bluuebunny/crossref_metadata_2025 huggingface dataset:
VolturnUS 1:8-Scale FRP Floating Wind Turbine Tower: Analysis, Design, Testing and Performance
Authors: Young Andrew C. , Hettick Steve , Dagher Habib J. , Viselli Anthony M. , Goupee Andrew J.
Published: September 2014

In May of 2013 the VolturnUS 1:8 floating semi-submersible wind turbine was successfully deployed off the coast of Castine, Maine, making the unit the first grid connected offshore turbine in the United States. The VolturnUS 1:8 structure features a 20 kW turbine, a post-tensioned and reinforced concrete semi-submersible base and a fiber reinforced plastic (FRP) tower (E-glass and polyester resin). The VolturnUS 1:8 structure is a geometrically 1:8-scale of a 6 MW floating turbine design and is used to demonstrate the feasibility of both the concrete base and FRP tower and validate the performance of the structure in a scaled environment. Data collected from the deployed 1:8-scale structure will be used for modeling and simulating the behavior of the system at full-scale. The effort was led by the University of Maine’s Advanced Structures and Composites Center (UMaine) and a consortium of industry partners, including FRP manufacturer Ershigs, Inc.

An overview of the process and methodology used in the analysis, design and testing of the 1:8 scale FRP floating wind turbine tower is presented. The use of an FRP tower on a floating wind turbine platform offers the benefits of reduced tower mass and maintenance requirements and has the potential to further reduce hull mass by lowering the global center of gravity of the structure. An FRP tower for use on the UMaine semi-submersible concrete VolturnUS 1:8 platform was developed that meets all strength and serviceability criteria and is robust enough to withstand the loading from both wind and waves. An overview of the tower loads analysis and FAST modeling, tower structural design, structural proof testing and preliminary analysis of performance are presented.

The VolturnUS 1:8 wind turbine tower is the first time FRP materials have been used in an offshore wind tower application. Further, the methodologies and procedures that were developed in the design of the pilot-scale tower are directly applicable to the design and analysis of composite wind turbine towers at the full-scale level. These “lessons learned” are already in use as Ershigs and UMaine work to design a full-scale composite tower over 80 meters tall for use on the VolturnUS platform with a 6MW wind turbine. The results of the 1:8-scale program demonstrate the successful use of an FRP wind turbine tower on a floating platform and highlights the potential for the use of an FRP tower at the full-scale (6 MW) level.
A Simplified Procedure for Global Hull Strength Analysis of Classic Spars
Authors: Teng Xiaoqing
Published: September 2014

Primary structural components of a spar hull are designed to resist lateral hydrodynamic and hydrostatic pressure and global loads. The scantlings of each primary component are usually determined based on the largest pressure it may encounter in various phases such as wet-tow, upending, and in-place operational conditions. The effect of global bending moment and shear force on the spar hull is often evaluated much later via laborious finite element analysis. This paper proposes a simple analytical tool for quickly assessing global hull strength of classic spars in the in-place condition. A spar platform undergoes steady, low-frequency, and wave-frequency motion of comparable magnitude at the same time in a storm event. The present approach separates the wave-frequency component from the steady and low-frequency response. A closed-form solution is developed for wave-induced motion and loads by taking advantage of the simple cylindrical shape of classic spar hulls. The theoretical solution is verified by comparing to numerical WAMIT results. The low-frequency response is approximated as one part of the steady response, since its dynamic effect is weak. The steady structural response is mainly a function of the heel angle. It is demonstrated that local effect of wind pressure and current load is not significant. The total response, as a summation of the wave-induced loads and the steady solution, is represented by global bending moment and shear force envelopes along the spar hull for a given sea state. Global bending and shear stresses of primary structural components can be further calculated and checked against code requirements. This procedure has been implemented through Matlab scripts. A comparison with global finite element analysis for a classic spar is made showing very good agreement. The present simple procedure allows us to evaluate primary structural components of a spar hull without resorting to expensive finite element modeling. It can help the scantling design by providing the global load. The approach can also be used to identify critical environmental conditions and structural components for detailed finite element analysis.
Analysis of Wave Interaction With Cylinders Using a 3D Numerical Wave Tank
Authors: Kamath Arun , Bihs Hans , Arntsen Øivind A.
Published: September 2014

Evaluation of flow around a cylinder placed in waves is a challenging task due to the complex nature of the flow. A good understanding of the flow physics involved here is important as coastal and offshore structures consist of horizontal and vertical cylindrical elements. This paper explores the use of Computational Fluid Dynamics (CFD) to evaluate the flow field around cylindrical structures. A 3D numerical wave tank is employed to study the free surface and fluid velocities around a vertical cylinder placed in waves and the total force acting on the cylinder is evaluated. The numerical results are compared with experimental data. Further, a simple representation of an offshore structure modelled as multiple cylinders in proximity is also simulated in the numerical wave tank. The presence of neighbouring cylinders has an effect on the flow field. This affects the force acting on each of the cylinders in the group. The forces acting on every cylinder in the group are evaluated and the free surface elevation in the flow field is also visualised. The numerical result is compared with the result from an analytical formula.

The numerical model uses the Reynolds-Averaged Navier-Stokes equations to evaluate the flow field. The convective terms are discretized using a 5th-order conservative finite difference WENO scheme. Time discretization is carried out using a 3rd-order Runge-Kutta scheme. Pressure discretization is carried out using Chorin’s projection method. The Poisson pressure equation is solved using a pre-conditioned BiCGStab algorithm. A sharp representation of the free surface is obtained using the level set method. Turbulence modeling is carried out using the k-ω model. Computational performance of the numerical model is improved by parallel processing using the MPI library.
Modeling and Analysis of a Multi Degree of Freedom Point Absorber Wave Energy Converter
Authors: Davis Andrew F. , Thomson Jim , Mundon Tim R. , Fabien Brian C.
Published: September 2014

This paper illustrates an approach to the modeling of a point absorbing Wave Energy Converter (WEC) with the intent of analyzing the sensitivity of the system response to variation in the model parameters. Using first principles, the nonlinear equations of motion are formed to describe the heave motion of a 3 body system. A linearized model is developed and used to simulate the system in both the time and frequency domains. The input to the model is a time series displacement and a time series velocity that describes the incident waves. A sensitivity analysis is then performed on the system parameters to show how the characteristics of the heave plate, the component masses, and the mass of the entrained fluid affect the performance of the system. The model is validated by numerically modeling a generation 1 device produced by Oscilla Power Inc., which is compared against experimental data from a field test on Lake Washington. The WEC is designed to provide tension along a series of tethers with connected power take off units. The wave input is specified using frequency spectra measured with a nearby Datawell Waverider MK III buoy during the field testing, from which time domain waves are reconstructed.
Response Surface Models for Analyzing Planing Hull Motions in a Vertical Plane
Authors: Sayeed Tanvir Mehedi , Lye Leonard M. , Peng Heather
Published: September 2014

A non-linear mathematical model, Planing Hull Motion Program (PHMP) has been developed based on strip theory to predict the heave and pitch motions of planing hull at high speed in head seas. PHMP has been validated against published model test data. For various combinations of design parameters, PHMP can predict the heave and pitch motions and bow and center of gravity accelerations with reasonable accuracy at planing and semi-planing speeds. This paper illustrates an application of modern statistical design of experiment (DOE) methodology to develop simple surrogate models to assess planing hull motions in a vertical plane (surge, heave and pitch) in calm water and in head seas. Responses for running attitude (sinkage and trim) in calm water, and for heave and pitch motions and bow and center of gravity accelerations in head seas were obtained from PHMP based on a multifactor uniform design scheme. Regression surrogate models were developed for both calm water and in head seas for each of the relevant responses. Results showed that the simple one line regression models provided adequate fit to the generated responses and provided valuable insights into the behaviour of planing hull motions in a vertical plane. The simple surrogate models can be a quick and useful tool for the designers during the preliminary design stages.
Effect of Pressure on Collapse Behaviour of Stiffened Panel
Authors: Jiang Lei , Zhang Shengming
Published: September 2014

During normal operations, ship and offshore structures, are subjected to combined lateral pressure and in-plane stresses. The effect of the lateral pressure is often ignored in hull girder ultimate assessments. This paper investigates the influence of the lateral pressures on the nonlinear collapse behavior of stiffened panels subjected to in-plane longitudinal stress. In this study, nonlinear finite element analyses were first conducted for the desired pressure alone; the longitudinal stress was then applied up to and beyond the collapse of the structures. Four representative stiffened panels taken from the bottoms of different double hull oil tankers were considered. The nonlinear analyses were performed using LR’s in-house finite element program VAST and following the procedure for nonlinear collapse analysis developed by LR. The numerical results indicated that the application of the initial pressure loads not only reduced the ultimate load carrying capacity of the panels significantly, but also changed the failure modes of the structures. The sensitivity of the ultimate strength to lateral pressure was dependent upon the panel geometry and whether the pressure was applied on the plate or the stiffener side. The numerical results and findings from this study are presented in this paper.
Rate Dependent Soil Resistance in FE Analysis of Pipeline Walking
Authors: Carneiro Daniel , Rathbone Andrew , Soon Kok Siong , Viecelli Graham
Published: September 2014

Pipeline walking is a complex phenomenon, governed by the axial pipe-soil interaction which is not yet fully understood, and which has been gaining increasing attention over the last few years. Axial friction (as any other aspect of soil mechanics) is usually bounded by drained and undrained responses, while in real life it will often be somewhere in the middle. For pipeline expansion it could be critical as while the ends might move a couple meters in the few hours it takes to heat up, somewhere close to the middle it will move a few millimeters only (so the level of drainage is totally different along the same line, during the same loading).

Geotechnical frameworks to address the different aspects governing the soil response to pipeline axial movement have recently been published. However, the current practice has been to lump all the time dependent effects back into a single equivalent friction factor, based on a representative pipeline velocity.

This paper presents the results of ‘true’ rate dependent pipeline walking analyses, and compares them to those obtained using constant equivalent frictions. While good agreement is observed for particular cases, differences up to 30% of the walking rate are observed. Examples show that the results of rate dependent pipeline walking analyses are significantly influenced by how the temperature changes over time along the pipeline length, thus realistic modeling of the heat up and cool down processes is crucial.

The rate dependent model employed describes the axial resistance as a hyperbolic function of the pipe velocity. Additional aspects which are expected to influence the soil response (e.g. consolidation time between sweeps, progressive compression and consolidation hardening) have been neglected, and shall be considered in further studies.
High Capacity Carousels: Modularity, Technical Development and Innovation
Authors: Tibbetts David , Dale Nicholas
Published: September 2014

Marine installation operations are becoming ever longer, larger and deeper. With campaigns regularly being carried out in the 2,000–3,000m water depth range and tieback lengths on the increase; installation, replacement or recovery works for this specification of product requires specialist equipment that can accommodate the increasing bulk of the flexible products being installed, without compromising the performance of the vessels which carry them.

In order to fulfil this technical challenge, new and innovative carousel designs are being developed. Placing the ‘reel’ horizontally allows maximum product capacity whilst minimising the impact on the vessel, thus maintaining operational efficiency in both shallow and deep waters and for projects such as subsea tiebacks and connections to processing platforms and offshore installations.

This paper explores recent developments in technology, where product loads of up to 1,500Te (Tonnes) are frequently required and ever shorter schedules are placed on contractors for initial spooling and subsequent installation. Technical innovations, operational solutions and commercial advantages will be assessed and the ability to open up new markets with the technology on offer will be scrutinised.

Alongside this, the benefits of modularity are evaluated to quantify the advantages of being able to disassemble and transport this type of hardware worldwide and the vessel utilisation improvements that can be made.
Semi-Empirical VIV Analysis of Full-Scale Deepwater Composite Risers
Authors: Kaboudian A. , Tan L. B. , Jaiman R. K. , Chen Y. , Tan V. B. C.
Published: September 2014

A global-local analysis methodology based on fluid-structure coupling is used to investigate the mechanical responses of both composite and steel risers. Since the design of the riser system can be a daunting task, involving hundreds of load cases for global analysis, semi-empirical fluid load models are considered for the reduced order computations of full-scale riser models. The structural performance of composite risers under real sea current conditions is investigated systematically and discussed with regard to the practical concerns in full-scale settings. The failure envelops of internal liners are found to be within that of the composite layers, which reveals that the liner is the weakest link for composite riser design. Results show that the composite risers can be more prone to vortex-induced-vibration (VIV) due to their lower structural frequencies. In the present study, the composite riser yields 25.5% higher RMS strains than the steel riser. Placement of buoyancy modules along the riser may be critical for the design against VIV, and our results show that the modules are not recommended at the top region of the riser, especially if a top-sheared current is expected. Instead, it is preferable to implement them at the bottom-half portion of the riser and as a continuously buoyed region rather than short discrete buoys separated with gap spaces.
Influence of Imperfection on Dynamic Response of Cross-Stiffened Deck Subjected to In-Plane Impact
Authors: Ji Zhen-hua , Wang De-yu
Published: September 2014

The motivation of this research is that the ship deck is subjected to uni-axial compression under sagging condition, when in the severe condition, the deck is dynamically loaded in nature, with impact type loads, which is the reason of deck collapse under severe condition. In addition, imperfection probably substantially influences the dynamic response of the impacted deck. Based on nonlinear explicit finite element method, the paper aims at studying the influence of three types of initial geometric imperfection on dynamic response of cross-stiffened deck subjected to in-plane impact. Three types of imperfection are local imperfection on deck plate between stiffeners, imperfection of overall positive deflection of the deck, and imperfection of overall negative deflection of the deck. Impact function is a half sine wave function with two parameters, impact duration and amplitude of the impact load. First order natural vibration period of the deck is selected as impact duration. Amplitude of the impact load is selected according to the value of ultimate strength of the deck, and varied by times of the ultimate strength. Strain rate effect and Strain hardening effect of the material are accounted in the analysis. Axial residual displacement of the end of the deck after impact is selected as dynamic response of the deck. The result shows that dynamic response of deck is sensitive to local imperfection, while not sensitive to overall imperfection under low or moderate level of imperfection magnitude. When imperfection is severe, residual axial displacement of the end of the deck is large, especially amplitude of impact load is somewhat larger than ultimate strength. Among 3 kinds of imperfection, dynamic response of overall negative deflection imperfection is the largest, followed by that of local imperfection, while that of overall positive deflection imperfection is the least.
Modeling Transient Circulating Mud Temperature in the Event of Lost Circulation and its Application in Locating Loss Zones
Authors: Chen Yuanhang , Yu Mengjiao , Miska Stefan , Ozbayoglu Evren , Kang Yongfeng , Zhou Shaohua , Al-Khanferi Nasser
Published: September 2014

Lost circulation is one of the most persistent and costly drilling problems that drilling engineers have been struggling with for decades. The main reason why some of the remedial procedures are not working as planned is the lack of information, such as the location of the loss zone. The pinpointing of the zone of loss will allow the treatment to be applied directly to the point of loss rather than to the entire open hole.

This paper presents an approach to predict the location of loss zone from the transient mud circulation temperature profile altered by the mud loss. A numerical model in estimating the transient mud circulating temperature profile during a lost circulation event is developed. The temperature profile in both the flow conduits (drillpipe and annulus) are modeled using mass and energy balance. The flow rate of drilling mud decreases in the annulus above the loss zone as part of the fluids lost into the fractures, which in turn alters the heat transfer between the drillpipe, annulus, and formation. The wellbore is divided into two multiple sections, which account for single multiple loss circulation zones. Rigorous heat transfer in the formation is included. Case studies are performed and numerical solution results are presented and analyzed. According to the results, temperature alterations induced by mud loss include: 1) Declines in both bottom-hole temperature (BHT) and mud return temperature over time, and 2) Discontinuity in the first order derivative of annulus temperature with respect to depth at the location of loss zone; meanwhile, the temperature alterations are mainly controlled by the mud loss rate and location of loss. By matching the simulated results with the distributed temperature measurements at different times, the depth of the loss zone can be identified. This piece of information is important for the spotting of LCM (lost circulation material) pills, the optimization of overbalance squeezing pressure, as well as the consideration of setting the cement plug or additional casing.
Implementation of a Visco-Elastic Model Into Slender Rod Theory for Deepwater Polyester Mooring Line
Authors: Ma Gang , Sun Liping , Wang Hongwei
Published: September 2014

Polyester mooring line is gradually becoming popular in deepwater engineering because of its lightweight property. In catenary mooring system, the horizontal restoring force comes from the gravity effect of the catenary mooring line which occupies the payload of the platform. Thus, the synthetic mooring line overcomes the drawback together with the taut mooring system which utilizes the axial elasticity to offer restoring force. The synthetic mooring line may only be one seventh weight of the steel mooring line and has low tensile stiffness which leads to a large stretch, and nonlinear material characteristics such as visco-elasticity. These two terms need to improve the slender rod theory which is proposed by Garrett for no stretched lines and improved by Paulling and Webster for small stretched lines. In this topic, a method for large stretched slender rod theory is introduced with the finite element method to deal with the problems of the large rotation and the large deformation. And then a linear visco-elastic model in the stretch-tension relation is utilized to simulate the properties of polyester line such as creep, strain-stress hysteresis and excitation period-dependent stiffness. Finally, an implementation method for integrating the visco-elastic model into slender rod theory is proposed with the numerical method and corroborated by a specified case which has the analytical results.
Structural Optimization for the Ice-Resistance Platform in Bohai Bay
Authors: Jinlin Hou , Weidong Shao
Published: September 2014

Bohai Bay is a major offshore oil and gas production area in China, a number of oil and gas fields are located there. The offshore facilities in this area are subject to ice loads in the winter, and the ice loads on the platform are one of the major concerns in the structural design in this area. The loads include ice impact, ice-induced-vibration and floating ice influence to the offshore operations. Compared to the loads combination of wind, wave and current, the ice load may be the governing loading condition in the structural design. The ice induced vibration to the production facilities and the living quarters may also seriously affect safety and the health of the operating personnel. This paper briefly introduces the development history of the ice-resistance platform design in Bohai Bay, and based on previous experience, the ice loads calculation method, the structure configuration of primary steel, the appurtenance arrangement and the conical structure at ice abrasive zones are discussed to optimize the ice-resistance capability. The material selection and different vibration mitigation methods are also discussed in this paper. Although significant progress has been made on the ice-resistance platform design, the problems still remain such as the floating ice accumulation around well slots. Further study on ice-induced vibration is necessary. This paper summarizes the main issues and challenges in the ice-resistance platforms and proposes the key points for future development. This paper also provides helpful references to the design and optimization of the offshore platform in an ice active environment.
Long-Term Extreme Response Analysis for a Fixed Offshore Wind Turbine Considering Blade-Pitch-Actuator Fault and Normal Transient Events
Authors: Li Qinyuan , Gao Zhen , Moan Torgeir
Published: September 2014

In this paper, the 50-year long-term 1-hour extreme responses of a fixed jacket-type offshore wind turbine with consideration of one-blade-pitch-actuator-stuck fault and the effect of normal transient events such as normal shut-down and start-up process is studied. The long-term extreme results are found based on each short-term extreme response distributions at different environmental conditions. Structure responses such as tower and jacket bottom shear and bending moments as well as blade root bending moments will be focused in this paper. To study the long-term effect of the fault and transient events, the service life of a wind turbine is divided into normal part, faulted part, and transient part. Normal part includes both normal operation and parking of the wind turbine at different wind speed range without any faults. Faulted part includes the parked and emergency shut-down condition of the wind turbine under the fault assuming that the faults are detected soon after they occur but require a longer time before fully repaired. Transient part includes the start-up and shut-down process during the normal operation when wind speed is beyond operation range. The contribution of each part to the long-term extreme response distribution is calculated by weighting factors based on the probability of occurrence of each part. From the results, it is found that in general, the blade-pitch-actuator-stuck fault and the normal transient events generally increase the extreme responses of the wind turbine. The jacket wind turbine is more affected compared to its land based counterpart. In this study since the wind direction is aligned with wind turbine, it is found that the fault primarily increases the tower bottom shear force perpendicular to the wind direction and the bending moments with the axis parallel to the wind as well as the torsional moment, while normal transient events, especially the start-up process at cut-out speed, causes a much greater increase compared to the fault. It contribute mostly to the shear forces parallel and bending moment with axis perpendicular to the wind direction. The azimuth of the blades is found to be very important for blade responses during start-up process especially at higher wind speed.
Analysis of Sea Shock Encountered by a Ship in North East Japan
Authors: Shiotani Shigeaki , Sasa Kenji
Published: September 2014

In general, earthquake motion experienced by a navigable vessel is called a sea shock (or seaquake). Sea shock is assumed that the vertical ground motion of the sea floor propagates as a compressional (longitudinal) wave in water.

Seaquakes are sudden, unexpected phenomena that are induced by undersea earthquakes. There are almost no examples of their systematic measurement and the phenomenon of seaquakes is said to not be thoroughly understood.

The effect of seaquakes was studied theoretically during the 1990s, during research on the wave response of structures for the construction of VLFS. However, There are a few experimental studies on the effect of seaquakes on floating bodies because of the problem of reproducibility.

We measured ship motion caused by sea shocks by examining seabed oscillation. Such measurements provided very valuable data. The conclusions obtained in the present study are that ship motion caused by sea shocks has high-frequency oscillation. In particular, the acceleration in the z direction is remarkable.
Upheaval Creep of Rock Dumped Flexible Flowline
Authors: Zhengmao Yang , Norland Kristian , Brown Neil , Karunakaran Daniel
Published: September 2014

For the protection from dropped object/fishing trawl impact, flexible flowlines are normally trenched or rock-dumped. And hence, upheaval buckling and lateral buckling may be promoted by the elevated temperatures and high pressures. Due to the unique properties of un-bonded flexible flowline, the flexible flowline may creep in the trench or rock berm when it is subjected to cyclic pressure and temperature changes due to start-up and shut-down of flowline in service.

In this paper, a finite element analysis model for the global buckling and upheaval creep of flexible flowline is proposed. In this model the effect of bending stiffness hysteresis are considered in addition to the temperature and pressure changes in each start-up/ shut-down cycle. A case study of a 10″ water injection flowline is performed by using finite element analysis software package ANSYS. The nonlinear general beam section is used to simulate the specific flexible pipe behavior, nonlinear bending behavior but linear axial behavior.
Plate Shape Effect on the Performance of the Vertical Axis Auto Rotation Current Turbine (VAACT)
Authors: Bakhshandeh Rostami Ali , Fernandes Antonio Carlos
Published: September 2014

The present paper explores experimentally the performance of two types of hinged plates which rotate about vertical axis when submitted to uniform current. A flat plate configuration and also a flapped plate (say, S shape) configuration have been investigated. The Vertical axis Auto rotation Current Turbine (VAACT) is one degree of freedom system (free to rotate in yaw direction). It is shown that a high efficiency for S shape type can be obtained of the order of 30 percent while flat blade type reaches approximately to 7 percent. Upper limit of tip speed ratio for flat blade type has been expanded approximately 0.9 whereas S shape approaches 1.3.
Development of a Prediction Formula for Ship Resistance in Level Ice
Authors: Cho Seong-Rak , Jeong Seong-Yeop , Lee Sungsu , Kang Kook-Jin
Published: September 2014

Icebreakers are mainly classified according to the breakable thickness of level ice, although they differ slightly according to each ship’s classification and by country. The breaking performance in level ice is a priority factor for icebreakers and is directly linked to ships’ safety and survival in extreme environments. In addition, some large cargo vessels with an icebreaking capability have recently been built, and various types of commercial icebreakers will appear when the Northern Sea Route is activated in the future.

In this paper, previous studies on ice resistance are reviewed and analyzed by arranging the merits and demerits of each study. We developed a new prediction equation of ice resistance by analyzing the resistance components, such as the breaking, buoyancy, and clearing resistance, and using dimensional and regression analyses on ice model tests. This procedure could be applied to large merchant vessels and is intended to contribute to the prediction of icebreakers’ ice performance.
Extreme Wave Condition at Doggerbank
Authors: Engebretsen Espen , Haver Sverre K. , Myrhaug Dag
Published: September 2014

In design of offshore wind turbines, extreme wave conditions are of interest. Usually, the design wave condition is taken as the sea state corresponding to an annual exceedance probability of 2·10−2, i.e. a return period of 50 years. A possible location for a future wind farm, consisting of bottom fixed wind turbines, is the Doggerbank area, see Figure 1. The water depth in this area varies from about 60m in the north to about 20m in the south.

The hindcast database NORA10 provides sea state characteristics from 1957 to present over a domain covering Doggerbank. Regarding the deeper areas just north of Doggerbank, this hindcast model is found to be of good quality. Larger uncertainties are associated with the hindcast results as we approach shallower water further south. The purpose of the present study is to compare sea state evolution over Doggerbank as reflected by NORA10 with the results of commonly used shallow water hindcast model SWAN.

The adequacy of the default parameters of SWAN for reflecting changes in wave conditions over a sloping bottom is investigated by comparison with model test results.

Extreme wave conditions for two locations 102.5km apart in a north–south direction are established using NORA10. This is done using both an all sea states approach and a peak over threshold approach. Assuming the extremes for the northern position to represent good estimates, the wave evolution southwards is analyzed using SWAN. The extreme condition obtained from NORA10 in the northern position is used as input to SWAN and the results from the two hindcast models are compared in the southern position. SWAN seems to suggest a somewhat faster decay over Doggerbank compared to NORA10.
Predicting Hydrostatic Collapse of Pipes Using Finite Element Analysis
Authors: Bastola Ajit , Wang Junkan , Mirzaee-Sisan Ali , Njuguna James
Published: September 2014

A pipeline’s resistance to collapse is governed by geometric imperfections, material properties and residual stresses. The offshore pipeline design code DNV-OS-F101 provides a method for predicting collapse of pipelines with diameter to wall thickness (D/t) ratios between 15 and 45. This paper examined the various factors that could influence the collapse resistance of several pipe geometries, such as ovality, eccentricity, material stress-strain behavior and residual stresses in the hoop and longitudinal directions. A total of 132 cases were carried out, using 2D and 3D Nonlinear Finite Element Analysis, to predict the collapse pressure of several realistic pipe geometries. Results of this study suggest that the DNV-OS-F101 predictions are conservative and applicable for a wide range of D/t ratios. While there is close correlation between Finite Element prediction and DNV-OS-F101 prediction, there is a degree of conservatism at low D/t ratios using DNV-OS-F101 equations. Hence there would be scope for further optimization of pipe wall thickness design against the collapse limit state at low D/t ratios.
Effect of Reeling Simulation on the Mechanical Properties of New Duplex Stainless Steel for Line Pipe
Authors: Shitamoto Hidenori , Sagara Masayuki , Amaya Hisashi , Hisamune Nobuyuki , Motoya Daisuke , Watatani Yuuki
Published: September 2014

Corrosion resistant alloys (CRAs) such as martensitic and duplex stainless steels (DSS) are used as a flow line material in corrosive wet gas environments (i.e., carbon dioxide and hydrogen sulfide environments). A new DSS which consists of 25mass%Cr - 5mass%Ni - 1mass%Mo - 2.5mass%Cu has been developed for line pipe usage in slightly sour environments.

There are several methods currently being used to install offshore oil and gas pipelines. The reel-lay process is fast and one of the most effective offshore pipeline installation methods for seamless, ERW, and UOE line pipes with outside diameters of 18 inches or less. In the case of the reel-laying method, line pipes are subjected to plastic deformation multiplication during reel-laying. Thus, it is important to understand the change of the mechanical properties of line pipes before and after reel-laying.

In this study, full-scale reeling (FSR) simulations and small-scale reeling (SSR) simulations were performed to investigate the effect of cyclic deformation on the mechanical properties of the new DSS for line pipe. Furthermore, investigation of the most susceptible temperature range to cracking and sulfide stress cracking (SSC) tests were performed in slightly sour conditions.
Experimental Comparison of Tensile Armor Wires Using Strain Gages and Fiber Bragg Grating Techniques
Authors: Vargas Felipe Arêas , Lopes Diogo Garcia , Kenedi Paulo Pedro , Clevelario Judimar , de Souza Pires Fabio
Published: September 2014

Flexible pipes are being installed and operated in more marginal and challenging offshore conditions related to deep-water environments. Especially important is the accurate assessment of the remaining life of a flexible riser so operators can avoid costly premature change outs. So, emerging inspection and monitoring technologies are being several developed to achieve a comprehensive flexible pipes integrity approach.

For unbonded flexible pipes, the primary challenges include fatigue life, collapse, axial compression loading of the tensile armor wires and end fitting development.

This paper is related to end fitting development area, due to necessary folding/unfolding process on the tensile armor wires during pipe assembling. This mechanical forming generates high levels of plastic strain on the wires, which may therefore reduce the fatigue life of the flexible pipe in the field.

This paper presents an analysis of the high strain level involved in the folding and unfolding process along tensile armour wire, using the strain gages and fiber Bragg grating techniques.

Besides that, this paper describes a comparison analysis, which correlates the performance of both methods to assess high strain levels.
The Effect of Bending Stiffness on Hydroelastic Response of VLFS
Authors: Wei Wei , Fu Shixiao , Guo Fei , Liang Yuanhua , Chai Shuhong
Published: September 2014

Owing to the flexibility of ocean structures with large dimension, the hydroelastic theory is more applicable than traditional method of treating the floating structure as a rigid body. To study the factors that influence the hydroelastic responses, a very large floating structure (namely, VLFS) model is chosen to conduct numerical calculations in regular waves with the aid of three dimensional linear hydroelastic code concerning varied bending stiffness and wave frequency. It is found that bending stiffness and wave frequency have a critical but complex influence on relevant hydroelastic results, including generalized displacement, vertical response amplitude and bending moment. More specifically, the effect of bending stiffness on hydroelastic parameters above can be categorized into different phases, and quite different tendencies are observed in each phase.
Application of Combination of Local Domain Free Discretization and Immersed Boundary Method (LDFD-IBM) to Numerical Simulation of 3D Flows Over a Circular Cylinder
Authors: Wu Yanling , Shu Chang , Gullman-Strand Johan
Published: September 2014

In this paper, the recent developed Local Domain Free Discretization method combined with Immersed Boundary Method (called LDFD-IBM) is extended from two-dimensional version to three-dimensional version. LDFD-IBM is a new member in the family of Cartesian mesh methods. The advantages of LDFD-IBM over other Cartesian mesh solvers includes: no cutting cell, easy to adaptive mesh refinement, easy to implement for moving boundary problems, truly second order accuracy over whole domain, no flow penetration into the solid wall.

LDFD-IBM three-dimensional solver is then used to simulate three-dimensional flow past a circular cylinder. Both oblique mode and parallel mode of vortex shedding of the cylinder in three-dimensional configuration are reproduced according to different end-conditions. Oblique shedding is one of the important three-dimensional features that could influence the amplitude, frequency and phase of the flow-induced forces.
Structural Damage Identification of Offshore Jacket Platform Based on the Pseudofree Response Data
Authors: Yansong Diao , Dongmei Meng , Yadong Cao
Published: September 2014

Due to the influences of the excitation and measurement noise, the misjudgments are inevitable when the coefficients of an autoregressive (AR) model in the time series combined with neural networks are used to identify the damage. So, in this study, the pseudofree response data are extracted from the acceleration responses data with the random decrement (RD) technique, the AR model is employed to fit the pseudofree response data, the Akaike information criterion (AIC) is used to estimate the order of AR model, the coefficients of the AR model will be changed if the structure is damaged, so we can determine if the structure is damaged according to the changes of the coefficients of the AR model. If the structure is damaged, the differences of the first 4 order AR coefficients pre and post damage are extracted and composed as damage sensitive vector which is put into back-propagation (BP) neural network to identify the damage location. The numerical model of a four-floor offshore jacket platform excited with white noise is used to testify the proposed damage identification method, the different amplitudes of the white noise excitation and the different level of the measurement noise are also considered. The simulation results show the proposed method is almost not affected by the changes of excitation amplitude, and when the noise level is no more than 5 percent, the damage location can be identified by the method correctly. The proposed method uses the acceleration responses to identify the damage directly, which is not dependent on the modal parameters (frequency, mode shape, damping), therefore, it is suitable for the on-line damage identification.
Single Point Mooring FPSO Monitoring and Forecast System Design
Authors: Qi Junliang , Lv Ligong , Su Zhiyong , Liu Chao , Shen Hui , Luo Yong
Published: September 2014

The proposed paper is going to address the development of single point mooring FPSO (Floating Production, Storage and Offloading) monitoring and forecast system design. With 17 FPSOs deployed in both Bohai Bay and South China Sea, CNOOC owns one of the largest FPSO fleet in the world. Most of those FPSOs have been or will be moored to the seabed for decades. The extreme response during storm conditions could cause serious environmental problem, asset loss, personnel safety etc. In order to timely understand the tanker operation conditions and avoid potential risk of system failure when experiencing hurricanes, a monitoring and forecast system is developed for FPSO to monitor the environment conditions, tanker motions, green water, mooring tensions, FPSO heading and to predict the extreme mooring tensions and global motions before typhoon coming. The forecast system could further suggest the optimum loading condition for minimizing the extreme mooring tension and tanker motions to enhance the safe operation. In this paper, we take the Internal Turret Mooring FPSO 111 and the Submerged Soft Yoke Mooring FPSO 112 as the examples to introduce the design technology of the system. Through the integrated onboard interface information, the personnel could proactively take actions to mitigate the tensions on mooring lines and vessel motions. Furthermore, the measured mooring line tension, motion and environment history could assist the numerical studies of global performance. The details of designing or selecting the measuring and monitoring equipment, theory background of forecast system and the integrated onboard interface will be described.
Research and Design on Internal Turret Mooring System of FPSO in South China Sea
Authors: Huaxiang Liu , Da Li , Xueping Bai , Jianping Wang , Baolei Zhang
Published: September 2014

This paper mainly introduced the research and design procedure, method and conclusion for internal turret mooring system of FPSO in South China Sea. According to the environmental condition characteristic and operating experience of FPSO in South China Sea and other reasons, the permanent internal turret mooring system with disconnected function is recommended as the final scenario after comparison with the other internal turret systems including disconnected system in typhoon condition and permanent system without disconnected function. For the gradually bad environmental conditions such as wind, wave and current in South China Sea, 500-year return typhoon condition is required as the extreme design condition of mooring system. According to the environmental condition distribution and classification requirement, the design standards, rules and corresponding design parameters for 500-year return condition are determined as the basis of mooring system design. The mooring system is very difficult to design because of very shallow water for catenary system and 500-year return typhoon environmental condition in South China Sea. For the problems appeared because of 500-year return extreme design condition and other parameters, a reliable solution was proposed from the aspect of wave characteristics research and got approval from classification society. The new design criteria for mooring system of FPSO in South China Sea is updated and the mooring system design is completed based on the updated design criteria. The research and design procedure and method for internal turret mooring system of FPSO in South China Sea are summarized too. This paper can be helpful to the similar projects in South China Sea.
A Very Simple Index to Determine Design Parameters of Wave Energy Converter for Wave Farm Cost Optimization
Authors: Okamoto Takashi , Higo Yasushi , Fukaya Yutaro
Published: September 2014

Expansion of the deployment site of wave power generation plant to lower wave energy sites is a key for the market size expansion, which contributes the reduction of the cost of electricity. However, it is not easy to take place from the economical view point since the revenue is limited by the low wave energy. Also, since current prototypes are designed for very energetic seas, they are too large for milder conditions. In order to improve the economic feasibility, design parameters of wave energy converter (WEC) have to be adjusted to the existing wave condition at the site. However, no method exists for this purpose. A new index to determine parameters of WEC for having better economic feasibility in wave farm is introduced in this work, so that the designer can see the relative superiority among parameter combinations without conducting complex economic feasibility studies. The index value was examined through an economic feasibility study of hypothetical wave farm project at Japan and found that it reasonably indicates the level of cost of electricity.
Modelling Issues for Nonlinear Time History Analysis of Offshore Platforms Subjected to Strong Ground Motions
Authors: Nezamian Abe , Morgan Patrick M.
Published: September 2014

The response of a platform subjected to a strong ground motion is highly affected by the method of modelling, and special attention should be paid to such advanced analysis.

While the potential of dynamic inelastic analysis and performance based design of offshore platforms is recognised in earthquake design and assessment, they have inherent shortcomings. This paper discusses the critical issues and their effects on the obtained results in the application of inelastic dynamic analysis. Areas of possible developments that would render the method more applicable to the prediction of dynamic response, and new developments towards a fully adaptive performance based design are explored. These developments lead to dynamic analysis results that are closer than ever to the true inelastic time-history behavior of the structures.

It is concluded that there is great scope for improvements of this powerful technique that would increase confidence in its employment, and encourage its use during design as the primary tool for reliability assessment under seismic loads. The results from a sample project indicated that inelastic time history seismic analysis provides a more realistic assessment of structural behavior and strongly recommended to be implemented for the design and reliability assessment of the fixed offshore structure under extreme seismic event.