bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of Higher Order Spectral Method for Deterministic Wave Forecast
Authors: Clauss Günther F. , Klein Marco , Dudek Matthias , Onorato Miguel
Published: September 2014

This paper addresses the Higher Order Spectral (HOS) method as very fast and accurate non-linear method for deterministic wave forecast. The focus of the paper lies on wave propagation, with the objective to draw conclusions on the applicability of the HOS method for deterministic wave forecast. Systematic numerical and experimental investigations are conducted. The investigations comprise exact solutions of the nonlinear Schrödinger equation (NLS) as special non-linear wave groups, which are used as initial conditions for the subsequent simulations. Different parameters such as relative water depth, wave steepness and propagation distance are varied to evaluate the applicability of the HOS method. The results obtained are validated by experiments as well as fully non-linear simulations. It is shown that the HOS method is capable for non-linear wave forecast due to high accuracy and fast calculation time at once.
Vortex Induced Vibrations of Deep Water Risers: Sensitivity to Current Profile, Shear and Directionality
Authors: Schiller Rafael Vergara , Caire Marcelo , Nóbrega Pedro Henrique Affonso , Passano Elizabeth , Lie Halvor
Published: September 2014

Slender offshore structures such as risers experience vortex induced vibrations (VIV) when they are exposed to currents and accumulate significant fatigue damage through that process. VIV will depend on several structural properties of the riser and on the current profile that the structure is exposed to. In deep water regions, risers will be subject to intricate circulation systems that impose currents profiles which may vary in intensity, shear and direction throughout the water column. The increased complexity of currents will make the prediction of VIV more difficult and represents a clear challenge to the Oil and Gas Industry. The objective of this study is to investigate how selected properties of a current profile affect the development and excitation of VIV for a deep water tensioned riser. We employ a semi-empirical frequency-domain program to perform a series of numerical sensitivity analyses where the riser model is subject to current profiles that vary in complexity and include uniform profiles, linearly-sheared profiles and more realistic profiles that represent offshore boundary current regimes from SE Brazil. We address the sensitivity of the VIV response to current intensity, shear and directionality. Our results demonstrate that those properties of the current profile have significant influence on the range of VIV modes that are excited and on the VIV response. Overall, uniform profiles produced the largest responses and the linearly-sheared profiles demonstrated the large range of VIV modes that can be excited. The realistic profiles also excited a broad range of VIV modes and variations between the profiles produced changes in the VIV response. This study highlights the need to further understand how complex current profiles in the offshore region affect VIV development in comparison to simpler profiles that are recurrent in model test conditions.
Additional Wind/Wave Basin Testing of the DeepCwind Semi-Submersible With a Performance-Matched Wind Turbine
Authors: Goupee Andrew J. , Fowler Matthew J. , Kimball Richard W. , Helder Joop , de Ridder Erik-Jan
Published: September 2014

In 2011 the DeepCwind Consortium, led by the University of Maine (UMaine), performed an extensive series of floating wind turbine model tests at the Maritime Research Institute Netherlands (MARIN) offshore basin. These tests, which were conducted at 1/50th scale, investigated the response of three floating wind turbine concepts subjected to simultaneous wind and wave environments. The wind turbine blades utilized for the tests were geometrically-similar models of those found on the National Renewable Energy Laboratory (NREL) 5 MW reference wind turbine and performed poorly in the Froude-scaled, low-Reynolds number wind environment. As such, the primary aerodynamic load produced by the wind turbine, thrust, was drastically lower than expected for a given Froude-scaled wind speed. In order to obtain appropriate mean thrust forces for conducting the global performance testing of the floating wind turbines, the winds speeds were substantially raised beyond the target Froude-scale values. While this correction yielded the desired mean thrust load, the sensitivities of the thrust force due to changes in the turbine inflow wind speed, whether due to wind gusts or platform motion, were not necessarily representative of the full-scale system.

In hopes of rectifying the wind turbine performance issue for Froude-scale wind/wave basin testing, efforts have been made by UMaine, Maine Maritime Academy and MARIN to design performance-matched wind turbines that produce the correct thrust forces when subjected to Froude-scale wind environments. In this paper, an improved, performance-matched wind turbine is mounted to the DeepCwind semi-submersible platform investigated in 2011 (also studied in the International Energy Association’s OC4 Phase II Project) and retested in MARIN’s offshore basin with two major objectives: 1) To demonstrate that the corrective wind speed adjustments made in the earlier DeepCwind tests produced realistic global performance behaviors and 2) To illustrate the increased capability for simulating full-scale floating wind turbine responses that a performance-matched turbine has over the earlier, geometrically-similar design tested. As an example of this last point, this paper presents select results for coupled wind/wave tests with active blade pitch control made possible with the use of a performance-matched wind turbine. The results of this paper show that the earlier DeepCwind tests produced meaningful data; however, this paper also illustrates the immense potential of using a performance-matched wind turbine in wind/wave basin model tests for floating wind turbines.
Variables Affecting Fracture Toughness of Welds in T-K-Y Connections
Authors: Panday Radhika , Zhang Shenjia , Ogborn Jon , Narayanan Badri K.
Published: September 2014

Fracture toughness of tubular welded joints is one of the critical factors affecting the structural integrity and reliability of offshore structures, such as platforms and subsea pipelines. The factors affecting the design fracture toughness of these structures are related to, both, the welding process as well as the chemical composition of the weld metal.

The welding process in this application typically comprises of depositing weld metal in the tubular joints of varying thicknesses through series of weld passes. The number of weld passes required for welding these joints subjects the weld metal to repetitive cycles of heating and cooling. The effect of the thermal cycling introduces significant heterogeneity in the microstructure. This is further exacerbated by the presence of micro-alloying elements such as Niobium (Nb) and Vanadium (V) that form complex carbides, nitrides and carbo-nitrides during post weld heat treatment (PWHT).

The focus of this work is to evaluate the effect of micro-alloying elements on the ductile to brittle transition temperature and the mode of fracture at temperatures relevant to offshore applications. A threshold Nb and V level has been determined for achieving acceptable weld metal toughness. The improvement in the fracture toughness using this approach has been quantified by Charpy V-Notch (CVN) and Crack Tip Opening Displacement (CTOD) measurements. The Ductile to Brittle Transition Temperature (DBTT) has been shown to be shifted to lower temperatures by 25 °C after post weld heat treatment in the welds where the total amount of Nb and V are controlled to less than 40 ppm.

A wet precipitate extraction technique was used to extract precipitates from the welds to establish the presence of fine Nb rich precipitates in the welds with the higher DBTT. The weld deposited with controlled levels of Nb and V was further tested in different joint configurations and base plate thickness. The fracture toughness was evaluated by CTOD testing of the weld in two different thicknesses (50 mm and 70 mm). Increased specimen thickness resulted in lower CTOD values.
A Theoretical and Experimental Analysis of the Bending Behavior of Unbonded Flexible Pipes
Authors: Vargas-Londoño Tatiana , de Sousa José Renato M. , Magluta Carlos , Roitman Ney
Published: September 2014

Due to its compound cross-section, the prediction of the structural response of flexible pipes to loads such as their self-weight, internal and external pressure, movements imposed by the floating system and environmental loads such as currents, waves and wind is quite complex. All these loads generate stresses and strains in the cross section of the pipe that have to be properly evaluated in order to ensure integrity of the line. Research has been done on the local behavior of flexible pipes under combined axisymmetric loads as well as under bending loads. However, there is a lack of research combining both axisymmetric and bending loads, as also in the study of the strains in the tensile amour layers of the pipes, aspects which are important for the calibration of theoretical models to predict such behavior. Based on that, this study aims to evaluate the local behavior of flexible pipes under combinations of axisymmetric (tension, and internal pressure) and bending loads via a series of experimental tests in a 9.13″ I.D pipe. In the experimental tests, the behavior of the pipe was studied for three load combinations: i) bending combined with tension; ii) bending combined with internal pressure; and iii) bending combined with tension and internal pressure. Based on these tests, the authors obtained the strains in the tensile armor layer, axial elongation due to tension, axial reaction forces due to internal pressure, and deflection due to bending. These measurements were used to calibrate a theoretical model devoted to simulate the pipe’s response, getting accurate results for stiffness and stresses of the pipe in each scenario.
Structural Dynamics and Load Analysis of Large Offshore Wind Turbines in Western Gulf of Mexico Shallow Water
Authors: Yin Ling Ling , Lo King Him , Wang Su Su
Published: September 2014

In this paper, a study is conducted on wind and metocean loads and associated structural dynamics of a 13.2-MW large offshore wind turbine in Western Gulf of Mexico (GOM) shallow water. The offshore wind turbine considered includes a rotor with three 100-meter long blades and a mono-tower support structure. Natural frequencies and mode shapes of the blades and the mono-tower are determined first and used subsequently to establish a Campbell diagram for safe wind turbine operation. The results show that hydrodynamic added mass has little effect on the natural frequencies and mode shapes of the support structure but it introduces, in part, appreciable effects on loads carried by the turbine when the blades are pitched at wind speeds above the rated speed. Also determined, for normal operation and extreme metocean conditions (i.e., 100-year return hurricanes), are normal thrust on the wind rotor, blade-tip displacement, overturning moment and tower-top displacement sustained by the wind turbine.
Cement Sheath Integrity Implications of Scaled-Down Wellbore Casing-Cement-Formation Sections
Authors: De Andrade Jesus , Sangesland Sigbjørn , Vrålstad Torbjørn , Golan Michael
Published: September 2014

Many offshore petroleum wells have serious cement sheath integrity issues that imply costly repairs, limitations with respect to short- and long-term use of planned or converted production and injection wells, and safety issues. The annular cement sheath is exposed to a wide variation of thermal and pressure loads that potentiality may result in zonal isolation failure throughout its lifecycle.

While several experimental and numerical work manage to map highly loaded scenarios that leads to radial cracks and micro-annulus generation, little effort has been taken to investigate and quantify the impact of moderate pressure and thermal loads on cement sheath stresses. This paper presents a 2D finite element assessment of impacts of numerous material properties, geometric parameters, pressure and thermal loading variables contributing to cement sheath stresses. A centralized wellbore section of casing-cement-formation is considered, representing a production casing string. This work is conducted as a preliminary effort in order to develop a down-scaled laboratory set-up, that represents realistically the impact of thermal and pressure loads on cement sheath stresses.

This paper introduces a discussion about the capabilities of a down-scaled well section to characterize the stress distribution around the annular cement sheath of a conventional 9 5/8″ production casing. Results indicate that a down-scaled configuration exposed to comparable internal pressure and temperature profile resembles the stress distribution associated with completion and production operations of wells. It is shown that a proper characterization of the cement stress regimes requires the combined effect of pressure and thermal variations.

Sensitivity studies conducted on cement sheath stresses, for both wellbore and down-scaled configurations, have assessed the relative influence of mechanical and thermal properties as drivers of cement stresses. For well operations with substantial wellbore temperature variations such as production and frac job, particularly uncertain parameters such as cement-formation-Young’s modulus and thermal expansion coefficient of cement have shown significant impact on annular cement sheath stresses. In terms of combined pressure and thermal loading, cement stresses have proved to be more sensitive to temperature differential than pressure differential variations.
Comparison of Numerical Simulations With Experimental Measurements for the Response of a Modified Submerged Horizontal Cylinder Moored in Waves
Authors: Costello Ronan , Davidson Josh , Padeletti Davide , Ringwood John V.
Published: September 2014

To facilitate commercially relevant numerical design optimization in wave energy conversion accurate and validated simulations of wave body interactions are necessary. Wave energy, more so than almost any other industry, can benefit from such numerical optimization because of the high cost and long period of design iteration in experimental and field testing. For the foreseeable future wave energy device design and optimization will continue to rely heavily on potential flow solvers. Two important prerequisites to successfully using simulations based on these codes are firstly a need to validate the simulation implementation by comparison with experiment and secondly a need to supplement the potential flow solution with experimentally (or CFD) derived coefficients for the forces that are neglected by the potential flow solver. This paper attempts to address both of these prerequisites. A comparison of numerical simulations and physical wave tank experiments on a submerged horizontal cylinder moored in waves is presented. Good agreement between numerical model and experiment is achieved. At operating points where the body response is linear a numerical model based purely on potential flow and linear mooring stiffness achieves excellent results and at operating points where the body response is non-linear a time domain model with frequency independent quadratic damping is shown to give good agreement for a wide range of wave periods and amplitudes.
Development of Deep Water Multicolumn Buoy
Authors: Barreira Rodrigo A. , Vileti Vinicius L. , Sales Joel S. , Sphaier Sergio H. , Esperança Paulo de Tarso T.
Published: September 2014

A new conceptual design of a deepwater MONOBUOY, named DeepWater MultiColumn Buoy (DWMCB), patent PCT/BR2011/000133, was developed by PETROBRAS/CENPES. The DWMCB was designed to be part of an offloading system for a Spread Moored Floating Production Offloading Unit (FPSO). The offloading system principle consists of Oil being exported from the FPSO to a Shutle tanker passing through Offloading Oil Lines (OOLs) that are supported by the DWMCB. The system is designed to operate at a water depth of 2,200 meters, with expected in site life duration of 25 years. The geometry of DWMCB was defined after an optimization process in order to minimize its motions. This paper describes the development of this concept and discusses the results from some design verifications done with the help of a model tests campaign. An equivalent traditional shaped monobuoy was also tested for comparison purposes.
New Construction of Reference Frame for Underwater Cable
Authors: Zhu Xiang Q. , Yoo Wan S.
Published: September 2014

The choice of reference frame applied in a multibody system and finite element method analysis assists in describing the motion of bodies. Two non-collinear vectors are prerequisite for the creation of a local reference frame for three-dimensional cases. The Frenet frame, which is constructed with tangential and normal vectors, is widely used for continuous, differentiable space curves, and the local reference frame for underwater cable systems. However, because underwater cables experience external loads originating from the surrounding environment, it is inappropriate to only treat an underwater cable as a spatial curve. Furthermore, the lumped-mass method involves classical discretized modeling, which is not suitable for continuous, differentiable space curves, and its application complicates the calculation of rotational transformation matrices. This paper proposes a new method for the construction of local reference frames which considers both the cable geometry and external environment. The model of the cable consists of discretized cable elements, of which the orientation of each cable element is described by vectors that describe the local reference frame. Taking advantage of the expression of hydrodynamic drag forces, this paper provides the prerequisite for two non-collinear vectors by replacing the normal vector with one representing the relative velocity of the fluid. The construction of local reference frames is meant to easily express all the loads acting upon a cable. To reveal the advantages of this new local reference frame, we create three classifications for the loads acting upon the cable. Because the axial extension occurs along the orientation of the cable element, the calculation of the first type of loads benefits from the Frenet frame according to the classification. Meanwhile, both the first and second types of loads are expressed easily and accurately with respect to the new local reference frame. Because of the participation of relative fluid velocity in our reference frame, the formulation of the hydrodynamic drag forces can be simplified and its accuracy improved. Moreover, the calculation of the rotational transformation matrix is made less complex and the computational error is reduced. The cable model created by this new reference frame is compared with that based on a Frenet frame and the commercial simulation code ProteusDS. The simulation results show that cable modeling using our novel reference frame is much more stable and accurate than that using a Frenet frame.
Numerical Simulation of Vortex-Induced Motion With Free Surface by Lattice Boltzmann Method
Authors: Miyamura Akitaka , Hirabayashi Shinichiro , Suzuki Hideyuki
Published: September 2014

In this study, numerical simulation of the fluid flow by using lattice Boltzmann method is carried out and the vortex-induced motion (VIM) of a cylindrical floating structure is calculated. The way of calculate the fluid flow, fluid force and floating body’s movement is introduced.

The fluid flow with free surface is also calculated. The height change of water surface exerts the effect to the evaluation of hydrostatic pressure and wave resistance. In this study, the method to express the movement of free surface is introduced.
Preliminary Design Through Analysis of a Bottom Weighted Rigid Riser for Subsea Mining
Authors: Viswanathan Savin , Selvam R. Panneer , Raphael Deepak C.
Published: September 2014

Polymetallic nodules found in the deep ocean basins contain many economically valuable metals and are viewed as potential resources to supplement depleting onshore reserves and tackle increasing demand. The present work focuses on the preliminary design through analysis of a bottom weighted rigid riser which would form a part of the future full scale prototype nodule mining system under development by the National Institute of Ocean Technology (NIOT), India, operating in a water depth of around 6500m in the Central Indian Ocean Basin. A semisubmersible platform is chosen as a surface support vessel and the hydrodynamic analysis of a conceptual semisubmersible platform is carried out with WAMIT [3] to determine the heave Response Amplitude Operators (RAOs). The initial riser diameter, material grade and wall thickness configuration are arrived at using basic engineering principles and semi-coupled dynamic analysis of the platform riser system is carried out using ORCAFLEX [8] software to determine the loads imposed on the riser. The wall thickness is optimized to meet requirements of offshore standard DNV OS F 201 [6]. Vortex Induced Vibration (VIV) and Fatigue Analysis are then carried out using ORCAFLEX.
Numerical Simulation of Fluid Resonance in Narrow Gap of Two Bodies in Close Proximity
Authors: Moradi Nima , Zhou Tongming , Cheng Liang
Published: September 2014

The resonant behavior of the fluid trapped in the narrow gap between a floating LNG and an LNG carrier in a side-by-side offloading operation is investigated in this study employing a numerical wave flume. The wave flume is based on the finite volume solution of the Navier-Stokes equations to account for the viscous dissipation. The waveFoam toolbox, a modified version of the standard OpenFOAM multiphase flow solver interFoam developed by Jacobson et al (2011) has been used for the purpose of wave generation and relaxation inside the computational domain. This method has a quite high efficiency as it takes advantage of the potential flow theory for wave generation purpose and the viscous flow theory for inside the wave tank, respectively. The volume of fluid (VOF) method first introduced by Hirt and Nichols (1981) is used to capture the free surface oscillations at the air and water interface. Water waves are generated at a reasonable distance from the inlet boundary and two rectangular relaxation zones at the inlet and outlet boundaries of the domain have been implemented to suppress wave reflection at the outer boundaries as well as waves reflected internally in the computational domain. The influence of incident wave frequency on resonance wave height and frequency is examined. Numerical results of free surface evolution at different incident wave frequency seem to agree well with the experimental results of Saitoh et al (2006) and numerical results of Lu et al (2008). In order to justify the effect of bilge keels on flow separation at the bottom corner of the ship, four different corner configurations have been investigated and compared to the base sharp edged case. It is observed that the magnitude of the free surface elevation at the resonance frequency increases significantly by about 10 times the incoming wave height while the peak of resonant frequency curves shifts to higher frequencies in the higher curvature modes compared with the base case.
Microstructure and Residual Stress Analysis of Dissimilar Metal Composite Plates Produced by Explosion Welding
Authors: Varavallo Rogério , de Melo Moreira Vitor , Paes Vinicius , Brito Pedro , Olivas José , Pinto Haroldo Cavalcanti
Published: September 2014

In the present work, different corrosion resistant materials (AL-6XN superaustenitic stainless steel, ZERON 100 superduplex stainless steel and Inconel 625 Ni alloy) were joined with ASME SA516-70 carbon steel by explosion welding to form bimetal composite plates. The microstructure of cladded plates was characterized by optical and scanning electron microscopy and the hardness variation across the interface was determined by applying Vickers microhardness. The residual stresses generated by the cladding process were determined by X-ray diffraction. All materials adhered well to the ASME SA516-70 base plate and the cladded interface exhibited the wavy morphology usually associated with strong bond strengths. Hardness increased near the interface due to strain hardening caused by plastic deformation and tensile residual stresses were found to develop on all clad metals.
The Small-Scale Distortion Model Design Method for Ultimate Strength Test of Hull Girder
Authors: Li Chenfeng , Ren Huilong , Xu Donghao , Ji Zeng
Published: September 2014

The objective of this paper is to develop a design method of small-scale distortion model for ultimate bearing capacity test. The ultimate moment is an important index of ship girder strength. Experimental analyses are regarded as the most straightforward and effective methods to predict ultimate moment of hull girder, especially full-scale and/or large-scale model testing. However, large-scale model test not only requires high loading capacity of test device, but it also costs a lot of human and material resources. Based on the similarity theory and failure mechanism of hull girder under bending, a small-scale distortion model design method is presented in this paper. In which, the transverse section is divided into several parts, such as main deck, middle deck, bottom and so on. Then, two similar criterions are required to follow. There are section modulus of each part and critical stress of main stiffened plats. The similar section modulus ensures a similar stress distribution of transverse section between model and real structures under bending; the similar critical stress ensures the same failure modes of stiffened plates. Based on this method, a real vessel is taken as example to design its small-scale (1/25 scale) distortion model. The simulating results indicate that this model design method is precise and reliable.
Experimental and Numerical Studies on an Underwater Towed Body
Authors: Krishnankutty Parameswaran , Anantha Subramanian , Francis Roni , Nair Prabhasuthan Prabhakaran , Krishnamachari Sudarsan
Published: September 2014

A streamlined form of underwater towed body is considered here for the experimental and CFD studies to verify its stability when towed in submerged condition. Its physical characteristics lead to different behavior in the hydrodynamic tow conditions. Influence of lifting fins and tow arrangements on the operational depth and stability of the tow body are studied. The main variants associated with the body, and which affect the tow characteristics, are identified as basic body shape, tow point location, fin size for stable tow and dynamic trim due to forward speed. In order not to disturb the flow around the sensor location, the fin is placed further away towards the lower aft end. Additionally, the stable tow is validated and demonstrated in the towing tank at IIT Madras using the scaled model. The drag and lift force values obtained from model tests are extrapolated to the prototype and compared with CFD results.
Mechanics of Dynamic Ice Failure Against Vertical Structures
Authors: Jordaan Ian , Barrette Paul
Published: September 2014

Ice interaction with vertical faces of structures can result in regular vibrations given certain conditions such as temperature and speed of interaction. The mechanism that can provide this regular behaviour is studied. Fracture in general does not offer a solution in compressive failure. An approach based on viscoelastic theory, with softening resulting from microstructural change, is given. The pivotal observation was a layer of microstructurally modified ice adjacent to the structure or indentor, together with high local pressures transmitted into the layer. The microstructural changes include microfracturing and recrystallization. A series of triaxial tests was performed to determine the inputs into the viscoelastic theory. The theory recognizes changes in the microstructure of the ice by means of a state variable, which is a function of prior stress history, and therefore of location within the ice mass. The theory and the calibration thereof are reviewed, and the results of triaxial tests examined. One feature of these tests is the occurrence of “runaway” strains and associated localization of damage. This appears to be sensitive to confining pressure, and is considered to be a key factor in the rapid load drops observed in ice-structure interaction. Temperature effects are also studied. Directions for future research are identified.
Thruster and FPSO Hull Interaction
Authors: Tian Ye , Kinnas Spyros A.
Published: September 2014

A hybrid method which couples a Vortex-Lattice Method (VLM) solver and a Reynolds-Averaged Navier-Stokes (RANS) solver is applied to simulate the interaction between a Dynamic Positioning (DP) thruster and an FPSO hull. The hybrid method could significantly reduce the number of cells to fifth of that in a full blown RANS simulation and thus greatly enhance the computational efficiency. The numerical results are first validated with available experimental data, and then used to assess the significance of the thruster/hull interaction in DP systems.
Propagation of a Radar Modulated Ocean Wave Field
Authors: Paalvast M. S. M. , Naaijen P. , Huijsmans H. R. M.
Published: September 2014

In this article a study has been carried out to explore the feasibility of a wave propagation model that is able to predict the wave field in a deterministic sense, based on remote observations of the sea surface. The surface is modulated in order to simulate images created by a marine radar operating at grazing incidence. The developed model uses an integral equation method, utilizing the frequency domain Green’s function which fulfills the linear free surface boundary condition. Synthesized observations of either the wave elevation or surface tilt at the source points are used to initialize the wave model. At each of the locations of the added remote free surface panels, time traces of the observed wave elevation or surface tilt can be recorded. A Fourier Transform (FFT) of these time traces yields the frequency domain description of the boundary condition that has to be satisfied by the wave potential. The derived Green’s function for the free surface source panels is then used to solve the source of strength at these panels. Once values have been found for the sources, the potential, and thus the surface elevation, may be calculated at the ship’s location.
New Insights in Turret Mooring Systems Yaw Dynamics
Authors: Caille François , Prieur Julien , Mamoun Naciri
Published: September 2014

Owing to friction in the weathervaning system and in the swivel stack of a turret-moored FPSO, the turret can be locked to the FPSO in day-to-day environments meaning that no relative yaw motions occur in such conditions. When the FPSO yaws sufficiently, the mooring yaw restoring moment builds up to a point where the friction torque is overcome freeing within seconds the turret from the FPSO which then yaws back towards its neutral yaw position. This phenomenon is referred to as a turret release event. Evidence of the existence of such a phenomenon has been confirmed for one North Sea FPSO. In this paper, the basic physics of turret release are investigated from first principles. Incorporation of a friction module in a readily available time domain simulation software is described. Application of this enhanced tool towards a better understanding of the FPSO/Turret relative yaw motions & velocities during the FPSO service life is then discussed in the context of existing SBM Offshore turret systems.
Straightener Settings for Under-Straight Residual Curvature of Reel Laid Pipeline
Authors: Roy Alan , Rao Venu , Charnaux Carlos , Ragupathy P. , Sriskandarajah T.
Published: September 2014

The pipeline inplace buckling design by residual curvature method for controlled thermal expansion during operation is a novel and cost effective method for initiating multiple expansion loops for a reel-laid pipeline. The controlled thermal expansion will be initiated and managed by planned buckle sites at regular intervals of say, 1km spacing with residual curvature corresponding to 0.20%–0.25% residual strain.

The pipelines installed by the reel Lay process are required to be straightened by a reverse curvature process which results in pipeline with nominally zero curvature and axial strain. In order to achieve a residual strain of 0.20%–0.25%, two options are available either to over-straight or under-straight the pipelines. This paper discusses the methodology used laying of pipeline with 0.20%–0.25% residual strain at pre-determined intervals by an under-straightening process with a combination of a central 40m under straight section with a residual strain of 0.2%–0.25% and a 15m transition section on either side of the central section, at 1km intervals.

The paper presents the methodology used to determine the settings for the top straightener module of reel lay vessel using finite element analyses, as the equipment settings for under or over straight pipes are not readily available from analytical methods. The optimum settings were obtained after substantial number of FEA simulations allowing for the statistical range in pipe material properties from “strong” to “weak” and these settings are used in reeling trials to make necessary adjustments. The residual strain, out-of-straightness and axial force distributions in pipe sections for straight, under-straight and transition sections are discussed.

The paper discusses how these settings were used on-board the vessel and the modifications and adjustments required to select the final settings for the under-straight curvature for the field development of Statoil Skuld project as a case study. This paper briefly discusses the vessel equipment specific features and limitations that may need to be taken into consideration in finite element analyses to optimise the straightener settings for residual strain.
Assessment of Pipeline Integrity Based on Successive Inspection Data
Authors: Salama Mamdouh M. , Maes Marc A.
Published: September 2014

The ultimate objective of performing in-line inspections (ILI) in pipelines is to obtain data to use in establishing the current and future integrity of a given pipeline system. The estimation of the progress of corrosion based on successive inspections in time is preferred to a purely empirical approach providing that the ILI inspection uncertainties are considered in the analysis. This paper provides recommended treatments for assessing the remaining life of a pipeline and proposes a simple yet consistent model that allows the use of ILI data in predicting the future state of a pipeline based on the results of multiple ILI inspections and accounting for the ILI uncertainties.
Fasteners in Subsea Applications: End User Experiences and Requirements
Authors: Rørvik Gisle , Haaland Bjørnar Øvrum , Kirkemo Finn
Published: September 2014

Fasteners used for critical applications subsea represent a challenge in terms of material selection, quality control, traceability and documentation for manufacture and delivery to ensure adequate performance, integrity and avoidance of costly failures. The definition of critical subsea fasteners are both pressure containing and primary load bearing members, but may in a wider sense also include fasteners which may affect functionality of valves, equipment and instruments for safe, reliable and cost effective operation.

The major failure mechanism experienced during the last decades is so-called hydrogen induced stress cracking (HISC) or hydrogen embrittlement (HE) primarily resulting from the hydrogen charging conditions of the cathodic protection (CP) system. The main bulk of fasteners used subsea is of low alloyed steel grade type (e.g. ASTM 320 Grade L7, L7M, L43 bolts with ASTM 194 Grade 4, 7M, 7 nuts) which relies on electrical continuity to the cathodic protection system to avoid sea water corrosion, but may be prone to HISC/HE if the fastener hardness and strength level exceed the specified and established limits. If higher strength materials are needed from design, either cold formed or precipitation hardened CRA’s are usually selected (e.g. Grade 59, 660, 625, 686, 718, 725), but these may also be susceptible to HISC/HE depending on strength level and manufacturing practice.

The present paper will give some field experience and typical examples of recent HISC failure case histories for low alloyed steel grades, but also for some CRA type of subsea fasteners. Furthermore, some in-house low alloyed steel grade (L7, L43) fastener HISC test results will be presented to indicate the robustness of today’s hardness and strength level restrictions, and at which loading and pre-tension level a given strength grade may be prone to HISC failure. Finally, some experience from manufacturing stage as well as focus areas in materials specification, quality assurance / quality control and design will be discussed.
CFD Modeling of Long Risers With Buoyancy Modules and Complex Shapes
Authors: Holmes Samuel , Constantinides Yiannis
Published: September 2014

Recent advances in computers and software have enabled the direct simulation of the vortex induced vibrations of long risers. Most of the problems solved to date have focused on bare or straked risers with a mild catenary shape. However fielded risers often make use of large buoyancy elements and may have a very general “lazy wave” shape. These features introduce new problems of solution stability and run time. In this paper we address some of these issues. We develop efficient meshing strategies for risers with buoyancy modules. The models are tested against published laboratory experiments. The CFD solutions show the same effects of changing excitation frequencies with changing buoyancy coverage and other effects as seen in the experiments. A general approach is proposed to build large problems with an L/D of 10,000 and a lazy wave shape. This approach demonstrated with an example problem.
Study of Turbulence Statistics in Large-Eddy Simulations of Ocean Current Turbine Environments
Authors: Alexander Spencer R. , Hamlington Peter E.
Published: September 2014

As ocean current turbines move from the design stage into production and installation, a better understanding of oceanic turbulent flows and localized loading is required by researchers and members of industry. Consideration of realistic ocean turbulence environments, in particular, is essential for obtaining accurate and reliable predictions of ocean turbine lifetime and performance. In this study, large eddy simulations (LES) are used to model the turbulent boundary layer in which an ocean current turbine operates. The LES model captures current driving due to winds, waves, and tides, thereby providing a high degree of physical realism. Inflow and boundary conditions are designed to represent conditions during an observational campaign at Admiralty Head in Puget Sound, and comparisons are made between the LES results and available observational measurements. Further statistical measures of the LES flow fields are outlined, including vertical profiles of Reynolds stresses, turbine loading, and two point correlations. The ability of the synthetic turbulence generator TurbSim to reproduce realistic ocean turbulence is qualitatively assessed through comparisons with LES results. Finally, preliminary simulation results are presented for an ocean current turbine represented by an actuator disk.
Near-Optimal Time-Domain Control of Small Buoys in Irregular Waves
Authors: Korde Umesh A. , Ertekin R. Cengiz
Published: September 2014

Within the linear theory framework, smooth optimal control for maximum energy conversion in irregular waves requires independent synthesis of two non-causal impulse response functions operating on the body oscillations near the free surface, and one non-causal impulse response function relating the exciting force to the incident wave profile at the body. Full cancellation of reactive forces and matching of radiation damping thus requires knowledge or estimation of device velocity into the future. As suggested in the literature, the control force can be synthesized in long-crested waves by suitably combining the ‘full’ impulse response functions with wave surface elevation information at an appropriately determined distance up-wave of the device. This paper applies the near-optimal control approach investigated earlier by one of the authors (Korde, UA, Applied Ocean Research, to appear) to small floating cylindrical buoys. Absorbed power performance is compared with two other cases, (i) when single-frequency tuning is used based on non-real time adjustment of the reactive and resistive loads to maximize conversion at the spectral peak frequency, and (ii) when no control is applied with damping set to a constant value. Time domain absorbed power results are discussed.
Ship Impact Behavior on Jacket Type Offshore Wind Turbine Foundation
Authors: Zhang Jianhua , Gao Dianwei , Sun Ke , Zhang Zhiyang
Published: September 2014

Jacket is an ideal supporting structure for large 4 to 6 MW offshore wind turbine (OWT) in deep sea. However, Due to the harsh marine environment, as well as offshore wind farms are mostly located near a busy waterway, it is inevitable that ships crash offshore wind power structures. Sometimes, collision accidents may result in severe damage in the support structure. To study the collision behavior of jacket offshore structure, dynamic characteristics of the jacket OWT foundation is analyzed in this paper, based on ADINA explicit dynamic analysis method, considering different ship speed, impact locations and joint thickness. Based on the numerical results, the joints are reinforced in special location to meet requirements. Structural performances of jacket type OWT foundation under ship impact are studied. The research results in the paper can provide the reference for practical engineering design.
Environmental Contours for Multicomponent Response
Authors: Leira Bernt J. , Lindstad Halvor Borgen
Published: September 2014

Approaches for derivation of joint design parameters in relation to multi-component environmental processes are briefly summarized. Within the context of structural design, this is frequently referred to as a “load combination problem”. It is focused on application of design contours for analysis of such load combinations. Contours of this type can be utilized for cases where the different load processes are both correlated and uncorrelated. Existing procedures for construction of such contours are summarized, and extensions to situations where the characteristic time scales for the different process components are widely different are outlined. Development of such tools based on application of “translation processes” (i.e. transformed Gaussian processes) is highlighted.

Analysis of mechanical design formulations which involve several interacting response quantities is further addressed. Utilization of so-called contour response tubes within such a framework is discussed. An example of application is given for a cross-section at the upper end of a flexible riser configuration where the interaction between the tension and curvature processes needs to be taken into accounted.
Systematic Accuracy Assessment of Two Frequently Used QTF Approximations in the Case of a Rectangular Barge
Authors: Monroy Charles , de Hauteclocque Guillaume , Chen Xiao-Bo
Published: September 2014

This paper presents numerical results of the monodirectional second order forces acting on a rudimentory FPSO hull shape. The exact calculation of second order loads through potential theory is computationally challenging and requires the evaluation of a slowly-convergent free surface integral. In practice, this integral is often neglected as it is widely assumed that it does not contribute significantly to the overall load and few commercial seakeeping software propose this option anyway. An even rougher and widely used approximation consists in expressing the second order loads occuring at a non-zero difference frequency from the mean drift loads. It is called the Newman’s approximation; it enables serious CPU time gains, but at the cost of a worse accuracy. The object of this paper is to investigate on a simple case the accuracy of each of these two approximations (without the free surface integral and Newman’s approximation) and the influence of parameters such as the water depth and the sea state wave length. Depending on the resonant period of the considered mooring system, it may give some insight on a good compromise between CPU time and accuracy when choosing the way of determing the second order loads.
The Arctic Ship Shape Platform for Well Maintenance, Survey and Workover
Authors: Verbitsky Sergey V. , Agafonov Alexey A. , Fomitchyov Eduard N. , Kovalev Maksim V. , Chesnokova Irina G. , Shinkarenko Olga V.
Published: September 2014

The conceptual design of floating ship shape platform for well maintenance, survey and workover in prospective fields of Russian Arctic and Sakhalin Island shelf (further The platform) was developed by Krylov State Research Centre specialists with the assistance of CDB “Korall”. The platform under design intended for operation in the Arctic conditions and designed to resist the ice formations such as continuous thin first-year ice and broken ice being the first one in the world practice. The form and chosen hull ice strengthening class allow ensuring safe operation in the mentioned ice conditions.

The main purpose for development of the platform was to minimize the offshore wells downtime providing the cost effective way for maintenance of subsea production systems comparing with floating drilling rigs and drilling vessels that takes a significant investment. The economic assessments have shown that operation of such platform will cost two to three times less than renting of a rig or drillship. Having the working deck space of about 1,000 m2 and the ability to download a variety of equipment the platform can perform a variety of testing and wells repairing procedures both for underwater wellhead and for wells operated on fixed platforms.
Concept to Design: A Novel Seawater Intake Riser System
Authors: Chakkarapani Venkat , Chaudhury Gautam
Published: September 2014

FLNG cooling process uses large volume of cold seawater drawn from 500–1000 feet below the mean sea level utilizing several vertical risers (20in–30in ID) hanging from the vessel. These risers have requirements very different from conventional risers supported at hang-off and seabed. The design challenges are hang-off loads (in both strength and fatigue), minimum bend radius (MBR) and interference. Hang-off loads can be lowered using low stiffness and light weight risers, but the associated deflections and riser motions will be excessive leading to low MBR and potential interference issues. On the other hand, interference and MBR issues can be addressed by using suffer and heavier weight risers, but this will increase hang-off loads and stresses at the platform base. Weight mass not only affects the dynamics but also provides valuable restoring moment thus helps to reduce static displacement due to current forces. Stiffness affects the bending deflections and helps achieve an acceptable MBR. Based on this, a novel alternative design for seawater intake is to use a single large diameter (60in-70in) steel caisson riser. This paper will briefly outline the preliminary concept verification and concentrate on the engineering of the concept in terms of design optimization, cost/schedule, construction, and installation. Other materials such as High Density Polyethylene Pipe (HDPE), Glass Fiber Reinforced Plastic (GFRP), will be examined for feasibility.

FLNG process technology is continuing to develop to be safe, compact, and cost-efficient which also increases the possibilities of developing a number of stranded deep water gas reservoirs around the world. FLNG processes will become more efficient and cost effective with the proposed novel seawater intake system. Cost effectiveness will be evaluated in a qualitative manner.