bluuebunny/crossref_metadata_2025 huggingface dataset:
Impact Analysis of a Vessel Exclusion Barrier
Authors: Chakrabarti Partha , Bose Kaushik , Chawan Abhijeet , Dhaware Sameer , McDole Mike
Published: January 2011

A vessel exclusion barrier (VEB) is used at a port to prevent an unauthorized boat from crossing it and entering an area of vital security concern. The VEB is a floating barrier extending several thousand feet constructed out of a number of units of modular construction most often made of HDPE material. These individual units are assembled together to make a segment of about 50 ft length interconnected by several wire rope cables. A number of these segments are placed next to each other to make up the full length. The connecting wire ropes provide the primary global strength to span the barrier between the supports. The VEB is anchored to the seabed at specific intervals using chain and/or wire rope mooring and gravity or pile anchors. There could be some gates that would allow entry to the authorized vessels. Sometimes the barrier is also equipped with wire nets to capture the encroaching vessel improving the effectiveness of the barrier. The primary objective of the present study is to perform analyses for impact by a small craft of about 30 ft length trying to breach the barrier. Through these analyses mooring configuration for the floating barrier will be selected and designed. These analyses will also provide estimation of the energy absorption capacity of the VEB. The size, grade and the number of cables interconnecting the modules, the freeboard height, and the anchoring system strength, all influence the energy absorption capability of the barrier. The energy absorption capacity would limit the size of the vessel and its speed that can theoretically be stopped by the barrier. The effect of various design parameters including interconnecting wire rope breaking strength, support spacing, mooring line strength, and mooring plan radius are needed to be studied to decide on the final design configuration for optimum impact energy absorption before taking critical design decisions. A computer model of the structure is developed for a specific application and nonlinear time domain dynamic analysis is performed to address the design problem. The modeling technique used is discussed in detail. Parametric studies are carried out to quantify the effect of each design factor. The primary critical factors and their effects that emerge from the study are discussed. To the authors’ best knowledge publication covering the analytical aspects of a vessel exclusion barrier does not exist in the public domain. Therefore, general conclusions made about the characteristics of the system are expected to help a designer for the proper design of a specific application of a VEB.
Hydrogen Induced Stress Cracking of Duplex Stainless Steel Subsea Components
Authors: Bahrami Amir , Woollin Paul
Published: January 2011

A small number of duplex and superduplex stainless steel components have failed in subsea service due to hydrogen induced stress cracking (HISC). The significance of these failures has led to research to define critical loading conditions for HISC, to allow confident design of components in future. Data relating to the Foinaven superduplex hub failures were published at OTC in 1999 and NACE Corrosion conference in 2001 and data from TWI Group Sponsored Projects were published at OMAE in 2004. DNV RP F112 has been based on these and other data, to provide a conservative approach to design. There are a number of gaps in the published literature and in the data available when DNV RP F112 was prepared, related to differences between small scale specimens tested at typical seabed temperature and actual components in operation, ie the operating temperature and pressure, and data from full-scale tests on pipe material with fine austenite spacing and hence good resistance to HISC. The paper presents new data on these issues and indicates where technology gaps remain.
Tests and Applications of a Numerical Model for Nonlinear Wave Propagation on Non-Uniform Current
Authors: Zhang Hong-sheng , Wang Wei-yuan , Feng Wen-jing , Yang Jian-min , Lu Shi-qiang
Published: January 2011

A Boussinesq-type wave model is used to investigate the interaction of waves with a non-uniform current field. The numerical model is first tested for wave propagation in a wave flume of uniform depth without current. It is then tested in a wave flume for a submerged bar. The validated numerical model is of higher accuracy than the fully-nonlinear fourth-order model, which is one of the best forms among the existing conventional Boussinesq models that do not incorporate breaking mechanism in one dimension. Comparison of model results with laboratory data shows the model is capable of representing wave-current interactions. The effect of current on wave is studied for different incident wave conditions, and the wave blocking is simulated successfully.
Numerical Simulations and Truncation Design of a Deepwater Internal Turret FPSO Before Model Test
Authors: Wang Hongwei , Luo Yong , Cheng Xiaoming
Published: January 2011

Model tests of an internal turret moored deepwater FPSO will be conducted in the deepwater offshore basin recently completed in Harbin Engineering University. Considering the limitations of the basin facilities and to reduce the complexity of fixing risers on the basin bottom, the 13 risers are simplified and combined to 4 risers. In addition, the horizontal dimension of the basin is not able to accommodate mooring system at the test model scale. Therefore, certain mooring lines have to be truncated. Two truncation methods are proposed according to the similitude of static characteristics of the mooring system. The first method only truncates the portion of the line that lies on the seabed and the second method truncates the suspended line portion. Numerical simulations have been carried out in the time domain with mooring lines coupled with FPSO. The calculated results are found to agree well with those of the prototype mooring system. Both truncation methods are found to be robust and reliable which provide a valuable reference for the forthcoming model tests.
The Effect of Whipping/Springing on Fatigue Damage and Extreme Response of Ship Structures
Authors: Mao Wengang , Ringsberg Jonas W. , Rychlik Igor
Published: January 2011

Wave-induced vibrations, also known as whipping and springing, are defined as the high frequency response of ship structures. In this paper, the fatigue damage caused by whipping and springing is presented by investigating the amidships section of a 2800 TEU container ship that operates in the North Atlantic Ocean. A simplified fatigue model, originally from the generalized narrow-band approximation for Gaussian load, is employed to include the damage contribution from wave-induced vibrations. In this model, the significant response range hs and the mean stress up-crossing frequency fz are simplified using only the wave-induced loading and encountered wave frequency, respectively. The capacity and accuracy of the model is illustrated by application on the measurements of the 2800 TEU container ship for different voyages during 2008. The whipping-induced contribution to the extreme response is investigated by means of the level crossing approach. It shows that the level crossing model for Gaussian load cannot be used for the prediction of extreme responses, such as the 100-year stress, based on a half-year full-scale measurement. It is found that a more complicated non-Gaussian model is required to consider the contribution from whipping.
A Method for the Verification of Subsea Equipment Subject to Hydrogen Induced Stress Cracking
Authors: Fabbrizzi Michelangelo , Di Sisto Paolo , Merlo Roberto
Published: January 2011

Subsea oil and gas production systems can be subject to Hydrogen Induced Stress Cracking (“HISC”) depending on the material, cathodic protection and other factors. A failure in this kind of systems can lead to safety issues as well as environmental hazards and high repair costs. The analysis of recent failures has led to the recognition of HISC as a very important issue related to local stress and strain. This has necessitated the extensive use of Finite Elements Methods for the analysis of all system components. Since HISC is a recent issue, there are very few cases of such assessments reported in the literature. This paper is based on the assessment of the susceptibility of subsea piping manifolds of Duplex stainless steel to Hydrogen Induced Stress Cracking, which was conducted during the Skarv project by General Electric Oil & Gas. A variety of cases consisting of different loads and configurations were considered to give a broad assessment using a recently developed code (DNV-RP-F112-October2008). This work has led to the development of a set of procedures and models for the assessment of the entire system which is described in the current paper. The proposed methodology is useful for both design purposes and also for the verification of parts, which, if found to be non-compliant, would require redesign. In general, parts that were determined to be non-compliant using a linear assessment were found to be compliant through non-linear analysis, in fact 3D plastic analysis leads to a redistribution of stress and strain and hence, to lower values. “Cold creep” was not considered since the levels of stress and strain were considered to be low enough to avoid this phenomenon. As a consequence of this experience, a new methodology was developed, which is able to speed up the analysis process and to predict local stresses from only pipe elements. The latter permits the use of a linear assessment for bends, T junctions and weldolet even with misalignment and erosion, avoiding the need to perform 3D analysis. The second part of the paper describes this method.
Dynamic Optimization of Steel Catenary Risers Based on Reliability Using Metamodel
Authors: Zheng Wenqing , Yang Hezhen
Published: January 2011

Reliability based design optimization (RBDO) of a steel catenary riser (SCR) using metamodel is investigated. The purpose of the optimization is to find the minimum-cost design subjecting to probabilistic constraints. To reduce the computational cost of the traditional double-loop RBDO, a single-loop RBDO approach is employed. The performance function is approximated by using metamodel to avoid time consuming finite element analysis during the dynamic optimization. The metamodel is constructed though design of experiments (DOE) sampling. In addition, the reliability assessment is carried out by Monte Carlo simulations. The result shows that the RBDO of SCR is a more rational optimization approach compared with traditional deterministic optimization, and using metamodel technique during the dynamic optimization process can significantly decrease the computational expense without sacrificing accuracy.
Heat Treatment Conditions of Low Carbon Steel Part Used in the Deep Sea
Authors: Lim Sang-Seop , Kang Chung-Gil
Published: January 2011

With increasing oil consumption, we have to find more oil resources in the deep sea. The extreme working condition of the deep sea requires high toughness and high strength values at low temperatures. Academic institutions limited the chemical composition of the requested casting steel to meet their requirements of fracture toughness and weldability. Thus, the carbon content was set approximately 0.10% based on classification societies which required specific mechanical properties of strength, elongation, reduction area and impact energy (−40°C). In this study, we find the optimal heat treatment condition of low carbon steel (0.10%C) to obtain the desired mechanical properties at low temperature (−40°C) according to different quenching parameters (heating times) and tempering parameters (heating temperatures, cooling methods).
Stress Concentration Factors in Tubular K-Joints Under Combined Loadings
Authors: Chen Tuanhai , Chen Guoming
Published: January 2011

A numerical method of stress concentration factors (SCFs) in tubular K-joints under combined loadings was proposed, and the loading effects on SCFs were investigated by using this method. Since it couldn’t present the state of stress concentration very well if the displacements were used as the boundary conditions, the load boundary conditions were selected to analyze the stress distribution of the tubular K-joints. In view of the disadvantages of the superposition of stress distributions from each uniaxial loading mode, an alternative method for the calculation of the hot spot stress under combined loadings was provided. In this method, three basic types of basic loadings namely, axial loading, in-plane and out-of-plane bending moments were applied to the tubular joints simultaneously by virtue of the submodeling technique in ANSYS system. Then the stress distributions along the intersection of tubular K-joints under combined loadings were developed directly, and the hot spot stresses were obtained through extrapolation. Finally, according to parametric study, the loading effects of ice thickness and drift direction on SCF distributions and values were studied through this method. The investigation provides the theoretical basis for determining SCF in tubular K-joints under different load cases in evaluating the ice-induced fatigue of offshore platforms, and has very important engineering application value.
A Systematic Approach for Quantitative Environmental Assessment of Seafloor Massive Sulfide Mining
Authors: Yamazaki Tetsuo , Ikemoto Masahito , Nakatani Naoki , Arai Rei
Published: January 2011

Seafloor massive sulfides have been a subject of interest for profitable commercial mining these ten years. However, less information is available for both the baseline ecosystems in the distribution areas and the environmental impacts on them caused by the mining. Owing to growing concern for the global and local environments, the quantitative understanding of the environmental impacts, the systematic environmental assessment, and the effective control and regulation methods of seafloor massive sulfide mining must be clarified. A systematic approach necessary for the clarification is discussed. An example design of artificial impact experiment and the monitoring are introduced. An international initiative is proposed.
Installation Vessel and Method for Offshore Wind Turbine in Ultra-Shallow Water
Authors: Fan Huiqu , Lin Jinbao , Shi Qingsong
Published: January 2011

Compared to onshore wind turbines, offshore wind turbines take advantage of wind speeds which are more constant and stronger than those on land. Since many large electricity load centers are located near coastline in China, larger wind turbines can be installed closer to these areas to supply energy in a more economical way. Wind turbine transportation and installation are key issues for offshore wind farm construction, especially for large size turbine installation in ultra-shallow water like intertidal zone with water depth less than 5m. The traditional installation vessels with large design drafts are likely to be trapped in shallow water zones. It is usually impossible to carry out turbine installation in shallow water. This paper presents a set of innovative installation vessel concept and corresponding methods for ultra-shallow water zone include ultra-shallow draft crane vessel and ultra-shallow draft barge. The main purpose is to simplify the installation procedures and reduce total investment.
Numerical Study of Air Gap Response and Wave Impact Load on a Moored Semi-Submersible Platform in Predetermined Irregular Wave Train
Authors: Liang Xiufeng , Yang Jianmin , Xiao Longfei , Li Xin , Li Jun
Published: January 2011

The importance of understanding air gap response and potential deck impact is well-known in the design stage of semi-submersible platform. The highly non-linear nature of wave elevation around large structures in steep waves makes it difficult to accurately predict wave field under the deck and wave run up along the columns. Present engineering tools for the prediction of air gap response generally based on simplified models. Even the models accounting for nonlinear wave diffraction is not free of uncertainties. A method adopted here couples a Navier-Stokes solver, VOF technique capturing violent free surface and DNV/Seasam predicting motions of moored semi-submersible platform. Air gap response at different locations of the hull was evaluated in predetermined irregular wave train. Wave run up was also measured by wave probes near the columns. Load cells were mounted under the deck of the platform to trace potential deck impact. The predetermined irregular wave train was simulated in a numerical wave tank and verified against physical tank results. Analysis of the air gap response, wave run up and impact loads on the semi-submersible platform were conducted.
Liner Wrinkling of Lined Pipe Under Compression: A Numerical and Experimental Investigation
Authors: Hilberink A. , Gresnigt A. M. , Sluys L. J.
Published: January 2011

A promising possibility to reduce costs in pipelines that require corrosion resistant alloys (CRA) is the use of lined pipe, consisting of a carbon steel load-bearing outer pipe that provides the structural capacity and a corrosion-resistant alloy (CRA) liner, protecting the carbon steel outer pipe from the transported corrosive product. The objective of the current research is to identify all variables that have an influence on the mechanical behaviour of lined pipe during bending and to determine their influence on this behaviour. The parameters that determine the resistance to pipe ovalisation and liner wrinkling are to be quantified, enabling safe and economic design of lined pipe for installation. This paper describes the approach and results obtained so far during the current research.
Wave Fields Diffracted by an Array of Porous Circular Cylinders
Authors: Zhao Fenfang , Kinoshita Takeshi , Bao Weiguang , Itakura Hiroshi
Published: January 2011

In the present work, the diffraction of waves by an array of porous circular cylinders with or without an inner porous plate is considered in detail based on the linear wave theory. For the sake of designing more effective floating breakwaters, the transmission rate of waves propagating through the array is also calculated. Each cylinder is partly made of porous materials. Namely, it possesses a porous sidewall and an impermeable bottom. In addition, an inner porous plate is fixed horizontally inside the cylinders, which works as obstruction to make wave dissipate more effectively and eliminates the phenomenon of sloshing mode. To deal with hydrodynamic interactions among a great number of bodies, a hierarchical interaction theory, which saves computation time efficiently, is adopted to compare with the approach of Kagemoto & Yue (1986). Consequently, a series of model test of an array of porous cylinder are carried out in the basin to validate the theoretical work and the calculation results. The draft of the cylinders, the location of the inner porous plate and the spacing between adjacent cylinders are adjusted to investigate its effect on the wave dissipation.
Computing Acceleration Loads on Free-Fall Lifeboat Occupants: Consequences of Including Nonlinearities in Water Waves and Mother Vessel Motions
Authors: Luxcey Neil , Fouques Se´bastien , Sauder Thomas
Published: January 2011

The safety of occupants in free-fall lifeboats (FFL) launched from a skid is addressed, and the focus is on numerical evaluation of acceleration loads during water impact. This paper investigates the required level of detail when modeling the physics of a lifeboat launch in waves. The first part emphasizes the importance of the non-linearity of the wave surface. Severity of impacts in linear (Airy) waves is compared to impacts in regular Stokes waves of the 5th order. Correspondingly, severity of impacts in irregular waves of the 2nd order is statistically compared to impacts in linear irregular waves. Theory of the two wave models are also briefly presented. The second part discusses the importance of a more detailed modeling of the launching system. This concerns especially cases for which damage to the mother vessel induces major lifeboat heel angles. A three-dimensional skid model is presented, along with validation against experimental measurements. In addition, the wave induced motion of the mother vessel is included. Consequences on the severity of the impact of the lifeboat in regular waves are discussed. This study is based on MARINTEK’s impact simulator for free-fall lifeboats, in which slamming loads are evaluated based on momentum conservation, a long wave approximation, and a von Karman type of approach. It is coupled here to the SIMO software, also developed at MARINTEK. Performance of this coupling is discussed.
VIV of Free Spanning Pipelines: Comparison of Response From Semi-Empirical Code to Model Tests
Authors: Passano Elizabeth , Larsen Carl M. , Wu Jie
Published: January 2011

The purpose of this paper is to compare predictions of vortex-induced vibrations (VIV) from a semi-empirical program to experimental data. The data is taken from a VIV model test program of a free span pipeline using a long elastic pipe model. Both in-line (IL) and cross-flow (CF) vibrations are compared. The Norwegian Ormen Lange field development included pipelines laid on very uneven seafloors, resulting in many free spans. As part of the preparations for this field development, VIV model tests of single- and multi-span pipelines were carried out at MARINTEK for Norsk Hydro, which later became a part of Statoil. The VIVANA program is a semi-empirical frequency domain program based on the finite element method. The program was originally developed by MARINTEK and the Norwegian University of Science and Technology (NTNU) to predict cross-flow response due to VIV. The fluid-structure interaction in VIVANA is described using added mass, excitation and damping coefficients. Default curves are available or the user may input other data. VIVANA originally included only cross-flow excitation but pure in-line excitation was later added. Recently, simultaneous cross-flow and in-line excitation has also been included. At present, the excitation in the cross-flow and in-line directions is not coupled. Coefficients for simultaneous cross-flow and in-line excitation have been proposed and are available in VIVANA. In this paper, response predicted by VIVANA has been compared to the Ormen Lange model tests for selected test series. The analyses with pure IL loading gave good estimates of IL response up to and beyond the start of CF response. The analyses with combined CF and IL loading gave good response estimates for the test series with a long span. The experiments with short spans tended to give CF and IL mode 1 response while the present version of the program gave IL response at higher modes. The present coefficient based approach is, however, promising. Further work should aim at establishing better coefficients and to understanding the interaction between CF and IL response.
Riser VIM Fatigue Design Induced by Deep Draft Semi-Submersible
Authors: Xiang Sherry , Cao Peimin , Rijken Oriol , Ma Jian , Chen Yongjun
Published: January 2011

SBM Atlantia Inc. (SBMA) designs and constructs DeepDraft Semi® Floating Production Units (FPU). Two of these units are installed in the South-Central Gulf of Mexico at water depths of 8,000-ft and 6,000-ft. Both units are operational. Each of these FPUs is characterized by four square columns interconnected by four rectangular pontoons. Model tests and field measurements have indicated that these types of vessels are susceptible to Vortex Induced Motions (VIM). Typical loop/eddy current conditions at these Gulf of Mexico locations are likely to induce semi FPU VIM response. The paper addresses the impact of the semi-submersible FPU VIM motion on the fatigue design of the Steel Catenary Risers (SCR). Semi VIM induced riser fatigue design is performed using time domain approach and the scatter diagrams for the surface currents. Riser fatigue damage rates consider the effects of both the inline and transverse semi VIM responses. A comprehensive sensitivity study has been carried out, and the results demonstrate that riser VIM fatigue is primarily affected by the combination of column shape, current heading and intensity, the individual riser configuration, mooring line pretention and polyester stiffness. A simplified approach based on quasi-static Stress Transfer Functions (STF) is proposed. The STF is developed based on the VIM motion amplitude without considering the effect of the mean FPU offset induced by the current. The STF allows one to quickly develop a preliminary map identifying the feasible riser configurations. Such map can be used during the Front End Engineering Design (FEED) studies to initiate viable riser configurations. Finally, it is important to note that the preliminary field VIM measurements are significantly less than those observed during the model tests. Therefore, the fatigue life estimates based on the model test observations are considered to be conservative.
Extreme Response Estimate of Steel Catenary Risers Using L-Moments
Authors: Cheng Jingyun , Spillane Mike , Bian Sean , Cao Peimin
Published: January 2011

The steel catenary risers (SCR) respond non-linearly to FPU motions, especially in extreme environments such as 100-year and 1000-year hurricanes. One of the design challenges is an accurate estimate of SCR response based on limited realizations. Due to the nonlinear nature of SCR response and the lack of enough sample data, traditional methods did not produce optimum extrema estimation. L-moments have been applied successfully in the statistical analysis of data, because they are linear combinations of probability weighted moments inherently and are less sensitive to outlying data values. Therefore, L-moments have been introduced in this paper to determine the candidate distribution of SCR responses and estimate extrema. Three typical SCRs connected to semi-submersible FPU have been analyzed using this methodology. Simulations confirmed L-moments can accurately determine the SCR responses distribution. Moreover, the extreme values predicted in this way were consistent and reliable as compared with current industry practice. Those observations have demonstrated that L-moments are robust and efficient methods in the design of steel catenary risers.
Operation Integrity Evaluations for Deepwater Drilling Riser System
Authors: Ge Long , Bhalla Kenneth , Stahl Matthew
Published: January 2011

This paper highlights concerns and operating issues for deepwater drilling risers operated from dynamically positioned or moored rigs, and discusses proper approaches and criteria to evaluate and ensure the operational integrity of drilling riser systems. As drilling water depths go deeper, more operation issues need to be considered. Major operating concerns of deepwater riser systems include: • collapse pressure rating, • handling and lift capacity, • deployment and retrieval, • planned or emergency riser disconnect, • connected operations, • drift-off or drive-off, • hangoff during storms, • fatigue due to vortex-induced-vibration or wave loads. In deepwater drilling, longer and heavier riser equipment, rig motions, weight mass ratio, hydrostatic pressure, mud flow, stiffness, drag, wave, and current all play important roles on riser system integrity, from well head to rig equipment. This paper provides general guidelines to evaluate the drilling riser system of various rigs under normal and extreme operational conditions. Issues for different major operations will be introduced, evaluation methods and associated criteria will be discussed, typical findings and remedial solutions will be presented.
Roll and Heave Response of Hull Sections of Variable Shapes in Waves
Authors: Yu Yi-Hsiang , Kinnas Spyros A.
Published: January 2011

This paper addresses the hull responses near resonance in beam seas. A 2-D analysis is performed, and the hull form is free to roll and to move in the vertical direction (2-DOF). A cell center based finite volume method is applied for solving the Navier-Stokes equations. The numerical scheme is utilized for analyzing the flow field around the hull section as well as for predicting the wave and floating hull interaction. The effect of the hull corner geometry and the effectiveness of using bilge keels on roll damping are examined. The results show that the maximum roll response is reduced when the hull is free to heave and to roll as compared to the roll-only case (1-DOF). In general, the maximum hull response decreases when the shed vortices are induced by the sharp edge, and the reduction increases as the keel width increases.
Efficient Computations of Fully-Nonlinear Wave Interactions With Floating Structures
Authors: Yan Hongmei , Liu Yuming
Published: January 2011

We consider the problem of fully nonlinear three-dimensional wave interactions with floating bodies with or without a forward speed. A highly efficient time-domain computational method is developed in the context of potential flow formulation using the pre-corrected Fast Fourier Transform (PFFT) algorithm based on a high-order boundary element method. The method reduces the computational effort in solving the boundary-value problem at each time step to O(NlnN) from O(N2∼3) of the classical boundary element methods, where N is the total number of unknowns. The high efficiency of this method allows accurate computations of fully-nonlinear hydrodynamic loads, wave runups, and motions of surface vessels and marine structures in rough seas. We apply this method to study the hydrodynamics of floating objects with a focus on the understanding of fully nonlinear effects in the presence of extreme waves and large-amplitude body motions.
Burst Prediction of Flexible Pipes
Authors: Gay Neto Alfredo , Martins Clo´vis de Arruda , Pesce Celso Pupo , Meirelles Christiano Odir C. , Malta Eduardo Ribeiro , Barbosa Neto Teo´filo F. , Godinho Carlos Alberto F.
Published: January 2011

Usually, when a large internal fluid pressure acts on the inner walls of flexible pipes, the carcass layer is not loaded as the first internal pressure resistance is given by the internal polymeric layer. The internal polymeric layer transmits almost all that loading to the metallic pressure resistant layer, or pressure armor layer. This layer must then be designed to ensure that the flexible pipe will not fail when loaded by a defined value of internal pressure. The present paper presents three different numerical models and one simple analytical model for determining the maximum internal pressure loading for a flexible pipe. The aim is to avoid pipe burst. The first numerical model is a ring approximation for the helically rolled pressure layer, considering its cross section as built. A second one is a full model for the same structure, considering the pressure layer laying angle and the cross section as built. The third numerical model is a 2D simplified version, considering the pressure layer as an equivalent ring. The first two numerical models consider contact nonlinearities and a nonlinear elastic-plastic material model for the pressure layer. The failure event and the corresponding stress state are discussed. Assumptions for each model are compared and discussed, with comparative results.
Characterizing Dangerous Waves for Ocean Wave Energy Converter Survivability
Authors: Hovland Justin , Paasch Robert , Haller Merrick
Published: January 2011

Ocean Wave Energy Converters (OWECs) operating on the water surface are subject to storms and other extreme events. In particular, high and steep waves, especially breaking waves, are likely the most dangerous to OWECs. A method for quantifying the breaking severity of waves is presented and applied to wave data from Coastal Data Information Program station 139. The data are wave height and length statistics found by conducting a zero-crossing analysis of time-series wave elevation records. Data from two of the most severe storms in the data set were analyzed. In order to estimate the breaking severity, two different steepness-based breaking criteria were utilized, one being the steepness where waves begin to show a tendency to break, the other the steepness above which waves are expected to break. Breaking severity is assigned as a fuzzy membership function between the two conditions. The distribution of breaking severity is found to be exponential. It is shown that the highest waves are not necessarily the most dangerous. Even so, waves expected to be breaking are observed being up to 17 meters tall at station 139.
Assessing the Global Wave Energy Potential
Authors: Mo̸rk Gunnar , Barstow Stephen , Kabuth Alina , Pontes M. Teresa
Published: January 2011

In this paper the evaluation of the global wave energy potential is presented based on data from a global wind-wave model (validated and calibrated against satellite altimeter data) and buoy data (the WorldWaves database). The theoretical potential was computed first using all the available wave data and, in a second step, areas in which the power level is very low (P≤5kW/m) were excluded. Finally, in the third step, areas impacted by sea ice were removed. Annual and seasonal power distributions are presented both in tables and maps. The technical resource was also assessed for the west coast of Iberian peninsula showing a significant power decrease from north to south within only 500 km.
Improvement of Torque Performance of a Vertical Axis Type Marine Turbine for a Water Current Generation System
Authors: Ikoma Tomoki , Fujio Shintaro , Masuda Koichi , Rheem Chang-Kyu , Maeda Hisaaki
Published: January 2011

This paper describes the possibility of an improvement of torque performance and hydrodynamic forces on a vertical axis type water turbine, used for marine current generating system. The water turbine analyzed here is based on a Darrieus turbine with vertical blades. We considered possibilities of controlling the angle of attack of blades in order to improve the starting performance and to reduce energy loss during the rotation of the turbine. We used blade-element/ momentum theory in order to investigate the variations appearing in torque performance when the angle of attack were controlled. We also proved the validity of our predictions of hydrodynamic forces on the blade and the turbine, made through CFD calculation, by comparing them with the results of corresponding model tests in a current channel. In the corresponding model test we investigated not only the hydrodynamic forces on the turbine with three fixed blades, but also the inline force and the cross-flow force on the rotating turbine with three blades. Regarding the cyclic pitching of turbine blades, results suggest that significant increase in average turbine torque is possible.
On Weibull Extreme Response of Offshore Structures
Authors: Chen Cheng-Yo , Mills Trevor
Published: January 2011

This paper investigates the so-called two-parameter and three-parameter Weibull distribution functions with regard to their technical formulation and application to prediction of extreme responses of offshore structures. The validity of predicted Weibull extreme responses is evaluated using time-history responses of two floating structures (a Spar and a Truss Semi) and a fixed-base compliant tower platform. All these structures were previously designed for harsh environments in deepwater beyond 1,700 ft. in the Gulf of Mexico. The evaluation results indicate that the maximum responses predicted by both two-parameter and three-parameter Weibull distribution functions with a single 3-hr simulation agree well with those obtained from a more lengthy and rigorous process with numerous 3-hr time-history responses. It also shows that the predicted maxima from the two-parameter Weibull distribution function are more reliable and have less statistical variability than those from the three-parameter Weibull distribution function, particularly for highly nonlinear wide-banded non-Gaussian responses such as minimum air gaps at the cellar deck of the topsides of the floating structures. The approach using the two-parameter Weibull function as presented in this paper is thus an appealing tool in predicting maximum responses and will greatly improve the process in the early design stage of a fast track project.
Two Methods to Obtain Analysis Solution of Beam on Winkler Foundation
Authors: Li Shun-qun , Gao Ling-xia , Chai Shou-xi
Published: January 2011

Based on the differential equation of beam section, two methods for beam on Winkler foundation in complex conditions are presented with displacement, angular rotation, moment and shear compatibility conditions of the two adjacent beam elements utilized. Together with the two moment boundary conditions at beam ends, (4n+2) equations can be got. In order to obtain analytic solution, two additional equations are required. The first method to obtain additional equations is depended on the principles of minimum potential energy and virtual work, and the second one makes advantage of boundary conditions of shear at two beam ends. The proposed methods in this paper can be used to compute beam on Winkler foundation in complex conditions which include complex beam conditions, such as variable Young’s modules and section, complex foundation conditions, namely variable coefficient of subgrade reaction, and complex loads consisting of concentrated force, moment and random distributed load. The displacement equations both for beam elements and for the whole beam, which have same structure as the classic Hetenyi solution, can be derived from the proposed method. Therefore, the equations for angular rotation, moment and shear of the beam can also be obtained from their differential relationship with displacement equation. The computation of an example shows that the results by the proposed method are consistent with that by the finite element method when the elements are small enough. Furthermore, element partition in the proposed method is thoroughly different from that in the classic finite element method even the former is belong to the later in the overall meaning.
Subsea Slurry Lift Pump for Deepsea Mining
Authors: Judge Robert A. , Yu Alan
Published: January 2011

Recent developments in subsea mining as well as oil & gas exploration and development drilling will require the use of a subsea pumping solution capable of handling slurries. These slurries are characterized by relatively large particle sizes and non-uniform flow conditions including fluctuating solids concentrations, densities, viscosities, and maximum particle size in both the mining and drilling applications. While challenging enough in surface applications, slurry transport problems are exacerbated by the vertical lift required in subsea use. This paper will first present the effect of each varying parameter on the overall horsepower required. Additionally, the pumping efficiency of some different pump types when responding to these changing conditions will be shown. From a control perspective, altering a pump’s speed in response to changing flow conditions presents some unique challenges. As an alternative to adjusting a rotary pump’s speed, an algorithm to control a positive displacement pumping solution to automatically adjust its output in this application is presented.
The Feasibility of Mini Coiled Tubing to Cleanout a Plugged Umbilical
Authors: Bhalla Kenneth , Gong Lixin
Published: January 2011

The purpose of this paper is to present an evaluation and assess the feasibility of various Coiled Tubing (CT) sizes to reach and remove a wax plug in a capillary in an umbilical. The wax plugs are located at a prescribed distance along the length of the umbilical. The equations governing the deformation of CT have been identified and modified. A field example is considered with the associated choices of mini coiled tubing that can be used to potentially achieve the objectives of the job. The main factors that influence the ability of the CT to reach a given location in the umbilical are: • Friction between the surfaces of the capillary in the umbilical and the CT; • Radial clearance between the capillary and the CT; • Weight of the CT per unit length; • Bending stiffness of the CT; • Inclination of the umbilical; • Curvature of the path of the umbilical; • The helical pitch of the capillary in the umbilical. These parameters affect the following: • The normal force between CT and umbilical; • The friction force between CT and umbilical; • The axial compression (or tension) on the CT; • The critical helical buckling load. When the compressive force on CT is greater than the critical helical buckling load, the CT will buckle into a helix, which leads to lockup. The equations that govern the normal force and the frictional force on the coiled tubing have been modified to account for the curvature of the umbilical and the helical pitch of the capillary. Previously, there has been no attempt to model the affect of the helical pitch from a capillary or even a tubing string. The exclusion of the pitch effect may have shown why previous simulations may have been successful, whereas the actual field job unsuccessful due to premature lockup. The inclusion of the helical pitch leads to additional normal forces which restrict the length/depth at which the tubing can be deployed. The premature lockup can now be accounted for by the additional normal/frictional forces induced by the helical pitch.
Cost-Effective Span Analysis Methodology for Different Pipeline Applications
Authors: Wang James , Xu Jianfeng , Jukes Paul
Published: January 2011

A pipeline laid on an uneven seabed may result in free spanning, which may cause strength and fatigue problems under static and dynamic loads, leading to vortex-induced vibrations. This can potentially result in expensive remediation and/or intervention works. An assessment of criticality of the spans can be complicated and time consuming due to complexity of the seabed profile, unknown pipeline residual tension, variability of the supporting soil stiffness and other uncertainties inherent in the problem. In this paper, a methodology for the span analysis is presented. It comprises a simple screening, with intermediate screening followed by detailed finite element analyses. Each of these analyses requires a different level of modeling effort and input data. Therefore, performing an efficient span analysis is important, because it not only ensures that all span issues are captured but also minimizes unnecessary calculations. Without proper planning of the analysis, critical pipeline spans may be overlooked or the number of analyses to be performed can be overwhelming. The analysis methodology described in the paper comprises the three levels of analysis mentioned above, and addresses how the work scope and specific application will control the required input and analysis levels. The applications discussed include existing versus new pipelines, selection of pipeline route versus as-installed analysis, pipelines with and without thermal buckles, and an analysis with complete versus incomplete seabed survey data. The applications also include scenarios with multiple spans, where resource allocation is of significance. The methodology presented in this paper presents the in-house experience and can serve as a guideline for a cost-effective span analysis.
Blade Response on Offshore Bottom Fixed Wind Turbines With Down-Wind Rotors
Authors: Reiso Marit , Moe Geir
Published: January 2011

Offshore wind turbines are becoming more common due to the scarcity of suitable land sites. By going offshore, maintenance costs become one of the driving expenses. Hence more reliable components should be implemented on offshore wind turbines. The down-wind rotor configuration does not require as powerful yaw drive as the upwind rotor configuration to align with the wind direction. Thus the yaw system can be simpler with fewer components that can fail and require maintenance. This paper presents numeric simulation studies of how the tower shadow impacts the blades when they pass through the wake behind the tower. The work concentrates on bottom-fixed offshore wind turbines designed for the specifications of the NREL offshore 5-MW baseline wind turbine. The blade response has been compared for a full truss tower and a conventional tubular tower to show how the different tower shadows influence the blades. The blades on the more transparent truss tower experience less root flapwise moment fluctuations due to the weaker tower shadow. The simulations were performed by means of GH Bladed, version 3.82.
Lift Coefficient Curves for Predicting Response Using Shear7
Authors: Jhingran Vikas , Vogiatzis Johnny
Published: January 2011

Vortex-Induced Vibration (VIV) is a complex, non-linear fluid-structure interaction problem with important consequences for offshore risers, tendons and other tubulars. The prevalent approach in the industry is to use semi-empirical formulations to estimate VIV amplitudes, frequencies and the resulting fatigue damage. These semi-empirical techniques estimate VIV response amplitude by considering the balance of power input into the pipe due to vortex-shedding and the loss of power from the pipe due to damping. At the heart of this method are lift coefficient curves, which are used to estimate power input into the pipe. Local lift coefficients are difficult to measure or derive for a flexible pipe and hence most of the lift curves used today have been developed using experiments with rigid cylinders. This paper discusses the development of a new family of lift coefficient curves using experimental data. Results of comparisons between predicted and measured data show the lift curves to effectively predict VIV response.
AC Ring Distribution: Architecture for Subsea Power Distribution
Authors: Huang Xianghui , McJunkin Samuel T.
Published: January 2011

Subsea power transmission and distribution is an emerging technology that may enable the oil and gas industry to produce hydrocarbon reserves in deeper and more remote offshore waters. The longer tieback subsea operations will likely require pumps and compressors driven by electric motors to be located on the sea floor to pressure boost the oil and gas to surface and/or onshore platforms. Existing HVAC and HVDC technologies are efficient means for subsea power transmission and distribution. However, they are subject to a variety of limitations, for instance, the single-point failures that would impact production uptime. Furthermore, it is challenging to implement subsea bulk power transmission and distribution by using existing architectures due to the footprint, weight and electrical insulation requirements. This paper describes a subsea power distribution architecture — AC ring. It can be used to interface a high-voltage bulk power transmission network, either AC or DC, to a subsea multi-load AC system. The new topology uses series-connected, by-passable, open-winding transformers to provide better modular design flexibility. The system is expected to be more reliable than conventional “hub and spoke” architectures and more technically feasible regarding practical subsea equipment designs.
An Attitude Survey on the Establishment of Offshore BSL-4 Facility as a NIMBY
Authors: Kobayashi Takako , Kondo Takeo , Miura Yuji , Yamamoto Kazukiyo , Miyazaki Wataru
Published: January 2011

We conducted a pilot attitude survey about attitude towards NIMBY (not in my back yard) facility with the students who major in ocean engineering and environmental engineering as subjects. The results found the following: facilities that are most often considered NIMBYs are a nuclear power plant, an industrial waste disposal facility, followed by a bio-research institute; if a NIMBY is established more than 50 km apart from the subjects’ residential area, its presence becomes acceptable; the degree of rejection lessens in the order of an isolated, uninhabited island, on a desert, and a submarine location. Also, the results showed that the best merit of building a BSL-4 facility offshore is that it can be relocated to another location if necessary. As a conclusion, a BSL-4 facility is most likely to be accepted to the local residents if it is built on a floating construction 50 km offshore, allowing to move to another location if necessary.