bluuebunny/crossref_metadata_2025 huggingface dataset:
Viscous Drift Forces on Semis in Irregular Seas: A Frequency Domain Approach
Authors: Lie Halvor , Kaasen Karl E.
Published: July 2009

For offshore floating structures, such as semi-submersible platforms, TLPs and SPARs viscous drag forces on columns and truss work may give significant contributions to the low frequency loads and responses. Calculating such loads based on just potential theory, which is common, can be significantly insufficient. The viscous loads depend nonlinearly on waves and current in combination. In particular, correct low frequency loading is important in the design of mooring systems for offshore structures. The paper describes some intermediate results from a project aiming at formulating dynamic frequency domain models for low frequency viscous excitation and damping of floating structures subjected to current and irregular waves. The basis for the models is the drag term in the Morison equation. Different models are formulated for the submerged parts of the structure and the splash zones. At present basic models are formulated for a narrow submerged strip of a vertical cylinder and the splash zone of the same. The loads are quantified in terms of power spectral density functions. To be able to formulate expressions for the power spectra the Morison drag force model is approximated by simple polynomials in the water speed. For the submerged strip an alternative method exists, that uses a series expansion for the force spectrum.
Wave and Current Induced Cumulative Fatigue Damage Assessment for Offshore Pipeline on Free-Span
Authors: Valipour Ali , Zoroufi Mehrdad , Yeganeh Bakhtiary Abbas , Valipour Reza
Published: July 2009

Suspending pipelines between two points over an uneven seafloor is called free-spanning which induces more vortices behind the pipe especially when the pipelines are exposed to the hydrodynamic effects of waves and currents. These influences cause a significant increase in the dynamic acting drag forces. In fact, the oscillatory acting drag forces, as a result of changes in the net pressure and velocity filed around the pipeline, increase the probability of fatigue damage. Since the fatigue life of offshore pipelines has a major influence on the useful life of a project, studying it for offshore pipelines laid on uneven sea beds has become very important. Both the consequence of vortex shedding as a result of wave and current interaction with the pipeline, and the selected suitable approach in assessing fatigue damage has made the design process sophisticated. In the present study, it is attempted to employ a proper finite element model with the capability of coupling the dynamic drag forces induced by wave and current conditions with estimating cumulative fatigue damage methods. The wave profile is simulated using Non-deterministic Spectral Amplitude in Gaussian sea state and using linear wave formulation to calculate wave -induced forces. Moreover, the current force is estimated by using the conventional Morison Equation as well as employing a new approach of estimating current -induced dynamic drag forces obtained by a numerical model that includes the VIV effects. Finally cumulative fatigue damage as a result of acting those forces is investigated for various conditions of laying pipeline on the seabed.
Interaction of Solitary Waves With a Group of Dual Porous Circular Cylinders
Authors: Sankarbabu K. , Sannasiraj S. A. , Sundar V.
Published: July 2009

Diffraction of solitary waves around a group of dual porous cylinders consisting of a thin and porous outer cylinder circumscribing an impermeable inner cylinder is investigated analytically based on the eigenfunction expansion method. The present formulation is an extension of the work of Zhong and Wang (2006), wherein; the interaction of solitary waves with a single concentric porous cylinder system was studied. This paper aims at investigating the influence of multiple interactions between the cylinders in a group on the hydrodynamic wave forces, wave run-up and free-surface elevation in their vicinity. Further, the study focuses on the variation of the forces on the individual cylinders within the group compared to that on isolated cylinders.
Non Linear Response of Fixed Offshore Platforms to Seismic Excitation Including Soil-Pile-Structure Interaction
Authors: Zargar Ehssan , Aghakouchak Ali Akbar , Aghakouchak Amin
Published: July 2009

A nonlinear seismic soil-pile-structure interaction (SSPSI) analysis of fixed offshore platforms constructed on pile foundations including both vertical and battered piles is presented. The analysis is carried out in time domain and the effects of soil nonlinearity, discontinuity at pile soil interfaces, energy dissipation through soil radiation damping, formation of soil layers on bed rock, structural material nonlinearity and geometrical nonlinearity are considered. A combination of FEM approach and BNWF approach is used in modeling pile (substructure), platform structure (superstructure) and soil media. Gapping in clay is modeled by a special connector configuration. To find out the ground motion of soil layers caused by earthquake excitations at bed rock, a nonlinear site response analysis is performed. The effects of soil-pile-structure interaction on nonlinear seismic analysis of offshore platforms are discussed. It is generally concluded that considering soil-pile-structure interaction causes higher deflections and lower stresses in the platform elements due to soil flexibility, nonlinearity and radiation damping and leads to a more feasible and realistic platform design. The sequence of generation of plastic zones in the structure and their distribution are also investigated. Sensitivity of results to soil layers configuration and soil material damping ratio are discussed.
Experimental Study of Sloshing Effects in Roll Motion of Floating Units
Authors: de Medeiros Higor Felipe , Rueda Silva Guilherme Espan˜a , Cheng Liang-Yee , Tsukamoto Marcio Michiharu , Nishimoto Kazuo
Published: July 2009

Usually, the hydrodynamic loads due to sloshing are considered in the design of liquid cargo ship or floating units concerning the structural. Owing to the increasing size of these structures, resonant sloshing motions may occur and result in the amplification or attenuation of motion of the vessel. In order to assess the effect of sloshing, traditionally the motion of the vessel is calculated at first without considering the dynamic of the liquid inside the tank. After that, this motion of the vessel is inputted as excitation motion acting on the tank and, finally, the sloshing effect is evaluated. In the other words, the coupling effects of sloshing and sea wave in the vessel’s motion are ignored. A bibliographical survey shows that there are few studies that consider the effect of sloshing on the ship motion, acting as a passive device of absorption of the movements. The main goal of this research is to investigate experimentally the roll motion amplification and reduction due to sloshing. The coupling effects of sloshing and sea wave in the vessel’s motion are taking into account by recording the motions, in regular waves, of a free floating model with a partially filled liquid tank. For this purpose, a two-dimensional model is designed to carry out measurements with fixed cargo and partially filled liquid cargo. The experimental results are evaluated by comparing the measured motion of the free-floating model with fixed cargo against the results obtained by traditional approach. Then, the effects of sloshing on floating units are shown by comparison of the measurements from free-floating model with fixed cargo and liquid cargo. The results shown herein provide data for the validation of new numerical approaches for the study of the coupled motions of the floating units and sloshing.
Simulation of Second-Order Roll Motions of a FPSO
Authors: Rezende Fla´via , Chen Xiao-Bo , Ferreira Marcos D.
Published: July 2009

The roll motions are a key parameter on the design of FPSOs that operate in moderate and severe environmental conditions. To reduce the magnitude of roll motions, some techniques based on changing the vertical position of gravity center are used to put the roll natural period outside of the frequency range of the linear waves. However, recent model tests and also numerical calculations have shown that the vessel may still experience large roll motions which are considered to be induced by second-order wave loads. Further to the work in Rezende et al. (2007) to compute the roll response in frequency domain, new developments to perform simulations in time domain are presented here. In this new method, variations of second-order roll moments dependent on the roll and heave motions are taken into account consistently. It is shown that, unlike the horizontal loads, the quadratic transfer functions of the vertical loads depend on the instantaneous position of the vessel. The variation of the roll moment with the heave position of the vessel has been considered more important than the variation obtained only with the inclination of the vessel. Furthermore, numerical results of roll simulations are compared with model tests results and presented in the paper.
The Need to Understand the Dynamic Interaction Between Wellbore and Reservoir in Liquid Loaded Gas Wells
Authors: Solomon Fouad A. , Falcone Gioia , Teodoriu Catalin
Published: July 2009

Liquid loading in gas wells is a phenomenon where the liquid content of the well is sufficient to create a back pressure (usually dominated by gravitational pressure changes) which restricts, and in some cases even stops, the flow of gas from the reservoir. Liquid loading is an all too common problem in mature gas fields around the world. It is estimated that in the U.S.A. alone, at least 90% of the producing gas wells are operating in liquid loading regime. The phenomenon is more detrimental in tight wells than in prolific wells and it poses a serious problem in subsea tie-backs, where back pressure effects through the risers and the flowlines may have an important role. Such is the importance of liquid loading; the oil and gas industry has devoted a lot of attention to the alleviation of the problem using various measures. However, the fundamental understanding of the associated phenomena is still surprisingly weak. This applies not only to the flows in the wells, but also to how these flows interact with those in the reservoir. It is this latter dynamic interaction that has received the least attention by the industry. Reliable predictive models to link the well dynamics with the intermittent response of a reservoir, that is typical of liquid loading in gas wells, remain unavailable. This paper introduces the complexity of liquid loading and critically reviews recent attempts to model liquid loading and the dynamic interactions between reservoir and wellbore. The paper then illustrates the need for a better understanding of the transient flow phenomena taking place in the near-wellbore region of the reservoir. This includes re-injection of the heavier phase, a phenomenon that has yet to be proven by fluid mechanics.
Investigation of Drilling Problems in Gas Hydrate Formations
Authors: Teodoriu Catalin , Falcone Gioia , Afolabi Amodu
Published: July 2009

Gas hydrates are ice-like crystalline systems made of water and methane that are stable under high pressure and low temperature conditions. Gas hydrates have been identified as strategic resources and may surpass all known oil and gas reserves combined. However, these resources will become reserves only if the gas contained therein can be produced economically. In the oil and gas industry, gas hydrates may be encountered while drilling sediments of the subsea continental slopes and in the subsurface of permafrost regions. They also represent a flow assurance issue, as they may form in the well and in the flowlines, causing blockages. Deepwater drilling programmes have experienced problems when encountering gas hydrate formations. A major issue is that of phase transition, where gas hydrate goes from a solid state to dissociated gas and water, as there are rapid changes in fluid volumes and pressure. This can cause drilling equipment failure, borehole instability and formation collapse. After dissociation of water and gas, hydrates may be prevented from forming in the well by using appropriate inhibitors in the drilling mud. There is a need to develop fluids specifically for drilling through gas hydrate formations, either to unlock the unconventional reserves trapped in the crystalline gas hydrate structures or to safely reach underlying conventional reserves. To drill wells in a gas hydrate formation, a conductor casing is needed to allow close loop circulation of the mud, if different from seawater. The search for the ideal mud for drilling through gas hydrate formations must start with a review of past experiences worldwide and of the lessons learned. This paper presents a review of the problems encountered while drilling through gas hydrate formations. It identifies the key requirements for drilling fluids, based on the interaction between the drill bit, the drilling fluid and the formation. An evaluation of the environmental risk associated with drilling through gas hydrate formations is also presented.
Effect of Polyester Rope Damping on Behavior of Offshore Floating Units
Authors: Rossi Ronaldo Ross , Fernandes Antonio Carlos , Novaes Gilson L. , Zanutto Jose Carlos
Published: July 2009

Today the use of polyester ropes for mooring floating units in deepwaters is increasing around the world especially in Campos Basin, offshore Brazil. Several papers pointed out the advantages of polyester ropes in deepwaters. However the effect of damping of the material is not completely studied. The aim of this paper is to evaluate the effect of polyester rope damping on behavior of offshore floating units. The hysteresis cycles have been studied using dynamic tests of long duration. There are two types of tests: variable and constant mean tension tests. For evaluating the energy dissipated in each cycle software for calculating the hysteresis cycle area was developed. Using dimensional analysis it is proposed a model that defines the behavior of line damping and using this model and simulation of a FPSO unit the line damping is calculated.
Two Dry Tree Semisubmersible Designs for Ultra Deep Water Post-Katrina Gulf of Mexico
Authors: Murray John J. , Yang Chan K. , Chen Cheng-Yo , Nah Edwin
Published: July 2009

The increased interest in floater designs for ultra-deep water has produced a number of dry tree semisubmersible designs that allow the use of top tensioned risers (TTRs). The primary advantage of the TTRs is that they facilitate direct vertical access to production wells and thereby offer access for well completions and interventions. The principle behind several of the dry tree semisubmersible designs is to reduce the motions of a traditional shaped semisubmersible to a level that can accommodate TTRs. This is accomplished by using heave plates that are positioned beneath the semisubmersible hull and are supported by a lower structure, such as a truss. To a certain extent, the motion responses above and below the response amplitude operator (RAO) cancellation period can be manipulated by designing the relative sizes of the pontoons and columns as well as the size and depth of the heave plates such that the sum of the forces interacts to minimize the heave motion. The paper presents and discusses two semisubmersible designs that assume a common topsides and riser payload. The two designs are sized and analyzed for the new Gulf of Mexico metocean criteria. The comparison is based on hull dimensions, including heave plate and structural support construction. In both cases, results of the hull performance predicted by numerical simulations from fully coupled models are compared and discussed.
Experimental and Numerical Evaluation of the Installation of Sub-Sea Equipments for Risers Support
Authors: Fujarra Andre´ L. C. , Tannuri Eduardo A. , Masetti Isai´as Q. , Igreja Haroldo
Published: July 2009

The installations of sub-sea equipments are very complex operations, requiring previous analysis in order to define the correct procedure and the environmental “window” for a safe operation. This paper addresses the installations of a Mid Water Arch (MWA) that consists of a structure to provide risers support. Connecting the risers to the MWA largely eliminates the dynamic forces that would otherwise cause friction and fatigue. Such structure is composed by the riser guides and several buoyancy tanks. It is kept in the water by means of tethers connected to an anchor. The MWA is to be installed 42m from the seabed. The installation procedure has several steps evolving the launching of each component of the MWA (anchor, main structure and the tethers). A tug boat with an A-frame is used during the whole launching, and an assisting vessel is required to keep the buoy away from the tether and the launching cable. The presence of exciting waves induces oscillatory motions in the whole system, and may cause large dynamic forces in the cables and tethers. Due to the complexity of the multi-body system, a comprehensive numerical and experimental analysis was then carried out in order to dimensioning the launching cables and to define the limit environmental condition. The numerical analysis was carried out in the Numerical Offshore Tank (TPN), a multi-processor offshore system simulator that considers the 6 DOF of each body and all environmental forces acting in them. The lines are modeled by finite-element method. Furthermore, a full set of small-scale experiments were carried out at the State of Sa˜o Paulo Technological Research Institute (IPT) towing tank, considering the system excited by a sinusoidal motion at the top, emulating the wave excitation. Comparisons between numerical and experimental results were performed, with good adherence between them. The validated numerical simulator was then used to make predictions of the behavior of the systems during the installation, considering several environmental conditions and configurations.
A Generic Method to Model Frequency-Direction Wave Spectra for FPSO Motions
Authors: van Zutphen Hermione J. , Jonathan Philip , Ewans Kevin C.
Published: July 2009

We report a new approach to model the frequency-direction spectrum, in which the frequency-direction spectra from measurements or hindcast studies are fitted simultaneously in two dimensions, frequency and direction. Depending on the amount of wind forcing on the partition, either a unimodal (swell) or bimodal (wind-sea) wave spreading function is adopted together with the spectral form which best fits the frequency spectrum. This paper describes the new method and presents the results on a measured dataset.
Finite Element Simulations and Experimental Investigations of Simple 2-D Geometries in Slamming
Authors: Constantinescu Adrian , Neme Alain , Jacques Nicolas , Rigo Philippe
Published: July 2009

This paper presents a numerical and experimental study of fluid structure interaction during the impact of a solid body on a water surface. The main request is the modeling of the slamming forces acting on the ship structure in severe sea conditions. The numerical work uses the finite element modeling of a structure impact with free water surface. The first analysis use the commercial finite element code ABAQUS/Standard and combines the assumption of small displacements for the ideal fluid and the solid with an asymptotic formulation for accurate pressure evaluation on the boundary of the wet surface. For deformable strickers, two methods are developed. The first method employs a weak fluid-structure coupling. The second method, more accurate, uses an implicit fluid-structure coupling using a convergence criterion. The second analysis is represented by the simulations of slamming with ABAQUS/Explicit. The simulation uses a viscous, compressible fluid and a soft-exponential law to manage the contact between fluid and solid. The results in term of pressure and total effort applied to the rigid structure are in good agreement with first numerical results and especially with the FLUENT CFD. In order to validate the numerical methods, slamming experimental tests were carried out with a new hydraulic shock press at the ENSIETA laboratory.
The Transient Effect of Tendon Disconnection on the Global Motion of ETLP
Authors: Yang Chan K. , Padmanabhan Balakrishna , Murray John , Kim M. H.
Published: July 2009

This paper deals with a numerical study of the transient effect of tendon disconnection on global performance of ETLP for harsh environmental condition in GoM (Gulf of Mexico). Twelve tendons support the platform with twelve production TTRs and one drilling riser attached by hydro-pneumatic tensioner. Charm3D, a program for global motion analysis of multiple floating hulls coupled with risers/mooring lines, is made to be capable of modeling the tendon disconnection at both top and bottom connection. The study includes the break due to high tension at the top and the unlatch due to negative tension at the bottom. A sudden disconnection of one or more tendons causes the unbalance of force and moment of the total system, only to cause the transient motion and tension as well as the mean offset. The breakage and the unlatch also make the different effects. The transient responses and the mean offsets are compared and discussed in the viewpoint of the safety of the system.
Design of the World’s 1st Gravity Actuated Pipe (GAP) for Murphy’s Kikeh Deepwater Development, East Malaysia
Authors: Lemoe¨l Marc , Brown Paul , Jean Philippe , Shepheard Barry
Published: July 2009

MURPHY Sabah Oil Co. Ltd. has developed the Kikeh Field located offshore Malaysia in the South China Sea in a water depth of 1325m. This field development is based on a Floating Production Storage and Offloading unit (FPSO) and a Spar Dry Tree Unit (DTU). Fluids are transported in fluid transfer lines (FTL) using SBM’s newly developed and patented Gravity Actuated Pipe (GAP) system. The GAP principle is based on a large diameter carrier pipe supporting a number of flow lines and a power/control umbilical. The carrier pipe serves as a structural support for the flow lines and umbilical. The flow lines are attached to the carrier pipe by supports allowing movement in the axial direction. The carrier pipe terminates at each end with a towhead, which channels the forces into the carrier pipe and supports flexible jumpers between the flowlines and the floating facilities. Tether chains are suspended from the floaters to support each end of the carrier pipe. Clump weights and buoyancy are mounted at the ends of the carrier pipe to generate vertical load. The action of the clump weights and buoyancy on the angled chains keeps the carrier pipe under tension. This tension minimizes the hog/sag deflections of the pipe bundle and keeps the system within acceptable limits. This paper presents a technical overview of the Kikeh FTL GAP system. Several challenging engineering and design aspects and their solutions are presented. Design of the GAP sub-components is highly interactive requiring significant hydrodynamic and structural analysis by experienced personnel. The Kikeh FTL GAP project commenced fabrication in 1st quarter 2006, and completed installation and achieved first oil in August 2007.
Model Testing the 3D Dynamic Behaviour of the GAP System and Comparison With Numerical Computations
Authors: Sergent Emmanuel , Thibault Bruno , Lenoble Alexis
Published: July 2009

MURPHY Sabah Oil Co. Ltd. is developing the Kikeh Field located offshore Malaysia in the South China Sea in a water depth of 1325m. This field development is based on a Floating Production Storage and Offloading unit (FPSO) and a wellhead Spar Dry Tree Unit (DTU). Fluids will be transported in fluid transfer lines (FTL) using SBM’s newly developed and patented Gravity Actuated Pipe (GAP) system to support a horizontal neutrally buoyant bundle of flow lines between two moored floaters. Fatigue and extreme analyses of the GAP system have been carried out with a fully dynamic numerical model. In order to verify the validity of the model, three dimensional tests have been performed by OCEANIDE in IFREMER basin in Brest (France). The complete GAP system was modelled at 1:37 scale by OCEANIDE (France). The model included the GAP bundle (carrier pipe with four flow-lines and one umbilical), two towheads located at each extremity of the bundle, a tether chain arrangement supporting the GAP bundle at its two attachments on the FPSO and the DTU. The paper will cover the salient results of the model test campaign. A comparison between tests-results and numerical simulations will also be presented. Furthermore, the occurrence of such dynamical behaviours as Heave Induced Lateral Motion (HILM) will be investigated.
Vortex Induced Vibration of the GAP Bundle: From Selection of Flow Lines Arrangement to VIV Fatigue Analysis
Authors: Sergent Emmanuel , De Wilde Jaap , Harrisson Richard , Helle Yann
Published: July 2009

MURPHY Sabah Oil Co. Ltd. is developing the Kikeh Field located offshore Malaysia in the South China Sea in a water depth of 1325m. This field development is based on a Floating Production Storage and Offloading unit (FPSO) and a Spar Dry Tree Unit (DTU). Fluids will be transported in fluid transfer lines (FTL) using SBM’s newly developed and patented Gravity Actuated Pipe (GAP) system to support a horizontal neutrally buoyant bundle of flow lines between two moored floaters. A mid-water bundle, comprising a carrier pipe, four flow lines and an umbilical, in a transverse current may experience Vortex Induced Vibration (VIV). This is a well-known phenomenon for a circular cylinder. Much less is understood however for a bundle arrangement. The methodology put in place to assess risks of VIV will be presented. Computational Fluid Dynamics (CFD) has been instrumental in defining the bundle arrangement minimizing the VIV amplitude. Two-dimensional model tests have been performed at a scale of 1/5 at MARIN facilities with the selected bundle configuration. In these experiments, a rigid bundle was subjected to controlled motions (towing and/or forced vertical oscillations). The salient results will be presented. Finally, a VIV fatigue analysis was performed using Shear 7 based on the results derived from the 2D model tests.
GAP Launch: Methodology, Constraints and Challenges
Authors: Morin Pierre , Martin Christophe , Alberts Ron
Published: July 2009

MURPHY Sabah Oil Co. Ltd. has developed the Kikeh Field located offshore Malaysia in the South China Sea in a water depth of 1325m. This field development is based on a Floating Production Storage and Offloading unit (FPSO) and a Spar Dry Tree Unit (DTU). Fluids are transported in fluid transfer lines (FTL) using SBM’s newly developed and patented Gravity Actuated Pipe (GAP) system. The launch of the first GAP took place on the 8th and 9th of May 2007 in Bintulu (Malaysia). This was the first major milestone in the GAP system installation campaign. The overall length of the GAP, the weight of both towheads, the constraints imposed by the selected site and the need to cope with the environmental data made it a very challenging operation. This paper describes the constraints, pre-launch activities and the onshore and offshore spread used during the main steps of the launch.
Preparation and Execution of the GAP Surface Tow
Authors: Morin Pierre , Jean Philippe , Melis Cecile , Helle Yann , Shepheard Barry
Published: July 2009

MURPHY Sabah Oil Co. Ltd. has developed the Kikeh Field located offshore Malaysia in the South China Sea in a water depth of 1325m. This field development is based on a Floating Production Storage and Offloading unit (FPSO) and a Spar Dry Tree Unit (DTU). Fluids are transported in fluid transfer lines (FTL) using SBM’s newly developed and patented Gravity Actuated Pipe (GAP) system. This paper highlights the challenges and solutions associated with the engineering and execution of the 188 nautical mile tow of the GAP from the SBM construction site in Bintulu (Malaysia) to the Kikeh Field. Substantial effort was invested at the engineering stage to study the tow-induced fatigue, including influence of wave direction, tow speed, trailing tug back tension, current and submergence of towheads in order to develop a tow strategy which was a compromise between fatigue damage reduction and realistic and cost effective marine operations. This process is detailed in the paper. Finally, all tasks associated with tow from the point at which the beach hold-back rigging was abandoned to the point at which field pre-entry tasks were carried out are also described in the paper.
Fender Selection for Reverse Lightering of Ship-to-Ship Transfer Operations
Authors: Sakakibara Shigeki , Yamada Shuu
Published: July 2009

The ship maneuvering procedures and fender selection of ship-to-ship transfer operation of crude oil are recommended in the OCIMF (Oil Companies International Marine Forum) guideline. The fender selection is mainly based on a berthing energy of two ships, and empirical rules of the mooring masters. Recently due to new oil trade requirements, various type lightering operations have been conducted at the actual offshore field, especially “Reverse-lightering operation”. In this paper, some considerations on fender selection for the reverse-lightering operation are investigated and pointed out in berthing and mooring of ships by using numerical simulation software (IAMOS).
Calibration of Mooring Line Damping for Taut-Leg FPSOs Model Testing
Authors: Fernandes Antonio C. , de Oliveira Allan Carre´ , Mineiro Fabio P. S. , Sales Joel S. , Rosa Andre´ Luis , Ramiro Andre´
Published: July 2009

The paper discusses alternatives to represent the MLD (Mooring Line Damping) in models tests with truncated mooring lines. The work has performed both numerical experiments and reduced model tests. The results for stiffness and damping have been compared. This allows further considerations for future designs.
Consequences of a Non-Linear Asymmetric Restoring Force on the Offshore Barge Rolling Motion
Authors: de Oliveira Allan Carre´ , Sales Joel Sena , da Silva Joa˜o Luis Batista , Fernandes Antonio Carlos , Orlowski Rene Thiago C. , Saad Arthur Curty
Published: July 2009

A certain barge widely used for offshore operations in Brazil has a ramp to allow riser and pipeline installations. This ramp crosses the free surface, leading to a non-linear restoring force. The restoring force is also asymmetric, since the ramp is placed in starboard side. Because of the irregular geometry, the predicted hydrodynamic behavior obtained considering potential theory is affected. It is usual consider using duffing equation (inclusion of a cubic term in restoring force) to reproduce GZ curve for analysis at large angles. Although non-linear, the behavior of Duffing model (limits of stability) has been stressed by scientific community and can be considered well-known. Unfortunately, for this analysis, duffing model is not the most appropriated to be used, because of the asymmetry. A quadratic model of restoring force may present better results describing roll evolution and should be investigated. Several studies have been done to improve the dynamic behavior of the barge. As a part of this effort, model tests have been conducted to allow the installation of bilge keels to control the rolling behavior. This was a good opportunity to access the non-linear behavior what will permit comparison with theoretical analysis.
Seismic Analysis of a Double-Hinged Articulated Offshore Tower
Authors: Hasan Syed Danish , Islam Nazrul , Moin Khalid
Published: July 2009

The response of offshore structures under seismic excitation in deep water conditions is an extremely complex phenomenon. Under such harsh environmental conditions, special offshore structures called articulated structures are feasible owing to reduced structural weight. Whereas, conventional offshore structure requires huge physical dimensions to meet the desired strength and stability criteria, therefore, are uneconomical. Articulated offshore towers are among the compliant offshore structures. These structures consist of a ballast chamber near the bottom hinge and a buoyancy chamber just below the mean sea level, imparting controlled movement against the environmental loads (wave, currents, and wind/earthquake). The present study deals with the seismic compliance of a double-hinged articulated offshore tower to three real earthquakes by solving the governing equations of motion in time domain using Newmark’s-β technique. For this purpose Elcentro 1940, Taft 1952 and Northridge 1994 earthquake time histories are considered. The tower is modeled as an upright flexible pendulum supported to the sea-bed by a mass-less rotational spring of zero stiffness while the top of it rigidly supports a deck in the air (a concentrated mass above water level). The computation of seismic and hydrodynamic loads are performed by dividing the tower into finite elements with masses lumped at the nodes. The earthquake response is carried out by random vibration analysis, in which, seismic excitations are assumed to be a broadband stationary process. Effects of horizontal ground motions are considered in the present study. Monte Carlo simulation technique is used to model long crested random wave forces. Effect of sea-bed shaking on hydrodynamic modeling is considered. The dynamic equation of motion is formulated using Lagrangian approach, which is based on energy principle. Nonlinearities due to variable submergence and buoyancy, added mass associated with the geometrical non-linearities of the system are considered. The results are expressed in the form of time-histories and PSDFs of deck displacement, rotational angle, base and hinge shear, and the bending moment. The outcome of the response establishes that seismic sea environment is an important design consideration for successful performance of hinges, particularly, if these structures are situated in seismically active zones of the world’s ocean.
Coupled Dynamic and Static Analysis of Typhoon TLP Accident During Extreme Environmental Conditions
Authors: Joelsas Timerman Gabriela , de Campos Marcio Araujo , Nishimoto Kazuo , Brito Oscar Augusto
Published: July 2009

An important path to reviewing technology is through failure investigation, which enables understanding of appropriate levels of safety and assurance of design criteria. In the occurrence of extreme environmental conditions in which most units respond well or at least satisfactorily, the assessment of exception cases can be taken as a good starting point for the failure investigation mentioned above. Hence, especially on account of hurricanes Rita and Katrina, the 2005 hurricane season, which had a notable impact on offshore industry at Gulf of Mexico region, can be taken as a promising scenario to assess the losses and bring out cases to be more carefully appraised. It is the case of the TLP Typhoon found upside down after the passage of Rita. Being the only unit located at a significant water depth which followed recent project standards, the case naturally came to the fore. This paper addresses an investigative analysis held out in order to establish and analyze the possible causes that resulted on the capsizing of the referred unit. The investigation focuses on two major paths which were selected as being the most probable and also had feasibility to be carried out — one that analyzes a possible lack of displacement due to the passage of a hurricane wave and the consequent loss of stability of the TLP and the other, concentrated on a dynamic analysis of the unit under the impact of the hurricane metocean conditions. This second analysis focuses on obtaining the tension on the tethers stating if either they suffered excessive loads causing them to break or lack of it, causing compression and possible buckling.
Development of a New Self-Stable Dry-Tree GoM TLP With Full Drilling Capabilities
Authors: Heidari Homayoun , Williams Neil , Large Sean , Byrne Jim
Published: July 2009

SBM Atlantia has developed a Dry-Tree FourStar TLP with full drilling capability for GoM applications. This paper focuses on the unique features of the Dry-Tree FourStar by considering a typical design example. The Dry-Tree FourStar TLP considered is a 100,000 ton displacement structure with a payload of approximately 40,000 tons, designed for 4300 ft water depth. Like its wet-tree predecessor, the hull geometry follows the battered column form, which leads to increased stability for quayside integration, wet tow of the integrated system, and self-supported installation. Furthermore, the large keel footprint of the hull maximizes the efficiency of the tendon system, resulting in a more economical solution. In order to enable quay side integration of the 22,000 ton topsides onto the FourStar hull, the deck is designed as three separate interconnecting modules. The Dry-Tree FourStar hull is structurally optimized for effective load transfer through the outer-shell of the structure, resulting in significantly lower hull steel densities when compared to industry standards. The design was confirmed through a comprehensive wave basin model test campaign performed at the Offshore Technology Research Center at Texas A&M University. The model tests were performed at 1:52 scale, and utilized the latest GoM MetOcean criteria. The top-tensioned risers were included in the physical model in order to provide more realistic estimates of the hydrodynamic behavior of the TLP. The model tests confirmed the performance of the GoM Dry-Tree FourStar as a stable drilling and production platform. This paper presents the development of this TLP with emphasis on the hydrodynamic and structural aspects that are unique to the concept. It discusses the observations from the wave basin model tests, including the results of a comparison with a theoretical global performance analysis.
Jack-Deck for FPSO Module Installation
Authors: Tcherniguin Nicolas , Cholley Jean-Marc
Published: July 2009

Technip has developed a new installation concept called the “Jack-Deck” which enables installation of topsides onto an FPSO hull in one or two “Integrated Modules” with weights of up to circa 21,000t each (See Figure 1). This installation method facilitates extensive pre-commissioning and commissioning of the modules at their construction yard. It considerably reduces the onshore hook-up and integration work required on top of the FPSO hull deck. Consequently, the overall FPSO construction schedule and cost are significantly reduced. This paper describes the “Jack-Deck” installation method and discusses the consequences on topsides layout and structural design. It also highlights the main benefits of the system.
Ultimate Strength Analysis of Jacket Type Offshore Platforms Due to Wave, Current and Wind Loading
Authors: Mirzadeh Niasar J. , Aghakouchak A. A. , Samadani S. , Aghakouchak A.
Published: July 2009

As the result of safety factors considered in load and resistant values for the design of new offshore platforms, a higher resistance compared to the 100 year return period environmental loading, is achieved in the structure. This type of design is usually performed using linear methods of structural analysis. In this case the structure practically has a high reserve capacity which is disregarded in the design phase but may be considered in special cases. One of these important cases is when an existing structure is assessed and evaluated after elapse of its structural life. In this case using the platforms reserve capacity, through a non-linear analysis such as push-over, would be logical and economic. In this paper initially the factors affecting a platform non-linear analysis and methods for considering non-linear characteristics of structures will be discussed. Then the results of an investigation on the behavior of three Persian Gulf platforms due to wave loading using the non-linear push over analysis method of USFOS software are presented. Finally, the results of these analyses are compared with the API RP2A’s assessment criteria [1].
Utilization of FMEA During the Preliminary Design of a Dynamic Positioning System for a Shuttle Tanker
Authors: Moratelli La´zaro , Tannuri Eduardo A. , Morishita He´lio M.
Published: July 2009

The Dynamic Positioning mode operation of vessels, especially those related to the offshore oil industry, requires rigorous reliability analysis in order to eliminate the dangerous consequence of loosing position or to create a procedure when the consequence can not be eliminated. In general, the reliability analysis of the Dynamic Positioning System (DPS) is performed through Failure Mode and Effect Analysis (FMEA). In this paper the FMEA procedure is proposed taking into account the failure data reported in IMCA DP Incidents Reports. The idea of this procedure is to analyze and improve the system configuration during preliminary design in terms of the reliability considering critical failures. To complete the analysis, the consequence is evaluated through dynamical simulations for offloading operation in tandem configuration. The simulations results indicate the need to improve the DPS configuration and three modifications are proposed to compensate and create redundancy to avoid occasional failures of the propellers are proposed.
An Investigation of Stability and Workability of ESEMI Under the Pre-Service Condition in Gulf of Mexico
Authors: Yang Chan K. , Murray John , Nah Edwin , Hussain Anis
Published: July 2009

This paper presents a global performance characteristics of an extendable draft semisubmersible (ESEMI) under the preservice condition in Gulf of Mexico (GoM) environment. The ESEMI consists of a deep draft semisubmersible hull with the second tier pontoon (STP) attached by eight extendable columns beneath the main hull. The STP is positioned just below the main pontoon bottom during the dock-side installation and the tow-out, and is installed once the ESEMI arrives at the in-place location. The STP increases vertical motion compared to the conventional deep draft semisubmersible when it is towed out, and the metacenter (GM) value keeps changing when the STP is lowered down. Typically, the GM becomes minimum value when there is a sudden change of water plane area, when the GM is supposed to be positive to assure the stability. The hydrostatic stability during tow-out and installation is investigated and is checked if it satisfies the criteria. The motions at tow-out and some typical installation stages are also simulated and discussed in the view point of workability.
Simulation of the Hydrodynamic Interaction Forces in Close-Proximity Manoeuvring
Authors: Sutulo Serge , Guedes Soares C.
Published: July 2009

A code for simulating hydrodynamic interaction forces in manoeuvring simulating systems has been created. The algorithm takes into account potential forces only and is based on the Hess and Smith panel method. Own inertial hydrodynamic forces were estimated through pre-calculation of the added masses followed by use of the Thomson–Tait–Kirchhoff equations. Comparative computations of the added masses, surge and sway interaction forces and yaw interaction moments with varying number of surface computational panels showed that on a typical modern PC, an acceptable accuracy in terms of the integrated loads can be reached with a relatively small number of panels permitting real-time simulations with the developed algorithm in the loop. Importance of the account for the local time derivative of the potential has been demonstrated on comparative calculations in simulation of a passing-by manoeuvre. The code can be used for predicting interaction loads with any number of moving objects and fixed obstacles.
Hydrodynamic Interaction and Drift Forces on a Rectangular Barge and Modified Wigley Hull Arranged Side-by-Side
Authors: Pessoa Joa˜o , Fonseca Nuno , Guedes Soares C.
Published: July 2009

The paper studies the hydrodynamic interactions between two bodies in close proximity oscillating on the free surface due to incident harmonic waves, inspired in the problem of offloading from a LNG floating platform to a tanker. It deals with a modified Wigley hull and a rectangular barge arranged side by side. The numerical results include hydrodynamic coefficients, exciting forces, absolute motions, and drift forces, and they are compared with published experimental data followed by further analysis of the effects of free surface elevation between the two bodies. The calculations are carried out with the WAMIT® code, which is based on a panel method, or boundary integral method.
Centrifugal Separators Used for Oilfield Multi-Phase Separation
Authors: Zhao Lixin , Hua Zhengrong , Wang Yue , Hu Yanqing , Ma Zhanzhao
Published: July 2009

The development of centrifugal separators used for oilfield multi-phase separation is introduced. The structural types and functional principles of the separators, which include static, adjustable, rotary and dynamic hydrocyclones, spiral separator, gas-liquid cylindrical cyclone, and spiral tube separator, etc., are described. The main characteristics, advantages, disadvantages, separating effect of the separators are analyzed respectively. The application condition of each type is introduced as well. The research state-of-the-art of the separators and some of their applications are also described. The right way for selecting an efficient separator is based on the actual operating condition in order to obtain satisfactory treating result. This paper would be beneficial for the selection, design, and application of different separators in oilfields, especially for offshore applications.
Structural Analysis of a Multi-Body Wave Energy Converter in the Frequency Domain by Interfacing WAMIT and ABAQUS
Authors: Taghipour Reza , Arswendy Arswendy , Devergez Me`lanie , Moan Torgeir
Published: July 2009

In this paper the structural response of a multi-body wave energy converter with power take-off is analyzed in the frequency domain. The device consists of a semisubmersible platform and 21 buoys. The buoys can slide along guides that are attached to the platform. The hydrodynamic and structural problems are solved by using boundary element and finite element software systems WAMIT and ABAQUS. The hydrodynamic analysis is carried out by a linear perturbation approach. A mode expansion method, with total number of 27 modes, is used to describe the dynamic behavior. Moreover, an idealized form of power absorption mechanism is considered herein. A general procedure is established to interface the relevant information between the two software systems. Such information includes the radiation, diffraction and restoring force pressures and inertia loads, etc. In this way, the interaction between the floating bodies is included in the solution in which the dynamic reaction forces are carried as external forces. The structural response is obtained by a quasi-static approach. The objective is to investigate the still water and wave induced internal loads in the column-deck and guide-deck connections in the form of transfer functions. To demonstrate the approach, calculations are made for a following- and oblique-sea wave condition.
Multiobjective Optimisation of a Floating LNG Terminal
Authors: Boulougouris Evangelos K. , Papanikolaou Apostolos D.
Published: July 2009

The paper presents a comprehensive multi-objective hydrodynamic optimisation procedure and its application to the early design of a Floating LNG Terminal for improved seakeeping and wave attenuation characteristics on its lee side. Genetic Algorithms are used to find the Pareto optima designs followed by Multi-Objective Decision Making procedures to select the optimum design among them. The paper addresses the definition of the relevant optimisation problem and the setup and interface of relevant software tools, presents results of systematic studies with respect to the minimization of the motion responses and the wave elevation on the leeward side of the free-floating terminal and concludes with the analysis and critical review of the obtained results and their impact on the terminal design.