the problem is obtained using a one-parameter shooting method. The results are then compared with the consolidated numerical solution for linear elastic material. It is concluded that the viscoelastic phenomena can lead to excessive curvatures on the upper terminations of risers and umbilical cables if the polymeric structure were designed considering elastic behaviour. The correct characterization of the viscoelastic properties of polyurethane used on bend stiffeners must be taken into account when accurate analysis is desired.

Commentary by Dr. Valentin Fuster
2005;():539-546. doi:10.1115/OMAE2005-67369.

Monitoring of the effects of hydrocarbon pipeline blockages such as may be caused by hydrates and waxes is receiving a higher level of consideration as the distributed sensing capability offered by the use of fibre optic technology matures. The extent of the hydrate or wax formation problem increases with pipeline length through the effects of cooling. The challenge is significantly greater when assuring flows in deep water and remote subsea locations. Commercially available strain and temperature sensing equipment such as discrete FBGs (Fibre Bragg Gratings) and fully distributed sensing techniques such as Raman DTS (distributed temperature sensor) and Brillouin OTDR (optical time domain reflectometry) typically offer sensing lengths of the order of 20–30km. Whilst this is in many instances a useful length, it is not sufficient to be able to monitor the whole of a pipeline which may be several hundreds of kilometres in length. The authors have developed and demonstrated a method for extending the reach of a Brillouin OTDR interrogating system such that sensing sections of conventional length (approximately 25km) can be successfully interrogated from distances well in excess of 100km without having to compromise on the performance. With a single instrument, more than 250km of sensing fibre can be monitored to within 1.5 metre resolution. By this means, temperature and strain profiles may be measured for the entire pipeline length which will enable active flow assurance measures to be taken including identifying the presence, nature and extent of blockages as they form. Consequently, any corrective action taken by the pipeline operators will be on an informed basis (such as the injection of an optimised quantity of inhibitor), and will incur a significantly lower level of risk than is currently possible. This paper describes the technology which has been developed to meet this requirement and provides results of simulated pipeline blockage effects which demonstrate this.

Commentary by Dr. Valentin Fuster
2005;():547-556. doi:10.1115/OMAE2005-67400.

The Petrobras Capixaba North Terminal - TNC is located in the state of Espirito Santo, in Brazil and is being built to receive the heavy and high viscosity oil produced onshore in the Fazenda Alegre field. This oil shall be heated prior to be pumped into the pipelines and it will be exported through a monobuoy and a tanker system. The two export pipelines are being laid to connect the onshore Terminal to a subsea PLEM to be installed under the monobuoy. The pipelines and PLEM were designed to operate with oil containing H2 S in cyclic high temperature. This paper addresses the special concerns defined by the design activity to cope with the TNC operation conditions. It also focuses the modifications imposed to the installation process to fulfill the design and operation requirements.

Commentary by Dr. Valentin Fuster
2005;():557-561. doi:10.1115/OMAE2005-67401.

The increased demand for high strength linepipe for onshore and offshore pipeline systems has been well documented over the past few years. The economic benefits have been demonstrated, and solutions have been developed to address the technical issues facing high strength linepipe use. However, there are still a few unanswered questions, one of which is addressed in this paper: what is the effect of thermal treatment during the pipeline coating process on the material behaviour of high strength linepipe? This paper presents the results of a thermal coupon study investigating the effects of low temperature heat treatment on the tensile and compressive stress strain curves of samples taken from X100 linepipe. Thirty axial test coupons and thirty circumferential test coupons were machined from a 52 inch diameter, 21 mm wall thickness UOE X100 linepipe. Some of the coupons were maintained in the as-received condition (no heat treatment) while others were heat-treated in a manner that simulates a coating plant induction heat treatment process. All coupons were subsequently tested in tension or compression, either at room temperature or at −18°C. This study has provided a number of interesting results. In regards to material strength, the heat treatment increased the tensile and compressive yield strengths in the longitudinal and circumferential coupons. Axial tensile, axial compressive and circumferential tensile yield strength increases ranged from 5 to 10%. Circumferential compressive yield strength increases ranged from 14 to 24%. A Y/T ratio increase of approximately 7% was observed for all heat-treated tensile coupons. The coupon tests conducted at −18°C were only slightly different than their room temperature counterparts; with an average yield strength increase of 4% in all directions and orientations and a slight reduction in Y/T ratio.

Commentary by Dr. Valentin Fuster
2005;():563-570. doi:10.1115/OMAE2005-67441.

This paper presents two high temporal resolution current measurement campaigns where the current is sampled and stored at a frequency of 1Hz. One of the campaigns also included high lateral resolution measurements. The measurements were part of the current measurement program conducted by the Ormen Lange project. The measurements are rather unique and the first successful campaigns of its kind on the Norwegian Continental Shelf. The main objective of these measurements was to measure the turbulence characteristics of the current flow. This was regarded as important input to the design of the numerous free pipeline spans in the Ormen Lange development resulting from the very uneven seabed in the area. The frequencies of particular importance were the frequencies of the first eigenmodes of the free pipeline spans being in the range 0.1–1 Hz. The measurements confirm the shape of the turbulence following the Kolmogorov –5/3 decay law in this frequency range. Measurements along the pipeline route did not reveal any significant differences in the near seabed turbulence intensity along the route. Extrapolating the measurements into a near seabed record mean current speed of 0.5m/s gives a turbulence intensity of approximately 0.1. The high lateral resolution measurements demonstrate local topographic influence on record mean current. This concerns topographic steering, damping and amplification of current speed.

Commentary by Dr. Valentin Fuster
2005;():571-578. doi:10.1115/OMAE2005-67453.

Free spans often become a challenge in pipeline design and operation due to pipeline installation on uneven seabed or seabed scouring effects. The costs related to seabed correction and span intervention are in many projects considerable. Therefore it is relevant to investigate whether such intervention work is necessary or not. Despite the complexity inherent in free span response, the spans are often designed applying presumably conservative concepts and very simple analytical tools. The DNV guideline no 14 (GL14) for free spanning pipelines was issued in 1998 and has later been updated and issued as an Recommended Practice (DNV-RP-F105) to account for recent technical research and development and accumulated experience applying GL14 in pipeline projects. This code allows vortex induced vibrations (VIV) as long as the pipeline integrity is ensured, by for example checking that the fatigue life is sufficient. By giving design methodology and acceptance criteria for fatigue, the DNV-RP-F105 approaches the real physics of free spans in a better way than older codes and makes it possible to select cost-effective methods both in the design phase and later when re-assessing spans in the operational phase. This paper will briefly discuss some experiences obtained by using the DNV-RP-F105 in free span design/re-assessment. Some examples of pipeline failures due to free span and vortex induced vibrations will also be presented.

Commentary by Dr. Valentin Fuster
2005;():579-590. doi:10.1115/OMAE2005-67472.

The BP West Sole gas field is located in Block 48/6 in the UK sector of the southern North Sea, about 70 km off the Holderness coast. Production from the gas field is exported to a shore terminal at Easington by two pipelines. Both pipelines were trenched at installation. Pipeline surveys over the last few years show that both pipelines are substantially exposed at the shore approach and inshore sections. This has occurred in part due to the retreat of the cliffs in Easington and subsequent lowering of the sea bed level and also the migration of sand from around the pipelines leaving them largely unburied and sitting on the local clay abrasion platform. It has been concluded that both pipelines require stabilisation sooner rather than later to reduce the risk of pipeline failure. Pipeline stabilisation options need to take account of the environment in which they have been placed. Easington is at a critical position along the Holderness coast. All net sediment transport from the Holderness coastline passes through this section. Any interruption to this movement could result in a change to the adjacent coast. Maintenance of the sediment budget is important to a wider area of the East coast of England. Stabilisation options must not reduce the net amount of sediment moving southwards past Dimlington and must not result in any long term negative impact on the coastal evolution. This paper outlines consultancy required and problems process regarding the geomorphological issues in getting acceptance from government and non-government bodies. A methodology has been developed that allows quantification of impacts of different options on the sediment budget and on the long-term coastal evolution (see also Chen et al 1998, 2001 and 2002). Application of this method aimed at providing understanding and information which is considered to be important in the process of selecting an optimal solution for the pipeline stabilisation in such an environmental sensitive coast.

Topics: Pipelines
Commentary by Dr. Valentin Fuster
2005;():591-596. doi:10.1115/OMAE2005-67477.

During S-laying a pipeline on the seafloor, the pipe is subjected to plastic strains over the stinger of the laying boat. The residue curvature may result in pipe axial rotation, which causes concerns about the orientation of the valves or connections. This paper presents two approaches to predict the pipe roll. The first approach idealizes the pipeline as a pre-bent helix by the residue curvature stretched on the solid floor. The second approach is a 3-D model, simulating the process of the pipe roll during the laying procedure in a quasi-static manner. The pipe loading path and deformation history are fully considered. Other factors, such as soil conditions, can be included. Empirical graphs for the pipeline axial roll versus a normalized residue curvature is established, by which the roll can be preliminarily estimated and the sensitivity of the roll to various parameters is explicitly expressed.

Topics: Pipelines
Commentary by Dr. Valentin Fuster
2005;():597-607. doi:10.1115/OMAE2005-67482.

In the recent years, the offshore pipeline industry has been under pressure to provide solutions for demanding material and line pipe technology problems, installation technology to safely tackle the ultra-deep waters challenge, quantitative prediction of reliable operating lifetime for pipelines under high pressure/high temperature conditions and remedial measures to tackle considerable geo-morphic and human activity related hazards. Future pipelines are being planned in very difficult environments, i.e. crossing ultra-deep water and difficult geo-seismic-morphic conditions. In these circumstances, it is of crucial importance (1) to adopt advanced design procedure and criteria, possibly based on limit state principles recently implemented in the design codes, and (2) to use advanced engineering tools for predicting the strength capacity and the pipeline behaviour during the installation and operational phase, in order to design the pipeline safely and to assess properly the technic-economical feasibility of the project. This paper discusses the relevant failure modes for offshore pipelines, the FE analysis results relevant to the sectional capacity of thick-walled pipes, and the FE analysis results relevant to the global and local response effect of a pipeline, laid on the sea bottom, and subject to a point-load force.

Commentary by Dr. Valentin Fuster
2005;():609-617. doi:10.1115/OMAE2005-67484.

Sandwich pipes have been considered feasible conceptions for ultra deepwater pipelines, since they are capable to work at low temperatures and withstand high hydrostatic pressures. Sandwich pipelines are composed by inner and outer metallic pipes and a suitable core material which must provide high compression strength and good thermal insulation. The aim of this ongoing research is to study the quasi-static propagation of buckles in sandwich pipes. In this paper, a three-dimensional finite element model considering material and geometric nonlinear behavior is presented. The mesh discretization is determined through a detailed mesh sensitivity analysis. Some experiments with small scale models combining aluminum pipes and polypropylene as core material were carried out to calibrate the numerical model. The propagation pressure is evaluated under different bonding conditions between pipe layers.

Commentary by Dr. Valentin Fuster
2005;():619-627. doi:10.1115/OMAE2005-67487.

In the last ten years, several studies were completed with the aim to define a design format for the local buckling of pipes subjected to differential pressure, axial load and bending moment. Experimental tests were carried out and simplified analytical solutions were developed in order to predict the pipe bending moment capacity and the associated level of deformation. Standard finite element (FE) structural codes, such as ABAQUS, ADINA, ANSYS, etc., were and are used as a “numerical testing laboratory”, where the model is suitably calibrated to few experimental tests. The outcomes of these research efforts were implemented in the design equations enclosed in international design rules, as DNV OS-F101. The local buckling design formats, included in these rules, give the limit bending moment and associated longitudinal strain as a function of the relevant parameters. The effect of the girth weld is introduced with a reduction factor only for what regards the strain at limit bending moment. This paper addresses the effects of the presence of the girth weld on both limit bending moment and corresponding compressive longitudinal strain. A 3-dimensional (3D) FE model developed in ABAQUS has been developed to perform a parametric analysis. The FE model results are shown to compare reasonably well with full scale experiments performed for on-shore pipelines. The limit bending moment is reduced by the weld misalignment and this reduction is also dependent on both internal pressure load and linepipe material mechanical strength. The FE results are compared with the limit bending moment calculated with DNV OS-F101.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2005;():629-636. doi:10.1115/OMAE2005-67502.

This paper discusses use of the effective axial force concept in offshore pipeline design in general and in DNV codes in particular. The concept of effective axial force or effective tension has been known and used in the pipeline and riser industry for some decades. However, recently a discussion about this was initiated and doubt on how to treat the internal pressure raised. Hopefully this paper will contribute to explain the use of this concept and remove the doubts in the industry, if it exists at all. The concept of effective axial force allows calculation of the global behaviour without considering the effects of internal and/or external pressure in detail. In particular, global buckling, so-called Euler buckling, can be calculated as in air by applying the concept of effective axial force. The effective axial force is also used in the DNV-RP-F105 “Free spanning pipelines” to adjust the natural frequencies of free spans due to the change in geometrical stiffness caused by the axial force and pressure effects. A recent paper claimed, however, that the effect was the opposite of the one given in the DNV-RP-F105 and may cause confusion about what is the appropriate way of handling the pressure effects. It is generally accepted that global buckling of pipelines is governed by the effective axial force. However, in the DNV Pipeline Standard DNV-OS-F101, also the local buckling criterion is expressed by use of the effective axial force concept which easily could be misunderstood. Local buckling is, of course, governed by the local stresses, the true stresses, in the pipe steel wall. Thus, it seems unreasonable to include the effective axial force and not the true axial force as used in the former DNV Pipeline Standard DNV’96. The reason for this is explained in detail in this paper. This paper gives an introduction to the concept of effective axial force. Further it explains how this concept is applied in modern offshore pipeline design. Finally the background for using the effective axial force in some of the DNV pipeline codes is given.

Commentary by Dr. Valentin Fuster
2005;():637-640. doi:10.1115/OMAE2005-67515.

The dynamic stress prediction methodology developed by Norton [1] for broad-band acoustic-induced vibration of piping systems is applied here to the failure data of Carucci and Mueller [2]. Proprietary noise and vibration prediction technologies are used in order to improve the accuracy and robustness of the predictions. This results in generalized dynamic stress predictions that clearly delineate the cases of acoustic fatigue and satisfactory operation documented by Carucci and Mueller, unlike other failure prediction procedures traditionally used by industry. Other advantages of the dynamic stress approach include (i) a theoretically sound approach to account for pipe wall thickness, pipe diameter and internal density; (ii) direct consideration of material fatigue properties and stress concentration effects; and (iii) the potential to evaluate fatigue life for transient blow-down conditions.

Commentary by Dr. Valentin Fuster
2005;():641-647. doi:10.1115/OMAE2005-67517.

Since modern pipelines usually display ductile fracture behaviour, fracture assessments accounting for ductile tearing should be used. In this work we use a simplified strain-based fracture mechanics equation in the probabilistic fracture assessments. Furthermore, we use the traditional tangency criterion between the crack driving force and the crack growth resistance, in calculation of the onset of critical ductile tearing. Additionally, two types of external load on the line-pipe are considered, namely strains due to external bending moments and internal pressure. We establish the probability of fracture for line-pipes with relevant diameter to thickness ratios, and thicknesses, for J-laid or S-laid offshore pipelines. The distinction between system effects, in which all defects are likely to be subject to the same loading, and cases where only a small part of the pipeline will experience high loading, is also discussed.

Commentary by Dr. Valentin Fuster
2005;():649-658. doi:10.1115/OMAE2005-67518.

In this paper a framework for a strain-based fracture mechanics crack driving force methodology for pipes with surface cracks, are presented. The model addresses the effects of crack depth, crack length, pipe diameter, wall thickness and yield to tensile ratio. Based on FE simulations, an equation to calculate the applied crack driving force, either through CTOD or J, has been derived. The equation is intended for use in cases where global plastic deformation occurs. A general approach to introduce the effects of biaxial loading, yield stress mismatch, and misalignment on the driving force, through definition of an effective wall thickness and an effective crack ligament height, is outlined. Models to quantify the effects of the different parameters are also derived. Finally, results are presented from comparison between 2D and 3D FE simulations and the predictions made by the proposed driving force equations.

Commentary by Dr. Valentin Fuster
2005;():659-668. doi:10.1115/OMAE2005-67519.

Recent development plans envisage the exploitation of very deep offshore reservoirs as well as transport of hydrocarbons at temperature and pressure conditions far more severe than in past projects. Technical feasibility of such projects requires higher material utilisation, and the design guidelines need to be improved to allow for the new design conditions. Fracture assessment methods have been used in the evaluation of pipeline integrity for several years. In particular, the verification of acceptable defect sizes for installation and operational loads are now widely used and assessment methods are referenced in pipeline standards and guidelines. However, design guidelines are still missing the calibrated safety factors and stringent design format required to let the fracture failure mode be consistent with the other failure modes in the pipeline design such as bursting, local buckling and fatigue. The Fracture Control Offshore Pipelines Project is a Joint Industry Research and Development Project, whose objective is to study the behaviour of defected girth welds in pipelines subject to construction and operational loads ever experienced before. Due to the envisaged high loading condition and the high costs of recent offshore pipeline projects it is important, with an accurate defect assessment analysis, to avoid delays caused by unnecessary repairs or failures because of flaws that should have been detected and repaired. The final objective is the development of specific design criteria in the form of a design guideline to be used in the verification and design of offshore pipelines against the fracture/plastic collapse failure of a defected girth weld. The design criteria are based on the application of reliability methods to calibrate the partial safety factors in compliance with the safety philosophy established by DNV OS-F101 and will include the rational application of new NDT techniques. The JI Project is carried over 5 years and has started in 2002. The JI project is sponsored by the industry (BP, ENI Norge, Hydro and Statoil) and by the Norwegian Research Council. This paper describes the current status of existing fracture assessment procedures for pipelines with particular attention to their limitations and the needs for development and a brief overview of the results obtained in the project so far as well as the challenges to be solved in the project.

Commentary by Dr. Valentin Fuster
2005;():669-674. doi:10.1115/OMAE2005-67520.

The following paper describes the experimental and analytical work that was carried out in the development of a continuous external trawling protection for the Direct Electrical Heating cables for the subsea flowlines on the Kristin Field in the North Sea. The developed trawl protection was able to absorb the required 5.0 kJ energy without damage to the cable. The Kristin field is the most extreme HP/HT subsea field developed in Norway to date (170 degC reservoir temperature and 911 barg pressure). This is the first time that a continuous trawl protection is applied to the full length of a flowline. The normal approach would be burying to avoid interaction with fishing activities, however since this is a HP/HT subsea flowline it was necessary with an extra cooling from unburied pipelines to reduce the riser temperatures.

Topics: Cables , Heating
Commentary by Dr. Valentin Fuster
2005;():675-681. doi:10.1115/OMAE2005-67521.

Surface cracks pose major challenges for the structural integrity of pipelines. In fracture assessment programs the use of constraint parameters, such as the T-stress, along with K, J or CTOD are important to account for the limitations of single-parameter fracture mechanics. However, the three-dimensional nature of surface cracks precludes detailed 3-D finite element modeling for routine calculations. Here line-spring/shell-element models are demonstrated to be an efficient and reasonably accurate tool for constraint estimation even under large deformation levels when general yielding prevails in the pipe. Envisaging the potential use of this procedure in fracture analysis of pipelines, a new software, LINKpipe, has been developed. The program has been developed as a part of the Joint Industry project Fracture Control Offshore Pipelines. The objective of this project is to study the behaviour of defected girth welds in pipelines subject to construction and operational loads ever experienced before. The calculations have been performed in close cooperation with the project participants; see presentations of project-colleagues at OMAE 2005: Bruschi et al (2005), O̸stby (2005), Nyhus et al (2005) and Sandvik et al (2005). In this paper the line-spring calculations are compared with 3-D FE calculations and computations according to BS 7910. A pipe geometry, with OD = 400mm, was selected for the comparisons. The line-spring calculations were close to the 3-D calculations, while BS7910 was very conservative for long cracks and unconservative for short cracks. In highly ductile materials, such as pipeline steels, considerably amount of stable crack growth can be tolerated prior to the final failure of the structure. A simple method for simulating ductile tearing in surface cracked pipes with the line-spring model has been developed. A detailed parametric study has been performed to examine the effect of ductile tearing for pipes loaded in tensile, bending and with internal pressure. A significant reduction in deformation capacity from the stationary case is noticed. As the crack depth increases, the effect of ductile tearing becomes more important. And under biaxial loading a significant reduction of the deformation capacity is found as the internal pressure is increased. The development of the line-spring methodology paves the way for a transition from to-days rule-based design to direct calculations.

Commentary by Dr. Valentin Fuster
2005;():683-690. doi:10.1115/OMAE2005-67523.

Recent development plans envisage the transport of hydrocarbons at temperature and pressure conditions far more severe than in past projects. Technical feasibility of certain inter field lines was put in doubt as a consequence of application of design guidelines currently in force. This fact gave rise to a critical review of design criteria. The HotPipe Project is a Joint Industry Research and Development Project, whose overall objective is to prepare a DNV Recommended Practice to be used in structural design of high temperature/high pressure pipelines. The DNV-RP will cover most practical cases where pipelines are subjected to high internal pressure and high temperature (HP/HT). The design criteria are based on the application of reliability methods to calibrate the partial safety factors in compliance with the safety philosophy established by DNV OS F101. The overall objective was pursued by performing the following subprojects: • Pipe Capacity: Experimental tests were carried out and FE models were developed aiming at establishing the failure mechanisms of thick pipes subjected to internal pressure, bending moment and axial compression under monotonic and cyclic loading conditions. • Pipe Response: Analytical tools and FE models were developed for studying the localization of buckling pattern for the envisaged pipeline scenarios i.e. pipelines laid on even sea bed, pipelines laid on uneven seabed and buried pipelines. • Mitigation Measures. Study of relevant mitigation measures and associated criteria for preventing/reducing/controlling additional pipeline bending. • Design Guideline. Preparation of the design guideline. The Joint Industry Project Hotpipe did recently finish the work and issued an internal confidential project guideline. This is in the process of being converted into a public DNV Recommended Practice, ref. DNV-RP-F110, which will be published later this year. It provides procedures and criteria to fulfill this functional requirement, to ensure the integrity of the pipeline in the post buckling condition. This paper will describe the procedures and criteria in the project guideline. It is expected to be identical in the coming RP issued for industry hearing. This paper describes the main structure and the covered design scenarios of the DNV-RP, particularly: • Pipelines exposed on even seabed, where thermal expansion may be accommodated by lateral snaking; • Pipelines on uneven seabed corresponding to even seabed; and • Pipelines on bottom of trenches/covered by natural or artificial backfill.

Topics: Design , Pipelines
Commentary by Dr. Valentin Fuster
2005;():691-699. doi:10.1115/OMAE2005-67524.

The HotPipe Project is a Joint Industry Project, whose overall objective is to prepare a DNV Recommended Practice to be used in structural design of high temperature/high pressure pipelines. The developed design criteria are based on the application of structural reliability methods to calibrate the partial safety factors involved. One of the three scenarios covered in this DNV-RP is buried pipes subjected to upheaval buckling which is discussed in this paper. The most significant factor in this scenario is uncertainty in the pipeline configuration and uncertainty in the pipe-soil interaction. The paper presents the background of the proposed soil capacities and the associated uncertainties for both uplift resistance and downward resistance in cohesive and non-cohesive soil. The paper links these soil models with the design requirements to upheaval buckling including: - Functional requirements i.e. survey data accuracy, smoothing of survey data, modeling of the pipeline, design conditions, soil cover etc.; - Trenching technology; - Qualification of the minimum soil cover, natural or artificial, with the aim to guarantee pipeline stability; - Assessment of pipeline response; - Pipe integrity checks and design criteria. The internal confidential project guideline has been completed and is currently in the process of being converted into an official DNV-RP-F110, to be published later this year.

Topics: Pipelines
Commentary by Dr. Valentin Fuster
2005;():701-714. doi:10.1115/OMAE2005-67525.

The HotPipe Project is a Joint Industry Research and Development Project, whose overall objective is to prepare a DNV Recommended Practice, to be used in the verification and design of high temperature/high pressure pipelines. The design guideline will cover most practical cases where pipelines are subjected to high internal pressure and high temperature i.e. exposed pipeline laid in a flat or uneven seabottom and buried pipes. The design criteria are based on the application of reliability methods to calibrate the partial safety factors involved. In this paper, the analysis methodology, the design procedures and relevant design criteria/functional checks for pipeline laid on uneven seabed, developed in the HotPipe Joint Industry Project, are discussed.

Commentary by Dr. Valentin Fuster
2005;():715-729. doi:10.1115/OMAE2005-67526.

In the last twenty years, experimental tests and FEM-based theoretical studies have been carried out to investigate the buckling mechanisms of thin-walled pipes subject to internal pressure, axial force and bending moment. Unfortunately, these studies do not completely cover the scope relevant for offshore pipelines i.e. outer diameter to thickness ratio lower than 50. In the HotPipe Phase 2 JI Project, full-scale bending tests were performed on pressurized pipes to verify the Finite Element Model predictions from HotPipe Phase 1 of the beneficial effect of internal pressure on the capacity of pipes to undergo large plastic bending deformations without developing local buckling. A total of 4 pipes were tested, the key test parameters being the outer-diameter-to-wall-thickness ratio (seameless pipes with D/t = 25.6, and welded UOE pipes with D/t = 34.2), and the presence of a girth weld in the test section. For comparison a Finite Element Model was developed with shell elements in ABAQUS. The test conditions were matched as closely as possible: this includes the test configuration, the stress-strain curves (i.e. using measured curves as input), and the loading history. The FE results very realistically reproduce the observed failure mechanisms by formation and localization of wrinkles on the compression side of the pipe. Good agreement is also achieved in the moment capacities (with predictions only 2.5 to 8% above measured values), but larger differences arose for the deformation capacity, suggesting that the DNV OS-F101 formulation for the characteristic bending strain (which is based on FE predictions from HotPipe Phase I) may be non-conservative in certain cases.

Commentary by Dr. Valentin Fuster
2005;():731-740. doi:10.1115/OMAE2005-67532.

Engineering critical assessment of offshore pipelines is usually very conservative if standardized single edge notch bend (SENB) specimens are used for the fracture mechanics testing. It is commonly accepted that the fracture toughness is dependent on the geometry constraint at the crack tip. The standardized SENB specimens have a high geometry constraint, and give lower bound fracture toughness for all geometries. For circumferential flaws in pipes the single edge notch tension (SENT) specimens is taken more into use, to establish more correct fracture toughness for the pipe in question. In this paper the effect of crack depth, misalignment and different wall thicknesses in SENT specimens have been studied. In addition the effect of crack depth and internal pressure in pipes have been studied with FE simulations.

Commentary by Dr. Valentin Fuster
2005;():741-745. doi:10.1115/OMAE2005-67540.

Pipelines on undulating seabed may be subject to free-span areas where interaction between VIV response of several modes occurs, including both single and multi-spans. Within the project Ormen Lange a model test program, as completed in 2004, was performed to investigate this phenomena. The implementation of the test results into detail design went via an Ormen Lange specific design guideline in which the general analysis flow is given. However, taking the mode interaction effects from the tests into the actual routing makes the need for additional evaluations and generalisation on dynamic system of interacting spans. A dynamic system represents a set of spans were the eigenmodes interact physically and not only mathematically in computation of the eigenmodes over a long pipeline section. For a single span case we find that the modes are well separated while for multi-span problems modes are close in frequencies. A clear criterion and limitation on dynamic system to be considered becomes vital. The relative effect of inline and cross flow VIV in actual design depends on the SN-curves at hand. The general trend is that inline generated fatigue takes over for corrosion sensitive problems covering operation phase, whereas preliminary phases may be governed by cross flow response. The modelling of pipe-soil interaction at shoulders is vital for the multi-span response characteristics, the experience being that softer soil gives minor span interaction. The soft soil gives deeper pipeline penetration and more uniform support stiffness all along the shoulder than the hard soil for which local reactions occur more like pinned supports.

Commentary by Dr. Valentin Fuster
2005;():747-753. doi:10.1115/OMAE2005-67548.

Explicit and implicit three dimensional Finite Element (FE) models of a pipe-in-pipe (PIP) system are developed. The models represent the casing pipe and the carrier pipe using shell elements. Frictional contact ensures kinematic compliance between the pipes while allowing unlimited axial relative movement. Steel for both pipes is modeled using a kinematic hardening material model capturing plastic deformations. The model allows large deformations and rotations. The developed models are used to study structural response of a PIP system under typical loadings experienced by pipelines. It is shown that a PIP system can be designed to avoid upheaval buckling without any external support. It is also shown that a PIP system has significantly superior structural response when measured in terms of strains experienced by the carrier pipe against the same size single pipe under some extreme axial and lateral loadings. Currently most PIP applications are motivated by the insulation properties of PIP systems or for ease of construction. The results presented here open opportunities for the application of PIP systems. Examples include offshore pipelines designed to protect against ice scour and onshore pipelines against unstable slopes. It is shown that explicit FE can be used to determine closely spaced chaotic solutions for the carrier pipe. It is known that the elastica problem has many closely spaced solutions. The results obtained compared to known chaotic solutions of elastica. Behavior of a PIP system presents interesting real life examples for bifurcations of elastica.

Topics: Pipes , Bifurcation
Commentary by Dr. Valentin Fuster

CFD and VIV

2005;():757-765. doi:10.1115/OMAE2005-67019.

Experiments were carried out at IFREMER Brest to obtain data on the displacement of Steel Catenary Risers (SCR) in the Touch-Down Zone (TDZ) induced by top motion. Measurements were conducted both in the section on the ground (2D motion) and in the section above the touch-down point (3D motion) with an optical tracking system. In the model test, the bottom 1/10 of the riser was represented at a scale of about 1/10. The present paper is focusing on Heave-Induced Lateral Motion (HILM) in the bottom part of the riser. The model riser is based on a full scale case which is first described. Based on Froude’s similitude, the geometrical characteristics of the model are derived. The tracking system and the various instrumentation are then detailed. In the last section, experimental results on one specific case are presented. Numerical calculations with a coupled fluid/structure solver are performed. A comparison for the amplitude and frequencies of HILM is presented. Comments are made on the analogy between the present experiments and some simpler experiments on a one-degree-of-freedom cylinder/spring system sinusoidally excited. Such an analogy should prove itself very fruitful in understanding and quantifying HILM.

Topics: Motion
Commentary by Dr. Valentin Fuster
2005;():767-774. doi:10.1115/OMAE2005-67036.

The present study is concerned with the free-surface wave flows around surface-piercing cylindrical structures. The volume of fluid method implemented in a Navier-Stokes computational fluid dynamics code is employed for test cases that involve general ship waves, spilling breaking waves, bubbly free-surface in separated regions, and interaction between free-surface waves and underlying viscous flow. The computational results are validated against existing experimental data, showing good agreement. The validation results suggest that the present computational approach provides a tool that is flexible and accurate enough to capture the outstanding flow physics associated with the free-surface wave flows around surface-piercing cylindrical structures.

Commentary by Dr. Valentin Fuster
2005;():775-783. doi:10.1115/OMAE2005-67044.

Large eddy simulations were carried out for the flow around a hydrodynamically smooth, fixed circular cylinder at two Reynolds numbers, one at a subcritical Reynolds number (Re = 1.4 × 105 ) and the other at a supercritical Reynolds number (Re = 1.0 × 106 ). The computations were made using a parallelized finite-volume Navier-Stokes solver based on a multidimensional linear reconstruction scheme that allows use of unstructured meshes. Central differencing was used for discretization of both convection and diffusion terms. Time-advancement scheme, based on an implicit, non-iterative fractional-step method, was adopted in conjunction with a three-level, backward second-order temporal discretization. Subgrid-scale turbulent viscosity was modeled by a dynamic Smagorinsky model adapted to arbitrary unstructured meshes with the aid of a test-filter applicable to arbitrary unstructured meshes. The present LES results closely reproduced the flow features observed in experiments at both Reynolds numbers. The time-averaged mean drag coefficient, root-mean-square force coefficients and the frequency content of fluctuating forces (vortex-shedding frequency) are predicted with a commendable accuracy.

Commentary by Dr. Valentin Fuster
2005;():785-792. doi:10.1115/OMAE2005-67048.

For the deterministic analysis of extreme structure behavior, the hydrodynamics of the exciting wave field, i. e. pressure and velocity fields, must be known. Whereas responses of structures, e. g. motions, can easily be obtained by model tests, the detailed characteristics of the exciting waves are often difficult to determine by measurements. Therefore, numerical wave tanks (NWT) promise to be a handy tool for providing detailed insight into wave hydrodynamics. In this paper different approaches for numerical wave tanks are introduced and used for the simulation of rogue wave sequences. The numerical wave tanks presented are characterized by the following key features: a) Potential theory with Finite Element discretization (Pot/FE); b) Reynolds-Averaged Navier-Stokes Equations (RANSE) using the Volume of Fluid (VOF) method for describing the free surface. For the NWT using the VOF method three different commercial RANSE codes (CFX, FLUENT, COMET) are applied to calculate wave propagation, whereas simulations based on potential theory are carried out with a wave simulation code developed at T echnical U niversity B erlin (WAVETUB). It is shown that the potential theory method allows a fast and accurate simulation of the propagation of nonbreaking waves. In contrast, the RANSE/VOF method allows the calculation of breaking waves but is much more time-consuming, and effects of numerical diffusion can not be neglected. To benefit from the advantages of both solvers, i. e. the calculation speed (Pot/FE-solver WAVETUB) and the capability of simulating breaking waves (RANSE/VOF-solver), the coupling of both simulation methods is introduced. Two different methods of coupling are presented: a) at a given position in the wave tank; b) at a given time step. WAVETUB is used to simulate the propagation of the wave train from the start towards the coupling position (case A) or until wave breaking is encountered (case B). Subsequently, the velocity field and the contour of the free surface is handed over as boundary (case A) or initial values (case B) to the RANSE/VOF-solver and the simulation process is continued. To validate these approaches, different types of model seas for investigating wave/structure interactions are generated in a physical wave tank and compared to the numerical simulations.

Commentary by Dr. Valentin Fuster
2005;():793-803. doi:10.1115/OMAE2005-67063.

This paper describes work done within the EU FP5 Project EXPRO-CFD to develop a system to couple commercial CFD software to existing hydromechanics tools to allow prediction of the response of floating structures in waves and currents, including viscous effects. Its focus is the use of this system to improve the prediction of CALM buoy response in waves. The Atkins EXPRO-CFD system is made up from the CFD code CFX, coupled to the AQWA-LINE and AQWA-NAUT hydromechanics codes. In this system, CFD provides the complete set of hydrodynamic forces and moments at each time step in the motions simulation, with the dynamics of the floating structure, its moorings and riser/export lines modeled in the AQWA-NAUT software. AQWA-NAUT returns the structure’s displacements and velocities to the CFD model and a moving grid algorithm uses these to couple motions and fluid flow in an accurate and stable manner. The motions of a CALM buoy were studied to test the capabilities of the system. The CALM buoy geometry (based on current designs by SBM) is 23m in diameter with a 2m wide skirt attached 1m above the keel; the effects of flow separation off this skirt and the associated viscous damping on the motions of the buoy were expected to be significant, especially around its natural period. A series of 1:40 model scale tests were carried out by Sirenha using a simplified mooring system with no risers. The results from the model tests, from AQWA-NAUT alone (carried out by SBM), and from the coupled EXPRO model were compared directly at model scale. The same AQWA-NAUT model was used in both the AQWA-NAUT-only simulations and the coupled simulations, allowing direct comparison between the results. The EXPRO-system simulations were carried out ‘blind’, i.e. without access to the experimental data. The three sets of RAOs showed reasonable agreement in long or short waves (within the limits of the specification of the mooring system). However, around the natural period, the AQWA-NAUT-only model significantly over-predicted the response in heave, and in particular in pitch. Although the EXPRO-CFD system slightly over predicted the heave and pitch responses, the results were close to the experimental measurements throughout. Further tests indicate that the weakness in the potential flow approach appears to be in the formulation of added viscous damping rather than the choice of model values for drag coefficients.

Commentary by Dr. Valentin Fuster
2005;():805-816. doi:10.1115/OMAE2005-67142.

The paper presents the development of BEM (Boundary Element Method) and FVM (Finite Volume Method) based models for the analysis of the flow around 2-D FPSO hull-sections fitted with bilge keels and subject to forced roll motions. Through these models an attempt is made to gain an insight into the two important aspects of the flow: separation around bilge keels and the effect of the free-surface. The effect of the free-surface and the resulting wave-body interaction is studied using a 2-D BEM model coupled with a Mixed-Eulerian-Lagrangian (MEL) time marching scheme for the free-surface boundary conditions. The separation around the bilge keels and viscous aspects of the flow are studied using a FVM based 2-D Navier Stokes (2DNS) solver with linear free-surface boundary conditions. The primary aim of the BEM model is to investigate the effects of the linear and nonlinear boundary conditions on the predicted flow within the scope of the parameters of the FVM model.

Commentary by Dr. Valentin Fuster
2005;():817-825. doi:10.1115/OMAE2005-67152.

The purpose of this paper is to develop a Fourier-Chebyshev collocation spectral method for computing unsteady two-dimensional viscous incompressible flow past a circular cylinder for low Reynolds numbers. The incompressible Navier-Stokes equations (INSE) are formulated in terms of the primitive variables, velocity and pressure. The incompressible Navier-Stokes equations in curvilinear coordinates are spectrally discretized and time integrated by a second-order mixed explicit/implicit time integration scheme. This scheme is a combination of the Crank-Nicolson scheme operating on the diffusive term and Adams-Bashforth scheme acting on the convective term. The projection method is used to split the solution of the INSE to the solution of two decoupled problems: the diffusion-convection equation (Burgers equation) to predict an intermediate velocity field and the Poisson equation for the pressure, it is used to correct the velocity field and satisfy the continuity equation. Finally, the numerical results obtained for the drag and lift coefficients around the circular cylinder are compared with results previously published.

Commentary by Dr. Valentin Fuster
2005;():827-835. doi:10.1115/OMAE2005-67176.

As the use of CFD in industrial applications increases, so does the need for verification and validation of the theoretical/numerical results. This paper focuses on tools for validation and in particular, on the use of Particle Imaging Velocimetry (PIV) as such a tool. Diffraction of regular waves due to a single, fixed vertical cylinder is investigated. Theoretical results of wave run-up and wave kinematics are compared to measurements from model tests. Theoretical results are obtained by second order potential theory and by fully non-linear CFD computations. The second order potential theory frequency-domain results are computed by the industry standard code WAMIT, while the fully nonlinear time-domain simulations are performed by the commercial CFD code Flow-3D. Measurements are obtained by means of wave probes, PIV and snapshots taken with a high-speed camera. The experiments are made with the model in place as well as without the model, for validation of the incident flow field. For the identification of non-linear effects, the steepness of the waves is varied. The surface elevation is measured by means of the wave probes, while the PIV equipment measures the kinematics. High quality photos taken by the high-speed camera give a detailed overview of the surface elevation for inspection. In addition to focusing on validation tools, the paper also addresses some critical aspects associated with the CFD computations, such as the modeling of boundary conditions. The work is based partly upon results from the WaveLand JIP, Phase 2.

Commentary by Dr. Valentin Fuster
2005;():837-846. doi:10.1115/OMAE2005-67197.

The paper reports results from two strip theory CFD investigations of the Vortex-Induced Vibrations of model riser pipes. The first investigation is concerned with the vibrations of a vertical riser pipe that was subjected to a stepped current profile. An axial spatial resolution study was conducted to determine the number of simulation planes required to achieve tolerably converged numerical solutions. It was found that six to seven simulation planes are required per half-wavelength of pipe vibration in order to obtain convergence. The second investigation is concerned with the simultaneous in-plane and out-of-plane vibrations of a model Steel Catenary Riser that was subjected to a uniform current profile. The pipe’s simulated vibrations were found to agree very well with those determined experimentally. This result was achieved despite the questionable usage of simulation planes at high angles to the flow direction.

Commentary by Dr. Valentin Fuster
2005;():847-852. doi:10.1115/OMAE2005-67202.

A three-dimensional numerical method combining solution of the incompressible Reynolds Averaged Navier-Stokes (RANS) equations with a rigid body structural dynamics response has been developed previously to aid in the prediction of the loads and motions of offshore structures. In this paper, we use the tool to compute the hydrodynamic flow around two tandem cylinders oriented perpendicularly to each other. The flow conditions and gap distances between the cylinders are chosen to match a set of water tunnel experiments carried out at the University of Queensland. Comparisons of Strouhal frequencies and example flowfield visualizations are presented between the experimental measurements and associated CFD results.

Commentary by Dr. Valentin Fuster
2005;():853-865. doi:10.1115/OMAE2005-67206.

The vibratory response of a long slender riser, made of composite materials and subject to an ocean current, is studied for a range of conditions. The influence of the number of modes of vibration is studied as is the influence of the mass ratio and the value of the damping coefficient. The flow past the riser is represented by a shear flow, ranging from Re = 8000 at the lower end of the riser to Re = 10,000 at the upper end of the riser. The riser vibration is treated as a coupled fluid-flow/vibration problem. The fluid-flow equations are represented by a Large Eddy Simulation model for the wake turbulence present in the flow. Strip theory is used to represent different forcing locations along the length of the riser. Since the composite riser has a variable damping coefficient, which decreases with increasing frequency, its response is different from, say, a steel riser with a constant damping coefficient. The composite riser, with variable damping, has a larger RMS displacement than a riser with constant damping. The vibration amplitude is found to increase with an increase in number of modes.

Commentary by Dr. Valentin Fuster
2005;():867-873. doi:10.1115/OMAE2005-67212.

This paper presents a novel two-way nested grid scheme that allows information to travel freely through the computational boundaries of the grid interface. The method tentatively assumes the pressure boundary conditions between subdomains, but uses a receding boundary approach to minimize the pressure errors. Initially, the domains overlap, but within a few time steps the overlap is eliminated by the receding boundaries, so repeated use of approximate boundary conditions is avoided. This prevents the accumulation of errors on the subdomain boundaries and, furthermore, decreases the transfer of errors to the rest of the domain by continuously discarding boundary data. To avoid continuous shrinking of the subdomains, the boundaries are reset to their original positions every few time steps. Successful applications of the method are presented for surface wave propagation, gravity currents and a combination of both surface and internal waves.

Commentary by Dr. Valentin Fuster
2005;():875-883. doi:10.1115/OMAE2005-67221.

During the last few years there has been a strong growth in the availability and capabilities of numerical wave tanks. In order to assess the accuracy of such methods, a validation study was carried out. The study focuses on two types of numerical wave tanks: 1. A numerical wave tank based a non-linear potential flow algorithm. 2. A numerical wave tank based on a Volume of Fluid algorithm. The first algorithm uses a structured grid with triangular elements and a surface tracking technique. The second algorithm uses a structured, Cartesian grid and a surface capturing technique. Validation material is available by means of waves measured at multiple locations in two different model test basins. The first method is capable of generating waves up to the break limit. Wave absorption is therefore modeled by means of a numerical beach and not by mean of the parabolic beach that is used in the model basin. The second method is capable of modeling wave breaking. Therefore, the parabolic beach in the model test basin can be modeled and has also been included. Energy dissipation therefore takes place according to physics which are more related to the situation in the model test basin. Three types of waves are generated in the model test basin and in the numerical wave tanks. All these waves are generated on basin scale. The following waves are considered: 1. A scaled 100-year North-Sea wave (Hs = 0.24 meters, Tp = 2.0 seconds) in deep water (5 meters). 2. A scaled operational wave (Hs = 0.086 meters, Tp = 1.69 seconds) at intermediate water depth (0.86 meters) generated by a flap-type wave generator. 3. A scaled operational wave (Hs = 0.046 meters, Tp = 1.2 seconds) in shallow water (0.35 meters) generated by a piston-type wave generator. The waves are generated by means of a flap or piston-type wave generator. The motions of the wave generator in the simulations (either rotational or translational) are identical to the motions in the model test basin. Furthermore, in the simulations with intermediate water depth, the non-flat contour of the basin bottom (ramp) is accurately modeled. A comparison is made between the measured and computed wave elevation at several locations in the basin. The comparison focuses on: 1. Reflection characteristics of the model test basin and the numerical wave tanks. 2. The accuracy in the prediction of steep waves. 3. Second order effects like set-down in intermediate and shallow water depth. Furthermore, a convergence study is presented to check the grid independence of the wave tank predictions.

Commentary by Dr. Valentin Fuster
2005;():885-894. doi:10.1115/OMAE2005-67238.

This paper examines the vortex induced motions (VIM) of a spar type floating production platform in uniform and sheared currents. The large draft of modern production platforms means that in some of the extreme current events the flow past the platform is highly non-uniform along the hull. We discuss the simulation of these stratified flows associated with hurricane events and loop currents and the implications for experiments and numerical simulations. Model testing options are reviewed along with the potential effects of buoyancy due to temperature and salinity variations in the current. Comparisons are made between experimental test results and numerical simulations of VIM at small scale and projections are made to full scale behavior using computational fluid dynamics (CFD) and detached eddy simulation (DES).

Commentary by Dr. Valentin Fuster
2005;():895-902. doi:10.1115/OMAE2005-67252.

Floating spar platforms are widely used in the Gulf of Mexico for oil production. The spar is a bluff, vertical cylinder which is subject to Vortex Induced Motions (VIM) when current velocities exceed a few knots. All spars to date have been constructed with helical strakes to mitigate VIM in order to reduce the loads on the risers and moorings. Model tests have indicated that the effectiveness of these strakes is influenced greatly by details of their design, by appurtenances placed on the outside of the hull and by current direction. At this time there is limited full scale data to validate the model test results and little understanding of the mechanisms at work in strake performance. The authors have been investigating the use of CFD as a means for predicting full scale VIM performance and for facilitating the design of spars for reduced VIM. This paper reports on the results of a study to benchmark the CFD results for a truss spar with a set of model experiments carried out in a towing tank. The focus is on the effect of current direction, reduced velocity and strake pitch on the VIM response. The tests were carried out on a 1:40 scale model of an actual truss spar design, and all computations were carried out at model scale. Future study will consider the effect of external appurtenances on the hull and scale-up to full scale Reynolds’ numbers on the results.

Commentary by Dr. Valentin Fuster
2005;():903-908. doi:10.1115/OMAE2005-67263.

It is widely acknowledged that the use of helical strakes for mitigation of vortex-induced motions (VIM) of surface piercing cylinders, such as spar platforms, is only partially effective. Using computational fluid dynamics tools, we compare the oscillation characteristics of a bare cylinder and a straked cylinder in uniform currents. Our model comprised of a straked cylinder with diameter of 0.741 m, aspect ratio of 1:1.9 and three helical strakes of height 13% of cylinder diameter. This geometry corresponds to the hard tank geometry of a scaled truss spar model known to exhibit VIM in tow tank testing. In the CFD simulations the cylinder is moored with linear springs to provide a range of reduced velocities. The fluid domain is made of an unstructured grid comprising of hexahedral elements. Fluid structure interaction utilizes grid stretching and a user defined function for solving the equations of motion. Turbulence modeling uses Detached Eddy Simulation (DES) and the boundary layer is modeled using a wall function with a surface roughness of 0.0003 m. Reynolds numbers are in the range of 50,000 to 100,000. Results for straked cylinder compares reasonably with published results, but under-predicts the peak response. In comparing with corresponding results for a bare cylinder without strakes, the spectral features of the transverse displacement show variations, which are found to be due to the spoiling effect of the strakes.

Commentary by Dr. Valentin Fuster
2005;():909-916. doi:10.1115/OMAE2005-67278.

The effect of surface tension on the evolution of weak spilling breakers is studied by performing large-wave simulations (LWS) of the free-surface flow developing by the interaction of a gravity free-surface wave and a surface shear-layer current. The flow models the evolution of gravity waves under the influence of wind shear. The surface tension modifies the dynamic free-surface condition and its effect depends on the dimensionless Weber number. The Euler equations are filtered according to the LWS formulation and solved numerically by a spectral method and a fractional-time-step scheme. The results indicate a stronger surface tension effect with decreasing Weber number values and increasing initial wave height. Specifically, decreasing the Weber number alters the size and shape of the characteristic bulge of spilling breakers and the toe position resulting in sharper slopes and angles of the free surface profile. The spiller wave height is reduced with decreasing Weber number.

Commentary by Dr. Valentin Fuster
2005;():917-922. doi:10.1115/OMAE2005-67318.

A 3-D Finite Volume method (FV3D) is developed and applied to green water problems. The Navier-Stokes (N-S) equations are discretized with the 3-D finite volume method on collocated Cartesian grids. The free surface motion is captured with the Volume of Fluid (VOF) method. The velocity and pressure fields are solved by the SIMPLER scheme with an alternating direction implicit solver. FV3D is validated against existing experimental and numerical results for tank sloshing and ship green-water-on-deck cases. This method is applicable to calculation of the green water effect on advanced wave-piercing hull forms.

Commentary by Dr. Valentin Fuster
2005;():923-926. doi:10.1115/OMAE2005-67339.

An effective numerical technique is presented to model turbulent motion of a standing surface wave in a tank. The equations of motion for turbulent boundary layers at the solid surfaces are coupled with the potential flow in the bulk of the fluid, and a mixed BEM-finite difference technique is used to obtain the wave and boundary layer characteristics such as bed shear stress. A mixing-length theory is used for turbulence modeling. Although the technique is presented for a standing surface wave, it can be easily applied to other free surface problems.

Commentary by Dr. Valentin Fuster
2005;():927-934. doi:10.1115/OMAE2005-67352.

Numerical prediction of vortex-induced vibrations requires employment of the unsteady Navier-Stokes equations. Current Navier-Stokes solvers are quite expensive for three-dimensional flow-structure applications. Acceptance of Computational Fluid Dynamics as a design tool for the offshore industry requires improvements to current CFD methods in order to address the following important issues: (i) stability and computation cost of the numerical simulation process, (ii) restriction on the size of the allowable time-step due to the coupling of the flow and structure solution processes, (iii) excessive number of computational elements for 3-D applications, and (iv) accuracy and computational cost of turbulence models used for high Reynolds number flow. The above four problems are addressed via a new numerical method which employs strong coupling between the flow and the structure solutions. Special coupling is also employed between the Reynolds-averaged Navier-Stokes equations and the Spalart-Allmaras turbulence model. An element-type independent spatial discretization scheme is also presented which can handle general hybrid meshes consisting of hexahedra, prisms, pyramids, and tetrahedral.

Commentary by Dr. Valentin Fuster
2005;():935-943. doi:10.1115/OMAE2005-67355.

Practical cases of flow-structure interactions involve ocean currents with turbulence. Ocean turbulence effects on offshore structures are of high interest due to the lack of information about its role regarding the hydrodynamic forces and fatigue load on structures. Current turbulence profiles are employed for various Reynolds numbers that are based on an isotropic geophysical turbulence model. These turbulence profiles are specified as the inflow condition for the numerical simulations over a circular cylinder. The present work consists of three parts: (i) determination of appropriate mesh and time step resolution to accommodate the turbulence, (ii) investigation of the effect of current profile turbulence on the amplitude and frequencies of the hydrodynamic forces acting on a cylinder, and (iii) study of the sensitivity of the hydrodynamic load exerted on the cylinder for a range of Reynolds numbers with a turbulent inflow profile. Spalart-Allmaras one-equation turbulence model is used for high Reynolds number simulations.

Commentary by Dr. Valentin Fuster
2005;():945-949. doi:10.1115/OMAE2005-67372.

The resonances among long and short waves have been studied intensively. The focus of this paper is on flow configurations with two short wave groups with slight detuning in group velocities. This detuning effect will introduce a much richer set of dynamics as the mismatch in group velocities will have profound effects on the motion of pulses. Exact solitary waves are calculated for the coupled set of nonlinear evolution equations by the Hirota bilinear transform. The Hopscotch method is applied to solve the set of equations numerically. Physically, the goal is to study the effects of mismatch in group velocities and the initial phase difference of arbitrary solitary pulses between wave guides. The outcome will depend on the strength of the group velocity differences, initial amplitudes and phase differences.

Topics: Simulation , Waves
Commentary by Dr. Valentin Fuster
2005;():951-963. doi:10.1115/OMAE2005-67422.

A robust Volume of Fluid (VOF) technique is presented together with an incompressible Euler/Navier Stokes solver operating on adaptive, unstructured grids to simulate the interactions of extreme waves and three-dimensional structures. The incompressible Euler/Navier Stokes equations are solved using projection schemes and a finite element method. The classic dam-break problem has been used to validate the computer code developed based on the method described above. The numerical simulations of a three dimensional dam-break wave interacting with a single cylinder and a cylinder array have been carried out. Computational results have demonstrated that the present CFD method is capable of simulating the interactions of extreme waves and three-dimensional structures, which are of great importance for the comprehension of many natural phenomena in marine, coastal and marine engineering.

Commentary by Dr. Valentin Fuster
2005;():965-972. doi:10.1115/OMAE2005-67426.

This paper investigates the hydrodynamical aspects of a fixed FPSO in regular waves. The emphasis is geared to the validation of state of the art CFD techniques, using particle image velocimetry measurements on the water velocities around a captive model of an FPSO in regular waves. The main focus is on the following issues: Determination of the water velocities around the vessel using PIV techniques and pressures on a girth below and just above the mean waterline of the vessel using pressure gauges. Comparison of the detailed RANS simulations of the flow around the bilges of the FPSO. The resulting pressure profiles along the girth, especially near the waterline, showed a significant non-linear effect in steep beam wave conditions. The velocities as measured near the bilges of the vessel indicated a strong separation at the bilges of the vessel. This leads to an overestimation of the water velocities near the bilges of the vessel as calculated using a linear diffraction programme. The resulting pressure profiles along the girth, especially near the waterline, showed a significant non-linear effect in steep beam wave conditions. The velocities as measured near the bilges of the vessel indicated a strong separation at the bilges of the vessel. This leads to an overestimation of the water velocities near the bilges of the vessel as calculated using a linear diffraction programme. RANS simulations will give a more realistic flow profile around the bilges.

Commentary by Dr. Valentin Fuster
2005;():973-980. doi:10.1115/OMAE2005-67430.

The present work deals with the numerical simulation of accelerating and decelerating motions of a submerged body of revolution, in order to investigate the influence of the thrust variation on the maneuvering characteristics. A finite volume method is adopted to solve the essentially 2D RANS, time-dependent equations in a body-fitted co-ordinate system. Two problems have been examined for a 50m body of revolution: (a) the stopping maneuver, i.e. crash stop from steady forward speed and then full astern and (b) full ahead at steady speed from rest and then full astern.

Commentary by Dr. Valentin Fuster
2005;():981-992. doi:10.1115/OMAE2005-67448.

This paper presents results from simulation of green water loading including vessel motions. The simulation is performed through a domain decomposition: the far field and ship motions are calculated by potential theory and are used to simulate the local flow around the deck of an offshore floater using a Navier-Stokes solver. In the solver the free surface is displaced using a Volume of Fluid based method, improved by introducing a local height function. First, simulations of an irregular wave, where the velocities at the boundaries of the domain are prescribed using results of a linear diffraction code are performed in order to check wave propagation properties. Then, the same code is used to initiate the simulation of an FPSO in high waves, resulting in green water on the deck.

Commentary by Dr. Valentin Fuster
2005;():993-1001. doi:10.1115/OMAE2005-67460.

With the trend towards offshore LNG production and offloading, sloshing of LNG in partially filled tanks has become an important research subject for the offshore industry. LNG sloshing may induce impact pressures on the containment system and may affect the motions of the LNG carrier. So far, LNG sloshing has been studied mainly using model experiments with an oscillation tank. However, the development of Navier-Stokes solvers with a detailed handling of the free surface allows the numerical simulation of sloshing. It should be investigated, however, how accurate the results of this type of simulations are for this complex flow problem. The paper first presents the details of the numerical model, an improved Volume Of Fluid (iVOF) method. The program has been developed initially to study the sloshing of liquid fuel in satellites. Later, the numerical model has been used for calculations of green water loading and the analysis of anti-roll tanks, including the coupling with ship motions. Recently, the model has been extended to incorporate two-phase flow. This extension improves its ability to simulate the effect of gas bubbles of different sizes. Gas bubbles are present in virtually all relevant offshore situations; not only at LNG sloshing but also during green water events, bow slamming and water entry. In a two-phase flow model, both the liquid and the gas phase can have their own continuity and momentum equations. The handling of the compressibility of the gas phase is a major issue in the design of a two-phase flow model. However, as a first step in the modeling process, the gas phase is considered as incompressible. For a dambreak experiment, results of the one-phase model, the incompressible two-phase model and model experiment results have been compared. It is shown that the physics are more accurately simulated with the incompressible two-phase model. Furthermore, the paper will show results of the incompressible model for LNG sloshing. The physics of LNG sloshing and several other applications can be approached better by taking the compressibility into account. Therefore, as a second step, a compressible model is currently under construction, involving adiabatic compression of the gas phase.

Commentary by Dr. Valentin Fuster
2005;():1003-1010. doi:10.1115/OMAE2005-67505.

This work forms part of an investigation into the non-linear interaction between steep transient waves and flared structures, using a coupled finite element and boundary element model. The use of a coupled approach is based on consideration of the relative strengths and weaknesses of the finite element (FE) and boundary element (BE) methods when implemented separately (e.g. efficiency of computation versus complexity of adaptive mesh generation). An FE model can be used to advantage away from the body, where the domain is regular, and a BE discretisation near the body where the moving mesh is complex. The paper describes aspects of the FE and BE models which have been developed for this analysis, each based on the use of quadratic isoparametric elements implemented in a mixed Eulerian-Lagrangian formulation. Initially the two approaches have been developed side by side, in order to ensure the use of robust components in the coupled formulation. Results from these methods are obtained for a series of test cases, including the interaction of an impulse wave with a circular cylinder in a circular tank, and non-linear diffraction by a cylinder in a long tank.

Commentary by Dr. Valentin Fuster
2005;():1011-1016. doi:10.1115/OMAE2005-67508.

A parallel finite element fluid-structure interaction free-surface solver is developed for numerical simulation of water waves interacting with floating objects. In our approach, the governing equations are the Navier-Stokes equations written for two incompressible fluids. An interface function with two distinct values serves as a marker identifying the location of the interface. The numerical method is based on writing stabilized finite element formulations in an arbitrary Lagrangian-Eulerian frame. This allows us to handle the motion of the floating objects by moving the computational nodes. In the mesh-moving schemes, we assume that the computational domain is made of elastic materials. The linear elasticity equations are solved to obtain the displacements. In order to update the position of the floating object, the nonlinear rigid body dynamics equations are coupled with the governing equations of fluids and are solved simultaneously. The mooring forces are modeled using nonlinear cables and linear spring models. The finite element formulation is implemented on Cray X1.

Commentary by Dr. Valentin Fuster
2005;():1017-1026. doi:10.1115/OMAE2005-67581.

The aim of the presentation is to demonstrate the possibilities of predicting loads on partially submerged, moving structures, in particular on tanks in ships and on offshore platforms. Since tanks can in reality move arbitrarily, we are using the moving grid approach and a Finite Volume solution method designed to cater for arbitrary motions of polyhedral control volumes. The motion of liquid is computed using an interface-capturing scheme which allows overturning and breaking of waves. By performing a coupled simulation of the flow and vessel motion, it is possible to obtain a realistic response of the liquid in a tank to external excitation, e.g. by sea waves. Results are first presented for tanks whose motion is prescribed in accordance with experimental setups, with which the solutions are compared. A very good agreement between pressures computed in simulations and experimental data at representative locations is obtained. It was possible by means of simulation to clearly distinguish between off-resonance and resonance cases, in which peak pressures reached the highest values, in accordance with experimental observations. Following validation by experimental data in laboratory setups, we demonstrate the plausibility of simulation of both vessel motion and the flow inside and outside vessel. The forces and moments exerted by the sea cause the vessel to move, which excites the sloshing of liquid in tanks. For the computation of vessel motion, both sea-induced forces and forces due to sloshing in tanks are taken into account. While there are no experimental data to compare with, the results look plausible and encourage further validation and application studies.

Topics: Simulation , Stress , Sloshing
Commentary by Dr. Valentin Fuster

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