bluuebunny/crossref_metadata_2025 huggingface dataset:
Compression Assessment of Deepwater Steel Catenary Risers at Touch Down Zone
Authors: Cheng Yongming , Song Ruxin , Mekha Basim , Torstrick Andrew , Liu Hugh
Published: May 2009

A steel catenary riser (SCR) is a widely used concept for deepwater floating production facilities. Severe motions of a floating host facility such as a semisubmersible or FPSO may cause a significant compression load on SCRs at the touch down zone (TDZ). This paper investigates how to assess the compression that could be experienced by deepwater SCRs, including methodology, failure modes considered, acceptance criteria, computer modeling, and describe the steps necessary for assessing the compression forces. To demonstrate the proposed methodology and criteria, a recent example of the Independence Hub 20-inch Gas Export SCR in ultra deepwater (i.e. 8,000 ft) is given to illustrate the compression and buckling phenomenon. The behavior of the SCR compression and buckling at the TDZ is investigated by using a nonlinear finite element method to determine the mechanism and governing factors. Both beam and shell elements are used in the detailed analysis for comparison purposes. In addition a strain-based criterion is implemented to determine if the compression level is acceptable. Short term fatigue damage is also calculated by using the time domain rain-flow method. In general, the paper presents an analysis procedure outlining the steps necessary for evaluating the compression and buckling phenomenon of deepwater SCRs.
Mooring Cost Optimization via Harmony Search
Authors: Ryu Sam , Duggal Arun S. , Heyl Caspar N. , Geem Zong Woo
Published: May 2009

A mooring system optimization program has been developed to minimize the cost of offshore mooring systems. The paper describes an application of the optimization program constructed based on recently developed harmony search optimization algorithm to offshore mooring design which requires significant number of design cycles. The objective of the anchor leg system design is to minimize the mooring cost with feasible solutions that satisfy all the design constraints. The harmony search algorithm is adopted from a jazz improvisation process to find solutions with the optimal cost. This mooring optimization model was integrated with a frequency-domain global motion analysis program to assess both cost and design constraints of the mooring system. As a case study, a single point mooring system design of an FPSO in deepwater was considered. It was found that optimized design parameters obtained by the harmony search model were feasible solutions with the optimized cost. The results show that the harmony search based mooring optimization model can be used to find feasible mooring systems of offshore platforms with the optimal cost.
Fatigue Design of the Atlantis Export SCRS
Authors: Vandenbossche Mike , McDonald Will , Xia Dingwu , Masson Craig , Fang Jie , Xu Jun
Published: May 2009

The Atlantis semi-submersible platform is located in 7040 feet water depth in southern Green Canyon, in the Gulf of Mexico. It supports the Mardi Gras 24-inch oil and 16-inch gas export steel catenary risers (SCRs), the deepest and largest diameter SCRs in the world. Fatigue performance was one of the critical and challenging aspects of the design due to the severe wave climate coupled with the large vessel motions and strong bottom currents below the Sigsbee Escarpment. Preliminary design showed that the fatigue life at the girth welds in the touch down point (TDP) region did not satisfy the design criteria. This paper presents the fatigue design measures that were adopted to improve the fatigue performance of the Atlantis export SCRs. These include the first-time use of manually relocating the host platform to reduce fatigue damage accumulation in the TDP region, and removing the weld caps over the most critical part of the TDP region and grind flush the caps and roots of the welds in the flexible joint assembly to achieve practical defect acceptance criteria. It is demonstrated that the narrow fatigue damage peak in the TDP region can be decreased significantly by spreading it over a wider region, and all the design requirements are satisfied.
Time-Domain Simulation of Riser VIV in Sheared Current
Authors: Huang Kevin , Chen Hamn-Ching , Chen Chia-Rong
Published: May 2009

Riser vortex-induced vibrations (VIV) have attracted significant attentions in recent years in offshore oil and gas industry. There is an increasing interest in using computational fluid dynamics (CFD) approach for deepwater riser VIV time-domain simulations. Our previous study has demonstrated that the long riser (L/D = 1400) VIV response in uniform current can be predicted with reasonable accuracy by time domain simulations with Chimera over-set data grid technique. This paper is to further that study and investigate the riser VIV in sheared current profiles. The riser studied in this paper is a long marine riser with constant tension distribution. Its prototype has an outer diameter (OD) of 0.027m and a mass ratio of 1.6. The fluid domain is discretised using approximately one million elements. A linearly sheared current is imposed in perpendicular to the riser, and the flow field is calculated using an unsteady Reynolds-Averaged Navier-Stokes (RANS) numerical method in conjunction with a Chimera domain decomposition approach with overset grids. The critical parameters including riser VIV root-mean-square (rms) a/D, vorticity, drag and lift coefficients are processed, and compared to those of uniform current and experimental data. The simulation results show that the riser VIV under sheared current behaves differently from uniform current. It is also shown that the presented CFD approach provides reasonable results and is suitable for long riser VIV evaluation in deepwater and complex current conditions.
A Pipeline Case Study for Ice Scour Design
Authors: Konuk Ibrahim , Yu Shenkai
Published: May 2009

The Arbitrary Lagrangian Eulerian (ALE) Finite Element (FE) ice scour model developed in the previous years is used to study the importance of some of the pipe, seabed soil, and scour parameters. The effect of some of these parameters on the pipeline response is investigated. The paper reveals that the influence of these parameters on the pipeline response is very different than predicted by the Winkler models and also very beneficial for the pipeline design. A pipe-in-pipe (PIP) system is analyzed using the same model. It is shown that a carrier pipe installed in a PIP system would experience stresses less than those of the same size single pipe installed in the same trench by about one order of magnitude.
Using Active Circuits to Model Drillstrings and Analyze the Effect of Phase Shift on Stability Analysis
Authors: Elsayed M. , Aissi C. , Yalamanchili A.
Published: May 2009

In recent years, deeper wells are drilled in order to reach untapped reserves. The long drillstrings needed for such wells are more susceptible to vibrations induced by instability. To study these vibrations, test rigs are employed. These rigs must represent the dynamics of the drillstring to be effective at predicting bit vibrations. In this paper, we show that active circuits can be used to simulate drillstring dynamics and study bit vibration characteristics. A circuit representing one such vibration mode was used to study the effect of phase shift on bit stability. It is also shown that such circuits can be incorporated in test rigs to provide real-time input to actuators replicating bit displacement.
Smoothed Particle Hydrodynamics for Water Waves
Authors: Dalrymple Robert A. , Rogers Benedict , Narayanaswamy Muthukumar , Zou Shan , Gesteira Moncho , Crespo Alejandro J. C. , Panizzo Andrea
Published: May 2009

Smoothed Particle Hydrodynamics provides a numerical method particularly well suited to examine the breaking of water waves due to the ability of the method to cope with splash. The method is a meshfree Lagrangian method that allows the computational domain to deform with the flowing liquid. Here we discuss the appropriate kernels used in the interpolation and the time stepping alogrithms. Applications to water waves are shown.
Spectral Fatigue Assessment of Agbami FPSO Hull for Onsite and Seagoing as per ABS’s SFA Guidance
Authors: Hwang Oe-Ju , Kwon Seung-Min , Park Gweong-Won , Kang Joong-Kyoo , Heo Joo-Ho
Published: May 2009

Agbami FPSO is a floating facility for production, storage and offloading of oil at OPL 216/217 offshore Nigeria of the central Niger delta. The FPSO is positioned with spread moorings in water depth of about 1500 m for the service life of 20 years. As per the design requirement, the hull of FPSO shall be designed to meet ABS’s SFA (Spectral Fatigue Analysis) notation and the seagoing condition. Seagoing is a mandatory condition additionally required by Client, assuming a navigating vessel of North Atlantic. But, to conservatively assess Agbami for 60 years at its Onsite except specific details was taken into account, which is related to the uncertainties in S-N data & Palmgren-Miner’s damage rule and etc. Normally the off-western Nigerian sea has swell-governed environment and the multi-peaked spectral characteristics with wave and swells. For the spectral fatigue analysis, Jonswap spectrum for wind wave & Gaussian spectra for swells were taken into consideration as a representative in case of Onsite. Thus, in order to achieve a single damage, the combined spectral method for Onsite was introduced on the basis of EMDC’s FPSO FMS (Fatigue Methodology Specification). On the contrary, Walden’s wave scatter diagram as recommended by ABS for Seagoing was applied. The forward speed of FPSO in Seagoing case was also taken into account by WASIM, a hydrodynamic tool of DNV and compared to the results of PRECAL, ABS’s hydrodynamic code to ensure the validity of the analysis. The post-process to calculate fatigue damage was carried out using the in-house program to analyze fatigue of FPSO. The results from the analyses were found that the Seagoing led consequentially critical fatigue damage for most of hull structures more than Onsite that has moderated sea state.
Propagation of Fully Nonlinear Waves in Deepwater, Transition Zone and Shallow-Water
Authors: El Safty Hoda M. , Mansour Alaa M. , Abul-Azm A. G.
Published: May 2009

In this paper, a fully nonlinear numerical wave tank model has been used to simulate the propagation of fully nonlinear waves in different water depths. In the numerical wave tank model, the fully nonlinear dynamic and kinematic free-surface boundary conditions have been applied and the boundary integral equation (BIE) solution to the Laplacian problem has been obtained using the Mixed Eulerian-Lagrangian (MEL) approach. The model solution has been verified through the comparison with the available experimental data. A convergence and accuracy study has been carried out to examine the time stepping scheme and the required mesh density. The nonlinearity effects were evident in the solution by the asymmetrical wave profile around both vertical and horizontal axis along with sharp high crests and broad flat troughs. Fully nonlinear wave propagation in deepwater, in transition zone and in shallow water has been simulated. The nonlinear solution has been compared to the linear solution for various waves. Shoaling coefficient and wave-number have been derived based on the nonlinear solution and compared to the linear theory solution for various wave characteristics.
Fatigue Testing and Analysis of Full Scale Girth Welded Tubulars
Authors: Wa¨stberg Stig , Salama Mamdouh M.
Published: May 2009

Heavy duty girth welded tubulars are used for many critical structural members offshore, e.g. free spanning pipelines, risers and tethers. The fatigue properties of those girth welds are often the design limiting factor. Today’s fatigue design rules for girth welded tubulars are mainly based on testing of segment specimens machined from a girth weld. Such specimens do strictly speaking not simulate all the features of a complete girth weld in a tubular. Lately some data have become available from rotating bending of full scale tubulars. However data from axial loading of full scale tubulars are rare. In this study full scale (OD 24”, 609.6 mm, thickness 0.812”, 20.6 mm) girth welded tubulars were fatigue tested in axial tension. Each tubular specimen contained three girth welds, equalling almost 6 m of weldment for each test. Both as-welded, TIG-dressed and ground weldments were included in the test program. The fracture surfaces were inspected for determination of the initiation point and its relation to welding flaws. The test results are compared to common fatigue design curves. The significance of embedded weld flaws in ground welds and the relationship to S-N curves for ground welds is discussed.
Application of Structural Reliability Analysis to UHB Pipeline Design
Authors: Griffiths Terry , O’Brien Dermot , Johnson Richard
Published: May 2009

The detailed design of pipelines susceptible to upheaval buckling (UHB) has been characterized by a wide range of industry approaches, especially in relation to selecting safety factors. DNV recently published the draft of their HT/HP design code (RP-F110), which aims to codify and unify approaches to this aspect of design. Sensitivity analysis using Structural Reliability Analysis (SRA) methods highlights the sensitivity of rock cover requirements to a small number of key parameters. Several of these, including: Young’s Modulus; coefficient of thermal expansion; rock cover height; and rock uplift resistance factor have very poor published public domain statistically measured information on mean and variance. It is clear that the early identification in a project of the potential cost benefits associated with an SRA approach maximizes the potential to collect good project-specific statistical data. The clear benefit is an improvement in overall safety by ensuring a more uniform probability of failure along the pipeline which meets target reliability levels.
Analyzing the Stabilizing Tank for the Control of Rolling Motion by Model Testing and the Dynamic Absorber Theory
Authors: Saad Arthur Curty , Fernandes Antonio Carlos , de Tarso T. Esperanc¸a Paulo , Sales Junior Joel Sena
Published: May 2009

The FPSOs are a type of offshore platforms that are directional in nature. Since a large ship (presently the use of a VLCC - Very Large Crude Carrier - is very common) is used, there is a great importance the direction the waves hit the hull. Differently from a real ship, the FPSO cannot resort to maneuvering to avoid waves. It has been found that there is a possibility, in certain cases, that a rolling resonant wave may reach some stationary FPSOs and consequently, a very high response may be obtained. Sometimes, it is not possible to use large bilge keel and the alternative is to consider the use of other devices such as stabilizing tanks and U-tubes. Faced with this problem, the present work performed model tests in a deepwater ocean basin showing the effectiveness of the stabilizing tank. On the other hand, it is clear that due to the presence of mass, restoring motion and damping that the design problem may be tackled by the use of classical dynamic absorber theory. For this reason, a simplified problem was formulated by replacing the stabilizing tank by a passive concentrated mass on board. The fully nine dimensional and non-linear model are then recovered. Six degrees of freedom are to describe the ship motion and the renaming three are for the mass on board. Based on these preliminary studies, the work describes the use of tests with reduced models showing the usefulness of the theory in practice. The test results together with 2 and 4 degrees of freedom system addresses the importance of the roll-sway coupling. Subsequently, a careful linearization is made for the purpose of identifying the commanding variables such as the mass, the position above the keel, the damping, the dynamic absorber natural frequencies, etc. After that, several parametric studies have been performed, identifying the range of applicability of the variables. Finally, this theoretical-experimental exercise addresses back the use of the applicability of the stabilizing tank.
The Spatial Analysis of an Extreme Wave in a Model Basin
Authors: Buchner Bas , van Dijk Radboud , Voogt Arjan
Published: May 2009

As a pilot study into the understanding of the occurrence of extreme waves, the spatial development of an extreme wave (Ac/Hs = 1.59) in a model basin was investigated. This wave occurred in a wave spectrum that was not extremely steep and non-linear. It is observed that the extreme wave develops in less than half the wavelength from a relatively normal wave into an extreme crest. The wave crest stays high and constant over a large distance (almost 75m). Linear dispersion is not able to predict the wave propagation towards the observed extreme wave crest. Second order theory improves the prediction of the crest amplitude, but not enough. The crest amplitude is still underestimated. This is confirmed by the plots of the probability of extremes. The linear Rayleigh distribution underestimates the crest amplitudes. The second order distribution follows the measurements much better, but also in this case typically the highest 10 crests in a 3 hours storm are underestimated.
Numerical Simulation of the Deployment of a Hybrid ROV Optical Fiber Tether
Authors: Grosenbaugh Mark A. , Whitcomb Louis L. , Bingham Brian , Gobat Jason I. , Young Chris
Published: May 2009

This paper describes a numerical code for simulating the dynamics of an unmanned, underwater vehicle system that is self-propelled and tethered to a surface ship through an optical fiber tether. The vehicle, called a hybrid ROV, uses a buffered, single-mode optical fiber with a maximum working load of 1.7 N for communication and data transmission. The vehicle is designed to go to the deepest parts of the ocean and for exploring beneath the Arctic ice cap. The optical fiber tether is stored in a pair of canisters, one mounted on the vehicle and one mounted on a garage that is lowered from the ship. The canisters each hold 20 km of fiber, which is pulled out during operations when the tension at the canister reaches a threshold value, which is set to the maximum working load of the fiber. The numerical simulation is based on the two-dimensional version of WHOI Cable, a finite-difference solver of the cable equations that includes bending stiffness to model low-tension effects. A velocity/tension based payout algorithm was incorporated into the code to model the behavior of the canisters. In the payout model, the payout velocity is set equal to zero below the threshold tension and varies linearly with tension above the threshold value up to a maximum pay out velocity. Hydrodynamic drag models for axial and normal fluid loading, whose values are a function of Reynolds number, are used to calculate local drag coefficients of the optical fiber. Examples of the vehicle being lowered to the sea bottom in uniform and shear currents are used to demonstrate the capabilities of the code and the performance of the tether and payout system.
Assessing the Effects of Impact Forces on Subsea Flowlines and Pipelines
Authors: Alexander Chris
Published: May 2009

Damage associated with external impact can be a critical component in operating subsea flowlines and pipelines. External damage typically involves impact with anchors, although consideration of dropped objects is also important. Historically, operators assess damage after it occurs in an attempt to determine and establish mechanical integrity. For more than 30 years research has been performed studying the effects of external damage on subsea pipelines. In recent years there has been an interest in proactively addressing the potential for damage and attempting to quantify the severity of damage in terms of impact energies associated with anchors and dropped objects. This paper presents insights garnered in assessing the severity of pipeline damage in the form of dents and gouges. Additionally, research associated with impact forces including experimental work is included as part of the presentation, as well as limit analysis techniques using finite element methods. The primary purpose of this paper is to communicate to offshore pipeline operators a methodology that can be employed to assess the severity of damage and quantify tolerance levels in terms of impact energy.
The Grouped SLOR: Design and Implementation
Authors: Dale Nicholas , Karunakaran Daniel , Hatton Stephen
Published: May 2009

Large deepwater developments typically have a complex and congested seabed layout immediately adjacent to the production vessel. This is due to the large number of risers and umbilicals often required to meet production and export requirements and the spatial constraints imposed by mooring lines and vessel offsets. This poses significant constraints on the riser design to achieve an acceptable riser arrangement whilst ensuring that clashing and interference is avoided. The freestanding hybrid riser, in both the bundle and single line arrangements, is a configuration that has been widely used in West Africa. It has been selected due to its excellent fatigue performance, the decoupling of the vessel motion via jumpers and the ability to be pre-installed, thus taking this activity off the critical path. However, both the bundled hybrid riser and the Single Line Offset Riser (SLOR) have field layout problems: The bundled riser, whilst able to efficiently incorporate 10–12 lines in a single structure, poses practical problems at the bottom and top ends where connections need to be made to flowline and flexible jumpers respectively. Due to the large number of lines terminating in a small envelope the problem is how to acceptably route flowlines and their associated jumpers, whilst accommodating pipe expansion, movements and installation tolerances and is further compounded by thermal and other flow assurance issues. Similarly at the top end, the offtake of dynamic flexible jumpers to the vessel can be challenging to achieve an acceptable arrangement that both facilitates installation and prevents clashing during operation. The field layout challenge presented by the SLOR is primarily as a result of its large deflections as a result of current loading. This requires each SLOR to have a large spatial clearance with the adjacent SLOR, mooring line or umbilical. So whilst the SLOR facilitates easy access, at the top and bottom end, for connection of jumpers the maximum number of SLORs that can be accommodated within a given field layout is often limited and insufficient to meet initial and future project requirements. This paper provides an introduction to the Grouped SLOR, a riser solution that is developed specifically with a view to optimize the riser/vessel interface and seabed layout. It uses a buoyant frame to guide the freestanding risers which constrains all risers to move collectively and this effectively eliminates the risk of clashing. As the riser spacing can be greatly reduced compared to a conventional SLOR the arrangement allows seabed real estate to be optimized without losing the many benefits of the freestanding riser concept. The Grouped SLOR is discussed and compared with current riser solutions from component level, up to the technical and operational advantages of the system and on to its application in a typical West of Africa deepwater development.
Cartesian Cut Cell Free Surface Capturing Solver for Wave Floating Body Interactions
Authors: Qian Ling , Causon Derek , Mingham Clive
Published: May 2009

A newly developed free surface capturing code based on a two fluid formulation of Navier-Stokes equations and Cartesian cut cell grid system has been applied to study wave/floating body interaction problems. The basic flow solver uses an efficient dual time-stepping artificial compressibility algorithm and modern Riemann solver based upwind schemes of the Godunov-type which capture the moving free surface accurately as part of the numerical solution. Mesh generation in the conventional sense is eliminated in favour of defining local cut cell data on a stationary background Cartesian grid. For wave/floating body interaction problems where bodies can undergo arbitrary movement in response to the incoming waves, the only required changes to the meshing procedures consist of local updates to the cut cell data at the boundaries that are in motion. A test case has been simulated and the results are in a good agreement with available experimental data. The numerical model is generic and can be applied to a number of engineering flow problems such as green water overtopping of vessels and offshore floating structures.
Structural Reliability-Based Control for Moored Vessels: Experimental Results
Authors: Berntsen Per I. B. , Aamo Ole M. , Leira Bernt J.
Published: May 2009

This paper addresses dynamic positioning of surface vessels moored to the seabed via a turret based spread mooring system, referred to as position mooring. The controller utilize a reliability index to determine the actuator force needed to operate safely. The structural reliability measures become an intrinsic part of the controller, automatically adjusting the allowed geographical region based on current weather conditions and structural properties of the mooring lines. The performance of the controller is demonstrated through laboratory experiments on a model vessel named CyberShip III.
Comparison of Incremental Dynamic and Pushover Analysis of Jacket Type Offshore Platforms
Authors: Asgarian B. , Raziei A.
Published: May 2009

Jacket Type Offshore Platforms show nonlinear behaviors under strong ground motions result from nonlinear behaviors of soil, pile and jacket members. Nonlinear behavior study isn’t carried out accurately unless an accurate and suitable analytical method is selected. The main focus of this paper is the soil-pile-structure interaction analysis of the jacket type offshore platforms subjected to strong ground motion. A nonlinear dynamic analysis shows a true response if a logical model and an accurate theory are selected. In addition to nonlinear dynamic analysis, nonlinear static analysis is also carried out in this paper and results of the static nonlinear and dynamic nonlinear analyses have been compared. It was concluded that nonlinear static analysis can be used under some conditions instead of nonlinear dynamic analysis.
Definition of the Best Position for the Turret in an FPSO Based on a Hydrodynamic-Structural Analysis
Authors: Matter Ge´rson B. , Sphaier Sergio H. , Sales Joel S.
Published: May 2009

This paper presents a methodology for the positioning of a turret based on the hydrodynamic behavior of the FPSO and the structural behavior of the mooring lines. It is presented a procedure for the simulation of the equilibrium and the dynamics of the FPSO’s for a range of environmental conditions and positions of the turret. The results are studied in order to define the best position for the turret. Experimental results are used in order to calibrate the physical models.
The Role of Offshore Monitoring in an Effective Deepwater Riser Integrity Management Program
Authors: Goldsmith Brittany , Foyt Elizabeth , Hariharan Madhu
Published: May 2009

As offshore field developments move into deeper water, one of the greatest challenges is in designing riser systems capable of overcoming the added risks of more severe environments, complicated well requirements and uncertainty of operating conditions. The failure of a primary riser component could lead to unacceptable consequences, including environmental damage, lost production and possible injury or loss of human life. Identification of the risks facing riser systems and management of these risks are essential to ensure that riser systems operate without failure. Operators have recognized the importance of installing instrumentation such as global positioning systems (GPS), vessel motion measurement packages, wind and wave sensors and Acoustic Doppler Current Profiler (ADCP) units to monitor vessel motions and environmental conditions. Additionally, high precision monitoring equipment has been developed for capturing riser response. Measured data from these instruments allow an operator to determine when the limits of acceptable response, predicted by analysis or determined by physical limitations of the riser components, have been exceeded. Regular processing of measured data through automated routines ensures that integrity can be quickly assessed. This is particularly important following extreme events, such as a hurricane or loop current. High and medium alert levels are set for each parameter, based on design analysis and operating data. Measured data is compared with these alert levels, and when an alert level is reached, further response evaluation or inspection of the components in question is recommended. This paper will describe the role of offshore monitoring in an integrity management program and discuss the development of alert levels based on potential failure modes of the riser systems. The paper will further demonstrate how this process is key for an effective integrity management program for deepwater riser systems.
SmartPipe: Self Diagnostic Pipelines and Risers
Authors: Knudsen Ole O̸ystein , Giertsen Egil , Schjo̸lberg-Henriksen Per , O̸stby Erling , Grythe Knut , Oldervoll Fro̸ydis
Published: May 2009

The principal objective with the SmartPipe R&D project is to develop a concept for online monitoring of the technical condition of offshore pipelines and risers, suitable for integration into E-field concepts and integrated process management approaches. This will be achieved by developing and linking different technologies into a system for condition monitoring to enable rapid detection of dangerous situations, optimal use of production facilities and improved safety control. Sub-goals include development of a distributed sensor network, local power supply, communication infrastructure, improve the models for materials degradation and to develop better numerical tools for structural integrity analyses. Parameters to be measured in order to establish a complete condition monitoring system for offshore pipelines have been identified. Based on this, the necessary sensor network and sensor principles are evaluated and adapted. Integration of sensors in the pipe structure, with emphasis on developing concept for packaging to withstand loads introduced during installation and operation is a major challenge. Adequate communication infrastructure and concepts for energy supply are also evaluated. Processing of data retrieved from the sensor network will be realized through a coupling of numerical analysis tools and improved models for materials degradation, reflecting the effects of local conditions and previous load history. The results will be presented through a graphical user interface, allowing for visualization and the possibilities to run simulations regarding future operation.
A Framework for Reliability-Based Fatigue Safety Factors for Flexible Risers
Authors: Leira B. J. , Farnes K. A. , Igland R. T. , Martin J. , Percy D. , Lemos C. , Clements R. , Nedrelid K.
Published: May 2009

In the present paper, a procedure for reliability analysis of wave-induced fatigue in the tensile and pressure armours of flexible risers is described. The procedure is based on performing a number of parametric studies with respect to variables that influence the fatigue lifetime. The results of these parametric studies are subsequently combined with models describing the statistical scatter of the same parameters. By application of this procedure, the safety factors which are required in order to reach specific target reliability levels can be computed. Different SN-curves which correspond to different corrosive environments are considered.
Failure Modes of Tapered Suction Caissons Under Vertical Pull-Out Loads
Authors: Nabipour Mahmood , Zeinoddini Mostafa , Abdi Mahmood R.
Published: May 2009

The pull-out performance of conventional upright suction caissons has been investigated by different researchers. However, no attention has been formerly paid to tapered suction caissons. Some numerical studies already conducted by the authors demonstrated that tapered caissons exhibit pull-out capacities well above than that from their corresponding upright caissons. This paper deals with different failure mechanisms of tapered suction caissons and discusses some reason for their superior performance. A numerical approach has been used and different combinations of caisson types/ soil categories have been examined. With tapered suction caissons two different modes of failure have been discerned. The first mode has been noticed to develop in weak clays and sands under drained conditions. This mode corresponds to a shear sliding failure in the soil plug along the caisson’s interior wall. Concurrently a soil wedge is formed in the soil body adjacent to the caisson. The second mode of failure has been observed in higher strength drained clays and undrained clays and sands. With this failure mode a local failure at the bottom of the soil plug has been noticed to happen. At the same time the failure is extended to the lower surfaces of a soil wedge outside of the caisson. The detached soil plug accompanies the caisson in its movement upward following the local failure.
Load on Turbine Blade Induced by Motion of Floating Platform and Design Requirement for the Platform
Authors: Suzuki Hideyuki , Sato Akira
Published: May 2009

Due to the limited land area and mountainous topography, Japan is not necessarily suited for land-based wind power generation. But potential of offshore wind energy around the country is huge and has ability to supply whole electricity of the country. Development of offshore wind energy is also a promising solution for establishing sustainable society in the country. Water depth around the country generally becomes sharply deeper with distance from the shoreline and floating platform is necessary to deploy wind turbines. This paper investigates effect of motion of floating platform on the strength of turbine blade, a key issue in designing floating wind turbine, and design requirement for floating platform was discussed. Inertial load induced in the turbine blade by the motion of platform and rotation of turbine was formulated. The formulated load on the blade was verified by experiment with rotating rod on the oscillating tower. Two analysis codes, structural analysis code of turbine blade and motion analysis code of SPAR type floating platform, were developed. The effect of platform motion on the bending moment induced in the blade was investigated using the codes and design requirements for the platform were investigated from a viewpoint of maximum load and fatigue load. From a series of analysis on 5MW wind turbine showed that maximum load on blade is increased by 10% for pitching with amplitude of 5degrees but sectional modulus must be increased by 50% for fatigue. It is concluded that the increase of maximum load on the blade due to the motion of floating platform is not serious but fatigue load can be significant. Design requirement for the motion of floating platform will be that the amplitude of pitching motion should be less than a few degrees so that the land-based wind turbine can be installed on the floating platform with minimum modification.
Mechanical Properties and Bending Behaviors of Low Temperature Toughness Linepipe Steels
Authors: Ahn Seong Soo , Cho Woo Yeon , Yoon Tae-Yang , Yoo Jang-Yong
Published: May 2009

API-X70 and X80 steel with good low temperature toughness were developed. The microstructure and mechanical properties of API-X70 steel plate and pipe were investigated and the buckling behavior of X80 steel pipe was evaluated through large scale deformation tester. API-X70 steels with 30 mm thickness were manufactured by finished rolling below Ar3. The microstructure was composed of polygonal ferrite with subgrain network, degenerated pearlite and bainite. The yield strengths of API-X70 pipes were lower than those of plates, while the tensile strengths were similar in both states. The Charpy upper shelf energy of API-X70 steel plate was about 350 J and the energy transition temperature was below −100 °C. The separations were observed on the DWTT fracture surface of API-X70 steel plate. The DWTT 85 SA% transition temperature of plate was below −30 °C. It was conjectured that the separation associated with the low temperature rolling might increase the strength without deterioration of DWTT properties. API-X80 steels with 19mm thickness were fabricated with finished rolling above Ar3 and pipes with 30” diameter were made with R/B process. The deformation capacity of X80 linepipe was evaluated by large scale deforming machine operating under the loading of bending and axial compression force. It was showed that 2nd moment term should be calculated more correctly to measure the accurate critical compressive strain of pipe in the loading of bending and axial compression force. The compressive axial force had a little effect on the peak moment but changed the deformation pattern and state of critical compressive strain of linepipe. It was found that X80 linepipe used in this study was within the specification of DNV and API codes in terms of buckling capacity.
Numerical Simulation of Engineering Wave for Zhongzui Bay
Authors: Shu Xiejun , Jiang Senhui , Li Ruijie
Published: May 2009

For providing a better shelter condition, it is necessary to build a breakwater in Zhongzui Bay. In order to know whether mooring area meets the requirement after engineering construction and compare the mooring area between solid breakwater and permeable breakwater, a numerical simulation method is used in the sheltering harbor of Zhongzui Bay. The used Mild-slope equation which describes wave refraction, diffraction and reflection, considers the steep slope bottom and effect of energy dissipation. It has been validated to fit for simulating wave transformation in the coastal zone. Under extreme high water level and design high water level, wave fields in the calculation area of three wave types in three different return periods are simulated by using this method respectively. In addition, wave height in front of breakwater can be provided. Then the wave parameters and the mooring area of two occasions, with and without breakwater, are gained in calculation area. Based on these results, some conclusions are presented in the end.
Comparison of Coupled and Uncoupled Analysis Methodologies in Towing Pipeline Installation Modeling
Authors: Mendes P. L. F. , Jacob B. P. , Vaz M. A. , Correˆa F. N.
Published: May 2009

This paper is focused on numerical simulations of a new pipeline installation method recently patented by Petrobras, in which the pipeline is manufactured onshore and towed to the production site by two tugboats. It summarizes the comparison of the results for uncoupled and coupled models; in the former the hydrodynamic response of the tugboats is first calculated, and then applied to the pipe ends; the latter is able to take into account the change of the tugboat response under environmental loadings due to the existence of the towed pipe. The comparison is carried out based on parametric dynamic analyses which investigate the influence of several parameters on the coupling effect.
Evaluation of Real-Time Torque-Turn Charts With Application to Intelligent Make-Up
Authors: Teodoriu Catalin , Kinzel Holger , Schubert Jerome J.
Published: May 2009

A wide spectrum of power tongs can accommodate the increasing interest in mechanized operations at the rig floor. The importance of a torque-turn recording has been stated in many papers as a good control method for the make-up process, and the reliability of a drill string can be improved using feed back of make-up recordings. State-of-the-art devices not only allow the precise mechanized make-up of connections, but they also provide sufficient data to analyze the quality of the connection make-up. The motivation behind the research performed is the desire to make use of the data that are already provided to increases the lifetime of the connections and the reduction of drill string failures, especially for drilling under extreme downhole conditions like HPHT or deep water. This paper presents a new algorithm to evaluate the slope of the torque-turn recordings and to extract information that can be used to predict the quality of the process, with application to extreme drilling situations. This work is based on more than 300 make-up and break-out laboratory tests.
Feasibility Stage Assessment of Side-By-Side LNG Offloading Operation
Authors: Wong Jun , Paton Colin , Morandini Cedric , Withall Timothy , Kilner Andrew
Published: May 2009

A key driver in assessing the economic viability of floating LNG terminals is the marine offloading operations uptime. Marine offloading operations uptime is the percentage of time on site for which weather conditions are such as to permit offloading operations to be undertaken. Physical model testing or time domain numerical simulation techniques can model these marine offloading operations to a very high level of fidelity. However it is not practical for reasons of time and cost to apply such high fidelity modeling to the long duration data sets necessary to make reliable uptime estimates. Simpler solution methods, which can be used to carry out rapid what if studies as well as provide uptime assessment based on very long data records are therefore required. This paper illustrates that a reliable and fast numerical approach based on frequency domain analysis can be developed and used as a pre-screening tool to identify key marine operations uptime drivers. In this method the process of determining the marine offloading operations uptime involves the following steps: 1. Collect and collate site-specific environmental data. The typical starting point for an uptime analysis will be 5 to 10 years of hindcast environmental data, consisting of records of the average wind, wave and current amplitude over successive 3-hour sea states. 2. Evaluate the expected vessel heading in each successive 3-hour sea state throughout the hindcast record. 3. For each 3-hour sea state, estimate the relative motions between the FPSO and LNGC, at the previously determined vessel heading. From the relative motions estimate the envelope of motions of the loading arms and the maximum tensions in the mooring lines between the FPSO and LNGC. 4. For each 3-hour sea state compare the estimated loading arm motion envelopes and maximum mooring line tensions with the maximum acceptable design values to determine whether offloading would be feasible in this 3-hour sea state. 5. Identify times when there are sufficient consecutive 3-hour sea states to allow the offloading operation to be completed (weather window). Determine the percentage uptime from the ratio of the total of these periods to the total environmental data length. A range of sensitivity analysis can also be performed using this methodology, thereby allowing critical cases to be identified for further examination using the high fidelity model testing or time domain numerical simulation programs.
Flow Induced Motions on Multi Column Floaters
Authors: Waals Olaf J. , Phadke Amal C. , Bultema Stephen
Published: May 2009

Recent years have shown an increasing interest in low frequency response of offshore floaters in current. Spar VIM behavior and Semi Submersible flow induced behavior is known from calm water tow tests. Recent tow test projects have also shown low frequency TLP response in steady current. The motions from the model tests are used in the global analysis of the mooring systems and risers for these platforms. This paper discusses the dynamic behavior in current of multi column floaters and the associated complex flow patterns. Shielding between columns is addressed as well as the effect of mass ratio (i.e. floater mass divided by displacement). It is shown that lower mass ratios such as for conventional TLP’s may result in larger sway response than for deep draft semi submersibles. The motion behavior is discussed as well as the increase in total mean current loads due to transverse motions.
Tank Side-Wall Effect Evaluated by Using Multi-Body Interaction Computation
Authors: Duan W. Y. , Shaheen H. , Chen X. B.
Published: May 2009

It is well known that hydrodynamic first- and second-order loads measured on ships or offshore structures in wave tanks exhibit large scatter compared to the expected results in open sea, due to wave reflections from the side walls. A number of recent works have given theoretical models for diffraction and radiation solutions including side-wall effects. One way consisting of defining a Green function satisfying the boundary condition on two parallel side walls can yield reliable solutions for a floating body of arbitrary geometry in any position of the tank. Unlike previous studies, we evaluate the side-wall effects by directly using the option of multi-body interaction in usual model of wave diffraction and radiation. The tank side-walls are considered as an independent fixed body. The wave diffraction and radiation around a floating body in tanks is solved by taking into account of interaction between the two bodies. The excellent agreement between numerical results of first-order quantities and experimental measurements validates the present method. It is shown that this direct method is very efficient and can be further applied to the case of two side walls in non-parallel position as well as to take into account of bathymetric variation of sea bottom.
Relative Importance of Defects in Girth Welds of Linepipes
Authors: O¨rjasaeter Oddvin , Haagensen Per J. , Knagenhjelm Hans Olav
Published: May 2009

The Ormen Lange deepwater gas field is located at water depth down to 1100 meters. The irregular seabed gives severe challenges to pipeline design and verification program was launched to demonstrate adequate fatigue capacity. The research included: modern welding techniques (5G and 2G welding positions), mapping of actual welding defects, misalignment (high/low) and lack of penetration. The thick walled pipe (35mm) showed low or even compressive residual stresses at the inside. This will to some extent be “protective” to the root of the weld. The exceptions to this pattern were the repair welds and the two-sided welds. The small scale test results fell close to the full scale pipe tests when taking into account the geometrical weld distortions, loading mode, and the distribution of weld defects. The importance of the parameters influencing the fatigue capacity could be ranged as follows, most detrimental first: large crack-like defects (LOP, undercut > 1mm), hi/lo, and V-shape (radial shrinkage at the girth weld).
A Recommended Approach for Deriving ISO-Compliant 10,000 Year Extreme Water Levels in the North Sea
Authors: Leggett Ian M. , Bellamy Nigel F. , Fox Joseph P. , Sheikh Rizwan
Published: May 2009

Extreme water level is a key parameter in the design and safe operation of offshore installations. It is defined as a combination of three components; wave crest, tidal elevation and surge height. Historically, extreme water levels have been derived through the direct superposition of crest heights calculated empirically from sea states of 100 year return period, a surge component of the same return period and a Mean High Water Spring tidal level. This approach is limited for two reasons; firstly due to a simplifying assumption to calculate an individual wave crest elevation and secondly no account is taken of the joint probability of occurrence between the three components. As a result of significantly greater temporal and spatial resolution in acquired oceanographic data together with advances in computing and recent developments in formulating short-term statistics potential shortcomings in the traditional approach have been recognised. This is addressed in the new ISO 19901-1 standard, which specifies that wave crest elevations should be derived by the convolution of both long and short-term wave distributions. This paper compares and contrasts three methods for deriving ISO-compliant extreme wave crest heights and proposes a new joint probability approach for the addition of tide and surge elevations.