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ASME Conference No-Show Policy and Archival Proceedings

2011;():i. doi:10.1115/OMAE2011-NS2.
FREE TO VIEW

This online compilation of papers from the ASME 2011 30th International Conference on Ocean, Offshore and Arctic Engineering (OMAE2011) represents the archival version of the Conference Proceedings. According to ASME’s conference no-show policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in the ASME Digital Library and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Structures, Safety and Reliability

2011;():1-6. doi:10.1115/OMAE2011-49006.

It is widely known that fire on board ships is one of the major ship accidents resulting in loss of life and property. There exist international rules and regulations for ship design with reference to fire safety. These rules are primarily prescriptive with a provision for alternative design. For any novel or new type of design which is particularly very common these days for the design of Cruise Liner or Large Passenger Ships, the prescriptive part of the rule is found to be inadequate, so there is a need to resort to the provision of alternative design. This alternative design can be based on Risk Based approach. It is now well recognized by ‘International Maritime Organization’ that Risk Based approach is the most potential way out for bringing in innovation and improving safety in Marine industry. Though, Fire safety encompasses many aspects of Ship Design and Construction, however, various systems installed on board ships for detection and suppression of fire play an important role in controlling and improving the level of fire safety. In general these systems are installed as per the requirement of applicable regulation without having clear understanding of the failure mechanisms, function of the system and their interactions. This understanding is essentially required in order to improve the system reliability as well as effectively dealing with various accident scenarios. This paper describes some of the techniques which can easily be used and implemented for the purpose improving the clarity of systems installed on board ships for effectively and reliably dealing with fire accident on board ships. Some Fault Tree Analysis (FTA) and Event Tree Analysis (ETA) for various systems and fire scenarios will be described/presented in this paper.

Topics: Safety , Fire , Risk analysis , Ships
Commentary by Dr. Valentin Fuster
2011;():7-12. doi:10.1115/OMAE2011-49008.

The critical structure demands a 3D model to recreate a real situation and optimize the asset maintenance with a visual management. Since 1995, Petrobras uses software able to do the maintenance management registering the structures degradation evolution, paints, and cathodic protection in all vessels and offshore platforms. This paper regards how technology can help to avoid disasters with timeline deployed information management applied to structure maintenance.

Topics: Maintenance
Commentary by Dr. Valentin Fuster
2011;():13-22. doi:10.1115/OMAE2011-49048.

We present a stochastic model of sea storms to predict the maximum height of the wave surface over a given area during storms. To do so, we exploit the theory of Euler Characteristics of random excursion sets combined with a generalization of Boccotti’s equivalent triangular storm model (Boccotti, 2000) that describes an actual storm history in the form of a generic power law (Fedele and Arena, 2010). An analytical solution for the return period of extreme wave events over a given area and the associated statistical properties are given. We then assess the relative validity of the new model and its predictions by analyzing wave measurements retrieved from NOAA-NODC buoys moored offshore of the Atlantic and Pacific coasts.

Topics: Spacetime , Storms , Seas
Commentary by Dr. Valentin Fuster
2011;():23-33. doi:10.1115/OMAE2011-49055.

Three types of imperfections are analysed in the current paper, and they are: (i) Initial geometric imperfections, i.e., deviations from perfect geometry, (ii) Variations in the wall thickness distribution, and (iii) Imperfect boundary conditions. It is assumed that cones are subject to: (a) axial compression only, (b) radial pressure only, and (c) combined loading, i.e., axial compression and external pressure acting simultaneously. Buckling strength of imperfect cones is obtained for all of the cases above. It is shown that the buckling strength is differently affected by imperfections, when cones are subjected to axial compression or to radial external pressure. The response to imperfections along the combined stability envelope is also provided, and these results are first of this type. The finite element analysis, using the proprietary code is used as the numerical tool. Cones are assumed to be from mild steel and the material is modelled as elastic perfectly plastic. Geometrical imperfection profiles are affine to eigenshapes. A number of them are tried in calculations, as well as the effect of them being superimposed. The results indicate that imperfection amplitude and its shape strongly affect the load carrying capacity of conical shells. Also, it is shown that the buckling loads of analyzed cones are more sensitive when subjected to combined loading as compared to their sensitivity under single load conditions. At the next stage, uneven thickness distribution along the cone slant was considered. Variation of wall thickness was assumed to vary in a piece-wise constant fashion. This appears to have a large effect on the buckling strength of cones under axial compression only as compared with that of cones subjected to radial external pressure only. Finally, the effect of variability of boundary conditions on failure load of cones was investigated for two loading conditions, i.e., for axial compression and for radial pressure, only. Results indicate that change of boundary conditions influences the magnitude of buckling load. For axially compressed cones the loss of buckling strength can be large (about 64% for the worst case (beta = 30 deg, the cone not restrained at small radius end). Calculations for radial pressure indicate that the loss of buckling strength is not as acute — with 34% for the worst case (beta = 40 deg, relaxed boundary conditions at the larger radius end). This is an entirely numerical study but references to accompanying experimental programme are provided.

Topics: Buckling , Shapes , Thickness
Commentary by Dr. Valentin Fuster
2011;():35-42. doi:10.1115/OMAE2011-49056.

The effect of plate stiffeners of double bottom during ship grounding with large contact surface was analyzed in this paper, by numerical simulation technique. It is always taken for granted that the effect of the stiffeners on the plate can be neglected, but they should not be dealt with in such a simple way. The objective ship is a double bottom tanker, and four different finite element models are made. Five numerical simulations are designed for each model, with different indentations from 10 percent to 50 percent of the double bottom height. With the simulation results, two sets of comparisons are made. The purpose of the comparison is to demonstrate how much role the stiffeners play in the structural performance during the grounding, and whether the smeared thickness method can predict the role of the stiffeners adequately. It is found that the vertical and horizontal resistances of the cases with stiffeners are all larger than those cases without stiffeners quite a while, which can prove that the stiffeners do have obvious influences on the resistance of the double bottom structure. The difference for horizontal resistance is more obvious than that for vertical resistance. Besides, the value of resistance in the model with stiffeners can be nearly double of that in the corresponding case in the model without stiffeners, especially for those cases with small indentation, which means the stiffeners play an important role in the structure deformation in mild grounding course. Furthermore, it is also indicated that the effect of smeared thickness can not predict the role of the stiffeners adequately, due to the perpendicular disposal of the stiffeners on the plate is of crucial importance in the structural deformation mode. A conclusion is drawn that the influence of the stiffeners on the plate should be taken into consideration and the deformation mechanism of the stiffeners should be included in the simplified analytical method in future.

Topics: Ships
Commentary by Dr. Valentin Fuster
2011;():43-51. doi:10.1115/OMAE2011-49081.

Collisions between ships contribute to ship losses even in modern time, and the International Maritime Organization (IMO) strives towards a more risk-based view on addressing the damage stability of ships. The current paper makes use of a methodology for computing the consequences of ship collisions. This methodology is comprised of structural analysis of a collision scenario followed by dynamic damage stability simulations of the struck ship. The emphasis of this investigation is on uncertainties related to structural computations of the collision event. Explicit finite element analyses are presented for a case study of a collision scenario. For comparison, the model of the bow of the striking ship is considered as either rigid or deformable in separate analyses. The consequences from the model representation of the bow, uncertainties in material properties and failure model (three different ones are presented) in terms of shape and size of the damage opening are studied. This variation also has an impact on the time to capsize of the struck vessel, which is studied in the subsequent damage stability computations for different sea states. One conclusion is that the methodology that has been used is important for future efforts in the research on safety at sea. The study presented demonstrates the importance of incorporating uncertainties in the analysis chain and studying the effect of these on the final results: the time to capsize.

Commentary by Dr. Valentin Fuster
2011;():53-62. doi:10.1115/OMAE2011-49099.

A significant number of damage events continue to occur to ocean going vessels, many of which remain afloat in need of assistance to evaluate remedial actions to minimise the risk of further damage and conditions for onward transit for a repair facility. Therefore, it is vital that the post damage situation is rapidly understood to reduce damage propagation occurring, reducing the cost of repair and allowing more vessels to be recovered safely, ensuring the safety of personnel onboard. After an incident, it is often difficult to determine and model the precise damage scenario due to the inability to survey the area. Each scenario will have variability in geometrical and material properties, which will affect the residual strength of the structure. Variations in these aspects are not accounted for in methods currently utilised in damage response scenarios. Therefore, to be able to more accurately analyse the damage and provide better guidance to the crew in real time, it is important that this variability can be analysed, allowing an understanding of worst and best case scenarios, the probabilities of these occurring and their affect on the structural strength. Finite element analysis has been used to model damage scenarios due to the high level of accuracy that can be achieved. This paper demonstrates the implications of damage aperture on the limit state of stiffened steel panels, investigating the residual strength of the damaged structures and their sensitivity to variations in damage event, geometric and material properties. The results are then compared with the Smith and Dow progressive collapse method, [1,2], with conclusions being drawn about the use of this method in damage situations. Further to this, the effects of variability in the ship are investigated for damage scenarios, showing that these influence the ultimate strength of the structure to a larger extent in the failure of damaged plates than intact scenarios; however, lack of knowledge relating to the area of the damage could overshadow other potential variability within these scenarios.

Topics: Steel , Ships
Commentary by Dr. Valentin Fuster
2011;():63-72. doi:10.1115/OMAE2011-49104.

An efficient Rayleigh-Ritz approach is presented for analyzing the lateral-torsional buckling (“tripping”) behavior of permanent means of access (PMA) structures. Tripping failure is dangerous and often occurs when a stiffener has a tall web plate. For ordinary stiffeners of short web plates, tripping usually occurs after plate local buckling and often happens in plastic range. Since PMA structures have a wide platform for a regular walk-through inspection, they are vulnerable to elastic tripping failure and may take place prior to plate local buckling. Based on an extensive study of finite element linear buckling analysis, a strain distribution is assumed for PMA platforms. The total potential energy functional, with a parametric expression of different supporting members (flat bar, T-stiffener and angle stiffener), is formulated, and the critical tripping stress is obtained using eigenvalue approach. The method offers advantages over commonly used finite element analysis because it is mesh-free and requires only five degrees of freedom; therefore the solution process is rapid and suitable for design space exploration. The numerical results are in agreement with NX NASTRAN [1] linear buckling analysis. Design recommendations are proposed based on extensive parametric studies.

Topics: Design
Commentary by Dr. Valentin Fuster
2011;():73-81. doi:10.1115/OMAE2011-49110.

In the aftermath of the PRESTIGE disaster, an innovative system for the prompt intervention on oil pollution sources (primarily ship wrecks) at great depths was conceived at the Joint Research Center of the European Commission. This system, with some re-engineering, could also serve for collecting oil and gas leaking after an offshore well blow-out and could constitute a reference method for prompt intervention on deep water oil pollution sources like ship wrecks and blown-out offshore wells. A large fabric dome, solidly anchored at the sea bed, covers entirely the pollution source and channels the leaking oil to a large open bell-shaped reservoir just under the sea surface so as not to be affected by the waves. Oil occupies the upper part of the bell and is periodically recuperated by a shuttle tanker while water escapes from the open bottom. The buoyancy of the reservoir keeps the whole system in tension. The concept was validated and optimized through detailed engineering, laboratory experiments and numerical simulations performed by a consortium of European institutes and industries in a dedicated collaborative research project called DIFIS (Double Inverted Funnel for the Intervention on Shipwrecks). The project resulted in a light, modular and easy to deploy system design. Such a system, suitably dimensioned and re-engineered to take account the gas flow, could also be used to cover the wellhead area so as to collect all leaking oil and gas until the implementation of the relief drilling. The present paper outlines the DIFIS system and elaborates on its advantages and risks in containing oil well blow-outs.

Commentary by Dr. Valentin Fuster
2011;():83-91. doi:10.1115/OMAE2011-49121.

In the present paper, effects of geometrical parameters on the SCF distribution along the weld toe of multi-planar tubular DKT-joints under the axial loads are investigated. In order to study the multi-planar effect, SCF distribution in multi-planar joints is compared with the distribution in a uni-planar joint having the same geometrical properties. Based on the multi-planar DKT-joint FE models which are verified against experimental results and the predictions of Lloyd’s Register (LR) equations, a complete set of SCF database is constructed. The FE models cover a wide range of geometrical parameters. Through nonlinear regression analysis, a new set of SCF parametric equations is established for the accurate and reliable fatigue design of multi-planar DKT-joints under axial loads. An assessment study of these equations is conducted against the experimental data and the acceptance criteria recommended by the UK DoE.

Commentary by Dr. Valentin Fuster
2011;():93-105. doi:10.1115/OMAE2011-49123.

The integrity of mechanical components, particularly when they experience considerable fatigue damage during its operating life, can be strongly influenced by the presence of residual stress fields and mechanical heterogeneity. Premature closure of crack flanks greatly influences fatigue crack growth rate. Extensive elastic-plastic finite element analyses have been carried out to investigate detailed crack closure behavior in center cracked welded compact tension (CT) specimens with one level of weld strength mismatch. The finite element results show that homogeneous, soft material has higher crack opening loads than heterogeneous material with 50% overmatch conditions. Fracture testing conducted on C(T) specimens to measure fatigue crack growth rates for an ASTM A516 Gr. 70 steel weldment provide the experimental data to support such behavior. The fatigue life can be reduced by more than 100% for a condition of 50% overmatch when compared with the evenmatch condition. It was verified that most of time spent in fatigue propagation life is consumed at the beginning of the propagation life.

Commentary by Dr. Valentin Fuster
2011;():107-111. doi:10.1115/OMAE2011-49134.

This paper reports the development of a simplified energy solution to a complex problem involving large inelastic deformation in thin clamped square ductile plates with a square hole under the action of transverse pressure loading. The work is part of a project to study blast loading of steel plates with penetrations as used for deck plating or bulkheads that may be required to resist loading far in excess of their design limit due to the effects of an accidental explosion. It is important to develop criteria for the ultimate load-carrying capacity of such structures and guidance for the industry. In seeking this goal, the project will utilize experimental methods for conducting 1/8 scaled pulse pressure tests on 0.5 m square plates with a central aperture. The data will be used to help develop both finite element numerical simulations and simplified analytical models. Initial studies show, based on comparisons with previous test data, that acceptable engineering solutions can be achieved with simplified methods provided the user is aware of the limitations of the method. Furthermore, preliminary results suggest no significant reduction in resistance due to the central hole, the load being reduced in proportion to the hole area. Detailed analysis of the local stresses and strains around the hole will be carried out in the next stage of the project.

Commentary by Dr. Valentin Fuster
2011;():113-121. doi:10.1115/OMAE2011-49143.

The jack-up rig is a movable offshore platform, which is basically being designed to work at any location, worldwide for given maximum water depths and sea states, combined with different sea bed conditions. Moreover the platform is often transported with the legs in fully raised position, from one offshore location to another, resulting in completely different loads on the structure. The combination of extreme environmental loads with a small crack in structural joints due to fabrication process or fatigue degradation may cause unstable crack extension and fracture failure. To calculate the failure probability through the fracture failure mode, two methods based on the failure assessment diagram of level 2 recommended by the British Standard Institute (BS7910[1]) and formulation recommended by Dijkstra et al. [2] are presented and investigated. The results of BS7910 gives the lower reliability index than the Dijkstra formulation for several secondary to yield stress ratios and service times but the maximum difference between them is restricted to 17% in the case of secondary stress to yield stress ratio equal one.

Commentary by Dr. Valentin Fuster
2011;():123-133. doi:10.1115/OMAE2011-49161.

Most common studies on the strength of plates under compressive longitudinal loading are related to plates having unrestrained edges which lead to a zero net load in the transverse direction. In ships, the framing system and the continuity of the plating in the transverse direction tend to induce rather different boundary conditions on the ‘unloaded’ edges, which results in a completely different state of stresses when the external loads are applied. This is due to the surge of a significant level of induced membrane stresses in the direction perpendicular to loading. In this work, the behavior of long restrained plates under compressive axial loading is analyzed and compared with the one of plates having other boundary conditions. The finite element method is applied for the nonlinear analysis of the plates using a commercial package. 56 cases are considered corresponding to different levels of plate slenderness which ranges between 0.35 and 3.46 covering the practical range of structural plates used in the shipbuilding industry. Various shapes of initial imperfections are considered in order to establish the minimum level of resistance. Also the influence of the magnitude of the distortions associated to each mode is discussed. The study conducted to the establishment of the minimum compressive strength of restrained plates and it defines the expected range of strength’s variation due to the magnitude level of distortions. The biaxial axial state of stresses resulting from these boundary conditions is characterized and its dependence from the plate’s slenderness is quantified for the most common type of the hull plating.

Commentary by Dr. Valentin Fuster
2011;():135-142. doi:10.1115/OMAE2011-49176.

In recent years there have been significant developments in seismic retrofit of building structures. For steel off-shore jacket structures, however, researches on the seismic performance evaluation and retrofit are relatively rare. In this study, the advantages of using Buckling-Restrained Braces (BRBs) in offshore jacket structures and its effect on the seismic performance of the platform are highlighted. The potential failure mechanisms of the structures using both normal braces and BRBs under seismic loads were investigated first. Pushover analyses of fixed jacket offshore platforms comprising the most common types of the platforms in practice were conducted. In those model structures, two different schemes of BRB retrofitting were utilized and compared with the normal bracing case. After that, an example for the seismic assessment of an existing 8 legged service platform located in the Gulf of Thailand was investigated. The model was designed based on loads according to the environmental conditions in Gulf of Thailand. Different retrofitting schemes using normal braces were investigated to improve the seismic performance of the structure and the results were compared with the results of the seismic retrofit using BRBs. The analysis results showed that the seismic performances of the jacket structures were greatly improved when bracing members were replaced with BRBs compared to the case with using normal bracing. Numerical results revealed that the plastic hinges were more uniformly distributed over the structures in case of retrofit using BRB, while they were concentrated at the top of the structures when normal bracings were used.

Commentary by Dr. Valentin Fuster
2011;():143-150. doi:10.1115/OMAE2011-49198.

Experimental and numerical investigations reveal that nonlinear modulational instability can significantly affect the probability of occurrence of extreme waves, especially if waves are sufficiently steep and narrow banded both in the frequency and directional domain. However, it is not yet completely clear whether numerical simulations can provide an accurate quantitative estimate of experimental results. Here the potential Euler equations are used to assess the ability of numerical models to describe the evolution of statistical properties of mechanically generated directional, random wave fields and in particular the evolution of the kurtosis. Results show that simulations provide a good quantitative estimate of experimental observations within a broad range of wave directional width.

Commentary by Dr. Valentin Fuster
2011;():151-163. doi:10.1115/OMAE2011-49228.

The ultimate limit state of structural tension members with stress concentrations due to geometrical (non-welding related) stress raisers is investigated. Examples of such members are pad eyes, brackets etc. The influence of the application of high strength steels (up to S690) is taken into account. The focus lies on members with a predominant static loading regime. Such members frequently occur in the marine environment as parts of lifting appliances and handling systems or as a structural detail of equipment foundations, located outside the fatigue-prone regions of the hull girder. Typically, design stresses at the stress concentration approach the yield limit of the material. Common yield criteria cannot be applied to such peak stresses, due to the small margin between design and yield. Usually, the strength integrity is based on the nominal stresses in the critical cross section. Goal of the study is to determine the ductile failure limit with a method suited for design purposes. This would enable an ultimate limit state design approach and improve the structural safety philosophy. Main question is how the post yield behavior up to failure of a notched section is influenced by the stress gradient and the properties of the high strength materials. For this purpose, the applicability of two damage models based on the work of Rice & Tracey [8] (void growth model) and Bonora [1] (damage mechanics) is studied. In combination with elastoplastic finite element analysis these models enable the prediction of local ductile crack initiation. Calculations are performed on slender tensile members with a geometrical stress raiser, assuming a range of structural steel qualities and using a static loading regime. The results are verified using small scale laboratory tests. It is shown that isolated (non-redundant) tensile members with stress raisers feature a static ductile failure mode similar to that of uniform tensile specimen. Their failure loads can be determined as the product of the material’s tensile strength and the net section area, in the same way as for uniformly stressed members. These findings are valid up to S690 materials and clear the path to a safe and sound application of such materials based on an ultimate limit state approach. It was found that the ultimate limit state is governing design for higher strength steel members with a relatively low stress concentration. A severe stress raiser may be beneficial for efficient design of high strength members, since it allows a design stress in the notch up to yield without compromising the safety up to failure. Damage calculations were found superfluous for isolated member ultimate limit state design. Damage results, however, compare well with the failure mode observed. This is useful for the design of highly stressed notches in details which are surrounded by a large main structure, providing a huge reserve strength capacity. For these so-called embedded stress raisers an ultimate load approach is not possible due to the absence of a critical cross section. Damage mechanics can then be applied to determine a failure point in terms of stress and strain, allowing an ultimate limit state design for these stress peaks as well.

Commentary by Dr. Valentin Fuster
2011;():165-175. doi:10.1115/OMAE2011-49239.

One of the key parameters influencing the acceptability of a pipeline girth weld defect subjected to remote plastic deformation is the strength mismatch between weld and base metal. However, no single definition exists for weld strength mismatch, as it can be defined either on the basis of yield stress, ultimate tensile stress or any intermediate flow stress. To investigate the relevance of such definitions, the authors have performed a series of analyses of curved wide plate tests, using a validated parametric finite element model. The results indicate that, whereas yield stress overmatch determines crack driving force for small plastic strains, ultimate tensile stress overmatch is the more important parameter for advanced plastic strains and determines the eventual failure mode. Further, the strain capacity and exact crack driving force curve are additionally determined by uniform elongation and crack growth resistance.

Commentary by Dr. Valentin Fuster
2011;():177-184. doi:10.1115/OMAE2011-49259.

Permanent means of access (PMA) of oil tankers and bulk carries consists of a wide platform for walk through inspection. Since PMA structures have a tall web plate, they are vulnerable to elastic tripping. A previous paper [1] proposed a Rayleigh-Ritz method to analyze elastic tripping behavior of PMA structures. The method is parametric formulated, mesh free, computational efficient, and is able to predict both the flange plate critical tripping stress as well as the web plate local buckling stress; therefore the solution process is suitable for design space exploration. In this paper, multi-objective optimization methods are used to determine the Pareto solutions of a PMA structure based on the proposed tripping algorithm. The objective is to solve a design problem aimed at simultaneously minimizing the weight of a PMA structure and maximizing its critical buckling stress. Three multi-objective methods, Pareto Simulated Annealing (PSA), Ulungu Multi-objective Simulated Annealing (UMOSA) and Multi-objective Genetic Algorithm (MOGA) are presented for a case study. The numerical results show that all three methods can efficiently and effectively solve such optimization problems within a short search time. The critical buckling stress of the final optimal designs is validated by the linear and non-linear buckling analysis of NX-NASTRAN [2] .

Commentary by Dr. Valentin Fuster
2011;():185-195. doi:10.1115/OMAE2011-49266.

Offshore structure global strength analysis based on finite element plate model is a requirement for today’s classification societies and designers. Wave, wind, current loads have to be applied to the global strength model as a pre processing step to allow the analysis to take place. After the analysis, code checking must be performed to verify if the structure meets class or other requirements. Due to its complex nature, a large amount of engineering hours have to be spent for the pre and post processing. This is not only lengthy if performed manually or semi-automatically, but also mistake prone. General guidelines from classification societies exist, but general purpose commercial software is scarce and often still requires significant amount of engineering time to perform these tasks. This paper shows a rational approach to automate the pre and post processing of offshore structure global strength finite element analysis. Utilizing the FEMAP Application Program Interface (API), a complete automatic pre and post processing is implemented in one integrated program, Exmar Design Suite (EDS). The program will load the model from WAMIT generated wave pressure, apply internal pressure induced from motions to internal tanks, and also apply other environmental loads. After the finite element analysis, the program can execute strength code checking including yielding and buckling for the model. Both beam and stiffened plate panels can be identified using an automatic search algorithm, which is not a function available for general finite element software. The panels and beams are then checked against various common codes such as API/AISC/ABS/DNV. In addition, fatigue analysis can also be performed in either spectral or simplified approach. The benefits of automation are timesaving, accuracy and reliability. It also makes the check of whole model possible. Instead of relying more on “screening” or experience based structure check, engineers will have more confidence in the results by going through the whole model.

Commentary by Dr. Valentin Fuster
2011;():197-203. doi:10.1115/OMAE2011-49278.

Several articles in the literature cover methods for the prediction of semi submersible airgap. In general, using these methods, the reader will have to take care of the implementation, because in most cases not all factors that influence the final airgap are covered. This paper will describe the practical implementation of a calculation methodology covering all environmental aspects that affect the airgap for semi-submersibles. The proposed method utilizes a frequency domain stochastic approach (spectral method) to combine all known factors that can be calculated and that affect the airgap. These effects include the first order vessel motions under an undisturbed wave field, diffracted wave elevation along the free surface under the platform, slow drift quadratic transfer function (QTF), vessel set down and heel effects due to mooring stiffness into a single airgap prediction. Wave run up is not accounted in this procedure due to its high non-linearity, its difficult prediction and measurement. This is a local effect that is considered in the design phase by assuming that wave run up will hit the structures near columns and these components should be designed accordingly. The method proposes the combination of these statistical processes to be utilized for both the initial and final stages of the design. Model test results are also presented which show good correlation to the proposed method. A discussion of the limitations of this stochastic method, time simulations and model tests is also presented.

Topics: Submersibles
Commentary by Dr. Valentin Fuster
2011;():205-212. doi:10.1115/OMAE2011-49314.

The main objective of this paper is to study the effects of general low cycle fatigue (LCF) damage on ultimate strength of unstiffened ship plates under uniaxial compression. A nonlinear LCF damage model has been derived on basis of continuum damage mechanics (CDM) coupled with plastic mechanics in order to define the deterioration of materials. The theoretical results show that the damage variable D can be measured by a simple procedure and it can be related to the mechanical property of material directly. Formulae predicting ultimate strength reduction have been proposed based on amended Faulkner’s simple and useful expression. Nonlinear finite element analyses (FEA) of panels with various material damages have been carried out. The results indicate that the deterioration of materials is important factors determining the amount of strength reduction. It was also found that the proposed formulae can accurately predict the residual ultimate strength of unstiffened plate with general fatigue damage.

Commentary by Dr. Valentin Fuster
2011;():213-217. doi:10.1115/OMAE2011-49333.

A structural analysis performed by Scandpower combining explosion simulations and structural analysis utilising the the computer tools USFOS and FLACS is described. As of today, the most common procedure for elastic and plastic explosion response analyses is to define the explosion load as a uniform pressure vs. time function for all surfaces and elements in the model. Capabilities of the computer tools FLACS and USFOS allow for a more refined approach, recognizing that for large geometries explosion pressure will vary in both time and spatial domain. USFOS (Ultimate Strength for Offshore Structures) (Ref. /1/) is a leading computer program for nonlinear static and dynamic analysis of space frame structures. The program accurately simulates the collapse process, from the initial yielding, through to the formation of a complete collapse mechanism and the final toppling of the structure. FLACS (FLame ACcellerator Simulator) (Ref. /2, 3/) is a computational fluid dynamics (CFD) tool for modeling of ventilation, gas dispersion and explosions in complex process areas. The FLACS code allows for monitoring pressures at user defined surface areas, which can be chosen to correspond with an USFOS model. Results from FLACS simulations may be defined as individual time/pressure load histories to different parts of a steel structure. This high resolution of the explosion loads was further utilized by combining the results with USFOS. The combination of FLACS and USFOS in a structural analysis has shown that there may be significant capacity reserves compared to a standard “equal area pressure” analysis when analyzing a structure for a spatially variable explosion load.

Commentary by Dr. Valentin Fuster
2011;():219-228. doi:10.1115/OMAE2011-49337.

Increase in global ship transport induces building of Ultra Large Container Ships (ULCS), which have a capacity up to 14000 TEU with length up to 400 m, without changes of the operational requirements (speed around 27 knots). Natural frequencies of such ships can fall into the range of encounter frequencies in an ordinary sea spectrum. Present Classification Rules for ship design and construction don’t cover such conditions completely and hydroelastic analysis of ULCS seems to be the appropriate solution for analysis of their response in waves. This paper deals with numerical procedure for ship hydroelastic analysis with particular emphasis on improvements of the present beam structural model. The structural model represents a constitutive part of hydroelastic mathematical model and generally it can be formulated either as 1D FEM or 3D FEM model. For the preliminary design stage hydroelastic model derived by coupling 1D FEM structural model and 3D BEM hydrodynamic one seems to be an appropriate choice. Within the paper the importance of hydroelastic approach and methodology of hydroelastic analysis are elaborated. Further on, structural model based on advanced beam theory is described in details. The improvements include taking into account shear influence on torsion, contribution of bulkheads to hull stiffness as well as determination of effective stiffness of engine room structure. Along with that, hydrodynamic and hydrostatic models are presented in a condensed form. Numerical example, which includes complete hydroelastic analysis of a large container ship, is also added. In this case, validation of 1D FEM model is checked by correlation analysis with the vibration response of the fine 3D FEM model. The procedure related to determination of engine room effective stiffness is checked by 3D FEM analysis of ship-like pontoon which has been made according to the considered ship characteristics.

Topics: Containers , Modeling , Ships
Commentary by Dr. Valentin Fuster
2011;():229-237. doi:10.1115/OMAE2011-49343.

In order to promote the reliable evaluation for the fatigue strength of ships, the validation of a direct load and strength computation was performed based on the tank test and the full scale measurement. The fatigue damage in short term sea state under various operation parameters was indicated quantitatively by the tank test in irregular wave utilizing the elastic model. The long term fatigue damage was evaluated based on the full scale measurement [1]. The fatigue damage inferred from the measured stress on deck structure is quite small compared with the direct computation utilizing a full ship finite element model. That is mainly caused by the difference of environmental wave condition. Moreover, the effect of operational condition through whole life was indicated by the direct computation quantitatively. To make more rational fatigue design, it is important to take the long-term wave condition into account.

Commentary by Dr. Valentin Fuster
2011;():239-246. doi:10.1115/OMAE2011-49346.

Authors developed a whole ship finite element analysis system from a nonlinear wave loads to a structural strength at real sea state. A methodology for the rational analysis of structural strength is examined. Firstly, wave pressure, which is the input for the present whole ship finite element analysis and has much effect on the accuracy of a whole ship analysis, is validated through the comparison with experiments. It is confirmed that the present computation can estimates wave pressure in various wave condition accurately. Secondly, the whole ship finite element analysis system by the combination with the computation of nonlinear wave pressure is verified. It is verified that the present method can evaluate a structural response in irregular waves with taking account of the nonlinear effect of ship motions and slamming induced impact loads explicitly. Finally, through the structural analysis in various ship forward speed and wave condition by means of the present computation, the importance to assess a structural strength taking account of the effect of operational condition is clarified.

Commentary by Dr. Valentin Fuster
2011;():247-256. doi:10.1115/OMAE2011-49349.

To prevent the brittle fracture of the extremely thick plate which is used in the deck plate and the hatch side coaming of the large container ships, it is important to detect surely and repair the inner defects, which are possible to lead the brittle fracture at an early stage. Now Ultrasonic Testing is used in order to inspect the inner defect in a thick plate, and it is necessary to select adequately the probe size and oscillating frequency to evaluate the defect size accurately. The estimation of the defect size in vertical direction to plate surface is important from the viewpoint of fatigue strength. But the vertical flaw length and also the inclined flaw length are hardly to be estimated accurately. In this study, to clarify the characteristics of reflected ultrasonic wave from the defect, the wave propagation behavior is simulated with the numerical simulation program by FEM which is developed by the authors. In this program, the governing equation of elastic wave propagation is calculated in time domain with explicit method utilizing the central-difference scheme. First, the effect of the probe size and oscillating frequency on the accuracy of defect sizing are investigated utilizing the developed program. The numerical simulation is performed for imaging to examine the length of flaw which is parallel to plate surface by normal beam technique. And, the applicability of 6dB method, which is one of the methods for estimating flaw length, is examined. Moreover, a new method for estimating flaw length which cannot be estimated by 6dB method is proposed. Secondly, in order to examine the inclined flaw length, the angel beam test is performed. The accuracy of numerical simulation for angle beam technique is confirmed by comparing experimental result. And, it is examined how the inclined angle of flaw affects the echo height, and it is shown that 6dB method and L level method are useful for the defect which is perpendicular to wave beam and the tip echo method is useful for the defect which is inclined to wave beam. The actual structure usually has a paint film. Therefore, the echo height level will be affected by paint thickness, and the paint film effects on the accuracy of defect sizing. Thirdly, the effects on echo height by film thickness are clarified by experiments and numerical simulations.

Commentary by Dr. Valentin Fuster
2011;():257-262. doi:10.1115/OMAE2011-49350.

Oil production in offshore areas has increased in recent years, especially with deeper water creating new challenges to be overcome. In this scenario, the mooring systems of floating units need to withstand higher operating loads. The mooring system is designed to keep the FPSO (Floating Production Storage and Offloading) properly positioned. Being fundamental to the integrity of risers and the platform itself, the failure can cause the overloading of other lines and generate the collapse of the whole mooring system in an extreme environmental situation, so the design and material selection are essential to the success of the venture. This work presents the investigation of the premature failure in some lines of the mooring system for an FPSO, in this case links in class R3. There were flaws in the weld between the link and the stud, with the subsequent complete failure of link. Some analyses were performed to understand the problem, including chemical composition, microstructural evaluation, hardness, corrosion potential, numerical analysis and fatigue tests. Finally, we concluded that the use of materials from different links to the stud generated a localized corrosion leading to increased local tensions and the failure by fatigue.

Topics: Failure , FPSO
Commentary by Dr. Valentin Fuster
2011;():263-271. doi:10.1115/OMAE2011-49357.

The main factor restricting admission of deep-draught ships to ports is the risk of bottom contact. In the approach channels to Rotterdam and IJmuiden such ships are subject to tidal windows because of the limited depths in the approach channels. Some decades ago probabilistic methods were introduced for the allocation of tidal windows for the Euro-Maas channel to Rotterdam and later also for the IJ channel to IJmuiden. These methods are being further developed. The increased accuracy of computational methods and of forecasted wave conditions and water levels may lead to an increased accessibility of the port. Recent developments have resulted in the new tidal window advice program Protide (PRObabilistic TIdal window DEtermination). The objective of the work presented in this paper is the verification and validation of Protide for the Euro-Maas channel to Rotterdam and for the IJ channel to IJmuiden. The probability of bottom contact during channel transit is simulated for time series of ten years of measured wave data and water levels. The fleet of ships is represented by a limited number of ships and for each ship all possible tidal windows for the ten year period are determined with Protide. A database is developed with motion response characteristics for each of the representative ships. For all possible arrival times in the ten-year period, indicated as safe by Protide, a channel transit is simulated. The probability of bottom contact during the simulated transit is computed from the motion response characteristics of the ship and with the measured wave spectra and the measured water level. Two safety criteria are applied. Firstly the probability of bottom contact during a single channel transit should not be excessive. Secondly the total probability of bottom contact during a long period of application of the admittance policy should be limited. To determine the probability that a certain ship is in a certain section of the channel some elements from queuing theory are applied. The probability that the ship is present in each channel section combined with the probability of bottom contact results in the probability of bottom contact for the ship in the channel. This leads to a long term probability of bottom contact for the channel. This paper presents the analysis method and selected simulation results for Euro-Maas channel in detail.

Topics: Safety
Commentary by Dr. Valentin Fuster
2011;():273-282. doi:10.1115/OMAE2011-49378.

Large container vessels are known to vibrate heavily in head sea storms due to its flexible hull girder, pronounced bow flare and high vessel speeds. Strong vibrations are mainly excited by bow impacts causing transient vibrations referred to as whipping. This whipping response increases both fatigue and extreme loading. Further resonance vibration by springing response is also well known to contribute to increased fatigue loading. On these large container vessels it may however not be only the vertical vibration mode that is excited but also horizontal and torsional vibrations (coupled). The current paper describes the measurement system installed on a vessel operating between East Asia and Europe. Sensors for global loading, local loading and transverse hatch opening distortions are supplemented by navigational and environmental data. The system is an extended hull monitoring system analyzing data onboard and providing decision support onboard as well as providing statistical and time series of data to shore for further assessment. The measurements confirm that the fatigue loading of critical details are dominated by the vibrations, and that the fatigue loading level in deck in a storm is higher than ever measured before, also leading to high extreme loads above IACS rule values. The full scale measurements do to some degree confirm previous model tests of the same vessel, but the real vessel has been operated at reduced speeds. So far the fatigue loading on this route is at a comfortable level, partly due to reduced speeds, but also the encountered sea states may be less severe than the route specific scatter diagram. The loading may however increase if the vessel speed is increased, and the loading may become uncomfortable high if the vessel is put on a more harsh trade like North Pacific (or North Atlantic). Also torsional and horizontal vibrations are observed, but the transverse hatch opening distortions are moderate. Stern slamming is not measured to any significant degree, but bow flare slamming is measured in the storms. Side shell fatigue loading is at a comfortable level due to the CSA-2 notation involving direct hydrodynamic load calculations in the design introducing a sufficient safety margin against fatigue cracks. Vibration is however contributing significantly also in the side shell. Only a few storms have been encountered so far. The fatigue damage is concentrated amidships and the affect of warping in front of the superstructure does not increase the fatigue loading to a level of concern.

Commentary by Dr. Valentin Fuster
2011;():283-290. doi:10.1115/OMAE2011-49393.

Nowadays, LNG Import Terminals (where the storage and regasification process is conducted) are mostly onshore; the construction of these terminals is costly and many adaptations are necessary to abide by environmental and safety laws. Moreover, an accident in one of these plants might produce considerable impact in neighboring areas and population; this risk may be even worse due to the possibility of a terrorist attack. Under this perspective, a discussion is conducted about a vessel known as FSRU (Floating Storage and Regasification Unit), which is a storage and regasification offshore unit, that can work miles away from de coast and, owing to this, can be viewed as an option for LNG storage and regasification facilities. The goal is to develop a method for using Bayesian Networks in the Risk Analysis of Regasification System of the FSRU, which will convert Fault Trees (FT) into Bayesian Networks (BN) providing more accurate data. Using BN is possible to represent uncertain knowledge and local conditional dependencies. In addition, FT models the failure modes as independent and binary events while BN may model a larger number of states. It is worth noting that BN does not require the determination of cut sets; however, given a failure, it is capable of providing the probability of each possible cut set. This method will provide information to define, in a future study, a maintenance plan based on the Reliability Centered Maintenance. The results intend to clarify the applicability of BN on risk assessment and might improve the risk analysis of a Regasification System FSRU.

Commentary by Dr. Valentin Fuster
2011;():291-298. doi:10.1115/OMAE2011-49396.

A quantitative risk assessment comprises some basic activities that have to be developed to allow the quantification of the risks involved in the operation of a system or process under analysis. Basically, the likelihood of the undesired events has to be identified as well as their consequences must be calculated. When the risks in the operation of a marine vessel are analyzed, the same process has to be followed. For each specific phase of the marine vessel mission, all of the undesired events must be accurately determined and evaluated. Many different types of undesired events must be investigated, such as: fire, explosion, collision, falling objects and marine hazards. There are many techniques involved in the estimation of the likelihood of the events and the same occurs for the evaluation of their consequences. The purpose of this paper is to study a leakage during a cargo offloading considering a LNG shuttle tanker and a Liquefied Natural Gas Floating Production Storage Offloading (LNG-FPSO) in the Pre Salt Brazilian coast. Once defined the feasible scenarios and the total quantity discharged, the consequences will be evaluated using physical models described in the literature and implemented in commercial softwares. The main idea is to identify all possible consequences and verify their magnitudes. As this is an isolated study, where the likelihood of the event will not be estimated, the final objective is to use the results obtained to predict mitigating measures to the system. This is not a simple task due to the complexity of the phenomena developed after the leakage. In order to verify the extension of possible spread events it is also necessary the identification of many variables, including the atmospheric conditions, the sea condition, the ignition sources, the vessels nearby, the shuttle tanker(s), and the positioning of the vessels involved in the offloading operation.

Commentary by Dr. Valentin Fuster
2011;():299-304. doi:10.1115/OMAE2011-49409.

Hindcasts of wave conditions can be subject to large uncertainties, especially in storms. Even if estimates of extremes are unbiased on average, the variance of the errors can lead to a bias in estimates of extremes derived from hindcast data. The convolution of the error distribution and wave height distribution causes a stretching of the measured distribution. This can lead to substantial positive biases in estimates of return values. An iterative deconvolution procedure is proposed to estimate the size of the bias, based on the measured distribution and knowledge of the error distribution. The effectiveness of the procedure is illustrated in several case studies using Monte Carlo simulation.

Topics: Waves , Uncertainty
Commentary by Dr. Valentin Fuster
2011;():305-311. doi:10.1115/OMAE2011-49421.

The general procedure is discussed for determining the interval estimation of N-year return period extreme wave height with maximum likelihood method (MLM). The specific interval estimation expressions of the N-year return extreme wave height with MLM are given when the extreme wave height series fits a sort of maximum value distribution, such as Weibull distribution, generalized extreme value distribution and log-normal distribution. This paper also proposes a sign test method (STM) to estimate the interval of return extreme wave height. It is a non-parametric method that does not depend on the type of maximum value distributions. By adopting both MLM and STM, a stochastic simulation of the interval estimation is conducted to estimate the quantiles of the above maximum value distributions. The results indicate that SMT behaves well only when the sample size is sufficiently large (generally above 100), and the advantage of simulation with MLM is apparent if the sample size is smaller. Finally, a case study has been carried out with the extreme wave height measured at Weizhoudao hydrological station in the northern area of South China Sea.

Commentary by Dr. Valentin Fuster
2011;():313-323. doi:10.1115/OMAE2011-49445.

This article presents a review of the state-of-the-art technologies used in assessing the risk of hydrocarbon explosions and fires in offshore installations. Both qualitative and quantitative risk assessment approaches are described, and the modelling techniques employed in the quantitative assessment of explosions and fires are presented. Procedures developed through a joint industry project on the explosion and fire engineering of floating, production, storage and offloading units (abbreviated hereafter as EFEF JIP) led by the authors are introduced. This article does not attempt to survey the literature, but rather to summarise recent advances and future trends in terms of the technological aspects of the risk assessment of offshore installations subject to explosions and fire.

Commentary by Dr. Valentin Fuster
2011;():325-333. doi:10.1115/OMAE2011-49456.

In the design of a vessel’s ultimate strength the extreme hogging condition is of great concern. Due to special properties of container ship structures, such as large bow flare and overhanging stern, wave-induced slamming makes the ship responses more skewed to sagging conditions. In particular in large sea states, the ratio between maximum sagging and hogging can be quite high. Hence, the sagging condition might be very crucial with respect to a ship’s ultimate strength. In this study, the extreme response caused by hogging and sagging is derived from upcrossing spectrums of ship responses. The Weibull fitting method and Rice’s formula for the computation of the upcrossing spectrum are discussed using full-scale measurements from a container vessel on the North Atlantic trade. The extreme ship responses are therefore predicted using the long-term upcrossing spectrum. In the case where the ship response can be approximately treated as a series of stationary Gaussian processes, the corresponding upcrossings are computed by the explicit Rice’s formula. For the non-Gaussian ship response, it is shown that the 4-moment Hermite transformation is an efficient approach to compute the corresponding upcrossing spectrums. The parameters in the transformation mainly depend on the wave environments and operation profiles. The relations between these parameters and the wave environments are needed if no measurement is available. However, according to the full-scale measurements, it is not possible to find general formulas to estimate the parameters in terms of wave environments or operation profiles for the practical applications.

Commentary by Dr. Valentin Fuster
2011;():335-346. doi:10.1115/OMAE2011-49458.

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. To this end, the conventional simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated response extreme values (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. A more efficient method (ETS) was recently introduced which takes advantage of the correlation between the extreme values of surface elevation and their corresponding response extreme values. The method has proved to be very efficient for high-intensity sea states; however, the correlation and hence the efficiency and accuracy of the technique reduces for sea states of lower intensity. In this paper, a more efficient version of the ETS technique is introduced which takes advantage of the correlation between the extreme values of the nonlinear response and their corresponding linear response values.

Commentary by Dr. Valentin Fuster
2011;():347-358. doi:10.1115/OMAE2011-49460.

Offshore structures are exposed to random wave loading in the ocean environment and hence the long-term probability distribution of the extreme values of their response to wave loading is of great value in the design of these structures. Due to nonlinearity of the drag component of Morison wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of the extreme response probability distributions are not available. However, it has recently been shown that the short-term response of an offshore structure exposed to Morison wave loading can be approximated by the response of an equivalent finite-memory nonlinear system (FMNS). In this paper, the approximate FMNS models are used to determine both the short-term and the long-term probability distribution of the response extreme values with great efficiency.

Commentary by Dr. Valentin Fuster
2011;():359-364. doi:10.1115/OMAE2011-49467.

This paper deals with conditional risk assessment considering hull girder failure of damaged vessels. Only damages amidships are investigated. Conditional risk in this paper is defined as the probability of a given damage multiplied by the probability of hull girder failure conditioned upon the given damage. Taking a double hull oil tanker at full load condition as an example, the 3-hour’s failure probabilities conditioned upon damage properties are estimated. The statistical database for collision event in the HARDER project is adopted to assess the probability of sea state and damage. The sensitivity of 3-hour’s failure probability to the sea state and damage properties is investigated. Furthermore, the conditional risk is also assessed.

Commentary by Dr. Valentin Fuster
2011;():365-374. doi:10.1115/OMAE2011-49468.

The structural reliability analysis of damaged vessels has up to now commonly been investigated by neglecting the effect of sloshing. This paper deals with the effect of sloshing in tanks on motions and hull girder responses of oil tankers in various damage conditions and represents a part of a study to assess the effect of sloshing on hull girder failure of damaged vessels, The flooded tanks are assumed to have a of rectangular shape and linear multimodal approach is adopted to deal with sloshing. It is concluded that even though the effect of sloshing in tanks on the roll motion of vessels can be neglected in certain damage conditions, the effect of sloshing on the horizontal bending moment cannot be neglected, especially when resonance motion occurs.

Topics: Motion , Girders , Sloshing , Vessels , Hull
Commentary by Dr. Valentin Fuster
2011;():375-383. doi:10.1115/OMAE2011-49471.

Maritime and air traffic through the Arctic has increased in recent years. Cruise ship and commercial jet liners carry a large number of passengers. With increased traffic, there is a higher probability that a major disaster could occur. Cruise ship and plane accidents could be catastrophic and may require mass rescue. Due to the remote location, limited search and rescue resources, time for these resources to get to the accident location and large number of survivors, the retrieval time could be several days. Therefore, survivors may be required to survive on their own for days while they await rescue. Recognizing that the International Maritime Organization does not have specific thermal performance criteria for liferafts and lifeboats and personal and group survival kits, the Maritime and Arctic Survival Scientific and Engineering Research Team (MASSERT) initiated a research project to improve safety and provide input for advances to regulations. The objective of the project is to investigate if the current thermal protective equipment and preparedness available to people traveling in the Canadian Arctic are adequate for surviving a major air or cruise ship disaster and to identify the minimum thermal protection criteria for survival. This project builds on the results and tools developed in other research projects conducted by the team on thermal protection of liferafts, lifeboats and immersion suits. The project is divided into three major phases — clothing ensemble testing with thermal manikins, a physiology experiment on sustainable shivering duration and ensemble testing in Arctic conditions with human subjects. A numerical model uses these data to simulate survival scenarios. In the first phase of this project, the thermal resistance values of the protective clothing typically available to cruise ship and aircraft passengers were measured using two thermal manikins. The ensembles included Cabin Wear, Deck Wear, Expedition Wear, Abandonment Wear and protective clothing from Canada Forces Major Air Disaster Kit (MAJAID). Tests were conducted on dry and wet ensembles at 5°C and −15°C with and without wind. There is very good agreement between the thermal resistances measured by the two manikins. The differences in thermal resistances observed are likely caused by variations in fit and wrinkles and folds in the ensembles from dressing. With no wind, the thermal resistance is lowest with Cabin Wear and highest with MAJAID clothing inside the down-filled casualty bag. The Expedition Wear, the Abandonment Wear and the MAJAID clothing have about the same thermal resistance. With 7 metre-per-second wind, the thermal resistance of all ensembles decreased significantly by 30% to 70%. These results highlight the importance of having a shelter as a windbreak. For wet clothing ensembles at 5°C, the initial wet thermal resistance was 2 to 2.5 times lower than the dry value, and drying times ranged up to 60 hours. This highlights the importance of staying dry. Preliminary predictions from the numerical model show that the survivors in Expedition Wear, even with sleeping bag and tent, can be mildly hypothermic and need to depend heavily on shivering to maintain thermal balance. In a shelter, the predicted metabolic rate is roughly double the resting rate; it is triple the resting rate without protection from the wind. Further research is required to study shivering fatigue and age effects. Research on mass rescue scenarios for cruise ships and airplanes survivors should ideally involve subjects of both genders and the elderly.

Topics: Arctic region
Commentary by Dr. Valentin Fuster
2011;():385-395. doi:10.1115/OMAE2011-49486.

This paper deals with the interaction of sea waves with a moored floating rectangular structure. The problem is of interest in various practical situations such as the design of floating breakwaters and of artificial islands, and the modelling of wave-ice sheet interaction. In the paper the incident wave field is modelled as a Gaussian stochastic process of a specified spectrum. In this context a diffraction problem and a radiation problem are posed. The related boundary value problem is discussed and solved by an eigen-functions expansion matching method. The method renders an estimate of the velocity potential at the first order in a Stokes’ expansion and, thus, of the wave pressure and of the wave forces. Several hydrodynamic quantities of interest are investigated. Specifically, reflection and transmission coefficients are derived and represented as a function of the structural dimensions. Further, the added mass and radiation damping terms are estimated. Then, the dynamics of the floating structure is examined by considering a 3-D-O-F system. Next, the wave-structure interaction is investigated in the context of a Quasi-Determinism theory. It is shown that the developed approach can be employed to account for both the dynamic and hydrodynamic effects of a structure in which diffraction is not negligible. The theoretical results are supplemented with data from pertinent Monte Carlo simulations.

Commentary by Dr. Valentin Fuster
2011;():397-408. doi:10.1115/OMAE2011-49507.

The offshore platforms shall be designed with deck elevation that maintains sufficient air-gap with maximum predicted wave crest elevations. However, due to compelling reasons of hydrocarbon process and changes to water levels/seabed subsidence may have structures in the close vicinity of the sea level or submerged. Hence the assessment of wave induced forces on the deck structures especially the old platforms become very essential. Review of literature indicates that the previous studies focused mainly on the circular sections and not much information is available on the non-circular sections. The current study focused on measurement of wave induced slam/slap loads in a laboratory wave flume and estimation of force coefficients based on drag based empirical equations. Further, it is of interest to focus on the flow around the open sections (non-circular) using a flow visualization technique (Particle Image Velocimetry) to understand the flow pattern around the elements. The results indicate that the slam and slap coefficients for I sections are smaller than the channel sections and the values of coefficients are few times higher than the circular sections.

Commentary by Dr. Valentin Fuster
2011;():409-415. doi:10.1115/OMAE2011-49540.

The paper is based on review of research articles by the authors, with the purpose to demonstrate that the modulational-instability mechanism is active in typical directional wave fields. If so, possible limits for the wave height due to such mechanism can be outlined. The modulational instability can lead to occurrence of very high waves, which either proceed to the breaking or appear as rogue events, but it was derived for and is usually associated with two-dimensional wave trains. There exists argument, both analytical and experimental, that this kind of instability is impaired or even suppressed in three-dimensional (directional) wave systems. The first part of the paper demonstrates indirect experimental evidences which relate the wave breaking in oceanic conditions to features of two-dimensional breaking waves due to modulational instability. The second section is dedicated to direct measurements of such instability-caused breaking in a directional wave tank with directional spread and mean steepness typical of those in the field. The last section provides conclusions on what is maximal height of an individual wave, depending on the mean wave steepness in a wave train/field, that can be achieved due to such non-linear evolution of wave trains.

Topics: Waves
Commentary by Dr. Valentin Fuster
2011;():417-429. doi:10.1115/OMAE2011-49545.

The existence of freak waves is indisputable due to observations, registrations, and severe accidents. The occurrence of extreme waves, their characteristics and their impact on offshore structures is one of the main topics of the ocean engineering research community. Real sea measurements play a major role for the complete understanding of this phenomenon. In the majority of cases only single point registrations of real sea measurements are available which hinders to draw conclusions on the formation process and spatial development in front of and behind the respective registration points. One famous freak wave is the “New Year Wave”, recorded in the North Sea at the Draupner jacket platform on January 1st, 1995. This wave has been reproduced in a large wave tank and measured at different locations, in a range from 2163 m (full scale) ahead of to 1470 m behind the target position — 520 registrations altogether. Former investigations of the test results reveal freak waves occurring at three different positions in the wave tank and these extreme waves are developing mainly from a wave group. The possible physical mechanisms of the sudden occurrence of exceptionally high waves have already been identified — superposition of (nonlinear) component waves and/or modulation instability (wave-current interaction can be excluded in the wave tank). This paper presents experimental and numerical investigations on the formation process of extraordinarily high waves. The objective is to gain a deeper understanding on the formation process of freak waves in intermediate water depth such as at the location of the Draupner jacket platform where the “New Year Wave” occurred. The paper deals with the propagation of large amplitude breathers. It is shown that the mechanism of modulation instability also leads to extraordinarily high waves in limited water depth. Thereby different carrier wave length and steepnesses are systematically investigated to obtain conclusions on the influence of the water depth on the modulation instability and are accompanied by numerical simulations using a nonlinear potential solver.

Topics: Spacetime , Waves
Commentary by Dr. Valentin Fuster
2011;():431-438. doi:10.1115/OMAE2011-49588.

Air gap statistics for offshore platforms is directly related to the extreme value statistics of the random ocean wave field. The present paper describes a new method for predicting the extreme values of a random wave field in both space and time. The method relies on the use of data provided by measurements or Monte Carlo simulation combined with a technique for estimating the extreme value distribution of a recorded time series. The time series in question represents the spatial extremes of the random field at each point in time. The time series is constructed by sampling the available realization of the random field over a suitable grid defining the domain in question and extracting the extreme value. This is done for each time point of a suitable time grid. Thus, a time series of spatial extremes is produced. This time series provides the basis for estimating the extreme value distribution using recently developed techniques for time series, which results in an accurate practical procedure for solving a very difficult problem. This procedure is applied to the prediction of air gap statistics for a jacket structure.

Commentary by Dr. Valentin Fuster
2011;():439-448. doi:10.1115/OMAE2011-49596.

In the study, the results of a statistical modeling of ice loads from drifting ice features on the ice-resistant platforms in Piltun-Astohsky and Lunsky oil&gas fields of “Sakhalin-I” and “Sakhalin-II” Projects are investigated. The authors made a comparative analysis of ice loads on various types of gravity-based concrete structures in ice conditions of the Sea of Okhotsk according to the standards, procedures and guidelines from different Codes of design. And also the probabilistic model of ice loads, developed by the authors in the previous studies, was considered for comparative analysis.

Commentary by Dr. Valentin Fuster
2011;():449-454. doi:10.1115/OMAE2011-49601.

In analysis of the removal of offshore jackets an important failure mode is buckling. In current practice, a buckling check involves manual determination of the buckling lengths of each frame member. It is estimated that 5 to 10% of the man-hours in structural analysis of removal projects is spend on checking and correcting buckling lengths. Fortunately, an alternative method is available that does not require determining buckling lengths. In this paper it is shown how this method can be derived from the NORSOK standard for tubular steel frame structures. The method is demonstrated in a removal analysis of an offshore jacket. It is concluded that this method can be successfully applied.

Commentary by Dr. Valentin Fuster
2011;():455-462. doi:10.1115/OMAE2011-49617.

To rationally assess the consequence of a ship’s hull girder collapse, it is necessary to know the post-ultimate strength behavior of the hull girder including the global deformation and motions under wave-induced extreme loads. In the foregoing research, the authors proposed a numerical analysis system to predict the collapse behavior in waves including the post-ultimate strength behavior. In this paper, an analytical solution to describe the post-ultimate strength behavior is proposed. The primary objective of the present research is to clarify the parametric dependencies of the severity of the collapse in a rational manner. The parameters may include those related to load-carrying capacity and those related to the extreme loads. By comparing the numerical results and the present results, the analytical solution is shown to be effective. Some important parameters to predict the severity of the collapse are derived based on the analytical solution.

Topics: Girders , Waves , Hull
Commentary by Dr. Valentin Fuster
2011;():463-474. doi:10.1115/OMAE2011-49650.

Numerical and experimental simulations for small scale damaged stiffened panels are performed. Six small scale stiffened panels were fabricated following special techniques to keep them in accordance with usual tolerances related to initial geometric imperfections due to fabrication. Ultimate buckling strength analyses were carried out in order to perform a correlation study to adjust the numerical model for further use in parametric studies. The damage was imposed with a local indentation on the panels. Measurements of the geometric imperfection distributions and damage shapes were accomplished using the equipment laser tracker. It is a portable contact measurement equipment that uses laser technology with sub-millimeter accuracy. The numerical models are represented by shell elements assuming finite membrane strains and large rotations, considering both geometric and material nonlinearities. The aim of the work is to study the failure behavior up to and beyond buckling to evaluate the strength loss due to the damage imposed to the panel. Additionally, some numerical simulations of damaged stiffened panels were performed. In these analyses the damage was done with explicit nonlinear finite element code from ABAQUS program. Therefore the distortions and the residual stresses due to the damage are both considered in subsequent compression analysis.

Topics: Buckling
Commentary by Dr. Valentin Fuster
2011;():475-486. doi:10.1115/OMAE2011-49653.

Load-out operation is one of the important processes for Spar construction and transportation. The Spar is towed onto the deck of a semi-barge, supported by special cradles during the whole process. Variation in weight distribution of the Spar, tidal changes, the barge draft level and positioning are main factors that affect the consistence of the barge deck and the dock surface, which will induce the Spar to sustain vertical bending moments and shear force. Transverse wind acting on the Spar also forms a slight transverse shift of the barge deck, which will induce the Spar to suffer horizontal bending moments. Nevertheless, structural components of the Spar are not allowed to endure any plastic deformations or buckling. So, the ultimate loading condition should be determined and the weakest structural components during the load-out process should also be ascertained. A truss Spar is chosen as the objective, and twenty steps are defined for the load-out process. The code SESAM is used to do the analysis. A finite element model is made, consists of the whole Spar and the cradle frames, which also take part in the load-out process. Two groups of load cases are defined. The initial forced displacement is applied on the cradle, simulating the difference between the deck and the dock surface. The initial forced displacement can be modified to the state when the Spar structural components endure ultimate elastic load. On the transverse wind condition, the initial forced displacement is modified in the same way introduced above. By the analysis results, two envelope curves for the vertical force applying on the barge are obtained, to provide guidance for the operation. The stresses and the internal forces of the Spar structural components are summarized, on purpose of giving the flexural performance and the shearing stress condition of the Spar during the load-out process. The ultimate elastic loads can be obtained for each of the twenty steps, and the characteristic of every ultimate elastic load cases is concluded. According to elastic-plastic theory, affordable displacement between barge deck and dock surface is obtained.

Commentary by Dr. Valentin Fuster
2011;():487-495. doi:10.1115/OMAE2011-49658.

Welded joints are important for fatigue strength evaluation of ship and offshore structures. However, current techniques for fatigue evaluation of welded joint under variable load is not accurate enough. Also, it cannot consider the effect of load history which is one of the important features for the variable loads. Therefore, many experimental attempts are conducted for storm model to consider the variable loading. However, studies of storm loading usually ignore the effect of calm sea loading which constitute a large portion of the marine phenomena. Because it has been believed that the contribution of calm sea loading is not dominant for fatigue life in storm loading. In this paper, fatigue tests are conducted for the specimens with transverse attachment made of high tensile steel under variable amplitude axial loading based on storm model. Considered loadings include repeated single storm, 6 or 8 kind storms sequenced randomly, and storms including calm sea condition while the mean stress and the maximum stress of loadings are changed. Moreover, the effect of three variables are investigated; 1) root mean square (RMS) value of stress amplitude, 2) mean stress shift and 3) maximum stress which can characterize the storm loading on fatigue life. In addition, experiments with calm sea loading are conducted and the effect of calm sea loading is also investigated. The storm and calm sea loadings are generated from IACS-34 wave scatter diagram. 5% strain drop criteria is introduced to define crack initiation life. Experimental results including the test results from previous study are evaluated and compared with DNV-CN 30.7 (2005) and Matsuoka’s method for the estimation of crack initiation and propagation life. From the result, it is concluded that the fatigue strength under storm loading can be evaluated by RMS value of stress amplitude. And mean stress shift is more likely to relate to fatigue strength than maximum stress. The effectiveness of the calm sea loading is depend on the existence of mean stress shift. Regarding fatigue life evaluated by DNV and Matsuoka method, both of them have almost same accuracy.

Commentary by Dr. Valentin Fuster
2011;():497-505. doi:10.1115/OMAE2011-49687.

Spar platform is a compliant floating structure used for exploration of oil and gas from deep sea. To ensure safe operations, reliability against mooring line failure is a major concern in design. Furthermore, the mooring lines have high investment costs and are normally not accessible for in-service inspection. The common approach for solving the dynamics of Spar system is to employ a decoupled quasi-static approach which ignores the platform and mooring lines interaction. Coupled analysis, used presently, considers the mooring lines and platform in an integrated single model. Hence, it effectively captures the damping effect due to Spar and mooring lines coupling. Finite element code ABAQUS is used to obtain the response of Spar-mooring system under long crested random sea with current. Limit state function is derived based on failure due to fatigue for probabilistic reliability assessment. Random variables, participating actively in the limit state function are identified and statistically modeled. The most probable points or the design points are found to be an effective parameter for estimating partial factors of safety for load and resistance variables. First Order Reliability Method (FORM) is used to calculate probability of failure and reliability indices. The results are later checked against Monte carlo simulation. Reliabilities of segmental length of mooring and of full length are determined as they may significantly differ if the mooring properties change along the length. Reliability indices of annual and life time sea states are calculated.

Commentary by Dr. Valentin Fuster
2011;():507-515. doi:10.1115/OMAE2011-49713.

It has long been experienced that Launching of lifeboat from rigs and Positioning of offshore supply vessel (OSV) near rigs are very critical and vulnerable operations as regards to the safety. In present days, sophisticated & expensive Dynamic Positioning (DP) Systems are used in order to avoid the collision of OSVs with rigs. However, it is observed that despite the provision of such sophisticated means, accidents are still occurring. Loss of control during positioning of OSVs can lead to severe accident (e.g. collision of OSV Samudra Shakti with Mumbai High North (MHN) platform in 2005). To avoid such accidents, considerable gap needs to be maintained between the OSV and the rig position. Also launching of lifeboat is known to be another safety critical operation. Particularly in rough weather, the landing of lifeboat requires to be at least 20 to 30m away from the platform. This is essential for preventing the lifeboat to drift under the platform and colliding with the structure & piping system there at. Lifeboat launching by davit lowering or freefall have got their own traditional problems as widely known in shipping industry. These are described in the paper “Safety of Lifeboat launching - Some Possible Improvements {P19J_AB27}” presented at Design for Safety Conference-2010 held in Italy. Considering both the issues as described above, it is evident that maintaining adequate distance from the rig is a crucial factor for improving safety of Life Boat launching and positioning of OSVs alongside the rigs. In order to achieve this, an additional structural part of the rig is proposed in this paper. This new structural entity is to be in the form of an inclined truss which will act as launching skid for the lifeboat. This arrangement will provide a guided slipway to enable landing of lifeboat at a considerable distance from the rig. Also this structure will be configured suitably to enable berthing & mooring of supply vessels alongside the rig. In this arrangement the DP system will be required for a short time only, at the initial stage of placing the vessel. After having the vessel positioned, she can be moored with the new proposed truss structure. Thus the dependency of DP System will reduce significantly to avoid collisions. So the authors believe that the proposed arrangement will improve the safety of offshore operation in a significant manner.

Topics: Lifeboats
Commentary by Dr. Valentin Fuster
2011;():517-530. doi:10.1115/OMAE2011-49716.

Bad weather and rough seas contribute significantly to the risk to maritime transportation. This stresses the importance of taking severe sea state conditions adequately into account, with due treatment of the uncertainties involved, in ship design and operation. Hence, there is a need for appropriate stochastic models describing the variability of sea states. This paper presents a Bayesian hierarchical space-time stochastic model for significant wave height. The model has been fitted by data for an area in the North Atlantic ocean and aims at describing the temporal and spatial variability of significant wave height in this area. It could also serve as foundation for further extensions used for long-term prediction of significant wave height and future return periods of extreme significant wave heights. The model will be outlined in this paper, and the results will be discussed. Furthermore, a discussion of possible model extensions will be presented.

Topics: Spacetime , Waves
Commentary by Dr. Valentin Fuster
2011;():531-544. doi:10.1115/OMAE2011-49731.

This paper addresses two questions critical for the successful real world application of the Cooperative Research on Extreme Seas and their Impact (CresT) Joint Industry Project (JIP) design methodology in harsh operating environments: (1) how accurately may very extreme sea states (VESS) be specified by modern numerical spectral wave models? About 20 storms in which VESS (defined as with significant wave height (HS) > 14 m) have been measured by various in-situ and satellite-mounted altimeters are hindcast and it is shown that when the meteorological forcing is accurately specified, a proven 3rd generation (3G) wave model provides skillful and unbiased specification of peak HS and by implication of the associated spectral properties. The second question addressed is: how do current 3G models behave when applied to even more extreme meteorological forcing than observed in the real storms studied? The same hindcast methodology is, therefore, applied to a population of synthetic hurricanes whose combinations of intensity and scale are predicted by deductive modeling studies of Gulf of Mexico hurricanes carried out following Hurricane Katrina (2004). The model results suggests that for a tropical cyclone to generate say peak HS > 20 m would require the peak wind intensity of a major hurricane (Category 3 or greater) combined with a larger size and faster translation speed than may be maintained by a tropical cyclone in tropical or subtropical settings. Large scale cyclonic and relatively rapidly translating storms with major hurricane force peak wind speeds indeed exist as a class of mid-latitude extratropical cyclones, dubbed “winter hurricanes”. Hundreds of such storms have been detected in global satellite altimeter data in virtually all major ocean basins. The peak sea states in the most extreme examples are also found to be simulated quite skillfully with the hindcast technology applied. The hindcast results are explored to infer the upper limit to the naturally occurring dynamic range of sea states in tropical and winter hurricanes.

Topics: Design , Modeling , Storms , Seas
Commentary by Dr. Valentin Fuster
2011;():545-550. doi:10.1115/OMAE2011-49737.

The Hot Spot Stress (HSS) distribution around welded tubular joints in truss legs is essential information for the fatigue assessment of jack-up platforms. Both Finite Element (FE) analysis and model test were carried out on a multi-planar tubular KK joints under asymmetric axial loads in our research. The study shows that the HSS distribution around welded joints is different from common K or T joints, and the maximum HSS appears between crown and saddle on the chord surface at the chord-brace intersection. Moreover, some sensitive factors which influence the locations of the maximum HSS were investigated by FEA in this paper. The analysis results are useful for the optimal structure designing and crack initial prediction of the offshore jack-up platforms legs.

Commentary by Dr. Valentin Fuster
2011;():551-558. doi:10.1115/OMAE2011-49758.

Fatigue assessment of ships using the direct calculation approach has been investigated by numerous researchers. Normally, this approach is carried out as either a global model analysis, or as a local model structural analysis. The current investigation presents a case study of a container vessel where the global and local analyses procedures are combined. A nonlinear time-domain hydrodynamic analysis followed by a global FE analysis is employed to screen for the most severe locations of the global ship’s hull with regard to fatigue damage. Once these locations have been identified, a sub-modelling technique is employed to transfer global loads from the global FE model to local FE models that have high resolution of elements for local structure details. Results from a selection of local FE model simulations are presented. Stress concentration factors at four critical locations are calculated and compared with values recommended by classification guidelines. Results are presented from a short-term fatigue analysis which has been carried out using the rainflow counting method. Finally, a long-term fatigue analysis is performed in time-domain using a designed wave scatter diagram of representative sea states.

Topics: Fatigue damage , Ships
Commentary by Dr. Valentin Fuster
2011;():559-570. doi:10.1115/OMAE2011-49770.

This paper is concerned with a response based method for determining metocean design criteria for offshore pipelines. The method determines a set of metocean parameters that are consistent with the extreme response of the pipeline, and hence, incorporates the dependence between them implicitly. However, there are a number of challenges in its application. Firstly, the loading on a pipeline is dependent on the previous wave cycle, and hence, the drag and inertia coefficients vary within a sea-state. Secondly, along many pipeline routes the waves are depth limited and the short-term distribution of wave induced velocity and pipeline response can be difficult to define. These challenges are overcome through a number of approaches that include a parametric representation of the distribution of the response and the application of multivariate extreme value analysis. Furthermore, the sensitivity of the method to assumptions about the pipeline design is examined, and the problems with using the combined wave and current induced velocity as a proxy for the response are discussed. The method is applied to a site in the Mediterranean Sea and the results are compared to those from the application of the first order reliability method.

Topics: Stability , Design , Pipelines
Commentary by Dr. Valentin Fuster
2011;():571-577. doi:10.1115/OMAE2011-49837.

Theory and simulations have shown that the maximum crest height in an area can be substantially greater than the single point maximum. Theoretical results have been tested and confirmed using data from a 10 by 10 array of wave probes in the Marin Offshore Basin. Estimation of the maximum crest height under a TLP’s deck is more complicated than it is in unobstructed waters. A TLP’s large columns diffract and radiate incident waves. Fortunately, the short term statistics of the diffracted waves have the same form as the short term statistics of the incident waves. The maximum crests under the TLP are estimated by using diffraction theory to find the wave spectra at each point. Then, linear simulations are used to find time series at each point. Finally, the linear maxima are multiplied by the ratio between the theoretical linear and second order crest heights. Comparisons with measurements from wave probes placed under a TLP model in the Marin Offshore Basin support the accuracy of this process. These results mean that the maximum crest heights in the area under a deck must be considered in air gap calculations.

Commentary by Dr. Valentin Fuster
2011;():579-588. doi:10.1115/OMAE2011-49844.

This paper describes the test facilities developed within the Peninsular Research institution for Marine Renewable Energy (PRIMaRE) group and discusses the approach of the group to mitigate risk for marine renewable energy installations. The main consideration is given to the reliability assessment of components within mooring configurations and towards power umbilical for typical renewable energy sites. Load and response data from sea trial will be used to highlight the importance of these research activities, and a Dynamic Marine Component Test rig (DMaC) is introduced that allows four degree of freedom fatigue or destructive tests. Furthermore it is discussed how this facilities could also aid in the reliability assessment of wider offshore applications.

Commentary by Dr. Valentin Fuster
2011;():589-604. doi:10.1115/OMAE2011-49846.

The objective of the CresT JIP was ‘to develop models for realistic extreme waves and a design methodology for the loading and response of floating platforms’. Within this objective the central question was: ‘What is the highest (most critical) wave crest that will be encountered by my platform in its lifetime?’ Based on the presented results for long and short-crested numerical, field and basin results in the paper, it can be concluded that the statistics of long-crested waves are different than those of short-crested waves. But also short-crested waves show a trend to reach crest heights above second order. This is in line with visual observations of the physics involved: crests are sharper than predicted by second order, waves are asymmetric (fronts are steeper) and waves are breaking. Although the development of extreme waves within short-crested sea states still needs further investigation (including the counteracting effect of breaking), at the end of the CresT project the following procedure for taking into account extreme waves in platform design is recommended: 1. For the wave height distribution, use the Forristall distribution (Forristall, 1978). 2. For the crest height distribution, use 2nd order distribution as basis. 3. Both the basin and field measurements show crest heights higher than predicted by second order theory for steeper sea states. It is therefore recommended to apply a correction to the second order distribution based on the basin results. 4. Account for the sampling variability at the tail of the distribution (and resulting remaining possibility of higher crests than given by the corrected second order distribution) in the reliability analysis. 5. Consider the fact that the maximum crest height under a complete platform deck can be considerably higher than the maximum crest at a single point.

Commentary by Dr. Valentin Fuster
2011;():605-615. doi:10.1115/OMAE2011-49866.

To design marine structures in deep water, currents must be modelled accurately as a function of depth. These models often take the form of T-year profiles, which assume the T-year extreme current speed occurs simultaneously at each depth. To better reflect the spatial correlation in the current speeds versus depth, we have recently introduced Turkstra current profiles. These assign the T-year speed at one depth, and “associated” speeds expected to occur simultaneously at other depths. Two essentially decoupled steps are required: (1) marginal analysis to estimate T-year extremes, and (2) some type of regression to find associated values. The result is a set of current profiles, each of which coincides with the T-year profile at a single depth and is reduced elsewhere. Our previous work with Turkstra profiles suggested that, when applied in an unbiased fashion, they could produce unconservative estimates of extreme loads. This is in direct contrast to the findings of Statoil, whose similar (“CCA”) current profiles have generally been found to yield conservative load estimates. This paper addresses this contradiction. In the process, we find considerable differences can arise in precisely how one performs steps 1 and 2 above. The net finding is to favor methods that properly emphasize the upper tails of the data—e.g., using peak-over-threshold (“POT”) data, and regression based on class means—rather than standard analyses that weigh all data equally. By applying such tail-sensitive methods to our dataset, we find the unconservative trend in Turkstra profiles to essentially vanish. For our data, these tail-fit results yield profiles with both larger marginal extremes, and broader profiles surrounding these extremes—hence the title of this paper.

Topics: Currents , North Sea
Commentary by Dr. Valentin Fuster
2011;():617-626. doi:10.1115/OMAE2011-49867.

Nonlinear effects beset virtually all aspects of offshore structural loading and response. These nonlinearities cause non-Gaussian statistical effects, which are often most consequential in the extreme events—e.g., 100- to 10,000-year conditions—that govern structural reliability. Thus there is engineering interest in forming accurate non-Gaussian models of time-varying loads and responses, and calibrating them from the limited data at hand. We compare here a variety of non-Gaussian models. We first survey moment-based models; in particular, the 4-moment “Hermite” model, a cubic transformation often used in wind and wave applications. We then derive an “L-Hermite” model, an alternative cubic transformation calibrated by the response “L-moments” rather than its ordinary statistical moments. These L-moments have recently found increasing use, in part because they show less sensitivity to distribution tails than ordinary moments. We find here, however, that these L-moments may not convey sufficient information to accurately estimate extreme response statistics. Finally, we show that 4-moment maximum entropy models, also applied in the literature, may be inappropriate to model broader-than-Gaussian cases (e.g., responses to wind and wave loads).

Commentary by Dr. Valentin Fuster
2011;():627-635. doi:10.1115/OMAE2011-49880.

Steel Catenary Risers (SCRs) are commonly employed in deepwater oilfields. The most problematic aspect of fatigue analysis is the touchdown point, in which the dynamic bending stresses are highest and the uncertainties are most pronounced. From a previous study conducted by the authors, it is found that the fatigue damage is extremely sensitive to several seabed parameters. Thus the uncertainties in the seabed parameters should be properly accounted in order to achieve a reliable and economical SCR design. In this work, reliability analysis is performed with three seabed random variables, namely soil stiffness, soil suction, and seabed trench. The efficient First Order Reliability Method (FORM) is employed in conjunction with the Response Surface Method to yield the probability of fatigue failure. The results are then compared with direct numerical integration. Further, the dependency between the different random variables is investigated.

Commentary by Dr. Valentin Fuster
2011;():637-641. doi:10.1115/OMAE2011-49897.

Since 1982, 115 collisions have been reported on the Norwegian Continental Shelf, with varying degree of severity. In the period 2001–2010 there have been 26 reported collisions. None of the collisions has caused loss of lives or personnel injuries. The economic consequences however have been significant, especially one collision in 2009. This paper will give statistical summaries of the events, and compare the development of events with previous periods. The focus will be on the six most severe cases, describing each case, the damage and emphasise the most common causes of these events.

Commentary by Dr. Valentin Fuster
2011;():643-650. doi:10.1115/OMAE2011-49919.

This study has been carried out on ultimate compressive strength of a cracked steel plate component, considering the effects of initial imperfections (transverse and longitudinal residual stresses and initial deflection, as well). The main objective of this paper is to numerically investigate the influence of crack location and crack length on ultimate strength of a steel plate under monotonic longitudinal compression. This investigation is performed through non-linear finite element (FE) analysis using ANSYS commercial finite element code in which is employed Newton-Raphson method. The FE results indicate that the length of transverse crack and especially its location can significantly affect the magnitude of ultimate strength where the steel plate is subjected to longitudinal compressive action.

Commentary by Dr. Valentin Fuster
2011;():651-663. doi:10.1115/OMAE2011-49924.

A novel Wave Acquisition Stereo System (WASS ) is proposed for the stereo reconstruction of oceanic waves both in space and time. To test the performance of such video observational technology, we have deployed WASS at the oceanographic tower Acqua Alta in the Northern Adriatic Sea, off the Venice coast in Italy. The analysis of WASS video measurements yielded accurate estimates of the oceanic sea state dynamics, the associated directional spectra and wave surface statistics that agree well with theoretical models. Further, the expected largest wave surface height over an area is estimated via both Piterbarg’s theory (Piterbarg 1995) and Adler’s Euler characteristics (Adler 1981, Adler and Taylor 2007). It is found that the largest surface height over the imaged area is considerably larger than the maximum crest observed in time at a single point, in agreement with theoretical predictions.

Commentary by Dr. Valentin Fuster
2011;():665-673. doi:10.1115/OMAE2011-49926.

During the lifetime of an existing gas well, located in the D15FA/FB field in the North Sea, the pressure has dropped and consequently production is reduced. A depletion compressor had to be added to this existing platform to increase the production. This sounds easy but has been very challenging due to physical, noise and vibration restrictions. For this platform it appeared that a reciprocating compressor was the best choice based on its flexibility with respect to the specified operating conditions, available power, and efficiency. However, despite of several advantages a disadvantage of a reciprocating compressor is that it generates additional vibrations and noise in the living quarters, which are located close to the compressor system. The specified requirements, not to exceed the allowable noise limits in the living quarters and the vibration limits of the complete compressor system, could in this specific case, not be met with straightforward solutions. This presentation will explain the dynamic analysis and the efforts taken in compressor, skid, motor, piping and deck design to meet the very stringent specified requirements and to ensure a safe and reliable system for the long term operation. Special attention will be given to the measures taken to reduce the excitations acting on the platform, and the mechanical and acoustical analysis that have led to the final design of this reciprocating compressor system. The solutions that have been developed can be regarded as non-standard and have resulted in new directions in solving very demanding system requirements.

Commentary by Dr. Valentin Fuster
2011;():675-684. doi:10.1115/OMAE2011-49927.

Gas-lifted oil wells are susceptible to failure through malfunction of gas lift valve assemblies (GLV). One failure mode occurs when the GLV check valve fails and product passes into the well annulus, potentially reaching the wellhead. This is a growing concern as offshore wells are drilled thousands of meters below the ocean floor in extreme temperature and pressure conditions, and repair and monitoring become difficult. Currently no safeguard exists in the GLV to prevent product passage in the event of check valve failure. In this paper a design and operational procedures are proposed for a thermally-actuated positive-locking safety valve to seal the GLV in the event of check valve failure. A thermal model of the well and GLV system is developed and compared to well data to verify feasibility of a thermally-actuated safety valve. A 3× scale prototype safety valve is built and tested under simulated failure scenarios and well start-up scenarios. Realistic well temperatures in the range of 20C to 70C are used. Results demonstrate valve closure in response to simulated check valve failure and valve opening during simulated well start-up.

Topics: Safety , Design , Valves
Commentary by Dr. Valentin Fuster
2011;():685-692. doi:10.1115/OMAE2011-49940.

The precise knowledge of loads and motions in extreme sea states is indispensable to ensure reliability and survival of ships and floating offshore structures. In the last decades, several accidents in severe weather with disastrous consequences have shown the need for further investigations. Besides the sea state behavior and the local structural loads, one key parameter for safe ship design is the vertical bending moment. Previous investigations revealed that different ship design criteria, such as bow geometry and wave board height, affect the global loads significantly. Investigations in regular waves as well as in single high waves of vessels with different bow flares and freeboard heights show that the vertical bending moment increases significantly with increasing bow flare and freeboard height. Furthermore it became apparent that critical loads and motions do not have to come along with the highest wave which results in the main question of this paper: What is the worst case scenario — the highest rogue wave or a wave group with certain frequency characteristics? Which sea states have to be taken into account for the experimental evaluation of limiting criteria? This paper presents investigations in different critical wave sequences, i.e. two real-sea registrations accompanied by results in regular waves to evaluate the influence of the encountering wave characteristics on the vertical bending moment. For the model tests in the seakeeping basin of the Technical University Berlin a segmented RoRo vessel with large bow flare has been built at a scale of 1:70 and equipped with force transducers. The paper proves that critical loads and motions depend most notably on combinations of wave height, wave group sequences, crest steepness, encountering speed and the ships target position: Even small wave heights with unfavorable wave lengths can cause a critical situation.

Topics: Stress , Waves , Seas
Commentary by Dr. Valentin Fuster
2011;():693-702. doi:10.1115/OMAE2011-49950.

In principle, the reliability of complex structural systems can be accurately predicted through Monte Carlo simulation. This method has several attractive features for structural system reliability, the most important being that the system failure criterion is usually relatively easy to check almost irrespective of the complexity of the system. However, the computational cost involved in the simulation may be prohibitive for highly reliable structural systems. In this study a new Monte Carlo based method recently proposed for system reliability estimation that aims at reducing the computational cost is applied. It has been shown that the method provides good estimates for the system failure probability with reduced computational cost. By a numerical example the usefulness and efficiency of the method to estimate the reliability of a system represented by a nonlinear finite element structural model is demonstrated. To reduce the computational cost involved in the nonlinear finite element analysis the method is combined with a response surface model.

Commentary by Dr. Valentin Fuster
2011;():703-709. doi:10.1115/OMAE2011-49952.

A bivariate Gumbel distribution is established based on transformation of an existing bivariate Rayleigh distribution. Application of this distribution in relation to reliability assessment of marine structures is subsequently addressed. Linear combinations of the two basic variables which are Gumbel distributed are further considered. The role of the Gumbel distribution in connection with reliability assessment of marine structures is discussed. Comparison with another class of bivariate Gumbel distributions (Gumbel Type A) is also made.

Commentary by Dr. Valentin Fuster
2011;():711-719. doi:10.1115/OMAE2011-49957.

This paper describes a new series of laboratory observations, undertaken in a purpose built wave flume, in which a number of scaled simulations of realistic ocean spectra were allowed to evolve over a range of mild bed slopes. The purpose of the study was to examine the distribution of wave heights and its dependence on the local water depth, d, the local bed slope, m, and the nature of the input spectrum; the latter considering variations in the spectral peak period, Tp , the spectral bandwidth and the wave steepness. The results of the study show that for mild bed slopes the statistical distributions of wave heights are effectively independent of both the bed slope and the spectral bandwidth. However, the peak period plays a very significant role in the sense that it alters the effective water depth. Following detailed comparisons with the measured data, the statistical distributions for wave heights in relatively deep water are found to be in reasonable agreement with the Forristall [1] and Glukhovskii [2] distributions. For intermediate water depths, the Battjes & Groenendijk [3] distribution works very well. However, for the shallowest water depths none of the existing distributions provides good agreement with the measured data; all leading to an over-estimate of the largest wave heights.

Commentary by Dr. Valentin Fuster
2011;():721-728. doi:10.1115/OMAE2011-49980.

The aim of the present paper is to investigate the effects of corrosion pits on the ultimate capacity of rectangular mild steel plates under biaxial compression. A series of non-linear FEM analysis on plates with partial depth pits are carried out, changing geometrical attributes of both pits and plates, i.e., the radius, depth, location and distribution of the pits and the slenderness of the plates. Possible interaction between transverse and longitudinal axial compression is studied applying different level of loading ratio and considering the effects of partial depth pitting corrosion. It is shown that biaxial loading ratio is a dominant factor affecting the behavior of pitted plates besides pits intensity and thickness loss at pits. When longitudinal compression is dominant load with loading ratio lower than 1, the interaction relationship curves for different DOP levels tend to be parallel with each other and the distance between every two parallel curves seems to be dependent mainly on the deviation of their DOP values and thickness loss at pits. Moreover, pits distribution along long and shirt edges could also affect the ultimate strength behavior of plates. The work done in the paper illustrates that the ultimate capacity of pitted plate could be derived from intact plate by introducing important influential parameters like DOP, thickness loss and possible pits distribution.

Commentary by Dr. Valentin Fuster
2011;():729-736. doi:10.1115/OMAE2011-49982.

We have developed an approximate solution to Zakharov’s equations for the evolution of ocean surface waves. We have applied it with a spectral response surface method to estimate the probability of exceedance of crest elevation in random seas. The method avoids time-marching and allows probabilities to be estimated on practical time scales. The results indicate that quartet resonance can produce very large amplification of extreme crests in uni-directional seas. However, the effect is very much smaller in spread seas. Although the solution is approximate it provides a great deal of physical insight into the mechanisms by which a freak wave might develop and explains the difference in results between uni-directional and spread seas.

Topics: Resonance , Waves
Commentary by Dr. Valentin Fuster
2011;():737-742. doi:10.1115/OMAE2011-50014.

In this note, the response of a Spar Platform is studied when subject to very high waves. The novelty of this paper consists in adopting the Quasi Determinism (QD) theory in its first formulation for the determination of the response of the structure. In fact, one of the peculiarities of QD theory is that it can be adopted under any boundary condition. The Spar Platform has been modeled as a two-dimensional 3-degree-of-freedom (3dof) rigid cylindrical floating body, moored to the seabed, floating in deep water. Further, it is supposed to be a slender body under the action of extreme waves neglecting any diffraction effect. Moreover, such a system is considered to be nonlinear, because of the nonlinear damping derived from the drag force and because of the stiffness of the moorings, which in the model have been studied as nonlinear springs. The non linear coupled set of differential equations of motion have been integrated in order to obtain the response of the structure by means of numerical methods commonly adopted in Literature. Thus, the response under the occurrence of very high wave crests is studied by using Quasi Determinism theory and then the results compared and validated with those obtained through Monte-Carlo simulations.

Topics: Waves , Spar platforms
Commentary by Dr. Valentin Fuster
2011;():743-749. doi:10.1115/OMAE2011-50079.

In this paper, nonlinear instability and evolution of deep-water rogue waves on following and opposing currents were described by numerical simulation for laboratory investigation. The generation of rogue waves in a numerical tank by means of wave focusing technique had been studied. Here a spatial domain model of current modified nonlinear Schrödinger (NLSC) equations in one horizontal dimension (1D) was established for describing the deep-water wave trains in a prescribed stationary current field. The transient water waves (TWW) was adopted as the initial condition of the NLSC equation. The steady current was added to see the effect of wave-current interaction on the energy concentration of gravity waves. The influence of current as well as other terms in the NLSC equations on wave height, inclination, particle velocity and acceleration are shown. Meanwhile, the focusing time/position of TWW influenced by the current field is investigated, which is of course a very important factor in experimental research when we generate rogue waves in the laboratory.

Commentary by Dr. Valentin Fuster
2011;():751-759. doi:10.1115/OMAE2011-50092.

Coupled time domain analyses of a semi-submersible wind turbine are performed with the intention to study motions affecting fatigue damage at the base of the tower. The software applied is SIMO/RIFLEX with the extension TDHmill, which gives the wind thrust force and gyro moment on the wind turbine as point loads in the tower top. Short term environmental conditions are chosen from a joint wind and wave distribution for a site in the Northern North Sea. Variance spectra, mean value, standard deviation, kurtosis, skewness and Vanmarcke’s bandwidth parameter are calculated for stresses at the base of the tower. Damage is calculated for each short term condition by two methods; rainflow counting and narrow band approximation. The accuracy of narrow band approximation estimates for fatigue are examined for the structure in question. Time domain simulations are carried out for different sea states and fatigue damage is calculated for each case. Simulations show that turbulent wind dominates the response at low wind speeds and the response spectral density function tends to be very wide-banded. For wave dominated response, spectra have lower bandwidth, and narrow banded approximation for fatigue damage gives estimates 20–50% above rainflow counted damage.

Commentary by Dr. Valentin Fuster
2011;():761-769. doi:10.1115/OMAE2011-50100.

This paper concerns the statistical distribution of both wave crest elevations and wave heights in deep water. A new set of laboratory observations undertaken in a directional wave basin located in the Hydrodynamics laboratory in the Department of Civil and Environmental Engineering at Imperial College London is presented. The resulting data were analysed and compared to a number of commonly applied statistical distributions. In respect of the wave crest elevations the measured data is compared to both linear and second-order order distributions, whilst the wave heights were compared to the Rayleigh distribution, the Forristall (1978) [1] empirical distribution and the modified Glukhovskiy distribution ([2] and [3]). Taken as a whole, the data confirms that the directionality of the sea state is critically important in determining the statistical distributions. For example, in terms of the wave crest statistics effects beyond second-order are most pronounced in uni-directional seas. However, if the sea state is sufficiently steep, nonlinear effects arising at third order and above can also be significant in directionally spread seas. Important departures from Forristall’s empirical distribution for the wave heights are also identified. In particular, the data highlights the limiting effect of wave breaking in the most severe seas suggesting that many of the commonly applied design solutions may be conservative in terms of crest height and wave height predictions corresponding to a small (10 −4 ) probability of exceedance.

Topics: Waves , Seas
Commentary by Dr. Valentin Fuster
2011;():771-782. doi:10.1115/OMAE2011-50112.

Non-linear effects caused by extreme crest loading might be governing for important structural limit states. For TLP’s, non-linear effects caused by extreme crest might lead to tether overload and tether slack. Simple scaling of ULS results is not necessarily adequate if wave in deck events (or other NL phenomena) might occur with annual probability >10−4 . In this case ALS analysis should be performed, and detailed knowledge of the physics and statistics of extreme crest events, and of the loading caused by these waves, is required. Ignoring the possibility for wave in deck events and establishing the characteristic tension from data where there are no wave in deck events, might lead to unrealistic low estimates for the extreme tether tension. The present study demonstrates effects of the CresT JIP (Cooperative Research on Extreme Seas and their impacT) findings regarding extreme and rogue waves through a simplified reliability analysis of a generic TLP defined by the project. In this paper, SRA for tether overload is discussed, with emphasize on the effect of tension Twid caused by wave in deck events. Data from TLP basin test undertaken as part of the CresT JIP are applied, and parametric models for tether tension from measured crests and measured tether tension data are established. The model for Twid is used in the SRA for analysis of tether overloads for design sea states and for long term analysis for different geographic locations. Special attention is given to assessment of the effect of higher order crest non-linearities beyond 2nd order. The crest models are based on results from other work packages of the Crest JIP. Recommendations for future research needs are given.

Commentary by Dr. Valentin Fuster
2011;():783-792. doi:10.1115/OMAE2011-50152.

The paper presents a major benchmarking exercise to demonstrate the accuracy of the formulations for the ultimate strength of stiffened panels that had been proposed for inclusion in ISO TS 18072-2. The complete set of formulations addressed the ultimate strength of plates, hull girders and support members, in particular, transverse frames and webs. The stiffened panel strength formulations have been in the public domain for some time and represent the most comprehensive set yet published. Their potential application is to a wide range of structural forms, e.g. floating dock gates, caissons, bridge decks, FPSOs, etc, and as such have much to recommend for other than just ship structures. The benchmark study presents comparisons between two implementations of the formulations and the results of non-linear FEA. The structures analysed represent a range of different stiffened panels with varying overall dimensions, plate dimensions and stiffener shapes, in particular, flats, angles and Tees. Close correlation is generally realized but where differences arise, these are readily related to the approximations used in developing the strength formulations. The comparisons between the strength formulations and non-linear FEA results provides the basis for quantifying statistical uncertainties in the formulations which can be used in subsequent structural reliability analysis and partial resistance factor derivation.

Commentary by Dr. Valentin Fuster
2011;():793-803. doi:10.1115/OMAE2011-50153.

The present study demonstrates effects of the CresT JIP (Cooperative Research on Extreme Seas and their impacT) findings regarding extreme and rogue waves through a simplified structure reliability analysis of a generic TLP structure defined by the project. The study demonstrates effects of wave nonlinearity beyond the second order, diffraction-radiation of incoming waves with the structure, spatial variations of crest statistics, a vertical displacement of the TLP system at the deck location as well as sea water level variations due to surge and tide. In order to account for these effects a stochastic model is suggested. Sensitivity studies are carried out to identify importance of analysed parameters and their stochastic behaviour to the failure probability. Uncertainties related to the analysis are identified and ranked given special attention to the effect of wave nonlinearities beyond the 2nd order. Recommendations for future research needs are suggested.

Commentary by Dr. Valentin Fuster
2011;():805-813. doi:10.1115/OMAE2011-50162.

Global warming and extreme weather events reported in the last years have attracted a lot of attention in academia, industry and media. The ongoing debate around the observed climate change has focused on three important questions: will occurrence of extreme weather events increase in the future, which geographical locations will be most affected, and to what degree will climate change have impact on future ship traffic and design of ships and offshore structures? The present study shortly reviews the findings of the Intergovernmental Panel on Climate Change Fourth Assessment Report, AR4, [1] and other relevant publications regarding projections of meteorological and oceanographic conditions in the 21st century and beyond with design needs in focus. Emphasis is on wave climate and its potential implications on safe design and operations of ship structures. A risk based approach for marine structure design accounting for climate change is proposed. The impact of expected wave climate change on ship design is demonstrated for five oil tankers, ranging from Product tanker to VLCC. Consequences of climate change for the hull girder failure probability and hence the steel weight of the deck in the midship region is shown. Recommendations for future research activities allowing adaptation to climate change are given.

Commentary by Dr. Valentin Fuster
2011;():815-826. doi:10.1115/OMAE2011-50168.

This paper concerns the description of the formation of a very large, quality-controlled dataset of raw wave measurements. As part of the CresT (Cooperative Research on Extreme Seas and their impacT) Joint Industry Project (JIP), the participants provided raw field measurements of water surface elevation from various installations across the globe. This paper will describe the data collection from the different installations, the strict quality control procedure employed to ensure a reliable dataset, and an overview of the occurrences of records containing freak wave events. Part 2 of this paper will then go on to describe the analysis performed on this dataset as well as the findings from the study.

Commentary by Dr. Valentin Fuster
2011;():827-837. doi:10.1115/OMAE2011-50169.

This paper concerns the analysis of a very large, quality-controlled dataset of raw wave measurements. It directly follows from paper 1, as part of work undertaken for the CresT (Cooperative Research on Extreme Seas and their impacT) Joint Industry Project (JIP), and describes the various analyses performed on the dataset. In particular numerous freak wave events are observed and various analyses are performed to gain an insight into conditions that are conducive to their formation. The examination of probability distributions, spectral and temporal parameters, degree of focusing and environmental conditions that lead to freak waves is performed and the findings are presented.

Topics: Waves
Commentary by Dr. Valentin Fuster
2011;():839-846. doi:10.1115/OMAE2011-50170.

A global secant relaxation (GSR)-based accelerated iteration scheme can be used to carry out the incremental/iterative solution of various nonlinear finite element systems of offshore structural mechanics problems. This computation procedure can overcome the possible deficiency of numerical instability caused by local failure existing in the iterative computation. Moreover, this method can efficiently accelerate the convergency of the iterative computation. This incremental/iterative analysis can consistently be carried out to update the response history up to a near ultimate load stage, which is important for investigating the global failure behaviour of a structure under certain external cause, if the constant stiffness is used. Consequently, this method can widely be used to solve general nonlinear problems. Mathematical procedures of Newton-Raphson techniques in finite element methods for nonlinear finite element problems are summarized. These techniques are the Newton-Raphson method, quasi-Newton methods, modified Newton-Raphson methods and accelerated modified Newton-Raphson methods. Numerical results obtained by using various accelerated modified Newton-Raphson methods are used to study the convergency performances of these techniques for material nonlinearity problems and deformation nonlinearity problems, separately.

Commentary by Dr. Valentin Fuster
2011;():847-854. doi:10.1115/OMAE2011-50186.

Formal safety assessment (FSA), as a structured and systematic risk evaluation methodology, has been gradually and broadly used in the shipping industry nowadays around the world. Concerns have been raised to navigational safety of Yangtze River, known as China’s largest and the world busiest inland waterway. With the national development of the Middle and Western parts of China, the throughput and the passing ships in Yangtze River have been rapidly increasing during the past few decades. Meanwhile, accidents such as collisions, groundings, overturns, oil-spills and fires occur repeatedly, causing serious consequences. In view of this, attempts made in this paper are to evaluate the navigational risk of Yangtze River using the FSA concept and a Bayesian Network (BN) technique, so as to enhance the navigational safety in Yangtze River.

Topics: Safety , Rivers
Commentary by Dr. Valentin Fuster
2011;():855-864. doi:10.1115/OMAE2011-50199.

For the design of ships and offshore structures the largest crest height which can be expected in their lifetime is of key importance. This was confirmed by several incidences e.g. in hurricanes in the Gulf of Mexico during the recent years. This is why MARIN started up the CresT JIP with a number of partners. The CresT JIP is now completed and some results of the extreme wave load and response mechanisms observed during model tests with a TLP will be presented in this paper. First an overview is given of the loading and response process during the most extreme event observed. As a next step the loading and response is related to the time and spatial characteristics of the waves, as it is not per definition the highest local crest or wave height that results in the most extreme dynamic response. Furthermore, the effect of different TLP design variations and short-crestedness will be discussed.

Commentary by Dr. Valentin Fuster
2011;():865-873. doi:10.1115/OMAE2011-50214.

Regional frequency analysis (RFA) based on L-moments is applied to the HIPOCAS hindcast data using daily maximum significant wave heights offshore Portugal to identify the homogeneous regions and to suggest the appropriate regional frequency distribution and extreme quantiles. Several statistics are computed at the various grid points in the area of study to classify the wave conditions of the regions. The daily maximum significant wave heights of the rough winter month January are used for this case study. The results of the study have shown that there are 3 homogeneous regions in the offshore region under investigation (35°–45°N, − 9.5°–− 11°W) comprising from 15 equally spaced grid points referring to an area of 0.25°×0.25°. It is interesting to observe that the algorithm is able to identify neighboring grid points as members of different regions. The maximum discrepancy between the at-sites’ extreme quantiles and their respective regional quantiles is 0.82 m for a return period of 100 years.

Commentary by Dr. Valentin Fuster
2011;():875-884. doi:10.1115/OMAE2011-50244.

Various safety analysis techniques have been generated in dealing with uncertain information over the past three decades. The safety assessment based on conventional risk methods may not be well suited with most problems possessing high level of contingencies in marine and offshore systems. In other words, the effect of less confidence, unreliable, missing or incomplete data in safety assessment is arduous to apply common safety and risk assessment techniques for maritime industry. Therefore, fuzzy logic based approaches are probably more appropriate to apply in carrying out safety assessment for maritime systems. This paper presents a survey of applications in fuzzy set theory for safety analysis to marine and offshore industries’ problems. The safety assessment methods are presented in detail by demonstrating their robust and weak points. Subsequently, the main results on the application of fuzzy set theory for safety assessment will be further explained. Finally, recommendations in undertaking developments into the field of maritime safety assessment will be given.

Commentary by Dr. Valentin Fuster
2011;():885-892. doi:10.1115/OMAE2011-50268.

In recent decades, natural gas has been gaining importance in world energy scene and established itself as an important source of energy. One of the biggest obstacles to increase the usage of natural gas is its transportation, mostly done in its liquid form, LNG – Liquefied Natural Gas, and storage. It involves the liquefaction of natural gas, transport by ship, its storage and subsequent regasification, in order to get natural gas in its original form and send it to the final destination through natural gas pipeline system. Nowadays, most terminals for receiving, storing and regasificating LNG, as well as sending-out natural gas are built onshore. These terminals, however, are normally built close to populated areas, where consuming centers can be found, creating safety risks to the population nearby. Apart from possible damages caused by its cryogenic temperatures, LNG spills are associated with hazards such as pool fires and ignition of drifting vapor clouds. Alternatively to onshore terminals, there are currently several offshore terminals projects in the world and some are already running. Today, Brazil owns two FSRU (Floating Storage and Regasification Unit) type offshore terminals, one in Guanabara Bay, Rio de Janeiro and the other in Pecém, Ceará, both contracted to PETROBRAS. The identification of the operation risks sources of LNG terminals onshore and offshore and its quantification through mathematical models can identify the most suitable terminal type for a particular location. In order to identify and compare the risks suggested by onshore and offshore LNG terminals, we have taken the example of the Suape Port and its Industrial Complex, located in Pernambuco, Brazil, which is a promising location for the installation of a LNG terminal. The present work has focused on calculating the distance to the LNG vapor cloud with the lower flammability limits (LFL), as well as thermal radiation emitted by pool fire, in case of a LNG spill from an onshore and from an offshore terminal. The calculation was made for both day and night periods, and for three types of events: operational accident, non-operational accident and worst case event, corresponding to a hole size of 0,75m, 1,5m e 5m, respectively. Even though the accidents that happen at an onshore terminal generate smaller vulnerability distances, according to the results it would not be desirable for the Suape Port, due to the location high density of industries and people working. Therefore, an offshore terminal would be more desirable, since it presents less risk to the surrounding populations, as well as for workers in this location.

Commentary by Dr. Valentin Fuster
2011;():893-902. doi:10.1115/OMAE2011-50272.

Floating drilling and production systems are subject to increasing levels of scrutiny by governments and the general public. This paper discusses practical methods for the reliability analysis and risk management of the moorings systems of such drilling and production systems.

Topics: Reliability , Mooring
Commentary by Dr. Valentin Fuster
2011;():903-913. doi:10.1115/OMAE2011-50293.

The results of four tests on narrow stiffened panels under axial compression until collapse and beyond are presented to investigate the ultimate strength of stiffened panels. Tension tests are used to evaluate the material properties of the stiffened panels. The tests are made on panels with two half bays plus one full bay in the longitudinal direction. Initial loading cycles were used to relief the residual stresses of the stiffener panels. The strain gauges are set on the plates and stiffeners to record the distribution of strain. The displacement load relationship is established. The ultimate strength behavior, modes of failure and load-carrying capacity of the stiffened panels are investigated with the experiment.

Topics: Collapse
Commentary by Dr. Valentin Fuster
2011;():915-922. doi:10.1115/OMAE2011-50294.

The behavior of long stiffened panels are simulated numerically and compared with test results of axial compression until collapse, to investigate the influence of the stiffener’s geometry. The material and geometric nonlinearities are considered in the simulation. The initial geometric imperfections, which affect the collapse behavior of stiffened panels, are also analyzed. The initial imperfections are assumed to have the shape of the linear buckling mode. Four types of stiffeners are made of mild or high tensile steel for bar stiffeners and mild steel for ‘L’ and ‘U’ stiffeners. To produce adequate boundary conditions at the loaded edges, three bays stiffened panels were used in the tests and in the numerical analysis.

Commentary by Dr. Valentin Fuster
2011;():923-933. doi:10.1115/OMAE2011-50313.

This paper investigates the effect of third-order nonlinearities on the statistical distributions of wave heights, crests and troughs of waves mechanically generated in a deep-water basin, simulating two crossing systems characterized by bimodal spectra. Observed statistics exhibit various effects of third-order nonlinearities in a manner dependent on both the distance from the wave-maker and the angle between the mean directions of the component wave systems. In order to isolate and demonstrate the effects of third-order nonlinearities by themselves, vertically asymmetric distortions induced by second-order bound waves are removed from the observed time series. It appears then that the distributions of wave crests, troughs and heights extracted from the non-skewed series clearly deviate from the Rayleigh distribution, suggesting that waves are characterized by nonlinear corrections of higher order than the typical of second-order waves. Nonetheless, some models developed for weakly nonlinear second-order waves can still be used in describing wave heights, crests and troughs in mixed seas, provided that relevant distribution parameters are modified so as to reflect the effects of third-order corrections and some basic characteristics of mixed seas.

Commentary by Dr. Valentin Fuster
2011;():935-943. doi:10.1115/OMAE2011-50314.

In this paper some abnormal or rogue wave events registered in the North Sea by means of the surface elevation measurement are reconstructed with numerical models. The time series of surface elevation measured at a single point although providing incomplete information, are used to restore the unidirectional wave dynamics under reasonable assumptions. Different approaches of defining the relation between the surface elevation and the fluid velocity field are considered, and different numerical models are used to simulate the wave dynamics in time and space. It is shown that for some abnormal or rogue wave records the result of the extreme event reconstruction is robust. In particular, the verification of approximate approaches versus the fully nonlinear numerical simulation is performed. At the same time, the reconstructed rogue wave is generally less steep than the measured one. Possible reasons for this discrepancy are suggested.

Commentary by Dr. Valentin Fuster
2011;():945-953. doi:10.1115/OMAE2011-50317.

The main goal of this work is to investigate the wave groups using data from a deep water basin. Available data are for unidirectional waves measured at several fixed points situated in different distances from the wave maker. Previous works of many authors show that such series describe a process which differs significantly from the Gaussian one. Omitting the usual envelope definition by the Hilbert transform an upper and lower envelopes are introduced. Then the mean high run, mean group length and their distributions are found and compared with the theoretical results for Gaussian process.

Commentary by Dr. Valentin Fuster
2011;():955-960. doi:10.1115/OMAE2011-50320.

Functional Data Analysis is a set of statistical tools developed to perform statistical analysis on data having a functional form. In our case we consider the one-dimensional wave profiles registered during a North-Sea storm as functional data. The waves are defined as the surface height between two consecutive downcrossings. Data is split into 20-minute periods and after registration of the waves to the interval [0,1], the mean wave is obtained along with the first two derivatives of this mean profile. We analyze the shape of these mean waves and their derivatives and show how they change as a function of the significant wave height for the corresponding time interval. We also look at the evolution of the energy, as represented by the phase diagram, as a function of significant wave height. The results show the asymmetry in vertical and horizontal scales for real data. To consider how the individual waves vary we perform a Functional Principal Component Analysis of wave profiles, dividing previously the waves into groups according to their height and comparing with waves measured during a non-storm period. The results suggest that the modes of variation of wave profiles do not depend on wave height or sea condition.

Topics: Waves , Storms
Commentary by Dr. Valentin Fuster
2011;():961-967. doi:10.1115/OMAE2011-50331.

The market of LNG (Liquefied Natural Gas) carrier is continuously in a prosperous condition, and a lot of LNG vessels are being built in many shipyards. Membrane-type MARK-III LNG CCS (Cargo Containment System) is used more and more in the construction of LNG carrier, and it has already taken considerable market share among the various LNG CCS products. This paper deals with a study on structural safety of LNG carrier whose inner hull structure is affected by cryogenic temperature of LNG. If the primary and secondary barriers are failed simultaneously, the inner hull structure comes to be in direct contact with LNG. It is well known that the cryogenic temperature exposes the inner hull structure to fatal risk of structural failure due to brittle fracture, but nevertheless it is quite difficult to find a precedent research which explains the degree of risk and severity with due consideration of the consequence caused by structural failure of inner hull. The heat transfer test has been performed using the specimen appropriate to realize test scenario while considering cryogenic liquid flow from primary and secondary barrier into inner hull structure, and at the same time, the specimen has been tested by applying proper deformation so as to examine the structural behavior of inner hull structure under cryogenic condition. The heat transfer analysis has been performed to simulate and verify the heat transfer test, and consequently it is possible to obtain actual distribution of temperature in the inner hull structure exposed to cryogenic temperature. The structural analysis has been performed to evaluate the damage of inner hull structure and as a result to assess overall decrease of hull girder strength of LNG carrier. Finally, consequence of the decrease of global strength has been discussed.

Topics: Temperature , Safety , Hull
Commentary by Dr. Valentin Fuster
2011;():969-975. doi:10.1115/OMAE2011-50335.

The Drag-Inertia method is one of the most popular time domain methods for estimating the contribution of dynamics to the response of a Jackup. The method calculates the Dynamic Amplification Factor (DAF) using the simulated response of the Jackup to a short, usually one hour, random storm. The Drag-Inertia method has been subject to some criticism as it produces results which are, in some cases, less conservative than other methods. Despite this, we feel the benefits of the Drag-Inertia method; efficient and repeatable assessment, are essential to a site assessment. Other methods, such as the Winterstein method, require much longer storm durations, while the variability in DAF results is considerably more. In this study, we propose that factoring the DAFs produced by the Drag-Inertia method is a practical way to improve conservatism, bringing the DAFs in line with the mean DAF results produced by the Winterstein method. The factor is proposed based on simulations of 18 different Jackup configurations and eight different wave conditions. The simple factor is a function of period ratio, Tn /Tp , allowing for efficient and repeatable factoring of DAF values without additional simulation time. When the factor is applied to the Drag-Inertia DAF values the resulting DAFs are within 0.05 of the mean Winterstein DAF in 61% of cases and in only 7% of cases is the Winterstein DAF more than 0.10 above the factored Drag-Inertia result.

Commentary by Dr. Valentin Fuster
2011;():977-982. doi:10.1115/OMAE2011-50340.

In offshore structural design, it is necessary to evaluate probabilistic risk so that the topside structure has sufficient capacity to resist the effects of accidental loads such as drop impact, helicopter impact, vessel collision, fire, and explosion. Most engineers in the offshore industry, however, have difficulties in estimating a reliable risk value because there are still too many uncertainties in computing the probability of exceeding a target structural damage where the guideline for hazard analysis, which provides design load, is quite well described. Therefore, a framework to compute reasonable probability of structural failure is proposed in this paper. 88 impact scenarios were applied to a pipeline protection system on a topside module of Nexus genetic FPSO in order to perform regression analysis for structural damage. For risk calculation, the hazard analysis in this study is based on the detail engineering report worked by Ramboll Oil & Gas.

Commentary by Dr. Valentin Fuster
2011;():983-991. doi:10.1115/OMAE2011-50344.

DP shuttle tankers performing offloading directly from fixed or geostationary floating offshore installations is addressed in this paper. It is important to ensure that disconnection of offloading hose can be achieved in time given shuttle tanker DP failure and position loss. The accident scenario is the hose fail-to-disconnect while shuttle tanker has an excessive position excursion. The consequence can be oil spill combined with the damage the offloading system. The spill amount can be as much as the crude oil volume in the hose, or over 1000 m3 if isolation and shutdown of oil export pump on the installation are not achieved timely. Various barriers to prevent oil spill have been developed over the past 30 years’ history of shuttle tanker offshore loading. However, the direct offloading is a new operational context to the traditional offloading. A quantitative frequency model for oil spill initiated by DP shuttle tanker position loss in direct offloading is presented in this paper. Case study results show that in the base case where only traditional barriers are used, the frequency for large oil spill up to 1000 m3 or more may reach 2.48E−03 per year, given 20 hours offloading cargo transfer time and 52 times offloadings per year. This frequency is not negligible, and risk reduction measures are viewed necessary. Novel safety barriers, i.e. Automatic Shutdown and Release (ASDR), as well as the HPR (Hydroacoustic Position Reference) and BLS (Bow Loading System) weak link mode, are analyzed as sensitivity cases. Results show that the frequency of large oil spill can then be reduced to 3.81E−05 per year, i.e. 1.5% of the base case value, and this is well within 1.0E−04 per year level. Recommendations to minimize oil spill risk during DP shuttle tanker direct offloading operations are proposed in this paper.

Topics: Tankers
Commentary by Dr. Valentin Fuster
2011;():993-1000. doi:10.1115/OMAE2011-50345.

The maintenance of structural integrity is a significant consideration in the safety management of offshore installations. This paper presents an integrated approach for fitness-for-service evaluation of a deteriorating offshore radio tower structure. The approach is intended to assist engineers in assessing the overall fitness and survivability of aged offshore structures. A 43 m tall radio tower on an oil and gas platform located offshore Australia was reported with areas of heavy and medium corrosion of structural members. Severe corrosion in one leg of the radio tower had caused an obvious hole (extensive damage) through the leg at approximately 36m above the main deck and raised structural integrity concerns with the tower. The platform had been shut down due to concerns of a possible collapse of the tower. An assessment/repair program was developed to assure the short term integrity of the tower with minimal repair works. The integrity of the critically damaged leg had been temporarily restored using a clamped sleeve repair to allow progress with the inspection / thickness measurement of the corroded areas of the tower. As part of the fitness-for-service assessment, the minimum thickness acceptance criteria for the suspected corroded structural members were developed to enable initial assessment of the measured remaining wall thicknesses of the corroded member. Fitness for service integrity assessment requirements were developed to assess the locations that did not meet the minimum thickness criteria. The integrity requirements were adopted based on the average measured wall thickness, sensitivity structural analyses for reduced wind speeds for shorter life spans, and stability/survival assessment of the tower. An inspection program was carried out for the suspected locations and any additional locations identified during the inspection process. The inspection measurements were assessed against the fitness-for-service criteria. Where the measurements indicated that members did not meet the acceptance criteria temporary repairs were specified. Consequently, the tower fitness-for-service was found sustainable for up to 12 months until a more permanent repair or replacement of the tower could be completed, thus enabling the platform to resume normal operations.

Commentary by Dr. Valentin Fuster
2011;():1001-1007. doi:10.1115/OMAE2011-50349.

In-line Inspection (ILI) surveys are periodically performed to determine the condition of the pipeline. Typical ILI surveys involve Magnetic Flux Leakage primarily to determine metal loss and simple single channel Calliper surveys to determine any signs of geometry imperfections. Additional surveys such as high-resolution multi-channel Calliper deformation tools are occasionally used to accurately record imperfections to enable a more accurate assessment of the integrity of the pipeline containing the imperfection. Such tools have had limited employment, and therefore little experience exists of using the data obtainable for the detailed assessment of defects. This paper presents a study of such a case. As part of an In-line Inspection (ILI) of an offshore pipeline, a high-resolution deformation survey recorded numerous dent anomalies which had potentially resulted from a single dragged anchor incident before the pipeline was trenched. This data set was correlated to Magnetic Flux Leakage inspection data to confirm external mechanical damage. Pipeline sections having anomalies that were either found close to girth welds, or had associated corrosion defects were automatically selected for repair. The remaining anomalies were assessed in order to determine their acceptability for the maximum allowable operating pressure using the approaches detailed in API-579. Due to the sharp nature of some of the dents, elastic-plastic finite element analyses (FEA) were performed using denting profiles generated from the calliper data of the ILI run. API-579 level 3 assessments were then carried out using the FEA results. This paper details the high-resolution deformation tool findings and the approach used in order to assess the fitness-for-purpose of the pipe with the recorded anomalies.

Commentary by Dr. Valentin Fuster
2011;():1009-1015. doi:10.1115/OMAE2011-50350.

All pipelines are susceptible to the possibility of corrosion damage. Corrosion is a time dependent process that leads to localised gradual thinning of the pipeline wall and if allowed to continue will eventually cause failure of the pipewall. Due to the progressive nature of corrosion the likelihood of failure increases with time. One means of mitigating the likelihood of such failures is to perform an in-line inspection using a metal loss detection tool. The frequency of inspection is an important parameter to operators since if it is too high, excessive costs will be incurred and if it is too low, failure involving loss of supply, threats to safety and the environment may follow. Operators therefore seek the optimum frequency. This paper describes a robust method for optimizing inspection intervals based on the use of structural reliability analysis.

Topics: Inspection
Commentary by Dr. Valentin Fuster

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