bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Weld Strength Mismatch on J and CTOD Estimation Procedures for SENT Fracture Specimens Based on Plastic Eta-Factors
Authors: Paredes Marcelo , Ruggieri Claudio
Published: November 2011

This work focuses on the evaluation procedure to determine the elastic-plastic fracture toughness J-integral and CTOD based upon the η-method for welded SENT fracture specimens with clamped ends. The primary objective is to gain additional understanding on the effect of weld strength mismatch on estimation techniques to determine J and CTOD fracture parameters for a wide range of a/W-ratios and mismatch levels. Very detailed non-linear finite element analyses for plane-strain and 3-D models of SENT fracture specimens with center cracked, square groove welds provide the evolution of load with increased crack mouth opening displacement and load line displacement which are required for the estimation procedure. The results show that levels of weld strength mismatch within the range 10∼20% mismatch do not affect significantly J and CTOD estimation expressions applicable to homogeneous materials. The present analyses, when taken together with previous studies, provide an extensive body of results which serve to determine parameters J and CTOD for different materials using SENT specimens with varying geometries and mismatch levels.
Effect of Low Cycle Fatigue Damage on Ultimate Strength of Aged Ship Structures
Authors: Tian Yu , Ji Zhuo-shang
Published: November 2011

The main objective of this paper is to study the effects of general low cycle fatigue (LCF) damage on ultimate strength of unstiffened ship plates under uniaxial compression. A nonlinear LCF damage model has been derived on basis of continuum damage mechanics (CDM) coupled with plastic mechanics in order to define the deterioration of materials. The theoretical results show that the damage variable D can be measured by a simple procedure and it can be related to the mechanical property of material directly. Formulae predicting ultimate strength reduction have been proposed based on amended Faulkner’s simple and useful expression. Nonlinear finite element analyses (FEA) of panels with various material damages have been carried out. The results indicate that the deterioration of materials is important factors determining the amount of strength reduction. It was also found that the proposed formulae can accurately predict the residual ultimate strength of unstiffened plate with general fatigue damage.
Spudcan Penetration in Clay-Sand-Clay Soils
Authors: Yu Long , Zhou Hui , Gao Wen , Liu Jun , Hu Yuxia
Published: November 2011

Multi-layered soil conditions often exist in offshore practice. In some sites a thin layer of medium dense sand lays between firm to stiff clay layers. In these cases the ultimate bearing capacity of foundations can be increased due to the strong sand layer by comparing with foundations in uniform clay. However, there is also a potential of reduction in foundation capacity when the foundation punches through the sand layer. The punch-through failure can occur during either pre-loading or storm loading. In this study, the continuous penetration of spudcan foundations on clay-sand-clay soils was investigated by large deformation finite element analysis. The numerical simulation was carried out using Remeshing and Interpolation Technique with Small Strain (RITSS) model. The clays obey Tresca failure criterion for undrained analysis and the sands obey Mohr-Coulomb yield criterion for drained analysis. The friction angle of the sand was taken as φ = 32° and 40° with its dilation angle ψ = 2° and 10° respectively. The effects of the relative height of the top soft clay and the relative thickness of the middle sand layer on the load-displacement responses were investigated. The soil flow mechanisms at various penetration depths were also discussed.
Improvements of Beam Structural Modelling in Hydroelasticity of Ultra Large Container Ships
Authors: Senjanovic´ Ivo , Vladimir Nikola , Malenica Sˇime , Tomic´ Marko
Published: November 2011

Increase in global ship transport induces building of Ultra Large Container Ships (ULCS), which have a capacity up to 14000 TEU with length up to 400 m, without changes of the operational requirements (speed around 27 knots). Natural frequencies of such ships can fall into the range of encounter frequencies in an ordinary sea spectrum. Present Classification Rules for ship design and construction don’t cover such conditions completely and hydroelastic analysis of ULCS seems to be the appropriate solution for analysis of their response in waves. This paper deals with numerical procedure for ship hydroelastic analysis with particular emphasis on improvements of the present beam structural model. The structural model represents a constitutive part of hydroelastic mathematical model and generally it can be formulated either as 1D FEM or 3D FEM model. For the preliminary design stage hydroelastic model derived by coupling 1D FEM structural model and 3D BEM hydrodynamic one seems to be an appropriate choice. Within the paper the importance of hydroelastic approach and methodology of hydroelastic analysis are elaborated. Further on, structural model based on advanced beam theory is described in details. The improvements include taking into account shear influence on torsion, contribution of bulkheads to hull stiffness as well as determination of effective stiffness of engine room structure. Along with that, hydrodynamic and hydrostatic models are presented in a condensed form. Numerical example, which includes complete hydroelastic analysis of a large container ship, is also added. In this case, validation of 1D FEM model is checked by correlation analysis with the vibration response of the fine 3D FEM model. The procedure related to determination of engine room effective stiffness is checked by 3D FEM analysis of ship-like pontoon which has been made according to the considered ship characteristics.
Material Issues in Ageing and Life Extension
Authors: Ho¨rnlund Erik , Ersdal Gerhard , Hinderaker Rolf H. , Johnsen Roy , Sharp John
Published: November 2011

A considerable number of the structures in the Norwegian part of the North Sea have passed or are close to their design life. Material degradation will play an important role in the ageing of these structures and the evaluation of their safety. An overview of research work initiated by the Petroleum Safety Authority (PSA) is presented. The paper focuses on various material aspects of ageing related to offshore facilities, the risks they represent to the integrity of a facility and how to deal with them in a life extension process. The paper presents and discusses expectations towards the industry with respect to evaluation of ageing materials in life extension.
Effect of Whipping on Fatigue and Extreme Loading of a 13000TEU Container Vessel in Bow Quartering Seas Based on Model Tests
Authors: Storhaug Gaute , Derbanne Quentin , Choi Byung-Ki , Moan Torgeir , Hermundstad Ole Andreas
Published: November 2011

Many large and ultra large container vessels have entered operation lately and more vessels will enter operation in the coming years. The operational experience is limited and one of the concerns is the additional effect of hull girder vibrations especially from whipping (bow impacts), but also from springing (resonance). Whipping contributes both to increased fatigue and extreme loading, while springing does mainly contribute to increased fatigue loading. MAIB recommended the industry to join forces to investigate the effect of whipping after MSC Napoli, a Post-Panamax container vessel, broke in two in January 2007. This has been followed up by a JIP initiated in 2008 with the following participants: HHI, DNV, BV, CeSOS and Marintek. In 2009 a new design 13000TEU vessel was tested in head seas and reported in [1]. The current paper deals with fatigue and extreme loading of the same vessel, but from realistic quartering sea conditions tested in 2010. Different headings and the effect of wave energy spreading have been investigated and compared to results from head seas. Further, the effect of the vibrations have been investigated on torsion and horizontal bending, as the model is also allowed to vibrate with realistic frequencies in other modes in addition to vertical bending. The findings suggest that changing the course is not effective to reduce the fatigue loading of critical fatigue sensitive details amidships. The effect of wave energy spreading did also not reduce the fatigue loading significantly. For the highest observed vertical bending moments in each sea state and for the three cross sections the wave energy spreading in average reduced the maxima, but for the highest sea state the effect of wave spreading did not consistently give reduced maxima. This is an important aspect when considering the available safety margin that may be reduced by whipping. The whipping gave also a considerable contribution to horizontal bending and torsion. This suggests that validation of numerical tools is urgent with respect to off head sea conditions and that these tools must incorporate the real structural behavior to confirm the importance of the response from torsional and horizontal as well as for vertical vibrations.
Feasibility Study of a Wave Energy Farm in the Western Mediterranean Sea: Comparison Among Different Technologies
Authors: Archetti Renata , Bozzi Silvia , Passoni Giuseppe
Published: November 2011

The cost of the energy extracted by ocean surface waves depends on many technological-physical factors like conversion principle, wave energy potential, water depth, shore distance, and ocean floor characteristics. Economic factors, like assumed discount rate, cost reduction due to technological improvement and fiscal incentives, are extremely important too. The Mediterranean Sea and the Tyrrhenian Sea in particular, possess an interesting energy potential. At some locations offshore Sardinia and Sicily the wave energy density is of the order of 10kW/m. In this paper the energy production has been computed for three different wave energy converters (AquaBuOY AWS and Pelamis), at two promising locations in the western Italian seas.. The energy cost has also been computed for the Pelamis device at Alghero site which has the highest wave energy potential. The results indicate that the existing Pelamis converter is oversized with respect to the typical wavelength of the investigated sites After proper scaling of the device, the performance in the energy conversion could become economically attractive also for Italian seas.
Synthesis and Optimization of Submarine Pipeline Routes Considering On-Bottom Stability Criteria
Authors: de Lima Mauro Henrique Alves , Baioco Juliana Souza , Albrecht Carl Horst , de Lima Beatriz Souza Leite Pires , Jacob Breno Pinheiro , Rocha Djalene Maria , Cardoso Carlos de Oliveira
Published: November 2011

Researchers from Petrobras and LAMCSO/COPPE/UFRJ are currently involved in the development and implementation of a computational tool, based in Evolutionary Algorithms, for the synthesis and optimization of submarine pipeline routes. In this tool, randomly generated candidate routes are evaluated in terms of several criteria, incorporated in an objective (or fitness) function to take into account the relevant aspects that should be considered in the design of a route. A previous work [1] described the initial steps taken towards the development of such tool. In that work, attention was dedicated to the geometrical representation of a route, and to some of the terms of the objective function associated with a preliminary, global step of the optimization process (such as total pipeline length, and geographical-topographical issues associated with the route geometry and to the seabottom bathymetry and obstacles). Now, this work focuses in other aspects related to the structural behavior of the pipe, under hydrostatic and environmental loadings; more specifically, special attention is dedicated to the implementation of On-Bottom Stability (OBS) criteria such as the proposed in the RP-F109 code [2]. Case studies are presented to illustrate the use of the optimization tool and to assess the influence of the OBS criteria.
Pipeline On-Bottom Stability Analysis Based on FEM Model
Authors: Bai Yong , Yu Zhimeng
Published: November 2011

Pipeline on-bottom stability is one of the sophisticated problems in subsea pipeline design procedure. Due to the uncertainty of the pipe-soil interaction and environment loads, including wave, current, or earthquake, etc., it is classified as the typical nonlinear problem. The Finite Element Method is introduced into pipeline engineering several years ago. More and more special engineering software such as AGA, PONDUS are available in market. However, when doing a project, some abnormal data was found when compared the DnV calculation results and AGA. In order to know the behavior of pipeline on seabed under wave and current load, finite element method – ABAQUS is introduced to do this analysis. The ABAQUS/explicit is used to simulate 600s pipeline dynamic response. The pipeline is supposed to be exposed on seabed and the selected seabed model is large enough to avoid the edge effect. ABAQUS calculation results are compared with the requirements in DnV rules to verify the validity of finite element model.
On-Board Monitoring, Analysis and Decision Support During Offshore Pipe Lay Operation
Authors: Taby Joakim , O̸kland Ole David , Giertsen Egil , Ye Naiquan , Morgan Melvyn , Holden Ole Magnus
Published: November 2011

For pipe lay operations parameters like heading and position of the lay vessel are available from the GPS system on the vessel. In some cases the current profile may also be monitored. Provided that a fast and robust numerical tool is available and can be integrated with the on-board system, the above parameters and a measure of tension in the pipe will be sufficient to simulate the appurtenant static configuration of the catenary and visualize it online during the lay operation. Such a system will provide valuable additional information such as: catenary shape including stresses in the pipe, lay tension, development of free spans, and lateral stability during installation. In the period April-August 2009 two field tests of the developed system were performed. Both field tests were related to Acergy installation campaigns for the Ormen Lange Phase II development in the North Sea; 1) ∼13 km infield MEG (IMEGDA) installation by Acergy Falcon, and 2) ∼13km infield umbilical (IUMB-DA) installation by Acergy Eagle (both at ∼860m water depth). In this paper the experiences from the field tests are discussed. This includes assessment of on-board data collection and storage of key parameters during installation, real-time online 3D finite element (FE) analysis and satellite link to shore for support during installation. The accuracy of the results obtained in the on-board monitoring system is assessed by comparison to available as-laid survey information.
Improved Dynamic Positioning Using Wave Feed Forward
Authors: Quadvlieg Frans , Hallmann Rink , Hughes Greg , Harris Rick
Published: November 2011

Jo Pinkster made the first attempt to estimate 2nd order wave drift forces. In his PhD thesis from 1980, the first practical application of wave feed forward in DP was demonstrated both theoretically and in model tests. Knowledge of the local wave field was used to estimate the 2nd order wave drift forces. The local wave field was converted in wave forces and fed back in the DP system. The use of this knowledge in a DP system should lead to a better position keeping. Since Pinksters’ thesis 30 years ago, this technique has been tried several times with varying success. However, in 2009, the ‘nut was cracked’ and a good success was undoubtedly demonstrated. In 2008–2009, this method has been developed and applied in DP model tests on a ship equipped with azimuthing thrusters. The use of Wave Feed Forward resulted in a reduction of the watch circle by a factor of two. Important for the success of wave feed forward was the filtering of the measured wave signals to predict the wave forces with a limited delay. The performance is demonstrated during model tests in MARIN’s Seakeeping and Manoeuvring Basin at two speeds of 0 knots and 4 knots, uni-directional and multidirectional seas. Besides the application of wave feed forward for a single ship, wave feed forward is used in a side-by-side condition at zero speed and ahead speed. For both speeds, wave feed forward did not provide a significant improvement in DP accuracy. The objective was to make wave feed forward applicable to: zero and forward speed; on a ship alone and on ships sailing side-by-side; in unidirectional and multi-directional waves, with a realistic amount of sensors and as target wave heights, sea state 3 and 4 were envisaged. To measure the local wave height, wave height measurement sensors as well as pressure sensors were used. The pressure sensors can be mounted below the waterline and deliver an accurate estimation for the wave drift forces as well.
Bayesian Networks on Risk Analysis of a Regasification System on an Offshore Unit
Authors: Schleder Adriana Miralles , Martins Marcelo Ramos , Souza Gilberto Francisco Martha
Published: November 2011

Nowadays, LNG Import Terminals (where the storage and regasification process is conducted) are mostly onshore; the construction of these terminals is costly and many adaptations are necessary to abide by environmental and safety laws. Moreover, an accident in one of these plants might produce considerable impact in neighboring areas and population; this risk may be even worse due to the possibility of a terrorist attack. Under this perspective, a discussion is conducted about a vessel known as FSRU (Floating Storage and Regasification Unit), which is a storage and regasification offshore unit, that can work miles away from de coast and, owing to this, can be viewed as an option for LNG storage and regasification facilities. The goal is to develop a method for using Bayesian Networks in the Risk Analysis of Regasification System of the FSRU, which will convert Fault Trees (FT) into Bayesian Networks (BN) providing more accurate data. Using BN is possible to represent uncertain knowledge and local conditional dependencies. In addition, FT models the failure modes as independent and binary events while BN may model a larger number of states. It is worth noting that BN does not require the determination of cut sets; however, given a failure, it is capable of providing the probability of each possible cut set. This method will provide information to define, in a future study, a maintenance plan based on the Reliability Centered Maintenance. The results intend to clarify the applicability of BN on risk assessment and might improve the risk analysis of a Regasification System FSRU.
Faecal Pellets in Deep Marine Soft Clay Crusts: Implications for Hot-Oil Pipeline Design
Authors: Kuo Matthew Y-H. , Bolton Malcolm D.
Published: November 2011

In recent years, the presence of crusts within near surface sediments found in deep water locations off the west coast of Angola has been of interest to hot-oil pipeline designers. The origin for these crusts is considered to be of biological origin, based on the observation of thousands of faecal pellets in natural crust core samples. This paper presents the results of laboratory tests undertaken on natural and faecal pellet-only samples. These tests investigate the role faecal pellets play in modifying the gemechanical behaviour of clayey sediments. It is found that faecal pellets are able to significantly alter both the strength and the average grain-size of natural sediments, and therefore, influence the permeability and stiffness. Hot-oil pipelines self-embed into and subsequent shear on crusts containing faecal pellets. Being able to predict the time required for installed pipelines to consolidate the underlying sediment and thus, how soon after pipe-laying, the interface strength will develop is of great interest to pipeline designers. It is concluded from wet-sieving samples before and after oedometer tests, that the process of pipe laying is unlikely to destroy pellets. They will therefore, be a major constituent of the sediment subject to soil-pipeline shearing behaviour during axial pipe-walking and lateral buckling. Based on the presented results, a discussion highlighting the key implications for pipeline design is therefore provided.
Correction for Bias in Return Values of Wave Heights Caused by Hindcast Uncertainty
Authors: Mackay Ed
Published: November 2011

Hindcasts of wave conditions can be subject to large uncertainties, especially in storms. Even if estimates of extremes are unbiased on average, the variance of the errors can lead to a bias in estimates of extremes derived from hindcast data. The convolution of the error distribution and wave height distribution causes a stretching of the measured distribution. This can lead to substantial positive biases in estimates of return values. An iterative deconvolution procedure is proposed to estimate the size of the bias, based on the measured distribution and knowledge of the error distribution. The effectiveness of the procedure is illustrated in several case studies using Monte Carlo simulation.
CFD Analysis of Deck Impact in Irregular Waves: Wave Representation by Transient Wave Groups
Authors: Lande O̸ystein , Johannessen Thomas B.
Published: November 2011

Analysis of wave structure interaction problems are increasingly handled by employing CFD methods such as the well known Volume-of-Fluid (VoF) method. In particular for the problem of deck impact on fixed structures with slender substructures, CFD methods have been used extensively in the last few years. For this case, the initial conditions have usually been treated as regular waves in an undisturbed wave field which may be given accurately as input. As CFD analyses become more widely available and are used for more complex problems it is also necessary to consider the problem of irregular waves in a CFD context. Irregular waves provide a closer description of the sea surface than regular waves and are also the chief source of statistical variability in the wave induced loading level. In general, it is not feasible to run a long simulation of an irregular seastate in a CFD analysis today since this would require very long simulation times and also a very large computational domain and sophisticated absorbing boundary conditions to avoid build-up of reflections in the domain. The present paper is concerned with the use of a single transient wave group to represent a large event in an irregular wave group. It is well known that the autocovariance function of the wave spectrum is proportional to the mean shape of a large wave in a Gaussian wave field. The transient nature of such a wave ensures that a relatively small wave is generated at the upwave boundary and dissipated at the downwave boundary compared with the wave in the centre of the domain. Furthermore, a transient wave may be embedded in a random background if it is believed that the random background is important for the load level. The present paper describes the method of generating transient wave groups in a CFD analysis of wave in deck impact. The evolution of transient wave groups is first studied and compared with experimental measurements in order to verify that nonlinear transient waves can be calculated accurately using the present CFD code. Vertical wave induced loads on a large deck is then investigated for different undisturbed wave velocities and deck inundations.
Study of Failure Modes of Suction Anchors
Authors: Zhang Qiyi , Dong Sheng
Published: November 2011

Suction foundations are widely used in deep sea and their ultimate bearing capacity which is closely related with failure modes of suction anchor at limit equilibrium state is a key technology in offshore engineering practice. Based on Coulomb friction theory, an exact finite element model is presented in this paper. On the basis of this FEM model, by use of the finite element analysis software ABAQUS, the effect of mooring point and aspect ratio of a suction anchor on the ultimate bearing capacity and its stability are researched in detail. The results show that the ultimate bearing capacity and stability of the suction anchor are affected vastly by the position of mooring point, and the variation of mooring point on the suction anchor can lead to different failure modes. Simultaneously, the results also shows that tilted rotation of the soil along the direction of the mooring force will occur when the mooring point is near the top of the suction anchor, and the soil near the bottom of the fixed anchor rotates around the center of a circle, so the failure mode is called forward-tilted rotation in this paper; A general translation slip of the soil in front of the anchor along the direction of the mooring force will occur when mooring point is below midpoint of suction anchor, so the failure mode is called the translation slip failure mode in this paper. Anticlockwise tilted rotation of the soil along the direction of mooting force will occur when the mooring point is near the bottom of the anchor, and the soil at the top of the anchor rotates around the center of a circle, so the failure mode is called backward-tilted rotation in this paper.
Inline and Vertical Wave Force Variation due to Burial of Submarine Pipeline in Random Wave Fields
Authors: Neelamani S. , Al-Banaa K.
Published: November 2011

Marine pipelines encounter significant dynamic forces due to the action of waves. In order to reduce such forces, they are buried below the seabed. The wave force on the pipeline at any depth of burial for the given hydrodynamic condition depends on the properties of the sea bed soil. Physical model is used for assessing the hydrodynamic force on the pipeline for a wide range of random wave conditions, for different burial depths and in four types of soils. It is found that for all the four soil types, the horizontal force reduces with increase in depth of burial, whereas the vertical force generally increases up to certain depth of burial, mainly due to the significant change in the magnitude as well as the phase lag between the pore water pressures in the vertical direction. Among the soils, well graded soil is good for half burial of pipeline, since the least vertical force occurs for this soil. On the other hand, uniformly graded and low hydraulic conductivity soil attracts the maximum vertical force for half burial. On the other hand, such soil is good for full burial or further increase of burial, since it attracts less vertical force when compared to the other soils. The results of this study will help the submarine pipeline design engineers to select the minimum safe burial depth in a range of cohesion-less soil.
Susceptibility to Lateral Buckling: What Hobbs Equations for What Flowlines
Authors: Le Maouˆt Yann , Brunet Philippe
Published: November 2011

The Hobbs equations have been used for years to evaluate the susceptibility of flowlines to lateral buckling. However these equations are sometimes used without a clear understanding of the driving mechanism and for some flowlines configurations care shall be taken for a direct application of these formulas. In particular, the infinite mode buckle shape, although widely considered in the industry, does not reflect the lateral buckling mechanism. Through a thorough review of the assumptions and validity of the Hobbs equations, this paper presents adequate formulations to evaluate the susceptibility to lateral buckling for specific flowlines configurations. The cases of short flowlines, medium flowlines and unbounded pipe in pipe systems are presented. The methodology described in Hobbs paper is analyzed in detail and the bases of the equations are rewritten for short and medium flowlines. The pipe length in the feed-in zone before and after buckle is calculated based on the strain profiles in the pipes. Then the critical buckling force is deduced by the application of an equation of continuity of displacements. For unbounded pipe in pipe the same principle is used with the addition of an uncoupled behaviour between the axial and lateral components and the share of axial force at the bulkhead location. The contribution of the centralizers is also incorporated in the formulation. Eventually, the criteria presented in the paper are compared with a more realistic probabilistic approach to evaluate their degree of conservatism.
Numerical Investigation of the Effect of a Pier’s Shape on the Mitigation of Scouring
Authors: Givi M. Hamed Besharati , Madihi Mohammad Saeid , Hakimzadeh Habib
Published: November 2011

Nowadays, scouring has become an important issue in coastal engineering. Exposed to a flow, piers of different structures will continually be in danger of scouring. Thus, design of cross-sections for piers in order to produce the least turbulence in the flow passing the pier is a significant concern from hydrodynamical and structural points of view, in so far as the status quo studies concerning optimum pier cross-sections have engaged a major portion of civil engineering researches. In this study, a comparison has been made between the flat-bed scouring pattern in the vicinity of cylindrical and conical piers by investigating the flow pattern around them. Thus, 3D numerical models of flow around a cylindrical pier and conical piers with a same bed diameter as the cylindrical pier and ratios of diameter in the water surface to bed diameters of 0.75, 0.5, 0.375 and 0.25 has been developed, with the models being meshed in GAMBIT and analyzed in FLUENT. The 3D numerical model results of flow pattern around the cylindrical pier were then compared to the experimental data available. These results and data have been used to verify the results of the 3D numerical models of the present study. Consequently, the 3D numerical models of the flow around four conical piers with different slopes have been analyzed and achieved using the FLUENT software, in which specifications of the channel, flow and the software parameters have been set to be same in all models.
Consistent VIV Fatigue Analysis Methodology of Umbilicals
Authors: So̸dahl Nils , Steinkjer Oddrun , Gjo̸lmesli Elisabeth , Hansen-Zahl Kay
Published: November 2011

The fatigue life assessment of umbilicals due to vortex induced vibrations (VIV) requires special attention to the structural properties of the umbilical due to the stick/slip behavior of helix elements in bending. Essential parameters for assessment of the VIV response are the structural damping in the stick regime (i.e. umbilical behaves as a solid cross section due to friction between the helix elements) as well as the additional damping introduced by the hysteretic damping due to the stick/slip behavior in bending. Furthermore, consistent fatigue stress recovery considering the stick/slip behavior in bending is essential for fatigue life predictions. The expected VIV response characteristics of different types of umbilicals (steel tube umbilicals, control umilicals and power umbilicals) are discussed with basis of typical mechanical properties of the different types. It is shown that these main types of umbilicals may experience distinctive different VIV response characteristics underlining the need for a consistent VIV fatigue analysis scheme. The overall objective of the paper is to establish consistent VIV analysis methodology for umbilicals in compliance with requirements given in ISO 13628-5 ‘Subsea Umbilicals’. A consistent VIV fatigue analysis scheme is outlined using commercially available state-of-the-art computer programs for global system modeling/eigenmode analysis (Riflex), VIV response analysis (Shear7) and cross sectional stress analysis (Helica). The performance of the calculation scheme is demonstrated by case studies of deepwater umbilicals in a complex long-term current loading environment. It is show that consistent treatment of the mechanical properties of umbilicals is essential for VIV fatigue life assessments. It is furthermore shown that the VIV fatigue damage in umbilicals may be significant underlining, the practical importance in design of deepwater dynamic umbilicals.
The Effect of Sloshing in Tanks on Motions and Hull Girder Responses of Damaged Vessels
Authors: Jia Huirong , Moan Torgeir
Published: November 2011

The structural reliability analysis of damaged vessels has up to now commonly been investigated by neglecting the effect of sloshing. This paper deals with the effect of sloshing in tanks on motions and hull girder responses of oil tankers in various damage conditions and represents a part of a study to assess the effect of sloshing on hull girder failure of damaged vessels, The flooded tanks are assumed to have a of rectangular shape and linear multimodal approach is adopted to deal with sloshing. It is concluded that even though the effect of sloshing in tanks on the roll motion of vessels can be neglected in certain damage conditions, the effect of sloshing on the horizontal bending moment cannot be neglected, especially when resonance motion occurs.
Buckling of Clad Pipes Under Bending and External Pressure
Authors: Vasilikis Daniel , Karamanos Spyros A.
Published: November 2011

The present paper concerns the structural behaviour of clad pipes. This is a double wall pipe, composed by two pipes that are in contact through an appropriate manufacturing procedure; a thick-walled carbon steel “outer pipe”, and a thin-walled corrosion-resistant inner pipe, referred to as “liner” pipe. To predict the bending response and the buckling curvature of the thin-walled liner, it is necessary to account for its contact with the confining thick-walled outer pipe. Because of this confinement, existing numerical solutions or analytical predictions for the bending buckling resistance of unconfined thin-walled tubes are inadequate to predict the buckling resistance of the bent liner. In the present work, the problem is solved numerically, using nonlinear finite elements to simulate the clad pipe, accounting for the interaction between the liner and the outer pipe. First, the manufacturing process of the clad pipe is simulated to determine the liner hoop prestressing. Subsequently, bending curvature is applied (with and without the presence of external pressure). Stresses and strains are monitored throughout the deformation stage with emphasis on possible detachment of the liner from the outer pipe and the formation of local buckling on the liner wall.
Tsunami Bore Forces on Walls
Authors: Robertson I. N. , Paczkowski K. , Riggs H. R. , Mohamed A.
Published: November 2011

A series of experiments have been carried out at Oregon State University to quantify tsunami bore forces on structures. Phase 1 of the tests was carried out in the Tsunami Wave Basin (TWB), while Phase II of the tests were carried out in the Large Wave Flume (LWF) at approximately twice the scale of the Phase I tests. These latter tests included ‘offshore’ solitary waves, with heights up to 1.3 m, that traveled over a flat bottom, up a sloping beach and breaking onto a flat ‘fringing reef’. Standing water depths on the reef varied from 0.05 m to 0.3 m. Resulting bores on the reef measured up to approximately 0.8 m. After propagating along the reef, the bores struck a vertical wall. The resulting forces and pressures on the wall were measured. The test setup for the Phase II tests in the LWF is described and the experimental results are reported. The results include forces and pressure distributions. Results show that the bores propagated with a Froude number of approximately 2, and that the forces follow Froude scaling. Finally, a design formula for the maximum impact force is given. The formula is shown to be an improvement over existing formulas found in the literature. The lateral forces are shown to be quite significant compared to traditional lateral loads on vertical wall elements.
A Ship Flotation Control System Based on Pump-Driven Tank
Authors: Guo Juan , Tang Meng , Zou Zaojian
Published: November 2011

Extensive development in ship motion control strategies and systems in recent decades has called for higher requirements in control system accuracy and reliability. In this paper, a ship flotation control system based on pump-driven active tank is established. A special case is discussed, where the ship is heeling under an asymmetric loading either by structural damage or asymmetric consumption of ammunition. The purpose of the control system is to keep the ship in upright floating position in waves by transferring liquid between the tanks. Kalman filter is designed to eliminate the wave disturbance, in order to identify the heeling angle caused by asymmetric loading change. The effect of wave disturbance at different wave encounter angles, wave heights, as well as ship speeds is analyzed. Tuning of filter parameters such as initial state variables, initial error covariance and noise covariance is performed to achieve better filtering performance for different parameters of waves and ship motion. To verify the control model, simulation is conducted for a 3340t ship and the simulation results are compared with the theoretical calculations. The research results show the applicability and efficiency of Kalman filter. The concept of the control system presented in the paper is helpful to improve ship stability and safety when ship upright floating condition is disturbed.
Wave Slam/Slap Loads on Structural Members in the Air-Gap
Authors: Sekhar Gopu R. , Nallayarasu S.
Published: November 2011

The offshore platforms shall be designed with deck elevation that maintains sufficient air-gap with maximum predicted wave crest elevations. However, due to compelling reasons of hydrocarbon process and changes to water levels/seabed subsidence may have structures in the close vicinity of the sea level or submerged. Hence the assessment of wave induced forces on the deck structures especially the old platforms become very essential. Review of literature indicates that the previous studies focused mainly on the circular sections and not much information is available on the non-circular sections. The current study focused on measurement of wave induced slam/slap loads in a laboratory wave flume and estimation of force coefficients based on drag based empirical equations. Further, it is of interest to focus on the flow around the open sections (non-circular) using a flow visualization technique (Particle Image Velocimetry) to understand the flow pattern around the elements. The results indicate that the slam and slap coefficients for I sections are smaller than the channel sections and the values of coefficients are few times higher than the circular sections.
Crushing of Flexible Pipes Under Traction: A Theoretical-Experimental Assessment
Authors: Franzini Guilherme R. , Pesce Celso P. , Takafuji Fernanda C. M. , Gonc¸alves Rodolfo T. , Tanaka Rafael , Silva Marcelo , Barbosa Teo´filo , Godinho Carlos A.
Published: November 2011

The paper presents a theoretical-experimental comparison concerning standard crushing-traction tests of flexible pipe prototypes. The theoretical model for crushing is analytical and based on classic assumptions of equivalent pipes, applying model previously published in OMAE2003, and OMAE2010. Such a model considers the combined action of squeezing, concentrated loads due to the caterpillar shoes as well as the effect of initial ovalization. The experimental measures include a detailed internal geometrical mapping of the deformed carcass, until plastic deformation becomes evident. Discussion is made on the pertinence of modeling hypotheses. Sensitivity analyses, regarding initial ovalization and helical pitch of the pressure armor are also addressed.
Numerical Simulation of the Backfilling Process of a Trench Using a Trailing Suction Hopper Dredge
Authors: van Rhee C.
Published: November 2011

Deposition of sediment by means of discharging via the bottom doors of a Trailing Suction Hopper Dredge is a method often used. This method can be deployed for land reclamation and backfilling of trenches for pipelines or outfalls. The sediment will leave the hopper as a mixture of sand and water. Sand will settle from this mixture. The shape of the under water body is governed by the dynamic settling process from a (sometimes highly concentrated) density current. Slopes angles and density of the sand body and possible segregation is depending on the discharge production, sediment characteristics, water depth and ambient current. In the paper the discharge process is simulated using a 2 dimensional numerical model based on the Reynolds Averaged Navier-Stokes equations. In this model the momentum and sediment transport equations are solved. The model includes a morphological module therefore slopes angles can be predicted. The computational results are validated using laboratory tests. The efficiency of the backfilling process, expressed as the ratio between the volumes of settled sediment in the trench and the discharged sediment, is determined as a function of the domination process parameters.
Accidents in Situations With Large Stability Values
Authors: Kru¨ger Stefan , Steinbach Christoph Michael , Rox Nicolas , Kaufmann Jo¨rg , John Frenec
Published: November 2011

In the last two years a number of accidents with several dead or heavily injured crew members have been reported to the German Federal Bureau of Maritime Casualty Investigation caused by excessive roll motions with large transversal accelerations. All accidents happened on board of container vessels in situations of large stability and partial draft like in ballast or in laid-up situations. Also the accident situations were similar in all cases. During harsh weather conditions the crew tried to keep the ship against the incoming sea at constant slow speed. It was possible to reconstruct the accident events and the occurrence of high transversal acceleration values by numerical seakeeping simulations. To figure out if container vessels have an inherent danger of such problems a numerical study on a group of fifteen container vessels of different sizes was carried out in the mentioned accident situations. In this paper the reported accidents and the numerical investigation on the accidents are described, furthermore the results of the extended numerical study are presented and a phenomenological conclusion is drawn from the results.
Power Quality Performance of the Tidal Energy Converter, SeaGen
Authors: MacEnri Joseph , Reed Matthew , Thiringer Torbjo¨rn
Published: November 2011

SeaGen is an in-stream 1.2MW tidal energy converter (TEC) located in Strangford Lough, Northern Ireland that has been in almost continuous service since May 2009. This technical paper first outlines the characteristic features of the TEC itself and its power take-off system, followed by the electrical and marine environments of its location, and finally it presents the results of its power quality performance assessment. The PQ assessment was carried out using the existing European Norm EN50160 for assessing permissible voltage parameter variations on public electricity networks and the conclusions drawn are from data collected over a number of months of operation of the device.
HHT Analysis on Pore Pressure Dissipation of Wave-Induced Fluidization in a Sandy Bed
Authors: Tzang Shiaw-Yih , Chen Yung-Lung , Ou Shan-Hwei
Published: November 2011

Wave-induced pore pressure variations during the stage of increasing excess pore pressure consist of the mechanism of generation of fluidization. Moreover, in post-fluidization stage, pore pressure variations not only reveal the dissipation mechanism of fluidization but also the wave-fluidized bed interactions. Past results from a series of lab flume tests have further illustrated that pore pressure variations in a fluidized response are nonlinear and nonsataionary. Hence, the HHT method was further applied to analyze the pore pressure measurements in this study. The results demonstrate that after the dissipation of excess pore pressures the amplitudes of fundamental and higher-frequency components begin to decay. Meanwhile, the amplified amplitudes of fundamental and higher-frequency components during fluidization response would decrease with decreasing thickness of fluidized soil-layer in consecutive tests.
Equivalent Design Wave Approach for Calculating Site-Specific Environmental Loads on an FPSO
Authors: Sarala Resmi , Hajiarab Mohammad , Bamford Richard
Published: November 2011

This paper demonstrates the method used by Lloyd’s Register (LR) to derive an equivalent design wave from a response based analysis (RBA) to represent extreme loads on a weather-vaning FPSO [1] and proceeds to compare the results with that of the industry practice of the response amplitude operator (RAO) based approach. The responses investigated include roll, pitch, vertical wave bending moments, vertical wave shear forces and vertical acceleration. The RBA is based on 3 hourly hindcast metocean data and uses the results of the heading analysis directly, considering the combined effect of wind, wind-sea, current and swell. An equivalent design wave is then derived based on the spectral characteristics of each response instead of the common practice for ship design [2] which uses only the characteristics of the RAOs. For each response the design wave for the RBA and RAO approaches is compared. Deriving equivalent design waves using only the RAO characteristics is found to give some non-conservative and unrealistic equivalent design waves in some cases.
Fatigue Performance of Sour Deepwater Riser Welds: Crack Growth vs. Endurance
Authors: Holtam Colum M. , Baxter David P.
Published: November 2011

Steel catenary risers (SCRs) are increasingly used in deepwater oil and gas developments. SCRs can be subject to low-stress high-cycle fatigue loading, for example from wave and tidal motion, vortex induced vibration (VIV) and operating loads, and corrosive environments (internal and external). When the production fluids are sour, higher fatigue crack growth rates (FCGRs) are expected and therefore shorter overall life compared to performance in air, as a result of the interaction between fatigue crack growth and sulphide stress cracking. Successful design of risers is critically dependent on the availability of appropriate experimental data to quantify the extent to which fatigue lives are reduced and rates of fatigue crack growth are increased. Historically there has been a discrepancy between experimental sour fatigue endurance data and fracture mechanics-based estimates of the corresponding stress-life (S-N) curves. This paper summarises the results of recent sour FCGR tests on C-Mn pipeline steel. Tests were performed under conditions of increasing applied stress intensity factor range (ΔK), on specimens containing shallow initial flaws and at very high stress ratios (R), to obtain data close to threshold. In many cases it is material behaviour at these low values of ΔK that dominate the fatigue life (e.g. VIV loading). The FCGR data are then compared to sour fatigue endurance data, both published and from a TWI Joint Industry Project (JIP). The observed environmental reduction factor (ERF) for endurance tests is compared to that expected from the difference in fatigue crack propagation rates, to examine whether FCGR data might provide an alternative means of predicting ERFs. This paper offers valuable insight into current best practice methods for generating sour FCGR data when qualifying girth welds for sour service, and the relationship between fatigue crack growth and fatigue endurance.