bluuebunny/crossref_metadata_2025 huggingface dataset:
An Empirical Method for the Estimation of Towing Resistance of a Life Raft in Various Sea States
Authors: Mak Lawrence , Kuczora Andrew , Simo˜es Re´ Antonio
Published: May 2009

Current IMO regulations require life rafts to be tow tested only in calm water. In real evacuation situations, life rafts are deployed in the prevailing environmental conditions, with wind and waves. Added wave resistance is small at low wave heights but increases nonlinearly with increased wave height. If life rafts are to be towed in moderate seas (up to 4 m significant wave height), tow force estimates based only on calm water tow resistance become less reliable. Tow patches, towline, towing craft etc. also need to be designed to withstand dynamic wave loading in addition to mean load. Therefore, mean tow force, tow force variation and maximum tow force are important. A full-scale 16-person, commercially available, SOLAS approved life raft was towed in the tank, in upwind, head seas with significant wave height of 0.5 m. The measured tow force showed that it could be treated as a linear system with wave amplitude, by demonstrating that tow force is mainly inertial and follows a Rayleigh distribution. Therefore, extreme-value statistics used for waves can be applied to developing equations for predicting tow force. A method is proposed to predict life raft tow force at different tow speeds and in various sea states, with waves and wind. The method involved using tank experiments to obtain tow force response for one sea state. The information can then be used to predict life raft tow force in wind and waves for different sea states. Three equations are proposed to demonstrate that a simple tank experiment could provide valuable information necessary to empirically estimate the mean tow force, tow force variation and maximum tow force for a specific life raft in different sea states. The equations are developed for upwind, head seas. These equations were extensively validated using tow force measured in the tank. They were partially validated with limited sea trial data, by towing the same 16-person life raft and a 42-person life raft in upwind, head seas with significant wave height of 1.3 m. The equations were able to predict maximum tow forces to within 15% of the measured.
A Simple Time Domain Structural Redundancy Analysis Procedure for Semi-Submersibles
Authors: Chakrabarti Partha , Maiti Manoj K.
Published: May 2009

Offshore design codes like ABS and IMO require some level of redundancy in semi-submersible drilling vessels to withstand the loss of a slender bracing member without overall collapse of the structure, similar to fixed structures. Wave induced dynamic forces on semi-submersibles include hydrodynamic forces on ‘large body’, and inertia forces due to rigid body motions in six degrees of freedom. The amplitudes and phases of each component of the motion are important in defining the total force. Therefore, unlike static ‘pushover’ type analysis used in a relatively dynamically insensitive fixed jacket structure, semi-submersibles require nonlinear dynamic redundancy analysis in the time domain to determine the safety against collapse due to environmental loading. A simple time domain nonlinear analysis procedure is suggested in this study to capture the realistic behavior of the structure under wave loading. Dynamic loads are generated from hydrodynamic analysis of the floating body using a diffraction-radiation analysis program which assumes that the wave excitation is harmonic and so is the response. These loads are transferred to the structural analysis model. Each wave frequency is analyzed to produce a pair of loading conditions — ‘in-phase’ and ‘out of phase’. Combining these two components, a time history of the wave loading is created. In nonlinear structural analysis, first static loads are applied. Then wave load time history is applied for a few wave cycles in small increments. Results show that nonlinear analysis for one single cycle or two can usually predict the safety against collapse. If the analysis continues for a cycle or two, the structure passes the redundancy test. If it does not, the structure has a deficiency that needs to be addressed.
Research on Collision Mechanism for a Ship Colliding With a Spar Platform
Authors: Hu Zhiqiang , Cui Weicheng , Xiao Longfei , Yang Jianmin
Published: May 2009

The collision mechanisms of spar platform haven’t caused so much attention as that of ships in the past, for the short of this kind of collision accidents reported. But this does not mean the impossibility of the collision accident in the future. The research on external mechanism and internal mechanism for a ship colliding with a spar platform is introduced in this paper. A model test is designed to study the external mechanism. The collision scenario is described as a ship colliding with a spar platform moored in 1500 meters water depth. The specifics of the spar’s motions and the tension forces of the mooring lines are gathered, to find the hydrodynamic characteristics in the collision scenario. It is found that the maximal displacements and the maximal pitch angles of the spar platform, and the maximal tension forces of mooring lines are all linearly proportional to the initial velocity of the striking ship basically. Mooring lines play elastic roles in the collision course. The internal mechanism of the ship colliding with the spar platform is achieved by numerical simulation method and the software used is MSC.DYTRAN. A Truss-Spar is taken as the object and a double hull structural design is adopted in the part of hard tank near water surface. The curves of collision characters and the structural damage are obtained. The crashworthiness of the double hull design is verified, through the numerical simulation results.
Forecast of Critical Wave Groups From Surface Elevation Snapshots
Authors: Clauss Gu¨nther F. , Testa Daniel , Kosleck Sascha , Stu¨ck Robert
Published: May 2009

Reports on damages of ships, cargo and structures during heavy seas have been increasing within the last years. The impact of single extreme waves or wave groups on marine structures and ships causes enormous forces often leading to critical situations or even loss of crew, ship and cargo. Dangerous situations can be predicted by a forecast of encountering wave trains and the identification of critical wave groups. The paper presents a method to calculate the wave train a ship will encounter from surface elevation snapshots of the surrounding sea, taken by the ship radar. The time-dependent surface elevation snapshot far ahead of the ship is transferred into frequency domain by the use of Fast Fourier Transformation (FFT). The resulting complex Fourier spectrum given over the inverse wave length 1/L is converted into an amplitude spectrum and a phase spectrum. By shifting the phase spectrum to the position of the cruising ship the encountering waves can in turn be calculated in advance — depending on speed. The permanent processing of incoming snapshots delivers a continuous prediction of the water surface elevation at the position of the cruising ship. Based on these data the expected ship motion behaviour can be calculated continuously in time domain. In addition the response spectra, resulting from the wave spectrum and the relevant RAOs, are also evaluated. As wave data far ahead of the ship are used, it allows a forward glance, and dangerous situations, particularly resonance and parametric resonance are detectable before the ship is encountering this wave train. Consequently, the procedure can be used by the master as an assistance support system.
An Approach to VIV Design of Marine Risers
Authors: Lee Li , Allen Don W.
Published: May 2009

This paper presents an approach to vortex-induced vibration (VIV) design of marine risers. It begins with a brief review of the issue and then proceeds to presenting the key elements needed to make VIV predictions for marine risers. Uncertainties and discrepancies between model prediction and field measurements are discussed. Examples are used to illustrate the approach presented herein. Conclusions are drawn at the end of this paper.
SeaWolf: Latest Generation Inspection AUV
Authors: Hornfeld Willi
Published: May 2009

Autonomous underwater vehicles (AUV) operate independently of human control. The more sophisticated types of AUV are able to make their own mission-conformal decisions including appropriate action in unforeseen situations. The latest AUV development from ATLAS ELEKTRONIK is the SeaWolf, at present in the advanced development stage, is such a sophisticated type of AUV and will be shortly introduced into the market. The SeaWolf’s mission spectrum extends from the inspection of 2- and 3-D underwater facilities, maritime security in the underwater domain and to applications for maritime science. The vehicle belongs to the lower weight class of AUVs, has a large payload capacity and a endurance of several hours with a speed up to 8 kts. It is able to hover and has a very good manoeuvrability, due to its 5 propulsors. The SeaWolf can also be equipped with a fiberoptic cable, length of up to some kilometres, for broad band on-line data transmission. The Seawolf AUV, with its unique combination of power and manoeuvrability, will be able to execute a comprehensive variety of inspection, surveillance and reconnaissance tasks, including in high current environments and covering extremely complex structures. As well, SEAWOLF is able to detect and classify anomalies without human support. Such anomolies can be so handled even in areas of extensive marine growth such as mussels and barnacles, and in regions of poor visibility. To meet these demanding performance challenges, the SeaWolf is equipped with: • a multi-sensor package, • an efficient image processing software, • a precise geographic long range navigation system, • a very precise short range navigation system relative to the object, which guides the vehicle constantly in each axis from the object to be inspected ensuring a 100% coverage of the object’s surface. The SeaWolf is also able to execute the inspection from an existing CAD data from the target object. If such data are not available, SeaWolf will figure out the data file by itself on the basic of the actual inspection result. This paper presents the current AUV system ATLAS SeaWolf, describing the configuration and performance including an overview of potential applications and discusses the planned future developments.
Revision of DNV Design Standard for Offshore Wind Turbine Structures
Authors: Wedel-Heinen Jakob , Ronold Knut O. , Madsen Peter Hauge
Published: May 2009

The first DNV-OS-J101 standard “Design of Offshore Wind Turbine Structures” [1] was issued in June 2004. The standard represented a condensation of all relevant requirements in DNV standards for the offshore oil and gas industry which were considered relevant also for offshore wind turbine structures, supplemented by necessary adaptation to the wind turbine application. Det Norske Veritas (DNV) plans to issue the next revision of DNV-OS-J101 [2] in 2007. The DNV revised standard now implements the requirements of the coming IEC 61400-3 standard [11], which was presented as a committee draft in 2006. Numerous practical guidelines have been included to help designers of offshore wind turbine structures to develop cost optimal designs. The present paper summarises the proposed revisions of DNV-OS-J101 [2]. The most important revisions cover new formulations for design load cases, modified partial safety factors, exclusion of transformer platforms, more information on wave loads in shallow water and a revised chapter for design of concrete structures.
A Finite Element Analysis for Unbonded Flexible Risers Under Torsion
Authors: Bahtui A. , Bahai H. , Alfano G.
Published: May 2009

This paper presents a detailed finite element analysis of a five-layer unbonded flexible riser. The numerical results are compared analytical solutions for various load cases. In the finite element model all layers are modelled separately with contact interfaces placed between each layer. The finite element model includes the main features of the riser geometry with very little simplifying assumptions made. The numerical model was solved using a fully explicit time-integration scheme implemented in a parallel environment on a 16-processor cluster. The very good agreement found from numerical and analytical comparisons validates the use of our numerical model to provide benchmark solutions against which further detailed investigation will be made.
An Efficient Numerical Model of Hyperbolic Mild-Slope Equation
Authors: Song Zhiyao , Zhang Honggui , Kong Jun , Li Ruijie , Zhang Wei
Published: May 2009

Introduction of an effective wave elevation function, the simplest time-dependent hyperbolic mild-slope equation has been presented and an effective numerical model for the water wave propagation has been established combined with different boundary conditions in this paper. Through computing the effective wave elevation and transforming into the real transient wave motion, then related wave heights are computed. Because the truncation errors of the presented model only induced by the dissipation terms, but those of Lin’s model (2004) contributed by the convection terms, dissipation terms and source terms, the error analysis shows that calculation stability of this model is enhanced obviously compared with Lin’s one. The tests show that this model succeeds to the merit in Lin’s one and the computer program simpler, computational time shorter because of calculation stability enhanced efficiently and computer memory decreased obviously. The presented model has the capability of simulating exactly the location of transient wave front by the speed of wave propagation in the first test, which is important for the real-time prediction of the arrival time of water waves generated in the deep sea. The model is validated against experimental data for combined wave refraction and diffraction over submerged circular shoal on a flat bottom in the second test. Good agreements are gained. The model can be applied to the theory research and engineering applications about the wave propagation in the coastal waters.
Continuum Finite Element Methods to Establish Compressive Strain Limits for Offshore Pipelines in Ice Gouge Environments
Authors: Fatemi Ali , Kenny Shawn , Taheri Farid
Published: May 2009

In the design process for offshore pipelines in ice gouge environments, compressive strain limits provide a basis to assess pipeline mechanical integrity for design load events. A parametric study, using the continuum finite element methods, has been conducted to assess the global pipeline moment-curvature response for displacement-based loading conditions through the post-buckling regime. The purpose of this study was to investigate the accuracy and efficiency of some computational parameters in simulating the stability characteristics of thick pipes. For that, the study used a pipe that has been the subject of a comprehensive and extensive experimental investigation. In specific, the study selected the exact geometric, material, loadings, boundary conditions and operational parameters similar to the BPXA Northstar pipeline system. The numerical analysis examined the effect of element type, mesh density, internal pressure, axial load, end moment, and geometric imperfection mode on the predicted post-buckling response. The analysis demonstrated the importance of element type, mesh density and characteristics of initial geometric imperfections on the post-buckling response of a thick-walled pipeline subject to combine loads. In addition, element performance and solution efficiency was examined.
Detecting Tether Self-Collisions in Tethered ROV Simulations
Authors: Roy Andre´ , Carretero J. A. , Buckham B. J.
Published: May 2009

Currently, the only viable means of providing power and maintaining human control during underwater ROV operation is through the vehicle’s tether. Due to the high cost of ROVs and their tethers as well as potential risks to equipment and personnel, a realistic simulator is needed to train their pilots. To accurately simulate the tether, it is important to detect collisions of the tether with the environment and with itself as well as to calculate the forces involved. The aim of this work is to present a computationally efficient and accurate method of detecting tether self-contact. To this end, a combinatorial global optimisation method is first used to determine the approximate separation distances. Then, a local optimisation scheme is used to find the exact separation distance and the location of the closest points. This information can then be used to determine whether or not a collision has occurred. If a collision is detected, a force is applied at the collision site to maintain separation.
Experimental Study on Wave Loads of Flooded Ship
Authors: Kim B. W. , Hong D. C. , Hong S. Y. , Kyoung J. H. , Cho S. K. , Kim J. H. , Kim Y. S. , Choi S. K.
Published: May 2009

This paper investigates wave loads of a flooded ship by model test. Model tests are performed in ocean engineering basin of MOERI (Maritime and Ocean Engineering Research Institute). Ship motions are measured by RODYM6D. Wave loads such as shear forces, bending moments and torsion moments are measured by ATI load cell mounted on segmented parts of the ship model. A 300 m-long barge ship with two flooded compartments is considered in model test. Responses of intact and flooded cases are compared. The test results are also compared with numerical analyses using boundary element method.
Numerical Simulation of Two-Part Underwater Towing System: A Lumped Mass-Spring System Approach
Authors: Lalu P. P.
Published: May 2009

The altitude and position of a submerged body being towed by a vessel is affected by wave induced motion of the latter. To reduce this disturbance, a two-part towing system is used. In this study a two-dimensional dynamic model of the two-part underwater towing system is described. Numerical modeling of the problem was attempted. The finite difference algorithm was used to formulate the problem. A lumped mass-spring system was used for modeling the cable. The formulation for the towed body was based on Kirchhoff’s equations for motion of a rigid body. For the demonstration of the model, a numerical example of the tow-ship traveling in a wave field is described. Subsequently, response of the towed body was simulated and is compared with the experimental values obtained from the literature.
Neural Networks Based Simulation of Significant Wave Height
Authors: Bazargan H. , Bahai H. , Aminzadeh-Gohari A. , Bazargan A.
Published: May 2009

A large number of ocean activities call for real time or on-line forecasting of wind wave characteristics including significant wave height (Hs). The work reported in this paper uses statistics, and artificial neural networks trained with an optimization technique called simulated annealing to estimate the parameters of a probability distribution called hepta-parameter spline for the conditional probability density functions (pdf’s) of significant wave heights given their eight immediate preceding 3-hourly observed Hs’s. These pdf’s are used in the simulation of significant wave heights related to a location in the Pacific. The paper also deals with short and long term forecasting of Hs for the region through generating random variates from the spline distribution.
Hydrodynamic Force Identification From Vortex Induced Vibration Experiments With Slender Beams
Authors: Wu Jie , Larsen Carl M.
Published: May 2009

The characteristics of hydrodynamic forces are much more complicated for a long flexible cylinder under vortex induced vibration (VIV) than that for a rigid pipe. However, it is difficult to get force measurement directly from experiment with flexible beams. The dynamic behavior of the cylinder under VIV is recorded by accelerometers or transducers for bending moment measurement along the cylinder. The present work provides a modal approach to estimate the hydrodynamic force from measurement. The method is able to estimate the force at the fundamental vortex shedding frequency and the third order harmonic force component as well.
Numerical Simulation of the Flow Around an Array of Free-Surface Piercing Cylinders in Waves
Authors: Muscari Roberto , Di Mascio Andrea , Broglia Riccardo
Published: May 2009

This work deals with the viscous flow around an array of cylinders impinged by an incoming wave. Different configurations are considered in order to evaluate the effects of both wave heading and wave height on the loads applied to the bodies and on the run-ups. The results are also compared to previous calculations obtained with the assumption of inviscid flow with the aim of evaluating the contribution of viscosity.
Design Parameter Estimation Under Multivariate Extreme Ocean Environmental Conditions in the Bohai Sea
Authors: Dong Sheng , Wang Liang , Fu Xinyu
Published: May 2009

On the basis of 21 years hindcasted data in the Bohai Sea, a Trivariate Nested Logistic Distribution model is utilized to calculate the joint probability of extreme environmental conditions, such as wind speed, significant wave height, and current velocity, which simultaneously occur during storm processes. Statistical analysis shows that this approach is simple in mathematical form, explicit in its expression, and can be easily applied to parameter estimation. It involves hierarchical dependence and can be applied to a wider field than the symmetric Logistic model. The ocean environmental contours in three demensions are created rather than the usual two. The contours as slices through return periods of wind speed is displayed to find the mode of wave height and current velocity. Based on the multivariate extreme value distribution theory, the determination of environmental parameter values will be objective and reasonable in engineering design.
Effect of Strain History on Steel Tube Umbilical
Authors: Dobson Alan
Published: May 2009

The effect of repeated plastic strain on the mechanical properties of reeled super duplex tube used in the construction of umbilical riser systems is assessed. The impact of typical plastic strain cycles on the fatigue properties of tubing, mechanical properties of the tubing and the susceptibility of hydrogen embrittlement of the tubing has been investigated. There is evidence that plastic strain cycles have an effect upon the fatigue life of the tube; which can be attributed to a low cycle fatigue mechanism. There is no evidence that plastic strain cycling to specified limits has an effect upon the susceptibility of the material to Hydrogen Induced Stress Cracking (HISC) and typical plastic strain cycles have no degenerative effect upon the mechanical properties of super duplex tube and radial welds.
Stochastic Earthquake-Analysis of Underwater Storage Tanks
Authors: Karadeniz H.
Published: May 2009

In this paper, the problem and analysis method of underwater storage tanks resting on a horizontal seabed is presented under stochastic earthquake loading. The tank is axi-symmetrical and has a flexible wall/roof. The finite element method is used for the response solution. A solid axi-symmetrical FE has been formulated to idealize the tank whereas an axi-symmetrical fluid element is used for the idealization of the fluid domain. The Eulerian formulation of the fluid system is used to calculate interactive water pressure acting on the tank during the free motion of the tank and earthquake motion. For the response calculation, the modal analysis technique is used with a special algorithm to obtain natural frequencies of the water-structure coupled-system. For the stochastic description of the earthquake loading the modified Kanai-Tajimi earthquake spectrum is used. Finally, the analysis method presented in the paper is demonstrated by an example.
In Line and Cross-Flow Behavior of a Free-Spanning Pipeline
Authors: Morooka Celso K. , Idehara Annelise Y. , da Costa Matt Cyntia G.
Published: May 2009

In an offshore petroleum production system, subsea pipelines have an important function in the production of oil and gas from subsea wells. They are primarily horizontal steel pipe laying on, near or beneath to the seabed. Usually, a subsea pipeline is used to transport gas or oil from a satellite subsea well to a manifold as well as in the transferring process of the petroleum production from the offshore facility to onshore installations. Frequently this pipeline is buried in the seabed, and sometimes it passes over uneven seafloor creating freespan lengths. The pipeline structure will need to withstand environmental forces caused by soil, current and waves. In the free spanning portion, for the in-line direction, which is defined as the direction parallel to the ocean current flow direction, drag and lift effects usually are present. The cross-flow, or cross-flow direction, the presence of vortex induced vibrations (VIV) forces and self-weight are observed. Free-span lengths of pipeline can be a concern to the overall pipeline system due to material fatigue resulting from induced dynamic behavior. Previous investigations have shown that vortex induced vibrations are important element in the reduction of life-time service due to fatigue. In the present work, hydrodynamic forces are calculated by a semi-empirical formulation based on the Morison type equations, for inline and cross-flow directions. Ocean current and wave forces are considered for calculations of the free spanning pipeline behavior. Fluid particle velocity and acceleration are calculated based on the 5th order Strokes and Airy wave theory. The pipeline is modeled as an Euler-Bernoulli beam. Different boundary conditions have been investigated in the present study and the results are discussed. Discussions are carried out in terms of the behavior of the free-spanning pipeline, through time domain simulations.
The Radiation Damping Coefficients for Surge, Heave and Pitch Motion in Free Surface
Authors: Lee Jong-Hyun , Incecik Atilla
Published: May 2009

In order to predict the ship motion in a seaway, we need to determine the wave excitation forces and hydrodynamic coefficients. Currently many commercial programs based on various numerical procedures are used to predict the hydrodynamic coefficients. However special knowledge and skills for accurate modeling are required to be able to use commercial programs. Also in addition, the run time is rather excessive. This paper shows a simplified analytical procedure to predict the hydrodynamic radiation damping coefficients. In the formulation of the wave damping force due to oscillatory rigid-body motion, the use of wave radiation component with wave pattern analysis and of the two-dimensional motion energy conservation principle are made. In this paper, a procedure will be described to predict the motions of a barge geometry in time domain. At the initial stage of the development, a rectangular barge form was used and the model developed is based on strip theory. Numerical results for the radiation damping coefficients are presented and compared with the results of experiments obtained by Vughts.
Drilling Riser Fairing Tests at Prototype Reynolds Numbers
Authors: Allen Don W. , Henning Dean L. , Lee Li
Published: May 2009

Tow tests have been performed on flexible circular cylinders, with and without short weathervaning fairings, towed in a basin at critical and supercritical Reynolds numbers. The tests were conducted in the David Taylor Model Basin and the Rotating Arm Facility, at the Carderock Division, Naval Surface Warfare Center, in West Bethesda, Maryland. Measurements were made of both the drag and acceleration (due to vortex-induced vibration) of the cylinder. A 5-9/16-inch diameter PVC pipe was used to achieve Reynolds numbers ranging from about 7×105 to 1.5×106, in uniform flow, for straight tow tests with the pipe experiencing first mode bending vortex-induced vibration. Fiberglass pipes with a 2.5 inch diameter were used to achieve high mode number vortex-induced vibration, in sheared flow, at Reynolds numbers as high as about 3.75×105. The test results illustrate the importance of conducting tests at prototype Reynolds numbers for drilling riser as well as the importance of conducting tests in sheared flows and at higher mode numbers to fully understand the performance of a suppression device.
Controlled Weld Toe Profiles for Fatigue Life Extension in FSO’s and FPSO’s
Authors: Rodri´guez-Sa´nchez J. Efrai´n , Rodri´guez-Castellanos Alejandro , Carbajal-Romero Manuel F. , Ayala-Uraga Efre´n
Published: May 2009

Application of controlled weld toe profiles can be considered an option to extend the fatigue life of welded connections when ongoing tankers are converted in dry docks to serve like offshore ships (FPSOs and FSOs). Very slim chances to implement such fatigue improvement will arise when these vessels are in service, since a converted ship is designed to be inspected, maintained and repaired in situ and not in dry dock as it is uneconomical to interrupt production. Codes recognize fatigue life extension by means of a controlled weld toe profile, e.g. [1]. Application of a controlled weld toe profile during conversion in selected areas previously identified by stress analysis of the hull structure can lead to extend the converted vessel fatigue life to comply with an expected field life. The American Bureau of Shipping S-N curves allow a credit of 2.2 on fatigue life when suitable toe grinding and NDE are provided. A controlled weld toe profile can be applied in fatigue crack repaired welds during ship conversion or even on those that during ship conversion are found in a non-cracked condition but were identified prone to fatigue cracking in a stress assessment analysis under in-service conditions. Credit on fatigue life in various codes and results from experimental data obtained from fatigue tested specimens with a controlled weld toe profile are given. Comments on the design of a controlled weld toe profiles and recommendations based on experimental experience for the implementation of equipment to perform a controlled weld toe profile are also given. A Fracture Mechanics approach for the assessment of controlled weld toe profiles for fatigue life extension purposes is described. Initially, a comparison of SCFs for a typical ship hull plate connection with and without weld toe profile control determined by Finite Element Analysis (FEA) is presented. Then, results obtained from the FEA connection such as through plate stress distribution are used in a Fracture Mechanics Analysis to compare the fatigue crack growth curve in as-welded condition to that with controlled weld toe profile.
A Comparison of Methods for the Coupled Analysis of Floating Structures
Authors: Low Ying Min , Langley Robin S.
Published: May 2009

The dynamic analysis of a deepwater floating platform and the associated mooring/riser system should ideally be fully coupled to ensure a reliable response prediction. It is generally held that a time domain analysis is the only means of capturing the various coupling and nonlinear effects accurately. However, in recent work it has been found that for an ultra-deepwater floating system (2000m water depth), the highly efficient frequency domain approach can provide highly accurate response predictions. One reason for this is the accuracy of the drag linearization procedure over both first and second order motions, another reason is the minimal geometric nonlinearity displayed by the mooring lines in deepwater. In this paper, the aim is to develop an efficient analysis method for intermediate water depths, where both mooring/vessel coupling and geometric nonlinearity are of importance. It is found that the standard frequency domain approach is not so accurate for this case and two alternative methods are investigated. In the first, an enhanced frequency domain approach is adopted, in which line nonlinearities are linearized in a systematic way. In the second, a hybrid approach is adopted in which the low frequency motion is solved in the time domain while the high frequency motion is solved in the frequency domain; the two analyses are coupled by the fact that (i) the low frequency motion affects the mooring line geometry for the high frequency motion, and (ii) the high frequency motion affects the drag forces which damp the low frequency motion. The accuracy and efficiency of each of the methods are systematically compared.
New Cold-Expansion Process for End Sizing and Dimensional Precision of Inside Geometry on Both Ends of Line Pipes
Authors: Kuroda Kouichi , Arita Tsutomu , Takano Takahiro , Oosako Hajime , Okui Tatsuya , Akiyama Masayoshi
Published: May 2009

Cold-expansion process has been developed for ensuring dimensional precision of inside geometry on the end portions of line pipe. For expanding the inside diameter of line pipe, a plug insertion method by cold press in the axial direction from the end side was finally adopted. As the nature of end sizing operation, only very small amount of cold plastic strain is applicable by cold expansion, and in order to realize the high dimensional precision of end portion a special geometry of the plug had to be designed. If a single-taper plug is adopted the expanded portion tends to overshoot and the inside diameter of expanded portion is larger than the plug diameter. The new plug design allows smooth contact of the inside surface expanded by the inlet portion of the plug on the bearing portion of the plug and no overshooting phenomenon occurs. One of the simplest designs of this type of plug for practical use is a double-taper plug, and finite element method was carried out in order to determine the practical range of the plug geometry referring to the deep insight on the mechanism of expansion process.
The Instability of a Flexible Sheet in Uniform Parallel Flow
Authors: Morris-Thomas Michael T. , Steen Sverre
Published: May 2009

When a flexible sheet immersed in a fluid is under the influence of uniform flow, instability can arise when the fluid velocity reaches some critical value. The fluid-elastic instability known as flutter is the focus of this present work. We present a fluid-elastic model for a flexible sheet whereby the fluid dynamic lift is accounted for by the classical slender body approximation of Lighthill [1] for an ideal fluid. We describe aspects of the system in terms of a mass ratio α and a tension to flexural ratio γ. The model is solved by a spectral method to determine the fluid velocity and frequency at which instability occurs. In addition, we consider the fluid friction and damping on the response of the flexible sheet. Moreover, we compare predictions for the flutter velocity and frequency with published results. The unique aspect of this work is an investigation into the effect of additional tension, in the form of the ratio γ, on stability.
Development of a New On-Board Tool for Planning Drilling Riser Operations in High Current Environments
Authors: Lang Donogh W. , Murphy Kieran , Lane Michael
Published: May 2009

With drilling and exploration occurring in ever deeper waters and harsh environmental conditions, the ability to properly plan critical operations such as deployment and retrieval of the drilling riser becomes increasingly important. Particularly in deepwater high current environments, deployment and retrieval operations can be limited by the length of time it takes to deploy or retrieve the riser and the danger of the riser joints binding in the diverter housing. These limitations can have significant impact on both operations and riser integrity. Drilling contractors have devised a number of means of conducting operations in high current environments in order to increase operational up-time and mitigate the risks involved. One approach is drift-running, where the drilling rig is positioned upstream of the well at the start of the deployment operation and the rig is allowed to drift in a controlled manner to the well as the riser is deployed. This procedure effectively reduces the current loading on the riser, thereby allowing the riser to be deployed without risk of binding in the diverter housing. This paper describes an on-board simulation tool designed for use on dynamically-positioned (DP) drilling rigs that can be used to plan deployment/retrieval and drift-running operations. The tool is a development of an on-board drift-off simulator that has been in operation on-board the latest generation of DP drillships for some time. The simulator uses a fully-coupled 3D finite element (FE) model of the riser system, thereby allowing accurate determination of the riser response to current loading. The simulator computes the angle of the riser in the diverter housing throughout the deployment/retrieval process. Additionally, the simulator can compute the optimum drift speed of the vessel to minimise the riser angle. Based on the results of these simulations, the on-board tool can determine the feasibility of deploying or retrieving the riser and can compute the required vessel track for carrying out drift-running operations. The tool allows on-board personnel to plan these operations using either prevailing or forecast metocean conditions. The key features of the on-board simulator are discussed, with particular emphasis on the procedure used to compute the drift-running track for the drilling rig during riser deployment. The benefits of the system in planning deployment/retrieval operations are illustrated by means of a number of case-studies.
Vortex Suppression Efficiency of Discontinuous Helicoidal Fins
Authors: Pinto Antonio , Broglia Riccardo , Ciappi Elena , Di Mascio Andrea , Campana Emilio F. , Rocco Pierpaolo
Published: May 2009

Vortex-Induced Vibration (VIV) is one of the most demanding areas in the offshore industry, and detailed investigation of the fluid-structure interaction is becoming fundamental for designing new structures able to reduce VIV phenomenon. To carry on such analysis, and get reliable results in term of global coefficients, the correct modelling of turbulence, boundary layer, and separated flows is required. Nonetheless, the more accurate is the simulation, the more costly is the computation. Unsteady RANS simulations provide a good trade-off between numerical accuracy and computational time. This paper presents the analysis of the flow past a cylinder with several three-dimensional helical fins at high Reynolds number. Flow field, vortical structures, and response frequency patterns are analysed. Spectral analysis of data is performed to identify carrier frequencies, deemed to be critical due to the induced vibration of the whole structure. Finally, helical strakes efficiency in reducing the riser vibrations is also addressed, through direct consideration on the carrier shedding frequency.
Certification of Marine Energy Converters: The Experience So Far
Authors: Bittencourt Ferreira Claudio
Published: May 2009

In the past few years, DNV has been involved in a variety of projects related to marine energy converters. All projects have been characterised for the handling of technical uncertainties due to the application of new technology or proven technology in different area of application. A systematic approach based on the DNV RP-A203 Qualification of New Technology [1] was applied combined with the Guidelines for Design and Operation of Wave Energy Converters [2] to steer the third party activity, but, more importantly, to allow developers to systematically identify and deal with the risks in a rational manner with traceability of decisions throughout the development of the energy converter. From the very start of our engagement, it was clear that the handling of technical uncertainties was affected, not only by the technical barriers, but also by financial and time constraints. The establishment of the safety and functional targets to be achieved by the energy converter are to be based, not only on the safety and asset integrity aspects, but also on the financial / business model. The experience of using the Qualification process and the Guidelines on these projects, achieving the right balance between the constraints, handling of uncertainties, financial targets and safety and functional requirements, are briefly described in this paper as well as the future steps to be taken to improve the process and consolidate the experience so far. In this paper, it is also addressed the use of the DNV OSS-312 [3] on the certification process of marine energy converters.
Fatigue Resistant Threaded and Coupled Connectors: The New Standard for Deep Water Riser Applications
Authors: Sches Celine , Hill Joe W. , Peter Thomas , Massaglia Jacky , Lepine Etienne
Published: May 2009

In the past decade, premium threaded and coupled (T&C) connectors were developed for top tension riser applications (TTR). Compared to weld-on connectors, the benefit of such T&C risers is that, thanks to the use of high strength material (up to 125 ksi in yield), significant weight savings can be achieved on the full riser system and consequently on the platform or the floating unit. This is essential at the time when the offshore oil production goes deeper and deeper. Corresponding developments can concern full product lines, where the design and the performance are homogeneous regardless of the pipe diameter, thickness or grade. There can also be fit for purpose developments when specific material, fatigue or sealing performance is required. After years of varied developments, the outcome is several concepts concerning effective fatigue enhancement techniques and premium fatigue compliant sealing devices. These concepts are derived from a rigorous design and qualification process, supported by a huge quantity of full scale tests on connectors, material characterization tests, and the continuous improvement of our assessment techniques. Based on development capability and TTR experience from the field, premium T&C risers can be considered the standard versus weld-on. Continued development and testing could make this a viable solution for xHPHT, flow lines, and steel catenary riser applications.
Influence of Generator Damping on Peak Power and Variance of Power for a Direct Drive Wave Energy Converter
Authors: Sta˚lberg Magnus , Waters Rafael , Danielsson Oskar , Leijon Mats
Published: May 2009

The first full-scale offshore prototype of a novel wave energy converter system has been launched off the Swedish west coast. The concept is based on a point absorber directly coupled to a linear generator located on the ocean floor. The wave energy converter is part of a research project that will study the electrical system of 10 units forming a small farm of wave power plants as they are linked and connected to an electric grid. A full scale farm will consist of a large number of interconnected units. The chosen direct drive system reduces the mechanical complexity of the converter but has repercussions on the electrical system. The output from the generator will vary with the speed of the point absorber, leading to large fluctuations of power on the second scale. This has implications on both the individual generator and on the system as a whole. The hydrodynamic behavior of the point absorber depends, to a large extent, on the damping of the generator. The damping, in turn, can be controlled remotely by changing the load resistance. It has previously been shown that this has a large influence on the power absorbed by the wave energy converter. This paper investigates the peak power, the translator speed and the variance of the power at different sea states and for different levels of damping. The peak power has an impact on the design of the generator and the required ability, for a single unit, to handle electrical overloads. The speed of the translator is directly proportional to its momentum. It is thus important for the design of the end stop. The variance of the power of one unit will have an impact on the farm system behavior. The study is based on two and a half months of experimental measurements on the prototype wave energy converter and a wave measurement buoy. The aim is to analyze whether load control strategies may influence the dimensioning criteria for the electrical system and the generator. The results are compared with previously investigated relationships between absorbed mean power and load resistance as a function of sea state. In the study it was found that the maximum power is approximately proportional to the average power while maximum translator speed and standard deviation decrease as the damping factor is increased.
Wave Setup in Inlets: Some Practical Considerations
Authors: Kerper Dale , Appendini Christian M. , Kofoed-Hansen Henrik , Bro̸ker Ida
Published: May 2009

For the determination maximum flood elevations, a number of components contributing to the total water level need to be considered. For instance, astronomical tide, storm surge, relative changes in mean sea level, wave setup, wave runup and wave splash. In this study, numerical models were used to evaluate under which conditions wave setup penetrates into an idealized inlet. A number of idealized inlet/lagoon configurations were tested. A coupled wave-current model was used to assess the static component of the wave setup. A Boussinesq wave model was used to assess the influence of the dynamic oscillating component of the wave setup. This study demonstrates how numerical modeling tools can be effectively used to assess how wave setup develops depending on a specific inlet configuration.
The Influence of Loading History on the Reliability of Welded Structures
Authors: Smith David , Muhammed Amin
Published: May 2009

The impact of prior loading on the structural performance of welded components is explored. In particular, probabilistic models are used to determine the change in the survival probability of a welded component where prior loading contributes to changes in both the fracture toughness and residual stresses. The effect of prior loading on toughness reveals that there are shifts in the mean toughness together with changes in the statistical distribution. While there is a large body of data that characterize residual stresses in welded components there is considerable uncertainty about the choice of the mean and statistical properties. Furthermore, there is only limited statistical evidence for changes in residual stress following prior loading. The evidence points to the adoption of a simple linear model to explain reductions in residual stress after prior loading. The combined experimental evidence for changes in toughness and residual stress are then incorporated into probabilistic models. The models are used to investigate the sensitivity of the input parameters to measures of the survivability of a welded component. Predictions are compared to experimental evidence from large scale tests. Importantly, it is revealed that by including prior loading into an assessment of the performance of a welded structure we may be able to claim that the structure’s reliability is consistently better than that expected at start of life.
Structural Reliability Assessment of Offshore Platforms Under Hurricane Events
Authors: Wang Liangsheng , Chen Kaisheng , Bucknell Justin
Published: May 2009

This paper presents a structural reliability assessment method to quantify the probability of platform failure for Trinidad offshore platforms subjected to hurricane events. Platforms are modeled as a series system composed of the topsides and jacket including foundation. The platform failure limit state function is defined in terms of environmental load and platform capacity. The platform capacity is evaluated by non-linear pushover analysis using USFOS program. A parametric relationship of wave load as a function of wave height is derived based on the offshore extreme environmental data. The first order reliability method (FORM) is used to estimate the annual failure probability. The relationship between the probability of platform failure and the reserve strength ratio (RSR) of platforms is investigated. The assessment results could be used to evaluate the level of risk associated with hurricane hazards and may be incorporated into the risk-based underwater inspection (RBUI) program as part of the structural integrity management (SIM) process.