bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Experiments Analysis of Wave-Induced Vertical Mixing’s Effects on Sea Surface Wind-Induced Momentum Transfer
Authors: Liang Bingchen , Liu Ying , Yang Lili
Published: January 2011

A hydrodynamic sediment coupled model COHERENS-SED, which has been developed by the present authors through introducing wave-enhanced bottom stress, wave dependent surface drag coefficient, wave-induced surface mixing, SWAN, damping function of sediment on turbulence and sediment model to COHERENS, is modified to account for wave-induced vertical mixing. One equation k–ε turbulence model is taken into account in calculating vertical viscosity coefficient. COHERENS-SED consists of sediment model SED, current model COHERENS and wave generation model SWAN. The model can also calculate one-dimensional, two-dimensional and three-dimensional current separately. One-dimensional model and three-dimensional model are adoptted to study the wave-induced vertical mixing’s effects. The horizontal current velocity profiles given by the model, with same input conditions as what to get analytical results, are in nice agreement with analytical velocity profiles. Therefore the model can be reliable to identify wave-induced vertical mixing’s effects on horizontal velocity profiles and momentum transferring. Two group numerical experiments are built based on 130m water depth and 20m water depth for the one-dimensional model. Results show that higher wave height can generate larger vertical eddy viscosity and lower horizontal velocity generally. In order to find out such effects on fresh water flume momentum transfer towards down in vertical section of estuary, Yellow River delta is chosen to study the effects of wave-induced vertical diffusion on sediment vertical mixing and the Yellow River estuary vertical cross-section is chosen to study fresh water disperse range in vertical section. The results of fresh water shows that wave-induced vertical mixing increases the momentum of fresh water transferring ability towards down to seabed. So fresh water flume length is compressed obviously.
Structural Safety Assessment Based on the Fatigue Analysis of the Ice-Resistant Jacket Platforms
Authors: Che Xiaofei , Zhang Dayong , Yue Qianjin
Published: January 2011

Most of the ice-resistant jacket platforms in Bohai Sea have been close to or reached their service life. In order to keep the platforms perform safely, a reasonable risk assessment of the existing platforms should be considered. The hazard of the structural fatigue failure under the ice-induced vibrations is so serious, which plays an important role in the safety of the platforms. In this paper, the general process of the structural safety assessment based on the fatigue analysis is established. Firstly, risk assessment of fatigue failure needs to determine whether the cyclic stress of hot-spot induced by ice-vibrations under normal ice condition is so significant. Then, the method of estimating ice-induced fatigue life of existing platforms is developed based on the field data and the safe life design method. Finally, as an application example, the safety assessment of one old jacket platform in Bohai Bay is carried on, which can provide a warrant for owners to repair or reject platforms.
Determining and Avoiding HAC on Pipeline Repair
Authors: Huismann Gerd , Woodward Neil , Armstrong Mike , Hoffmeister Hans , Knagenhjelm Hans Olav
Published: January 2011

Hydrogen Assisted Cracking (HAC) is influenced by the microstructure, deformation, and hydrogen concentration. Fusion welding changes all three influence factors. Tests have been made to solve the problem by FEM analysis. But practical conditions do incorporate many factors. Only very few of them can be steered directly. Therefore, a practical approach was chosen for identifying the critical situations. The first stage was determining the reaction of the construction on stresses which are induced by shrinkage of the weld zone after joining. A mockup was set up for this purpose. The intensity of restraint became very high for the last part of the bead, where the joint between the two workpieces became nearly complete. About 11kN/(mm*mm) were measured. Test welds were done on the mockup under this restraint. 50% of the welds showed large cracks. To become more flexible and less expensive, such high restrained conditions were set up in an IRC device (Instrumented Restraint Cracking). The workpieces are clamped in the IRC device into a stiff frame. The frame becomes closed by the test weld. The use of such a device gives the chance of exploring the different influence factors under similar restraint conditions as on the original setup, but much easier and with the record of shrinkage force and moments. The tests showed, that a high possibility for forming a crack is present, when no pre- and postweld heat treatment was done. Cracking took place in the root bead, especially when the first bead gave a connection to the other workpiece. The cracks were formed in the time range 15min to 3h. Under high restraint it becomes obvious that even low hydrogen levels offer a high chance of receiving HAC. Therefore, methods for avoiding critical conditions were looked on. Preheating reduces the cooling speed. Lower hardness can be received. Unfortunately, the preheating expands the workpieces and after cooling down higher stresses are present (influence from the restraint). Therefore, preheating gave more HAC. For avoiding high stresses during the critical times, a combined pre- and postweld heat treatment was done. Even with low temperatures HAC was avoided easily. The sequence of analysing the mechanical conditions, transfer them into a test device, and optimisation of the welding procedure gave the possibility to study the HAC behaviour. The knowledge of the mechanical behaviour offered the solution for avoiding cracking.
Load Characteristics of Steel and Concrete Tubular Members Under Jet Fire: An Experimental and Numerical Study
Authors: Park Jeong Hyo , Kim Bong Ju , Seo Jung Kwan , Jeong Jae Sung , Oh Byung Keun , Kim Sung Hoon , Park Chang Hee , Paik Jeom Kee , Czujko Jurek
Published: January 2011

The aim of this study was to evaluate the load characteristics of steel and concrete tubular members under jet fire, with the motivation to investigate the jet fire load characteristics in FPSO topsides. This paper is part of Phase II of the joint industry project on explosion and fire engineering of FPSOs (EFEF JIP) [1]. To obtain reliable load values, jet fire tests were carried out in parallel with a numerical study. Computational fluid dynamics (CFD) simulation was used to set up an adiabatic wall boundary condition for the jet fire to model the heat transfer mechanism. A concrete tubular member was tested under the assumption that there is no conduction effect from jet fire. A steel tubular member was tested and considered to transfer heat through conduction, convection, and radiation. The temperature distribution, or heat load, was analyzed at specific locations on each type of member. ANSYS CFX [2] and Kameleon FireEx [3] codes were used to obtain similar fire action in the numerical and experimental methods. The results of this study will provide a useful database to determine design values related to jet fire.
Experimental Study on Regular Wave-Induced Scour and Settlement of a Submerged Obstacle Upon a Fluidized Sandbed
Authors: Tzang Shiaw-Yih , Chen Yung-Lung , Ou Shan-Hwei
Published: January 2011

Bed scouring and structure settlement due to interactions of regular waves and a submerged rectangular obstacle in the shallow water regions upon a fluidized fine sandy bed are experimentally investigated. Both impervious and pervious structures are installed on the test soil bed and synchronous measurements of water surface, bed surface, suspended sediment concentration and pore pressures are carried out at locations upstream and downstream. The measurements illustrate that interactions between waves and structures have relatively mild affects on the bed scouring in an unfluidized response. The interactions become more intense in the fluidized responses with significant wave decay over and increasing suspended sediments near above the fluidized beds oscillating with similar period to the loading waves. Meanwhile, the submerged structure starts to settle into the fluidized bed and significant scouring occurs. The interactions are typically greater in a RF test than in a NRF test. More than 90% of the total settlement occurs in a single event of the RF response so that the impervious and pervious structures become half buried and totally buried after the event. The settlements further affect the interactions of following loading waves and the fluidized beds. As a result, scouring on both sides of the half buried impervious obstacle is greater than those of the totally buried pervious structure.
Dynamic Stability Response of Piggyback Pipelines
Authors: Zeitoun Hammam , Brankovic´ Masˇa , To̸rnes Knut , Wong Simon , Hollingsworth Eve , Pearce Andrew , Sabavala Hosi
Published: January 2011

One of the main aspects of subsea pipeline design is ensuring pipeline stability on the seabed under the action of hydrodynamic loads. Hydrodynamic loads acting on Piggyback Pipeline Systems have traditionally been determined by pipeline engineers using an ‘equivalent pipeline diameter’ approach. The approach is simple and assumes that hydrodynamic loads on the Piggyback Pipeline System are equal to the loads on a single pipeline with diameter equal to the projected height of the piggyback bundle (the sum of the large diameter pipeline, small diameter pipeline and gap between the pipelines) [1]. Hydrodynamic coefficients for single pipelines are used in combination with the ‘equivalent diameter pipe’ to determine the hydrodynamic loads on the Piggyback Pipeline System. In order to assess more accurately the dynamic response of a Piggyback Pipeline System, an extensive set of physical model tests has been performed to measure hydrodynamic forces on a Piggyback Pipeline System in combined waves and currents conditions, and to determine in-line and lift force coefficients which can be used in a dynamic stability analysis to generate the hydrodynamic forces on the pipeline [2]. This paper describes the implementation of the model testing results in finite elements dynamic stability analysis and presents a case study where the dynamic response of a Piggyback Pipeline System was assessed using both the conventional ‘equivalent diameter approach’ and the hydrodynamic coefficients determined using model testing. The responses predicted using both approaches were compared and key findings presented in the paper, in terms of adequacy of the equivalent diameter approach, and effect of piggyback gap (separation between the main line and the secondary line) on the response.
Analysis of Spring-Back Behavior in the Cold-Forming Process of Three-Dimensionally Curved Metal Plates
Authors: Paik Jeom Kee , Kim Jeong Hwan , Kim Bong Ju , Tak Chang Hyo
Published: January 2011

The present paper is part of the study to develop the advanced computer aided manufacture (CAM) system called the changeable die system (CDS) that applies the cold-forming technique to produce curved metal plates with complex, three-dimensional geometry. This paper focuses on the algorithm of predicting the spring-back behavior using nonlinear finite element method, which is a key element within the framework of the CDS process. The validity of the algorithm is confirmed by comparison with experimental results obtained by the CDS machine in the cold-forming process of curved steel plates.
Effects of Wind on the Heat Flow of FPSO Topsides Subject to Fire: An Experimental and Numerical Study
Authors: Yoon Joon Young , Kim Seong Hwan , Yu Gwon Cheol , Seo Jung Kwan , Kim Bong Ju , Paik Jeom Kee , Czujko Jurek
Published: January 2011

The aim of this paper is to examine the effect of wind on the thermal diffusion characteristics of floating production storage and offloading (FSPO) topside models subject to fire. It is motivated by the need to identify the fire loads on FPSO topsides, taking into account the effects of wind speed and direction. The results of an experimental and numerical study undertaken for these purposes are reported here. This paper is part of Phase II of the joint industry project on explosion and fire engineering of FPSOs (EFEF JIP) [1]. An experiment was performed on a 1/14-scale FPSO topside model using a wind tunnel test facility. The locations of the heat source of the fire were varied, as were the speed and direction of the wind, and the temperature distribution was measured. Computational fluid dynamics (CFD) simulations using the ANSYS CFX program were performed on the test model, with the results obtained compared with the experimental results. It is concluded that wind has a significant effect on the thermal diffusion characteristics of the test model and that the CFD simulations are in good agreement with the experimental results. The insights developed in this study will be very useful for the fire engineering of FPSO topsides.
Phase Analysis on the Mechanism of TMD and Mitigation of Ice-Induced Vibrations for Jacket Platforms With TMD
Authors: Li Dequan , Zhang Dayong , Yue Qianjin
Published: January 2011

TMD (Tuned mass damper) which has been widely used in civil engineering, is evaluated to see if it is valid to eliminate ice-induced vibrations of jacket structures in Bohai Sea. Analytical derivation of equations is conducted to determine the parameters of TMD primarily, and FEM analysis is performed to check the effect on specific jacket platforms. However, because of the randomness of dynamic ice forces, it is difficult to evaluate the applicability of TMD on ice-resistant structures using conventional methods. Accordingly, a time domain phase analysis is used to analyze the equivalent forces of TMD to the main structures, and a criterion based on phase analysis is presented to describe the effect of TMD under random ice excitations, based on this, frequency domain and time domain analysis is used to discuss the effectiveness of TMD, it is concluded that TMD system is valid to reduce steady state vibration under ice actions. On the other hand, according to the time domain analysis of the mechanism of TMD.
Experimental Study on Strength Weakening of Silty Soil of Subaqueous Yellow River Delta Under Cyclic Loadings
Authors: Liu Huixin , Xu Guohui , Zhao Qingpeng , Wang Xin , Yu Yueqian
Published: January 2011

Silty soil seabed of Yellow River Delta is prone to sliding and liquefaction, which usually leads to geological hazards including landslides of seabed and failures of marine structures. The failure of silty soil is closely correlated with its strength change induced by wave actions. In this paper, silty soil samples of various clay contents were prepared, and cyclic triaxial experiments modeling wave actions were carried out on the samples to study the relationship between cyclic shear stress and number of cycles of silty soil taken from Subaqueous Yellow River Delta. Research results indicated that there existed a critical value in the cyclic stress exerting on the soil; the cyclic stress could cause failure of soil only when it’s value was higher than that of the critical stress; in this paper, critical cyclic stress ratio was defined as Kcr = ((σd+σ1)/σ3)cr, where Kcr is critical cyclic stress ratio, σd is axial pressure, σ1 is cyclic stress, σ3 is confining pressure. For the sake of the safety of design and construction of marine engineering structures, 1.30 was taken as the critical cyclic stress ratio of remolded silty soil in the Yellow River Delta, and 1.40 as that of the undisturbed silty soil. Also the results show that the dynamic strength weakening of silty soil followed the rule of power function attenuation. In addition, cyclic loading vibration experiments in a soil tank were carried out to study the weakening law of shear strength and penetration resistance with the variation of oscillation times.
Coupled Aerodynamic and Hydroelastic Analysis of an Offshore Floating Wind Turbine System Under Wind and Wave Loads
Authors: Iijima Kazuhiro , Kim Junghyun , Fujikubo Masahiko
Published: January 2011

A numerical procedure for the fully coupled aerodynamic and hydroelastic time-domain analysis of an offshore floating wind turbine system including rotor blade dynamics, dynamic motions and flexible deflections of the structural system is illustrated. For the aerodynamic analysis of wind turbine system, a design code FAST developed by National Renewable Energy Laboratory (NREL) is employed. It is combined with a time-domain hydroelasticity response analysis code ‘Shell-Stress Oriented Dynamic Analysis Code (SSODAC)’ which has been developed by one of the authors. Then, the dynamic coupling between the rotating blades and the structural system under wind and wave loads is taken into account. By using this method, a series of analysis for the hydroelastic response of an offshore large floating structure with two rotors under combined wave and wind loads is performed. The results are compared with those under the waves and those under the winds, respectively, to investigate the coupled effects in terms of stress as well as motions. The coupling effects between the rotor-blades and the motions are observed in some cases. The impact on the structural design of the floating structure, tower and blade is addressed.
Finite Element Analysis of Flexible Riser Impact
Authors: Alsos Hagbart S. , Laksa´foss Bogi , Rasmussen Per D. , Kristensen Claus , Paulsen Gunnar
Published: January 2011

Flexible risers and pipelines are complex structures composed by several layers of steel and plastic sheaths. Many of these layers are intricate structures; having complex shape and self interaction. Take for instance the helically cross winded tensile armor or the interlocked pressure armor. Each of these components are individual structures which interact by internal contact and friction. Prediction of cross sectional riser response caused by external loads is therefore not straight forward. This is especially the case if flexible pipes are subjected to impacts and accidental actions. The article presents analyses and basic results from a riser impact study performed by Reinertsen AS and NKT Flexibles I/S. The study is highly relevant for impact scenarios such as: Flexible risers subjected to actions from dropped objects, and riser clashing against offshore structures or riser to riser clashing. A detailed model of a flexible production riser is generated and analyzed using the non-linear explicit finite element (FE) software LS-DYNA. The focus of the study is twofold: First, the cross sectional indentation and damage resistance of the riser is investigated. This implies that the riser/pipe response to local deformation is established. This is very similar to the dropped object impact scenario. Next, the local deformation response is coupled with global deformations. In other words, the riser is allowed to deflect with a bending mode during impact. This involves a model which allows local cross sectional deformations as well as global beam-like deflection, e.g. as will be the case during riser clashing with offshore or subsea structures. All analyses are performed dynamically and involve nonlinear-geometry, material response and contact interaction. The impact response of flexible pipes is discussed in details. Focus is on cross sectional deformation and how this is distributed through the structural layers. In addition the effect of global riser response is discussed.
Coupling Between Flexible Ship and Liquid Sloshing Using Potential Flow Analysis
Authors: Lee Youngbum , Tan Mingyi , Temarel Pandeli , Miao Shihua
Published: January 2011

The significant increase in demand for Liquefied Natural Gas (LNG) and the economic aspects of its transportation resulted in increases in the number and size of LNG carriers. One of the design issues for LNG carriers is the sloshing phenomenon because containment systems widely used nowadays have no internal structures. Furthermore, because the weights of ship and cargo are comparable and ship operators want more flexible operations allowing partial fillings in tanks, the coupling effect between ship motions and sloshing requires further investigation, including the effect of ship distortion. The previous study on coupling between rigid body and sloshing shows good agreement between methods of prediction and measurements [1,2]. Hence, in this paper the potential flow approach adopted for the coupling effect between rigid body ship motion and sloshing is extended to flexible ship-partially filled tank system, using the desingularised Rankine source method. In this case, the global deflection of the flexible ship is used for application of the body boundary condition on the partially filled tank. The aim of this paper is to investigate the influence of hull flexibility on the hydrodynamic forces and moments associated with liquid sloshing and vice versa, as well as the dynamic characteristics (e.g. resonance frequencies) of the whole system. As there are no experimental results available, the method is validated by comparing hydrodynamic forces from sloshing obtained using rigid and flexible body approaches. The coupling effect between flexible ship and sloshing in partially filled tanks is investigated for an idealized LNG Carrier in beam regular waves, considering different partial filling scenarios.
On the Ductile Response of Stiffened Panels Subjected to Lateral Indentation: Experimental and Numerical Analysis
Authors: Alsos Hagbart S. , Amdahl Jo̸rgen
Published: January 2011

Reliable prediction of ductile fracture is essential in analysis of accidental response of ships and offshore structures. The consequences of fracture are significant. It may imply a significant reduction in structural capacity. It may also pose a potential hazard for human safety, as well as lead to an environmental and economical loss, e.g. caused by tanker collision or grounding. A series of five steel-plate indentation tests were conducted at the Norwegian University of Science and Technology (NTNU), Department of Mariner Technology, during late fall 2007. These are performed quasi-statically on various configurations of stiffened panels. The tests represent hull or deck plates in ships or platform structures subjected to accidental actions from ship-ship collisions, ship grounding or dropped object impacts. Various configurations of stiffened panels are tested, all laterally by a cone shaped indenter until fracture occurred. The specimen dimensions represent a 1:3 scale of the dimensions found in medium sized tankers, i.e. plate thickness of 5 mm. Naturally, because damaged hull and cargo tanks may cause severe environmental consequences, focus is on the plastic deformation and fracture resistance of the panels. The panel tests are primarily intended to serve as verification for advanced finite element simulations using a failure criterion based on instability mechanisms, i.e. local necking. This is implemented into the non linear explicit finite element code LS-DYNA and is referred to as the BWH instability criterion. In addition, the influence of the element size with respect to onset of failure is studied using three different element sizes for the various test cases. Although, attention is primarily placed on accidental scenarios, such as ship collision and grounding, the experimental results are of considerable relevance for other types of abnormal actions, e.g. dropped objects on deck and subsea structures, and stiffened panels subjected to explosion or ice actions.
A Master S-N Curve Representation of Subsea Umbilical Tube Weld Fatigue Data
Authors: Dong Pingsha , Yang Xinhua
Published: January 2011

In this paper, some of the available duplex and super-duplex stainless steel umbilical tube fatigue data are analyzed using an equivalent nominal stress range parameter, which shares a great deal of similarity to the one used in the 2007 ASME Div 2 Code. The proposed fatigue parameter consists of nominal stress range, a size correction term RTt with RT being the tube radius and t being tube wall thickness, and a stress ratio term (1 − R) with R being applied stress ratio. With it, the available girth weld fatigue test data are shown to collapse into a narrow band. The equivalent nominal stress range parameter is also shown to be effective in correlating seam weld data collected from seam-welded umbilicals, but with a different slope in log-log based S-N plot. Coiling/uncoiling low-cycle effects can be addressed by introducing a pseudo elastic nominal stress definition. Miner’s rule can be used to combine both coiling/uncoiling effects with subsequent high-cycle fatigue loading.
Application of Environmental Contour Lines on a Flexible Riser
Authors: Baarholm Gro Sagli , Haver Sverre
Published: January 2011

This paper is concerned with estimating the response value corresponding to an annual exceedance probability. In principle, this response needs to be obtained by combining the response statistics in all sea states and wave headings. The probability distribution for a given sea state and wave direction, specified by the significant wave height, Hs, the peak period, Tp, and wave direction, β, can be obtained by frequency domain analysis for linear response. Time domain simulations are, in general, required to obtain the stochastic structure of a non-linear response quantity. To limit the statistical uncertainty, the time domain simulations must be sufficiently long. Therefore, a simplified method is necessary to improve the efficiency of the direct calculation of the long-term response value in the non-linear case. A method to determine the long-term extremes by considering only a few short term sea states is outlined. The sea states have a certain probability of occurrence and are identified by a contour line in the (Hs,Tp) plane. This will make possible practical estimation of the extreme loads the structure is exposed to. The contour line approach is merely suggested as a method for predicting load- and response maxima corresponding to a given annual exceedance probability without having to carry out a full long term analysis. The advantage with this concept is that the environmental and response analysis is decoupled. This is very convenient if the problem under consideration is of a very non-linear nature — in particular if characteristic values for design are to be established directly from model tests. The method is an approximate method, but seems to give results of reasonable accuracy for most problems. The purpose of the present paper is to present the contour line method applied to estimate responses on a flexible riser, lazy wave configuration, located in the North Sea.
On the Numerical Simulation of Fluid-Structure Interaction to Estimate the Fatigue Life of Subsea Pipeline Spans: Effects of Wall Proximity
Authors: Pontaza Juan P. , Menon Raghu G.
Published: January 2011

Subsea pipelines laid on uneven terrain typically have segments of unsupported spans, referred to as “free spans”. Alternatively, subsea pipelines lying on loose and fine gravel or sand may develop free spans due to sea bottom being scoured out due to current action. This paper is concerned with the numerical simulation of fluid-structure interaction (FSI) to predict the response of free spans exposed to sea bottom currents. When exposed to sea bottom currents these spans may experience vortex-induced vibrations (VIV), which may cause fatigue damage to the pipeline. The VIV response of the pipe span is predicted by coupling a three-dimensional viscous incompressible Navier-Stokes solver with a nonlinear beam finite element solver. Parameters such as turbulence in the flow, proximity of the seabed, pipe sagging due to submerged weight, and pipe-soil interaction, are all accounted for in the FSI simulation. We pay special attention to the effect of seabed / wall proximity on VIV. Design guidelines for free spans are typically based on VIV amplitude and frequency responses for isolated pipes, with little regard to effects of seabed / wall proximity. This may result in overly conservative designs and/or expensive span remediation recommendations, when in reality no span remediation is required. Two examples of field applications are presented.
Concept Selection Philosophy for Floating Control Facilities
Authors: Liu Tuanjie , Chen Xiaohong , Ye Wei
Published: January 2011

Floating control facilities (FCFs) are designed to support fields using only subsea production system. Potential concepts, including Tension Leg Platform (TLP), Spar and semi-submersible can be selected as the FCFs. This paper discusses the philosophy of concept selection for FCFs. Two basic considerations for concept selection for FCFs are functional requirements and costs. Different structure types are first to be sized to meet the functional requirements for given water depths, environmental conditions and soil conditions. Then the costs for the procurement, construction, installation, hook up and commissioning for different concepts are estimated based on the sizes. Finally, the most cost-effective concept is chosen for further study to confirm the feasibilities of the concept by detailed analyses. Global sizing tools assisting concept selection have been developed by Ocean Dynamics LLC (Limited Liability Company) (ODL) over the past years. For a given concept, these sizing tools can find the optimized size based on costs to meet the functional requirements. Capabilities of the sizing tools cover the general layout, weight estimate (hull and deck), riser weight and stiffness estimate, loading conditions (transportation, normal operating, extreme, survival), hydrostatics and stability, natural periods, motions, air gap, environmental loads (wind, current, and mean wave forces), mooring sizing, cost estimate, and optimization based on the total costs. Two case studies in two different water depths, 140m and 1200m, are presented to illustrate the procedures and considerations in concept selection for floating control facilities. The FCFs used in the case studies are to support a Super Puma helideck, satellite communication equipment, an umbilical riser and option for a 3.5″ pipeline riser, with a total estimated topside payload as 400 Metric Ton (MT).
Thunder Hawk Riser System Design: An Integrated Experience
Authors: Cao Peimin , Xiang Sherry , Chabot Luc , Fourchy Paul , Boubenider Rafik
Published: January 2011

The Thunder Hawk field was developed through a Steel Catenary Riser (SCR) system connected to a DeepDraft Semi® Floating Production Unit (FPU) installed in approximately 6,060-ft. water depth in the central Gulf of Mexico (GoM). This paper presents an integrated riser design experience, focusing on the significant design challenges and solutions. The Thunder Hawk FPU was one of the first deepwater projects to address the enhanced GoM environment criteria. These criteria required the integrated design team to configure a feasible and robust hull, mooring and riser system. Model tests were performed to validate the design and calibrate the analysis model. Other challenges included the SCR fatigue and fracture design, titanium stress joint and receptacle design for the high pressure production risers, flex joint over-rotation design, and porch fatigue design. A significant amount of sensitivity analyses were performed to cover the uncertainty of the various hull, mooring, and riser design parameters that affected the SCR performance. The SBM Atlantia (SBMA) riser team worked closely with the Murphy Integrated Project Team (IPT) to achieve a robust and comprehensive riser design 16 months before the first SCR was installed. The highly integrated engineering approach enabled prompt and full considerations of the system interactions, and provided instant design evaluation throughout the project. The sensitivity analyses established the system design envelopes, and resulted in significant flexibility in offshore installation and operation.
Theoretical Analysis of Average Wave Steepness Related to Peak Period or to Mean Period
Authors: Arena Felice , Guedes Soares Carlos , Petrova Petya
Published: January 2011

The average wave steepness may be defined as the ratio between the significant wave height Hs and the wavelength associated to either the zero-up-crossing mean period Lm or the peak period Lp (Bitner Gregersen et al., 1998). This parameter may be calculated from wave data at fixed locations, as well as by starting from theoretical spectra. In this paper the average wave steepness is firstly analyzed by considering a JONSWAP spectrum. It is shown that for this spectrum the ratio Sm = Hs/Lm, as well as Sp = Hs/Lp, depends upon the values of the spectrum parameters. The theoretical values are then compared with wave data in the Mediterranean Sea, Pacific Ocean, Atlantic Ocean and North Sea. The values of Sm and Sp are deeply investigated for severe sea states, with large values of Hs. It is obtained that in severe sea state the observed values of wave steepness, defined as either Sm or Sp, are always in the range defined by theoretical spectra; therefore, the extreme values of Sm and Sp, which are of interest for naval architecture, may be obtained from theoretical analysis, as a function of extreme values of Hs.
Reliability Evaluations of an Offshore Platform With Pile-Soil Foundation System Due to Random Wave and Seismic Forces
Authors: Park Minsu , Kawano Kenji , Choi Yoonrak , Koo Weoncheol
Published: January 2011

It has been well recognized the importance of dynamic soil-structure interaction for several structures founded on soft soils. In order to examine the effects of soil-structure interaction, the substructure method is applied to the dynamic response evaluations of offshore platform. Since the offshore platform is generally subjected to severe dynamic forces such as wave, current and seismic forces, it is very important to clarify the dynamic response characteristics for the reliable design of the platform. For the idealized three-dimensional offshore platform subjected to random waves and seismic forces with the pile-soil foundation system, the dynamic response evaluations were carried out through the modal analysis. On the other hand, the uncertainty effects of dynamic forces and structural properties play very important roles on the reliability evaluations of offshore platform. If the limit state function is given by the most critical situations of dynamic responses, the reliability evaluations of the platform can be effectively calculated by the reliability index with the results obtained from Monte Carlo Simulation (MCS) method. Since the uncertainty of the random wave and seismic forces is critical for dynamic response evaluations, it is necessary to clarify the effects of uncertainties for the reliable design of the offshore platform.
Nonlinear Resonant Response of Deep Draft Platforms in Surface Waves
Authors: Liu Yuming , Yan Hongmei , Yung Tin-Woo
Published: January 2011

To minimize body motions, floating marine structures are often designed with natural frequencies far away from the spectrum of ocean waves. Such design considerations led to a class of deep draft caisson vessels (DDCV or spars). Even so, large resonant responses may still be generated by excitation from nonlinear interactions of waves with body motions. Past experiments indicated that a DDCV experiences large-amplitude heave and pitch resonant motions when the incident wave frequency is much larger than the heave and pitch natural frequencies. Such resonant motions are not predicted by classical theories without considering nonlinear effects. This nonlinear mechanism has received little attention because of the complex nonlinear wave-body dynamics involved. In this work, we investigate nonlinear wave-wave and wave-body interaction effects on dynamic instability of such marine structures. We first perform a linear stability analysis of the wave-frequency body motion. From the analysis, we find that at certain incident wave frequencies the body motion is unstable with natural heave and pitch motions growing exponentially with time by taking energy from the incident wave through nonlinear wave-body interactions. The condition for the occurrence of instability and the key characteristic features of unstable natural heave and pitch motions, predicted by the analysis, agree well with the experimental measurement and our full-nonlinear numerical simulations. As time-domain fully nonlinear numerical simulations are computationally expensive, we further develop an approximate time-domain analytic model, by including the second-order body nonlinearity only, for predicting the onset of instability and ultimate response of DDCVs in both regular and irregular waves. We use this model to systematically investigate the dependence of unstable motions on frequency detuning, damping, body geometry, and wave parameters.
Use of Finite Element Analysis for Stud Pre-Tension Determination of Large Diameter Integral Flanges With Ring Joint Gaskets
Authors: Panapitiya Upali , Wang Haoyu , Jafri Syed , Jukes Paul
Published: January 2011

Large diameter integral steel flanges are widely used in many applications in the oil and gas industry. The flanges of nominal pipe sizes, 26-inch and above with ring-joint gaskets as specified in ASME B 16.47 Standard, are used in the offshore applications for the transportation of oil and gas from production facilities. These pipelines require flanged connections at end terminations, mid-line tie-ins and expansion loops. The conventional design of large diameter steel flanges is based on one-dimensional analytical methods similar to the procedure in ASME VIII Boiler and Pressure Vessel Code, Division 1 Appendix 2. The effects of axial forces and bending moments are approximated by calculating an equivalent pressure. This usually results in conservative designs for the large flanges because it estimates the required stud pre-tension based on the assumption that the gasket will be unloaded entirely to a minimum stress, whereas only a small section of the gasket is subjected to low stress. This technical paper presents the quasi-static, nonlinear, and three-dimensional finite element models of large diameter steel flanged joint for the determination of stud pre-tension and change of stud tension under various loading conditions. The finite element analysis results are compared with the results obtained by using the equivalent pressure method and flange “Joint Diagram”.
Research and Development of Offshore Platforms for SMS Mining: Part 2—Conceptual Design of the Platform
Authors: Masanobu Sotaro , Kato Shunji , Ishida Shigesuke , Uto Shotaro
Published: January 2011

National Maritime Research Institute of Japan (NMRI) has been developing the integrated design support tool, called “Harmonic Design Tool”, for evaluating economical / safe / environmental aspects of the offshore platform. We are also conducting the conceptual design of the offshore platforms for assessing the availability and accuracy of this design support tool. Seafloor massive sulphides (SMS) are high-grade hydrothermal deposits rich in copper, zinc and lead with a high gold and silver content. The Japanese Economic Exclusive Zone (EEZ) is well known to have high potential of SMS. Thus we decided the SMS mining as one of the most promising applications for the offshore platform technologies. Based on the published potential map of resources and the metocean database established in this project, we decided Izu-Bonin Arc as the target sea area and derived the design and operation conditions for the offshore mining platform. Then we proceeded to the basic concept such as the production rate and storage capacity, dynamic positioning system for station keeping and operation availability. This paper presents the review of the conceptual design of the offshore platform for SMS mining.
Comparison Investigations of Numerical Simulations of Incompressible Viscous Flows by SPH and MPS
Authors: Zhang Yuxin , Wan Decheng
Published: January 2011

Comparison investigations of numerical simulations of incompressible viscous flows by the SPH (smoothed particle hydrodynamics) and the MPS (moving particle semi-implicit) are presented. A dam-break problem is chosen as the test case. In the calculation with the SPH method, weakly compressible model is used, i.e. WCSPH, which describes water as a nearly incompressible fluid, while in MPS method, the pressure Poisson equation is introduced to keep the density of fluid to be constant. The numerical results show that the two particle methods are robust and flexible, numerical results qualitatively agree with the experimental data. It can be seen that both the SPH method and the MPS method can be easily applied to the complex free surface flow problems.
An Experimental and Numerical Study on the Axial Compression Response of Flexible Pipes
Authors: de Sousa Jose´ Renato M. , Viero Paula F. , Magluta Carlos , Roitman Ney
Published: January 2011

This paper deals with a nonlinear three-dimensional finite element (FE) model capable of predicting the mechanical response of flexible pipes subjected to axisymmetric loads focusing on their axial compression response. Moreover, in order to validate this model, experimental tests carried out at COPPE/UFRJ are also described. In these tests, a typical 4″ flexible pipe was subjected to axial compression until its failure is reached. Radial and axial displacements were measured and compared to the model predictions. The good agreement between all obtained results points that the proposed FE model is efficient to estimate the response of flexible pipes to axial compression and, furthermore, has potential to be employed in the identification of the failure modes related to excessive axial compression as well as in the mechanical analysis of flexible pipes under other types of loads.
Wave-in-Deck Impact: Comparing CFD, Simple Methods, and Model Tests
Authors: Kendon Timothy E. , Pakozdi Csaba , Baarholm Rolf J. , Berthelsen Petter A. , Stansberg Carl-Trygve , Enger Sven
Published: January 2011

The slamming of waves on the lower deck of large volume offshore platforms has received increased attention over recent years. For many existing platforms, the problem of insufficient air-gap clearance has become more acute of late due to more extreme weather conditions than was used in their original design basis and/or due to issues such as subsidence of gravity based structures. To investigate this problem, MARINTEK’s Wave Impact Loads JIP has, in one of its sub-tasks, focused towards an idealised model test setup of a rectangular block in regular waves. The block is fixed at a distance h above the calm water line. Both 2D and 3D model test experiments of the block in regular waves have been carried out in Phase 1 of the JIP (2008). This paper considers results from the 2D model test setup, and compares the measured vertical loading on the deck against two simple potential theory based methods (Baarholm, OMAE 2009-79560) and against results from a CFD code (STAR-CCM+). The results demonstrate that a second impact event closely following a first impact event can have a much flatter free-surface profile (and stronger water entry force) as a result of its interaction with the (deck) diffracted wave from the first impact event. The importance of resolving this diffracted wave in the CFD analysis is demonstrated. The paper concludes that for isolated impact events the simple potential flow based models, which do not consider the influence of one impact event on another, are adequate to predict the vertical loading on the deck. However, from a design basis criteria, if there is the strong likelihood of steep wave groupings resulting in closely following wave-in-deck impact events, then the presented simple methods may be non-conservative, and a CFD (Computational Fluid Dynamics) analysis or model test may be advisable to predict the vertical wave-in-deck loading. However the horizontal loading was significantly under-predicted in the CFD analysis compared to the measurements, so more work still needs to be done in this respect.
Temperature Measurements in a Linear Generator and Marine Substation for Wave Power
Authors: Bostro¨m Cecilia , Rahm Magnus , Svensson Olle , Stro¨mstedt Erland , Savin Andrej , Waters Rafael , Leijon Mats
Published: January 2011

This paper analyzes temperature measurements acquired in offshore operation of a wave energy converter array. The three directly driven wave energy converters have linear generators and are connected to a marine substation placed on the seabed. The highly irregular individual linear generator voltages are rectified and added on a common DC-link and inverted to 50 Hz to facilitate future grid-connection. The electrical power is transmitted to shore and converted to heat in a measuring station. First results of temperature measurements on substation components and on the stator of one of the linear generators are presented from operation in linear and in non-linear damping. Results indicate that there might be some convective heat transport in the substation vessel. If high power levels are extracted from the waves, this has to be considered when placing components in the substation vessel to avoid heating from neighbouring components. The results also indicate that the temperature increase in the linear generator stator is very small. Failure due to excessive heating of the stator winding PVC cable insulation is unlikely to occur even in very energetic sea states. Should this conclusion be incorrect, the thermal conductivity between the stator and the hull of the WEC could be enhanced. Another suggested alteration would be to lower the resistive losses by reducing the linear generator current density.
The Effect of Sampling Rate on the Statistics of Impact Pressure
Authors: Repalle Nitin , Truong Tam , Thiagarajan Krish , Roddier Dominique , Seah Robert K. M. , Finnigan Timothy
Published: January 2011

Sloshing model tests have been performed to estimate the sloshing loads for design of LNG containment systems. The experiments have revealed that the sloshing phenomenon is highly stochastic and impact pressure varies significantly even for a simple harmonic excitation in one direction. It is important to select an appropriate sampling rate and duration to capture the true pressure peaks in order to obtain a reasonable statistical estimation. In this pursuit, experiments have been performed on a 2D model scale tank with sway motion for duration of 30 minutes at different sampling rates of 20, 40, 60, 80 and 100 kHz. Comparison of statistical quantities like maximum pressure, rise time, decay time and impulse for various sampling frequencies are presented. Exceedence probability is also evaluated for each case and reported. The high sampling rate runs are down sampled to see the effect on the magnitude of the pressure peaks. Also the 30 minutes runs are split into a set of three 10 minute runs to see how the statistics change for each segment. The paper makes recommendations on required sampling rate and test duration for model scale to capture the various local effects such as breaking waves and spray, pronounced during the liquid sloshing impact.