ASME Conference Presenter Attendance Policy and Archival Proceedings

2014;():V06AT00A001. doi:10.1115/OMAE2014-NS6A.

This online compilation of papers from the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering (OMAE2014) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Pipeline and Riser Technology

2014;():V06AT04A001. doi:10.1115/OMAE2014-23010.

Induction pipe bends are essential multi-functional components in offshore applications performing not only as fluid conductors but also as structural members providing flexibility to the entire pipeline. The deforming mechanism of bends minimizes the effects of pipe walking, length changes due to thermal expansion/contraction, etc. However, the extent to which the bend deforms to counteract the pipeline deformation, prior to reaching plastic collapse, is dictated by the design variables. The pipe bend design variables include the geometry of the bend, the inelastic material properties, and the operating loads. The study of the influence of these variables is central to improving upon existing bend designs and is the focus of this research.

The certification process for bends typically involves ensuring the pipe bending moment is within limits set by agencies such as DNV, ASME, etc. Closed form solutions for the bending moment do exist but they often do not consider the effects of large deformation and the material nonlinearity of the bends. Since it is impractical to perform physical tests for every possible design, numerical techniques such as the finite element methods are an attractive alternative. Furthermore, for a given bend design, the design variables are prone to deviation, due to manufacturing process, operating conditions, etc., which introduces variation in the structural response and the resulting bending moment.

In this paper, a nonlinear finite element analysis of induction bends is discussed followed by a presentation of a simulation workflow and reliability analysis. The finite element analysis utilizes a nonlinear Abaqus model with an user-subroutine prescribing precise end loading and boundary conditions. The workflow utilizes the design exploration software, Isight, which automates the solution process. Thereafter, reliability analysis is performed by varying the design variables, such as bend angle, ovalization, etc. and the results of the simulation are presented.

The objective is to illustrate a solution technique for predicting the induction bend load carrying capacity and to examine design robustness. An automated workflow is demonstrated which allows for quick design variable changes, there by potentially reducing design time. The reliability analysis allows analysts to measure the variation in the load carrying capacity resulting from the deviation of design variable specifications. These demonstrations are intended to emphasize that to ensure the success of a bend design, it is important to not only predict the load carrying capacity accurately but also to perform reliability analysis for the design.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A002. doi:10.1115/OMAE2014-23103.

Tensile armor wire breaks in flexible risers have been studied by measuring strain in wires during a series of full-scale dynamic pipe tension tests. This has resulted in an understanding of wire breaks described by parameters such as time evolution, spatial distribution and the scale of the measured strain change. With the measured strain data wire break events in flexible risers can be detected and discriminated from other events in the pipe structure. Hence, by instrumenting risers with a grid of fiber optical strain sensors embedded in the tensile wires a Wire Break Detection system can be implemented. The test data and the resulting knowledge of how strain variations from a wire break event propagate in a flexible riser are presented.

Topics: Wire , Flexible risers , Armor
Commentary by Dr. Valentin Fuster
2014;():V06AT04A003. doi:10.1115/OMAE2014-23106.

During the productive phase of an oil well, there may be jointly production of groundwater, called formation water, which has different dissolved salts which may, during production, form a solid phase (precipitate), causing partial or total obstruction of the well, due to scaling of precipitation. The main inorganic scales in oil industry are: calcium sulfate (CaSO4), barium sulfate (BaSO4), strontium sulfate (SrSO4) and calcium carbonate (CaCO3). The carbonate is precipitated due to changes in temperature, pressure and water flow associated with oil. These variations happen due to fluid displacement which originally are in equilibrium conditions to the flow conditions. As the removing process is extremely expensive and often irreversible, mathematical tools are used to predict the saline scale. This work will present three methods: software Multiscale, Multiple Linear Regression and Taguchi Approach. Those methods will help predict saline scales of CaCO3, by calculating saturation index. The other salts will not be used in this monograph. After realization of the experiments, comparing the values found in the calculation of the CaCO3 saturation index using the three methods, satisfactory results are obtained when the pressure ranges up to 20% of the experimental pressure, with a reduction in the number of experiments, using the method known as Taguchi Approach.

Topics: Oil wells
Commentary by Dr. Valentin Fuster
2014;():V06AT04A004. doi:10.1115/OMAE2014-23111.

Numerous leak detection systems (LDS) using a variety of technologies are on the market. Since LDS are designed for a particular use, then the evaluation and selection process requires suitable metrics and involvement from all stakeholders. This paper proposes to use15 criteria for selection of LDS. Each criterion is first rated for their importance by a number of experts. Then, the same experts are asked to rate a list of candidate LDSs. Fuzzy TOPSIS is used to aggregate Experts’ judgment. A case study is presented to demonstrate the application of the method. This method would indicate how a good compromise might look like. This could aid the decision-maker to weigh options and set priority and decide on a system.

Topics: Leakage
Commentary by Dr. Valentin Fuster
2014;():V06AT04A005. doi:10.1115/OMAE2014-23116.

Offshore pipelines installed in the Arctic and other cold regions are often buried to reduce the risk of damage from ice gouging, upheaval buckling, and other loading challenges specific to the region. Pipeline burial is normally achieved through trench excavation and backfill.

Pipelines have been buried using a wide variety of technologies including conventional excavation equipment, hydraulic dredges, ploughs, mechanical trenchers, and jetters. In order to determine a preferred trenching method for a particular route, consideration must be given to a variety of factors. The water depth range and maximum trench depth required along a route are primary considerations when evaluating the various trenching technologies. These are “show stopper” route parameters, which have a direct impact on the ability to complete a particular trench.

If multiple trenching technologies satisfy the primary considerations, a variety of secondary considerations must be used to determine the preferred solution. These include parameters such as seabed geology, backfill method, seabed slopes, and environmental sensitivity. The preferred solution may not always be the only method of excavating the trench, but it may have an advantage compared to other technologies for the route under evaluation.

As developments are proposed for areas that experience relatively deep ice gouging (up to 5m), burial depth requirements will exceed the capabilities of current technologies. New technologies capable of working in deeper water, achieving greater burial depths, achieving reasonable trenching advance rates, operating in harsh environments, and trenching through variable and difficult seabed soils will be required.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A006. doi:10.1115/OMAE2014-23117.

With the oil industry’s continued quest for oil and gas in frontier offshore locations, several developments have taken place in regions characterized by seasonal ice cover including the US Beaufort, North Caspian, and Sakhalin Island. In these projects, pipeline systems have been used, which are a cost-effective, safe, and reliable mode of hydrocarbon transport.

For pipeline development in Arctic, several years of data need to be collected to support the pipeline design and construction planning, and may be required by regulations. Therefore, Arctic offshore pipeline projects generally require repetitive mapping surveys and geotechnical programs to verify design loads, soil properties, and thaw settlement potential.

The major design loads that are considered for Arctic projects include ice gouging, strudel scour, upheaval buckling as well as thaw settlement. These issues can have a significant influence on the pipeline engineering considerations such as strain based design, target burial depth requirements, cost, and safety.

While important to the design of the pipeline, these issues account for just a few of the many criteria that must be considered when routing a pipeline; criteria which can be categorized as either engineering, environmental, social, administrative, or infrastructural.

The pipelines which are currently operational in the Arctic are located in shallow water depths and close to shore but were influenced by the unique Arctic environmental loading conditions. The experience from these past projects provides a significant base for the design, and operating of future offshore arctic pipelines.

Pushing the limits to developments further offshore in deeper water will require that additional consideration be given to aspects related to pipeline design, in particular with respect to burial for protection against ice gouging.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A007. doi:10.1115/OMAE2014-23137.

As has been noted in industry publications and conferences in the recent past the use of more modern deepwater capable 5th and 6th generation semisubmersible drilling rigs in relatively shallow water applications (when compared to design water depth) is likely to become more commonplace. Water depths of 500m or less will necessitate the use of mooring systems in order to maintain position close to the well centre whilst drilling. For fatigue assessments of moored MODUs, the current industry practice to estimate fatigue damage in the drilling riser and the wellhead, using global riser analysis techniques, is to consider both wave and VIV fatigue effects. Standard wave fatigue analysis considers two key response parameters, firstly the impact of the hydrodynamic loading on the riser joints due to drag forces, inertia and added mass effects, and secondly the effects of vessel motions on the riser system and wellhead loading. Standard practice for wave fatigue analysis is to consider only first order motion effects as described by the vessel RAO (response amplitude operator). However, for a moored MODU low frequency (100s-200s period) vessel response can have a significant impact on the overall vessel motions.

The actual response and magnitude of MODU motion will be influenced by the size and displacement of the vessel in addition to the configuration of the mooring system. First order lateral motions for a semisubmersible tend to increase as wave period is increased and therefore at lower periods first order motions can be quite low. However, the opposite can be said of wave drift forces that contribute to second order response. Although the wave drift forces are largest for lower wave periods, these low period drift forces have a significant influence on the resulting long period second order response of a moored MODU. This has important implications for drilling riser and wellhead fatigue analysis as in many cases the critical seastates for fatigue damage are low period seastates with a large number of occurrences. Thus the current global analysis techniques for fatigue calculations may lead to an underestimation of fatigue damage contribution from low period seastates.

The purpose of this paper is to present the key conclusions and findings of a study carried out in order to determine the effects of low frequency moored MODU motions on wellhead fatigue. These results are derived from a case study of a moored 6th generation semi-submersible drilling vessel in 500m water depth.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A008. doi:10.1115/OMAE2014-23146.

Rigid steel jumpers are used in a subsea flow line system to connect subsea components. They provide a certain flexibility with respect to installation and operating conditions. This flexibility makes the jumper susceptible to slug flow induced vibrations. Slug flow can be described as an alternating flow of long oil and gas bubbles which flow at the gas velocity. The alternation between oil and gas density causes loads on the jumper which causes the jumper to vibrate. Two excitation mechanisms can be identified; 1) The variation in weight along the straight sections and 2) the difference in impact loads on the bends. Due to the cyclic nature of these loads fatigue can cause the jumper to fail.

As a main contractor of SURF-projects (Subsea Umbilicals Risers and Flowlines) Heerema Marine Contractors (HMC) is responsible for the engineering, procurement, construction and installation (EPCI) of the entire project scope, including the design of the subsea jumpers. Hence this paper has been set up by HMC and the Delft University of Technology to study slug flow induced fatigue in subsea jumpers and in order to find new design considerations.

In the early design phase of a subsea jumper the offshore industry commonly uses, to authors knowledge, a static analysis to predict the fatigue damage caused by slug flow. Since the vibrations caused by slug flow are not incorporated in a static analysis an accurate tradeoff between flexibility and fatigue lifetime cannot be made during the design phase. As this tradeoff during the design phase is desirable, a new dynamic and more accurate analysis method has been developed which takes these vibrations into account. A comparison between this new methodology and the common industry method is made in order to quantify the difference in analyzed fatigue damage due to slug flow induced vibration. Additionally the effects of a pressure drop over a passing slug is also investigated to determine if a pressure drop should be incorporated as a design factor for slug flow induced fatigue. The new dynamic method will also be used to investigate the relation between jumper configuration and high slug flow velocity. It will show what excitation mechanisms are dominant and how this affects the fatigue behavior. Since is be the first time, to authors knowledge, such an extensive analysis of geometries and velocities is undertaken it will provide new insights into slug flow induced fatigue in subsea jumpers in general. The newly found amplification and attenuation of the vibration by the successive impacts on the bends of a subsea jumper are investigated.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A009. doi:10.1115/OMAE2014-23151.

The bending stiffness response is an important parameter in the lifetime assessment of unbounded flexible risers. Its behavior is governed by interlayer friction mechanisms leading to a non-linear moment x curvature relationship that is highly dependent on the internal pressure. In order to investigate its influence on the critical bend stiffener hang-off region response, a detailed finite element analysis is carried out using a specialized tool for a short segment length of a selected 2.5″ ID riser cross section. Different internal pressures are numerically analyzed and the resulting local hysteretic bending response is then adjusted and directly incorporated into a global dynamic analysis tool that uses an equivalent elasto-plastic formulation with a hardening parameter that controls the behavior of the slippage mechanism. A fully coupled irregular wave dynamic analysis is then carried out and the flexible riser curvature distribution response in the bend stiffener region compared for different bending hysteresis models adopted.

Topics: Flexible risers
Commentary by Dr. Valentin Fuster
2014;():V06AT04A010. doi:10.1115/OMAE2014-23192.

Axial compressive loads can appear in several situations during the service life of a flexible pipe, due to pressure variations during installation or due to surface vessel heave. The tensile armor withstands well tension loads, but under compression, instability may occur. A Finite Element model is constructed using Abaqus in order to study a flexible pipe compound by external sheath, two layers of tensile armor, a high strength tape and a rigid nucleus. This model is fully tridimensional and takes into account all kinds of nonlinearities involved in this phenomenon, including contacts, gaps, friction, plasticity and large displacements. It also has no symmetry or periodical limitations, thus permitting each individual wire of the tensile armor do displace in any direction. Case studies were performed and their results discussed.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A011. doi:10.1115/OMAE2014-23196.

A study was conducted to have a deeper understanding to the statistical characteristics of response of flexible riser in global dynamic simulation with irregular wave. If consider the numerical simulation model as a system and the input wave train as an excitation to it, the time histories of riser load should be the response of the system to the excitation. In order to look the effect of riser configuration and water depth, the study was conducted with three kinds of configuration: Free-Hanging, Lazy-S and Tethered-Wave, which were in different water depths. In order to examine the stationarity and ergodicity of riser response, 100 simulations were performed. Each simulation was performed with a 3-hours-long storm. Except the seeds used to generate the random phases to the wave components, the 100 irregular wave processes for each riser are completely the same. When the number of wave components is enough large, the input irregular wave train should be a stationary normal process. Since the software used for the dynamic simulation is high nonlinear, however, the time history of riser response may not be perfectly stationary normal process. Then different probability distribution theories were applied to fit these time histories and the most fitting one was found out for different riser responses and for different riser configurations. The ensemble autocorrelation functions and the time autocorrelation functions were also examined for both irregular waves and the riser responses. Then the study indicated that both irregular waves and riser responses as random processes should be ergodic stationary. Finally the cross correlations between the irregular waves and riser responses were also examined and it was found that the irregular wave for each riser should be jointly stationary with each response of the riser.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A012. doi:10.1115/OMAE2014-23201.

Flexible Joints have been an essential part of many steel catenary risers (SCRs) as one of the main means for connecting SCRs to the floating facilities in deep water. Since 1994 when the first Flexible Joints were installed on the Augur Tension Leg Platform (TLP), many Flexible Joints have been designed and installed for 6 inch to 24 inch SCRs on all kinds of floating production facilities and in a range of water depths.

Woodside Energy Limited (Woodside), as operator of the Browse LNG Development on behalf of the Browse Joint Venture participants for their original Browse LNG Development concept (Browse to Kimberley subsea development and onshore processing concept), had contemplated using Flexible Joints as top termination units for the high temperature 24 inch Wet Gas Export SCRs. Woodside contracted Oil States Industries, Inc. (OSI) to design, manufacture and test a prototype 24 inch Flexible Joint that would meet the project’s stringent operational and fatigue requirements for 40 years design life. This high temperature 24 inch Flexible Joint was the largest Flexible Joint ever manufactured since its design required bellows and a thermal barrier.

A state-of-the-art qualification program for the high temperature 24 inch Flexible Joint was undertaken to demonstrate its feasibility for use on the 24 inch wet gas export SCRs for the Browse project in approximately 600m water depth. The Flexible Joint prototype was subjected to a comprehensive test program, including axial and rotational tests, hydrotests and a fatigue test, which was developed to cover all aspects of the Flexible Joint design and its components.

In this paper, the details of the high temperature 24 inch Flexible Joint qualification program will be presented with emphasis on the steps taken to successfully complete the program and the lessons learned from the challenges that were encountered. The paper will also provide a summary of the challenges and steps taken to complete the program which presented a unique opportunity to expand on the capability of the industry to go beyond what has been previously accomplished. The program was very challenging in all aspects but succeeded in achieving all of its goals.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A013. doi:10.1115/OMAE2014-23202.

The decomposition process of gas hydrate in sediments is actually the dynamic phase transition process of solid hydrate in sediments after absorbing heat decomposition. According to the phase equilibrium characteristics of gas hydrate, there are three basic development methods, including heating, chemical injecting and depressurization. Currently, there is no good commercial software used to simulate heat transmission and mass transfer in the gas hydrate decomposition process. So in this paper, based on typical gas hydrate sediment in South China Sea, microcosmic, mesocosmic (fractal theory) and macrocosmic scales are respectively used to successfully reveal the heat and mass transfer mechanism of three basis development methods. Molecular dynamics simulation shows heat injection is the best method for heat and mass transferring, and chemical injecting is better than depressurization. Fractal theory is successfully used to describe the complex structure of the porous sediments with gas hydrate occurrence, and can realize the prediction of heat and mass transfer law of hydrate dissociation in porous media. Macrocosmic numerical simulation of depressurization for gas hydrate sediment in South China Sea shows gas hydrate reservoir geological model has a large influence on the gas hydrate decomposition, and permeability and hydrate saturation of the upper cover layer have great effect on gas hydrate decomposition. It is poor development efficiency for only depressurization development and the problem of water drainage should be paid attention during development process.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A014. doi:10.1115/OMAE2014-23222.

A considerable challenge in deep water field developments is the possibility of Vortex Induced Vibrations (VIV) of flexible risers due to presence of high currents. The most common remedy is the use of VIV suppression devices such as helical strakes.

The background for this study is the lack of guidelines for estimation of hydrodynamic force coefficients for straked risers. In particular there seems to be no publicly available experimental data on tangential force coefficients for straked risers. Analyses indicate that tangential drag may be important for compliant steel risers, e.g. SCRs and steel lazy-waves risers on high-motion platforms in harsh environments. For large vertical motion of the platform there will be a substantial tangential relative velocity along the riser. Adding strakes to the riser will then effectively increase the tangential drag.

Simplified formulations for hydrodynamic coefficients for straked risers have been derived, including added mass and drag coefficients for both normal and tangential flow. The simplified formulations are derived from basic hydrodynamic theory without experimental calibration except for the drag coefficients for normal flow where the proposed drag coefficients are empirical based on experiments reported in the literature.

An example of a global analysis of straked risers in deep water is presented to demonstrate the effect of tangential drag. Derived formulas for hydrodynamic coefficients to be used in global analysis of straked risers will be implemented in DNV GL Recommended Practices.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A015. doi:10.1115/OMAE2014-23250.

The paper focuses on a Finite Element (FE) model developed at IFPEN, denominated 3D-Periodic, which is dedicated to flexible riser studies. It takes full advantage of the geometric and loading periodicities to reduce the model length and the CPU cost. The model is developed in a commercial FE software with dedicated pre- and post-treatment packages. The model can represent standard cyclic bending with internal pressure and axial tension as well as external pressures load cases to investigate the risk of lateral buckling of tensile armors or of pipe collapse.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A016. doi:10.1115/OMAE2014-23266.

Critical sections of work over strings, with respect to integrity, are components located close to end terminations, near well heads and above drill floors, where recurring bending moments are prevalent. The lifetime of these components are strongly dependent on the stiffness in the components of the string. Connections between these components are often complex, and of a type where the stiffness is unknown, or hard to reveal based on theoretical analysis. This paper considers the feasibility of applying state-of-the-art measurement technology for testing of the physical behavior of specific connections on a landing string to be used for work over operations in harsh environmental conditions, where low fatigue life of components have proven to be a recurring problem. Behavior of joint-connections revealed through measured response from physical testing serve as input for the global finite element (FE) analysis, where accumulated fatigue damage for each sea state is calculated based on site specific met-ocean data. The present work was carried out in advance of an operation on the Norwegian shelf, where a four-point bending test of the actual landing string to be used during the offshore campaign were performed on a section containing two critical couplings, in order to reveal the actual stiffness of the connections. The test string was subject to variable internal pressure, axial tension and bending loads, representative for the applicable work-over riser operational loads. The performance of the system was monitored through strain, displacement and force sensors, in order to relate applied loads to structural response. The results from these tests where later recreated from local FE analysis, where non linear springs was implemented and modified to fit the experimental results at the connections of interest. These springs was later input to the global fatigue analysis, where the complete system, including marine riser and inner work over string, was implemented in one model. Results from the fatigue assessment where used to determine the operational criteria for the work over operations.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A017. doi:10.1115/OMAE2014-23290.

This paper presents a numerical method to predict the behavior of unbonded flexible risers under symmetrical loads, and takes an eight-layer unbonded flexible riser for the case study. In order to accurately simulate geometric properties of the riser and contact interaction between layers, carcass and zeta layers are modeled as the actual complicated cross-sections, and all layers are modeled by solid element, both contact and friction between layers are taken into consideration. ABAQUS/Explicit is adopted to avoid convergence problem caused by geometric and contact nonlinearities, mass scaling is adopted to avoid long computational time caused by the very detailed finite element model of carcass and zeta layers. Load cases considered in this paper are external pressure coupled with axial tension, axial compression under different lay angles of helical strips and boundary conditions. The results show that numerical results agree well with theoretical results, lay angles of tensile armor layers have a great impact on the axial displacement, but boundary conditions have little effect on the axial displacement.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A018. doi:10.1115/OMAE2014-23306.

Although the scope and use of flexible pipe systems in deepwater developments is expanding, the mechanical behavior for these environments is not fully understood. This is due to the complex response and interaction between multiple layers within the pipe system that introduces significant difficulties and constraints into the engineering analysis. As future developments look to extend the use of this technology to greater water depths and harsher operating conditions there is a need to develop advanced numerical tools that can evaluate the mechanical integrity of these complex hybrid pipe systems. Availability of increasingly advanced computational packages has enabled substantial improvements to be made in the complexity of simulation tools for combined loading, external pressure collapse and fretting.

This study establishes a foundation for the development of advanced numerical modeling procedures to assess the collapse failure of composite flexible pipe systems for deepwater applications. Here, a continuum finite element model is constructed using the software package ABAQUS/Standard, and studied using non-linear (arc length) methods. The carcass, pressure armor and corresponding polymer layers are represented in detail and modeled with three dimensional solid brick elements in order to examine the interlayer relationships influencing collapse initiation. In many recent studies, an initial geometric imperfection in the form of general ovality is explored as the predominant bifurcation mode. A similar approach is adopted here, coupled with case studies chosen such as to facilitate validation against existing analytical and numerical data. The importance of element selection, contact mechanics, interface properties and initial imperfections on the system mechanical response and performance is presented and compared to the available literature.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A019. doi:10.1115/OMAE2014-23309.

In this article, we present the numerical analysis of a Free Standing Riser. The numerical simulation was carried out using a commercial riser analysis software suit. The numerical model’s dimensions were the same of a 1/70 reduced scale model deployed in a previous experiment. The numerical results were compared with experimental results presented in a previous article [1]. Discussion about the model and limitations of the numerical analysis is included.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A020. doi:10.1115/OMAE2014-23331.

Global buckling is a behavior observed on subsea pipelines operating under high pressure and high temperature conditions which can jeopardize its structural integrity if not properly controlled. The thermo-mechanical design of such pipelines shall be robust in order to manage some uncertainties, such as: out-of-straightness and pipe-soil interaction. Pipeline walking is another phenomenon observed in those pipelines which can lead to accumulated displacement and overstress on jumpers and spools. In addition, global buckling and pipeline walking can have strong interaction along the route of a pipeline on uneven and sloped seabed, increasing the challenges of thermo-mechanical design.

The P-55 oil export pipeline has approximately 42km length and was designed to work under severe high pressure and high temperature conditions, on a very uneven seabed, including different soil types and wall thicknesses along the length and a significant number of crossings. Additionally, the pipeline is expected to have a high amount of partial and full shutdowns during operation, resulting in an increase in design complexity. During design, many challenges arose in order to “control” the lateral buckling behavior and excessive walking displacements, and finite element analysis was used to understand and assess the pipeline behavior in detail.

This paper aims to provide an overview of the lateral buckling and walking design of the P-55 oil export pipeline and to present the solutions related to technical challenges faced during design due to high number of operational cycles. Long pipelines are usually characterized as having a low tendency to walking; however in this case, due to the seabed slope and the buckle sites interaction, a strong walking tendency has been identified. Thus, the main items of the design are discussed in this paper, as follows: lateral buckling triggering and “control” approach, walking in long pipelines and mitigate anchoring system, span correction and its impact on thermo-mechanical behavior.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A021. doi:10.1115/OMAE2014-23351.

Developments of oil and gas reservoirs in Bohai Sea, South China Sea etc., are presently accelerated, to cope with the significant increase in energy demand from the mainland, China. In recent developments in Bohai Sea, fluid temperature and pressure have been found dramatically being increased up to 100 °C and 20 MPa respectively. The fact that High Temperature and High Pressure (HTHP) in Bohai area brings design challenges, especially to jacket risers and spool pieces. Pipe-in-Pipe (PIP) flowline systems have been widely employed in this region and are continuously being considered for further developments. This is due to its significant thermal insulation capacity to deal with the High Temperature and High Pressure (HTHP) issue. To cope with the challenges induced by HPHT and structural complexity of PIP, COTEC Offshore Engineering Solutions, together with its mother company, China Offshore Oil Engineering Company, have developed a approach by using ABAQUS and AutoPIPE. This paper describes the relevant experience obtained during one development in Bohai Sea, BZ34-2/4 project containing dozens of risers and spool pieces. Two main parts are presented. Firstly, a beam-element based expansion calculation model adopting ABAQUS has been developed to achieve accurate HPHT induced expansions. The structural behavior of PIP can be represented in the developed model, meanwhile with minimum increase in modeling complexity. Secondly, practical and extensive parametric studies have been carried on the riser and spool flexibility against HPHT induced expansions. Since Bohai Sea has been developed extensively, many risers are post-installed and the existing of restriction areas practically enlarges the difficulties of anchor clamp and spool arrangements. Key parameters of these arrangements, such as Z/L shape, the length between two bends, the combinations of bend angles, the length of protection pipe on the riser etc. have been comprehensively investigated. “Gold” rules for rigid riser accessories arrangements and spool piece layout have been suggested accordingly.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A022. doi:10.1115/OMAE2014-23366.

Accidental loads, for example, due to heavy dropped objects, impact from the trawl gear and anchors of fishing vessels can cause damage to pipelines on the sea bed. The amount of damage will depend on the impact energy. The indentation will be localized at the contact area of the pipe and the impacting object, however, an understanding of the extent of the damage due to an impact is required so that if one should occur in practice an assessment can be made to determine if remedial action needs to be taken to ensure that the pipeline is still serviceable. There are a number of parameters, including the pipe cross section and impact energy, which influence the impact behaviour of a pipe. This paper describes the response, and assesses the damage, of mild steel pipes under high mass low velocity impacts. For this purpose full scale impacts tests were carried out on mild steel pipe having diameter of 457 mm, thickness of 25.4 mm and length of 2000 mm. The pipe was restrained along the base and a 2 tonnes mass with sharp impactor having a vertical downward velocity of 3870 mm/sec was used to impact the pipe transversely with an impact energy of 16 kJ. It was found from the impact tests that a smooth indentation was produced in the pipe. The impact tests were then used for validation of the non-linear dynamic implicit analyses using the finite element analysis software ABAQUS. Deformations at the impact zone, the rebound velocity, etc, recorded in the tests and the results of the finite element analysis were found to be in good agreement. The impact tests and finite element analyses described in this paper will help to improve the understanding of the response of steel pipes under impact loading and can be used as a benchmark for further finite element modelling of impacts on pipes.

Topics: Steel , Pipes
Commentary by Dr. Valentin Fuster
2014;():V06AT04A023. doi:10.1115/OMAE2014-23373.

Heerema Marine Contractors (HMC) is entering a new era of pipe laying with the new Deep water Construction Vessel (DCV) Aegir, which is designed to be able to reel/J-lay pipelines for a wide range of pipe dimension and water depth combinations. The DCV Aegir has a moonpool close to the center of the vessel in which the Hang Off Module (HOM) for pipeline installation is positioned. The HOM is utilized to hold the pipeline and its structures during installation [1]. Pipelines and its structures will be lowered through the moonpool. When a pipeline structure is lowered through the moonpool the HOM is retracted, thus clearing the moonpool to allow the free pass of the structure.

The behavior of the water in the moonpool will induce loads on the structures when these are being lowered. These loads will induce motions and bending moments on the attached pipeline. The effect of the moonpool on the structure has to be known in order to assess the integrity of the pipe (fatigue and local buckling limit state) and the optimal procedure for structure installation.

The assessment of the moonpool effect on the structure during the lowering through the splash zone is complex because it is governed on one side by the motions of the vessel and on the other side by the motion of the water inside the moonpool.

This paper will focus on the moonpool effect assessment in a Pipe Line End Termination (PLET) installation analysis. Two different software packages are utilized; Flexcom of MCSKenny and STAR-CCM+ of CD-Adapco.

Flexcom is used to reproduce the vessel motions which affect the PLET motion behavior. Pipe properties (OD, WT and length) and PLET together with pipelay equipment; such as Upper Tensioner (UT), HOM, Pipe Wire Centralizer (PWC) and winch, are modelled.

To assess the hydrodynamic loads on the PLET inside the moonpool a Computational Fluid Dynamics (CFD) approach is required. STAR-CCM+ is a CFD program which uses Finite Volume Method (FVM) to solve the flow field motion equations. With this software it is possible to accurately model the geometry of the PLET and simulate the interaction between water in the moonpool and the PLET.

The paper will present an overview of the models and the analyses performed, together with the methodology used to combine the outcome of both software packages.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A024. doi:10.1115/OMAE2014-23403.

Free span assessment has more and more become an important part of modern pipeline design. The reason for this is partly that the remaining hydrocarbon reservoirs are located in more challenging places, e.g. with very uneven seabed. Another explanation is that the pipeline design codes a few decades ago did not allow for vibrating free spans, while the modern, state-of-the-art pipeline codes, such as DNV-OS-F101 “Submarine Pipeline Systems” (2013) and its Recommended Practices, opens for long spans that are allowed to vibrate as long as the structural integrity is ensured.

In presence of non-cohesive soils and high on-bottom flow velocities significant free span development may occur over the design life, e.g. due to scouring. Such spans may be associated with a fatigue life capacity less than the design life if the spans are assumed stationary. For non-stationary spans with occasional long span lengths this may not be true since the criticality is strongly linked to the persistence of long spans and the prevailing environmental condition. A realistic fatigue assessment must account for the history of the span (i.e. stress cycles encountered for a critical weld) including predictions into the future development.

High costs related to span intervention puts focus on minimizing these costs while still ensuring integrity of the pipeline with respect to vortex induced vibrations (VIV) and associated fatigue damage. On the other hand the potential costs related to fatigue failure of a pipeline (recovery costs, economical loss and environmental consequences) are enormous. Therefore it is essential to ensure that the probability of failure for free spans is within acceptable limits.

One frequent challenge faced with old pipelines in operations survey reports are that they report several free spans. Old pipelines were not designed to allow any vibration and usually there is scanty information about different parameters such as soil conditions, operational parameters, lay tension, environmental data, etc., thus it’s difficult to determine whether it’s necessary to intervene the span or not.

State-of-the-art free span codes are deterministic in their nature. If the new codes are used to evaluate such old pipeline spans, considering all the before mentioned uncertainties in the input parameters, this would eventually lead to over conservative very low time to failures. The outcome will be that many spans need to be fixed immediately or should have failed already. Such a situation leads to a mistaken conclusion about the conservatism of the codes and not on the way they were applied.

This paper discusses some of the challenges often seen with free spans during the operational phase. The objective of the paper is to demonstrate that for in-service pipelines the lack of reliable information about the free spans is the main source of commonly low life encountered and not the methodology used to evaluate the free span. Some of these challenges are discussed in detail and potential ways forward are outlined.

Topics: Arches , Pipelines
Commentary by Dr. Valentin Fuster
2014;():V06AT04A025. doi:10.1115/OMAE2014-23423.

Oil and gas production systems are continually moving into more challenging environments. Therefore having a precise understanding of the flexible pipeline behaviour for various configurations is becoming even more important. This paper presents a medium scale test of a flexible riser in a wave configuration aiming at representing the bending stiffness characteristic of a flexible riser. Results are then compared tothose of a dynamic analysis software to validate the experimental modeling.

This paper addresses the problem of scaling a flexible steep wave riser for model tests, where the global bending response is considered under quasi-static current loading. A methodology for cost-effective and robust model construction is presented, including an effective method of determining the bending stiffness of the model.

A 1/15 scale model of a 8″ flexible riser was constructed from silicon hose and foam for testing within the Australian Maritime College’s Circulating Water Channel (CWC). Variances in the model’s buoyant section length, system offset and bending stiffness were tested at different flow velocities to observe the changes in curvature experienced by the riser. The model test regime was formulated specifically to compare the results of CWC testing with numerical modelling results. Comparison between model test and numerical results demonstrate good agreement.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A026. doi:10.1115/OMAE2014-23424.

Stick-slip is a major problem in offshore drilling because it may cause damage to the drill bit as well as crushing or grinding the sediment layer, which is crucial problem in scientific drilling because the purpose of the scientific drilling is to recover core samples from the layers.

To mitigate stick-slip, first of all it is necessary to establish a model of the torsional motion of the drill bit and express the stick-slip phenomenon. Toward this end, the present study proposes a model of torsional waves propagating in a drillstring. An analytical model is developed and used to derive a neutral delay differential equation (NDDE), a special type of equation that requires time history, and an analytical model of stick-slip is derived for friction models between the drill bit and the layer as well as the rotation speed applied to the uppermost part of the drill string. In this study, the stick-slip model is numerically analyzed for several conditions and a time series of the bit motions is obtained. Based on the analytical results, the appearance of stick-slip and its severity are discussed.

A small-scale model experiment was conducted in a water tank to observe the stick-slip phenomenon, and the result is discussed with numerical analysis. In addition, utilizing surface drilling data acquired from the actual drilling operations of the scientific drillship Chikyu, occurrence of stick-slip phenomenon is discussed.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A027. doi:10.1115/OMAE2014-23488.

Flow assurance is an important aspect of offshore, particularly deepwater pipeline design and operation, since one of the critical issues is the eventual initiation and growth of hydrate or paraffin blockages under certain conditions. Ideally, operators would benefit from online information regarding position and extent of an eventual blockage in a pipeline. The aim of this work is to apply acoustic technology to design and make a prototype that can be used in a pipe to efficiently identify and measure blockages. The technique uses a short duration sound pulse that is injected into the pipe. When the acoustic pulse encounters an impedance discontinuity, a portion is reflected back towards the acoustic source and microphones or hydrophones. Analysis of the measured signal reflections can provide valuable data related to location and size of the blockages. An experimental setup with a pipe of 4″ internal diameter and length of 100 m was constructed, and different excitation signals for the impulsive response function measurements were conducted. Microphones and hydrophones measurements were recorded using a fit-for-purpose data acquisition system with sampling rates of up to 1kS/s per channel. The tests were performed in air and water using different sizes of blockages and in different positions in the pipe. In parallel, finite element analyses were performed using the commercial software Abaqus to simulate the same conditions. The experiments were numerically reproduced with good correlation proving the potential of the technique.

Topics: Acoustics , Pipelines
Commentary by Dr. Valentin Fuster
2014;():V06AT04A028. doi:10.1115/OMAE2014-23493.

Severe slugging may occur at low flow rate conditions when a downward inclined pipeline is followed by a vertical riser. This phenomenon is undesirable for offshore oil and gas production due to large pressure and flow rate fluctuations. It is of great technological relevance to develop reliable and economical means of severe slugging mitigation. This study aims to develop an automated control system to detect and mitigate the formation of severe slugging through a choke valve and a series of sensors. As a first step, an overall flow map is generated to indicate the region within which severe slugging may occur based on Boe’s criterion [1] and Taitel’s model [2, 3]. It was possible to obtain different flow patterns by controlling the rate of water and gas injection.

The aim of this paper is, however, the formation of severe slugs and study of mitigation techniques. In the control part, we used a choke valve controlled by software which is in feedback with data from a system with pressure, temperature, flow, which are able to measure even small changes in the relevant parameters to the model. A two-phase flow loop was built for the study of severe slugging in pipeline-riser system with air and water as work fluids. The inner diameter of riser and flowline is 76.2 mm. The riser is 20 meters high and the flowline is 15 meters long and could be inclined upward or downward up to 8-degree. It has been shown by experiments how riser slugging can be controlled by automated control system.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A029. doi:10.1115/OMAE2014-23500.

A number of oil and gas projects encounter significant costs to achieve subsea pipeline stabilization using present methods. The standard procedure to estimate pipeline stability is to consider the worst combination of amplitude and direction of the current and waves that the pipe will undergo during its operational lifetime. To calculate the hydrodynamic forces a common approach is to consider only the component of the fluid velocity perpendicular to the pipe axis according to the independence principle. The hydrodynamic coefficients are then taken from a case where the fluid flow is perpendicular to the pipe for similar flow characteristics.

A substantial amount of research has been carried out to assess the hydrodynamic forces on pipelines with the current and wave directions collinear and perpendicular to the pipe. However, only limited information is available on pipeline hydrodynamic forces for highly oblique current and wave flow. A Computational Fluid Dynamics (CFD) analysis was carried out to investigate the effect on pipeline hydrodynamic forces for highly oblique collinear and non-collinear current and wave directions. The work was carried out as part of the STABLEpipe JIP (1) (with participation by Woodside, Chevron, The University of Western Australia, and Wood Group Kenny) which aims to achieve a step-change improvement in the approaches to stability design, especially on mobile or erodible sea beds.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A030. doi:10.1115/OMAE2014-23507.

Sandwich pipes (SP) combining high structural resistance with thermal insulation have been considered as an effective solution for using in ultra deepwater pipelines. Research has been conducted at COPPE/UFRJ with different core materials aiming to develop qualified pipes to transport deepwater oil and gas, especially for the pre-salt reservoirs in offshore Brazil. SPs using SHCC material are easy to manufacture and cost-effective. Moreover, the composition of the SHCC material can be controlled to achieve structural requirements along with good thermal insulation. Investigation on the buckling under external pressure and feasibility of installation by reel-lay method is required. This study presents numerical analysis of the collapse, collapse propagation and bending of sandwich pipes with different geometries. The Drucker-Prager formulation is employed for SHCC constitutive model and it is calibrated through small-scale tests. Model geometries match full scale specimens manufactured and tested in bending apparatus and hyperbaric chamber. Numerical/experimental correlation is also presented.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A031. doi:10.1115/OMAE2014-23512.

Studies on sandwich pipes, which comprise two concentric high strength steel tubes and a suitable annular material have shown good results due to their improved strength under external pressure and therefore they have been indicated as a good alternative for application in brazilian pre-salt scenario as gas and oil exportation pipelines. After installation on seabed a sandwich pipe can undergo mechanical damage caused by possible impacts of an anchor or some dropped heavy objects. When this happens, its strength under external pressure can be significantly reduced. This paper presents a numerical and experimental study of the reduced strength under external pressure (initiation pressure) of a damaged sandwich pipe. To this end, small scale sandwich pipe models were manufactured using steel tubes and PVDF in the annular space. Mechanical damages were simulated on the external surface of the models to collapse them under external pressure. The test results were compared to a three dimensional FE model simulating both the initiation pressure and the propagation pressure of the tested samples. A parametric study was carried out to analyze the sandwich pipe performance under damaged conditions.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A032. doi:10.1115/OMAE2014-23520.

In deep water, pipelines are usually laid directly onto the seabed, in which they embed becoming partially buried. The depth a pipe will embed during installation depends on its weight and on the soil strength, but also on many other features of the lay process. The embedment along the pipeline length will govern many aspects of its behavior, for example the axial and lateral stiffness in thermo-mechanical expansion process. Despite its importance, foreseeing this penetration in design stage is still a difficult task, for which many models have been proposed over the years.

Verley & Sotberg [1] and Verley & Lund [2] have presented a thorough dimensional analysis of the pipe-soil interaction, as part of a broad study on the lateral stability of pipelines. Their equations for the initial embedment (in particular the last for embedment in undrained conditions) have been often used in design, but usually receiving additional correction factors to account for aspects not originally considered by the authors. Unlike the original dimensional analysis, the calibration of these correction factors has been done in trial and error basis, often leading to unreasonable results.

This paper revisits the dimensional analysis of the pipeline embedment during installation, aiming at widening the set of variables involved. The objective is to put together a dimensionally consistent framework in which additional aspects of the embedment process can be included in a rational way.

The paper is limited to the desktop study of selecting a new set of dimensionless groups. This might be later used for deriving a new, more accurate, model for foreseeing pipeline embedment levels in design stage. This would though require extensive testing. Which is beyond the scope of the current work.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A033. doi:10.1115/OMAE2014-23521.

Pipeline walking is a complex phenomenon, governed by the axial pipe-soil interaction which is not yet fully understood, and which has been gaining increasing attention over the last few years. Axial friction (as any other aspect of soil mechanics) is usually bounded by drained and undrained responses, while in real life it will often be somewhere in the middle. For pipeline expansion it could be critical as while the ends might move a couple meters in the few hours it takes to heat up, somewhere close to the middle it will move a few millimeters only (so the level of drainage is totally different along the same line, during the same loading).

Geotechnical frameworks to address the different aspects governing the soil response to pipeline axial movement have recently been published. However, the current practice has been to lump all the time dependent effects back into a single equivalent friction factor, based on a representative pipeline velocity.

This paper presents the results of ‘true’ rate dependent pipeline walking analyses, and compares them to those obtained using constant equivalent frictions. While good agreement is observed for particular cases, differences up to 30% of the walking rate are observed. Examples show that the results of rate dependent pipeline walking analyses are significantly influenced by how the temperature changes over time along the pipeline length, thus realistic modeling of the heat up and cool down processes is crucial.

The rate dependent model employed describes the axial resistance as a hyperbolic function of the pipe velocity. Additional aspects which are expected to influence the soil response (e.g. consolidation time between sweeps, progressive compression and consolidation hardening) have been neglected, and shall be considered in further studies.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A034. doi:10.1115/OMAE2014-23545.

Steel reinforced bonded vulcanized rubber loading hoses are frequently used in offshore oil loading systems. The hoses are used as offloading system between platforms and shuttle tankers and represent a flexible economical feasible solution for hydrocarbon transfer. The reliability of these hoses throughout the service life is an important issue both with respect to operational availability and with respect to environmental safety. Hence, the load bearing capacity of these hoses and the fatigue endurance become matters of concern and important design topics. Due to the large deformation of these hoses and the complicated composite structure in the hose wall, the hoses are difficult to analyze with respect to the strain and stress response when subjected to the various load cases.

The present paper presents extreme load capacity assessments and a fatigue life prediction methodology for bonded loading hoses subjected to high pressure, tension and bending in a catenary configuration and in repeated reeling under high hose tension. The load effects on the hose during the reeling operations and the fatigue life predictions methodology for both steel components and rubber are emphasized in the present work. A combination of advanced finite element modeling and full scale testing to corroborate the analyses results has been carried out to qualify the hoses to be fit for service and to fulfill the API17K requirements. The present paper presents results from advanced finite element models and full scale testing for a 20″ bonded hose with steel end fittings. The investigation brings new knowledge in the field of hose design methodology and the results are discussed in light of current requirements and regulations such as OCIMF and API 17K.

A consequence of the comprehensive testing and analysis work is that the requirement for testing in future projects is reduced, and calibrated analysis models can be used to predict hose capacities.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A035. doi:10.1115/OMAE2014-23551.

Modeling flexible pipes in the local level is not a trivial task and many authors have employed a great amount of time in such task. The non-triviality arises from the various layers and their interaction, which are pretty tough to correctly model. The possible approaches to solve the problem are divided in to major categories: analytical models and numerical models. The analytical ones rely on a great number of hypotheses and, after a great effort, result in a system of algebraic equations. The numerical ones can be further differentiated in the ones developed using commercial software and the other ones using proprietary models.

The authors choose the second way to approach the problem and presented in previous works a group of elements called macro-elements, including a cylindrical element for orthotropic layers, a three dimensional curved beam for helical elements, a rigid connection and a contact element, both dealing with different node displacement natures. These elements take into account the physical and geometrical characteristics of the components. In this paper a pipe model, with a flexible internal core, two tensile armors and an external sheath, will be simulated and its the results will be checked against commercial software and commented.

Topics: Modeling , Pipes
Commentary by Dr. Valentin Fuster
2014;():V06AT04A036. doi:10.1115/OMAE2014-23567.

This paper presents a coupled design analysis for a Pipe-in-Pipe (PIP) Steel Catenary Riser (SCR) and Flowline (FL) system in 5000ft of water under high pressure and high temperature (HP/HT) conditions in the Gulf of Mexico (GOM).

The finite element program ABAQUS is used to model the inner and outer pipes of the PIP system, the centralizer, the SCR hangoff, the flowline lateral and end supports, and pipe-soil interactions on the seabed. Thermal stresses and stress ranges caused by repeated temperature transients, axial and lateral displacements of SCR and flowline, as well as by the dynamic responses of the coupled riser and flowline system, are presented. Comparisons are also made with results by ABAQUS models with beam and pipe elements, and results obtained by coupled and de-coupled analysis. The sample vessel motion was obtained for an example Spar platform in GOM.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A037. doi:10.1115/OMAE2014-23573.

The polymer barrier is the most important component in unbonded flexible pipe, providing the leak-tight boundary for transporting hydrocarbon medium. Premature failure of the barrier during service can be costly and may lead to disastrous environmental consequences. Design of the barrier for 25 years’ service integrity is therefore a major requirement in the flexible pipe design process. However, the API design code does not give a specific procedure for the design of the barrier and is mainly concerned with the design of other layers in the pipe which are intended to provide integrity to the polymer barrier.

The selection of barrier material depends on many factors including the service temperature/pressure range and pipe bending requirements. Polyvinylidene fluoride (PVDF) is used as a barrier material in cases where high pressure and relatively high temperature applications are involved. However, a hard polymer such as PVDF can be susceptible to crazing and cracking under specific conditions and therefore the use of PVDF in flexible pipe barriers requires critical consideration of the above issues.

This paper discusses the general design requirements of a single layer barrier, and different barriers in relation to static and dynamic applications. The details of a qualification test program performed to establish service integrity of single layer Solef 60512 PVDF barriers is discussed. The unique testing facilities developed to test the integrity of the barrier are presented.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2014;():V06AT04A038. doi:10.1115/OMAE2014-23574.

End fitting is an essential component of flexible pipes enabling their connection between moving structures and pipes to create a complete pipe infrastructure. Designing the end fitting requires careful consideration of many factors including creating an effective barrier seal, termination of all the polymer and metal layers and anchoring armor layers so that the external loads of the pipe are transferred to the connecting structure without disturbing the seal. The most difficult challenge in designing end fittings for ultra-high pressure pipes is creating the necessary barrier sealing performance. In general, the seal needs to perform to approximately twice the design pressure of the pipe so that it can sustain the burst condition of the pipe. This requires the sealing components to be subjected to extreme loading where most of the sealing components are subjected to severe plastic deformation. Understanding seal deformations and quantifying and demonstrating sealing performance are important in the development of a reliable seal. This paper describes a method of evaluating the sealing performance of end fitting seals. Design requirements of end fitting seals are also described.

The barrier seals in end fittings are normally created by swaging a metal seal ring in between the polymer barrier and the body of the fitting. This creates two leak paths, one between the metal ring and the barrier and the other at the metal-to-metal contact interface between the seal ring and the body. Achieving sufficient compression on the polymer barrier and creating sufficient contact pressure over a reasonable distance at the metal-to-metal interface are the key requirements in developing a reliable high pressure barrier seal. During this study, finite element models have been used to evaluate the contact pressure at seal contacts. These results, with a specifically developed leak criteria, have been used to evaluate the sealing performance. A brief description of the models and some specific results are presented. The FE model predictions are compared with the actual contact pressure measurements of a sealing setup.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A039. doi:10.1115/OMAE2014-23577.

Pipelines and flowlines that carry corrosive hydrocarbons are often protected by lining them internally with a thin layer of a corrosion resistant material. In the most economic method, the liner is brought in contact with a carbon steel carrier pipe by mechanical expansion. In applications involving severe plastic bending, such as reeling, such a liner can wrinkle and collapse while the carrier pipe remains intact. A numerical framework for establishing the extent to which lined pipe can be bent before liner collapse was presented in [1,2]. This framework, suitably extended is used here to examine the effect of girth welds on liner collapse. The modeling starts by simulating the expansion process that plastically deforms the two tubes bringing them into contact. Bending plastically the composite structure leads to differential ovalization of the two tubes and detachment of the liner. The girth weld locally prevents this detachment creating a periodic boundary effect in the liner. With increasing bending this local disturbance grows leading eventually to a diamond-shaped liner collapse mode. The problem is first investigated using a 12-inch carrier pipe base case followed by a parametric study of the factors that influence collapse.

Topics: Pipes , Collapse
Commentary by Dr. Valentin Fuster
2014;():V06AT04A040. doi:10.1115/OMAE2014-23579.

Thermoplastic composite materials are very advantageous as component layers in subsea risers due to their inherent properties such as high strength, low density, fatigue and chemical resistance. However, response of composite materials to applied loading is complex and three-dimensional in nature. The heterogeneous structure of the composite material induces irregular distribution of stress/strain over the cross-section and thus, it is essential for design to use analytical methods capable of determining the stress-strain relationship in three-dimensional space. Currently, most methods rely upon one-dimensional or two-dimensional data collection techniques with macro scale stress / strain observations for experimental validation. In order to ascertain the correct load to the failure, a complete understanding of the material failure at the micro-scale is essential.

In this work, X-ray computed tomography is employed for the in situ observation of micromechanical failure of the composite material under a compressive load. The observed results are compared and validated with the traditional stress-strain data and finite element analysis. It is observed that the damage in the composite material initiates by delamination which grows as the loading progresses. Moreover, the properties and failure modes are highly dependent on the manufacturing process. By gaining further understanding of the failure modes using these methods, the findings can be utilized in optimizing the design of composite riser structures.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A041. doi:10.1115/OMAE2014-23625.

Current design standards and codes do not provide specific guidance how to perform engineering criticality assessment with bi-metallic girth weld in lined or clad pipe. Recently, Bonora et al. (Proc. ASME 2013 32nd OMAE conf.) proposed the equivalent material method (EMM) which allows one still to use current design assessment routes. The method consists in considering instead of three materials in the weld joint, a single “equivalent” material with a flow curve defined as the interpolated lower bound of the three weld joint material flow curves. In this work, the applicability of the EMM was verified considering the effect associated with inner pressure loading and weld residual stresses. To this purpose, two flaw geometry configurations have been investigated. Particular relevance was given to the multi-pass weld process simulation. Numerical results indicate that the EMM always provides conservative results in terms of applied J with respect to those estimated considering the effective multi-materials configuration in the weld joint.

Topics: Welded joints , Pipes
Commentary by Dr. Valentin Fuster
2014;():V06AT04A042. doi:10.1115/OMAE2014-23628.

Bimetallic girth welds are characteristics of clad pipe technology. When dealing with propagation issues, fracture mechanics concepts usually are no longer applicable as a result of the extensive and non-homogeneous plastic deformation along bi-material interface that occur at the crack tip even below design allowables. In this study, ductile crack initiation and propagation in bi-material girth welds was investigated using a Continuum Damage Mechanics (CDM) model proposed by Bonora [1]. For the base, weld and clad metal, ductile damage model parameters have been determined by means of inverse calibration technique using fracture data obtained on smooth and round notched tensile bar specimens. Firstly, the damage model was validated predicting ductile crack growth occurring in single end notch (SEN(T)) geometry sample comparing the applied load vs crack mouth opening displacement with experimental measurements. Successively, the model was used to investigate ductile crack initiation and propagation for under clad circumferential weld crack under remote tension.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A043. doi:10.1115/OMAE2014-23640.

Spread moored FPSO (Floating Production and Storage Offloading) vessels are generally used for the large West African oil fields. The oil is transferred from the FPSO to shuttle tankers via an Oil Loading Terminal (OLT). 2 to 3 large diameter flexible lines are connecting the FPSO to the OLT. The final connection between the OLT and the shuttle tanker is made by floating hoses.

The single length of each flexible pipe can be typically 2,300 meters or higher, and the internal diameter is generally very large in the order of 15_23″ to minimize the pressure drop and the offloading time.

Conventional flexible pipe is the most suitable solution for this application. However, its long length and large diameter require a large number of buoyancy modules which are necessary to support the substantial weight generated by the steel armor wires.

An alternative to steel is Carbon Fiber Composite (CFC). This material is not only twice as strong and five times lighter than a high strength steel but it is also characterized by its exceptional performance in fatigue. As the weight of the composite armor flexible pipe is significantly reduced, the use of buoyancy is no longer necessary. The pipe can also be manufactured in a single length without intermediate connection.

A qualification program based on a 19″ internal diameter prototype has been launched. This is the first time that a large internal prototype with Carbon Fiber Composite Armor (CFA) and end-fittings have been designed and manufactured. The main goals are to confirm the suitability of the CFA flexible pipe for oil offloading application in accordance with the design tools. The mechanical behavior responses of the CFA are monitored by strain gages when the flexible pipe is in straight and curved positions under internal pressure and bending cycles.

The paper will present the main mechanical properties and the overall performance of the flexible pipe designed and tested.

The economic viability will be demonstrated by showing how the CFC material cost is positively offset by the removal of the buoyancy modules and a faster offshore installation.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A044. doi:10.1115/OMAE2014-23656.

Solid particle erosion in piping systems is a serious concern of integrity management in the oil and gas production, which has been widely predicted by the numerical simulation method. In the present work, every step of the comprehensive procedure is verified when applied to predicting the bend erosion for gas flow, and improvements are made by comparing different computational models. Firstly, five turbulent models are implemented to model the flow field in a 90 degree bend for gas flow and examined by the static pressure and velocity profile measured in experiments. Secondly, the particle velocities calculated by fully coupling and one-way coupling are compared with experimental data. Finally, based on the knowledge of flow modeling and particle tracking, four classic erosion equations are introduced to calculate the penetration rates in a 90 degree bend. By comparing with the experimental data available in the literature, it indicates that the kε model is the most accurate and effective turbulent model for gas pipe flow; the fully coupling makes the simulation of particle motion closer to measured data; and the Grant and Tabakoff equation presents better performance than other equations.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A045. doi:10.1115/OMAE2014-23660.

Codes and standards for oil&gas industry, as OS-DNV-F101, recommend the use of single edge cracked plate in tension (SEN(T)) for the experimental determination of the material critical CTOD. For clad pipe welds, this specimen geometry is difficult to be obtained for the weld material or clad corrosion resistance alloy due to the specimen shape and minimum dimensions. Alternatively, circumferential cracked bar geometry, CCB(T) could be used. This geometry configuration can be machined when limited material quantity is available and used for both quasi-static and dynamic fracture characterization. In this paper, an extensive elastic-plastic finite element investigation has been carried out on both SEN(T) and CCB(T) geometries in order to select equivalent configurations in the J-Q space. Ductile crack initiation and growth has been simulated using continuum damage mechanics model. Numerical simulation results indicate that CCB(T) with a crack depth ratio r/a = 0.2 realizes constraint loss similar to that of SEN(T) with a crack depth ratio a/W = 0.5. Similar crack resistance curves have been obtained for these two configurations confirming the equivalence of the selected sample geometries.

Topics: Geometry
Commentary by Dr. Valentin Fuster
2014;():V06AT04A046. doi:10.1115/OMAE2014-23673.

For the protection from dropped object/fishing trawl impact, flexible flowlines are normally trenched or rock-dumped. And hence, upheaval buckling and lateral buckling may be promoted by the elevated temperatures and high pressures. Due to the unique properties of un-bonded flexible flowline, the flexible flowline may creep in the trench or rock berm when it is subjected to cyclic pressure and temperature changes due to start-up and shut-down of flowline in service.

In this paper, a finite element analysis model for the global buckling and upheaval creep of flexible flowline is proposed. In this model the effect of bending stiffness hysteresis are considered in addition to the temperature and pressure changes in each start-up/ shut-down cycle. A case study of a 10″ water injection flowline is performed by using finite element analysis software package ANSYS. The nonlinear general beam section is used to simulate the specific flexible pipe behavior, nonlinear bending behavior but linear axial behavior.

Topics: Creep , Rocks
Commentary by Dr. Valentin Fuster
2014;():V06AT04A047. doi:10.1115/OMAE2014-23683.

The exploration and extraction of offshore hydrocarbon is currently facing stricter requirements in environmental conditions, structural integrity, and dynamic performance. Vibration control may be a critical part of mitigating the excessive dynamic responses of the offshore floating structures. If the structural responses can be monitored and controlled, then smart-platform technology can greatly widen the applicability of current technology toward deeper waters and more severe environmental conditions. This paper is focusing on the numerical simulations and analyses of top-tension risers in a tension-leg platform (TLP), incorporated with a bang-bang controlled magneto-rheological (MR) damper and variable stiffness (VS) system. The specific characteristics of the innovative system in alternating the damping forces and system stiffness show great potential to interactively change the structural behaviors corresponding to various external loadings. This research is expected to provide a robust and cost-effective solution for greatly expanding the capability of future smart offshore-platform technology.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A048. doi:10.1115/OMAE2014-23690.

A pipeline’s resistance to collapse is governed by geometric imperfections, material properties and residual stresses. The offshore pipeline design code DNV-OS-F101 provides a method for predicting collapse of pipelines with diameter to wall thickness (D/t) ratios between 15 and 45. This paper examined the various factors that could influence the collapse resistance of several pipe geometries, such as ovality, eccentricity, material stress-strain behavior and residual stresses in the hoop and longitudinal directions. A total of 132 cases were carried out, using 2D and 3D Nonlinear Finite Element Analysis, to predict the collapse pressure of several realistic pipe geometries. Results of this study suggest that the DNV-OS-F101 predictions are conservative and applicable for a wide range of D/t ratios. While there is close correlation between Finite Element prediction and DNV-OS-F101 prediction, there is a degree of conservatism at low D/t ratios using DNV-OS-F101 equations. Hence there would be scope for further optimization of pipe wall thickness design against the collapse limit state at low D/t ratios.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A049. doi:10.1115/OMAE2014-23694.

Corrosion resistant alloys (CRAs) such as martensitic and duplex stainless steels (DSS) are used as a flow line material in corrosive wet gas environments (i.e., carbon dioxide and hydrogen sulfide environments). A new DSS which consists of 25mass%Cr - 5mass%Ni - 1mass%Mo - 2.5mass%Cu has been developed for line pipe usage in slightly sour environments.

There are several methods currently being used to install offshore oil and gas pipelines. The reel-lay process is fast and one of the most effective offshore pipeline installation methods for seamless, ERW, and UOE line pipes with outside diameters of 18 inches or less. In the case of the reel-laying method, line pipes are subjected to plastic deformation multiplication during reel-laying. Thus, it is important to understand the change of the mechanical properties of line pipes before and after reel-laying.

In this study, full-scale reeling (FSR) simulations and small-scale reeling (SSR) simulations were performed to investigate the effect of cyclic deformation on the mechanical properties of the new DSS for line pipe. Furthermore, investigation of the most susceptible temperature range to cracking and sulfide stress cracking (SSC) tests were performed in slightly sour conditions.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A050. doi:10.1115/OMAE2014-23706.

Flexible pipes are being installed and operated in more marginal and challenging offshore conditions related to deep-water environments. Especially important is the accurate assessment of the remaining life of a flexible riser so operators can avoid costly premature change outs. So, emerging inspection and monitoring technologies are being several developed to achieve a comprehensive flexible pipes integrity approach.

For unbonded flexible pipes, the primary challenges include fatigue life, collapse, axial compression loading of the tensile armor wires and end fitting development.

This paper is related to end fitting development area, due to necessary folding/unfolding process on the tensile armor wires during pipe assembling. This mechanical forming generates high levels of plastic strain on the wires, which may therefore reduce the fatigue life of the flexible pipe in the field.

This paper presents an analysis of the high strain level involved in the folding and unfolding process along tensile armour wire, using the strain gages and fiber Bragg grating techniques.

Besides that, this paper describes a comparison analysis, which correlates the performance of both methods to assess high strain levels.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A051. doi:10.1115/OMAE2014-23731.

The operational requirements for subsea pipeline systems have progressed towards higher design temperatures and pressures (HTHP). To address flow assurance requirements, pipe-in-pipe systems have been developed.

For pipelines laid on the seabed, or with partial embedment, the potential for lateral buckling; in response to operational loads, external forces and boundary conditions, has become a major factor in engineering design. The effective axial force is a key factor governing the global lateral buckling response that is influenced by parameters such as internal and external pressure, and operating and ambient temperature. Other design parameters that influence lateral buckling include global imperfections or out-of-straightness, pipe/soil interaction characteristics and installation conditions. Global buckling reduces the axial load capacity of the pipeline that may impair operations and exceed serviceability limit states.

Results from a numerical parameter study on lateral buckling response of a subsea pipe-in-pipe (PIP) pipeline are presented. The parameters examined include pipe embedment, pipe out-of-straightness (OOS), soil shear strength, soil peak and residual forces and displacements, variation in soil properties distributed along the pipeline route, and external pressure associated with the installation depth. The observed pipe response was a complex relationship with these parameters and kinematic boundary conditions.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A052. doi:10.1115/OMAE2014-23764.

Design recommendations for high frequency mechanical impact (HFMI)-treated welds have been proposed based on available experimental fatigue data of axially-loaded high strength steel specimens which include longitudinal, cruciform and butt welds. Test specimens were of a size appropriate for laboratory study. However, in reality, structures in civil, offshore and ship industries generally include large-scale and more complicated components, such as bridges, cranes, platforms, excavators etc. This paper presents a further validation of the design proposals by considering fatigue data sets which are obtained from large-scale components. The extracted fatigue data from the available literature includes bridge, crane and beam like components. In total, 65 published test results of weld details with various yield strengths (250 ≤ fy ≤ 725 MPa) and stress ratios (−1 ≤ R ≤ 0.56) are presented. All the data are found to be in good agreement with the previously-shown design curves.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A053. doi:10.1115/OMAE2014-23778.

Controlling subsea oil and gas production systems requires fibre optics, power cables and tubes for the transportation of chemical for injection and fluid for hydraulic controls. These are arranged in helical bundles in successive layers. Shaped thermoplastic fillers are used to separate components and to fill out voids in the cross section. If necessary, armour is added to provide strength for the dynamic loads. This paper describes the application of 3D finite element method to assess the structural Integrity of subsea umbilicals, including the interaction between the constituent elements. It is suggested to uses a global analysis to determine the internal forces and local analyses (sub-modelling) for detailed stress evaluation. Abaqus is used for this purpose, due to its capability with large size problems under severe discontinuities due contact conditions. This paper also discusses the umbilicals interaction with seabed. The focus of this paper is on the fatigue life calculation.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A054. doi:10.1115/OMAE2014-23837.

In 2012 and 2013 Heerema Marine Contractors (HMC) performed full installation testing (bend, roller, tensioner and friction clamp) on pipes with different types of coating ranging from three layer polypropylene to thick insulation coating. The material property data as supplied by the coaters and/or the material suppliers appeared to have insufficient details for HMC to develop a model for the coated pipes. In order to obtain the missing details, HMC undertook a program of material finger printing for all coating materials used in the recent full scale testing in order to establish our own baseline for material properties. The reasons for doing so were; i) the data sheets from different suppliers of similar products were based on different test procedures and the results were not directly comparable, ii) initial testing indicated that the results quoted on the data sheets could not always be achieved by HMC and iii) the data as provided by the suppliers appeared to be not sufficient to be used for material models for finite element analysis. The focus to date has been on polyurethane based insulation materials, both for line pipe coating and for field joint coating, although the plan is to continue with polypropylene based insulation materials. The purpose of this paper is to discuss the setup of the full scale level winder and bend tests, the measurements and observations from the tests, the preliminary finite element analyses of the coating and the findings from the finger printing testing to date.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A055. doi:10.1115/OMAE2014-23869.

Comprising 4 pipelines over 900 km in length, with 32-in diameter and traversing water depths over 2200 m, the South Stream project requires a step-out in technology application. Following several years of preparation, the project is now approaching its implementation.

In order to document the reliability of the collapse resistance for South Stream, an extensive material development program was initiated and executed, including small scale, medium scale, and full scale testing on over one hundred purposely manufactured pipe joints by world’s 5 leading mills.

Testing performed included plate tests, full scale collapse tests on various combinations of plate sources, steel grades, and thermal ageing condition, pressure-bend tests, and reverse bending tests. A large number of medium and small scale tests were performed to allow the development of a suitably reliable statistical database for the probabilistic wall thickness design.

In addition, programs were developed and executed for weldability tests, performing over one hundred trial welds, and for H2S resistance tests.

This material development program was built on INTECSEA’s extensive experience with deep water large diameter pipelines (i.e. Oman-India, Blue Stream, Medgaz, Mardi Gras, IGI, etc.). Due to its extent and rigorous approach the South Stream material development program was able to conclusively prove the feasibility of the selected technological approach at an industrial scale.

This paper provides an overview of the key design issues that were successfully addressed and the major technological advances that have been implemented as part of the linepipe material development process for deepwater pipelines in an H2S containing environment.

The practical significance of this program is to optimize the wall thickness to a level that is manufacturable by the industry and hence enables the South Stream Project to proceed with its unprecedented depth and diameter combination.

Topics: Pipelines , Pipes
Commentary by Dr. Valentin Fuster
2014;():V06AT04A056. doi:10.1115/OMAE2014-23872.

Due to depleting sources of oil and gas reserves in shallow water depths, exploration and production activities have moved into ultra-deep offshore oil fields. Risers are an essential part of any offshore drilling facility. A riser tensioner located on the drilling platform has to provide an adequate vertical tension to maintain the stability of the riser. It is essential for a successful operation. Composite risers in deep sea conditions require much lower top vertical forces due to their high strength to weight ratio. Carbon/epoxy composite has been considered in the present study to carry out the burst analysis and to assess the safety of the composite riser under internal and external pressures and other environmental loads due to random sea currents. In order to ensure the permissible pressure and no fluid leakage, composite risers are provided with an internal steel liner. Initiation and propagation of debonding between the liner and composite has been studied and probability of failure is obtained. In burst analysis, maximum internal pressure is applied to a riser section and the stresses in all (hoop and longitudinal layers) the composite layers are checked against the failure. High pressures are incremented in small steps until fiber rupture occurs due to bursting. Maximum normal stress theory is employed for checking the failure. The same theory provides the limit-state to assess the safe pressure considering uncertainties associated with random input parameters involved. A finite element analysis has been carried out in ABAQUS/AQUA for random sea motion and fluctuating axial tension considering salient non-linearities. A small riser section modelled as a hybrid beam element (for global analysis) has been considered to study the bursting and debonding behavior. It is further discretized into thin shell elements (S4R). Steel liner and composite pipes are modeled separately and assembled together to ensure the overlapping various layers and sharing nodes. The composite body sustains the major stresses in the inner layers that diminish on moving outwards radially. An implicit time domain analysis has been carried out to obtain the response. The debonding through circumference and length are studied. The stresses obtained are compared with their ultimate strength.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A057. doi:10.1115/OMAE2014-23882.

The riser system selection and sizing is one of the most challenging design issues of the subsea world specifically for deepwater applications. Bundle riser towers have been used for few deep water developments since the late 1990s and have demonstrated their capacity to meet the operational constraints in terms of field layout congestion, flow assurance, mechanical design, fabrication and installation etc.

From the pioneer application in the Gulf Of Mexico (Green Canyon 29 – 1988) to the 2nd generation systems in Western Africa (GIRASSOL to Rosa or CLOV), the design of Hybrid Riser Towers has evolved to improve mechanical robustness, installation efficiency and operability. This article reviews the past evolutions and gives an overall view of the performance achieved so far, mainly in West African sectors. The move of the offshore industry towards ultra-deep (WD > 1800m) and/or marginal fields may trigger some additional design evolutions which are highlighted in the article, with a focus on the water depth increase considering environmental conditions met on previous projects. The feasibility in harsher conditions is not addressed in this document.

In addition, the challenges raised by the engineering of such deepwater Hybrid Riser Tower concepts are discussed:

• Structural models of different scales shall be used to properly account for potential couplings between the global behavior of the whole tower with the local responses of each individual component;

• Top and bottom assemblies as well as the rigid or flexible spools, the piping and connectors shall be introduced into the FE models to define correct boundary conditions to the HRT;

• Slug or severe internal flows shall be considered for fatigue issues requiring complex models which combine CFD and structural calculations;

• Risks of hydrodynamic instabilities shall be assessed like plunge and torsion galloping for non-cylindrical sections and VIV for bare peripherals;

• A specific attention shall be paid to the fatigue budget spent from towing and Installation to the In-place configuration.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A058. doi:10.1115/OMAE2014-23911.

Pipe-In-Pipe (PIP) systems are increasingly used in subsea oil and gas production where a low Overall Heat Transfer Coefficient (OHTC) is required. A PIP system is primarily composed of an insulated inner pipe which carries the production fluid and an outer pipe that protects the insulation material from the seawater environment. This provides a dry environment within the annulus and therefore allows the use of high quality dry insulation system. In addition, from a safety point of view, it provides additional structural integrity and a protective barrier which safeguards the pipeline from loss of containment to the environment.

Genesis has designed a number of PIP systems in accordance with the recognized subsea pipeline design codes including DNV-OS-F101 [1]. In section 13 F100 of the 2013 revision, a short section has been included in which PIP systems are discussed and overall design requirements for such systems are provided. It has also been stated that the inner and outer pipes need to have the same Safety Class (SC) unless it can be documented otherwise.

This paper looks at the selection of appropriate SC for the outer pipe in a design of PIP systems based on an assessment of different limit states, associated failure modes and consequences. Firstly, the fundamentals of selecting an acceptable SC for a PIP system are discussed. Then, different limit states and most probable failure modes that might occur under operational conditions are examined (in accordance with the requirements of [1]) and conclusions are presented and discussed. It is concluded that the SC of the outer pipe of a PIP system may be lower than that of the inner pipe, depending on the failure mode and approach adopted by the designer.

Topics: Safety , Design , Pipes
Commentary by Dr. Valentin Fuster
2014;():V06AT04A059. doi:10.1115/OMAE2014-23922.

In the North Sea, carcass failures have over the last years been the largest failure category for flexible pipes, causing multiple riser replacements [1]. The carcass failures could be split into two groups: axial tear-off and collapse.

In order to determine the resistance to collapse, a 3D analysis model has been built using the explicit finite element (FE) program LS-Dyna.

In addition, the LS-Dyna results were compared with a simplified 2D FE approach, using the non-linear implicit solver MARC.

The 3D model allows for more complex sensitivities, such as curved pipe and carcass tension.

The FE analyses were based upon measured carcass profile geometries and material data from hardness measurements at several positions along the carcass profile, including effects of strain hardening during manufacturing.

Three different carcass profiles of three different risers were analysed. The study includes sensitivities on straight and curved pipe sections, axial preloaded carcass, carcass ovality, radial gap between carcass and pressure sheets and pressure increase velocity.

Collapse resistance of axial strained carcass and bent pipe is not well documented and this paper will highlight some of the observed effects.

The results from the FE analyses showed good correlation between the vendor data [2] both in shape of the buckling mode and capacity.

Commentary by Dr. Valentin Fuster
2014;():V06AT04A060. doi:10.1115/OMAE2014-23928.

The stress evaluation of tensile armor layer in bent unbonded flexible pipe due to end fitting termination is investigated. A preliminary mathematical stress analysis model which accounts for the boundary condition at the end fitting interface is developed. Two shifting parameters are introduced to correct the friction field acting on the tensile wires. Closed form solutions are obtained for the modified friction force, axial stress and displacement. The validity of this model is investigated through a comparison with results obtained using a 3D finite element model. The relationship between the shifting parameters and the initial hoop position of wires is discussed. Studies of local stress elevation on wires with different initial hoop positions are conducted. The results show that end fitting effect could causes a significant stress elevation in a large amount of tensile wires if heavy pipe bending is permitted at the end fitting vicinity.

Topics: Stress , Pipes , Fittings , Armor
Commentary by Dr. Valentin Fuster
2014;():V06AT04A061. doi:10.1115/OMAE2014-23969.

In this work, an approach to predict the remaining fatigue life of flexible pipes with damages in their tensile armor wires is proposed. This approach relies on a previous proposed approach to calculate the fatigue life of intact flexible pipes. By relying on results from theoretical and experimental investigations, the previous proposed expressions were modified in order to account for damages in the tensile armor wires of these structures. Furthermore, the computation of the fatigue life was also modified in order to account for results from inspections in these pipes thus allowing the estimative of the remaining fatigue life of the pipe. The use of this methodology is illustrated in the analysis of a 9.13″ flexible pipe considering different conditions in its outer tensile armor wires: intact and with one up to ten wires broken along time. The results obtained indicate that the rupture of the tensile armor wires may significantly reduce the fatigue life of flexible pipes and, consequently, may lead to the premature failure of the pipe.

Topics: Wire , Pipes , Fatigue life , Armor
Commentary by Dr. Valentin Fuster
2014;():V06AT04A062. doi:10.1115/OMAE2014-23984.

This work focuses on the structural analysis of a damaged 9.13″ flexible pipe to pure and combined axisymmetric loads. A set of experimental tests was carried out considering one up to ten broken wires in the outer tensile armor of the pipe and the results obtained are compared to those provided by a previously presented finite element (FE) model and a traditional analytical model. In the experimental tests, the pipe was firstly subjected to pure tension and, then, the responses to clockwise and anti-clockwise torsion superimposed with tension were investigated. In these tests, the induced strains in the outer armor were measured. Moreover, the axial elongation of the pipe was monitored when the pipe is subjected to tension, whilst the twist of the pipe was measured when torsion is imposed. The experimental results pointed to a slight decrease in the stiffness of the pipe with the increasing number of broken wires and, furthermore, a redistribution of forces among the intact wires of the damaged layer with high stress concentration in the wires close to the damaged ones. Both theoretical models captured these features, but, while the results obtained with the FE model agreed well with the experimental measurements, the traditional analytical model presented non-conservative results. Finally, the results obtained are employed to estimate the load capacity of the pipe.

Topics: Pipes
Commentary by Dr. Valentin Fuster
2014;():V06AT04A063. doi:10.1115/OMAE2014-23987.

In support of its commitment to safe and reliable operations, BP has been continuously developing a program to assess and maintain structural integrity for offshore drilling risers and conductors. This paper presents recent efforts by BP, in conjunction with 2H Offshore, to develop a new fatigue monitoring methodology for drilling riser systems due to both wave and vortex-induced-vibration (VIV) damage.

BP has been monitoring structural response, including the fatigue damage, of riser systems in the Gulf of Mexico over the past ten years. To date, the focus has predominantly been on determining the fatigue damage due to VIV, since VIV and its effects on structural response are considered a not well-understood phenomenon. In addition to VIV fatigue, direct wave loading and vessel motions also contribute to the total fatigue damage, and sometimes wave fatigue may have a larger contribution than VIV fatigue damage. Therefore, it is necessary to determine fatigue due to both wave and VIV effects to confirm the long-term fatigue integrity of the drilling risers.

To take full advantage of the accumulated monitoring data, a new fatigue monitoring methodology was developed using an analytical solution to account for the damage due to both wave and VIV effects. With this method, the measured acceleration data are converted into curvature, and then fatigue damage along the length of riser and conductor are calculated. This new methodology has been validated with both finite element analysis (FEA) and field data, and sensitivities to various parameters have been considered.

Commentary by Dr. Valentin Fuster

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