bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Study on Mean Velocity Profiles Under Wave-Current Interactions
Authors: Zheng Jinhai , Li Tongfei , Yan Yixin , Hu Jinchun
Published: July 2009

A series of experiments are carried out in a laboratory flume for combined wave-current flows, to investigate the characteristics of vertical structure of current profile in a wave-current co-existing water areas. Changes induced in the mean velocity profiles are considered for a range of wave heights, wave periods, water depths and flow velocities which are based on those typical data in the Yangtze River Estuary, China. Preliminary tests are conducted on the unidirectional current and on the wave alone. These show that the current mean velocity profiles agree well with the logarithmic law, and that the waves are approximated closely by the Stokes’ second-order theory. For the combined wave and current tests, the mean velocity profiles generally differ from those suggested by a linear superposition of wave and current velocities. In the case of waves following a current, the velocity distributions exhibit a relatively greater velocity near the bed and a smaller velocity above a certain depth relative to the logarithmic law, and the maximum velocity is observed at a lower location. In the case of wave opposing a current, the velocity distributions depart from the logarithmic law with a relatively smaller velocity near the bed and a greater velocity above a certain depth. Experimental data can be used to validate the theoretical or mathematical models associated with the combined wave-current motions.
Strain Based Failure Assessment for Offshore Structures Under Seismic Loading
Authors: Zhao Wangwen , Turner Richard , Liang Jian
Published: July 2009

Under seismic loading, structural hot spots can experience very high levels of stress and many random stress reversals. Conventional stress based methods cannot assess the failure state in detail when stress is beyond the elastic limit and nominal stress reversals are more than double the yield stress. A method has been created to fully reproduce the true stress/ strain history by using 1) generalised Masing’s rule with equivalent cyclic energy dissipation to model cyclic stress/strain relation, 2) Neuber’s method to calculate inelastic strain concentration factor, and 3) relative effective notch factor determined from comparing S-N curves of different joint classes. From this reproduced strain history, strain cycles can be counted and low cycle fatigue analysis can be conducted by using Miner’s rule and by estimating damage from the strain based failure criteria such as Coffin-Mason method. This method has been implemented in a numeric procedure and coded in a FORTRAN program called CYSTRA (as for CYclic STRain Analysis). It takes input of “nominal” random stress history directly from general structural software, linear or non-linear, local or global, and calculates extreme strain and strain cycles at multiple hot spots for the whole structure efficiently. Thus it greatly facilitates failure assessment for offshore structures which can have a large number of hot spots within the structure, unlike mechanical devices commonly assessed in strain based analysis where detailed FE based methods can be used.
Nonlinear Response Characteristics of Discus Data Buoy Hull
Authors: Balaji R. , Sannasiraj S. A. , Sundar V.
Published: July 2009

The response characteristics of the discus hull shaped data buoy under the influence of nonlinear wave conditions was studied in an experimental investigation. The measured wave elevation and the buoy response time histories were analyzed by phase-portraits as well as through the spectral representations. The details of the model, instrumentation, testing conditions and the analysis are presented and discussed in this paper.
A Numerical Simulation for the Prediction Movement of Gas Condensate From Spill Accidents in the Assalouyeh Marine Region, Persian Gulf, Iran
Authors: Habibi Shahla , Bidokhti Ali Akbar
Published: July 2009

This paper presents a three dimensional numerical model of flow and movement of gas condensate spills based on Navier-Stokes and continuity equations with Boussinesq approximation involving various surface wind forcings. The model simulates the surface movement of gas condensate slick from spill accidents in Assalouyeh Marine Region. For the advection term an upwind weighted, multidimensional positive definite advection transport algorithm (MPDATA) was used. This algorithm uses an explicit finite difference scheme with an antidiffusive velocity for equilibrium diffusion. It also uses a generalized-conjugate-residual (GCR) method for the solution. The model is run for gas condensate spill accidents in Assalouyeh Marine Region in summer and winter of 2005. Numerical results show that gas condensate particles spread torwards the shore in summer, while in winter it mostly spreads towards east. The spreadings follow the flow fields that are in good agreement with flow field observations. Diffusion of gas condensate particles in the water due to more turbulence in winter is larger, while gas condensate particles are observed on the water surface due to more stability and buoyancy force in summer.
Probabilistic Presentation of the Still Water Loads: Which Way Ahead?
Authors: Ivanov Lyuben D. , Wang Ge
Published: July 2009

This paper analyzes the state-of-the-art concerning this subject. The objectives are: 1) to provide data of still water bending moments and shear forces that were collected from loading manuals of dozens of tankers 2) conduct statistical analysis of the loading cases and 3) discuss possible ways for further improvement of knowledge about the variability of still water loads during real ships’ operation.
Managing Vortex Induced Vibration in Well Jumper Systems
Authors: Bhalla Kenneth , Gong Lixin
Published: July 2009

The purpose of this paper is to present a method that has been developed to identify if vortex induced vibration (VIV) occurs in well jumper systems. Moreover, a method has been developed to determine when VIV mitigation measures such as strakes are required. The method involves determining the in-plane and out-of-plane natural frequencies and mode shapes. The natural frequencies are then used, in conjunction with the maximum bottom current expected at a given location to determine if suppression is required. The natural frequency of a jumper system is a function of many variables, e.g. span length, leg height, pipe diameter and thickness, buoyancy placement, buoyancy uplift, buoyancy OD, insulation thickness, and contents of the jumper. The suppression requirement is based upon calculating a lower bound lock-in current speed based upon an assumed velocity bandwidth centered about the lock-in current. The out-of-plane VIV cross-flow response is produced by a current in the plane of the jumper; whereas the in-plane VIV cross-flow response is produced by the out-of-plane current. Typically, the out-of-plane natural frequency is smaller than the in-plane natural frequency. Jumpers with small spans have higher natural frequencies; thus small span jumpers may require no suppression or suppression on the vertical legs. Whereas, larger span jumpers may require no suppression, suppression on the vertical legs or suppression on all the legs. The span of jumper systems (i.e. production, water injection, gas lift/injection ...) may vary in one given field; it has become apparent that not all jumper systems require suppression. This technique has allowed us to recognize when certain legs of a given jumper system may require suppression, thus leading to a jumper design whose safety is not compromised while in the production mode, as well as minimizing downtime and identifying potential savings from probable fatigue failures.
Sustained Load Cracking Resistance of Grade 23/29 Titanium GTA-Welded Pipe Joints
Authors: Leonard Andrew J. , Schutz Ronald W. , Porter Richard L.
Published: July 2009

Grade 23 and/or 29 titanium alloy pipe and forgings are typically butt-welded together in the fabrication of offshore riser components such as tapered stress joints (TSJs) for top-tensioned risers and as hang-offs for dynamic catenary risers. Although Grade 29 titanium base metal in a relevant wrought/forged product form has already been evaluated in regards to sustained-load cracking (SLC) resistance, minimal data is available to ensure that the SLC resistance of typical GTA butt-welded joints in these thicker-wall titanium alloy components will also meet design requirements. As part of a TSJ production weld qualification, conservative fracture-mechanics based SLC tests were conducted at room temperature on 1G-position machine GTA butt-welded Grade 23 titanium pipe utilizing Grade 29 titanium filler metal. Test results revealed no significant SLC susceptibility in the weld and a minor effect in HAZ metal, producing KSLC values similar to KQ values. These values safely meet typical TSJ fracture mechanics requirements, and are consistent with published SLC information on this alloy system.
Effect of Roof Impacts on Coupling Between Wave Response and Sloshing in Tanks of LNG-Carriers
Authors: Molin Bernard , Remy Fabien , Ledoux Alain , Ruiz Nicolas
Published: July 2009

An experimental campaign is reported on the wave response of a rectangular barge supporting two rectangular tanks partly filled with water. Flat and chamfered tank roofs are successively tested, at varying heights above the free surfaces inside the tanks. The tests are carried out in irregular wave systems coming from abeam. The measured barge roll and sloshing motions in the tanks are compared with numerical results from a linearized potential flow model. Good agreement is reported in mild seastates. Nonlinear effects, associated with large amplitude sloshing motion and/or roof impacts, are investigated.
Offshore Drilling Riser VIV Suppression Devices: What’s Available to Operators?
Authors: Taggart Samuel , Tognarelli Michael A.
Published: July 2009

VIV suppression and drag reduction are key issues for improved operation in offshore drilling. Properly designed helical strakes are effective in the mitigation of VIV fatigue damage for many riser applications. However such strakes tend not to be applicable to offshore drilling riser applications. This is due to increases in drag force due to increased apparent diameter as well as workability problems for drilling operations. For these reasons, effective devices are sought that would mitigate VIV and reduce, or at least not increase, drag for drilling applications. Along with yielding good hydrodynamic performance, a drilling riser VIV suppression device must be compact and robust enough to be used in a drilling-rig environment. It needs to be deployable and recoverable in declared operational sea states. It must also be easy to store and assemble. Finally, and most importantly, it must be efficient to deploy and recover during normal riser operations. This last point is vital to drilling operations in deepwater in hurricane-prone areas. Weather conditions can change quickly and even a non-faired deepwater riser takes 2 to 3 days for a full retrieval. BP continues to research and document suppression device types and to assess their practical performance. A supply choice in the market place is important so that the correct device can be used for particular situations. To this end, we have recently worked with cooperative partners to demonstrate the hydrodynamic performance of a handful of the most promising devices. This paper is a tailored synopsis of previous suppression concepts and the philosophical pathway toward what is available on the market today. At its core are recent circumstances which precipitated a need to quantify and qualify for operational acceptance the performance of two commercially available short aspect ratio fairing devices. (i.e. a dual-fin splitter and an airfoil-shaped fairing). This paper discusses the results of the large-scale model acceptance tests over prototype Reynolds number for these devices. In addition to rigid devices, a relatively newer suppression product that “inflates” in the direction of the relative flow was also assessed by BP for expected hydrodynamic performance. This device shows particular promise for the mitigation of VIV during drilling operations surprises in high currents along with appearing potentially commercially viable.
Sour Service Corrosion Fatigue Testing of Flowline and Riser Welds
Authors: McMaster Fraser , Bowman Jonathan , Thompson Hugh , Zhang Michelle , Kinyon Steven
Published: July 2009

Standard X-65 carbon steel pipe material is to be utilized for a flowline and riser system for an on-going project within the Gulf of Mexico. The project-produced fluids are corrosive with CO2 and produced water chemistry being the primary corrosive driving forces. The oil field will utilize water injection for reservoir pressure maintenance and as a consequence souring of the field in the later stages of life could occur. An experimental program of work was undertaken to measure the fatigue “knock-down” factor of sour/brine environments compared to that of laboratory air. Stress-life (S-N) samples were removed from segments of pipe with an outside diameter of 9.625 inch (wall thickness of 1.26 inch) containing fully inspected, production-quality circumferential welds. The environment examined included laboratory air conditions as well as deoxygenated brine supplemented by a gas mix of H2S and CO2. The primary variable changed during the test program was the test frequency. Fatigue testing in the high stress, or flowline, region was performed at 0.01Hz compared to a test frequency of 0.33Hz, utilized for the lower stress region, indicative of riser conditions. For all environmental tests performed, the dissolved oxygen levels were maintained at less than 10 ppb during testing. The measured fatigue life decrease in the curved pipe segments was in the range of 3–12 times when tested under sour brine environmental conditions compared to lab air. The results of this work clearly illustrates the importance of performing environmentally relevant fatigue testing when considering material selection for offshore applications that may contain a sour environment.
Bent Collapse of an Unbonded Rough Bore Flexible Pipe
Authors: Lu Jing , Ma Frank , Tan Zhimin , Sheldrake Terry
Published: July 2009

This paper investigates the collapse behaviour of an unbonded rough bore flexible pipe as it adopts a straight and bent configuration respectively. An unbonded flexible pipe typically consists of several layers, where each layer is designed to provide a specific structural function. The collapse strength of an unbonded rough bore flexible is provided mainly by its Flexbody layer, and enhanced by the Flexlok layer through the Flexbarrier layer in between. A 3D detailed FEA model was developed to simulate the most susceptible collapse situation, where the outermost Flexshield is assumed bleached and the external hydrostatic pressure acts directly on the outer surface of the Flexbarrier layer. This is often referred to as the wet collapse. The model was calibrated against the collapse test data performed on a 6 inch ID pipe in both straight and bent configuration. The equivalent test data shows about 5.0% reduction in the collapse strength under the bent configuration. The paper proposed a practical approach for estimating the collapse pressure for a pipe bent to various bend radii. The work presented is part of an ongoing research and development project.
On Influence of Turbulence Modelling on Cavitation Prediction for Flow Around P1356 Ship Propeller
Authors: Lifante Conxita , Frank Thomas , Rieck Karsten
Published: July 2009

The goal of this work is to get a deeper understanding of the structure of the flow around a propeller of a passenger ship, specially the pressure field, where turbulent fluctuations and cavitation can lead to pressure oscillations, vibration and noise. The accurate prediction of cavitation has been found out to be intrinsically related to the accurate resolution of turbulent structures of the flow. Therefore, a thoroughly analysis of the turbulence modeling in this kind of application was performed. Following the Best Practice Guidelines (BPG) [1] different grids and turbulence models have been investigated. The numerical results obtained have been compared to the experimental data generated at SVA Potsdam, which includes transient pressure signals as well as cavitation patterns. A highly satisfactory agreement between numerical solutions and experiments is observed for the finest grids and a scale-resolving turbulence modeling approach (DES/SAS).
Consistent Hydro-Structure Interface for Evaluation of Global Structural Responses in Linear Seakeeping
Authors: Malenica Sˇime , Stumpf Estelle , Sireta Franc¸ois-Xavier , Chen Xiao-Bo
Published: July 2009

The difficulties related to the equilibration of the 3D FEM structural model, in the context of hydro-structure interactions in linear seakeeping are discussed. Different philosophies in modeling the structural and hydrodynamic parts of the problem, usually lead to very different meshes (hydro and structure) which results in unbalanced structural model and consequently in doubtful results for structural responses. The procedure usually employed consists in using different kinds of interpolation schemes to transfer the total hydrodynamic pressure from hydrodynamic panels to the centroids of the structural finite elements. This approach is both, very complex for complicated geometries, but also rather inaccurate. The method that we propose here is based on two main ideas: 1. Pressure recalculation instead of interpolation; 2. Separate transfer of different pressure components (incident, diffraction, radiation & hydrostatic variation). The first point removes the difficulties related to the interpolation techniques, and allows for a very robust method of pressure transfer. The second point ensures the perfect equilibrium because the body motions are calculated after integration over the structural mesh, which means that the equilibrium is implicitly imposed. It should be noted that this procedure is not completely straightforward and several numerical “tricks” need to be introduced. However, once these difficulties are solved, the final numerical code is extremely robust and can be easily coupled with any of the general 3D FEM packages.
Analysis of Dynamic Stress Propagation in Subsea Pipeline and Flow Line Systems
Authors: Olunloyo Vincent O. S. , Osheku Charles A. , Ogunmola Bayo Y.
Published: July 2009

Offshore pipeline and flow line systems define a variety of subsea architectures associated with Floating Production Storage and Offloading units (FPSOs) or Floating Storage and Offloading units (FSOs) that are usually employed for oil and gas production in deep and ultra deep waters. The design of such transmission facilities, must satisfactorily account for various phenomena such as hydrodynamic wave loading, fluid transport velocity, operating pressure and temperature of the internal fluid as well as limitations imposed by the seabed subsoil layer geotechnical properties. In fact the transverse and longitudinal dynamic responses of these pipeline and flow line systems are strongly modulated by these effects. In this paper, the effect of transverse and longitudinal vibrations on the dynamic stresses induced by the fluid flow, is studied with special reference to onset of buckling or bursting of such pipes. For this purpose, an offshore pipeline is idealized as a fluid conveying elastic beam on an elastic foundation. In particular, by employing integral transforms, an analytic solution for the induced stresses is computed and simulated for design applications while comparison with corresponding formulae currently in use in the field is also carried out.
Risk Indicators for Major Accidents on Load Bearing Structures and Maritime Systems in the Petroleum Industry
Authors: Kvitrud Arne , Ersdal Gerhard , Vinnem Jan-Erik
Published: July 2009

To monitor the development of the risk of major accidents of structures and maritime systems on the Norwegian Continental Shelf a set of incident indicators has been established. These indicators are given a weight related to the fatality risk predictions in order to develop a total risk indicator. Aggregated and weighted indicators as well as individual indicators are presented and evaluated. In addition indicators related to barriers are established giving a method to monitor the performance as a function of time, and to benchmark the participants in the industry. The present paper describes the method used in the risk level project to monitor the risk related with emphasis on structures and maritime systems, including hazards as wave in deck, vessel collisions and cracks on the structures. The paper further presents the actual development using the methodology, and the identified major contributors to the risk level.
Experimental Validation of Hydroelastic Analysis of Pontoon-, Semisubmersible- and Hybrid-Type VLFS
Authors: Utsunomiya Tomoaki , Watanabe Eiichi , Hiraishi Tetsuya , Noguchi Takatoshi , Yamamoto Syuji
Published: July 2009

This paper presents a hydroelastic analysis of pontoon-, semisubmersible-, and hybrid-type VLFS and its experimental validation. In the analysis, the detailed configuration of the experimental model is considered by using three dimensional finite element (FE) method. Experimental models measuring 15m in length, 3m in width and 0.03–0.23m in draft (depending on whether it is the pontoon and semisubmersible part) are built. The experiment has been made in the wave-basin with the bottom-slope of 1/75. At the same time, a hydroelastic analysis is carried out using the same models as the experimental ones in a wave-basin with flat as well as slanting bottom with slope of 1/75. By comparing the numerical results and experiment, the effect of bottom slope is verified. In the analysis of the semisubmersible part, the effect of viscous damping is considered by using the drag force formula with assumed drag force coefficient of 2.0. A comparison between the experiment and the analytical results indicates that numerical results from the semisubmersible part with additional effects of viscous damping agree better with the experimental results than those without. An experiment using irregular waves is also carried out and compared with the numerical predictions. Finally, the steady drift forces are analyzed using the far-field method and compared with the experiment.
Experimental Study of Taper Piles With Different Taper Angles
Authors: Ghazavi Mahmoud , Kalantari Ashkan Behmardi
Published: July 2009

Piled foundations are normally used in offshore engineering. Among various piles, tapered piles have normally greater cross sectional are around the head than the pile toe. Therefore they have greater potential for substantial cost advantages in static loading conditions. The objective of this study is to explore the characteristics of the axial response of tapered piles. Laboratory facilities for testing model piles were prepared. Four polyamide piles with different angles of taper from 0° to 1.5° were used in this study. The soil was sand. It has been found that tapered piles have more bearing capacity than cylindrical piles of the same length and volume. This is interesting and may be considered for offshore piling since in this situation a large number of piles is routinely used.
Hydroelastic Aspects of Large Container Ships
Authors: Senjanovic´ Ivo , Malenica Sˇime , Tomasˇevic´ Stipe , Tomic´ Marko
Published: July 2009

The importance of hydroelastic analysis of large and flexible container ships of today is pointed out. A methodology for investigation of this challenging phenomenon is drawn up and a mathematical model is worked out. It includes definition of ship geometry, mass distribution, structure stiffness, and combines ship hydrostatics, hydrodynamics, wave load, ship motion and vibrations. Based on the presented theory, a computer program is developed and applied for hydroelastic analysis of a flexible segmented barge for which model test results of motion and distortion in waves have been available. A correlation analysis of numerical simulation and measured response shows quite good agreement of the transfer functions for heave, pitch, roll, vertical and horizontal bending and torsion. The developed tool is furthermore used for hydroelastic analysis of a large container ship.
Load Generation for Structural Strength Analysis of Large Containerships
Authors: Ro¨rup Jo¨rg , Schellin Thomas E. , Rathje Helge
Published: July 2009

Many modern ships, particularly large containerships, are characterized by extreme bow flare, large stern overhang, and low torsional rigidity due to an open deck structural configuration. Software package GL ShipLoad was developed as an aid to assess the structural integrity of such ships. This software tool became the standard method to generate rule based loads for a global strength finite element analysis of sea going displacement ships. It efficiently generates loads based on first principles. A graphical user interface facilitates the convenient application of ship and cargo masses to the finite element model and aids in the selection of relevant design wave situations. User defined selection criteria, such as maximum values of rule based bending moments, shear forces, or torsional moments, specify which waves have to be chosen for the global strength analysis. This approach yields a reduced number of balanced load cases that are sufficient to dimension the hull structure. To adequately simulate roll motion, additional roll angles are analyzed that simulate realistic distributions of torsional moments over the ship length. A strength analysis of a typical post-panamax containership demonstrated the load generation procedure. First, efficiently modeled mass items were grouped into reusable assembled masses for the ship at hydrostatic equilibrium. Second, regular design wave scenarios were estimated, and hydrodynamic pressures for a large number of regular waves were computed. Third, a reduced number of relevant wave situations were automatically selected, and balanced hydrostatic, hydrodynamic, and inertia loads were applied to the finite element model. Enforced roll angles were found to contribute significantly to the initial torsional moment in the fore holds. Finally, based on a locally refined FE submodel of the hatch corners in way of the ship’s fore hold, a fatigue analysis was performed to assess effects of critical loading under enforced roll angles.
Multidisciplinary Design Optimization: A New Way to Bring Softer Bows
Authors: Hu Zhiqiang , Cui Weicheng , Yang Jianmin
Published: July 2009

It is well known that sharp bulbous bow has a good performance on ship resistance reduction, but it is also threatens the struck ships and the environment greatly. For their own economy profit, ship owners would like the bulbous bow to be designed sharp and rigid. However, from the viewpoint of environmental protection, the bulbous bow should be designed blunt and soft. Multidisciplinary Design Optimization (MDO) is a prosperous design concept and technique, to reconcile this problem effectively. The basic concept and theories of MDO are introduced in this paper. An optimization analysis is accomplished on the bulbous bow design for a container ship, using Collaborative Optimization Method. The characters of the bulbous bow on resistance reduction, collision force density and structural strength requirement are all considered at the same time. A compatible bulbous bow can be obtained by this way.
Linear Diffraction and Radiation of Surface Waves by a Hollow Suspended Cylindrical Shell
Authors: Mitchell Lewis , Zhu Song-Ping
Published: July 2009

A first-order analytical solution for the diffraction and radiation of ocean surface waves around a suspended hollow cylindrical shell is presented. We revisit the problem first studied by Garrett (1970), using a new and more direct technique to obtain the solution for the wave potential. Numerical results are presented for the combined diffraction and radiation problems, showing the wave field inside and around the structure. The results give fundamental insight into both the ideal design and placement of an Oscillating Water Column (OWC) device in the ocean, so as to extract the maximum possible energy from an incident plane wave field.
Probabilistic Assessment of Extreme Events Subject to Epistemic Uncertainties
Authors: Nishijima Kazuyoshi , Faber Michael Havbro , Maes Marc A.
Published: July 2009

Over the years the modeling and treatment of aleatory and epistemic uncertainties in probabilistic assessments has repeatedly been an issue of discussion and also some controversy. The philosophical and mathematical aspects may be said to be well appreciated, however, there are cases in practice where principles seem to be violated and frequently the effects of the epistemic uncertainty are treated inconsistently in the probabilistic modeling. The present paper first reviews the general principles for the modeling and treatment of uncertain characteristics subject to both aleatory and epistemic uncertainties. Thereafter, the general principles are applied considering three examples concerning the probabilistic modeling of extreme events; 1) the n-year maximum distribution, 2) the corresponding return period and 3) the exceedance probability in hazard analysis. Through these examples typical inconsistencies made in practical probabilistic assessments are pointed out. The results from the examples are interpreted and discussed from a structural design perspective and from a rational risk-based decision perspective. Finally, a practical solution to avoid the inconsistencies is suggested emphasizing the analogy of the analysis of extreme events with the analysis of portfolios.
Probabilistic Fatigue Reliability Analysis of Deepwater Steel Catenary Risers
Authors: Xia Jie , Das Purnendu K.
Published: July 2009

In recent years, offshore reservoirs have been developed in deeper and deeper water environments, where floating production, storage and offloading (FPSO), semi-submersibles, spars and TLPs are considered to be the most economically viable platforms. Steel Catenary Risers (SCRs) are being considered for these production units in deepwater development such as Northern North Sea. A variety of uncertainties are associated with material behaviour, environmental loading, hydromechanics modelling, structural modelling, and fatigue / corrosion / wear characteristics, especially at hang-off and touch down points. To improve the understanding of SCR behaviour and increase the confidence in the design of such systems in deep water environment, a probabilistic reliability-based methodology that systematically accounts for the inherent uncertainties is needed. By using a probabilistic mechanics approach, the existing deterministic design/analysis methods are improved with introduction of uncertainties in model parameters. This paper concentrates on the probability of failure associated with the current design practice of fatigue analysis of SCRs. A probabilistic methodology for fatigue reliability is developed, which utilizes deterministic cumulative fatigue damage indicators calculated from DeepC, namely the stress levels and cycles associated with the various sea states and the fatigue strength of the members. Uncertainties in structural load and material properties are accounted for by assigning probability distributions and standard deviations to the deterministic stress levels. Furthermore, fatigue strength parameters, Miner’s indices, and capacities are modelled as random variables. First order reliability method (FORM) and second order reliability method (SORM) are employed for estimating fatigue reliability. The methodology is applied to two deterministic case studies, involved either a semi-submersible or a FPSO platform. The effect of uncertainties in parameters on fatigue reliabilities is investigated. Additional insight is gained from the parametric sensitivity studies on the fatigue strength parameters.
A Parametric Study for the Design of Steel Catenary Riser System in Deepwater Harsh Environments
Authors: Xia Jie , Das Purnendu K. , Karunakaran Daniel
Published: July 2009

In recent years, offshore reservoirs have been developed in deeper and deeper water environments, where floating production, storage and offloading (FPSO), semi-submersibles, spars and TLPs are considered to be the most economically viable platforms. Steel catenary risers (SCRs) are being considered for these production units in deepwater development such as Northern North Sea. A variety of uncertainties are associated with material behaviour, environmental loading, hydromechanics modelling, structural modelling, and fatigue / corrosion / wear characteristics, especially around hang-off and touch down areas. SCRs used in conjunction with a semi-submersible or a FPSO in deepwater harsh environments present significant design challenges. The large vertical motions at the FPSO or semi induce severe riser response, which results in difficulty meeting strength and fatigue criteria at the hang-off and touch down point locations. To improve the understanding of SCR behaviour and increase the confidence in the design of such systems in deepwater harsh environments, a parametric study was carried out in this paper to deal with the factors that mainly influence the loading condition and fatigue life of the riser. Two cases, one steel catenary riser connected to a semi-submersible and one steel catenary riser connected to a FPSO, were studied and compared. And weight-optimized configurations were applied for both risers. Riflex combined with DeepC was the primary analysis tool used for the long-term response of the nonlinear structure SCR’s simulations, which is high computer time consuming. Hence, the parameters affecting the efficiency and accuracy of the simulations have also been studied during the analysis process.
Forecast of Critical Situations in Short-Crested Seas
Authors: Clauss Gu¨nther F. , Kosleck Sascha , Testa Daniel , Hessner Katrin
Published: July 2009

The impact of single extreme waves or wave groups on marine structures and ships causes enormous forces often leading to critical situations or even loss of ship, cargo and crew. One approach to avoid dangerous situations is to adjust heading and cruise speed. To identify critical situations well in advance the forecast of the incoming wave train is essential. Concerning the method to predict the wave train a ship will encounter within the near future — some minutes ahead — the so far unidirectional WAVE FORECAST method, pre-calculating an encountering wave train from surface elevation snapshots of the surrounding sea — taken by radar — has been improved. This paper presents a method to predict the entire sea state within the surrounding area of the vessel considering multidirectional waves. Thus the evolution of critical waves coming from various directions can be predicted. In addition the SHIP MOTION FORECAST method — pre-calculating the vessel response — has also been enhanced. Taking into account the encounter angle of the incoming wave components, depending on time and course angle of the vessel, the ship-fixed compass rose is divided into a number of sectors. The corresponding encountering wave train for every sector is derived by superimposing all wave components coming from certain directions. With a set of directional Response Amplitude Operators (RAOs) for the six degrees of freedom the sector-wise vessel responses can be calculated as well. The response spectra are derived in frequency domain and transferred into time domain by the use of Inverse Fast Fourier Transformation (IFFT). Thus the overall vessel response is obtained by superimposing the time domain responses for every sector and degree of freedom, delivering a comprehensive data base for the analysis of critical situations in advance.
Development of Key Performance Indicators for Offshore Structural Integrity
Authors: Sharp J. V. , Ersdal G. , Galbraith D.
Published: July 2009

Key performance indicators (KPIs) are widely used to assess performance against targets, whether these be technical, environmental or financial. Offshore KPIs are used by both duty holders and regulators to assess the reliability of equipment and systems, often they relate to safety systems and the regulator’s interest relates to such systems. The most obvious KPIs include number of fatalities, fatal accident rate, lost time injury frequency and total recordable incident rate, as well as hydrocarbon release incident rates associated with maintaining safety. Many of the “non-headline” KPIs relate to systems that could be critical in the event of an accident and these are of great importance. However KPIs have not yet been developed for the performance of the offshore structural system. Performance standards are a requirement of current UK offshore legislation, although these again are more normally associated with fire and explosion. Since many offshore installations are now in the ageing phase performance measures are increasingly important. This paper described the background to developing KPIs for offshore structures, relating to aspects which are important for both safety and asset integrity. This has been achieved based on a hazard approach, which includes extreme weather, fatigue, corrosion and accidental damage. KPI’s need to be measurable and this aspect has been incorporated in their development. It is proposed that these KPIs will have significant use in providing a basis for measuring structural performance, particularly for ageing installations where a case for life extension needs to be made.
Three-Dimensional Large Deformation Analyses of Plate Anchor Keying in Clay
Authors: Wang Dong , Hu Yuxia , Randolph Mark F.
Published: July 2009

Plate anchors are a relatively new type of offshore anchorage in deep water. Plate anchors are usually installed vertically, followed by rotation under eccentric pulling force. In normally consolidated clay, the embedment loss during keying may reduce the anchor capacity significantly. The keying behaviour of rectangular anchors is investigated here using a three-dimensional large deformation finite element approach. This approach adopts mesh regeneration to avoid severe distortion of soil elements, as well as mapping the stresses and material parameters from the old to the new mesh. To study the factors affecting the embedment loss, the loading eccentricity ratio was varied from 0.17 to 1.5 for aspect ratios of the anchors of 1 and 2. The strength profile and rigidity of soil and loading orientation were also varied. The numerical results agree well with centrifuge testing data. It is found the embedment loss depends strongly on the eccentricity ratio and orientation of pulling force, while the effects of soil rigidity and anchor aspect ratio are minimal.
Combination Criteria of Joint Extreme Significant Wave Height and Wind Speed in Weizhoudao Offshore Area
Authors: Dong Sheng , Liu Wei , Xu Pengjing
Published: July 2009

With the data of annual maximum wave heights and their corresponding wind speeds acquired at Weizhoudao Observation Station in China from 1961 to 1989 as a statistical sample, the joint probabilities of extreme wave heights and wind speeds at the station are estimated by using the following distribution models: (1) the bi-variable Pearson-III distribution, (2) the conditional distribution, and (3) the independent distribution. To examine the dependence of design load criteria on the assignment of distribution model, design base shears of a jacket platform are computed by assigning one of the three models, respectively. It is found that the 100yrs base shear resulting from the model (1) is less by 7% than that resulting from the model (2), and is less by 18% than that resulting from the model (3). This suggests that the design load criteria could be reasonably reduced by considering the correlation between environmental conditions, so as to reduce the engineering investment of marine structures.
On the Novel Fast Ocean Platform
Authors: Yoshida Motoki , Iwashita Hidetsugu , Kihara Hajime , Itakura Hiroshi , Bao Weiguang , Kinoshita Takeshi
Published: July 2009

The mission of the large trans-ocean fast ship is to transport the high-valued and time-sensitive goods punctually at reasonable price. Accordingly, the speed reduction, additional resistance or slamming, which are caused by the large amplitude of ship motions, should be restricted completely. Some seaworthiness, such as no speed reduction and absolutely no slamming, is especially required for ships running fast in ocean waves. The final intention of the study is to compare the transport efficiency, including accuracy of time schedule and transport quality like damage of goods caused by slamming, among three types of ship hulls, i.e. mono-hull, catamaran and trimaran. As a first step, a “Resonance-Motion-Free SWATH (RMFS)” hull is considered as an example of catamaran type in the present work. As a result of very small water plane, the restoring moments are near zero for this type of hull, which leads to free of resonance in the pitch responses. Experiments in towing tank and calculations based on potential theory are carried out to examine hydrodynamic forces and motion responses in waves. The results are compared with those of typical mono-hull and trimaran ships. The predominance of RMFS regarding seaworthiness will be pointed out in the conclusion.
Numerical Modelling of Wave Run-Up on a Wind Turbine Foundation
Authors: Nielsen Anders Wedel , Mortensen Simon Brandi , Jacobsen Vagner , Christensen Erik Damgaard
Published: July 2009

This paper presents the results of a CFD model of the wave run-up on a monopile. The monopile is widely used as the foundation unit for offshore wind turbines. The aim for the calculations is to make a detailed investigation of the effect of three-dimensional (3D) waves on the run-up and to determine the maximum wave run-up. The CFD results are compared with the results of physical model tests conducted under the same conditions. The model tests were conducted under idealized conditions: The tests were carried out on a horizontal bottom using phase and directional focused waves to obtain a 3D effect and at the same time being able to control the breaking. The key objective of this part of the numerical analysis is to develop a model capable of reproducing the results of the physical model tests.
Fatigue Design Criteria as Function of the Principal Stress Direction Relative to the Weld Toe
Authors: Lotsberg Inge
Published: July 2009

For fatigue design it is necessary to provide guidelines on how to calculate fatigue damage at weld toes based on S-N data when the principal stress direction is different from that of the normal direction to the weld toe. Such stress conditions are found at details in different types of plated structures. Some different fatigue criteria for these stress conditions are presented in design standards on fatigue design. Criteria used by the International Institute of Welding (IIW), Eurocode, British Standard and in the DNV standards have been assessed against some relevant fatigue test data presented in the literature. Only proportional loading conditions have been considered here. (By proportional loading is understood that the principal stress direction is kept constant during a load cycle). An alternative equation for calculation of an equivalent or effective stress range based on stress normal to the weld toe and shear stress at the weld toe has been proposed. The proposed methodology can be used for nominal S-N curves and it can be used together with a hot spot stress S-N curve with stresses read out from finite element analysis. The different design criteria are presented in this paper together with recommendations on analysis procedure.
Towards Environmental Risk Acceptance Criteria
Authors: Sames Pierre C. , Hamann Rainer
Published: July 2009

Risk evaluation criteria related to safety of human life have been available in the maritime industry for some time. However, only recently these criteria became formally accepted by including the CAF and ALARP-boundaries into the Formal Safety Assessment guidelines of the IMO. Risk evaluation criteria related to the protection of the environment are not yet agreed. A proposal for a cost effectiveness criterion related to accidental oil spills called CATS was suggested by the project SAFEDOR. However, a societal risk acceptance of environmental damages from shipping is not yet proposed. And, to effectively apply a cost-effectiveness criterion related to environmental protection, societal risk acceptance and the associated ALARP area need to be defined. To contribute to the ongoing discussion on environmental risk evaluation criteria, this paper presents a societal risk acceptance criterion related to oil spills of tankers which can be used within risk-based ship design and approval as well as for rule-making. The presented work adds to SAFEDOR’s contribution to risk evaluation criteria for the maritime transport in providing an ALARP area for risk assessment of oil transport by tankers. The paper first presents the current state of oil transportation by tankers and continues with providing suggestions how the ALARP boundaries may be derived in this context.
Experimental Analysis of Rudder Contribution to Roll Damping
Authors: Etebari Ali , Atsavapranee Paisan , Bassler Christopher , Carneal Jason
Published: July 2009

Measuring and modeling the forces on the appendages of surface ships is important for understanding roll-damping and validating numerical simulations. In recent years, Atsavapranee et al (2007) showed that the bilge keel damping component can be modeled using the flat plate theory established by Keulegan and Carpenter (1958). This model treats the bilge keels as a flat plate that generates viscous damping, as well as added mass. The model comes as an improvement to models used in computational codes used for predicting roll damping, due to the fact that the added mass component is significant. In this study, uncoupled roll motion is investigated to quantify the rudder forces on a fully appended DTMB model #5415 with instrumented appendages at Froude numbers of 0 and 0.138. The objective of the current effort is to decompose the rudder force into its steady, symmetric, and antisymmetric components using Fourier analysis. In the force analysis the rudders are treated as flat plates for the Fr = 0 tests, using the model described by Keulegan and Carpenter (1958). The drag and lift forces are consistent with the flat plate model. The anti-symmetric term, however, does not show a clear trend. For a flat plate model, the anti-symmetric term should resemble a negative sine wave with respect to roll. However, the rudders represent a higher aspect ratio flat plate, and thus require a modification to the added mass formulation. Furthermore, during a normal roll period they tend to interact with the free surface, which can lead to wave damping, which should resemble a positive sine wave with respect to roll. Thus, the two components of the anti-symmetric portion of the signal are superimposed upon one another. In an attempt to decouple these two components, the added mass was artificially removed from the antisymmetric component of the force. This paper will detail the methods used to model the rudder forces for both the standstill and positive Froude number cases.