bluuebunny/crossref_metadata_2025 huggingface dataset:
Bend Stiffener Design for Umbilicals
Authors: So̸dahl Nils , Ottesen Torfinn
Published: November 2011

A crucial design issue for compliant risers and umbilicals for dynamic applications is termination of the compliant structure to a rigid structure. A practical way to solve this problem is to introduce a properly designed bend stiffener to limit the stresses in the compliant structure due to bending at the supports. The bend stiffener provides a gradually increase of the bending stiffness from the rather small value of the compliant structure to a significantly larger value that can be rigidly connected without compromising the structural integrity of the compliant structure. Hence, the bend stiffener geometry needs to be designed to fulfill the design requirements for extreme as well as fatigue loading conditions for the compliant structure. Furthermore, it is required that the bend stiffener is as short as possible to limit costs, support forces, and enable fabrication and installation. The main focus of this paper is to outline an optimization scheme for bend stiffeners to meet design criteria for extreme loading conditions. Measures to provide an adequate fatigue performance of bend stiffeners are also discussed. The loads on the bend stiffener are governed by effective tension and relative angle close to the support (i.e. direction of effective tension relative to the longitudinal direction of the compliant structure at the support). Combinations of effective tension and relative angle aggregated for all relevant extreme load conditions define the design loads on the bend stiffener. The capacity of the compliant structure is governed by a capacity curve expressing the allowable curvature as function of the effective tension. A general optimization scheme is outlined accounting for a general description of the design loads as well as the capacity curve. The optimization methodology is based on a general purpose optimization algorithm utilizing a tailor made non-linear static finite element solver to describe the response of the bend stiffener and the compliant structure. Non-dimensional design curves are also presented based on a simplified conservative description of the design loads and the capacity. This allows for easy practical sizing of bend stiffeners without the need for sophisticated optimization software.
The Study on the Ice Sea Trial in Chukchi Sea Using Korean Icebreaker “Araon”
Authors: Kim Hyun Soo , Lee Chun-Ju , Choi Kyungsik
Published: November 2011

The ice sea trial measurement in Chukchi Sea using research vessel Araon was performed on July 2010. It was the first voyage to the Arctic Sea. The latitude of the route was between 73 degree north to 80 degree north. Araon is the first Korean Ice breaking research vessel. The principle dimension is 110m length, 19m beam and 7.3m draft. Araon was designed to break 1.0m level ice of 630 kPa flexible ice strength. Four attempts to know the performance of the ship in Arctic region were carried out and the results were summarized in this paper. The basic datum for the sea trial such as ship speed, power of engine, wind speed, location of the ship, air temperature, drafts, heading of the ship etc., were measured during the trail in every second by the video recording. Simultaneously the ice information such as ice thickness, compressive strength, temperature of ice, snow depth, free board of ice floe were measured in each field by the coring tool, auger and compression test equipment. The ice sea trial was performed in large ice floes rather than level ice because the sea ice condition on July and August in Chukchi Sea has no uniform level ice and starts to melt. The size of four ice floes is about 100m to 300m length and 100m to 200m wide beam. It was some second year ice and most of first year ice floes. The mean flexible strength of ice was less than 250 kPa. The analysis result of the ice sea trial shows the relation between the ice thickness, ice strength, ship speed and power of engine. Araon is possible to operate at 1.5 knots in 2.5m ice thickness with 5 MW engine power when the strength of ice floe is 250 kPa. The speed reaches 3.1 knots at same ice condition if the power is increasing up to 6.6 MW. She has good performance compare to the design target (3 knots in 1.0m level ice and 630 kPa of flexible strength) but it’s come from the different ice types and low flexible ice strength. The more detail analysis result was discussed in this paper.
Upheaval Buckling Analysis of Partially Buried Pipeline Subjected to High Pressure and High Temperature
Authors: Sun Jason , Shi Han , Jukes Paul
Published: November 2011

Offshore industry is now pushing into the deepwater and starting to face the much higher energy reservoir with high pressure and high temperature. Besides the significant impacts on the material, strength, and reliability of the wellhead, tree, and manifold valve; high Pressure (HP) also leads to thicker pipe wall that increases manufacturing and installation cost. High Temperature (HT) can have much wider impact on operation since the whole subsea system has to be operated over a greater temperature range between the non-producing situations such as installation, and long term shut down, and the maximum production flow. It is more concerned for fact that thicker wall pipe results in much greater thermal load so to make the pipeline strength and tie-in designs more challenging. Burying sections of a HPHT pipeline can provide the advantages of thermal insulation by using the soil cover to retain the cool-down time. Burial can also help to achieve high confidence anchoring and additional resistance to the pipeline axial expansion and walking. Upheaval buckling is a major concern for the buried pipelines because it can generate a high level of strain when happens. Excessive yielding can cause the pipeline to fail prematurely. Partial burial can have less concern although it may complicate the pipeline global buckling behavior and impose challenges on the design and analysis. This paper presents the studies on the upheaval buckling of partially buried pipelines, typical example of an annulus flooded pipe-in-pipe (PIP) configuration. The full-scale FE models were created to simulate the pipeline thermal expansion / upheaval / lateral buckling responses. The pipe-soil interaction (PSI) elements were utilized to model the relationship between the soil resistance (force) and the pipe displacement for the buried sections. The effects of soil cover height, vertical prop size, and soil resistance on the upheaval and lateral buckling response of a partially buried pipeline were investigated. This paper presents the latest techniques, allows an understanding in the global buckling, upheaval or lateral, of partially buried pipeline under the HPHT, and assists the industry to pursue safer but cost effective design.
Numerical Study of Oscillatory Flow Past Four Cylinders in Square Arrangement for Pitch Ratio Equal to 4
Authors: Anagnostopoulos P. , Dikarou Ch. , Seitanis S. A.
Published: November 2011

The results of a numerical study of the viscous oscillating flow around four circular cylinders are presented herein, for a constant frequency parameter, β, equal to 50, and Keulegan-Carpenter numbers, KC, ranging between 0.2 and 10. The cylinders were placed on the vertices of a square, whose two sides were perpendicular and two parallel to the oncoming flow, for a pitch ratio, P/D, equal to 4. The finite-element method was employed for the solution of the Navier-Stokes equations, in the formulation where the stream function and the vorticity are the field variables. The streamlines and the vorticity contours generated from the solution were used for the flow visualization. When the Keulegan-Carpenter number is lower than 4, the flow remains symmetrical with respect to the horizontal axis of symmetry of the solution domain and periodic at consecutive cycles. As KC increases to 4 the flow becomes aperiodic in different cycles, although symmetry with respect to the horizontal central line of the domain is preserved. For KC equal to 5 asymmetries appear intermittently in the flow, which are eventually amplified as KC increases still further. These asymmetries, in association with the aperiodicity at different cycles, lead to an almost chaotic configuration, as KC grows larger. For characteristic cases the flow pattern and the traces of the hydrodynamic forces are presented. In addition, the mean and r.m.s. values of the in-line and transverse forces and the hydrodynamic coefficients of the inline force were evaluated for the entire range of Keulegan-Carpenter numbers examined.
Simulation of Ship Manoeuvring in the Proximity of a Pier by Using Support Vector Machines
Authors: Luo Wei-lin , Zou Zao-jian , Xiang Hong-liang
Published: November 2011

Ship manoeuvrability has a close relationship with navigation safety. For a ship sailing in the proximity of a pier, its manoeuvrability is different from that in unrestricted waters. To predict the ship manoeuvrability in the proximity of a pier, the method of simulation based on mathematical model is used. The mathematical model of ship manoeuvring is obtained by using Support Vector Machines (SVM) based system identification, combined with free-running model tests. The interaction between the pier and the ship are considered in the mathematical model of ship manoeuvring motion. By analyzing the test results, SVM is applied to identify the hydrodynamic coefficients and the interaction coefficients in the mathematical model. By using the regressive models, the ship manoeuvring motions in the proximity of a pier are predicted. Comparisons between the predicted results and the test results demonstrate the validity of the proposed SVM method. Identification results, model test results as well, also confirm that navigation safety of the ship in the proximity of a pier can be guaranteed if the distance between the pier and the ship is under control.
Application of “Nonlinear Fourier Transform (NLFT)” for the Analysis of Soliton Fission Behind Submerged Reefs With Finite Width
Authors: Bru¨hl Markus , Oumeraci Hocine
Published: November 2011

The conventional Fourier transform (FT) is the most common analysis method in the frequency domain. The measured signal is decomposed into sine and cosine waves as linear basic components. Every basic component is represented in the Fourier spectrum by its wave amplitude, frequency and phase. As an extension to the conventional FT, this paper presents the advanced “nonlinear Fourier transform (NLFT)” as an alternative nonlinear approach for the analysis of nonlinear shallow water waves. The NLFT uses nonlinear cnoidal waves as basic components and considers their nonlinear wave-wave interactions in the analysis. The application of the NLFT to the analysis of numerical test data is shown using examples of soliton fission over and behind submerged reefs. The determined nonlinear Fourier spectra illustrate the capability of this method and its advantages as compared to the conventional FT.
Fatigue Reliability Assessment of Coupled Spar-Mooring System
Authors: Jameel Mohammed , Ahmad Suhail
Published: November 2011

Spar platform is a compliant floating structure used for exploration of oil and gas from deep sea. To ensure safe operations, reliability against mooring line failure is a major concern in design. Furthermore, the mooring lines have high investment costs and are normally not accessible for in-service inspection. The common approach for solving the dynamics of Spar system is to employ a decoupled quasi-static approach which ignores the platform and mooring lines interaction. Coupled analysis, used presently, considers the mooring lines and platform in an integrated single model. Hence, it effectively captures the damping effect due to Spar and mooring lines coupling. Finite element code ABAQUS is used to obtain the response of Spar-mooring system under long crested random sea with current. Limit state function is derived based on failure due to fatigue for probabilistic reliability assessment. Random variables, participating actively in the limit state function are identified and statistically modeled. The most probable points or the design points are found to be an effective parameter for estimating partial factors of safety for load and resistance variables. First Order Reliability Method (FORM) is used to calculate probability of failure and reliability indices. The results are later checked against Monte carlo simulation. Reliabilities of segmental length of mooring and of full length are determined as they may significantly differ if the mooring properties change along the length. Reliability indices of annual and life time sea states are calculated.
Riser Management System Performance in Ultra-Deep Water With Rapidly Changing Current
Authors: Gundersen Gerhard , Jensen Gullik A.
Published: November 2011

The largest loading in terms of bending for a top tensioned riser in ultra-deep waters, subject to strong and rapidly changing ocean currents, are found to be close to the upper and lower extremeties. This has been demonstrated by the analyses of various dynamic current scenarios including strong cross currents and currents with rapidly changing direction, resembling eddies and loop currents based on metocean data from Brazilian waters. The riser response in terms of deflections and bending is not found to be critical at any location along the riser for the investigated load cases. Monitoring a riser based on the upper and lower flex-joint angles are hence sufficient to safely operate and control the drilling riser under such conditions. This implies that Riser Management Systems (RMS) that are widely used in intermediate and deep waters with slowly varying currents can safely be applied for top tensioned risers in ultra-deep waters with strong and rapidly changing currents. In fact they may prove to be more important under these conditions, to reduce the wear and tear, improve safety, and as a decision support tool for when to safely disconnect the riser.
Simulation and Prediction of Loads in Marine Current Turbine Full-Scale Rotor Blades
Authors: Senat Junior , Arockiasamy Madasamy
Published: November 2011

The paper presents an analysis for predicting the torque, thrust, and bending moments at the root of rotor blades in a horizontal axis marine current turbine using the blade element-momentum theory combined with linear wave theory. The measured current velocities taken near the core of the Florida Current offshore Fort Lauderdale, Florida are used in the evaluation of the loads on the rotor blades. The periodic loading on the blade due to the steady spatial variation of current speeds over the rotor swept area is determined by a limited number of parameters, including tip-speed ratio, Reynolds number, lift and drag coefficients, thrust and torque coefficients, and power coefficient. In the simulation modeling, numerical calculations are carried out for every time step on the assumption of quasi-steady flow. Lift and drag coefficients are calculated as function of incident angles taking into account the 3D effects. The results from this study include simulated thrust and torque when rotor operates in calm water and waves. The variation in the thrust and torque is predicted for certain parameters including wave height and wave frequency. The thrust and torque are evaluated by varying both the rotor’s rotational speed and current speed. It has been found both thrust and torque increase as the current speed increases, and in longer waves the torque is relatively sensitive to the variation of wave height. Both in-plane and out-of-plane bending moments fluctuate significantly and can be predicted by linear wave theory with blade element-momentum theory.
Metocean Design Criteria for Pipeline On-Bottom Stability
Authors: Gibson Richard
Published: November 2011

This paper is concerned with a response based method for determining metocean design criteria for offshore pipelines. The method determines a set of metocean parameters that are consistent with the extreme response of the pipeline, and hence, incorporates the dependence between them implicitly. However, there are a number of challenges in its application. Firstly, the loading on a pipeline is dependent on the previous wave cycle, and hence, the drag and inertia coefficients vary within a sea-state. Secondly, along many pipeline routes the waves are depth limited and the short-term distribution of wave induced velocity and pipeline response can be difficult to define. These challenges are overcome through a number of approaches that include a parametric representation of the distribution of the response and the application of multivariate extreme value analysis. Furthermore, the sensitivity of the method to assumptions about the pipeline design is examined, and the problems with using the combined wave and current induced velocity as a proxy for the response are discussed. The method is applied to a site in the Mediterranean Sea and the results are compared to those from the application of the first order reliability method.
Damage Detection in Offshore Free Span Pipelines Using Wavelet Packet Transform
Authors: Shahverdi Sajad , Lotfollahi-Yaghin Mohammad Ali , Aminfar Mohammad Hossein , Valizadeh Ramin
Published: November 2011

During the service life, offshore structures continually accumulate damage as a result of applying the various environmental forces. Clearly the development of strong techniques for early damage detection is very important to avoid the possible occurrence of a disastrous structural failure. Most of vibration-based damage detection methods require the modal properties that are obtained from measured signals through the system identification techniques. However, the modal properties such as natural frequencies and mode shapes are not such a good sensitive indication of structural damage. The wavelet packet transform (WPT) is a mathematical tool that has a special advantage over the traditional Fourier transform in analyzing non-stationary signals. In this study, a damage detection index called wavelet packet energy rate index (WPERI), is used for the damage detection of offshore free span pipelines. The measured dynamic signals are decomposed into the wavelet packet components and the wavelet energy rate index is computed to indicate the structural damage. It is observed that this method can be used for damage detection of this kind of structures.
Wave Interaction With an Infinite Long Horizontal Elliptical Cylinder
Authors: Song Hao , Tao Longbin
Published: November 2011

Wave-structure interaction in ocean engineering is a major source of unsteady loading and vibration of offshore structures including platforms, risers and long cables. Many efforts focus on vertical structures in which solution procedures can usually be simplified in the plane of mean free surface as the variable in the direction of gravity can be separated. In this paper, wave interaction with an infinite long horizontal elliptical cylinder is solved by a semi-analytical method, namely, the scaled boundary finite-element method (SBFEM). The solution domain is divided into two bounded domains and two unbounded domains with parallel side-faces. The governing partial differential equation (Helmholtz equation) is weakened and transformed to ordinary matrix differential equations in radial direction and are then solved analytically by SBFEM.
Offshore Pipeline Installation: 3-Dimensional Finite Element Modelling
Authors: Marchionni Lorenzo , Alessandro Lombardi , Vitali Luigino
Published: November 2011

The future offshore pipeline development projects envisage the installation of medium to large diameter pipelines (16″ to 32″ ND) transporting gas from the deep waters to the shallow water areas. The development of these deep water projects is limited by the feasibility/economics of the construction phase using the J-lay or the S-lay technology. In particular, the S-lay feasibility depends on the applicable tension at the tensioner which is a function of water depth, stinger geometry (length and curvature), and installation criteria. In this paper: – The challenges of future deep water offshore pipelines are briefly presented; – The installation criteria at the overbend, stinger tip and sagbend are discussed; – The ABAQUS FE Model, developed to simulate pipeline installation, is presented together with the pre- and post-processing program put in place; – The results of the developed ABAQUS FE Model are given considering two typical examples of deep water pipelines installed in the S-lay mode.
Statistical Analysis on the Extreme Events of Big Waves Under Wave Climate Change Around Taiwan Waters
Authors: Hwang Ching-Her , Lee Wen-Ching , Hsieh Wen-Fang , Tsai Ching-Piao , Chien Hwa
Published: November 2011

This study aimed to analyze the statistical characteristics of wave heights, wave energy and wave steepness, in order to investigate the wave climate changes around Taiwan Waters, especially for extreme events of big waves. The operational observation of Taiwan sea waves was initiated by the Central Weather Bureau in 1998; however, due to insufficient data length and low data space coverage, the data are unable to serve as references for long-term wave climate change research. Hence, this study adopted the SWAN (Simulation of Wave in Nearshore) Numerical Wave Hindcasting Method, which is a common method used in many studies, to hindcast the history of a wave field. The re-analysis on wind field data of the last 60 years (1948∼2008), published by the National Centers for Environmental Prediction (NCEP), was employed to make the wind field grid consistent with the hindcast wave field grid. Moreover, the Typhoon Wind Field Grid Down Scaling technique proposed by Winter & Chiou (2007) was applied to interpolate a U10 analysis field that better fits an actual typhoon wind field. The hindcast wave data were compared and validated with directional spectra, which were observed by the meteorological/oceanographic data buoys set up by the Central Weather Bureau and Water Resources Agency since 1997. Longdong, Hualien and Hsinchu Stations were chosen to represent the wave characteristics of sea areas around the island of Taiwan. According to observation data, model parameters were adjusted so that the hindcast results could be closer to observed data in Taiwan sea areas.
Influence of Wave Group Characteristics on Loads in Severe Seas
Authors: Clauss Gu¨nther F. , Dudek Matthias , Klein Marco
Published: November 2011

The precise knowledge of loads and motions in extreme sea states is indispensable to ensure reliability and survival of ships and floating offshore structures. In the last decades, several accidents in severe weather with disastrous consequences have shown the need for further investigations. Besides the sea state behavior and the local structural loads, one key parameter for safe ship design is the vertical bending moment. Previous investigations revealed that different ship design criteria, such as bow geometry and wave board height, affect the global loads significantly. Investigations in regular waves as well as in single high waves of vessels with different bow flares and freeboard heights show that the vertical bending moment increases significantly with increasing bow flare and freeboard height. Furthermore it became apparent that critical loads and motions do not have to come along with the highest wave which results in the main question of this paper: What is the worst case scenario — the highest rogue wave or a wave group with certain frequency characteristics? Which sea states have to be taken into account for the experimental evaluation of limiting criteria? This paper presents investigations in different critical wave sequences, i.e. two real-sea registrations accompanied by results in regular waves to evaluate the influence of the encountering wave characteristics on the vertical bending moment. For the model tests in the seakeeping basin of the Technical University Berlin a segmented RoRo vessel with large bow flare has been built at a scale of 1:70 and equipped with force transducers. The paper proves that critical loads and motions depend most notably on combinations of wave height, wave group sequences, crest steepness, encountering speed and the ships target position: Even small wave heights with unfavorable wave lengths can cause a critical situation.
Development of a Prototype Test Rig for a Cryogenic, Marine, Heavy Lift, Buoyancy System
Authors: Nicholls-Lee R. F. , Turnock S. R. , Tan M. , McDonald P. C. , Shenoi R. A.
Published: November 2011

There is a continual search for new methods of marine salvage and decommissioning of structures in the open sea in order to improve control and lower operational costs. The concept design of a lightweight, cryogenic, marine, heavy lift, buoyancy system has been investigated. The objective is to be able to raise or lower high mass objects controlled solely from a surface support vessel. The overall design concept and associated system development issues have been discussed previously. This work concentrates on the development of a test rig for the cryogenic, marine, heavy lift, buoyancy system and associated test procedures. The main area of concern in the design process is the cryogenic Dewar. This is required to operate at temperatures as low as −196°C but also at pressures exceeding 35bar. A more detailed design of the Dewar, including cryogenic composite materials testing, is considered. The prototype will be assessed through three different testing scenarios; shallow water, open water deep sea, and a hyperbaric chamber for very high pressure testing. The procedures for each of these test scenarios is discussed.
Sparse Spectrum Model of the Sea Surface
Authors: Charnotskii Mikhail
Published: November 2011

Traditionally statistical modeling of the sea surface is based on the Fast Fourier Transform (FFT) technique. In this approach the surface is represented as a sum of sinusoids with equally spaced frequencies and random amplitudes and phases. Statistic of the amplitudes and phases is prescribed to match the required observation-based spectral density. Higher spatial resolution and/or larger coverage domain is achieved by increasing the number of the spectral components in the model. For some remote sensing applications the required number of components can be as high as 1011. The conventional FFT technique is based on the Dense Spectrum (DS) model: namely on conjecture that at any given time moment all spectral components that are allowed by the spectral density are actually present at the surface. However it was realized by M. S. Longuet-Higgins as early as in 1956 [1], that DS is just an assumption. We propose a novel Sparse Spectrum (SS) model of the sea surface where each surface realization contains a finite, possibly random, number of sinusoidal components with random frequency, phase and amplitude. Unlike the FFT-based model, the number of spectral components forming the surface is determined by the sea state, but not the desired spatial resolution and domain size. A single constraint on the probability distributions of the wavenumbers, and the amplitude variances of the components allows the SS surface to conform to any prescribed spectral density. This affords a great flexibility in designing Monte-Carlo simulation algorithms that produce the surface samples, and provides substantial reduction of the computation efforts compare to the conventional FFT-based models. We designed and tested several versions of the Monte-Carlo surface simulation models that generate samples of the surface conforming to the Elfouhaily [2] spectral model. Visual inspection of the model-generated dynamic samples suggests that about 3000 spectral components are sufficient to generate the realistic surface samples capturing the scales from 2mm to 300m. In addition to the computational savings, the SS model is capable of providing a well-defined statistics of the individual waves that are important for the marine engineering applications.
Breaking Wave Impact on a Platform Column: An Introductory CFD Study
Authors: Pakozdi Csaba , Kendon Timothy E. , Stansberg Carl-Trygve
Published: November 2011

The slamming of breaking waves on the legs of large volume offshore platforms has received increased attention over recent years. To investigate this problem, MARINTEK’s Wave Impact Loads JIP has, in one of its sub-tasks, focused towards an idealised model test setup of a rectangular cylinder in breaking waves. The model consists of a vertical column with a fragment of a horizontal deck attached. The model is fixed at a distance L ahead of the wave maker. Physical scale model test experiments of the block in regular waves and in wave groups have been carried out in Phase 1 of the JIP (2008). The objective of this study is the CFD simulation of a long crested breaking wave and its impact on the aforementioned cylinder and deck structure in order to find out the feasibility of the numerical reconstruction of such events. The commercial CFD tool Star-CCM+ V5.03.0056 (www.cd-adapco.com) is used in this study. This paper considers results from the test setup, and compares the measured wave elevation against results from the CFD code. The position of the cylinder in relation to the breaking wave front is investigated in the numerical simulation in order to analyze its effect on the slamming force. Use of an unsteady wave boundary condition, matching the exact motion history of the wave-maker with the measured free surface elevation at the wave maker gives an almost exact matching between the computed wave profile and the measured wave profile. The improvement in the numerical tool of Star-CCM+ which makes it possible to use higher order time integration scheme for VOF significantly decreases the numerical diffusion of the wave propagation. This new scheme also enables the use of a time step 10 times larger than the first order scheme which reduces the computational time. Because a large time step can be chosen it is important that the time step is small enough to capture the correct time evolution of the physical phenomena of interest. Capturing the pressure evolution at a slamming event demands very high spatial resolution. Spatially averaged slamming pressures look fairly similar to the model test observations, while further work is needed for a more detailed comparison.
Wet Tree Semi-Submersible With SCRs for 4,000 ft Water Depth in the Gulf of Mexico
Authors: Cheng Jingyun , Cao Peimin , Xiang Sherry
Published: November 2011

This paper presents a design of a deep draft wet tree semi-submersible with steel catenary risers (SCRs) for 4,000 ft water depth in the Gulf of Mexico (GoM). The integrated system of hull, mooring, and SCRs is discussed. The design challenges of SCRs are highlighted and results of SCR strength and fatigue performance are presented. A comparison study on strength performance of various types of risers under the GoM environment criteria is performed. The assessment of extreme strength responses from various riser and hull configurations provide guidelines for the best hull selection. Sour service requirement creates challenges in the fatigue design of the production riser system at such water depth. Integrated mooring and riser design provides an optimum solution. It’s found that the majority of riser fatigue damage at touch down zone is generated by wave loading & resultant vessel motion and vortex induced vessel motion (VIM). Several fatigue mitigation methods are suggested to improve the riser fatigue performance, such as planned vessel repositioning. The conclusion of this study is that deep draft wet tree semi-submersible with SCRs can be a cost effective solution for field development at 4,000 ft water depth in the Gulf of Mexico.
Experimental Study of Erosion Threshold of Reconstituted Sediments
Authors: Ye Zhihui , Cheng Liang , Zang Zhipeng
Published: November 2011

Understanding of fundamental erosion characteristics of seabed sediments on which pipelines and other structures are founded is critical for the design of these facilities. The erosion threshold condition of cohesive sediments is not well understood because of the complexity and variability of natural sediments. Most of the existing methods for evaluating the erosion behaviours of seabed sediment are often applicable to the certain particular sediment types and test conditions. There appears to be a need for more research efforts in this area. In present research, the threshold of motion of four moderately consolidated mixtures was tested under unidirectional currents using a testing facility. Three threshold shear stress increase modes were observed, including initial increase mode, steady increase mode and equilibrium mode as mass content of mud Pm increases. A dimensionless threshold shear stress τ* is proposed to quantify the shear strength of these reconstituted mixtures, coupled with variation of coarse particle size. Finally, a predicting model is proposed to illustrate the trend of erosion threshold of the four mixtures as a function of mud weight content. Further understanding of erosion threshold can rely on the various consolidation conditions and different coarse and fine materials inputted in the mixture.
Long-Term Reliability Analysis of a Spar Buoy-Supported Floating Offshore Wind Turbine
Authors: Sultania A. , Manuel L.
Published: November 2011

Most offshore wind turbines constructed to date have support structures for the turbine towers that extend to the seabed. Such bottom-supported turbines are confined to shallow waters closer to the shore. Sites farther offshore provide a better wind resource (i.e., stronger wind and less turbulence) while also reducing concerns related to visual impact and noise. However, in deeper waters, bottom-supported turbines are less economical. Wind turbines mounted atop floating platforms are, thus, being considered for deepwater sites. Several floating platform concepts are being considered; they differ mainly in how they provide stability to counter the large mass of the rotor-nacelle assembly located high above the water. One of these alternative concepts is a spar buoy floating platform with a deep draft structure and a low center of gravity, below the center of buoyancy. The reliability analysis of a spar-supported 5MW wind turbine based on stochastic simulation is the subject of this study. Environmental data from a selected deepwater reference site are employed in the numerical studies. Using time-domain simulations, the dynamic behavior of the coupled platform-turbine system is studied; statistics of tower and rotor loads as well as platform motions are estimated and critical combinations of wind speed and wave height identified. Long-term loads associated with a 50-year return period are estimated using statistical extrapolation based on loads derived from the simulations. Inverse reliability procedures that seek appropriate load fractiles for the underlying random variables consistent with the target return period are employed; these include use of: (i) the 2D Inverse First-Order Reliability Method (FORM) where an extreme load is selected at its median level (conditional on a derived critical wind speed and wave height combination); and (ii) the 3D Inverse FORM where variability in the environmental and load random variables is fully represented to derive the 50-year load.
Numerical Simulation on Nonlinear Evolution of Rogue Waves on Currents Based on the NLS Equation
Authors: Hu Hanhong , Ma Ning
Published: November 2011

In this paper, nonlinear instability and evolution of deep-water rogue waves on following and opposing currents were described by numerical simulation for laboratory investigation. The generation of rogue waves in a numerical tank by means of wave focusing technique had been studied. Here a spatial domain model of current modified nonlinear Schro¨dinger (NLSC) equations in one horizontal dimension (1D) was established for describing the deep-water wave trains in a prescribed stationary current field. The transient water waves (TWW) was adopted as the initial condition of the NLSC equation. The steady current was added to see the effect of wave-current interaction on the energy concentration of gravity waves. The influence of current as well as other terms in the NLSC equations on wave height, inclination, particle velocity and acceleration are shown. Meanwhile, the focusing time/position of TWW influenced by the current field is investigated, which is of course a very important factor in experimental research when we generate rogue waves in the laboratory.
High Frequency Loading and Response of Offshore Structures in Steep Waves
Authors: Johannessen Thomas B.
Published: November 2011

Offshore structures such as the TLP or the GBS have natural frequencies which are much higher than the frequencies of the incident waves in the survival conditions. Nevertheless, many offshore structures experience significant resonant response of modes with periods in the range of 2s to 5s, particularly in steep waves. In particular the ringing response of offshore structures characterised by sudden, large and isolated resonant response packets, has been a concern for many years. The loads which give rise to these events are difficult to describe both because they are small in magnitude relative to the load level close to the wave spectral peak and also because they are nonlinear in nature. In the present paper, available theoretical methods for high frequency loading is employed for irregular waves and compared with model tests. The methods which are used in the present are first and second order diffraction methods as well as a third order loading model for slender cylinders applied to irregular waves with continuous wave spectra. The results are compared with measurements of tether response and overturning moments on a TLP and a GBS respectively. Provided that the incident waves are treated carefully and care is taken in treating the high frequency tail of the incident wave, it is found that methods which are presently available give a good representation of the resonant response for the GBS structure. The GBS structure has a relatively low natural frequency and a mode shape which is excited easily by horizontal loading in the surface zone. In contrast, weakly nonlinear theory does not capture the high frequency loading on a TLP which has resonant frequencies at more than five times the spectral peak in the survival seastates. For this case it is found that wave impact with both the columns and the deck gives significant contributions to the resonant tether response. This is the case even if no significant horizontal deck impact is observed and highlights the need for a reliable deck impact load model.
Development of a Computer Program for the Optimization of Deck Accommodation Arrangements of Ships and Craft Applying GRASP and VNS Meta-Heuristics Methods
Authors: Schachter Richard David , de Abreu Nair Maria Maia , de Melo Valdir Agustinho , Loiola Eliane Maria
Published: November 2011

This work presents a computer program developed for the definition of Deck Accommodation Arrangements of ships and craft that allocates spaces in an automated process, in order to help the designer with options and initial ideas for its definition or to optimize arrangements propositions. This optimization process is usually done based on experience, using maritime regulations and practices. It is an evolution of previous works [14] which adopted the Quadratic Assignment Problem (QAP), that allowed the user to select units to be assigned to demarcated spaces and the program provides optimized options of arrangements, which can be transformed, in turn, into drawings. In this work, a hybrid meta-heuristic has been incorporated to the program, namely the Greedy Randomized Adaptive Search Procedures — GRASP [7], and for the local search of solutions, the Variable Neighborhood Search — VNS [8], to improve the results. The program incorporates the mathematical model, containing menus of types of accommodation/compartments or deck units, for the user’s selection. Visualization and manipulation screens for friendly use were created, that allow the designer to quickly assemble deck arrangements in spaces, with an indirect tri-dimensionality feature. The program processes position and flow matrices, being the later ones determined through calibration (being modifiable) as a function of applicable Regulations of ILO 147 Convention, SOLAS and NORMAM (Brazilian Administration), incorporating as well maritime practice and usual ergonomic criteria for requirements of safety and habitability. The results can be used for General Arrangement drawings and can aid in the calculations of light weight and center of gravity, sail area, etc. A small discussion of recommendations for accommodation arrangements of the related regulations, rules of thumb derived from design practice and ergonomic criteria is provided.
A Benchmark Study of ISO 18072-2 on the Stiffened Panel Ultimate Strength
Authors: Frieze Paul A. , Abbattista Martino , Vallascas Mirella , Paik Jeom K.
Published: November 2011

The paper presents a major benchmarking exercise to demonstrate the accuracy of the formulations for the ultimate strength of stiffened panels that had been proposed for inclusion in ISO TS 18072-2. The complete set of formulations addressed the ultimate strength of plates, hull girders and support members, in particular, transverse frames and webs. The stiffened panel strength formulations have been in the public domain for some time and represent the most comprehensive set yet published. Their potential application is to a wide range of structural forms, e.g. floating dock gates, caissons, bridge decks, FPSOs, etc, and as such have much to recommend for other than just ship structures. The benchmark study presents comparisons between two implementations of the formulations and the results of non-linear FEA. The structures analysed represent a range of different stiffened panels with varying overall dimensions, plate dimensions and stiffener shapes, in particular, flats, angles and Tees. Close correlation is generally realized but where differences arise, these are readily related to the approximations used in developing the strength formulations. The comparisons between the strength formulations and non-linear FEA results provides the basis for quantifying statistical uncertainties in the formulations which can be used in subsequent structural reliability analysis and partial resistance factor derivation.
Identification of Favourable Free Fall Parameters for the AGaPaS Rescue Catamaran
Authors: Clauss Gu¨nther F. , Kauffeldt Andre´ , Otten Nils , Stuppe Sven
Published: November 2011

The joint research project “AGaPaS” (Autonomous Galileo-supported Rescue Vessel for Persons overboard) aims to significantly raise the chances of survival for people who have gone overboard. Its main objective is to develop a self activating, partially autonomously operating rescue system, able to search, find and rescue a drifting person [1]. A crucial part of the system is a remotely operated vessel, which is released by free fall from a mother ship. This type of launch offers a minimum response time but comes with the disadvantage of high loads for the rescue vessel and its equipment. A particular challenge is posed by the catamaran shaped hull, being dropped from a cruising ship. The goal is to find an optimum in the occurring accelerations and pressure loads applied to the hull. While the pressure loads have a direct impact on the strength of the hull, the accelerations mainly affect the boat equipment and its mountings. The hull optimization has already been conducted [2], therefore, this publication focuses on the identification of favourable launch parameters. Within the scope of this investigation crucial parameters such as cruise speed, drop height and launch angle are varied. All free falls are analyzed by means of computational fluid dynamics (CFD), whereas selected cases are validated by experimental data.
Examining a Comprehensive Dataset Containing Thousands of Freak Wave Events: Part 2—Analysis and Findings
Authors: Christou Marios , Ewans Kevin
Published: November 2011

This paper concerns the analysis of a very large, quality-controlled dataset of raw wave measurements. It directly follows from paper 1, as part of work undertaken for the CresT (Cooperative Research on Extreme Seas and their impacT) Joint Industry Project (JIP), and describes the various analyses performed on the dataset. In particular numerous freak wave events are observed and various analyses are performed to gain an insight into conditions that are conducive to their formation. The examination of probability distributions, spectral and temporal parameters, degree of focusing and environmental conditions that lead to freak waves is performed and the findings are presented.
Fatigue Assessment of Aluminum Ship Details by Hot-Spot Stress Approach
Authors: Tveiten Ba˚rd Wathne , Berge Stig , Wang Xiaozhi
Published: November 2011

This paper presents a robust methodology for fatigue assessment of aluminum ship details using a hot-spot stress range approach. A series of fatigue tests of a typical aluminum ship detail was carried out to obtain a design S-N curve. The test detail was analyzed by the finite element method (FEM) using several modeling techniques and element types. Recommendations are given on finite element modeling practice, extrapolation methods and design S-N curves.
Offshore Pipeline Embedment in Cohesive Soil: A Comparison Between Existing and CEL Solutions
Authors: Shi Han , Sun Jason , Hossain Kabir , Eltaher Ayman , Jukes Paul
Published: November 2011

A common issue confronted by engineers in analyzing high pressure high temperature (HPHT) pipelines for installation and operating conditions is pipe-soil interaction. For installation, a key concern is whether the soil can generate sufficient resistance to allow the pipeline to be laid on the curve. For operation, a concern is whether the pipeline structural stress can be controlled and mitigated, for the given soil condition, under conditions of thermal expansion and potential global buckling. In both scenarios, pipeline embedment is a critical parameter as it is directly related to soil resistances to the pipeline stability. Previous studies have used experimental, analytical and numerical methods to provide estimates to the pipe embedment during the laying operation. The recently developed Coupled Eulerian-Lagrange (CEL) finite element analysis (FEA) method provides a promising numerical technique in analyzing large-deformation geotechnical problems, such as pipeline embedment analysis. This paper uses this approach, together with currently available embedment solutions, to cross-validate these methods for cohesive soils.