bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficient Derivation of the Probability Distribution of the Extreme Values of Offshore Structural Response: Comparison of Three Different Methods
Authors: Abu Husain M. K. , Mohd Zaki N. I. , Najafian G.
Published: November 2013

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. To this end, the conventional (Monte Carlo) time simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated extreme responses (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. In this paper, three different versions of a more efficient time simulation technique (ETS) are compared by exposing a test structure to sea states of different intensity. The three different versions of the ETS technique take advantage of the good correlation between extreme responses and their corresponding surface elevation extreme values, or quasi-static and dynamic linear extreme responses.
Finite-Memory Nonlinear System Modelling of Offshore Structural Response Accounting for Extreme Values Residues
Authors: Mohd Zaki N. I. , Abu Husain M. K. , Mallahzadeh H. , Najafian G.
Published: November 2013

Offshore structures are exposed to random wave loading in the ocean environment, and hence the probability distribution of the extreme values of their response to wave loading is of great value in the design of these structures. Due to nonlinearity of the drag component of Morison’s wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of the probability distribution of extreme responses are not available. Monte Carlo time simulation technique can be used to derive the probabilistic properties of offshore structural response, but the procedure is computationally demanding. Finite-memory nonlinear system (FMNS) modeling of the response of an offshore structure exposed to Morison’s wave loading has been used to reduce the computational effort, but the predictions are not always of high accuracy. In this paper, further development of this technique, which leads to more accurate estimates of the probability distribution of the extreme responses, is reported.
Comparative Study on Fatigue Life of Drillship Operating for GoM and Brazil Sectors Based on Spectral Fatigue Analysis
Authors: Kim Tae-Hoon , Hwang Oe-Ju , Oh Yeong-Tae
Published: November 2013

A drillship is an exceptional vessel to be capable of drilling of new oil or gas wells, and carrying out maintenance or completion work in deep water. The drillship also has ability to take aboard much more variable loads and save the time of sailing between oil fields worldwide than Semi-submersible rig. Sometimes, it is required from design specifications that a drillship is to be designed to be satisfactory to several fields at the same time to maximize such advantages. This paper describes results of the comparative study on fatigue life of a drillship, which was developed by DSME, operating for both GoM (Gulf of Mexico) and Brazil sectors. It was assumed in the calculation that the drillship spends 80% of her life in On-site (GoM and/or Brazil sector) and other percentage in transit condition. Screening works for choosing fatigue sensitive locations such as moon pool corners, discontinuous areas, topside module foundations, etc., were performed referring to the local strength analysis. A whole ship model along with every topside module was generated and the very fine meshes with thickness by thickness as per DNV RP-C203 were embedded therein. Stress RAOs of critical hotspot locations of the drillship were directly obtained through its motions from 3D potential-theory-based analysis, i.e. DNV WADAM. In the analysis, wave scatter diagrams given by BMT were used. As results of the study, it was found that Brazil sector gives comparatively worse results to the drillship in terms of fatigue endurance and finally, Brazil sector was decided in calculating fatigue lives of the drillship as a representative sea state. The fatigue life was computated based on spectral fatigue analysis by means of DSME’s in-hose tool (i.e. D-SFAS, DSME Spectral Fatigue Analysis System).
Comparison of Extreme Responses From Wheeler and Vertical Stretching Methods
Authors: Mohd Zaki N. I. , Abu Husain M. K. , Najafian G.
Published: November 2013

Linear random wave theory (LRWT) is frequently used to simulate water particle kinematics at different nodes of an offshore structure from a reference surface elevation record. However, it is well known that LRWT leads to water particle kinematics with exaggerated high-frequency components in the vicinity of mean water level (MWL). To avoid this problem, empirical techniques such as Wheeler and vertical stretching methods are frequently used to provide a more realistic representation of the wave kinematics in the near surface zone. In this paper, the Monte Carlo time simulation technique is used to investigate the effect of these two different methods of simulating water particle kinematics on the probability distribution of extreme responses. It is shown that the difference could be significant leading to uncertainty as to which method should be used.
Short-Term Distribution of the Extreme Values of Offshore Structural Response by Modified Finite-Memory Nonlinear System Modeling
Authors: Mohd Zaki N. I. , Abu Husain M. K. , Wang Y. , Najafian G.
Published: November 2013

Offshore structures are exposed to random wave loading in the ocean environment, and hence the probability distribution of the extreme values of their response to wave loading is of great value in the design of these structures. Due to nonlinearity of the drag component of Morison’s wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of the probability distribution of extreme responses are not available. However, it has recently been shown that the short-term response of an offshore structure exposed to Morison wave loading can be approximated by the response of an equivalent finite-memory nonlinear system (FMNS). Previous investigation shows that the developed FMNS models perform better for high Hs values and their performance for low Hs value is not particularly good. In this paper, the modified version of FMNS models is referred to as MFMNS models is discussed. The improvement of MFMNS model is simply by dividing the structure into two zones (Zones 1 and 2) so that the horizontal distance between the nodes in each zone is relatively small compared to the wavelength. The modified version of MFMNS is used to determine the short-term probability distribution of the response extreme values with great efficiency.
Laboratory Experimentations for New Hydrothermal Monitoring Systems Using ADCPs
Authors: Komaki Kanae , Shimazu Mitsuru , Kondo Shunsuke , Onishi Yosuke , Furuta Satoshi , Arai Rei
Published: November 2013

Deep ocean mining in a hydrothermal area needs careful environmental impact assessments in terms of preservation and mitigation of biodiversity. The General Environmental Technos Co. Ltd., or KANSO TECHNOS, for short, has participated in environmental impact assessments in hydrothermal areas in the Izu-Ogasawara and the East China Sea areas (Ishida et al., 2011). Through the experience, we suggest a method of using acoustic systems such as acoustic Doppler current profilers (ADCPs) for monitoring of suspended matters and benthos in hydrothermal areas. Thus, we try to do in-situ observations, called Tow-yo (or Towing) observations with ADCPs (Komaki and Ura, 2009; Komaki et al., 2010). This system has a great advantage in enabling the measurement of great environmental factors, echo intensity and current velocity in a large range. To confirm exactly what the substances are and how large they are from the measured echo intensity data, we tried laboratory experiments in water tanks with echo sounders and turbidity sensors. These results will finally be integrated in a simulation model to predict substances from in-situ data in deep water for future monitoring systems.
Numerical Simulation of Vortex Induced Vibrations for Marine Risers Subjected to Sheared Flow
Authors: Van den Abeele F. , Vande Voorde J. , Kara F.
Published: November 2013

The increasing demand for oil and gas, currently estimated at 135 million barrels of oil equivalent per day, keeps pushing the boundaries of offshore engineering into ever deeper waters. Exploration and production activities in the Gulf of Mexico, for instance, are performed in water depths exceeding 3000 meters. For such deepwater developments, the suspended length of the marine risers adds up to several kilometers. When designing and installing risers in (ultra)deep water, the length/diameter aspect ratio of the marine riser can exceed L/D > 1000, and the features of the fluid flow in depth direction can no longer be neglected. Indeed, both the magnitude and the direction of the current change with water depth, giving rise to higher harmonics in the VIV response.

The prediction of vortex induced vibrations for deepwater risers is very challenging, owing to the fact that the incident flows are non-uniform and the associated fluid structure interaction phenomena are highly complex. These complex conditions give rise to a non linear coupled system with a large number of degrees of freedom, which depends on several physical and mechanical parameters.

In this paper, 3D CFD calculations are performed to evaluate the effect of the third dimension for risers subjected to uniform flow and sheared currents. For a uniform current velocity at the inlet boundary, it is shown that vortex shedding in the wake of long slender tubulars can give rise to the development of vortices with horizontal axis, resulting in a fluctuation of the flow in the Z-direction. These three dimensional vortices are strong enough to modulate the vortex shedding on the riser as a function of depth. The 3D simulations with uniform current velocity are then compared to marine risers subjected to sheared currents. It is shown that the presence of sheared currents invokes a shift in both phase and frequency of the vortex shedding.
An Integrated Approach for the Representation of Concrete Gravity Based Foundations for Offshore Wind Turbines
Authors: Philippe Maxime , Borgarino Bruno , Kotronis Panagiotis , Ducrozet Guillaume
Published: November 2013

This paper describes a novel approach to efficiently simulate the structural dynamics of a concrete Gravity Based Foundation (GBF). In this time-domain analysis, the GBF is subjected to loads applied by the turbine, wave loads and the influence of the soil structure interaction is taken into account. Wind turbine loads are computed using the aeroelastic software FAST and expressed at the connection point between the turbine and the GBF Wave loads on the GBF are computed using a potential, nonlinear wave model. Nonlinear soil-structure interaction is modelled with the use of a macro-element specifically developed for shallow foundations. Finally, the structure itself is modelled using an Euler-Bernoulli multifiber beam, which allows representing the reinforced concrete sections.

It is shown that the numerical model is able to efficiently simulate the behaviour of a GBF foundation under nonlinear irregular wave forces and loads transmitted by the turbine. It reproduces nonlinear phenomena such as a decrease in material stiffness due to damage and permanent strains but also the GBF displacements considering soil structure interaction.
Comparison Between Static, Forced Oscillation and Free Oscillation Model Tests for the Assessment of Galloping Instability at High Reynolds Numbers
Authors: Cinello Alexandre , Pétrié François , Rippol Thierry , Molin Bernard , Damblans Guillaume
Published: November 2013

Galloping may take place for non-circular cross sections, such as an ice-coated electric power line or a riser bundle, under current action. This type of instabilities occurs at lower frequency than Vortex Induced Vibrations but with unbounded amplitude, and might be detrimental for riser integrity.

In a recent joint industry project, the CITEPH “Gallopan” project, galloping instabilities were investigated for two types of cylinders: an academic square cylinder and a generic riser tower cross section. Model tests and numerical computations were performed to assess the propensity of both cylinders to gallop.

Experiments on the square cylinder are reported here. Three types of tests were performed in steady flow: loads measurement on fixed cylinder, at various headings; loads measurement on the cylinder with over imposed cross-flow harmonic oscillations; free transverse oscillations. By using analytical calculations, the ability to predict galloping instability occurrence and amplitude, of each of the three above methods, was compared.

Compared to typical results found in literature, these experiments were conducted at a larger scale, and thus with Reynolds number closer to on-site values, i.e. over 105.
Sea State Estimation Based on Ship Motions Measurements and Data Fusion
Authors: Drouet Céline , Cellier Nicolas , Raymond Jérémie , Martigny Denis
Published: November 2013

In-service monitoring can help to increase safety of ships especially regarding the fatigue assessment. For this purpose, it is compulsory to know the environmental conditions encountered: wind, but also the full directional wave spectrum. During the EU TULCS project, a full scale measurements campaign has been conducted onboard the CMA-CGM 13200 TEU container ship Rigoletto. She has been instrumented to measure deformation of the ship as well as the sea state encountered during its trip. This paper will focus on the sea state estimation.

Three systems have been installed to estimate the sea state encountered by the Rigoletto: An X-band radar from Ocean Waves with WAMOS® system and two altimetric wave radars from RADAC®.

Nevertheless, the measured significant wave height can be disturbed by several external elements like bow waves, sprays, sea surface ripples, etc… Furthermore, ship motions are also measured and can provide another estimation of the significant wave height using a specific algorithm developed by DCNS Research for the TULCS project.

As all those estimations are inherently different, it is necessary to make a fusion of those data to provide a single estimation (“best estimate”) of the significant wave height. This paper will present the data fusion process developed for TULCS and show some first validation results.
A Low Motion Floating Platform for Offshore Wind Resource Assessment Using Lidars
Authors: Nicholls-Lee Rachel
Published: November 2013

With offshore wind becoming a key source of renewable energy there exists a requirement for the acquisition of meteorological information at the sites allocated for development. Installation of a conventional, static, meteorological mast is costly. Multiple masts are required to obtain data at several positions in a large offshore wind farm, which further increases the cost of gathering such data. A structure that has mobility for relocation about the site has the potential to reduce costs whilst improving data capture coverage. As such, an instrumentation platform in the form of a floating structure which can be moved easily is desirable.

This work discusses the development of a low-motion, lightweight, floating platform with tunable motion response as a basis for a repositionable meteorological measurement station. Wind speed and direction measurements are acquired at a range of heights in the atmosphere through the use of a pulsed Lidar (light detection and ranging) system. The motions of the platform have been analyzed both numerically and experimentally, and the performance of the platform in a range of seas is good.
New Platform to Monitor the Environmental Impacts of Seafloor Mining
Authors: Maeda Nobuhiro , Onishi Yosuke , Kato Satoshi , Yamazaki Tetsuo
Published: November 2013

Various unconventional natural resources (e.g. seafloor massive sulfide, methane hydrate and manganese nodules) existing on and under the seafloor will possibly be mined in the near future. When mining these deep sea resources, it is necessary to monitor the environmental impacts of seafloor mining. Environmental impacts have been usually monitored by mooring systems or stations put on the seafloor. Mooring systems consist of floats, sinker and sensors. Their heights rang e from one hundred meters to several kilometers. In order not to entangle in the mining machine and the support cable, it is impossible to put the mooring systems within the distance of their heights from points of the operation safety. The stations consist of a frame and sensors. Their heights are about a few meters. Therefore, we are able to put the stations near the mining point, but we can’t observe the upper water layer. In this study, we have developed a new platform to observe the environmental condition near the mining point and the upper layers of the seafloor. The developed platform consists of frame, sinker, releaser, winch, battery and sensor assembly. Because the size of this is 2 × 2 × 2 m, we can put this near the mining point. The sensor assembly is moved from the seafloor up to 150 m by the winch. This platform can automatically observe the environmental condition for several months without external power supply by using the internal battery. Therefore, we can automatically observe the environmental condition near the mining point and the upper layer by using this new developed platform. The platform is also effective for environmental monitoring near deep-sea petroleum exploitation points.
Structural Performance of the Optimum Floating Structure for Reduced Motion
Authors: Lee Du-Ho , Jeong Youn-Ju , You Young-Jun , Park Min-Su
Published: November 2013

In order to design a reliable floating structure, the hydrodynamic motion and structural performance under wave loadings should be reduced with the effects of wave-induced hydraulic pressure acting on the floating structure. In this study, analytical studies were carried out for optimum type to reduce the hydrodynamic motion and pressure of concrete floating structure. The optimum floating structure is combined with pontoon-type and hybrid-type floating structures, called combination-type floating structure. In order to verify reducing motion and improving structural performance of combination-type floating structure, analytical studies were carried out for the floating structures. After hydrodynamic analysis, the six degree motions of structure are investigated for fifth periods in shallow water. The hydrodynamic motions of combination-type are lower than other type of floating structures. It meant that the combination-type floating structure can be very efficient to reduce the wave forces acting on structures and be slightly influenced by the incident waves. In addition, to evaluate structural performance of floating structures under the critical wave load that presents maximum motion of floating structure. As the results of this study, the combination-type floating structure identified reducing hydrodynamic motion and excellent structural performance than other floating structures. However, high concentrated stress occurred at the edge of the bottom slab of the bow and stern parts where cylinder wall was connected to the bottom slab. Therefore, some alternatives which can be easily obtained from a simply modification of structural details are proposed to overcome these problems.
Random Wave Forces on Monopile Wind Turbine Foundations: A Comparison of Wave Models
Authors: Zheng Xiang Yuan
Published: November 2013

The wave forces on a monopile are compared among the classical random wave models and the state-of-the-art fully nonlinear random wave model. Forces are calculated in terms of Morison’s equation up to the wave crest zone. Linear random wave model with constant stretching, second-order random wave model with linear extrapolation from the still water level and, the third-order nonlinear wave model are adopted in time-domain simulations.

The simulation results show that the third-order random model is able to produce accurate predictions of extreme wave crest and quasi-static wave forces when compared with the fully nonlinear model. The linear random model with constant stretching behaves better than the commonly used Wheeler stretching. The second-order random wave model is suggested to enter the mainstream application of industrial design for shallow-water offshore structures as it provides wave forces whose quality is only inferior to the third-order model, but with much less computational effort than both the third-order model and the fully nonlinear model.
A Detailed Investigation of the Nearshore Wave Climate and the Nearshore Wave Energy Resource on the West Coast of Ireland
Authors: Gallagher Sarah , Tiron Roxana , Dias Frederic
Published: November 2013

The western coast of Ireland possesses one of the highest wave energy resources in the world and consequently is a promising location for the future deployment of Wave Energy Converters (WECs). Most wave climate studies for this region have focused primarily on the offshore area since it enjoys higher energy densities. However, recent studies have shown that nearshore locations offer a similar potential for the exploitation of wave energy as offshore sites [13]. Furthermore, the proximity of WEC devices to the shore will likely reduce losses in power transport, and facilitate access for maintenance activities.

In this context, we analyse the wave climate over a ten year period for several nearshore sites off the Irish West Coast. The wave climate is estimated using a spectral wave model, WaveWatch III, forced with wind and spectral wave data from the ECMWF (European Centre for Medium Range Weather Forecast) operational archive. The wave model is validated with wave buoy data from intermediate to shallow depths (< 60 m).

Our focus is on two aspects of the wave climate resource assessment. Firstly, we characterise the directionality of the wave energy resource (mean direction, directional spread) which affects the site selection, design and performance of nearshore WECs. Secondly, we discuss the climate data from the perspective of accessibility for maintenance. When selecting sites for the deployment of WECs, a balance needs to be found between two opposing criteria: the existence of sufficiently long, continuous time intervals of calm sea states (weather windows) which are necessary for maintenance activities to take place, and a high, consistent level of wave energy density, essential for economically viable wave energy extraction.
Interference Effect in the Flow Around Two Aligned Cylinders and the Reduction of a Helical Strakes Suppressor Effectiveness
Authors: Freire Cesar M. , Korkischko Ivan , Meneghini Julio R.
Published: November 2013

It is well known that the interference effect reduces the effectiveness of vortex induced vibration (VIV) suppressors such as helical strakes. Usually the major concern about structures fitted with VIV suppressors is its own vibration. In this paper it is shown how an upstream structure fitted with helical strakes can induce higher vibrations than bare cylinders in a structure mounted downstream in a tandem configuration. Experiments were conducted in a water channel facility and demonstrate that the vortex wake generated by an cylinder fitted with strakes can induce higher amplitudes of oscillation than a bare cylinder. Particle image velocimetry was employed to measure the velocity field in the wake behind a smooth cylinder and behind a cylinder fitted with strakes. The Reynolds number for the experiments goes from 1000 up to 10.000. The flow field visualization was conducted for fixed cylinder at Re = 10.000.
Online Safety Monitor Design of the Jacket Platform Based on Structural Members Failure Study
Authors: Xu Yuwang , Fu Shixiao , Zhong Qian , Fan Dixia , Zhang Yu , Li Runpei
Published: November 2013

Currently, the Jacket platform plays pivotal role in offshore oil exploitation. However, since many fixed platforms become ageing, the periodic detection and safety assessment is necessary for the platform structures during the service. In this paper, a sensitivity diagnose method is proposed, based on the fact that some members of jacket platform are very sensitive to the damage of the structure. At first, we numerically calculated the dynamic response of the platform with failure members in extreme sea environment, where the pile-soil interaction was considered. Then the sensitive members and the sensitivity distribution were concluded. The statistics show that the group of sensitive members is entirely different in different destructive cases. In other words, the change of stress on the members, caused by the member failure, is unique. Therefore, the sensitivity diagnose method is feasible and we can find the failure members based on the change of stress.
A Probabilistic Metocean Model for Mooring Response in Gulf of Mexico Hurricanes
Authors: Mathisen Jan , Hørte Torfinn
Published: November 2013

Hindcast data for a specific location is utilised to develop a joint probability function for the metocean variables that are expected to have a significant effect on mooring line tensions for a floating platform moored at that location. The main random variables comprise: peak significant wave height, peak wind speed, peak surface current speed, peak wave direction, peak wind direction and peak current direction, where “peak” indicates the maximum intensity of the metocean effect during a random hurricane. The time lead of peak wind relative to peak waves and the time lag of peak current after peak wind are included as random variables. It is also necessary to describe the time variation around the peak events. Simple models are assumed based on inspection of the time variations during severe hurricanes. Only the part of the hurricane during which the significant wave height exceeds 80% of the peak value is taken into account. The duration of this interval is included. Linear variation is assumed for the directions, hence the rates of change of the 3 directions are included. A linear (triangular) plus parabolic model is assumed for the time variation of the intensities of the 3 metocean effects around their respective peaks. A single parameter is required to define the proportion of linear and parabolic models for each effect and the values of this parameter for each of the 3 metocean effects are also included as random variables.

A random hurricane can be drawn from this metocean model, such that the time variation of the metocean actions is deterministic once the values of the random variables have been selected. The overall duration of the hurricane is split into short intervals, each of 15 minutes duration, such that stationary response may be assumed during each short interval. The extreme value distribution of line tension during each short interval is obtained. These distributions are combined to obtain the extreme distribution of line tension during the hurricane. Second order reliability methods are applied to integrate over the distribution of the metocean variables and obtain the distribution of extreme tension during a random hurricane. The annual frequency of hurricanes is used to derive the annual extreme value distribution of line tension.

The model is intended for the reliability analysis of the ultimate limit state of mooring lines, but may also be applicable to other response variables. The present paper is primarily concerned with the metocean model, but it is intended to include sample results for the extreme line tension.
Finite Element Analysis of Subsea Pipelines Subjected to Underwater Explosion
Authors: Van den Abeele F. , Verleysen P.
Published: November 2013

Underwater mines and explosives, left in ports and harbours after World War II, can still pose a threat to subsea pipelines. In case of an accidental explosion, or even during controlled detonation, such explosives can cause significant damage to subsea pipelines. To assess the safety of pipelines exposed to an underwater explosion, finite element analyses are performed to predict the transient response of the pipeline to an acoustic pressure shock wave.

This type of problem is characterized by a strong coupling between the structural response of the pipe and the acoustic pressure on the wetted interface between the pipe surface and the surrounding seawater. The spherical pressure wave induced by an underwater explosion is characterized by a very steep wave front, where the maximum pressure is attained over an extremely short rise time. The pressure then drops off exponentially over a significantly longer period of time. As a result, the structural behaviour is a combination of a long time response, dominated by an added mass effect (low frequency), a short time response, governed by radiation damping (high frequency), and an intermediate time-frequency response, where both added mass and radiation damping effects are present.

In this paper, a finite element model is presented to simulate the transient response of a subsea pipeline subjected to an underwater explosion. The close coupling between acoustic pressure and structural response gives rise to numerical challenges like the accurate formulation and representation of the shock wave, the mesh requirements for the acoustic domain, and the position of the surface based absorbing radiation boundaries. An explicit dynamic solver is used to tackle these challenges, and to predict the behaviour of subsea pipelines exposed to an underwater explosion. The numerical results are compared to published experimental data, and can be used to assess the safety of submerged pipelines in the vicinity of explosives.
Wave Drift Forces on Three Barges Arranged Side by Side in Floatover Installation
Authors: Xu Xin , Yang Jianmin , Li Xin , Lu Haining
Published: November 2013

Floatover is a new method for installing integrated topside of a spar platform. It has several obvious advantages such as less time and cost compared with derrick lifting. In general, the floatover installation consists of three procedures: firstly a single barge is used for long-distance transportation of the topside in order to get good stability; secondly two barges take place of the single barge for floatover installation near the operating site; finally the topside is transferred from the two barges to the spar hull and the installation is completed. Between the first and second procedures, the case occurs that the single transportation barge is sided left and right by two floatover barges in the second procedure with close proximity. This case is concerned by many designers and operators for the security problem brought by possible large relative motions and forces of the three barges in side by side configuration.

The hydrodynamics of side-by-side barges are much more complex than that of a single barge in waves. In numerical simulation, it is a challenge to consider all effects including the hydrodynamic interactions, the shielding effects, the viscous effects and the wave resonance effect which has been observed in the gaps between the barges and has a significant impact on wave drift forces. In this paper, motion responses and wave drift forces were calculated in frequency domain for both the multi-body system and the single body. Far-field, middle-field and near-field method were all carried out to calculate wave drift forces. Numerical analysis was executed using potential flow code WAMIT. Corresponding model tests were also performed in the Deepwater Offshore Basin in Shanghai Jiao Tong University. Comparison between numerical and experimental results shows that numerical results agree well with the experiment and that middle-field method has better convergence than near-field method. The comparison between the multi-body system and single body shows that the hydrodynamic interactions (including wave shielding effect and Helmholtz resonance of water in the gaps) are remarkable and motion responses in the multi-body system are larger than single barge at some frequencies.
Riser Resistance to In-Service Fatigue Loading
Authors: Iob Francesco , Mecozzi Elisabetta , Di Vito Luigi , Darcis Philippe , Marines-Garcia Israel , Quintanilla-Carmona Hector M.
Published: November 2013

Riser systems are subjected to complex loading spectra induced by sea states. The complex loading spectrum differs sensibly from the constant amplitude loading commonly adopted for qualification of the product (riser pipe and its girth welds).

The present work deals with numerical calculations of the actual loading spectrum experienced by a Steel Catenary Riser when adopted in different scenarios, Gulf of Mexico (GoM) and West of Africa (WoA). Sensitivity analyses have been performed to evaluate the influence on applied fatigue loading of different parameters, like: water depth, seabed stiffness, hang-off angle.

The variable amplitude loading, derived from in-service conditions, has been applied to laboratory scale, full thickness specimens from X65 SCR girth welds. The fatigue performance of these samples has been compared to analogous samples subjected to constant amplitude loading. The samples subjected to in-service loading conditions exhibited a lower fatigue performance than in constant amplitude.

To allow a comparison with common qualification procedure under constant amplitude, full scale testing of SCR girth welds have been performed on resonance machine by in-service loading spectra rearranged in constant amplitude blocks. Laboratory full thickness specimens have been adopted to compare the girth weld fatigue performance when subjected to the original cycle-by-cycle sequence and the in-blocks rearranged sequence.
C-HyP: A Combined Wind and Wave Energy Platform With Balanced Contributions
Authors: Soulard Thomas , Babarit Aurélien , Borgarino Bruno , Wyns Mickael , Harismendy Migel
Published: November 2013

This paper synthesizes the technical feasibility study carried out for a hybrid ocean energy converter, with balanced wind and wave contributions. The solution envisaged involves a 100m diameter circular barge equipped with floating oscillating wave surge converters (OWSCs). This floating structure is mounted with a 5MW wind turbine. The present study covers power performance estimations, structural analysis and mooring design calculations. The first section describes the “Wave to Wire” model programmed in both frequency and time domain. The mathematical and hydrodynamic assumptions are highlighted together with the numerical model. The second part starts with the assessment of the performances of this device, carried out on in-house simulation codes. Based on combined wave and wind resources, the annual average absorbed power figures are compared with published results for existing ocean energy converters. The total rated power of the combined system reaches 10MW. Eventually, the last section approaches practical topics, directly related to the capital and operational costs inherent to an industrial development phase. The total steel mass is estimated first, from structural calculations carried out for a selection of 3D static loads cases. Then, a technical solution for the mooring system is presented together with the envisaged installation procedure.
Seabed Interaction Modelling Effects on the Global Response of Catenary Pipeline: A Case Study
Authors: Elosta Hany , Huang Shan , Incecik Atilla
Published: November 2013

A steel catenary riser (SCR) attached to a floating platform at its upper end encounters oscillations in and near its touchdown zone (TDZ), which results in interaction with the seabed. Field observations and design analysis of SCRs show that the highest stress and greatest fatigue damage occurred near the touchdown point where the SCR first touches the seabed soil. The challenges regarding the fatigue damage assessment of an SCR in the TDZ are primarily because of the non-linear behaviour of SCR–seabed interaction and considerable uncertainty in seabed interaction modelling and geotechnical parameters. Analysis techniques have been developed in the two main areas: SCR–seabed interaction modelling and the influence of the uncertainty in the geotechnical parameters on the dynamic response and fatigue performance of SCRs in the TDZ. Initially, this study discusses the significance of SCR–seabed interaction on the response of an SCR for deepwater applications when subjected to random waves on soft clay using the commercial code OrcaFlex for non-linear time domain simulation. In the next step, this study investigates the sensitivity of fatigue performance to geotechnical parameters through a parametric study. It is proven that employing the improved lateral SCR-seabed interaction model with accurate prediction of soil stiffness and riser penetration with cyclic loading enables us to obtain dynamic global riser performance in the TDZ with better accuracy. The fatigue analyses results prove that the confounding results indicated by the previous research studies on the SCR in the TDZ are due to different geotechnical parameters imposed with the seabed interaction model. The main benefit of employing non-linear seabed approach is to capture the entity of realistic soil interaction behaviour in modelling and analysis and to predict the likelihood of the fatigue damage of the SCR with seabed interaction, thereby minimising the risk of the loss of the containment with the associated environmental impact.
Validation of CFD for the Determination of Damping Coefficients for the Use of Wave Energy Converters Modelling
Authors: Thilleul Olivia , Babarit Aurélien , Drouet Aurélien , Le Floch Sébastien
Published: November 2013

Diffraction-radiation codes enable to model the behaviour of Wave Energy Converters (WEC) and seakeeping of ships on many sea-states with very little computational time. However, the viscous effects are neglected and therefore the simulations lead to relatively inaccurate values. The inaccuracy mainly occurs at the resonance frequency, especially in roll motions for which viscous effects are of major importance. Classically, the viscous effects are represented by adding viscous damping coefficients obtained either from experimental data or analytical approaches based on numerous approximations.

In order to improve the accuracy of the diffraction-radiation solvers, the damping coefficients can also be calculated from Computational Fluid Dynamics (CFD) simulations. The first part of this paper presents the three CFD solvers and turbulence models used in this validation study: ICARE and ISIS-CFD are developed by Ecole Centrale de Nantes and Star-CCM+ is a general purpose solver developed by CD-adapco. For each case, a preferred solver is chosen and a second solver is used for verification in most cases.

The second part briefly presents the theory that obtains drag coefficients in oscillatory flows, which are closely related to damping coefficients in waves. Each of the three following parts introduces the experimental test cases to which numerical results are compared to. The numerical parameter convergence study leads to a choice of around 200 timesteps per period with an adapted mesh enabling to obtain drag coefficients with errors lower than 5%. A mesh convergence study in the wake area leads to a mesh refinement of around 2 to 2.5 % of the body characteristic length. In order to reduce the computational time, the total number of cells can be decreased by mainly refining locations where specific flow detachment occurs, such as body corners or sharp edges. Turbulence models are also varied. Validation results are finally presented in terms of single or coupled damping coefficients and added mass coefficients. They are presented for various non-dimensional numbers such as Keulegan-Carpenters and Reynolds number.
Optimisation of Conductor System Design to Minimise Wellhead Fatigue Issues
Authors: Williams Dara , Purcell Kevin
Published: November 2013

Current market trends in the construction of newbuild drilling rigs indicate that the market is driven by demand for ultra-deepwater capacity semi-submersible rigs and drillships. These drilling vessels have capacity to drill in water depths of up to 12,000ft and possibly beyond in the near future. With increase in water depth capacity, more complex and heavier BOP stacks are required. Many modern drilling vessels are now incorporating BOPs with capacities of 20ksi pressure and up to 7 shear/seal rams incorporated. This leads to increased height and weight in the BOP. Whilst newbuild drilling vessels will be required to operate in water depths from 1,500ft to 12,000ft whilst on DP mode, deepwater semi-submersible drilling rigs will also have capability for operation in water depths <1,500ft using conventional mooring. Recent experience with modern deepwater rigs with large BOP stacks in water depth of 1,500ft or less suggests increased risk of fatigue when compared to 3rd generation rigs. If future trends continue with larger BOP stacks being designed then the problem of wellhead fatigue with modern deepwater drilling vessels is likely to become more acute.

As noted in previous studies the water depth at drillsite has a major impact on the level of fatigue accumulated in the wellhead system. The main driver for this has been found to be the height and weight of the BOP. With requirements for newbuild drilling rigs for 12,000ft water depth capacity being the industry norm, and with increased requirements for BOP functionality, the gap between wellhead loading from 3rd generation and 6th generation rigs is widening. Given that many 3rd generation rigs will likely be decommissioned in the coming years then the usage of 6th generation rigs for shallow water operations will only become more commonplace due to rig availability. Thus, unless market conditions dictate the construction of smaller and lighter BOP stacks, the design of shallow water wells will be critical to ensure fatigue loading on the wellhead and conductor is kept to a minimum.

This paper presents a summary of the results of a series of parameter studies carried out to assess a range of options for optimisation of casing and conductor design for 6th generation rigs in shallow water. Various recommendations are made as part of this study as to the addition of supplemental casing and conductor strings of varying sizes and wall thickness to ensure a robust conductor system design for fatigue performance.
Preliminary Cross-Validation of Wave Energy Converter Array Interactions
Authors: Folley Matt , Whittaker Trevor
Published: November 2013

The development of wave energy for utility-scale electricity production requires an understanding of how wave energy converters will interact with each other when part of a wave farm. Without this understanding it is difficult to calculate the energy yield from a wave farm and consequently the optimal wave farm layout and configuration cannot be determined. In addition, the uncertainty in a wave farm’s energy yield will increase the cost of finance for the project, which ultimately increases the cost of energy.

Numerical modelling of wave energy converter arrays, based on potential flow, has provided some initial indications of the strength of array interactions and optimal array configurations; however, there has been limited validation of these numerical models. Moreover, the cross-validation that has been completed has been for relatively small arrays of wave energy converters. To provide some validation for large array interactions wave basin testing of three different configurations of up to 24 wave energy converters has been completed. All tests used polychromatic (irregular) sea-states, with a range of long-crested and short-crested seas, to provide validation in realistic conditions.

The physical model array interactions are compared to those predicted by a numerical model and the suitability of the numerical and physical models analysed. The results are analysed at three different levels and all provide support for the cross-validation of the two models. The differences between the physical and numerical model are also identified and the implications for improving the modelling discussed.
Structural Modeling and Analysis of a Wave Energy Converter Applying Dynamical Substructuring Method
Authors: Zurkinden Andrew S. , Damkilde Lars , Gao Zhen , Moan Torgeir
Published: November 2013

This paper deals with structural modeling and analysis of a wave energy converter. The device, called Wavestar, is a bottom fixed structure, located in a shallow water environment at the Danish Northwest coast. The analysis is concentrated on a single float and its structural arm which connects the WEC to a jackup structure. The wave energy converter is characterized by having an operational and survival mode. The survival mode drastically reduces the exposure to waves and therfore to the wave loads. Structural response analysis of the Wavestar arm is carried out in this study. Due to the relative stiff behavior of the arm the calculation can be reduced to a quasi-static analysis. The hydrodynamic and the structural analyses are thus performed separately. In order to reduce the computational time of the finite element calculation the main structure is modeled as a superelement. The structural detail, where the stress analysis is carried out, is connected with the superstructure by interface nodes. The analysis is conducted for two different control situations. Numerical results will be presented which can be further used to carry out fatigue analysis in which a more refined FE model is required to obtain the stress concentration factors.
Improving Offshore Platforms Wave Measurements
Authors: Forristall George Z. , Aubault Alexia
Published: November 2013

Offshore platforms are equipped with wave instrumentation at deck extremities to measure incoming wave elevations. When those instruments are close to large structural members they record the diffracted wave as well as the incident wave. This paper studies the effect of the diffracted wave on the measured wave height. First and second order diffracted wave elevations are computed for a model Tension Leg Platform (TLP) that was tested in the Offshore Basin of the Maritime Research Institute Netherlands (MARIN) offshore basin as part of the CresT Joint Industry Project (JIP). Their respective contributions to the wave spectrum are compared at locations near the structure. These calculations are useful for identifying the best locations for wave probes. The diffraction solution is used in forward calculations to compute the wave height and wave crest at locations under the deck from the undisturbed wave. These calculations can be used to set the air gap under the deck. Conversely, this paper introduces an inverse method to retrieve the undisturbed wave height and crest from the measured data by inverting the diffracted wave coefficients. The calculations are verified using measurements of undisturbed and diffracted waves under the TLP model. This work was sponsored by the Climatology and Simulation of Eddies (CASE) JIP.