bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Sloshing Analysis of FLNG and LNGC Considering Two-Body Motion Effect
Authors: Park Jong Jin , Kawabe Hiroshi , Kim Mun Sung , Kim Byung Woo , Eom Jae Kwang
Published: November 2011

Side by Side arrangement is considered for the LNG-FPSO offloading operations. In that case, two-body coupled effects are important for LNG-FPSO and LNGC motion and sloshing analysis. The present study is concerned with a ship motion and sloshing analysis considering two-body motion and sloshing-motion coupled effects. The methodology is based on three-dimensional potential theory on a coupled model of LNG-FPSO and LNGC in the frequency domain. To calculation sloshing impact pressures, the violent liquid motion inside tank is treated with three-dimensional numerical model adopting SOLA-VOF scheme.
Modulational Instability in Directional Wave Fields, and Extreme Wave Events
Authors: Babanin Alexander V. , Waseda Takuji , Shugan Igor , Hwung Hwung-Hweng
Published: November 2011

The paper is based on review of research articles by the authors, with the purpose to demonstrate that the modulational-instability mechanism is active in typical directional wave fields. If so, possible limits for the wave height due to such mechanism can be outlined. The modulational instability can lead to occurrence of very high waves, which either proceed to the breaking or appear as rogue events, but it was derived for and is usually associated with two-dimensional wave trains. There exists argument, both analytical and experimental, that this kind of instability is impaired or even suppressed in three-dimensional (directional) wave systems. The first part of the paper demonstrates indirect experimental evidences which relate the wave breaking in oceanic conditions to features of two-dimensional breaking waves due to modulational instability. The second section is dedicated to direct measurements of such instability-caused breaking in a directional wave tank with directional spread and mean steepness typical of those in the field. The last section provides conclusions on what is maximal height of an individual wave, depending on the mean wave steepness in a wave train/field, that can be achieved due to such non-linear evolution of wave trains.
Formation of Extraordinarily High Waves in Space and Time
Authors: Clauss Gu¨nther F. , Klein Marco , Onorato Miguel
Published: November 2011

The existence of freak waves is indisputable due to observations, registrations, and severe accidents. The occurrence of extreme waves, their characteristics and their impact on offshore structures is one of the main topics of the ocean engineering research community. Real sea measurements play a major role for the complete understanding of this phenomenon. In the majority of cases only single point registrations of real sea measurements are available which hinders to draw conclusions on the formation process and spatial development in front of and behind the respective registration points. One famous freak wave is the “New Year Wave”, recorded in the North Sea at the Draupner jacket platform on January 1st, 1995. This wave has been reproduced in a large wave tank and measured at different locations, in a range from 2163 m (full scale) ahead of to 1470 m behind the target position — 520 registrations altogether. Former investigations of the test results reveal freak waves occurring at three different positions in the wave tank and these extreme waves are developing mainly from a wave group. The possible physical mechanisms of the sudden occurrence of exceptionally high waves have already been identified — superposition of (nonlinear) component waves and/or modulation instability (wave-current interaction can be excluded in the wave tank). This paper presents experimental and numerical investigations on the formation process of extraordinarily high waves. The objective is to gain a deeper understanding on the formation process of freak waves in intermediate water depth such as at the location of the Draupner jacket platform where the “New Year Wave” occurred. The paper deals with the propagation of large amplitude breathers. It is shown that the mechanism of modulation instability also leads to extraordinarily high waves in limited water depth. Thereby different carrier wave length and steepnesses are systematically investigated to obtain conclusions on the influence of the water depth on the modulation instability and are accompanied by numerical simulations using a nonlinear potential solver.
Influence of Heave Plate on Hydrodynamic Response of Spar
Authors: Sudhakar S. , Nallayarasu S.
Published: November 2011

Spar technology has been used in offshore oil and gas exploration successfully for number of years for drilling, production and storage in deepwater. The motion response of floating structures especially the heave response in particular, is very important to the selection of suitable drilling and production equipments. Reduction of heave response can be achieved by attaching a heave damping plate to the keel of a Spar. This has been used in the past. Experimental and numerical studies on such devices of various diameters under regular waves has been carried out and presented. The experiments were conducted on a 1: 100 scale model of Spar designed for a water depth of 245m with a payload of 10000 tonnes and the numerical analysis was carried out using ANSYS AQWA software. Numerical and experimental values of RAOs for surge, pitch and heave compares reasonably well. Measured and numerical results of RAOs for surge, pitch and heave and the effect of diameter ratio, wave steepness, and mooring line pretension were presented and discussed. An optimum heave response is achieved when the heave plate diameter is 20% to 30% larger than the diameter of the spar.
Development of 6% Nickel Steel for LNG Storage Tanks
Authors: Furuya Hitoshi , Saitoh Naoki , Takahashi Yasunori , Kurebayashi Katsumi , Kayamori Yoichi , Inoue Takehiro , Uemori Ryuji , Okushima Motohiro
Published: November 2011

9% Ni steel has been used for LNG storage tanks for more than four decades although 5.5% Ni steel (N-TUF CR196) was developed in the 1970’s using a special heat treatment method named L-treatment. The reason why the actual application of 5.5% Ni steel has not been attained to LNG storage tanks is mainly because the requirement of fracture properties is not confirmed for the tanks. Under the circumstances of expanding demand for natural gas and double-integrity in LNG storage tanks, we restarted developing low Ni steel for LNG storage tanks by using both conventional and advanced techniques. For the application of low Ni steel to the present LNG storage tanks, both fracture initiation and propagation properties of base metal plates and welded joints should be concerned. The fracture initiation and propagation properties of base metal were compensated with the intercritical reheating process (L-treatment), and the propagation property was additionally enhanced by combining TMCP with L-treatment. In addition, the chemical composition adjustment and the homogenization treatment of solute elements were conducted for improving the fracture initiation and propagation properties of welded joints. 6% Ni steel plates were manufactured by the process of continuous casting, reheating, hot rolling, direct quenching (TMCP), L-treatment, and tempering, and their chemical composition was 0.05C-0.06Si-1.0Mn-6.3Ni-Cr-Mo. As the results of fracture property evaluation including large-scale fracture tests such as the duplex ESSO test and the wide plate tensile test, it was demonstrated that 6% Ni steel has good characteristics regarding brittle fracture initiation and propagation in base metal plates and welded joints.
Numerical Investigation of Scale Effects in Modelling Scour Below Offshore Pipelines Under Steady Currents
Authors: Zhao Xu , Zhao Ming , Cheng Liang
Published: November 2011

Scale effects must be considered when results of small-scale laboratory tests on scour below offshore pipelines are extrapolated to real-life situations. In the present study, a 2D Finite Element Method (FEM) numerical model for scour simulation was employed to evaluate scale effects in modelling scour below a single pipeline subjected to steady currents. In the model, the Reynolds-Averaged Navier-Stokes (RANS) equations with a k-ω turbulence closure are supplemented by a morphological model with both suspended load and bedload sediment transport rates included. Scour simulations were conducted at a number of scales with various Shields parameter values and sediments of two median grain sizes. The numerical results indicate that for the range of parameters investigated in this study, the normalized equilibrium scour depth below the pipe centre decreases in a linear fashion as the pipe diameter increases. Accordingly, an expression delineating scale-effect trends that relate deviations in normalized equilibrium scour depths to those in pipeline diameters is proposed, thereby contributing to an accurate extrapolation of small-scale model test results to prototype scale in modelling scour below pipelines in currents.
Expected Values of Wave Power Absorption Around the Japanese Islands Using OWC Types With Projecting Walls
Authors: Ikoma Tomoki , Osawa Hiroyuki , Masuda Koichi , Maeda Hiisaaki
Published: November 2011

This paper describes performance improvement of wave power absorption by using a new concept. Basic system proposed is an oscillating water column (OWC) type. An artificial harbor surrounded by projecting walls is installed. The type is called as PW-OWC in this paper. Standing waves occur in the artificial harbor, the absorbing device consequently has a resonance frequency differing from that of OWC. From the effect, the system is able to absorb wave power in very wide range of the wave frequency. From the experimental results, PW-OWC types are very good performance of wave power absorption comparing with conventional OWC types. In addition, the performance of the PW-OWC type is insensibility to the nozzle ratio of an orifice. The performance can be easily improved by installing the harbor part even if the performance of a base OWC device is not good. Finally, we investigate the expected values of acquirable wave power in not only a year but also every season. The expected values of PW-OWC types are greater than that of conventional OWC ones in seas around the Japanese islands.
Comparison of Time and Frequency Domain Simulations of an Offshore Floating Wind Turbine
Authors: Philippe Maxime , Babarit Aure´lien , Ferrant Pierre
Published: November 2011

Time domain simulations of an offshore floating wind turbine have been performed. Hydrodynamic impulse responses of the floating platform are calculated with linear hydrodynamic simulation tool ACHIL3D. A user defined module for the wind turbine design code FAST has been developed to calculate hydrodynamic and mooring loads on the structure. Resolution of the movements of the system is done with FAST. Simulation results in time domain are compared with frequency domain results. In the frequency domain model, the whole system is linearized. In the time domain model, the wind turbine model is not linearized. A good agreement between time and frequency domain calculations is observed, even for the pitch motion. Furthermore we observe a non linearity in the response of sway, roll and yaw degrees of freedom around 0.3 rad.s-1. The effect of viscous damping on the movements of the floating wind turbine system has been studied with the time domain model, and a non linear hydrostatic and Froude-Krylov load model has been developed. Effects of these non linear terms are shown.
A Conceptual Design Method for Parametric Study of Blades for Offshore Wind Turbines
Authors: Fro̸yd Lars , Dahlhaug Ole G.
Published: November 2011

This article presents a simplified, integrated method for design studies of blades for offshore wind turbines. The method applies to variable speed horizontal axis wind turbines with pitch control, and allows designing the rotor blades based on a very limited set of input parameters. The purpose of the method is to allow parametric studies of different design configurations of the rotor at a reasonable effort. The resulting wind turbine models are at a level of detail suitable for preliminary design considerations using e.g. aero-elastic simulations in the time domain. The aerodynamic design is based on blade element momentum (BEM) considerations using a distribution of 2D airfoil characteristics. The structural design of the blades is based on aerodynamic forces calculated from a small number of load cases. The design procedure is facilitated by using simplified cross-section definitions and iterative approaches. The resulting blade designs are shown to compare well with data from available turbine models.
USP Active Absorption Basin: Absorption of Irregular Waves
Authors: Carneiro Mario Luis , de Mello Pedro Cardozo , Tannuri Eduardo Aoun
Published: November 2011

The implementation of active absorption of irregular waves in USP Active Absorption Basin is described. The algorithm control is based on the method proposed by Scha¨ffer. A digital filter is applied as time realization of a frequency domain absorption transfer function using the measurement of surface elevation at each wavemaker as the hydrodynamic feedback. Motor dynamics and delay due to signals time travel were compensated to increase the absorption efficiency. Experiments with regular and irregular waves were carried out, where the results show a good agreement between predicted theoretical reflection coefficient and experimental values. Reflection coefficient smaller than 10% were obtained for waves in the frequency range of 0.4 to 1.1 Hz.
Structural Health Monitoring (SHM) of Offshore Jacket Platforms
Authors: Lotfollahi-Yaghin Mohammad Ali , Shahverdi Sajad , Tarinejad Reza , Asgarian Behrouz
Published: November 2011

In the present paper, Structural health monitoring has become an evolving area of research in last few decades with increasing need of online monitoring the health of large structures. The damage detection by visual inspection of the structure can prove impractical, expensive and ineffective in case of large structures like offshore platforms, multistoried buildings and bridges. Structural health monitoring is defined as the process of detecting damage in a structural system. Damage in the system causes a change in dynamic properties of a system. The structural damage is typically a local phenomenon, which tends to be captured by higher frequency signals. Most of vibration-based damage detection methods require the modal properties that are obtained from measured signals through the system identification techniques. However, the modal properties such as natural frequencies and mode shapes are not such a good sensitive indication of structural damage. Structural damage detection and damage localization of jacket platforms, based on wavelet packet transforms is presented in this paper. Dynamic signals measured from the structure by the finite element software package ANSYS are first decomposed into wavelet packet components. Component energies are then calculated and used for damage assessment. The results show that the WPT-based component energies are good candidate indices that are sensitive to structural damage. These component energies can be used for damage assessment including identifying damage occurrence and location.
Modelling Risers With Partial Strake Coverage
Authors: Resvanis Themistocles L. , Vandiver J. Kim
Published: November 2011

This paper compares model test data for risers with partial strake coverage to predictions made with the Vortex-Induced Vibration (VIV) prediction program SHEAR7 Versions 4.5 and 4.6. It is shown that new features in Version 4.6 substantially enhance the capability for predicting the VIV response of risers with partial coverage. Experimental data is taken from two large L/D tests: the NDP 38 m long riser tests and the Deepstar-Miami, 500 foot long riser. New methods are described for modeling risers partially covered with helical strakes. Key SHEAR7 parameters are recommended, based on parametric investigations to calibrate the model against the available experimental data. Recommended modeling procedures are described so as to facilitate implementation by SHEAR7 users in their VIV modeling tasks.
Design and Efficiency Analysis of a Mechanical Wave Energy Converter
Authors: Chandrasekaran Srinivasan , Harender
Published: November 2011

Wave energy is the most promising natural energy resource that is gaining momentum in the recent years. Successful attempts are made by several researchers to harness wave energy by heave, surge and sway motion of the devices; however no successful commercial model is launched till date leaving this domain as a research potential. Among the proposed technologies, point absorbers are found to be commercially viable to a greater extent primarily due to its simplicity. The present study discusses a new mechanical wave energy converter (MWEC) using point absorber as a wave energy capturing device. Heave motion of a floating buoy due to incident wave field is harnessed to produce power. The conversion takes place in four different stages namely: i) motion of a gear rack, attached to floating buoy results in heave motion; ii) this vertical reciprocating motion is converted to oscillatory rotation of a shaft by a rack and pinion arrangement; iii) alternative rotary motion is converted in to continuous unidirectional rotation using a unidirectional chain assembly; and iv) unidirectional rotation is converted in to other usable energy form. MWEC employs numerous operating advantages over other systems such as: (i) the rack and pinion gear arrangement enabling the buoy to float in line to the changing water levels automatically. (ii) use of RPM multiplier enables rotation of generator shaft at high RPM even for small displacement of float; (iii) the free wheel sprockets of unidirectional chain assembly enable the gear rack to produce a positive upward stroke and a positive downward stroke for every passing wave impact. Further, rpm multiplier shall be easily adjusted to rotate the generator at desired rpm while the whole operation shall be shut down on emergency. The paper presents a detailed analysis of the mechanical system to arrive at the efficiency of the developed MWEC. Based on the studies conducted, it is seen that the overall efficiency of the MWEC is about 19% while maintaining maximum possible efficiency of the mechanical systems involved in the design.
Riser Analysis of a New Dry Completion Concept for a Monocolumn Platform
Authors: de Sousa Fernando Jorge Mendes , de Siqueira Marcos Queija , Dantas Claudio Marcio Silva , Sagrilo Lui´s Volnei Sudati , Masetti Isai´as Quaresma
Published: November 2011

In recent years, PETROBRAS has developed the concept of a monocolumn platform to be used as a floating production unit in deep and ultra-deep water oil fields. This platform, referred as MonoBR, is characterized by low heave and pitch induced motions, even in hurricane conditions, allowing its use as an alternative for locations such as Campos Basin and the Pre-Salt Area in Brazil, and also for the Gulf of Mexico. Despite their low vertical motions, this platform is not generally regarded as a dry completion unit. However, PETROBRAS and two public institutions in Brazil, UFRJ/COPPE and USP/POLI, have studied and designed an innovative mechanism to be installed in MonoBR, called Inverted Cup, which is a sliding floating body installed inside the MonoBR moonpool. In such way, the platform transmits only horizontal motions and rotations to the Inverted Cup; the heave responses of both bodies are decoupled. The Inverted Cup is then designed and dimensioned to receive dry completion X-mas trees and risers. The aim of this paper is to present the results of the riser analysis performed under this new concept — Inverted Cup system — to allow dry completion in the MonoBR. The environmental conditions refer to fields in the Gulf of Mexico and in the Tupi Field (Pre-Salt). The results point to a technically feasible mechanism to allow dry completion, according to adopted criteria.
Design of a Thermally-Actuated Gas Lift Safety Valve
Authors: Gilbertson Eric , Hover Franz , Arellano Jose , Freeman Bryan
Published: November 2011

Gas-lifted oil wells are susceptible to failure through malfunction of gas lift valve assemblies (GLV). One failure mode occurs when the GLV check valve fails and product passes into the well annulus, potentially reaching the wellhead. This is a growing concern as offshore wells are drilled thousands of meters below the ocean floor in extreme temperature and pressure conditions, and repair and monitoring become difficult. Currently no safeguard exists in the GLV to prevent product passage in the event of check valve failure. In this paper a design and operational procedures are proposed for a thermally-actuated positive-locking safety valve to seal the GLV in the event of check valve failure. A thermal model of the well and GLV system is developed and compared to well data to verify feasibility of a thermally-actuated safety valve. A 3× scale prototype safety valve is built and tested under simulated failure scenarios and well start-up scenarios. Realistic well temperatures in the range of 20C to 70C are used. Results demonstrate valve closure in response to simulated check valve failure and valve opening during simulated well start-up.
Statistics of Greater Sea States
Authors: Fullerton Anne M. , Fu Thomas C.
Published: November 2011

Accurate representations of seaway statistics are important for physical and computational predictions of ship motions. The spectra that are most typically used in these applications are the Pierson-Moskowitz or Bretschneider. While these spectra are useful for fully developed seas, the larger sea states (Sea State (SS) 7 and higher) are typically not fully developed. In these cases, other spectral models may be more appropriate. It is critical to ship motion prediction, for both physical and numerical models, to accurately capture the frequency range for the sea state of interest. Sea state statistics, including wave heights, periods, and spectral bandwidths from various buoys and a platform in the North Sea are collected and compared with statistics from lower sea states. The spectral data are then averaged to generate a typical spectrum under the measured conditions. These developed spectra are compared with the ideal spectra mentioned previously.
Development of Fatigue Testing of Full-Scale Girth Welded Pipes Under Variable Amplitude Loading
Authors: Zhang Yan-Hui , Maddox Stephen
Published: November 2011

In service the great majority of structures and components are subjected to stresses of variable amplitude (VA). The fatigue design of welded joints in such structures is based on fatigue data obtained under constant amplitude loading, used in conjunction with a cumulative damage rule to estimate the damage introduced by cycles of varying magnitude in the service stress history. There are two major concerns with fatigue design of deepwater steel catenary risers (SCRs): the validity of cumulative damage rule and the damaging effect of stresses below the constant amplitude fatigue limit (CAFL). It is known that SCRs can experience very high numbers of low stress cycles due to vortex induced vibration (VIV) with the result that the choice of method for accounting for the fatigue damage due to stresses below the constant amplitude fatigue limit can be highly significant in terms of the estimated fatigue life. These two fundamental issues have been addressed in a recent group sponsored project. By successfully establishing a loading spectrum representative of that experienced by risers and developing the testing procedures for VA loading using the resonance testing rigs, the fatigue performance of full-scale girth welded pipes under VA loading were investigated. These loading spectra had the same peak/maximum stress histograms but different minimum stresses. Many tests lasted over 108 cycles to investigate the fatigue damage of small stresses in these spectra. This paper describes the development of the method by which fatigue testing of full-scale girth welded pipes under VA loading had been successfully performed.
Optimization of Mooring Configuration Parameters of Floating Wave Energy Converters
Authors: Vicente Pedro C. , Falca˜o Anto´nio F. O. , Justino Paulo A. P.
Published: November 2011

Floating point absorbers devices are a large class of wave energy converters for deployment offshore, typically in water depths between 40 and 100m. As floating oil and gas platforms, the devices are subject to drift forces due to waves, currents and wind, and therefore have to be kept in place by a proper mooring system. Although similarities can be found between the energy converting systems and floating platforms, the mooring design requirements will have some important differences between them, one of them associated to the fact that, in the case of a wave energy converter, the mooring connections may significantly modify its energy absorption properties by interacting with its oscillations. It is therefore important to examine what might be the more suitable mooring design for wave energy devices, according to the converters specifications. When defining a mooring system for a device, several initial parameters have to be established, such as cable material and thickness, distance to the mooring point on the bottom, and which can influence the device performance in terms of motion, power output and survivability. Different parameters, for which acceptable intervals can be established, will represent different power absorptions, displacements from equilibrium position, load demands on the moorings and of course also different costs. The work presented here analyzes what might be, for wave energy converter floating point absorber, the optimal mooring configuration parameters, respecting certain pre-established acceptable intervals and using a time-domain model that takes into account the non-linearities introduced by the mooring system. Numerical results for the mooring forces demands and also motions and absorbed power, are presented for two different mooring configurations for a system consisting of a hemispherical buoy in regular waves and assuming a liner PTO.
Assessment of Ageing Structures: Case Studies
Authors: Landet Einar , Oma Narve , Ersdal Gerhard , Sigurdsson Gudfinnur , So̸rensen Trond
Published: November 2011

In all phases of the service life of a structure, uncertainties will occur, hence probability based methodologies may be an important and valuable tool in order to verify structural integrity and the corresponding expected safety level. This paper gives a brief description of an approach for how to demonstrate the safety of facilities in the life extension phase by the use of risk based inspection (RBI) plans. This is illustrated with four separate cases used for assessment and life extension of FPSOs operating in harsh environments on the Norwegian continental shelf (NCS). The assessments are partly based on the methodology introduced in NORSOK (2009).
Towards a General-Purpose Open Boundary Condition for Wave Simulations
Authors: Duz Bulent , Huijsmans Rene H. M. , Wellens Peter R. , Borsboom Mart J. A. , Veldman Arthur E. P.
Published: November 2011

For the design of FPSO’s in harsh environments an accurate assessment of the ability of the platform to survive in extreme sea conditions is of prime importance. Next to scaled model tests on the FPSO in waves also CFD capabilities are at the disposal of the designer. However even with the fastest computers available it is still a challenge to use CFD in the design stage because of the large computational resources they require. In that respect to use a small computational domain will improve the turn around time of the computations, however at the expense of various numerical artifacts, like reflection on artificial boundaries in the computational domain. In order to mitigate the reflection properties new absorbing boundary conditions have been developed. The work in this paper is constructed on the previous study about the generating and absorbing boundary condition (GABC) in the ComFLOW project. We present a method to apply the GABC on all the boundaries in a three dimensional domain. The implementation of the GABC in ComFLOW is explained in detail.
Full and Large Scale Wave Impact Tests for a Better Understanding of Sloshing: Results of the Sloshel Project
Authors: Bogaert Hannes , Kaminski Miroslaw Lech Mirek , Brosset Laurent
Published: November 2011

This paper outlines the progress made within the Sloshel User Group in the analysis of unidirectional breaking wave impacts on transverse walls in flume tanks at full (1:1) and large (1:6) scales. These tests, carried out during the Sloshel project, were intended to help understanding the physics of sloshing impacts in tanks of LNG carriers and Floating LNG terminals (FLNGs). Two test campaigns were performed at full scale involving respectively the NO96 and the MarkIII containment systems. The latter was performed recently (April 2010) and the analysis is in progress. This paper describes the physical phenomena observed during the impacts on the MarkIII containment system. This helps understanding the difficulties inherent to the scaling of sloshing pressures measured during model tests. The paper also shows how important local peak pressures are for the structural response of the MarkIII system (same kind of conclusion had already been demonstrated for NO96 in Brosset, Mravak, Kaminski, Collins and Finnigan, 2009). According to very preliminary analysis, reduction of impact pressures due to hydro-elasticity with the MarkIII containment system seems to be moderated if real.
Full Scale Measurement on Level Ice Resistance of Icebreaker
Authors: Suyuthi Abdillah , Leira Bernt J. , Riska Kaj
Published: November 2011

This paper presents results from the investigation of ship resistance on first year level ice in the Barent Sea. The basis for the work is the availability of full scale measurement data obtained on board of KV Svalbard in 2007. Measurements were made of the ice thickness, ship speed over ground in addition to setting power. The ice thickness was measured by means of an electromagnetic device, which enables careful selection of the time sequences for which level ice is present. By utilizing Newton II law and conservation of energy, the total resistance can be determined. The ice resistance in level ice was then obtained by subtracting the open-water resistance from the total resistance. The open-water resistance was measured when the ship was traveling in open water during the expedition. The relationship between the ship resistance and the ship speed over ground in level ice was finally obtained and compared with the Lindqvist formulation of estimation of ice resistance.
FPSO Operations on Dynamic Positioning
Authors: Tjallema Arjen , Brugts Hielke
Published: November 2011

As of March 2010, Bluewater’s FPSO Munin has been operating on Dynamic Positioning (DP) in the South China Sea. The station keeping is performed by the vessel’s DP system only; no anchor leg mooring is used. Offloading from the FPSO is done to conventional (non-DP) shuttle tankers. See Fig. 1. This means that the DP system has to deal with a relatively large external force, and that the tandem has to respond appropriately to changing environmental conditions. To prepare for this operation, extensive FMEA proving trials have been performed prior to start-up of production. Furthermore a bridge simulation program has been conducted in which the offloading operations have been trained. From the FMEA (Failure Mode and Effect Analysis) proving trials, the offloading simulations and operational experience, a lot of knowledge has been gained on this specific DP operation. This knowledge is not only applicable to this single operation, but can also be used to enhance many other DP operations. This paper describes the Munin’s unique DP operations and the lessons learned from the FMEA proving trials, the offloading simulation program and operations in the field.
Detecting the Phytoplankton Bloom From Satellite Images
Authors: Kuo Nan-Jung , Ho Chung-Ru , Huang Shih-Jen , Lo Yao-Tsai
Published: November 2011

Phytoplankton is the base of the marine food web. However, the phytoplankton bloom may deplete the dissolved oxygen and shade aquatic life, and even damages the marine environment. In this study, the satellite-derived sea surface chlorophyll-a concentration (Chl-a) and sea surface temperature (SST) are considered to examine the behavior of the phytoplankton bloom in the sea around the western side of the Luzon Strait in the northeastern South China Sea (SCS). Meanwhile, the related sea surface wind (SSW) is also included to understand the possible mechanism to induce the high Chl-a plume around there. The results indicate that the clear high Chl-a water is mainly around the northeastern SCS in winter, and spreads out westward from the northern edge of the Luzon island, Philippines. The external force of this blooming is probably from monsoon wind driving, the strong and diverse winter monsoon in the Luzon Strait can develop a very clear positive wind stress curl in the northwestern side of Luzon island, and then induce the cold and high nutrient water upward. Meanwhile, the strong winter monsoon can also spread this cold water plume out to develop a very clear and large phytoplankton blooming around there.
On the Structural Response of a Flexible Pipe With Damaged Tensile Armor Wires
Authors: de Sousa Jose´ Renato M. , Campello George C. , Bueno Antoˆnio Fernando B. , Vardaro Eduardo , Ellwanger Gilberto B. , Strohaecker Telmo R.
Published: November 2011

This paper studies the structural response of a 6.0″ flexible pipe under pure tension considering two different situations: the pipe is intact or has five wires broken in its outer tensile armor. A three-dimensional nonlinear finite element model devoted to analyze the local mechanical response of flexible pipes is employed in this study. This model is capable of representing each wire of the tensile armors and, therefore, localized defects, including total rupture, may be adequately represented. Results from experimental tests are also presented in order to validate the theoretical estimations. The theoretical and experimental results indicate that the imposed damage reduced the axial stiffness of the pipe. High stress concentrations in the wires near the damaged ones were also observed and, furthermore, the stresses in the inner carcass and the pressure armor are affected by the imposed damage, but, on the other hand, the normal stresses in the wires of the inner tensile armor are not.
Marine Scour and Offshore Wind: Lessons Learnt and Future Challenges
Authors: Harris John M. , Whitehouse Richard J. S. , Sutherland James
Published: November 2011

The drive for developing marine offshore renewables has led to specific requirements for scour hazard assessment relating to the associated foundation structures and the cabling necessary for in-field transmission and power export. To date within the United Kingdom (UK) a number of demonstrator projects have been constructed covering wind, wave and tidal generation. However, only offshore wind has been developed at large-scale at present as part of two rounds of commercial development of offshore wind farms (OWFs). In June 2008, The Crown Estate, responsible for licensing seabed use, announced proposals for a third round of offshore wind farms to develop an additional 25 GW of energy to the 8 GW already planned for under Rounds 1 and 2. The size of these Round 3 developments will vary, but the largest of these zones will involve the construction of around 2500 seabed foundation structures. Under Round 1 and 2 developments monopile and jacket type foundations have been used, although several other European (non UK) wind farms have been built using gravity base foundations. For a wind turbine the foundations may account for up to 35% of the installed cost. Therefore, one of the future challenges for large volume installation of offshore wind is the control and minimization of these costs. For tidal energy devices one of the principal requirements for many of the devices proposed is their placement in areas of strong tidal energy, and this has implications not only for the stability of the foundation option, but also for the construction methodology. Similarly wave energy devices are designed to be located in shallow, coastal environments as either floating or bottom mounted systems. These devices, by design, are intended to be located in environments with strong wave action. This may be substantial during storm events, which has implications for the integrity of the anchoring system keeping the wave device on station or the design of the device if it is seabed mounted. This paper will explore the lessons learnt from existing offshore wind farm developments as this represents the principal body of collected monitoring data. Using these data the paper will outline some of the challenges facing the offshore renewable industry in respect of the foundation designs and specifically the requirements for scour hazard assessment using the combined experience from those developments currently operational or under construction.
Decentralised Control Design for Load Mitigation in Horizontal Axis Wind Turbines (HAWTS)
Authors: Sandquist Fredrik , Moe Geir , Anaya-Lara Olimpo
Published: November 2011

In modern MW-size machines it has become a common practice to introduce controllers that provide active damping of turbine components to reduce blade, tower and drive-train loads, whilst optimising energy capture. However, as wind turbines become larger and more flexible, these controllers have to be designed with great care as the coupling between flexible modes increases and so does the potential to destabilise the turbine. The most direct method to address the above issues has been to exploit the pitch control capabilities. Individual Pitch Control (IPC) has been proposed many times over the last few years for load mitigation. Bearing this in mind, this paper investigates two different approaches to design a controller to pitch each blade individually in the wind turbine operating region III. The first one is a decentralised control algorithm and the second one is an H∞ loop shaping design. A controllability analysis of the wind turbine is also included in the paper. The investigation is conducted based on the NREL 5MW benchmark wind turbine. Turbine modeling and control is conducted in FAST and Simulink.
Vibration Assessment Methodology for Subsea Pipework
Authors: Chakkarapani Venkat , Nair Aravind , Whooley Alan , Chauvet Christian , Eltaher Ayman , Jukes Paul
Published: November 2011

Vibration is an important cause of fatigue damage in subsea piping systems (jumpers, rigid spools, tree pipework etc.). Presently, guidelines are available for a qualitative and quantitative vibration assessment for process pipework, but these are more applicable to topside and refinery piping. Currently, there are limited guidelines specifically developed for vibration assessment for subsea piping. For subsea pipework, additional guidelines may be necessary to check for possible interaction between current induced VIV, wave induced VIV (for shallow waters) and slugging, and other mechanisms that result in flow induced vibration. This paper provides augmented guidelines for subsea piping, which takes into account the interaction between piping and its subsea environment. At the same time, suggestions are provided to trim the guidelines of mechanisms that does not occur subsea. The Likelihood of Failure (LOF) assessment based on the proposed guidelines provides an indication whether further detailed vibration analysis (such as CFD and/or FEA) is necessary. Based on the LOF from this assessment, suitable detailed analysis can be performed to determine the fatigue life. This enhanced methodology enables the user to perform vibration assessment and analysis in the least computational time by identifying 1) the most critical flow conditions over the production life of the field and 2) the most critical vibration mechanisms that should be analyzed. This results in a reduction in the number of conditions or load cases which require detailed analysis, without decreasing the accuracy of the assessment and analysis.
The Effect of Partial Drainage on Measurements by a Piezoball Penetrometer
Authors: Mahmoodzadeh Hamed , Boylan Noel , Randolph Mark , Cassidy Mark
Published: November 2011

Full-flow penetrometers such as the T-bar and ball are now widely used in offshore site investigations to measure penetration resistance and then infer the strength properties of the seabed. To provide additional data on flow properties of the soil and increase the number of parameters that can be obtained from these tests, ball penetrometers fitted with pore pressure transducers, known as piezoballs, are now being used. Depending on the soil characteristics and rate of penetration, the soil conditions during penetration may vary from undrained to fully drained. The drainage condition during penetration will influence both the measured penetration resistance and excess pore pressure response and therefore influence any parameters interpreted. This paper examines the effect of drainage condition on piezoball measurements during penetration in a muddy silt sample collected from offshore Australia. Tests were conducted in the beam centrifuge at the University of Western Australia using a miniature piezoball, with pore pressure measurement at the equator position. The effect of drainage condition was examined by conducting tests at various rates of penetration, ranging from undrained to fully drained conditions. Methods of interpreting the results of a partially drained penetration test in order to deduce the undrained penetration resistance are also discussed.
Quantitative Risk Analysis and Comparison for Onshore and Offshore LNG Terminals: The Port of Suape - Brazil Case
Authors: Ramos Marilia A. , Droguett Enrique L. , Martins Marcelo R. , Souza Henrique P.
Published: November 2011

In recent decades, natural gas has been gaining importance in world energy scene and established itself as an important source of energy. One of the biggest obstacles to increase the usage of natural gas is its transportation, mostly done in its liquid form, LNG – Liquefied Natural Gas, and storage. It involves the liquefaction of natural gas, transport by ship, its storage and subsequent regasification, in order to get natural gas in its original form and send it to the final destination through natural gas pipeline system. Nowadays, most terminals for receiving, storing and regasificating LNG, as well as sending-out natural gas are built onshore. These terminals, however, are normally built close to populated areas, where consuming centers can be found, creating safety risks to the population nearby. Apart from possible damages caused by its cryogenic temperatures, LNG spills are associated with hazards such as pool fires and ignition of drifting vapor clouds. Alternatively to onshore terminals, there are currently several offshore terminals projects in the world and some are already running. Today, Brazil owns two FSRU (Floating Storage and Regasification Unit) type offshore terminals, one in Guanabara Bay, Rio de Janeiro and the other in Pece´m, Ceara´, both contracted to PETROBRAS. The identification of the operation risks sources of LNG terminals onshore and offshore and its quantification through mathematical models can identify the most suitable terminal type for a particular location. In order to identify and compare the risks suggested by onshore and offshore LNG terminals, we have taken the example of the Suape Port and its Industrial Complex, located in Pernambuco, Brazil, which is a promising location for the installation of a LNG terminal. The present work has focused on calculating the distance to the LNG vapor cloud with the lower flammability limits (LFL), as well as thermal radiation emitted by pool fire, in case of a LNG spill from an onshore and from an offshore terminal. The calculation was made for both day and night periods, and for three types of events: operational accident, non-operational accident and worst case event, corresponding to a hole size of 0,75m, 1,5m e 5m, respectively. Even though the accidents that happen at an onshore terminal generate smaller vulnerability distances, according to the results it would not be desirable for the Suape Port, due to the location high density of industries and people working. Therefore, an offshore terminal would be more desirable, since it presents less risk to the surrounding populations, as well as for workers in this location.
Fatigue of High Quality Girth Welds
Authors: O¨rjasaeter Oddvin , Aas Sigmund , Haagensen Per J. , Tveiten Ba˚rd Wathne
Published: November 2011

Several studies on fatigue strength of high quality girth welds are summarized and discussed. The fatigue performance of such welds is consistently above the common design classes, as long as key set of influencing parameters are controlled. Fatigue life and crack initiation depend on loading mode, and weld defects, weld geometry, residual stresses, and degree of weld distortions. The welding method, especially for the root, has also proven to be important, e.g. TIG and high quality STT are often superior to Cu-backed roots. In practice the most important factor will be the surface breaking flaws as root LOP, etc. Such flaws are rarely found at the cap side. Thick pipe walls will also reduce the fatigue capacity, e.g. a 45mm wall thickness with 25mm reference, will reduce the with the same amount as above. High fatigue performance requires absence of any weld discontinuity above certain critical sizes which may be a challenge for the accuracy and resolution of NDT systems. In the present study, some important factors that influence the fatigue strength are examined. Based on results and theoretical calculations, the effects of the various crack-like discontinuities are described and compared to current design standards. The geometrical misalignment of the joint (hi/lo) will also influence the fatigue capacity. In bending, the weld cap toe and the weld root are the critical locations. The residual stress distribution in the welded region may, however, alter this. With pipe wall thickness larger than ∼25mm residual stresses can be beneficial to the weld root area. However, due to the scatter in the measurements it is difficult to assess the influence on fatigue life exactly. FE modeling of the welding process is therefore used to supplement this discussion. Improvement methods are available to suppress the critical influence of surface cracks/discontinuities. Grinding of the cap weld toe has in many cases shown significant improvements of girth welds. Also, TIG-dressing, high quality STT, or fillers with high nickel content, have potential for improving the root performance.
Time Domain Comparison With Experiments for Ship Motions and Structural Loads on a Container Ship in Abnormal Waves
Authors: Rajendran Suresh , Fonseca Nuno , Guedes Soares C. , Clauss Gu¨nther F. , Klein Marco
Published: November 2011

The paper presents experimental results from model tests with a containership advancing in abnormal wave conditions and comparisons with numerical simulations. A nonlinear time domain method based on strip theory is used for the calculation of vertical ship responses induced by abnormal waves. This code combines the linear diffraction and radiation forces with dominant nonlinear forces associated with vertical response arising from Froude-Krylov forces, hydrostatic forces and shipping of green water. The time domain simulations are compared directly with experimental records from tests with a model of a container ship in deterministic waves for a range of Froude numbers. Extreme sea conditions were replicated by the reproduction of realistic abnormal waves like the New Year Wave and abnormal wave from North Alwyn. Head sea condition is considered and the comparisons include the wave elevation, the vertical motions of the ship and the vertical bending moment at midship.