bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation on the Burst Strength Capacity of Aging Subsea Gas Pipeline
Authors: Kim Dong Woo , Mohd Mohd Hairil , Lee Byeong Joon , Kim Do Kyun , Seo Jung Kwan , Kim Bong Ju , Paik Jeom Kee
Published: November 2013

Precisely evaluation of the reliability of aging structure is essential, particularly in the oil gas industry where inaccurate predictions of structural performance may have significant hazardous consequences. Related to this issue, it is important to predict the corrosion behavior of the gas pipeline structure used in the production of gas in subsea area. As corrosion is concerned, the effects of pipeline failure due to significant reduction of burst strength will make it hard for the pipeline operator to maintain the serviceability of pipelines. Therefore related to this problem, the resistance service of the pipeline is assessed by means of burst strength capacity. In this study, the critical part of the corrosion along 2.4 km pipeline is assessed using two approaches; empirical design codes formula and ANSYS numerical analysis. The future integrity of the pipeline is then assessed to predict the remaining year in service for the aging pipeline. The results and outcomes of the present study will be useful for evaluating the pipeline integrity as well as the prediction of the remaining life of in service aging pipeline structures.
Time Domain Analysis Approach for Riser Vortex-Induced Vibration Based on Forced Vibration Test Data
Authors: Wang Kunpeng , Tang Wenyong , Xue Hongxiang
Published: November 2013

As oil and gas exploration and production are pushed into deepwater area, the offshore industry is facing more challenges for riser vortex induces vibration (VIV). Although frequency domain approach has been widely used for the riser VIV prediction and fatigue design, several assumptions need to be made. In addition, frequency domain approach cannot account for the variable current and riser nonlinear boundary conditions, such as top boundary response, the interaction between riser and guides in the hull and soil-SCR interaction.

Considering above cases, several time domain codes have been developed for riser cross-flow (CF) VIV prediction. This paper presents a time domain approach based on forced algorithm. The exciting force is derived from the non-dimensional amplitude and frequency dependent lift coefficients from forced vibration test. The hydrodynamic damping model consists of empirical model and the extension of the lift curves. At each step, the displacement and velocity of each element would be obtained to calculate the response amplitude and frequency for the lift coefficient and damping. Expect for CF VIV, the mean drag force is also considered, which would be magnified by CF VIV.

The model test at Delta Flume of Delft Hydraulics is simulated using the proposed approach, and the CF VIV responses and the mean drag displacement are predicted. The results match well with the measured data.
Simulation of Dynamic Coupling Between Waves and a Free-Floating Rectangular Box by Smoothed Particle Hydrodynamics
Authors: He Ming , Ren Bing , Jiang Feng , Ma Chunli
Published: November 2013

An implementation of the smoothed particle hydrodynamics (SPH) method is presented to solve dynamic coupling between waves and a free-floating rectangular box. A 2D numerical wave tank (NWT) is established in this paper. Governing equations of weakly compressible smoothed particle hydrodynamics (WCSPH) are modified by CSPM and approximately Riemann solver. Boundaries of the NWT and the floating box are built with dynamic boundary particles (DBPs) described by Dalrymple. An active absorbing wavemaker is arranged at the upstream boundary of the NWT and an artificial damping layer is set on the other side. A comparison between SPH result and boundary element method (BEM) solution of wave forces on a fixed rectangular box is conducted to validate the numerical model. The interaction between waves and a floating box is simulated using the present model, and wave forces on the floating box and its motion responses under wave action are analysed.
Safety at Sea: Improving Search and Rescue (SAR) Operations in the Barents Sea
Authors: Berg Tor Einar , Holte Even Ambros , Ose Grethe Osborg , Færevik Hilde
Published: November 2013

In this paper, we select some of the crucial issues for future search and rescue (SAR) operations in the Barents Sea. The different nations that are involved and the resources necessary to build emergency preparedness due to the climatic conditions are thus important factors. This paper summarizes the state of the art within these areas while also indicating future development needs. The special requirements for life saving equipment on vessels due to the climate and requirement on personal protective equipment related to accidental immersion are also essential and thus presented in this paper. In addition, safe haven designs where the vessel itself is designed to provide shelter for personnel in distress is also a topic chosen to be addressed.
Finite Element Modeling of Flexible Pipes Under Crushing Loads
Authors: Malta Eduardo Ribeiro , Martins Clóvis de Arruda , Gonçalves Silas Henrique , Gay Neto Alfredo
Published: November 2013

The launching procedure can be one of the most critical stages of the operational lifetime of a flexible pipe. From the beginning of the pipe unrolling off the reel to the moment of its separation from the launching vessel, the flexible pipe is subjected to severe loads such as crushing and tension. This paper focuses on the crushing load applied to the flexible riser by the shoes of the caterpillars on the launching vessel. The objective is to present an effective methodology to evaluate the stresses at the structural nucleus of a flexible pipe during launching using the Finite Element Method. Firstly, a tridimensional ring model is used to represent the structural nucleus of the flexible pipe. In that model, the geometry of the interlocked carcass and the pressure armor is accurately represented. Then, similar models are constructed including a series of geometry simplifications. Those simplified models are compared to the baseline in order to evaluate the relevancy of an accurate representation of the geometry of the metallic layers. The results of these comparisons are presented and discussed.
CFD Analysis of a Unidirectional Axial Turbine for Twin Turbine Topology in OWC Plants
Authors: Pereiras Bruno , Valdez Pablo , Castro Francisco , Garrido Julio C.
Published: November 2013

OWC devices are widely known among researchers in ocean energy. It is well-known that the efficiency of the device is closely related to the efficiency of the Power-Take-Off (PTO) which is usually a turbine. Traditionally, self-rectifying turbines are the most widely considered for working in an OWC because unidirectional turbines require a system of valves to rectify the flow. However, another option recently proposed is the use of the “twin turbine” configuration. This paper focuses on the performance of the turbines used in this configuration.

A numerical model has been developed and validated with data from the bibliography. This model has been used to analyze the flow field of the turbine when working in both performance modes: direct and reverse. Flow angles and loss distribution have been analyzed and interesting conclusions can be extracted.

Once the flow field has been analyzed, changes in the turbine geometry are proposed in order to improve the efficiency of the whole system by increasing the blockage made by the turbine in reverse mode. These changes, focused on the solidity of the rotor and guide vanes, were implemented and new simulations were carried out. The results obtained are the core of this work.
Study on Weld Fatigue Evaluation Under Sour Service Environment Using Battelle Structural Stress Method
Authors: Hong Jeong K. , Forte Thomas P.
Published: November 2013

Risers, pipelines and flowlines for deep water applications are subject to corrosive environments. Especially, in the presence of hydrogen sulfide which makes the field sour, their fatigue performance becomes significantly degraded. In order to quantify the sour degradation effect, a knock-down factor has been introduced. This factor is defined as the fatigue life reduction relative to the in-air fatigue life.

Several sets of fatigue test results in sour service environments have been published. These include strip specimens of different sizes, e.g., diameters, wall thicknesses, and arc lengths. Naturally, the knock-down factor must be based upon a statistically valid number of fatigue test results obtained from the same specimen geometry and the same loading conditions tested in air and in sour conditions. Currently, the database available in the open literature is too limited to properly define a knock-down factor. Moreover, there is a great deal of scatter within the database and each test in a sour environment is costly and time consuming. Thus, it is difficult to establish a statistically valid database upon which to base the knock-down factor.

A mesh-insensitive structural stress method has been developed by Battelle researchers and has been proven to be highly effective in correlating the fatigue behavior of welded joints. In 2007, the Battelle structural stress based weld fatigue master S-N curve was included in ASME Section VIII Div. 2 because it successfully consolidated more than 800 fatigue test results for weld toe failures onto a single master S-N curve with very little scatter, regardless of specimen shape, size, loading type, and steel alloy [1–2].

A knock-down factor is derived by applying the Battelle structural stress method to the existing database for sour environment tests and by using the current in-air database as the reference condition. This approach will reduce the uncertainty in the knock-down factor because it allows a wider range of sour environment data from specimens of different sizes, types, and loading conditions to be combined, while simultaneously reducing scatter. As such, a unified knock-down factor can be determined with greater statistical validity and wider applicability for design recommendations in sour conditions.
State-Space Realization of the Wave-Radiation Force Within FAST
Authors: Duarte Tiago , Alves Marco , Jonkman Jason , Sarmento António
Published: November 2013

Several methods have been proposed in the literature to find a state-space model for the wave-radiation forces. In this paper, we compared four methods, two in the frequency domain and two in the time domain. The frequency-response function and the impulse response of the resulting state-space models were compared against those derived from the numerical code WAMIT.

A new state-space module was implemented within FAST, an offshore wind turbine computer-aided engineering tool, and we compared the results against the previously implemented numerical convolution method. The results agreed between the two methods, with a significant reduction in required computational time when using the new state-space module.
Performance and Characteristics of Take-Off Power of a Vertical Axis Marine Turbine With Variable-Pitch Blades
Authors: Ikoma Tomoki , Masuda Koichi , Nakamura Yasunori , Rheem Chang-Kyu , Maeda Hisaaki
Published: November 2013

This paper describes performances and characteristics of a vertical axis marine turbine (VAT) on which variable-pitch blades are equipped. Then, model experiments in a current channel and CFD calculations were carried out in this study. In the model testing, a variable-pitch type turbine model was used. Accuracy of the CFD computation is validated by comparing with experimental results of torque and number of rotation of the turbine model. The CFD calculation is three-dimensional as well as two-dimensional. The corresponding turbine model has three straight blades, which diameter is 0.3 m. Three kinds of tests were performed for model testing in the channel and CFD calculations as well. They were the starting-driving test, the braking test and the forced-rotation test in currents. The forced-rotation test was performed in still water as well in order to know a simple thrust and to estimate ideal efficiency of the turbine.

Form results of model experiments and numerical experiments, the CFD calculation was satisfactory to estimate performance of the turbine. The starting performance could be definitely improved by applying the variable-pitch system. For driving performance, it was very useful to control the pitch angle a little bit even in very fast velocity currents.
Protection of Subsea Pipelines Against Ice Ridge Gouging in Conditions of Substantial Surface Ice
Authors: Duplenskiy Stanislav , Gudmestad Ove Tobias
Published: November 2013

The development of Arctic offshore hydrocarbon fields involves transportation systems for oil and gas, which comprise their shipping by tankers or pipeline systems. The later have sustained behavior with respect to hydrocarbons delivery and relatively non-sophisticated operational requirements.

However, some important challenges regarding Arctic conditions have to be assessed before the pipeline is constructed. Attention is given to the conditions of a specific hydrocarbon field off the Sakhalin Island, and the design of an offshore pipeline. The issue of the pipelines’ interaction with first year ice ridges is described, a design ridge is selected and two theoretical models for evaluation of the maximum scour depth produced by the ridge ploughing the seabed are introduced.

Thereafter a beam model of a pipeline exposed to bending and tension in terms of combined transverse and lateral loadings is established and analyzed. Based on the limit state design criteria the required trench features for different soil conditions for the safety of the pipeline are specified. Following the required pipeline failure probability, it is documented that the pipeline might be buried just below the probable scour depth if certain conditions of a “sandwich” backfilling with a weak soil layer on the bottom are met.

Conclusions summarizing the acquired findings provide reasonable recommendations for offshore pipeline design in the Arctic regions and suggest a scope for future works and studies.
Theoretical Modelling of Free Falling Wedges Entering the Water Surface: Part I — Wedge Motions
Authors: Wang Jingbo
Published: November 2013

As the first of two companion papers, theoretical models are proposed to describe the motions of free falling wedges vertically entering the water surface at Froude numbers: 1 ≤ Fn < 9 (Here, the Froude number is defined as Fn=V0/gc0). The time evolutions of the penetration depth, the velocity and the acceleration are analyzed and expressed explicitly The maximum and average accelerations are predicted. The drag (slamming) coefficients are extensively studied. It is found that for the light wedge the transient drag coefficients have slow variation in the first half stage and rapid variation in the last half stage, and for the heavy wedge the transient drag coefficients vary slowly during the whole stage and can be treated as constant. The theoretical results are compared with numerical simulations by nonlinear BEM (Wang & Faltinsen (2010, 2013)), and good agreements are obtained.
Full Scale Thruster-Hull Interaction Improvement Revealed With CFD Analysis
Authors: Bulten Norbert , Stoltenkamp Petra
Published: November 2013

The performance of two steerable thruster configurations has been compared: a more conventional straight unit and a tilted-shaft-line unit. Based on a theoretical approach, it has been shown that the full scale bollard pull performance of both units is close to the realistic optimum. The open water performance of the units revealed comparable performance at bollard pull for the straight and tilted unit. A more pronounced difference in favor of the straight unit is found in free sailing condition. When the thruster-hull interaction effects are taken into account, the trends change however. Some typical cases have been analyzed in detail with RANS-CFD numerical analysis methods: a drill ship in free sailing and bollard pull condition and a drill rig in bollard pull condition.

The thrust-deduction factors are reduced significantly for all analyzed cases when the tilted unit is compared to the straight unit. This leads to improved performance of the complete vessel. In order to capture the full potential of the tilted-shaft line thruster units, a detailed review of the currently used DP-capability calculation methods needs to be made. Besides the reduced thrust-deduction factors due to hull interaction, also the forbidden-zones due to thruster-thruster interaction need to be reviewed. The downward deflection of the jet in case of tilted units will reduce the zones of interaction with other thrusters.

Based on the currently presented research, it should be clear that the tilted thruster concept has a large potential for drill ships and drill rigs. A very good DP-capability will be achieved with the currently installed powers.
Vortex Induced Vibration of Circular Cylinder With Two Degrees of Freedom: Computational Fluid Dynamics vs. Reduced-Order Models
Authors: Wang Enhao , Xiao Qing , Srinil Narakorn , Zanganeh Hossein
Published: November 2013

Computational fluid dynamics (CFD) studies capturing vortex-induced vibration (VIV) phenomena in a wide range of both the hydrodynamics and the structural parameters are important, because the analysis outcomes can be applied to numerical prediction codes, complement experimental measurement results and suggest a modification of some practical design guidelines. Nevertheless, in spite of many published studies on VIV, CFD studies for two dimensional coupled cross-flow/in-line VIV even with two degrees of freedom (2-DoF), are still quite limited. More CFD studies which can control the equivalence of system fluid-structure parameters in different directions with reduced uncertainty are needed to improve the numerical model empirical coefficients and capability to effectively match numerical predictions and experimental outcomes.

This paper presents a CFD study on the 2-DoF VIV of elastically mounted circular cylinder with a low mass ratio (m* = 2.55). The Reynolds number is fixed to be 150 and the reduced flow velocity parameter is varied by changing the cross-flow natural frequency. To model the problem, two-dimensional Navier-Stokes equations coupled with linear structural equations in the in-line and cross-flow directions are solved. Particular attention is paid to the determination of maximum attainable amplitudes and the associated instantaneous lift and drag forces and hydrodynamic coefficients. These results are compared with the obtained results from alternative numerical prediction outcomes using new reduced-order models with four nonlinearly coupled wake-structure oscillators (Srinil and Zanganeh, 2012). Some qualitative and quantitative aspects are discussed. Overall, the important VIV characteristics are captured including the dual-resonance and figure-of-eight trajectories. Through the flow visualization study, it is found that as the dual-resonance is excited, a P+S wake pattern appears.
Simulation of Hooking Event in Fish Trawling Operation
Authors: Wu Xiaopeng , Longva Vegard , Sævik Svein , Moan Torgeir
Published: November 2013

Interference between trawl gears and subsea pipelines is an important issue. A special case called hooking, defined as the situation when the trawl gear gets “stuck” under the pipeline, should be a rarely occurring situation. In this case, however, the warp line tension could be high as its breaking strength. This may be detrimental with respect to both fishing vessel safety and pipeline integrity. This calls for a better understanding of the hooking phenomenon.

The goal of this study is to develop a proper numerical model to describe the hooking event. The proposed model is based on the finite element method. A special penalty-based contact element that includes the friction effect is utilized to deal with the trawl board and pipeline interaction. The trawl board and seabed (span shoulder) contact is also accounted for in order to simulate the hooking event. To validate the proposed model, numerical simulation results are compared with previous model test results. A rectangular type trawl board was selected as the target object. The pull-over cases with different span height were firstly tested and compared. Then, the hooking event set-ups were modeled.

Based on the model tests, there are two most likely scenarios for hooking: 1) a de-stabilized trawl board with small span height; 2) small crossing angle with large span height. The above two cases were both tested by the proposed model. In the first case, the trawl board is towed flat on the seabed. Permanent hooking was successfully obtained in the simulation. Then, two cases with small crossing angle were studied. Hooking event was reproduced in the case of a 20 degree crossing angle by introducing a disturbance on the trawl board. It shows that the proposed model could reproduce the hooking event, provided that the trawl board motion similar to the model test could be obtained. This gives a good basis for further studies.
Assessment of Effective Stiffness Formulation of Concrete Coated Rigid Pipeline
Authors: Chellakat Satyaraj Joe-Joe , John Manoj , Puthiyaparambath Vinod , Durairaj Senthilkumar
Published: November 2013

Application of concrete coating on rigid pipelines alters the stiffness of the pipeline. Estimation of the effective stiffness of the concrete coated pipe is challenging as it depends on the diameter and thickness of the pipeline, thickness of the concrete coating, the corrosion coating and the cut back lengths.

Realistic estimation of the stiffness is essential in order to define the pipelay profile and for the estimation of pipeline stresses, which in turn governs the limiting weather window for the installation. This paper describes the effect of concrete stiffness of the rigid pipeline using three different methods of stiffness calculation, namely Mogbo, Lund and DNV methods.

Comparison of results from each of the method is presented with the aim of choosing the appropriate method for installation. The objective of this paper is to provide a better insight into the significant operational and economic influences these alternative approaches can have during the installation campaign.
Constructing the Shields Curve: Part C — Cohesion by Silt, Hjulstrom, Sundborg
Authors: Miedema Sape A.
Published: November 2013

The ‘standard’ Shields curve is intended for determining an erosion criterion for non-cohesive particles. Non-cohesive in this respect means that the particles are subject to drag and lift forces and subject to turbulent instantaneous velocities. The particles are not subject to inter-particle attraction or repulsion forces such as van der Waals forces and electro-chemical forces. The bed also is not subject to shear strength or yield stress. A cohesive sediment however is subject to these phenomena, resulting in higher critical shear stresses and higher Shields values. The cohesive effect can result from the presence of a silt (quartz) fraction or the presence of a clay fraction in the sediment. Here only the presence of a silt fraction will be considered. The silt particles in general are small enough to be subject to van der Waals forces. These attraction forces are strong enough to act like glue between the larger sand particles. In order to determine these attraction forces a Virtual Attraction Particle Diameter (VAPD) is introduced. The VAPD is the diameter of a virtual silt particle that can explain for the attraction forces in combination with the d50 of the sand. The VAPD will be in the range of the d1-d5. The van der Waals forces (if strong enough) increase the critical shear stress and thus the Shields parameter with a factor, which is inversely proportional with the d50 and inversely proportional with the VAPD (the diameter of the smallest fraction of the silt particles) to the third power. The relation often found in literature for this factor, inversely proportional with the d50 to the second power, can be explained by the fact that there is often a relation between the d50 and the VAPD. The smaller the d50, the smaller the VAPD. This however can lead to inverse proportionalities with different powers between the first power and the third power, depending on the coincidental choice of the diameter of the silt fraction. The model developed also shows that there does not exist a single Shields curve for sands with a cohesive silt fraction, but for a given set of the sediment density, the maximum sediment density (minimum porosity) and the VAPD, a Shields curve can be constructed. Using a density of 1.95 ton/m3, a minimum porosity of 0.32 (a rather uniform PSD) and a VAPD of 3 μm, the Brownlie equation can be approximated very closely. If the silt does not contain particles with a diameter smaller than 10 μm, there is hardly any cohesive effect. If the silt however contains a fraction of particles with a diameter around 1 μm, the cohesive effect is huge and already influences sand particles with a diameter of 1 mm. The model developed has been verified and validated with experiments from literature and gives a very good match, both quantitatively and qualitatively. The model developed also gives a good explanation of the famous Hjulström and Sundborg diagrams and gives these diagrams a more fundamental basis.
Reassessment of Dynamically Stable Pipelines Accounting for Potential Clashing With New Assets
Authors: Carneiro Daniel , Powell Sabine , Timmins David , Bruzzo Pier
Published: November 2013

Since mid-1980’s, guidelines permit the design of rigid subsea pipelines allowing significant lateral displacements under extreme environmental conditions. Such a design criterion might bring issues not foreseen in the original design when future assets are to be installed in close proximity — for example pipeline crossings, hot-tap tie-ins or parallel lines using the same corridor. Although not limited to any particular region of the globe, this issue has been showing increasing relevance in the Australian North West Shelf over the last few years. The combination of environmental and geotechnical conditions in that region has led to significant advances in engineering design techniques, and to audacious projects which made viable a number of important subsea pipelines which have been designed to be dynamically stable. As this oil and gas province matures, the installed infrastructure in the region forms part of an increasingly complex subsea network.

The design of new subsea assets close to, crossing, or tying into these dynamically stable pipelines require the reassessment of the existing infrastructure for potential clashing conditions. This paper discusses the techniques and integrity criteria available for reassessing dynamically stable pipelines whose originally expected displacements may be incompatible with new required infrastructure. Although the criteria for confirming the structural integrity is, and has to be, grounded on established design codes, the assessment often require advanced engineering and state-of-the-art analysis techniques.
Hydroelastic Challenges for Wave-Impact Phenomena in Sloshing Flow
Authors: Lugni Claudio , Bardazzi Andrea , Faltinsen Odd M. , Graziani Giorgio
Published: November 2013

Depending on the local flow features before the impact, several and complex scenarios can characterize the physical evolutions of a wave impact in a sloshing flow. As a consequence, flip-through event or wave impact with gas-entrapment may happen causing localized and large loads. In all these cases, when the typical temporal duration of the local load is comparable with the lowest natural period of the structure, hydroelasticity matters affecting the integrity of the structure. Here, the experimental investigation about the impact of a wave trapping air against a deformable plate in shallow-liquid conditions is performed. Several sizes of the cavity trapped at the wall will be considered by varying the amplitude of the tank motion, measuring the stresses and deformations of the elastic plate. The effect of the tank ullage pressure variation on the maximum local load are also considered. The study, though at a preliminary stage, emphasizes that the hydroelasticity may be triggered either during the occurrence of the first structural peak load or after the complete closure of the air-cavity.
Scour Development Through Time: Modelling Scour in Layered Soils
Authors: Harris John M. , Whitehouse Richard J. S. , Porter Kate , Simons Richard R.
Published: November 2013

This paper describes the extension of an engineering model (Harris et al., 2010) developed to predict the time evolution of scour around a monopile in uniform granular soils in the marine environment to layered granular soils. The model is capable of looking at cylindrical, square and rectangular structures. It is capable of representing both the scouring and backfilling process and has been modified to incorporate the ability to predict the effect of scouring in layered soils making use of the results from recent experimental studies into scouring in non-uniform sediment beds. Output from the model is compared with the laboratory results and extended to look at potential implications at field-scale.

The model is of use on those occasions when knowledge of the likely variation in depth of scour is important. Examples include predictions of scour development to inform the evaluation of the duration of time after pile driving for a fully developed scour hole to form for installation of scour protection, or to determine the likely exposure of cables entering and exiting offshore turbine foundations.
The Numerical Investigation of Plastic Collapse Loads in Cylinders Containing Circumferential Flaws Under a Combined Loading of Internal Pressure, Tension and Bending Moment
Authors: Wei Liwu
Published: November 2013

Fracture assessment diagram (FAD) based fracture assessment procedures are universally adopted by standards/documents including BS7910, R6, API579-1/ASME FFS-1 and FITNET. In the use of a FAD for structural integrity assessment, one important consideration is to determine the load ratio (Lr) which is defined by two equivalent definitions: Lr is either defined as the ratio of reference stress (σref) to yield strength (σY) as in BS7910, or as the ratio of applied load to plastic limit load as in R6. The solutions of reference stress or limit load are given in the assessment procedures for commonly encountered flawed structures such as a plate containing a surface crack and a cylinder containing an external surface crack. Although the solutions given in the various standards are not all the same, they were invariably derived on the basis of analysis of the force and moment equilibrium with regard to a flawed section and few of them has taken into account the effects of bi-axial stressing on a flawed section, thus remaining a question whether these solutions are still valid in situations involving bi-axial loading such as the presence of pressure in a cylinder in addition to axial tension and bending.

In this work, finite element analysis (FEA) of plastic collapse was systematically performed on circumferential internal surface cracks in a cylinder subjected to various combined loads, including combined tension and pressure, combined bending moment and pressure, and combined tension, bending moment and pressure. The focus was on understanding the effects of bi-axial stressing due to pressure on plastic limit load. The investigation of these cases has demonstrated a significant effect in plastic limit load arising from the application of pressure introducing a state of bi-axial stressing. Comparison of the results of plastic limit load obtained from FEA with those derived from BS 7910 reference stress solutions was carried out to assess the applicability when the standard solutions of plastic collapse are used in the applications containing bi-axial stresses.
Validation of a Hybrid Code Combining Potential and Viscous Flow With Application to 3D Moonpool
Authors: Kristiansen Trygve , Sauder Thomas , Firoozkoohi Reza
Published: November 2013

When operating with a moonpool, a main concern is the large-amplitude piston-mode motion at resonance. This limits the time-window for operations inside the moonpool. Longer time-windows are desired. Further, the moonpool size is expected to increase for dedicated vessels. There has therefore recently been an increased attention to moonpool design. Potential theory highly over-predicts the water motion at moonpool resonance, and may not be used for analyzing moonpool. Viscous damping has been shown to be important, and hence vital for the moonpool functionality. We present new numerical results with a hybrid method that combines potential and viscous flow. The simulations are done with a newly implemented code called PVC3D (Potential Viscous Code). The free-surface motion is governed by potential theory, while a Navier-Stokes solver provides the solution in the main bulk of the water. With the presently considered set-up with simple geometries, the computational time remains similar to that of pure potential flow time-domain solvers, while the important flow separation that provides viscous damping is captured. The application is to a 3D moonpool set-up. The inlet of the moonpool has sharp corners, and viscous damping is significant. Good agreement with experiments is demonstrated.
Assessment of Parameters Influencing Mechanical Response of Constrained and Unconstrained Dents Using Finite Element Modelling
Authors: Hanif W. , Kenny S.
Published: November 2013

Pipelines may experience damage (e.g. dent, gouge) during handling, installation and normal operations due to external interference. Pipelines in offshore environment may be prone to mechanical damage from events such as ice gouging, frost heave, and seismic fault movement. Damage mechanisms can be associated with deformation or metallurgical/metal loss that may include pipe dent, pipe ovality, ice gouging, pipe buckling, corrosion etc. The type and severity of pipe damage may influence operational, repair and intervention strategies.

For conventional pipelines, the assessment of mechanical damage plays an important role in the development of integrity management programs that may be of greater significance for pipeline systems located in remote harsh environments due to remote location and logistical constraints.

This study examines the effects of plain dents on pipe mechanical response using continuum finite element methods. ABAQUS/Standard (6.10-1) environment was used to simulate damage events and pipe response. Modelling procedures were developed and calibrated against physical and numerical data sets available in public domain. Once confidence in numerical procedures was achieved, an analysis matrix was established to account for a range of influential parameters including Diameter to wall thickness ratio (D/t), indenter diameter to pipe diameter ratio (ID/OD), hoop stress due to internal pressure to yield strength ratio (σh/σy), and kinematic boundary conditions.

The results from this study provide a basis to support a broader initiative for developing an engineering tool for the assessment of damage interaction with pipeline girth welds and development of an engineering performance criterion.
Different Ways of Modeling Ice Drift Scenarios in Basin Tests
Authors: Haase Andrea , Jochmann Peter
Published: November 2013

One known scenario from full scale sea ice investigations is a drifting managed ice field. This ice field may be driven by winds or currents or both and may eventually hit a vessel or an offshore structure. In case of a moving vessel the relative motion between vessel and ice may be determined by the vessels direction of motion or even its ambition to hold position against the drifting ice. All the above described scenarios deal with relative motions between several bodies. Along with the relative motion come the contact forces between the interacting bodies and last but not least the question of the failure of either of the bodies. As ice model tests are in general state of the art procedures to investigate the behavior of a vessel and the related loads in sea ice the question of how to model drift scenarios is of relevance here. Typically in ice model tests a drifting managed ice field is simulated by moving a model ship through a resting ice field. This paper addresses the differences in modeling the ice drift as described above and when moving the floes against a stationary vessel. For this purpose ice model tests of each kind are investigated and theoretical efforts are made to enlighten the topic. Also it is distinguished between the vessel being driven by its own propulsion system or by an external force. In summer 2011 and 2012 a comprehensive set of ice model tests was performed in the large ice tank of the Hamburg Ship Model Basin (HSVA). The tests are related to the research and development project DYPIC — Dynamic Positioning in Ice. Within the project two phases of model tests have been performed. The first phase has been documented and presented in [1] while the second phase is presented in [2]. The model setups described and analyzed in this paper all relate to tests performed within the scope of DYPIC.
Load History for SSFU Under Multimodal Wave Spectra
Authors: Mattioli Matteo , Drago Michele , Quondamatteo Federico , Bruschi Roberto
Published: November 2013

This paper presents results from an ongoing development project which aims at increasing the knowledge for the estimation of extremes conditions and fatigue assessment of structural element (i.e. topside) of Ship Shaped Floating Units (SSFU) in a complex environment-load conditions. The method can be extended for the calculation of total bending moment to connections and hull-girder system.

In some world geographical context, such as those of West Africa and Offshore Brazil, the environmental conditions are characterized by co-existence of waves and swells propagating in different directions. In these conditions load history for SSFU are strictly linked to comprehension knowledge of site specific environment. Loads pertain to various frequency ranges, for instance the long-period response of a morning, quasi-static response to swell, dynamic response to shore-crested sea and structural resonance of flexible components as flares etc.

This paper aims at developing a new method for establishing load history at site in case of metocean climate including combination of several (up to 3 or 4) sea state components, such as those of main swell, secondary swell and wind sea. The method is applicable to both extreme conditions and fatigue assessment as results of a “combined-event approach”.

The results are compared and discussed. They show that the use of different approaches, long and a short term analysis with deterministic and probabilistic computation of vessel heading, provide a reasonably conservative estimate of the vessel responses. Differences from the conventional method, i.e. unimodal spectrum and equivalent sea state, are also presented.
A Simple Approach for Determining the Holding Capacity of Torpedo Anchors Embedded in Cohesive Soils
Authors: de Aguiar Cristiano S. , de Sousa José Renato M. , Sagrilo Luís V. S. , Ellwanger Gilberto B. , Porto Elisabeth C.
Published: November 2013

Torpedo anchors have been used in various offshore applications especially due to its low cost installation and the ability to withstand high inclined loads. This anchor consists of a shaft in which flukes are welded in order to increase the soil-anchor contact region and, consequently, its holding capacity. Since this anchor presents a singular geometry, different from a regular cylindrical anchor/pile, the computation of the holding capacity of a torpedo anchor is not straightforward. In a previously presented work, the holding capacities of typical torpedo anchors were assessed with a finite element (FE) model in which both the anchor and the surrounding soil are represented with three-dimensional finite elements. However, this FE model demands a significant computational effort and, consequently, simpler approaches would be desirable in order to design these anchors. Relying on the FE model and a parametric study, this paper presents simple formulae to predict the holding capacities of torpedo anchors embedded into cohesive soils. These formulae are employed to predict the holding capacities of two different torpedo anchors, which are compared to those estimated with the FE model. Results agreed very well indicating that this simpler approach may be employed to quickly evaluate the holding capacities of these anchors.
The Impact of Winter Northwest Monsoon on Gust Factor
Authors: Nishida Tomoya , Waseda Takuji
Published: November 2013

This paper examines temporal variation of ocean wind gust factor and turbulence intensity in the Northwest Pacific. Observation data at coastal and open oceans near Japan are analyzed to investigate the relationship between gust factor and turbulence intensity, and the seasonal variation of probability distribution of gust factor. The contrast of the winter monsoon cold air and relatively warmer sea surface temperature is the trigger for growth of the marine boundary layer. Downdraft by developed convection enhances sea surface gustiness. The relative significance of sensible heat flux to momentum flux is considered to be the cause of higher gust factor in winter. In view of deploying offshore wind turbines around Japan, the result of this study may be more relevant on the Sea of Japan side because the winter monsoon influence there is stronger than that on the Pacific side.
A Fast Numerical Method for Internal Flood Water Dynamics to Simulate Water on Deck and Flooding Scenarios of Ships
Authors: Janßen Christian F. , Bengel Sebastian , Rung Thomas , Dankowski Hendrik
Published: November 2013

The paper reports the extension of a Lattice Boltzmann model for the nonlinear viscous shallow water equations (NSW) and its application to the simulation of internal flood water dynamics. The solver is accelerated with the help of NVIDIAs CUDA framework to access the computational power of graphics processing units (GPGPUs). The model is validated with typical tank sloshing and cross flooding scenarios and the results are compared to analytical solutions and the results of a state-of-the-art shallow water solver on the basis of Glimm’s method.
Direct Hang-Off Model to Evaluate Fatigue Damage at Riser Hang-Off
Authors: Hou Yucheng , Yuan Jiabei , Zhang Yanqiu , Tan Zhimin , Sheldrake Terry
Published: November 2013

Fatigue damage assessment at the flexible riser hang-off location, where the pipe frequently endures maximum tension and curvature variations, is key to verify design integrity for service life.

Traditionally, the fatigue analysis is performed in a separate local structure model, as the commercial global analysis software lacks the capability of handling the local behavior of the riser structural component, which is dependent on materials and manufacturing processes. During global fatigue analysis, the riser configuration is built with pinned connected at the hang-off point. The resulting tension and angle responses at the hang-off location, are then input to a local model to perform the stress and fatigue analysis, where the detailed pipe layer structure and bend stiffener are modeled. This traditional approach is conservative, time costly and is often limited to regular wave approach.

Wellstream developed an external function to work with specialized commercial riser dynamic analysis software. The external function simulates the detailed behaviour of flexible pipe structure components and the resulting bending hysteresis during dynamic simulation in the time domain. Therefore, the stress time history of the tensile armour becomes available at the end of global simulation in the time domain and is ready for fatigue damage assessment by rain flow counting.

This paper presents a study case where the fatigue assessment is performed directly at the hang-off region within the riser global dynamic simulation. The riser hang-off is situated at the top of the I-tube and a bend stiffener is fixed at the bottom of the I-tube. I-tube and pipe section are precisely modeled as pipe-in-pipe facility, where the interaction of riser/I-tube can be captured.
Pipeline Shore Approach Installation by Horizontal Directional Drilling
Authors: da Silva Danilo Machado L. , Solano Rafael F. , de Medeiros Antonio Roberto , Rodrigues Marcos V. , de Azevedo Fabio B.
Published: November 2013

Whenever an export pipeline coming from offshore fields to onshore facilities is designed, a shore approach solution needs to be provided, once it can become a very complex project in terms of offshore pipeline installation. At this phase the pipeline on-bottom stability is analyzed for surf zone, the possibility of using concrete coating is verified as well as the necessary burial depth. In addition, the pipeline installation stress analysis is performed, the potential for local scour is verified, among other things. In this context, horizontal directional drilling (HDD) emerges as an alternative method in which, in addition to overcome the technical aspects mentioned above, the environmental issues can also be minimized.

Many factors determine the success of an HDD project. Failure to complete the borehole is often the main concern, as the project would not be attempted if the pipeline could not be installed. However, the successful design and construction of an HDD is measured in more than a successful pullback. The achievements, as in any project, include the completion of the project for a reasonable cost with minimal environmental impact and according to the schedule. And of course, the pipeline integrity shall be ensured. That is the focus here in this work.

This paper presents discussions regarding to the proper design of the export pipeline section installed by HDD in the shore approach area. To ensure a proper design and pipeline integrity are important parts in the success of a shore approach HDD crossing. It must be noted that there are no methods for in situ repair of damaged pipelines installed by HDD. The point is a proper design, construction and installation, which includes, for instance, do not overstress the pipeline during installation, mainly pullback operation, as well as the proper selection of the drill path in order to place the pipeline within stable ground and isolated from obstacle’s active conditions, to properly consider the corrosion protection, etc., for the design life of the product pipe.