bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of Design Wave Approach and Short Term Approach With Increased Wave Height in the Evaluation of Whipping Induced Bending Moment
Authors: Derbanne Quentin , Bigot Fabien , de Hauteclocque Guillaume
Published: August 2013

The evaluation of extreme bending moment corresponding to a 25 years return period requires very long simulations on a large number of sea states. This long term analysis is easy to do with a linear model of the ship response, but is impractical when using a time consuming model including non linear and slamming loads. In that case some simplified methods need to be applied. These methods are often based on Equivalent Design Waves (EDW) which are calibrated on the extreme linear value.

The general practice is to define the EDW as a regular wave. A very simple method is to compute the non linear bending moment applying the pressure correction on the hull without recomputing the ship motions. A better method is to recompute in time domain the non linear ship response on this Design Wave. It is even possible to define a more realistic Design Wave, taking into account the frequency and directional content of the sea states used in the long term analysis: those waves are called Response Conditioned Wave and Directional Response Conditioned Waves.

The different methods are applied to an Ultra Large Container Ship (ULCS). Hydro-structure calculations are carried out on a severe design sea state, taking into account Froude-Krylov pressure correction, slamming forces and whipping response. Results of a very long computation are compared to the results of the Design Wave approaches.

Another method is proposed to compute very rare events. It is based on an artificial increase of the significant wave height of the sea state, and the assumption of the independence of the non linear effects to the significant wave height. Using this method it is possible, with a simulation of only a few hours, to predict a very rare short term event, corresponding to a very long return period. The results are compared to the Design Wave results and appear to be much more precise.
Techniques for Predicting Tensile Armour Buckling and Fatigue in Deep Water Flexible Pipes
Authors: Sævik Svein , Thorsen Mats J.
Published: August 2013

Free hanging deep water flexible risers are associated with high static top tension. In addition comes significant dynamic tension due to the tangential drag forces mobilised along the riser due to floater motions and wave loads. At the upper end fitting where conditions of direct metallic contact between tensile armours might occur, this may result in the fretting fatigue failure mode. This mechanism may be additionally triggered by the increase in longitudinal stress resulting from local bending at the end fitting fixation.

At the touch down point, the pipe cross-section is exposed to high external pressures resulting in true wall compression possibly initiating local buckling modes in the tensile armour wires. The buckling process may be initiated by cyclic bending effects leading to a gradual reduction of each tendon’s capacity resulting in excessive transverse displacements and cross-section failure.

The present paper presents a finite element formulation and analytical models addressing both of the above topics. A case study is further carried out to document the performance of the FE model and to investigate effects related to the transverse bending stress at the end fitting and under which conditions one single armour tendon will fail in different buckling modes.
Investigating the Use of the Hilbert-Huang Transform for the Analysis of Freak Waves
Authors: Maris Jan , Christou Marios , Huijsmans René
Published: August 2013

This paper presents the Hilbert-Huang Transform (HHT) with Empirical Mode Decomposition (EMD), which is an opportune method to analyse non-stationary traces of nonlinear waves, with freak waves as a case in point. Comparison to conventional methods such as the Fourier, Wavelet and Stockwell Transform is undertaken. Furthermore, an investigation of different EMD schemes (Ensemble EMD or Smoothed EMD) is undertaken to improve the interpretability of the results. Ultimately the Draupner New Year Wave is examined by using the HHT. After all, on successfully applying the HHT, a new perspective on analysing waves is suggested.
Collaborative CFD Exercise for a Submarine in a Steady Turn
Authors: Toxopeus Serge , Atsavapranee Paisan , Wolf Eric , Daum Stefan , Pattenden Richard , Widjaja Ronny , Zhang Jian Tao , Gerber Andrew
Published: August 2013

The application of viscous-flow solvers to calculate the forces on ship hulls in oblique motion has been studied for a long time. However, only a few researchers have published work in which the flow around ships in steady turns was studied in detail. To predict ship manoeuvres, an accurate prediction of the loads due to rotational motion is also required. In a collaborative CFD exercise, the Submarine Hydrodynamics Working Group (SHWG) performed calculations on the bare hull DARPA SUBOFF submarine to investigate the capability of RANS viscous-flow solvers to predict the flow field around the hull and the forces and moments for several steady turns. In the study, different commercial as well as bespoke flow solvers were used, combined with different turbulence models and grid topologies. The work is part of a larger study aiming to improve the knowledge and understanding of underwater vehicle hydrodynamics. In this paper, the results of the exercise will be presented. For several cases, verification studies are done to estimate the uncertainties in the results. Flow fields predicted by the different members of the SHWG are compared and the influence of the turbulence model will be discussed. Additionally, the computed forces and moments as a function of the drift angle during the steady turns will be validated. It will be demonstrated that using sufficiently fine grids and advanced turbulence models without the use of wall functions will lead to accurate prediction of both the flow field and loads on the hull.
A Nonlinear Numerical Algorithm for Time-Domain Hydrodynamic Simulations of Vessel Motions in the Presence of Waves
Authors: Pasquetti Eduardo , Coelho Luiz Cristóvão G. , Neves Marcelo A. S. , Oliveira Mauro C. , Esperança Paulo T. T. , Rodríguez Claudio A. , Celis Miguel Angel , Polo Julio Cesar F.
Published: August 2013

Linear approaches have been traditionally employed to simulate the dynamic behavior of floating vessels and its interaction with regular or irregular waves. Some difficulties arise when large waves and vessel motions occur. Under these circumstances, the linear assumptions to compute the restoring and wave forces, which are computed on the mean position of body and water surface, are not capable of accurately representing the physics of the interactions between waves and vessels.

Hydrostatic analysis of generic hull forms has already been implemented with a geometrical face representation of the hull and also internal ballast and oil tanks [1]. With the goal of improving the modeling the non-linear computation of hydrostatic in waves (at the instantaneous free surface) is implemented, thus using a generic geometric modeling of the hull to perform hydrodynamic simulations of vessel motions in the presence of waves. Additionally, for the computation of the instantaneous non-linear Froude-Krylov force (6 DOF time-domain model) the contribution of each geometrical face to the global Froude-Krylov force is calculated at the exact relative position of the vessel and the incident waves. After computing the relative position of each face, possibly being cut at the free surface, the pressure at the wetted face centers determines the contribution to the integral calculation.

The paper presents the main aspects of the proposed methodology and highlights its capabilities and differences with respect to the linear approach. Complementarily, comparisons with model experiments are discussed.
Conventional and Linear Statistical Moments Applied in Extreme Value Analysis of Non-Gaussian Response of Jack-Ups
Authors: do Nascimento Leonardo Sant’Anna , Sudati Sagrilo Luis Volnei , Ellwanger Gilberto Bruno
Published: August 2013

This work investigates numerically two different methods of moments applied to Hermite derived probability distribution model and variations of Weibull distribution fitted to the short-term time series peaks sample of stochastic response parameters of a simplified jack-up platform model which represents a source of high non-Gaussian responses. The main focus of the work is to compare the results of short-term extreme response statistics obtained by the so-called linear method of moments (L-moments) and the conventional method of moments using either Hermite or Weibull models as the peaks distribution model. A simplified mass-spring system representing a three-legged jack-up platform is initially employed in order to observe directly impacts of the linear method of moments (L-moments) in extreme analysis results. Afterwards, the stochastic response of the three-legged jack-up platform is analyzed by means of 3-D finite element model. Bias and statistical uncertainty in the estimated extreme statistics parameters are computed considering as the “theoretical” estimates those evaluated by fitting a Gumbel to a sample of episodical extreme values obtained from distinct short-term realizations (or simulations). Results show that the variability of the extreme results, as a function of the simulation length, determined by the linear method of moments (L-moments) is smaller than their corresponding ones derived from the conventional method of moments and the biases are more or less the same.
Methodology for Fatigue Analysis in Flexible Pipes Based on Structural Reliability Considering S-N Bi-Linear Curve
Authors: dos Santos Loureiro Filho Fernando , de Lima Edison Castro Prates , Sudati Sagrilo Luís Volnei , de Lemos Carlos Alberto Duarte
Published: August 2013

Flexible pipes are largely used by oil and gas companies all over the world to exploit oil and gas reserves located into the sea. These pipes are composed by different layers, each one with a specific function. The environmental loads can induce different types of failure modes in a flexible pipe. One important failure mode is associated with the fatigue damage in the tension armours. Fatigue analysis depends on various parameters that are uncertain. A reliability-based procedure to take into account these uncertainties in the fatigue analysis of flexible pipes has been recently proposed [1]. In this methodology the S-N curves have been modeled by a one-slope model. The present work expands this methodology in order to consider S-N bi-linear models.
Reliability Analysis of Hydraulic Directional Control Valve Installed in Subsea Control Module
Authors: Yue Yuanlong , Zuo Xin , Wang Lijun
Published: August 2013

This paper explores hybrid life distribution, instead of constant failure rate, based on the exponent and Gamma model to analyze the reliability of hydraulic directional control valve (HCV). The article shows that HCV reliability has not only to do with the pulse frequency of pilot valve but also to the hydraulic latching time of main valve, the failure rate not to be constant is with respect to the parameters of the hybrid life distribution model. This paper discusses the cold standby and the 1oo2 configuration applied to dual redundant coils, concluding that the latter is priority to the former and can extend the service life of the HCV. A simulation example shows the effectiveness.
Numerical Assessment of the Mean Power Production of a Combined Wind and Wave Energy Platform
Authors: Soulard Thomas , Babarit Aurélien
Published: August 2013

This paper synthesizes the performance evaluation of a hybrid ocean wave energy converter. The case of interest represents a singular approach to combine a 5MW wind turbine with floating oscillating wave surge converters (OWSCs). The first section describes the comprehensive set of equations of motion 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 initial performance of this device, carried out on in-house simulation codes. The first stage of numerical validation is based on the linear transfer functions (RAOs) results. The analysis of these RAOs also exposes interesting mechanical properties of this particular device. Eventually, the annual average absorbed power figures are extracted from the power matrices calculated for a few different sites.
Full-Scale Measurements of Wave Impact Loading on a Flat Plate
Authors: Drazen David , Terrill Eric , Walker Don , Hazard Joel , Cook Tom , Scott Shannon
Published: August 2013

Full scale measurements of wave impact loads and their statistics in real sea states are desirable for validation of numerical simulations and for application to marine engineering design problems. Measuring and/or estimating wave forces on flat plates are especially problematic due to statistics of large waves in a given sea state, the intermittent statistics of wave breaking, the sensitivity of the loading relative to the phase of the incoming wave and scaling issues when translating from model scale data to full-scale. To increase our understanding of wave hydrodynamic pressures on a flat plate, an instrumented plate was deployed from the Scripps Institution of Oceanography’s research pier. The instrumented plate is exposed to a wide range of wave conditions with Hs ranging from 3–4 m in the winter and with Hs in the 1–2 m range in the summer. The instrumented flat plate is composed of three discrete modules containing 6 pressure gages. Data are being collected over a extended period, nominally 12 months, to characterize extreme value distributions due to wave impact loading.
Breaking Wave on a Slender Cylinder: Comparison of Experimental Data and Numerical Results
Authors: Ramirez Jorge , Frigaard Peter , Andersen Thomas Lykke , Christensen Erik Damgaard
Published: August 2013

CFD models are promising in predicting non-linear wave loads on fixed and floating offshore structures. The NS3 model is described in this paper and it has been validated by means of model test such as wave run-up on monopiles in regular waves. The goal for the use of the NS3 model is to make a detailed investigation of the effect of 2D waves on the run-up height. The focused wave is designed by choosing the phases of the linear harmonic components such that they are in phase at a certain target location. The aim of this paper is the approach on the ability of NS3 model to reproduce these focused wave groups and compare with the results of the experimental tests carried out at Grossen WellenKanal (GWK).
Evaluating the Long-Term Fatigue Response of Mooring Lines Using an Asymptotic Approximation
Authors: Low Ying Min , Cheung Sai Hung
Published: August 2013

A major challenge in the design of a mooring line or riser is the evaluation of the fatigue damage accumulated over the lifetime of the structure. The long-term environmental condition is usually represented by a scatter diagram, or a joint probability density of the significant waveheight and a characteristic wave period. Since it is computationally impractical to consider a large number of sea states, a common practice is to condense the sea states into a small number of blocks, but this procedure inevitably introduces significant errors owing to the coarse discretization. In view of the need for efficient but accurate approaches, this paper investigates the application of an asymptotic approximation, which is an established technique for estimating probability integrals, but it has so far not been applied to the fatigue design of moorings and risers. In addition, a classical method known as the perturbation approach is examined. The above approximate methods are implemented on an FPSO floating system, and direct numerical integration is carried out to ascertain the accuracy of the approximate solutions.
Grid Impact of a Wave Farm on its Local Network: Analysis of Voltage and Flicker Levels
Authors: Blavette Anne , O’Sullivan Dara L. , Alcorn Ray , Lewis Anthony W. , Egan Michael G.
Published: August 2013

Most oscillating wave energy converters without significant amounts of energy storage capacity generate significant electrical power fluctuations in the range of seconds. Because of these fluctuations, a wave farm may have a negative impact on the power quality of the local grid to which it is connected. Hence, the impact of these devices on both distribution and transmission networks needs to be well understood, before large scale wave farms can be allowed to connect to the grid.

This paper details a case study on the impact of a wave farm on the distribution grid around the national wave test site of Ireland. The electrical power output of the oscillating water column (OWC) wave energy converters was derived from experimental time series produced in the context of the FP7 project “CORES”. The results presented in this paper consider voltage fluctuation levels and flicker levels for a typical time series. Simulations were performed using DIgSILENT simulation tool “PowerFactory”.
Three-Dimensional Pushover Analysis of a Jack-Up Platform
Authors: Cheng Ning , Kimiaei Mehrdad , Cassidy Mark J.
Published: August 2013

Jack-ups, as the name indicates, are self-elevating units consisting of a floatable hull and, usually, three truss work or circular legs. As a mobile structure, jack-ups are mainly utilized for oil and gas exploration and maintenance purposes in shallow to medium water (up to 120 meters), though they have recently seen application in the construction of offshore wind energy infrastructure.

The use of the finite element method for structural analyses of jack-ups is a common practice. However, most jack-up models remain elastic with the yielding of structural members or even the soil around the spudcan not accounted for. This paper describes the development of a comprehensive and integrated elasto-plastic jack-up model in ABAQUS [1]. This model is representative of a modern jack-up structure, and it can capture geometrical nonlinearities and plastic behaviour of the structural and soil materials.

In this study, the discretisation of the structural elements, the choice of elements, the elasto-plastic behaviour of the material, and the mesh generation are described. Numerical results of a series of static pushover analyses for this sophisticated model under extreme loads are presented. The sensitivity of the results to the structural configurations is discussed. For instance, the choice of sectional properties of the chord member and the assumption of the behaviour of the spudcan (jack-up foundation)-soil interaction are shown to be critical to the prediction of the ultimate strength of the platform and the progressive failure mechanism.

In conclusion, generic issues associated with static pushover analyses of jack-ups are discussed and possible numerical solutions are proposed.
Practical Structural Assessment of Offshore Structures for Wave Slap
Authors: Moon Joong Soo , Park Tae Hyun , Sim Woo Seung , Shin Hyun Soo
Published: August 2013

By the combination of theoretical and empirical approach, the methodology for practical structural assessment of offshore structures for wave slap is proposed. It is developed for engineers in the sense that the precise design pressure is easily obtainable and quickly applicable in early and detail design stage. For impact load prediction, the Pressure-Impulse theory that was well developed and validated in coastal engineering field is applied. The impact pressures are classified into three types (traditional, sharp, and immersed slap) according to model tests and BP Schiehallion FPSO’s bow monitoring. The time histories of impact pressures for the classified impact types are generated with the pressure impulse predicted by the Pressure-Impulse theory. Nonlinear transient structural analyses are performed using the time series of impact pressures to obtain equivalent static pressure factors. Finally, the design pressure is determined by multiplying the maximum peak pressure by the equivalent static pressure factor. The results are validated through the comparison with model tests and dedicated reports.
The Effect of Plate Length on the Behaviour of Free-to-Rotate VIV Suppressors With Parallel Plates
Authors: Assi Gustavo R. S. , Rodrigues Julia R. H. , Freire Cesar M.
Published: August 2013

Experiments have been carried out on free-to-rotate parallel plates fitted to a rigid section of circular cylinder to investigate the effect of plate length on the stability of this type of VIV (vortex-induced vibration) suppressor. Measurements of the dynamic response and trajectories are presented for models with low mass and damping which are free to respond in the cross-flow and streamwise directions. It is shown that, depending on a combination of geometric and strucutral parameters, parallel plates might not be able to completely suppress VIV for the whole range of reduced velocities investigated. Plates with length between 1.0 and 2.0 diameters showed instabilities and induced high-amplitude vibrations for some specific reduced velocities. Rotational friction was increased for a second run and all plates stabilised and suppressed VIV for the whole range of reduced velocities tested. An undesirable steady lateral force was also observed to occur for all configurations. Experiments with a plain cylinder in the Reynolds number range from 1,000 to 20,000 have been performed to serve as reference.
Collision Consequence Assessment of ROPAX Vessels Operating in the Baltic Sea
Authors: Tabri Kristjan , Ehlers Sören , Kõrgesaar Mihkel , Ståhlberg Kaarle , Heinvee Martin
Published: August 2013

A Ship collision accident represents a daily threat for vessels operating in dense traffic zones. The collision consequences may include loss of life or severe injuries if passengers are on board. The latter would be the case for ROPAX vessels, which are fairly dominant in the Baltic Sea connecting various member states. Furthermore, their routes tend to be in cross-traffic with the cargo vessels travelling through the full extent of the Baltic Sea. Therefore, it is of utmost importance to be able to assess the collision consequences for ROPAX vessels operating in the Baltic Sea with sufficient accuracy. This will result in an overview of possible damage scenarios for the actual traffic situation at a given location. As an example location the dense cross traffic between Helsinki and Tallinn will be analyzed and discussed.

The analysis procedure combines three steps: (1) determination of possible accidental scenarios based on traffic statistics; (2) assessment of the structural resistance of the colliding ship and (3) the evaluation of selected accidental scenarios using a time-efficient semi-analytical approach. The level of structural resistance of the chosen ships is assessed in a quasi-static manner using finite element method. This information is the basis for the calibration of a semi-analytical collision simulation model used to simulate large number of the accidental scenarios typical to the selected location. The presented results will be limited to the initial choice between vessels and dimensions, respectively masses, but the procedure can easily be extended to cover a vast amount of colliding vessels. However, the actual collision risk can be obtained using the presented results if the traffic along the vessels route is known.
Latching Control of an OWC Spar-Buoy Wave Energy Converter in Regular Waves
Authors: Henriques J. C. C. , Falcão A. F. O. , Gomes R. P. F. , Gato L. M. C.
Published: August 2013

The present paper concerns an OWC spar-buoy, possibly the simplest concept for a floating oscillating-water-column (OWC) wave energy converter. It is an axisymmetric device (and so insensitive to wave direction) consisting basically of a (relatively long) submerged vertical tail tube open at both ends, fixed to a floater that moves essentially in heave. The length of the tube determines the resonance frequency of the inner water column. The oscillating motion of the internal free surface relative to the buoy, produced by the incident waves, makes the air flow through a turbine that drives an electrical generator. It is well known that the frequency response of point absorbers like the spar buoy is relatively narrow, which implies that their performance in irregular waves is relatively poor. Phase control has been proposed to improve this situation. The present paper presents a theoretical investigation of phase control by latching of an OWC spar-buoy in which the compressibility of air in the chamber plays an important role (the latching is performed by fast closing and opening an air valve in series with the turbine). In particular such compressibility may remove the constraint of latching threshold having to coincide with an instant of zero relative velocity between the two bodies (in the case under consideration, between the floater and the OWC). The modelling is performed in the time domain for a given device geometry, and includes the numerical optimization of the air turbine rotational speed, chamber volume and latching parameters. Results are obtained for regular waves.
Analyses of Different FE Models for HP/HT-Pipeline Walking With Sliding End Structures
Authors: Castelo Adriano , Galgoul Nelson Szilard
Published: August 2013

Non-buried subsea pipelines subjected to high internal pressures and high operational temperatures (HP/HT) might experience significant axial expansion. If this movement is restrained by an end structure, considerably high loads can be imposed to the system. Sliding foundations have been used to minimize this effect, allowing free end displacements.

Regarding the aforementioned loads, the thermo-mechanical behavior of HP/HT pipelines interacts with the end restraints in a complex manner. Axial displacements can accumulate over the operational cycles, in a phenomenon known as “pipeline walking”. If the sliding foundation design does not account for these accumulated displacements, axial loads (not considered in the pipeline design) might be imposed. As a result, the overall thermo-mechanical behavior in terms of lateral buckling and walking can change significantly.

Two recently published papers present the results of different analysis methodologies for the same structure. The corresponding analyses were performed using two different tools: (1) a non-linear three-dimensional finite element model considering pipe-soil interaction with full 3D seabed bathymetry and (2) a simplified one-dimensional model published in OMAE 2011.

In both cases, the limited sliding range was imposed to the model ends. Both calculation tools show similar overall results of pipeline global behavior, but the results of the end reaction after a few operational cycles are somewhat different.

Stimulated by these recently published papers, which present the results of different performed finite element analyses; this paper was developed to investigate the same problem, but now starting off with a third software, and rerunning the two previous analyses in the same software and adding an intermediate modeling level.
Study on Deformation Law About Six Degrees of Freedom Circular Foundation of Seabed
Authors: Zhang Qiyi , Wang Qinghua , Zhang Ying , Wang Qi
Published: August 2013

Some foundations of Ocean engineering structures are always in addition to withstand the vertical gravity loads, but also to horizontal loads induced by sea wave and current, which makes the foundation subjected to concentrated force, overturning loads and torque loads, the above loading state including vertical, horizontal and overturning moment loads are termed combined loading. So it is very necessary for the safety of ocean engineering structures to study the deformation law of submarine seabed due to combined loading. On the basis of the three-dimensional elastic mechanics solution of circular foundations, the deformation law of submarine clay under circular foundation with six degrees of freedom is analyzed in detail using numerical calculation methods. Based on the above numerical results, this paper gives the elastic deformation law of clay in three dimensional spaces, gives a modification to the traditional elasticity theoretical solutions, and gives clay coupled deformation mechanism under three-dimensional circular foundation with six degrees of freedom.
VIV Simulation of 2-D Deformable Cylinder Using Coupled FDM/FEM Method
Authors: Hu Changhong , Liao Kangping , Duan Wengyang
Published: August 2013

In this work a coupled finite difference method and finite element method (FDM/FEM) is developed for numerical simulation of vortex induced vibration (VIV) phenomenon with an elastically deformable circular cylinder. The algorithm is based on a FDM with CIP (Constraint Interpolation Profile) method to fluid dynamics and a FEM to solve structural dynamics. Coupling between FDM and FEM is realized by BGS (Block Gauss-Seidel) procedure. IB (Immersed Boundary) method is applied for data transferring on the interface between fluid and structure. In this paper, numerical simulations of a 2-D circular cylinder at low Reynolds number are carried out. The effect of stiffness of the cylinder shell on the vortex shedding is investigated.
Optimisation of Heat Pressure Welding of Steel Wire in Flexible Riser Manufacture, Applying Taguchi Design-of-Experiment Approaches
Authors: Rafferty John G.
Published: August 2013

The safety, quality and overall integrity of flexible risers has a high dependence on the robustness of joining processes used in their manufacture. One such process used in this industry is the heat–pressure welding process, used in the joining of high tensile steel wires.

The optimisation of such welding processes now demands that a statistically–valid and systematic approach be taken. The application of Taguchi methodology to the welding of high tensile steel wires described in this paper is a relatively new departure for Wellstream Flexibles. This paper describes the approaches taken to optimise the heat pressure welding of new steel grades in this demanding application, and the subsequent confirmation of the optimised parameters in real–world environments.

Quality response factors are defined and analysed for welds in a range of carbon steel wire materials, leading to the subsequent optimisation of target quality for the manufacture of flexible riser pipe.
A Discussion of Practical Aspects of Reeled Flowline Installation
Authors: Manouchehri Soheil
Published: August 2013

Reeling is a fast, reliable and cost effective method for installing subsea flowline systems up to 20 inch in diameter. The individual flowline joints are lined up and welded together onshore to make long stalks of pipe before being spooled onto the large reels of reel lay vessels. On arrival in the field offshore, it is spooled off, straightened and laid on the seabed up to 10 times faster than conventional S-lay or J-lay methods. The ability to carry out the fabrication and welding onshore, off-line from the vessel critical path, allows very high quality welding and coating to be achieved.

Over the last few years, it is evident that the engineering fundamentals, mechanics and cost effectiveness of the reeling process are now well understood within different quarters of the offshore industry as many major pipelaying Contractors own, or are planning to own, a reel lay vessel. What is apparently missing, however, is the implications and practical aspects of the reeling process when it comes to implementation during project execution.

This paper reviews the engineering fundamentals of the reeling process first and then discusses the practical aspects and applications of reeling mechanics from spoolbase fabrication to spooling-on process up to the offshore campaign and spooling-off process and subsea installation.
Lateral Stability of Umbilical in Intermittent Rock-Berms
Authors: Hoff Ronny , Asklund Per Olav
Published: August 2013

The most common ways to ensure stability of umbilicals are post-lay trenching and rock-dumping. For a number of reasons, exposed sections of the umbilical on seabed may sometimes be part of the design, e.g. as a temporary condition prior to finalizing intervention measures. Light umbilicals in shallow waters may be subject to hydrodynamic forces resulting in continuing dynamic motions of the exposed sections. If the sea state is sufficiently severe, the umbilical may experience a break-out effect and gradually displace out of the rock-berm over time. Such a gradual degradation of effective rock-berm length will increase the projected drag area of the umbilical. This will in turn render the system susceptible for continued break-out at less severe sea states. The potential consequences of rock-berm failure are damage to the umbilical as well as other assets in close proximity.

This paper presents an approach to evaluate the required length of rock-berm to avoid initiation of the break-out effect. A dynamic FE model is established, using time series of water particle velocities for load calculations. The time series correspond to full 3 hr sea states of irregular waves for a range of significant wave heights. Non-linear lateral resistance curves are applied for the sections of umbilical within a rock-berm, including the hysteretic effect of displacement. The ambition of this paper is to facilitate evaluation of intervention measures for the benefit of the industry, providing a design methodology in accordance with the DNV rules system.
Dual Stiffness Approach for Polyester Mooring Line Analysis in Time Domain
Authors: Tahar Arcandra , Sidarta Djoni , Ran Alex
Published: August 2013

Polyester mooring lines have been used in the offshore industry since the late ’90s. With increasing oil exploration and production in deeper waters, using polyester lines provides greater benefit than using traditional steel wires and chains. Some advantages of using polyester include a reduction of mooring line weight, a reduction in vessel offset and a reduction in the dynamics of the line tensions. However, unlike steel, polyester lines exhibit axial stiffness characteristics that are nonlinear and vary with time and loading history.

Tahar (2001) developed a comprehensive theory and numerical tool to capture this behavior. The formulas allow relatively large elongation and nonlinear stress-strain relationships, as typically observed in polyester fibers. The mooring line dynamics are based on a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system.

Since this theory is computationally intensive, the benefits outweigh the costs less than they do for the practical approach recommended by API. Therefore, the fully coupled dynamic analysis tool CHARM3D has been modified to incorporate the API-recommended approach. Two axial stiffnesses (EA), post installation (static) stiffness and storm (dynamic) stiffness, have been convoluted into a dual stiffness to represent the total response of the floating platform in a single run. In the traditional method, the analyses are done twice, one run for each stiffness. Then, the extremes from each run are used as governing values for design. This paper presents the global performance comparison between the dual stiffness method and the traditional method. The effect of motions on SCR strength is also investigated using ABAQUS software.
Simulating Ice-Sloping Structure Interactions With the Cohesive Element Method
Authors: Lu Wenjun , Løset Sveinung , Lubbad Raed
Published: August 2013

The major processes that occur when level ice interacts with sloping structures (especially wide structures) are the fracturing of ice and upcoming ice fragments accumulating around the structure. The cohesive zone method, which can simulate both fracture initiation and propagation, is a potential numerical method to simulate this process. In this paper, as one of the numerical methods based on the cohesive zone theory, the cohesive element-based approach was used to simulate both the fracturing and upcoming fragmentation of level ice.

However, simulating ice and sloping structure interactions with the cohesive element method poses several challenges. One often-highlighted challenge is its convergence issue. As an initial attempt, the mesh dependency of the cohesive element method was alleviated by both creating a mesh with a crossed triangle pattern and utilizing a penalty method to obtain the initial stiffness for the intrinsic cohesive elements. Furthermore, two potential methods (i.e., introduction of a random ice field and bulk energy dissipation considerations) to alleviate the mesh dependency problem were evaluated and discussed.

Based on a series of simulations with the different aforementioned methods and mesh sizes, the global ice load history is obtained. The horizontal load information is validated against the test results and previous simulation results. According to the comparison, the mesh objectivity alleviation with different approaches was discussed. As a preliminary demonstration, the results of one preliminary simulation are summarized, and the load contributions from different ice structure interaction phases are illustrated and discussed.
Uncertainty of Long Term Fatigue Load of Subsea Well Heads
Authors: Grytoyr Guttorm , Steinkjer Oddrun
Published: August 2013

Well heads (WH) are exposed to dynamic loading from waves and floater motions through the marine drilling riser and the drill string. Risers are known to have a pronounced nonlinear response characteristic, this influences the dynamic loading of the well heads. Non-linear time-domain finite element analyses are in general required to give an adequate description of the non-linearities involved. Analyses of a large number of short-term environmental conditions considering stochastic wave loading are required to give a representative description of the long-term fatigue loading on the structure.

The objective of this paper is to give an assessment of the sufficient simulation time of each short term seastate in order to get a required level of the statistical uncertainty of the long term fatigue load of the well head and conductor. The analyst can actually influence this statistical uncertainty by selecting proper analysis methodology. A statistical uncertainty measure can be used to evaluate the robustness in the estimated fatigue life. Assessment of statistical uncertainty in fatigue damage estimate is demonstrated by a case study, using typical North Sea conditions, and for varying boundary conditions.

Short term fatigue damage is established by means of rainflow cycle (RFC) counting in each stationary short-term condition. It has been experienced that significant statistical uncertainties can be present in the short-term fatigue damage estimates. This is because the accumulated fatigue damage in a stationary condition normally has significant contribution from the largest stress cycles in the realization. Selection of sufficient simulation length is hence essential to obtain reliable fatigue life estimates.

Applicable codes and standards for risers provide Design Fatigue Factors (DFF) to secure adequate safety against failure due to wave induced fatigue. The total uncertainty in the calculated fatigue damage comes from various sources and the DFFs in e.g. DNV-OS-F201 “Dynamic Risers” and API-RP-2RD corresponds to a certain uncertainty level in the fatigue damage estimate. A recommended target value for the statistical uncertainty of the fatigue damage estimates is given with basis in these design codes. There are no similar target values for statistical uncertainty of long term fatigue damage given in the applicable design codes for well heads and drilling equipment, hence the values from the riser codes have been selected as a starting point.

The recommendations and methodologies presented in this paper, will be included in the upcoming DNV RP “Fatigue design and analysis of drilling and well equipment”, at present this is available only as a draft, ref. [1].
State-of-the-Art Review of Floating Platform Concepts for Offshore Wind Energy Generation
Authors: Thiagarajan K. P. , Dagher H. J.
Published: August 2013

Literature relating to offshore wind energy generation is produced at a significant rate, as research efforts are diverted to the emerging area of future clean energy. This paper presents an overview of recent research in the specific area of floating offshore structure design for wind energy. Earlier literature has broadly grouped these platforms into three categories based on their source of stability:

- Ballast stabilized (low center of gravity), e.g. spar

- Mooring stabilized, e.g. Tension Leg Platform

- Buoyancy or water-plane stabilized, e.g. semi-submersible.

These concepts were modifications of similar structures used in the offshore oil and gas industry. Recent papers have presented further improvements to these designs including active ballasting and control systems. These are examined for stability and global performance behavior and ease of operability and maintenance.

The paper also attempts to examine efforts to bring such concepts to fruition. Combined with this, the authors highlight activities that are ongoing in a consortium of industry and research institutions that is headed by the University of Maine. The origin of such a consortium is explained with reference to the future wind energy potential offshore Maine. This paper sets the stage for other papers in the Special Session on UMaine/ DeepCWind Consortium within the Offshore Renewable Energy Symposium at OMAE 2012.
Comparison Between Numerical Modeling and Experimental Testing of a Point Absorber WEC Using Linear Power Take-Off System
Authors: Zurkinden Andrew S. , Kramer Morten , Teimouri Mahdi Teimouri , Alves Marco
Published: August 2013

Currently, a number of wave energy converters are being analyzed by means of numerical models in order to predict the electrical power generation under given wave conditions. A common characteristic of this procedure is to integrate the loadings from the hydrodynamics, power take-off and mooring systems into a central wave to wire model. The power production then depends on the control strategy which is applied to the device. The objective of this paper is to develop numerical methods for motion analysis of marine structures with a special emphasis on wave energy converters. Two different time domain models are applied to a point absorber system working in pitch mode only. The device is similar to the well-known Wavestar prototype located in the Danish North Sea. A laboratory model has been set up in order to validate the numerical simulations of the dynamics. Wave Excitation force and the response of the device for regular and irregular waves were measured. Good correspondence is found between the numerical and the physical model for relatively mild wave conditions. For higher waves the numerical model seems to underestimate the response of the device due to its linear fluid-structure interaction assumption and linearized equation of motion. The region over which the numerical model is valid will be presented in terms of non-dimensional parameters describing the different wave states.
An Integrated Methodology for the Design of Mooring Systems and Risers of Floating Production Platforms
Authors: Cruces Girón Aldo Roberto , Corrêa Fabricio Nogueira , Jacob Breno Pinheiro , Senra Stael Ferreira
Published: August 2013

Nowadays, coupled analysis tools that allow the simultaneous modelling of the hydrodynamic behaviour of the hull and the structural behaviour of the lines of floating production platforms have been increasingly used.

The use of such tools is gradually allowing the introduction of some feedback between the design of risers and mooring systems. In the current practice, that comprises the so-called “hybrid” methodologies, mooring designers have been using these tools to consider the influence of the risers on the platform motions. On the other hand, riser designers can use motions that result from coupled simulations for the analysis of each riser. Such integration is already being implemented in the design practice of Petrobras; however, elsewhere the design of risers and mooring systems may still be performed separately, by different teams, therefore not fully exploiting the benefits that the coupled analysis tools can provide.

In this context, this work describes an innovative, fully integrated methodology for the design of mooring systems and risers of floating production systems (FPS). This methodology considers different design stages (from preliminary to advanced), integrating the design activities of mooring lines and risers in a single spiral, allowing gains in efficiency and cost reduction. The initial design stages already include a feedback between riser and mooring analyses. The integrity of the risers can be considered in the mooring design by determining their safe operational zones, and therefore, mooring line pretensions can be modified to improve its structural performance. Then, in advanced stages critical design cases for both mooring and risers systems can be identified and rigorously verified by using fully coupled models.

The application of the proposed methodology is illustrated with a case study of a typical FPS, representative of the platforms that have been recently considered for deepwater applications.

It should be stressed that the methodology described here does not reflect the current design practice of Petrobras. Presently it is merely a proposal that is being studied and assessed; this work comprises the first draft of the methodology, which will be enhanced and consolidated as the result of current and future studies.
A New Nonlinear Coupled Analysis Tool for Floating Structures
Authors: Li Zhiling , Llorente Carlos , Chen Cheng-Yo , Kang Chang Ho , Muehlner Edmund , Banumurthy Surya
Published: August 2013

For the global performance analysis of a floater, the traditional semi-coupled method models mooring lines/risers as nonlinear massless springs and ignores 1) the inertial effects from mooring lines/risers, 2) the current and wave load effects on mooring lines/risers, and 3) the dynamic interaction between mooring lines/risers and the floater. However, these effects are deemed critical for deepwater and ultra deepwater floating structures as they may have a significant impact on the floaters’ motions and mooring line/riser tensions.

This paper presents the development and verification of a time-domain nonlinear coupled analysis tool, MLTSIM-ROD, which is an integration of a recently developed 3D rod dynamic program, ROD3D, with the well-calibrated floater global performance analysis program, MULTISIM (Ref [9]). The ROD3D was developed based on a nonlinear finite element method and merged with MULTISIM by matching the forces and displacements of mooring lines/risers with the floater at their connections. MLTSIM-ROD can thus predict the floater’s large displacement/rotation motions and mooring line/riser tensions including all the coupled effects between the floater and mooring lines/risers. In this paper, global performance predictions for a SPAR in the Gulf of Mexico in deepwater were carried out using MLTSIM-ROD. The results were then verified with those from other coupled analysis programs.

The paper also presents the results of motions and mooring line/riser tensions of the SPAR using both the coupled and semi-coupled methods. The results from the coupled and semi-coupled analyses indicate that the floater’s motions and mooring line/riser tensions could be significantly influenced by the dynamic interactions between the floater and mooring lines/risers. Hence, the coupled method needs to be considered for deepwater floating structures.