bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-Domain Simulation of Motions of Large Structures in Nonlinear Waves
Authors: Sen Debabrata
Published: March 2009

In this paper, we discuss development of a time-domain motion simulation method for studying the interaction of nonlinear waves with large offshore structures. The computational algorithm follows a simplified numerical wave-tank approach based upon a boundary-integral method and time-integration of boundary conditions. The simplifying approximations include linearization of the interaction hydrodynamic effects (radiation and diffraction) while the incident wave effects are considered in full. The main aim is to develop a method that will consider all important nonlinear effects associated with a large-amplitude incident wave, and yet practical enough to be applied routinely by the industry. In the time-integration of motion equations, numerical instabilities usually arise if difference rules are applied for determining pressures, due to coupling between forces and motions. To avoid this, an algorithm has been developed for the pressure evaluation. The resulting computational scheme is numerically stable for all conditions. The method can incorporate effects of other forces such as Morison forces, forces from mooring lines etc. which can be nonlinear. After providing a description of computational scheme, force and motion results for the interaction of large amplitude regular waves as well as irregular waves with two practical semisubmersible configurations are presented.
Integrated Diver Display Mask (IDDM) System for Special Diving Applications
Authors: Gallagher Dennis G. , Olstad William D.
Published: March 2009

The United States Navy’s Coastal Systems Station (CSS) - Panama City, Florida has developed an Integrated Diver Display Mask (IDDM) system for special diving applications. The IDDM contains a miniature liquid crystal display (LCD) panel, multi-element optics, microcontroller, depth transducer, RF (radio frequency) receiver, replaceable battery, and controlling software – all built directly into a dive mask. The miniature LCD displays the diver’s depth, bottom time, and bottle pressure. In addition, the display has a number of annunciators that are illuminated when certain warning, or limit conditions are reached (i.e. ascent/descent rate too fast, low bottle pressure, or low battery). The multi-element optical lens provides a clear, highly magnified image of the LCD which is viewable regardless of environmental conditions. A commercial-off-the-shelf (COTS) RF transmitter module transmits the diver’s bottle pressure to the mask. The system is operational to depths of 135 FSW. The IDDM Project was funded by the Office of Special Technology (EOD/LIC Program), Ft. Washington, Maryland. It was conducted as a joint-development project under a Cooperative Research And Development Agreement (NCRADA - NSWCCSS-98-015) between the US Navy’s Coastal Systems Station of Panama City, Florida; and American Underwater Products, Inc. (parent company of Oceanic USA and Pelagic Pressure Systems) of San Leandro, California. The paper will describe the development of the IDDM system and suggest military, scientific, and other special applications for this unique diving technology.
Reliability-Based Assessment of Vibrations on an Offshore Living-Quarters Platform in Mexico
Authors: de Leon David
Published: March 2009

Structural safety, integrity and reliability of offshore marine platforms under extreme loading is generally a widely studied topic [2, 4, 9]. However, ordinary environmental forces on particularly flexible components may be important sources of dynamic excitation and significant vibrations. For those cases where the structural system of the superstructure is totally different of the one used for the substructure, the non-uniform vertical distribution of the stiffness may impact the dynamic response by adding significant amplifications on the resulting motions of the more flexible structure. Significant motions were reported on a Mexican living-quarters platform. The platform was monitored and its dynamic characteristics were identified in order to prevent excessive displacements and accelerations. The platform was modeled as a single degree of freedom system with elasto-plastic behavior and the dynamic properties were calibrated from records obtained throughout a monitoring campaign of response displacements and accelerations on the platform. Fourier amplitude response spectra were generated and the fundamental period and damping percent were estimated. On the basis of Monte Carlo simulation of time history accelerations, the probability to exceed prescribed thresholds for lateral displacements, accelerations and lateral strength under several given wave heights were estimated. As these probabilities are conditional to the occurrence of the wave height exciting the platform, the unconditional probabilities were obtained by convolving the conditional probabilities over the probability of occurrence of the wave heights prescribed for the site of the platform.
Wave Induced Hydrodynamic Forces on Vertical Piggyback Risers
Authors: Olsen Robert , Huang Shan
Published: March 2009

The wave induced hydrodynamic forces on a pair of vertical cylinders of different diameters in close proximity are investigated experimentally. The smaller cylinder is placed at various positions around the larger one. The wave forces, including both drag and lift, are measured on each cylinder independently at two different depths below the mean water level for each cylinder. The Keulegan-Carpenter numbers vary from 0.4 to 14 based upon the larger cylinder diameter and the Reynolds numbers are in the sub-critical regime. It is found that there is significant interference effect upon the cylinder drag and inertia coefficients.
Investigation of a Simplified Fatigue Reliability Model for Optimising the Design of Tubular Joints
Authors: Cole G. K. , Pinna R. , Ronalds B. F. , Romagnolo P.
Published: March 2009

While there has been considerable research effort over the last decade aimed at refining risk based fatigue inspection planning techniques, there has been relatively less effort aimed at using the same techniques to optimise the design of welded joints. This paper discusses the use of a simplified, semi-probabilistic technique for optimising the design of tubular joints on the basis of their total through-life costs. A model is developed which combines the probability of fatigue failure with initial construction costs, inspection costs, and repair and failure costs. An example study of a critical joint in a minimum structure is provided. In this example, where costs associated with joint failure were assumed to be purely financial, an optimum target probability of failure (threshold for inspection) was found to be 10−4 to 10−3. Provided the target probability of fatigue failure used in inspection planning is kept below this level, design in accordance with API RP2A appears to provide a reasonably optimised joint. The sensitivity of the total cost to this target failure probability, and a range of other important cost variables, was also investigated.
A Tethered Multiple Oscillating Water Column Wave Energy Device: From Concept to Deployment
Authors: Chudley John , Dai Y. Ming , Johnson Fraser
Published: March 2009

This paper discusses the research and design methodology employed in the development of a tethered Multiple Oscillating Water Column (MOWC) wave energy device – from concept to deployment. The fundamental aim of the project was the design and deployment of a scaled floating MOWC wave energy device capable of generating physical data from sea trials. The MOWC collector component incorporated oscillating columns connected to a self-rectifying impulse turbine via individual settling chambers. The device has a water draught of 12m and an air draught of 3m. It is of cylindrical design with an overall diameter of 4.4m, displacing 10t The present unit is rated to at 5 kW power output through restrictions of the internal airflows. Research indicates that a full-scale unit 5 times bigger than the scaled device would be capable of generating 500 – 750 kW in a moderately rough sea. The paper addresses the complex problems associated with floating MOWC devices and suggests methods to enable accurate modeling and matching of internal components. Topics discussed include: concept recognition, hydrodynamic motion interaction with OWC, local resource evaluation, turbine selection, power generation and dissipation, moorings, data monitoring, telemetry and performance evaluation. Mathematical simulations and tank testing were used to develop the concept to a stage where an engineering design could be generated. The use of mathematical modelling presented the project with several specific problems that have been highlighted within the paper. Tank testing enabled the project to overcome these difficulties and developed an engineering design tuned to the local wave climate. Initial research has indicated that the combination of individual Oscillating Water Columns (OWC) of different draughts increases the efficiency of this design when compared to a typical single OWC device. Results also indicated that the channeling of individual air masses through a self-rectifying impulse turbine would produce a self-regulated electrical output via phase locking of the individual columns.
Subsidence of the Ekofisk Platforms: Wave in Deck Impact Study — Various Wave Models and Computational Methods
Authors: Iwanowski Bogdan , Grigorian Henrik , Scherf Ingar
Published: March 2009

Subsidence of the Ekofisk platforms creates several operational challenges. For safety of the platforms, it is of great importance to find the wave impact loads acting on the platforms’ decks. The paper describes how such loads can be computed. Three theoretical wave models are discussed in the paper: the Airy wave, Airy wave modified through Wheeler stretching and the 5th order non-linear Stokes wave. The wave loads for these wave models are computed by various methods. The method based on momentum displacement approach and Morison-type equation developed by Dr. Kaplan is used as a reference point. The loads are also computed through a solution of complete Navier-Stokes equations, with the Volume of Fluid (VOF) method used to trace motion of the fluid’s free surface. Results of different wave models and different computational methods are compared and discussed.
Flow Due to a Moving Pressure Distribution on Free-Surface With Finite-Depth Bottom
Authors: Sahin Iskender , Okita Noriaki
Published: March 2009

Surface elevation and dynamic bottom pressure profiles caused by a moving pressure distribution over the free-surface are obtained. A direct numerical integration approach for the linear, two-dimensional, and steady-state solution has been developed. The behavior of the surface elevation and bottom pressure profiles along with wave resistance for increasing Froude number or depth are presented. The agreement of the wave resistance calculations using the profiles obtained by the current method and the expression given by Newman and Poole (1962) indicates that the current method can be used as a reliable tool for prediction as well as validation for other numerical approximation techniques.
Stochastic Modeling of Cracked Solids and the Related Size Effects
Authors: Shi Jianxu , Ghanem Roger G.
Published: March 2009

This paper examines the crack size effect of homogenization and microstructure on the materials. It is proposed that the size of the microstructure should not only affect the mean response of the material, which would correspond to the conventional deterministic concept of material size effect, but should also affect its whole probabilistic structure. As an example, a non-stationary stochastic tensor field is characterized as an equivalent continuum model of a solid medium with a random distribution of micro-cracks. The crack size dependence of the variance and covariance functions of the tensor field is verified and fitted to a proposed functional form. Once the correlation structure is determined, the corresponding stochastic field can be synthesized and standard computational algorithms can then be used to predict the behavior of the material.
A Consistent Analytical Model to Predict the Structural Behaviour of Flexible Risers Subjected to Combined Loads
Authors: Ramos Roberto , Pesce Celso P.
Published: March 2009

This work presents an analytical model for the structural analysis of flexible risers subjected to combined loads of bending, twisting and tension, as well as internal and external pressures applied to the riser. Flexible risers, either umbilical cables or flexible pipes, are complex structures used in offshore oil exploitation activities. Such structures are composed of several concentric polymeric and steel armour layers, which withstand static and dynamic loads applied by the floating production vessel and by the ocean environment. The complexity imposed mainly by geometry renders a finite element analysis of these structures practically unfeasible, even if we are to consider that all the materials obey a linear elastic behaviour. So, in order to calculate the stress distribution in the layers, as well as axial, torsional and flexural stiffness values of these structures, analytical methods have been proved to be, till now, a better choice. Using sets of equations, which comprise equilibrium conditions, constitutive equations and geometrical relations, it is possible to solve the problem for all the unknowns. This paper presents a consistent and comprehensive formulation leading to the solution in terms of stresses and deformation components in a flexible riser subjected to the above mentioned combined loads. It is based on the assumption of full-slip of the helically armoured layers after bending is imposed to the pipe. Other main modeling hypotheses are also highlighted in this work. The presented analytical model is, therefore, rather comprehensive and recovers, asymptotically, many results previously published in the technical literature. Comparisons between analytical results using the full-slip model and experimental results obtained in literature are shown and discussed. Some proposals leading to improvement of the presented model are drawn in the conclusions.
Elastoplastic Analysis of the Residual Stress in Chain Links
Authors: Lopes Pacheco Pedro M. Calas , Kenedi Paulo Pedro , Ferreria Jorge Jorge Carlos
Published: March 2009

Mooring lines of offshore oil exploitation platforms consist of long lengths of steel chain links, wire ropes and other accessories. Usually, these lines are designed for an operational life of about 20 years and periodic inspections are mandatory for monitoring the structural integrity of these components. The failure of a single element in a mooring line can cause incalculable environmental damage and severe economic losses. The ocean adverse environment loading produced by the combination of the wind, waves and currents leads to a complex alternate loading that can promote fatigue and crack propagation. Residual stress plays a preponderant part in the structural integrity of a mechanical component subjected to such loading. Offshore mooring line components as chain links enter in operation with a residual stress field created by the proof test dictated by offshore standards. However, the traditional design of such mechanical components does not consider the presence of residual stress. This study concerns about predict the residual stress field present in stud and studless chain links prior to operation to compare the fatigue life predicted by the traditional design methodology with the one predicted considering the residual stresses states present before operation. Numeric simulations with an elastoplastic finite element model are used to estimate the residual stress along the chain link that are present after the proof test and before operation. The results indicate that the presence of residual stresses modify significantly the fatigue life of the component.
A Prediction Method for Horizontal Plane Behavior of FPSO Under the Single Point Mooring
Authors: Masuda Koichi , Asanuma Takayuki , Maeda Hisaaki , Ikoma Tomoki , Rheem Chang-Kyu
Published: March 2009

It is well known that in single point mooring or anchoring the slowly varying oscillation of a ship is caused by action of current and wind. During the slowly varying oscillation, extraordinary tension occurs in the mooring line when the ship’s yaw angle becomes nearly maximum, and incurs, as the case may be, in breakdown of mooring lines or unforeseen drift of anchors. Floating Production, Storage and Offloading (FPSO) systems are often moored as Single Point Mooring (SPM) systems. SPM systems can be Catenary Anchor Leg Mooring (CALM) systems or Single Anchor Leg Mooring (SALM) systems. It has been required to predict and evaluate performance of horizontal plane behavior of FPSO in current, wind and waves, since the workability and safety of FPSO become important from the stand point of the Life Cycle Engineering. Numerical simulation is one of the practical methods for prediction of FPSO performance and it needs quite accurate values of hydrodynamic coefficients in the mathematical model. Recently some attempts on improvement of accuracy in prediction of the hydrodynamic coefficients were made and approximate formulae for hydrodynamic derivatives including the interaction effect of main hull form and appendages were also proposed. Recently extensive studies for numerical models which describe components of hull, propeller, rudder, thruster, wind and waves separately, and these interactions have been made successively. In this paper, first, the basic equations of maneuvering motion are explained. And, an estimation method of slender body theory for hydrodynamic force acting on the hull is outlined. The authors explain numerical models to obtain FPSO coefficients for the horizontal plane behavior from mathematical model of ship maneuverability. And, numerical test of FPSO under the slowly varying oscillation is carried out. Finally, a new mathematical model is proposed to describe the current forces acting on FPSO under the slowly varying oscillation.
Prediction of Relative Motion and Probability of Contact Between FPSO and Shuttle Tanker in Tandem Offloading Operation
Authors: Chen Haibo , Moan Torgeir , Haver Sverre , Larsen Kjell
Published: March 2009

Tandem offloading safety between FPSO and shuttle tanker is under concern. A few collisions between the two vessels have happened in the North Sea in recent years. In these incidents, excessive relative motions (termed as surging and yawing in this paper) between FPSO and tanker are identified as “failure prone situations” which have contributed to the initiation of most collision incidents. To quantitatively assess the probability of surging and yawing events, and more importantly, to effectively reduce their occurrence in tandem offloading operation, we present a simulation-based approach in this paper, which is carried out by a state-of-the-art time-domain simulation code SIMO. The SIMO simulation models are setup and calibrated for a typical North Sea purpose-built FPSO and a DP shuttle tanker. This 2-vessel system motion in tandem offloading is simulated. The simulated relative distance and relative heading between FPSO and tanker are analyzed by fitting their extreme values into statistical models. This gives out probabilities of surging and yawing events. Sensitivity studies are performed to analyze contributions from various technical and operational factors. Measures to minimize the occurrence of surging and yawing from design and operational point of view are proposed.
Evolution of Tidal Vorticity in Stratified Coastal Flow
Authors: Pawlak Geno , MacCready Parker , Edwards Kathleen A. , McCabe Ryan M.
Published: March 2009

The lifespan of a tidal eddy generated by flow around a coastal headland is examined. Field observations of a tidal headland eddy at Three Tree Point, WA (USA) are presented that examine the temporal evolution of the flood tide separation eddy from its generation, through the eddy release at the turn of the tide, until its dissipation during subsequent tidal cycles. Ship-based acoustic profiling examines the vertical structure of the velocity field and subsurface drogued drifters are used to track the horizontal motion of the flow structure. Drifter tracks from successive days at similar phases of the tide indicate that flow structure is repeatable. The combined set of drifter tracks is used to obtain an estimate of eddy lifetime. Time scales for vorticity decay of less than a tidal period are significantly shorter than simple estimates using boundary friction would imply. This finding suggests that the internal wave response of the stratified flow over the sloping headland plays a significant role in the dissipation of vorticity. Field observations are compared with results from numerical modeling that also suggest that baroclinic effects are significant.
Nonlinear Dynamical Mechanisms Connecting Ocean Giant Waves and Wave Groups
Authors: Su Ming-Yang , Tsai Cheng-Han , Lin Yin-Chern , Tseng Hsiang-Mao
Published: March 2009

It is well known by direct observations that large ocean waves occur, more frequently among a wave group or group waves, than individually. Several statistical relationships among these two wave features have also been discussed in the past thirty years. However, the dynamical reasons (or mechanisms), why these two ocean wave features are often related, have seldom been discussed in sufficient details. The purpose of this paper are two fold: (1) to review various observations of wave groups and giant waves obtained from deep seas, coastal zones and laboratory experiments, and (2) to provide several significant physical and dynamical mechanisms for explaining the observed close relationship between these two important wave features. (3) to provide an example of a giant wave (called mad-dog waves) event, which resulted in the drowning of fishermen, occurred on the coast of Taiwan.
Surface Production System Options for Deepwater
Authors: Ronalds Beverley F.
Published: March 2009

Selection of the optimum production system for a deepwater field requires a detailed understanding of the advantages and limitations of the various alternatives. In this paper the key features of five proven deepwater platform types – FPSOs, semisubmersibles, spars, TLPs and compliant towers – are compared. Particular emphasis is placed on the interactions between wells, risers and hull and how they vary with water depth. Large fields in ultra-deepwater are a challenging combination for which few systems are ideally suited: they are constrained in their water depth, riser count, or topside capacity. The exception is the spread-moored FPSO. In future, semi’s or other multi-column floaters (with either wet or dry trees) may also see greater application, especially where well access is important or in less benign environments.
Computation of Interaction Between Deepwater Risers: Collision Statistics and Stress Analysis
Authors: Herfjord Kjell , Holma˚s Tore , Leira Bernt , Bryndum Mads , Hanson Tor
Published: March 2009

A methodology for computing the dynamics of interacting risers is presented. During the time simulations, riser impacts (if any) are recorded. The relative velocity and angle between the axes of the risers are recorded at each hit, so that the actual stress and accumulated fatigue damage may be computed in a post-processing operation. Detailed material properties, such as properties of coating, the actual composite material properties, etc. are used in that process. The methodology is based on a pre-established database of forces acting on the cylinders. At each time step, all riser elements are loaded with a combination of in-line and transverse forces, depending on local relative distance to the neighbouring riser. This imply that the methodology is based on quasi-static assumptions. The paper presents a validation of the computed force database by comparison with measured results. Examples of results from simulation with top tensioned risers on a TLP are given.
Improved Current Profile Criteria for Deepwater Riser Design
Authors: Jeans Gus , Grant Colin , Feld Graham
Published: March 2009

New types of current profile criteria are described that are designed to take into account the vertical coherence of deepwater currents in offshore engineering. Reasoning is given why traditional types of current profile criteria can be inappropriate for deepwater locations. The paper describes current profile occurrence criteria that consist of a discrete number of characteristic profiles, each of which has an associated percentage frequency of occurrence. These criteria are appropriate for riser fatigue calculations (including Vortex Induced Vibration modelling) and operability assessments. Use of these criteria can reduce the level of over-conservatism associated with the traditional criteria, potentially leading to significant cost savings. Two possible methods of deriving these criteria are described, one of which uses Empirical Orthogonal Function (EOF) analysis to simplify the vertical structure of current profiles. The relative merits of each analysis technique are assessed.
A Numerical Method for the Calculation of an Oil Spill Spreading
Authors: Betancourt Quiroga Fabian Omar , Pe´rez Arturo Palacio , Rodri´guez Valdes Alejandro
Published: March 2009

A revision to mathematical and numerical to use in order to determine the trajectory and growth of a spill of petroleum in water was done. A numerical model was compared with some field data (11,12) and mathematical models. This comparison allowed us to determine the technical viability of using the numerical model for the prediction of the spill and evaluating the grade of adjustment of the studied models.
Mass Loss Evaluation in Oil Spill
Authors: Betancourt Quiroga Fabian Omar , Pe´rez Arturo Palacio , Rodri´guez Valdes Alejandro
Published: March 2009

A revision of the models used to study the behavior of the mass loss processes associated with petroleum spills on water and to compare those models with experimental data (3,23). The processes of mass transfer studied in this work are evaporation, dissolution, vertical dispersion, emulsification and the changes of properties associated with these. The comparison of the estimations with the field data allowed determining the utility and the degree of adjustment of the expressions.
Model Experiment on an OWC-Type Wave-Energy Converter With Hydraulic Power Take-Off
Authors: Bjarte-Larsson Torkel , Lillebekken Per Magne , Hals Jo̸rgen , Falnes Johannes
Published: March 2009

A wave-energy converter of the OWC type is described, in which the absorbed wave energy is converted to useful energy by means of a hydraulic power take-off. Means are provided to enable the float to be latched for phase control. The float is connected to a piston pump, which pumps water from the level of the water in the wave channel to a higher level, which is adjustable. By means of measurements from three wave gauges (two on the upstream side and one on the downstream side) the incident wave energy and the absorbed wave energy are derived. For a down-scaled laboratory model, resonance is obtained with an incident sinusoidal wave of period 1 s. With optimum load, the converted useful hydraulic energy is a fraction of 0.2 of the incident wave energy. The absorbed wave energy is then 0.6 units of the incident wave energy. With wave period 2 s and optimum load, these energy fractions are 0.03 and 0.13, which are increased to 0.05 and 0.21, respectively, when latching control is applied.
Wave Scattering Around a Vertical Cylinder: Fully Nonlinear Potential Flow Calculations Compared With Low Order Perturbation Results and Experiment
Authors: Trulsen Karsten , Teigen Per
Published: March 2009

A detailed description of a fully nonlinear numerical method for computing wave interaction effects around arbitrary marine structures is presented. The paper highlights application to one specific geometry: A single, fixed vertical truncated cylinder. The fully nonlinear computations are compared with linear and second-order nonlinear results obtained with the perturbation approach, as well as with experiments.
“Useful Mean Duration”: An Alternative to the “Mean Duration” Statistic for Wave Height and Other Metocean Parameters
Authors: Jenkins Alastair D.
Published: March 2009

The probability distribution of the duration of events where the wave height or other metocean parameter X(t) is under or over a specified level X = u is of great importance for the planning of offshore engineering operations. The mean duration, estimated by adding up the durations of the events and dividing by the number of such events, is an often-used measure, but it depends on the recording interval (the time interval between successive measurements). We show that the dependence of the estimated mean dura-tion on the recording interval Δt is related to the behavior of the mean absolute discrete derivative E[|X(t + Δt) − X(t)|/Δt] as a function of Δt, and also to the fractal dimension of the level set of points where X(t) crosses X = u. An alternative useful mean duration is proposed, which weights each event by its own duration. Analysis of offshore wave data confirms the stability of this parameter to changes in recording interval.
Fracture Mechanics Analysis of a High Strength Steel for Offshore Application
Authors: Lundberg Arild , Nyhus Ba˚rd , O̸rjasæter Oddvin
Published: March 2009

One of the main bearing elements of the Siri jack-up is three tubular legs; outside diameter 3.5m and an overall length of 104m. The main material grade is high strength steel with SMYS≥690MPa. This strength was required due to contact forces during the jacking phase. One of the major issues concerning integrity of the platform was associated with the fatigue life of the legs. The fatigue life of the legs WAS to be 60 years for inaccessible locations, 40 years for accessible submerged locations and 20 years for accessible dry locations. To achieve the required fatigue lives, wall thickness of 65mm, 75mm and 90mm were utilized for the 690 steel. Furthermore, to meet the 40-year requirement, lives for the lower circumferential welds were designed to Class C, i.e. they needed to be improved by grinding. Based on performed research, it was aimed at minimum CTOD values of 0.20mm and 0.25mm in the PWHT and AW conditions respectively. Although the aim was achieved for the base material and sub critical HAZ, the weld metal and the coarse grained HAZ values were significantly below these values, i.e. 0.12mm and 0.02mm respectively. In order to verify the integrity of the platform a fracture mechanics analysis was performed. The assessment comprised evaluation of critical defect sizes and fatigue life calculations using PD6493. Wide plate testing, residual stress measurements and fatigue testing have been performed. The assessment showed that the integrity of the platform was maintained. In order to obtain additional safety an inspection program for the critical welds has been implemented in the operational phase.
A Practical Estimation Method of the Skin Plate Distortion Caused by Fillet Weld
Authors: Gotoh Koji , Toyosada Masahiro
Published: March 2009

Development of a useful estimation method of the welding deformation about welded built-up structures is developed in this paper. Improvement of matching accuracy of each hull block element is essential to introduce the economical automatic welding machines in an assembly stage. Precise estimation method of the welding deformation is also necessary to achieve the aim. Equivalent inherent strain method is highlighted as a convenient estimation method about welding deformation in large welded built-up structures. Inherent strain can be seen as the deformation source generated by welding and derives from the thermal cycles under welding processes. This deformation source can be given if the amount of welding heat input is known. The deformation can be calculated by elastic finite element analysis. Shell element is applied to simplify the mesh idealization. However, this method did not give a good estimation result in case that out-of-plane deformation such as a skin plate distortion occurs. Improvement of equivalent inherent strain method to advance the accuracy of estimation about the skin plate distortion caused by fillet weld is performed in this paper. Beam element, which expresses the fillet weld bead and has inherent strain along the bead to generate tendon force, is incorporated with former equivalent inherent strain method. The skin plate distortion measured are compared with the calculated one by applying the proposed method. As a result, the validity of the improvement is confirmed.
Evaluation for Abnormal Fracture Appearance in Drop Weight Tear Test With High Toughness Linepipe
Authors: Muaoka Ryuji , Ishikawa Nobuyuki , Endo Shigeru , Kondo Joe
Published: March 2009

The West-Jefferson type full scale partial gas burst test was carried out in order to investigate appropriate evaluation method for resistance to brittle fracture propagation in high toughness linepipe materials that exhibits abnormal fracture appearance by the Drop Weight Tear Test (DWTT). Shear area fraction (SA%) of the DWTT that had been derived from the way regarding or disregarding the abnormal fracture appearance was compared with the shear area fraction obtained from the fracture surface by the full scale burst test. It was shown that SA% obtained by the burst tests corresponded well with that by the pressed notch DWTT for the cases of disregarding abnormal fracture appearance. On the other hand, SA% in the DWTT was lower than that in the burst test when the abnormal fracture appearance was treated in the same manner as the brittle fracture that occurs at the notch tip of the specimen. Therefore, it can be stated that the evaluation by regarding the abnormal fracture surface can be conservative and much relevant evaluation can be possible by disregarding the abnormal fracture appearance. SA% of the fracture surface in the Chevron notch DWTT showed slightly lower value than that in the burst test, regardless of whether abnormal fracture appearances was regarded or disregarded. This means the Chevron notch DWTT is also severe testing method, as well as the pressed notch DWTT with regarding the abnormal fracture surface.
The Use of Mixed Element FEA for Predicting Tubular Frame Behaviour
Authors: Pinna Rodney , Ronalds Beverley F.
Published: March 2009

Pushover analysis of offshore framed structures is usually performed on simplified structural models, using finite element analysis. To assess the typical assumptions made concerning joint and member behaviour, various numerical models of increasing complexity are considered in this paper. These range from simple beam models, to models including a detailed replica of the critical joint connected to a beam model, to a complete shell model of the critical compression member and end connections. Phenomenological models of member behaviour, in this case a Marshall Strut representation, are also examined. Numerical results are compared with experimental data from two studies [1, 2]. It is found that detailed modelling of the joint stiffness is sufficient to accurately model the peak frame response, while a detailed model of the complete compression member, which includes cross section distortion, is needed to match the residual strength of the frame. Marshall strut elements are found to provide a robust solution, provided that the variables which describe the model can be defined with sufficient accuracy. Detailed modelling of the frame is also found to be able to provide the required input data for such special purpose elements.
PETROBRAS 40: Technical Concepts and Solutions for the World’s Greatest Dry Tow
Authors: Santos Marco Antonio , Machado Christiane Lopes
Published: March 2009

The world’s greatest dry transportation already done is the floating production unit PETROBRAS 40 from Singapore to Brazil. PETROBRAS 40 is a 42000 tons semi-submersible floating production unit of 150000 b.p.d. which was converted from the derrick barge DB100 hull. All the naval design for this conversion was developed by PROJEMAR S.A. The transportation from the yard at Singapore to the South Marlim field at Campos Basin, Brazil was done in a heavy lift ship to accelerate the delivery of the PETROBRAS 40. This study had several analysis performed to guarantee the feasibility of the operation. Due to the three-pontoon configuration of the hull and its large weight, the loads induced by the hull girder analysis of the heavy lift ship were mandatory to the analysis of the unit. The configuration of the semi-submersible structure did not allow it to be supported only at the side pontoons, leaving the center pontoon free to deflect. This situation would lead to large deflection on the hull and, consequently, high stresses would be installed on the PETROBRAS 40 hull. The separate analysis of the vessel and the PETROBRAS 40 hull generated a very conservative interface load and/or deflection. Therefore, the hull girder analysis of the heavy lift ship had to be detailed for a very specific critical wave conditions that the dry transportation could face in the 45 days trip. In order to minimize the contingency factors in a safe condition, several studies were performed on the weather routing and consequently in the wave height and associated period. Then, this paper presents all the technical aspects used in the dry tow analysis and discuss the possible solutions found to make it feasible.
Syntactic Foam Thermal Insulation for Ultradeep High Temperature Applications
Authors: Wang Wen-Tsuen , Watkins Lou
Published: March 2009

This paper addresses a major challenge facing deepwater production of oil and gas: how to assure continuous flow of product under the pressures and temperatures found on the ocean floor. Syntactic foam promises to overcome the limitations exhibited by conventional insulation materials in the past. New hybrid glass and polymer chemistries with improved “hot, wet” performance survive in conditions that were formerly thought impossible. This paper presents the latest laboratory test data on these new materials, and proposes models for predicting long-term performance.
Response Characteristics of Semisubmersible-Type-Megafloat in Waves and Accuracy of Hydroelastic Response Analysis Program VODAC
Authors: Suzuki Hideyuki , Yoshida Koichiro , Iijima Kazuhiro , Kobayashi Kentaro
Published: March 2009

Technical feasibility and practical design method of SSMF, a Semi-Submersible-Type-Megafloat, was studied under research funding from Corporation for Advanced Transport and Technology. A SSMF which serves as a airport for local air transportation of isolated islands was assumed in the research. Typical deck size of the SSMF is 2200m long, 300m wide and 9m deep. In the design of the SSMF, column supported type structure was chosen to satisfy the strength and functional requirements in the environmental condition around Japanese islands in the pacific ocean. The deck structure is supported by 320 columns with draft of 16m. In the design of structural dynamics of the SSMF, a computer code VODAC was adopted to calculate hydro-elastic response. VODAC is an analysis program of hydro-elastic response of Very Large Floating Structure (VLFS) which has been developed in University of Tokyo. This paper presents a series of experiments and calculations carried out to investigate response characteristics of SSMF and to confirm capability and accuracy of VODAC. Basin experiments were carried out using a scale model, which is elastically and dynamically similar to the designed SSMF airport, and the response characteristics were clarified. A simple numerical model was also proposed as a simplified model of dynamic response of VLFS. This model is a simple beam on elastic foundation, but it is shown that basic response characteristics of the structure can be relatively accurately expressed by this model. Design parameters were discussed using this model. Relationships between major design parameters and dynamic response characteristics were clarified. Furthermore it was shown that this model is not just a qualitative model but gives relatively accurate estimation of the response. It is shown that this model gives upper limits of response of real structure and a good safe side estimation.
Parametric Investigation of the Stability of Vessels for Float-Over Deck Transportation
Authors: O’Neill Lee , O’Brein Shaun , Fakas Emmanuel
Published: March 2009

The technical challenge of transporting large size integrated decks, especially float-over decks at high elevation, is well recognised. The cost of such operation usually forms a significant part of the overall development cost, especially in remote parts of the world where the transport routes are longer and the transport vessel choice is limited. In this respect, suitable vessel selection is very critical in terms of the economic viability of oil and gas developments, especially of marginal fields. A cost-effective working method is developed for selecting suitable vessels for a wide range of transport options (deck weights and transport elevations) without the requirement of extensive analyses. This method may be used at the very early stages of any study to assess the feasibility of the transport concept and also provide a wider choice of feasible options to be considered at the next stage of the development, where a more accurate analysis may be justified. Some potential ways for improving vessel stability are also investigated. Findings from the study can also be applied to integrated deck salvage transport.
Hydrodynamic Coefficients for Risers With Strakes
Authors: Lambrakos Kostas F. , Trianafyliou Michael S. , Moros Thanos
Published: March 2009

Drag and inertia coefficients for offshore risers with strakes have been derived from model tests, and are reported in this paper. The coefficients correspond to flow conditions of forced oscillations with tow in the transverse direction, and without tow. For the forced oscillations the Keulegan-Carpenter (KC) range was 1 to 5. The drag coefficients are higher than generally assumed, and decrease monotonically with KC in the absence of current. In the presence of current the drag coefficient becomes dependent on the ratio of current velocity to wave velocity. Low KC numbers strongly affect the calculated riser fatigue damage caused by vessel motions in storms. The present data fill an important need in riser design since in many situations fatigue damage governs riser design, and frequently the assumed coefficients are overly conservative.
Consideration of Wave-Induced Loads for Direct Strength Calculation Under Extreme Waves
Authors: Zhu Tingyao , Kumano Atsushi , Shigemi Toshiyuki , Matsunami Ryoju
Published: March 2009

Relationship between extreme values in long-term distribution and the short-term values under each sea-state in which ships may encounter through their lifetime is discussed regarding wave-induced loads such as ship motions, hull girder moments and hydrodynamic pressures. Based on the discussion, a simplified formula of the wave-induced loads equivalent to the extreme value in the long-term distribution at the exceedance probability of 10−8 is proposed together with the corresponding sea-states by a series of direct load analyses of the tankers and bulk carriers. Furthermore, nonlinear characteristics with respect to waveheight on wave-induced loads are discussed based on the comprehensive experiment results of a blunt ship model in regular/irregular extreme waves. Finally, a correction method of the observed nonlinear phenomenon is proposed.
Improvements to the Convected Co-Ordinates Method for Predicting Large Deflection Extreme Riser Response
Authors: O’Brien Patrick J. , Lane Michael , McNamara John F.
Published: March 2009

The geometric nonlinearity associated with large motions of compliant riser structures is catered for by defining an orthogonal triad of axes, known as the convected co-ordinate axes, for each beam element. This triad rigidly translates and rotates in three-dimensional space so as to closely follow the motions of the deforming element. The method enables equilibrium equations to be written by expressing the total nodal solution vector as the sum of rigid body and deformation terms. This paper presents and develops newly defined measures of beam deformation that provide a precise measure of beam rotational deformation relative to the convected axes. The use of these new terms no longer depends on the assumption that the deformation rotations behave as true vectors. A mathematical expression that accurately predicts the nonlinear coupling between torque and bending that occurs along the convected axes is also developed. This upgraded definition of rotational deformation also increases the robustness of the analysis technique, enabling larger time-steps during a dynamic simulation and with rapid convergence being achieved at each solution step.
Hydrodynamic Interactions for Small Arrays of Wave Energy Devices
Authors: Justino P. A. P. , de O. Falca˜o A. F.
Published: March 2009

The main object of this work is to develop accurate solutions for hydrodynamics coefficients of arrays of simple-geometry wave-energy devices. Use is made of a computationally very efficient method, to model the wave energy absorption by a rigid sphere moving in heave and sway in water of constant (finite) depth, that employs a series of pulsating multipoles located at the centre of the sphere; a monopole or source can be added to simulate the effect of pulsating volume. In the present work, this is extended to model the interaction between spheres in a finite array, by locating series of multipoles at the centre of each sphere and computing the multipoles intensities from the boundary conditions at the spheres’ surfaces in order to find the matrices of damping and added-mass coefficients in several oscillating modes. Numerical results are obtained for small arrays. This may be used to validate results from boundary-element methods.
Extreme Waves Detected by Satellite Borne Synthetic Aperture Radar
Authors: Lehner Susanne , Schulz-Stellenfleth Johannes , Niedermeier Andreas , Horstmann Jochen , Rosenthal Wolfgang
Published: March 2009

Within the last 20 years at least 200 supercarriers have been lost, due to severe weather conditions. In many cases the cause of accidents is believed to be ‘rouge waves’, which are individual waves of exceptional wave height or abnormal shape. I situ measurements of extreme waves are scarce and most observations are reported by ship masters after the encounter. In this paper a global set of synthetic aperture radar (SAR) images is used to detect extreme ocean wave events. The data were acquired aboard the European remote sensing satellite ERS-2 every 200 km along the track. As the data are not available as a standard product of the Europea Space Agency (ESA), the radar raw data were focused to complex SAR images using the processor BSAR developed by the German Aerospace Center. The entire SAR data set covers 27 days representing 34000 SAR imagettes with a size of 5km×10km. Complex SAR data contain information on ocean wave height, propagation direction and grouping as well as on ocean surface winds. Combining all of this information allows to extract and locate extreme waves from complex SAR images on a global basis. Special algorithms have been developed to retrieve the following parameters from the SAR data: Wind speed and direction, significant wave height, wave direction, wave groups and their individual heights. The satellite ENVISAT launched in March 2002 acquires SAR data with an even higher sampling rate (every 100 km). It is expected that a long-term analysis of ERS and ENVISAT data will give new insight into the physical processes responsible for rogue wave generation. Furthermore, the identification of hot spots will contribute to the optimization of ship routes.