bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficient Modal Decomposition and Reconstruction of Riser Response due to VIV
Authors: McNeill S. I. , Agarwal P.
Published: November 2011

Vortex-Induced-Vibrations (VIV) due to ocean currents can consume a sizable portion of the allotted fatigue life of marine risers. Vibration monitoring and concurrent estimation of fatigue damage due to VIV can significantly enhance the safe and reliable operation of risers. To this end, riser response can be characterized by using sensors (e.g. accelerometers and/or angular rate sensors) to measure the motion of the riser at a few locations. Fatigue damage can be predicted along the entire length of riser from measured data using the method of modal decomposition and reconstruction. In this method the structural response of interest, such as stress and fatigue damage, is expressed by modal superposition, where the modal weights are estimated using measured data and analytical modeshapes. However the accuracy of this method declines as the sensor density (number of sensors per unit riser length) decreases, especially when the riser vibrates in high-order modes and exhibits traveling wave behavior. In this paper, an efficient frequency-domain methodology allowing for accurate reconstruction of the riser response along the entire riser using a limited number of sensors is proposed. We first identify the excited VIV modes (natural frequency and modeshape) using principal vectors of the cross spectral density. Modal decomposition and reconstruction is performed separately for each VIV band surrounding each excited mode. This allows us to use several (as many as the number of sensors) participating modes in each band, and thus improve the accuracy. Since the stress distribution is sensitive to the chosen set of participating modes, we optimize over several candidate sets, selecting the set of modes that result in the lowest prediction error. In order to improve the reconstruction of complex modes, particularly traveling waves, the modeshapes can be augmented with additional basis vectors. The additional basis vectors are obtained by shifting the phase of the normal modes by 90 degrees at every wave number using the Hilbert transform. Though developed in the context of VIV, the method can be used to estimate fatigue damage due to vibrations regardless of the excitation mechanism. The methodology is demonstrated using the NDP (Norwegian Deepwater Program) test data on a 38 meter long slender riser, using data from eight accelerometers. Results show that the proposed algorithm can reconstruct stresses and fatigue damage accurately along the length of the riser in the presence of traveling wave behavior using relatively few sensors.
Vortex-Induced Vibration of Two Mechanically Coupled Cylinders of Different Diameters in Steady Flow
Authors: Rahmanian Mehran , Zhao Ming , Cheng Liang , Zhou Tongming
Published: November 2011

Vortex-induced vibrations of two mechanically coupled cylinders of different diameters in steady incompressible flow are studied. The diameter ratio of the cylinders is fixed at 0.1. Petrov-Galerkin finite element method is used to solve the two dimensional Reynolds-averaged Navier-Stokes equations equipped with SST k–ω turbulence model closure. The numerical model is evaluated against available experimental results. Following that, the effects of cylinders’ gap and their angular orientation relative to the free stream on oscillation amplitude and force coefficient variation of the bundle are investigated. It is found that small changes in the arrangement of the cylinders can lead to considerable changes of oscillation amplitudes.
Dynamics and Hydrodynamics of a Moored Floating Rectangular Structure Under the Action of Random Sea Waves
Authors: Malara Giovanni , Arena Felice , Spanos Pol D.
Published: November 2011

This paper deals with the interaction of sea waves with a moored floating rectangular structure. The problem is of interest in various practical situations such as the design of floating breakwaters and of artificial islands, and the modelling of wave-ice sheet interaction. In the paper the incident wave field is modelled as a Gaussian stochastic process of a specified spectrum. In this context a diffraction problem and a radiation problem are posed. The related boundary value problem is discussed and solved by an eigen-functions expansion matching method. The method renders an estimate of the velocity potential at the first order in a Stokes’ expansion and, thus, of the wave pressure and of the wave forces. Several hydrodynamic quantities of interest are investigated. Specifically, reflection and transmission coefficients are derived and represented as a function of the structural dimensions. Further, the added mass and radiation damping terms are estimated. Then, the dynamics of the floating structure is examined by considering a 3-D-O-F system. Next, the wave-structure interaction is investigated in the context of a Quasi-Determinism theory. It is shown that the developed approach can be employed to account for both the dynamic and hydrodynamic effects of a structure in which diffraction is not negligible. The theoretical results are supplemented with data from pertinent Monte Carlo simulations.
A Conceptual Design on the BSL-4 Location and Management
Authors: Kobayashi Takako , Miyazaki Wataru , Yamamoto Kazukiyo , Miura Yuji , Kondo Takeo
Published: November 2011

In the event of an outbreak of an infectious disease, the pathogen of which is highly contagious, or when there are patients suspected to be infected with such a pathogen, it is essential to start treating the patients and controlling infection immediately. As the transportation system has been well developed, there are more worries about terrorist attacks using biological agents now than ever before. It is a real possibility that even the most dangerous pathogens that are the causes of Class 1 infectious diseases specified in the Infectious Diseases Act, such as hemorrhagic fever or smallpox, be brought into Japan. Those most dangerous pathogens can be dealt with only by special laboratories that satisfy BSL-4 (Bio-Safety Level 4) specified by WHO (BSL-4 laboratories)(1). As there are no BSL-4 laboratories in operation in Japan at present, it is an immediate requirement to develop a suitable environment for the operation of laboratories of this kind, in order to maintain effective health risk management for the Japanese people. In this research, in order to put BSL-4 laboratories in operation in Japan, we will clarify what relevant environmental factors are in different types of possible locations. Based on that, we will propose to utilize those laboratories not only in emergencies, but also in normal times. We will also discuss the necessary conditions in the social environment of contemporary Japan for BSL-4 laboratories’ operation.
CFD Calculations of Wave-in-Deck Load on a Jacket Platform: Impact Pressure Decrease due to Air Pocket Formation
Authors: Wemmenhove Rik , Lefranc Marc
Published: November 2011

The industrial problem of a jacket platform subjected to Wave-In-Deck load due to an extreme wave is studied numerically by a CFD technique. In particular, details of local flow and slamming-like hydrodynamic impact on structural members are studied. The applied CFD code ComFLOW is a Navier-Stokes equation solver with an improved Volume of Fluid (iVOF) method employed to displace and re-construct fluids free surface. Two different fluid models, single-phase (liquid+void) and two-phase (liquid+compressible gas) can be used, the latter model being capable of simulating gas entrapped in liquid. Local air pockets are formed in corners and nooks of the structure as the incoming wave front approaches. The study presents a comparison of hydrodynamic impact pressures found with and without the air entrapment. Numerical realisation of the two-phase model is considerably more expensive computationally and the study shows possibility and various aspects of its simulation. Accuracy of the numerical solution and relevance of the air pocket formation on the impact pressures and therefore on the exerted structural load are discussed.
Viscous Flow Effects of Passing Ships in Ports
Authors: Bunnik Tim , Toxopeus Serge
Published: November 2011

This paper presents a method to compute the effect of passing ships on moored ships. The method is in principle based on the method originally developed by Pinkster. He developed a method to quantify these effects based on linear potential flow. The disturbance of the passing vessel on the port geometry and moored ship is analyzed using a double-body flow. The reflection of this disturbance on the geometry of port and moored ship is analyzed using a linear diffraction method. This method has shown to give good results for ships sailing straight ahead, but is expected to be less accurate for passing ships sailing under a drift angle, in which case the viscous wake related to the lift force on the vessel becomes more dominant. Therefore, the method by Pinkster is extended with viscous effects. The disturbance caused by the passing ship on the moored ship and port geometry is computed by a RANS method instead of a potential method (double-body flow). The diffraction effects due to the presence of the moored ship and port geometry is accounted for in the same way as Pinkster did by means of a linear diffraction analysis. A comparison is made with model tests for a ship passing a moored ship along a quay at various drift angles. Results are shown for the traditional method (double-body flow disturbance) and for the considerably improved method (RANS disturbance).
System Identification of a Floating Oscillating Water Column Wave Energy Converter
Authors: Stappenbelt Brad , Fiorentini Massimo , Cooper Paul , Zhu Song-Ping , Nader Jean-Roch
Published: November 2011

One of the objectives of studies regarding the performance of floating oscillating water column (OWC) wave energy conversion devices is the prediction of the heave motion of the chamber and the water column. This paper presents a method to evaluate the parameters involved in the dynamics of partially submerged bodies in order to predict the coupled movement of the chamber and the water column in the frequency domain. System identification was performed and a lumped parameter model of the heave motions of a floating OWC was proposed. A novel approach utilising the reverse SISO method was employed to allow frequency dependent parameters for both the floating structure and the oscillating water column to be determined from several forced vibration experiments. Experimental results under forced vibration and wave excitation agree reasonably well with the dynamic model established.
A Modelling Framework for Describing the Corrosion Fatigue Behaviour of Carbon Steel Pipelines and Risers
Authors: Baxter David
Published: November 2011

The fatigue performance of pipeline or riser girth welds is often a critical factor during design. Under corrosive conditions the detrimental influence of an environment on anticipated fatigue lives needs to be taken into account. Fatigue design codes provide advice for the case of loading in seawater at typical wave frequencies, but in other environments, or at other cyclic loading frequencies, the required approach is less certain, and laboratory testing is often needed to provide suitable guidance. In addition to the time and expense of such an exercise, in some cases there are physical limits to the nature of data that such programmes can provide. The provision of data associated with very low cyclic loading frequencies is one such case, as in this instance the time taken for each test becomes unmanageable. This leads to the need for extrapolation of test data into the region of interest, for instance by carrying out tests at either a higher cyclic loading frequency, or a higher stress range, than that anticipated in service. Such extrapolation requires careful consideration in order to avoid potential non-conservatism. It is suggested that the availability of a model which accounted for the influence of these two key parameters (frequency and stress range) would greatly improve confidence when extrapolation is needed, and more broadly would provide a rational basis for designing experimental testing programmes and applying determined data to pipeline design. Such a model should be based on a description of the perceived underlying mechanisms of a material’s interaction with the environment. This paper provides a review of relevant test data and describes the basis for such a model. A generalised framework is developed and fitted to numerous published test data for carbon steels in seawater or sour environment.
VIV Response Inference From Tension Measurements
Authors: Lee Li , Gerretsen Heine
Published: November 2011

A vortex-induced vibration (VIV) inference technology, termed as VIV detector, has been developed at Shell. The underlying idea is to use the tension measurements of a tubular to infer its bending responses due to vortex-shedding. A prototype was developed for monitoring VIV of the tendons of a tension leg platform (TLP). The theoretical frame work, including key parameters and design curves for commonly used offshore tubular, is discussed herein. Examples are provided, which indicate the agreement of the inferred and the measured responses of models in laboratory conditions. The benefits and potential applications to other structures are suggested at the conclusion.
Numerical Evaluation of Observation Algorithm for Sea Surface Remote Sensing by Doppler Radar
Authors: Yoshida Takero , Rheem Chang-Kyu
Published: November 2011

Algorithms of sea surface remote sensing are based on changes of Doppler shifts, which are measured by a Doppler radar. Microwave irradiation width on the sea surface and time taken to collect data for frequency analysis influence Doppler spectra. In order to evaluate the influences of these parameters in observing algorithms, a simulation of microwave backscattering from numerical sea surface was done in time domain to obtain Doppler spectra. Doppler spectra have been simulated in the case of various numerical regular waves. In the case of the microwave irradiation width is larger than the wavelength of the numerical regular wave or the Fourier transform time for the frequency analysis is longer than the period of the numerical regular wave, the peak value of Doppler spectra shows the phase velocity of the Bragg resonance wave. The results show the principle of measuring sea surface current. In the case of the microwave irradiation width is smaller than the wavelength of the numerical regular wave or the Fourier transform time is shorter than the period of the numerical regular wave, Doppler spectra vary with the orbital motions of the regular wave. As the result, when the sea surface wavelength is five times or more as long as the microwave irradiation width, the time fluctuations of Doppler velocity which shows a mean value of Doppler spectrum are good agreement with the orbital motions of the numerical regular wave. Also in such condition, the wave height of the sea surface waves can be observed accurately by analyzing the changes of Doppler velocity.
Collapse Capacity of UOE Deepwater Linepipe
Authors: Aamlid Olav , Collberg Leif , Slater Simon
Published: November 2011

Whereas the wall thickness for most pipelines is governed by internal pressure, the wall thickness of pipelines at very deep waters may be governed by external pressure and the failure mode is collapse. This paper will firstly summarise the work performed in the early 90ties in the SUPERB project that constitutes the basis for the collapse equation adopted in DNV Rules for Submarine Pipeline Systems. This work documented a comparison between various expressions for collapse prediction (Timoshenco, Murphy and Langner (Shell) and Haugsmaa (BSI)) to available experimental results. This work made it possible to select the formulation deemed to be most appropriate as a design equation as well as calibrating safety factors. Secondly, the paper will discuss the well documented detrimental effect that pipe forming can have on the compressive yield strength in the hoop direction and thus the collapse capacity of pipes. This effect led to the introduction of the so-called fabrication factor in DNV-OS-F101 that reduces the compressive yield strength by 7–15 per cent for pipes manufactured using cold forming. However, DNV-OS-F101 states “The fabrication factor may be improved through heat treatment or external cold sizing (compression), if documented” and the paper will summarise various published work, experimental and analyses, that has, during the last 15 years, been performed in several pipeline projects to document the beneficial effect that mainly light heat treatment but also optimised forming in the UOE process have on the compressive yield stress and collapse capacity.
Irregular Wave Loads on a Gravity Based Foundation in Shallow Water
Authors: Christensen Erik D. , Lohmann Iris P. , Hansen Hans F. , Haerens Piet , Mercelis Peter , Demuynck Annelies
Published: November 2011

In order to achieve a safe but cost-effective foundation design of offshore structures, it is important to include effects of run-up and wave breaking in the estimation of wave loads on structures in relatively shallow water. This study presents results from a method applied to estimate wave loads on a gravity based foundation (GBF) coming from irregular waves which are potentially subjected to wave breaking. The objective of the study is to analyse the loads on gravity based foundations for wind turbines on the Thornton Bank, Belgium, due to irregular breaking waves. This study focuses on uncertainties in estimation of maximum loads based on the same wave condition, i.e. (Hs, Tp, Wave Spectrum). To this end three different synthetic irregular wave time series elaborated from the same wave condition are used to simulate the wave load on the GBF. The simulations result in time-series of wave loads and wave elevations on the GBF. The loads obtained from the model indicate a small difference (below 10% in peak values) between the wave-induced inline force for the three simulations, and differences up to 15–20% on the peak values of the obtained wave induced overturning moments. From the simulation results it is also possible to investigate flow patterns and run-up around the structure.
Numerical Simulation on Vortex-Induced Vibration of an Elastically Mounted Circular Cylinder With Two Degrees-of-Freedom
Authors: Huang Zhiyong , Larsen Carl M.
Published: November 2011

A two-dimensional numerical simulation is applied to study the forces and responses associated with vortex-induced vibration of an elastically mounted circular cylinder with two degrees-of-freedom, i.e. the cylinder vibrates in in-line and cross-flow directions. This work could be regarded as a first step to carry out the prediction of vortex-induced-vibration responses of a long flexible beam with a number of two-dimension sections along the spanwise based on strip theory. A direct comparison has been made between the numerical results and measured data from the experiment by Jauvtis and Williamson in 2004. The peak cross flow response reaches 1.28 diameters in the present simulations. The profiles between the displacement and transverse force are found to have a good match with the experimental results, and a typical figure of ‘8’ trace is observed between the lift and drag forces in the initial and super-upper branches. Two typical in-line wake structures SS mode and AS mode are well reproduced in the low reduced velocity range. The newly discovered wake pattern 2T mode corresponds to the super-upper branch is also recaptured. Comparison shows that most features of the experiment can be reproduced by the present numerical model, and this model can be regarded a robust tool to investigate the responses, forces and the basic mechanics of vortex induced vibrations of an elastically mounted cylinder with two degrees-of-freedom.
Kinematics of Experimentally Generated Severe Wave Conditions and Implications for Numerical Models
Authors: Minnick Lisa , Kent Christopher , Bassler Christopher , Percival Scott , Hanyok Lauren
Published: November 2011

An experiment was performed to measure and characterize wave kinematics in a large-scale experimental basin. The primary objective was to measure and characterize the wave kinematics of both regular waves of varying steepness and scaled irregular seas with embedded large-amplitude wave groups. The secondary objective was to provide a validation data set for wave theory and numerical simulation tool development. Measurements included free surface elevations and velocity field measurements under the free surface using particle image velocimetry (PIV). A discussion of free surface elevation data, the effect of wave steepness on the velocity profile of regular waves and an introduction to the analysis of the wave kinematics of the embedded wave groups was presented in a previous paper by the authors (OMAE2010-20240). The current paper expands on the analysis presented in the previous paper, providing a discussion of the effect of wave group composition, wave group location within irregular seas, and seaway scaling on the kinematics. This experiment is part of an ongoing effort at the Naval Surface Warfare Center Carderock Division (NSWCCD) to improve predictions and measurement of ship motions in waves and assess the dynamic stability and seakeeping performance of naval ships. NSWCCD has developed a deterministic wave generation method to be used in its seakeeping basin facility. The exploration of the above factors provides a better understanding of the employed method and its effectiveness for creating wave groups. Specifically, to model realistic severe conditions that a ship may encounter. These findings, in turn, lead to the possibility of improvements to current model testing methods at NSWCCD. In addition to assessing the experimental methods employed, the experimental data set can also be used to validate and improve current numerical wave models for ship motions prediction. A comparison of the measured wave elevations and kinematics with a pseudo-spectral numerical wave model is also presented and discussed.
Prediction of Airgap Statistics for Fixed Offshore Platforms
Authors: Naess A. , Gaidai O.
Published: November 2011

Air gap statistics for offshore platforms is directly related to the extreme value statistics of the random ocean wave field. The present paper describes a new method for predicting the extreme values of a random wave field in both space and time. The method relies on the use of data provided by measurements or Monte Carlo simulation combined with a technique for estimating the extreme value distribution of a recorded time series. The time series in question represents the spatial extremes of the random field at each point in time. The time series is constructed by sampling the available realization of the random field over a suitable grid defining the domain in question and extracting the extreme value. This is done for each time point of a suitable time grid. Thus, a time series of spatial extremes is produced. This time series provides the basis for estimating the extreme value distribution using recently developed techniques for time series, which results in an accurate practical procedure for solving a very difficult problem. This procedure is applied to the prediction of air gap statistics for a jacket structure.
Unstable Motion of a Floating Structure in Surface Waves
Authors: Yan Hongmei , Liu Yuming , Li Yile
Published: November 2011

Unstable resonant heave and pitch motions of a floating deep draft platform, under the action of a regular wave with the frequency equal to the sum of the heave and pitch natural frequencies, can be developed by nonlinear instability (Liu, Yan & Yung 2010). The instability is associated with difference-frequency interactions between the body motion and the ambient wave. In this work, we study the effect of the nonlinear instability upon floating platforms with relatively shallow drafts whose wave damping at heave/pitch natural frequencies may not be small. Direct time-domain numerical simulations of wave-structure interactions, which can take into account different levels of nonlinearity effects, are applied to understand the characteristics of the unstable coupled heave/pitch (or heave/roll) resonant motion and its dependence on the key physical factors. In particular, it is found that such a nonlinear instability at other wave conditions involving sum-frequency interactions between the body motion and the ambient wave can also occur. For practical applications, long-time nonlinear simulations with irregular waves are also performed. The results show that depending on the sea conditions and damping in the system, the unstable resonant motion associated with the nonlinear instability can be significant for platforms with shallow drafts.
Numerical Simulation of Installed Torpedo Anchors Embedded in Cohesive Soil
Authors: de Aguiar Cristiano S. , de Sousa Jose´ Renato M. , Ellwanger Gilberto B. , Porto Elisabeth de Campos , Fernandes Jane V. V. , Henriques Paulo Roberto D. , Costa Rachel G. B.
Published: November 2011

Torpedo anchors have proven to be one of the most important alternatives for mooring systems, especially in Brazilian offshore fields. Their installation involves many hydrodynamic and soil-structure interaction aspects and, therefore, their vertical stability cannot be always assured. This often results, after their installation, in an inclination between the anchor and a vertical axis called tilt angle. The traditional approach to assess the holding capacity of torpedo anchors relies on the hypothesis that the relative angle between the load direction and the axis of the anchor may be used in conjunction with a finite element model in which a perfectly vertical anchor is considered. In this work, this assumption is discussed. A parametric study in which four different tilt angles are combined to various load inclinations was conducted. The study relied on a previously proposed FE model. The results obtained indicate that the use of the traditional approach is safe for relative angles higher than 40° and lower than 140° For other angles, the approach leads to values slightly higher than those predicted with the direct consideration of the tilt angle.
Towards an H2S Free Environment in Flexible Pipe Annulus Thanks to a New Anti-H2S Polymer Layer
Authors: Epsztein Thomas , Demanze Frederic , Lefebvre Xavier , Jarrin Jacques
Published: November 2011

Due to H2S pressure build-up in annulus, the use of sour service steel grades is required in some cases. For ultra deep water fields where the flexible pipe top tension is critical, high weight of the structural steel layers combined with lower mechanical properties (about 950MPa) of the sour grade materials is a disadvantage. The generic flexible pipe design avoids any direct contact between the bore environment (production fluids, injection fluids, etc) and the steel layers of the flexible pipe. However, diffusion of active components such as water, carbon dioxide, hydrogen sulfide, etc, through the thermoplastic sheaths could lead to corrosion exposure of metallic layers between the thermoplastic sheaths. In order to reduce the weight of the flexible pipe structure to reduce riser top tension, high strength grade steel layers (>1200MPa) can be used with reduced thickness, considering that the steel layers will not be exposed to H2S coming from the bore fluid. This is now achievable even when H2S is present in the bore by use of a new reactive polymer layer, which will ensure a H2S free environment in the flexible pipe annulus. The new “Anti-H2S” layer will chemically react with the H2S, leading to a complete absence of H2S exposure to steel layers during the service life of the pipe. In addition, the “Anti-H2S” layer makes it possible to propose flexible pipes for applications with high H2S content for which no acceptable solution would otherwise be found. This paper will present the theoretical background supporting the new Anti-H2S layer concept, along with the qualification works carried out both at laboratory scale and on full flexible pipe structures. It will also focus on the design/calculation methodology applicable to this new sheath. Introduction of the “Anti-H2S” layer results in that: - Flexible pipes can now be proposed for applications with high H2S contents; - Sweet service high strength steel grades can be used in flexible pipe annulus even with the presence of H2S into the bore of the pipe; - The weight of the flexible pipe is reduced.
Numerical Study on Hydrodynamic Behavior of an Underwater Glider
Authors: Phoemsapthawee Surasak , Le Boulluec Marc , Laurens Jean-Marc , Deniset Franc¸ois
Published: November 2011

Underwater gliders are AUVs used in ocean exploration and observation. They use small changes in their buoyancy to dive and to return to the ocean surface. During the change of altitude, they use the hydrodynamic forces developed by their wings to move forward. Their flights are controlled by changing the position of their centers of gravity and their buoyancy to adjust their trim and their heel angles. For better flight control, the understanding of the hydrodynamic behavior and the flight mechanics of the underwater glider is necessary. A 6-DOF motion simulator is coupled with a BEM code for this purpose. In some specific cases, the numerical study demonstrates that an inappropriate stabilizer dimension can cause a counter-steering behavior. The simulator can be used to improve the automatic flight control. It can also be used for the hydrodynamic design optimization of the devices.
Higher Order Wave-Current Elevations in Deep Water
Authors: Bratland Anne Katrine , Bo̸rresen Ragnvald , Berntsen Per Ivar Barth
Published: November 2011

Wave-current interaction refers to the interaction between surface gravity waves and ocean current flow. This interaction implies an exchange of energy, i.e. both the waves and the current are affected. The present paper describes the calculation of wave elevations in higher order unidirectional, irregular waves with a uniform current in deep water. Results for regular waves are compared with those obtained for Stokes second and third order waves with uniform current according to the method described by Fenton [1]. The results for higher order wave elevations in irregular waves have been compared with waves and current generated in a model test basin and reasonable agreement has been found.
Advanced Numerical Simulation to Meet Design Challenges of XHPHT Metallurgically Clad PIP Platform Riser
Authors: Zhao Tao , Lee Dan , Farahani Kasra , Cooper Philip
Published: November 2011

Pipe-in-pipe (PIP) systems are proposed for platform risers subjected to extra high pressure high temperature (XHPHT) shut-in condition, to meet the flow assurance and stringent strength and thermal criteria, and to mitigate design issues associated with wet insulation application. To further satisfy the corrosive fluid environments, the inner pipe of the PIP system is metallurgically clad with a Corrosion Resistant Alloy (CRA). These complex design challenges require advanced numerical simulation to correctly capture the complex PIP behaviour and clad-pipe effects in order to avoid overly conservative design, and to provide a robust and optimised solution. The equivalence of CRA clad pipe was investigated and analytically deduced, especially on the thermal expansion behaviour under the XHPHT environments. An advanced numerical simulation based on Finite Element Analysis (FEA) was subsequently carried out. A systematic family of FE models was developed to meet the design complexity, namely: global PIP platform riser model to capture the global behaviour, local PIP centraliser model to address contact behaviour, local bulkhead design model for PIP bulkhead design and optimisation, local girth weld model to address mismatches (high-low misalignment, thickness and material strength). In addition, a modal analysis was conducted based on a PIP model to ensure that the analysis accounts for centralisers, pre-stress and deformation effects. The eigenvalue computing is then used for free span analysis. Due to lack of limit state design codes for pipe bends and the fact that the allowable stress criteria can be overly conservative, a bend collapse capacity deduced from FEA was applied in accordance with DNV local buckling criteria. The analysis procedures developed are outlined and a XHPHT PIP platform riser design is presented. This paper aims to provide a robust solution to aid design by the application of advanced numerical simulation.
Performance Analysis of a Semicircular Free Surface Breakwater
Authors: Teh Hee Min , Venugopal Vengatesan , Bruce Tom
Published: November 2011

In the present study, the hydrodynamic performance of a semicircular free surface breakwater (SCB) has been investigated through a systematic experimental programme. Three semicircular breakwater models were tested: one with impermeable front and rear walls; a second with perforated front wall and impermeable rear wall; and a third with perforated front and rear walls. The models were tested for three submergence depths with reference to the still water level in a wave flume under irregular seas with different significant wave heights and peak periods. The performance of the breakwaters was evaluated in the form of coefficients of transmission (CT), reflection (CR) and energy dissipation (CL). The measured wave modification in front of the structure and in the breakwater’s chamber were quantified and presented in the form of a ratio relative to the incident wave height, respectively. Also, the measured horizontal wave forces acting on the SCB were analysed and reported in a dimensionless form. Empirical equations were then developed using nonlinear multiple regression models to estimate the hydrodynamic characteristics of the SCB models.
Ratcheting, Wrinkling and Collapse of Tubes Induced by Axial Cycling Under Internal Pressure
Authors: Jiao Rong , Kyriakides Stelios
Published: November 2011

A buried offshore pipeline is essentially axially constrained by the soil cover. Heating by the passage of hot oil at high pressure can plastically deform it. The deformation involves expansion of the diameter, which for thinner pipes can be accompanied by axisymmetric wrinkling. During a lifetime of 20 or more years, lines experience regular startup and shutdown cycles. This study examines how this cycling affects wrinkling and the hoop expansion of such lines. A set of experiments on super-duplex tubes with D/t of 28.5 was conducted using the following idealized cyclic loading history. A tube is first pressurized and then compressed into the plastic range to a level that initiates wrinkling. It is then cycled under stress control about a compressive mean stress while the pressure is kept constant. The combined loads cause simultaneous ratcheting in the hoop and axial directions as well as a gradual growth of the wrinkles. At some stage the amplitude of the wrinkles starts to grow exponentially with the number of cycles N leading to localization and collapse. The rate of ratcheting and the number of cycles to failure depend on the initial compressive pre-strain, the internal pressure and the stress cycle parameters. The problem is modeled as a shell with initial axisymmetric imperfections. A challenge in the simulations is that the cyclic plasticity model that is used must be capable of capturing correctly the type of biaxial material ratcheting that develops. The Dafalias-Popov two-surface nonlinear kinematic hardening model, enhanced and suitably calibrated is shown to capture the prevalent ratcheting deformations correctly leading to predictions that are in good agreement with the experimental results. The model is then used to evaluate the ratcheting behavior of pipes under thermal-pressure cyclic loading histories seen by buried pipelines.
A Modified Structural Arrangement of Rigs for Safe Launching of Life Boats and Berthing of OSVs
Authors: Kar Apurba , Patil Sandip
Published: November 2011

It has long been experienced that Launching of lifeboat from rigs and Positioning of offshore supply vessel (OSV) near rigs are very critical and vulnerable operations as regards to the safety. In present days, sophisticated & expensive Dynamic Positioning (DP) Systems are used in order to avoid the collision of OSVs with rigs. However, it is observed that despite the provision of such sophisticated means, accidents are still occurring. Loss of control during positioning of OSVs can lead to severe accident (e.g. collision of OSV Samudra Shakti with Mumbai High North (MHN) platform in 2005). To avoid such accidents, considerable gap needs to be maintained between the OSV and the rig position. Also launching of lifeboat is known to be another safety critical operation. Particularly in rough weather, the landing of lifeboat requires to be at least 20 to 30m away from the platform. This is essential for preventing the lifeboat to drift under the platform and colliding with the structure & piping system there at. Lifeboat launching by davit lowering or freefall have got their own traditional problems as widely known in shipping industry. These are described in the paper “Safety of Lifeboat launching - Some Possible Improvements {P19J_AB27}” presented at Design for Safety Conference-2010 held in Italy. Considering both the issues as described above, it is evident that maintaining adequate distance from the rig is a crucial factor for improving safety of Life Boat launching and positioning of OSVs alongside the rigs. In order to achieve this, an additional structural part of the rig is proposed in this paper. This new structural entity is to be in the form of an inclined truss which will act as launching skid for the lifeboat. This arrangement will provide a guided slipway to enable landing of lifeboat at a considerable distance from the rig. Also this structure will be configured suitably to enable berthing & mooring of supply vessels alongside the rig. In this arrangement the DP system will be required for a short time only, at the initial stage of placing the vessel. After having the vessel positioned, she can be moored with the new proposed truss structure. Thus the dependency of DP System will reduce significantly to avoid collisions. So the authors believe that the proposed arrangement will improve the safety of offshore operation in a significant manner.
Effect of Higher Stress Harmonics and Spectral Width on Fatigue Damage of Marine Risers
Authors: Price R. , Zheng H. , Modarres-Sadeghi Y. , Triantafyllou M. S.
Published: November 2011

Recently, experimental data from vortex induced vibrations (VIV) of long flexible cylinders have revealed the existence of (i) higher harmonic stress components, and (ii) chaotic response characteristics, in addition to the well-known first harmonic responses. To assess the effect of these phenomena on fatigue damage of long flexible structures, we generate alternative stress time series, with and then without higher harmonic components; as well as time series, which have either sharply peaked power spectral densities (PSD), or broad-banded PSDs, characteristic of chaotic response. We show that for stresses containing higher harmonics, the predicted fatigue damage to the riser can increase by a factor of 2.5 relative to the damage caused by stresses containing only the first harmonic and having equivalent total power. Likewise, for stresses with a significantly spread PSD, an indicator of chaotic response, the damage to the riser is increased by a factor of 1.8, relative to a stress with narrow-band PSD and the same total power. Thus, it is found that the increase in fatigue damage caused by chaotic response can be as significant as that caused by the stress higher harmonic components.
Dry and Wet Tree Damper Chamber Column Semisubmersible Design in Harsh Environment
Authors: Mansour Alaa M. , Upston Laurence , Wan Yaming
Published: November 2011

The Damper Chamber Column Semi is a new semisubmersible conceptual design that is developed to provide a cost effective solution that can be used in deepwater dry or wet tree applications. The new semisubmersible design resembles the conventional design with a ring pontoon. However, in the new design, the columns have been configured with free flooding inner damper chambers; hence the name Damper Chamber Column Semi (DCC Semi). At quayside and during wet tow, the column configurations provide large water plane area for stability purposes, while for the in service conditions the inner chambers are flooded and the trapped water provides considerable amount of “free” added mass that significantly reduces the semisubmersible motion response. In this paper the performance of the DCC Semi located in Gulf of Mexico (GoM) environment has been simulated using fully coupled time domain analysis. Numerical results are presented for the platform motion and the tensioner stroke to demonstrate the platform performance feasibility. Structural design is performed to confirm the steel weight and establish the column connection details of the new design. The paper also presents the topside to hull quayside integration approach for this case study.
Reliability-Based Fatigue Life Estimation of Ocean Current Turbine Rotor Blades
Authors: Hurley Shaun , Arockiasamy Madasamy
Published: November 2011

The study presents a reliability-based fatigue life prediction model for the ocean current turbine rotor blades. Using the short-term distribution of the bending moment ranges, the 2-parameter Weibull distribution is used to fit the bending moment range data to obtain the long-term distribution. The long-term distribution is used to determine the number of load cycles for any given bending moment range. The published laboratory test data in the form of the ε-N curve is used in conjunction with the long-term distribution of the bending moment ranges in the prediction of the fatigue failure of the rotor blade using Miner’s rule. Future work on the reliability analyses and partial safety factors are included in the paper.
Simple Numerical Models for Pipeline Walking Accounting for Mitigation and Complex Soil Response
Authors: Carneiro Daniel , Murphy David
Published: November 2011

Non-buried subsea pipelines subjected to high internal pressures and high operational temperatures (HP/HT) may experience significant axial expansion. Asymmetries in the loading and unloading in startups and shutdowns (e.g. due to seabed slope, temperature transients or riser tension) may cause the axial displacements to accumulate over operational cycles, in a ratcheting process often called “pipeline walking”. Despite the complexity of the pipe-soil interaction governing this behavior, several analytical and simple numerical models have been used for estimating the total accumulated pipeline axial displacement. These simple models are powerful tools in preliminary phases of a pipeline design, although their use is limited due to the simplifications. This paper presents results of a simple numerical model able to account for additional features in the preliminary walking assessment, such as loads on mitigation systems. The models were originally prepared to assess walking mitigation for some rigid flowlines in a recently installed subsea system, and remarkable agreement with complex three-dimensional finite element models was observed. The effect of different types of mitigation systems on the global behavior of the pipelines is presented and discussed. The influence of the pipe-soil interaction model employed is also investigated.
Conceptual Design of Navigation of an AUV for Monitoring CCS Site at Deep Sea Bottom
Authors: Watanabe Yoshitaka , Yoshida Hiroshi , Ochi Hiroshi , Hyakudome Tadahiro , Ishibashi Shojiro , Nakano Yoshiyuki , Omika Shinobu , Matsuura Masami
Published: November 2011

We, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), are developing an autonomous underwater vehicle (AUV) whose main mission is monitoring a site at the sea bottom for the carbon dioxide capture and storage (CCS). The AUV cruises very near the sea bottom, and is equipped with chemical sensors in order to detect escape of CO2 from sub-bottom. Of course, the position information of the AUV is critical information for the monitoring. In this paper, a conceptual design of navigation of the AUV is described. Recently, navigation of AUV is implemented by integrating multiple navigation devices including inertial navigation system (INS), Doppler velocity log (DVL), depth sensor, acoustic navigation system, and others. The AUV under construction will be equipped with these navigation sensors, and will integrate those sensors’ outputs to navigate herself. In order to measure the absolute position of the AUV the acoustic method is one of fundamental technique. At the first step of development of the AUV, three acoustic methods are considered to adopt. The three methods are super short baseline (SSBL) method which is a tracking from support ship or other surface station, long baseline (LBL) which is navigation based on preplaced acoustic transponders, and virtual LBL (VLBL) which is navigation based on only single transponder. These acoustic methods are integrated with the navigation result of INS, depth sensor, and DVL. The three methods are used in each appropriate case. Which feature of observation is desired simplicity, accuracy, or independence from support ship and time efficiency? The acoustic method is influenced by environment, and also output of other sensors is depending on the environment, for example the DVL miss the data when the terrain is with many up-hills and down-hills. The integration or filtering parameters of the navigation should be adjusted depending on the influential environmental factor.
Effect of Strain Ageing on Mechanical Properties of Pipeline Girth Welds
Authors: Narayanan Badri K. , Ogborn Jon
Published: November 2011

Pipeline girth welds for on-shore and off-shore pipelines use a variety of arc welding processes. The trend towards strain based designs for line pipe installation and the effect of coatings for off-shore pipelines have resulted in evaluation and testing of pipe material subjected to strain ageing. However, very little work has been done to systematically study the effect on ferritic weld metal. This work details some initial work done on evaluating the effect of strain ageing on ferritic weld metal deposited with a 1.2 mm diameter flux cored wire under 75% Ar −25% CO2 shielding gas. Pipeline girth welds were welded on API Grade X-70 pipe and tested to get all weld metal tensile and Charpy V-Notch properties. The weld metal strength overmatched the base material by 7–9%. The ductile to brittle transition temperature for the weld metal was −40°C. The effect of strain ageing on weld metal properties was evaluated. All weld metal tensile samples were subjected to varying levels of pre-strain and ageing treatments to evaluate the effect on yield strength and post-yield behavior. An increase in yield strength after straining and ageing as well as the re-appearance of yield point is observed. Increase in pre-strain decreases elongation. Increase in ageing temperature delays the appearance of dynamic strain ageing. The activation energy for the increase in strength after strain ageing has been measured by assuming a diffusion controlled mechanism. Charpy V-Notch samples were taken to generate transition curves of weld metal after strain ageing and compared to the as-welded condition.