bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial Effects in Risk-Based Design and Maintenance of Pipelines
Authors: Maes Marc A. , Faber Michael Havbro
Published: May 2009

Pipelines are to a large extent spatially continuous systems having a system-component relationship that is not as clearly articulated as for other structural systems. Reliability-based design methods for pipelines often provide conflicting views about the spatial extent of limit states, the effect of spatial correlation, the applicability of target risks and target reliabilities (for instance on a per unit length basis), the link with lifecycle cost methods, and risk acceptability in general. The present paper first reviews probabilistic design and assessment approaches for pipelines, ranging from partial factors and limit state design, to reliability based and consequence-based methods. Subsequently we identify the various types of limit states from the point of view of their spatial characteristics. The paper also reviews the possible approaches to target risks and target reliabilities in view of the different spatial extent of the limit states. The role of spatial correlation as it impacts on different kind of pipeline limit states and on the risk acceptance process is discussed. The role of inspection, repair and maintenance can easily be included in many of the reliability-based pipeline design and assessment approaches as the lifetime costs of mitigative actions are fairly well defined, together with the spatially distributed consequences of failure, but they do add some additional challenges to the spatial modeling of the system.
Deep Water Riser Collision Avoidance by Top Tension Control
Authors: Rustad Anne M. , Larsen Carl M. , So̸rensen Asgeir J.
Published: May 2009

For tensioned riser arrays in deep waters interference between individual risers in strong ocean current is a key design and operational concern. The lateral deflections are likely to be large, and the risers may experience collision with fatigue or surface damage as a consequence. In this paper a system consisting of a tension leg platform (TLP), a pair of risers, environmental forces and hydrodynamic interaction is presented. The control system is described, and a set of control objectives with corresponding control strategies are suggested. The collision avoidance effects of the different control objectives are shown through simulations.
Static and Dynamic Behavior of Highly-Deformed Risers and Pipelines
Authors: Santillan Sophia T. , Virgin Lawrence N. , Plaut Raymond H.
Published: May 2009

This paper models flexible risers and pipelines as slender elastica structures. The theoretical formulation leads to a type of nonlinear boundary value problem that can be solved numerically given appropriate boundary conditions. The offsetting effects of gravity and buoyancy are included in the analysis. These forces can provide considerable axial loading (as can thermal changes) and hence stability (buckling) is a major concern. Initial studies are based on the planar problem. A free-vibration analysis is also conducted for small-amplitude oscillations about various deflected equilibrium configurations in terms of natural frequencies and corresponding mode shapes [1]. Energy dissipation and fluid forces are a key issue in the forced problem, especially when large deformations are involved [2, 3]. Free vibration information is a vital prerequisite in understanding the response of these types of structures in practice [4].
Evaluation of IACS Common Structural Rules in Terms of Ultimate Limit State Design and Assessment of Ship Structures
Authors: Paik Jeom Kee , Kim Bong Ju , Seo Jung Kwan
Published: May 2009

The aim of the present paper is to evaluate the ultimate limit state performance of an AFRAMAX-class hypothetical double hull oil tanker structure designed by IACS CSR (Common Structural Rules) method, compared with the same-class/type tanker structure designed by IACS pre-CSR method. The ultimate strengths of stiffened plate structures in deck and bottom parts under combined in-plane and out-of-plane actions, and hull girder against vertical bending moment, are computed for the two designs, and the resulting computations are compared. ALPS/ULSAP program is used for the ultimate limit state assessment of stiffened plate structures, while ALPS/HULL program is employed for the progressive hull collapse analysis. ANSYS nonlinear FEA method, which uses more refined technology, is also used for the same purpose. The insights and developments obtained from the present study are addressed.
Pipe-Soil Interaction: An Evaluation of a Numerical Model
Authors: Colicchio Giuseppina , Colagrossi Andrea , Brocchini Maurizio , Venturi Marco , Drago Michele
Published: May 2009

The investigation of the evolution of the pipe/sea-bottom configuration has a large relevance for pipeline design as the occurrence and persistence for a certain period of a free-span causes the accumulation of fatigue on the pipeline which can lead to unacceptable damage. Of great importance is, therefore, the possibility of accurately predicting both generation and evolution of dangerous scour-induced free-spans. The onset of scouring is intimately connected with the dynamics of the bottom boundary layer and with the localized sediment transport. To this purpose various numerical tools are becoming available which allow for an accurate quantitative assessment of the local flow-sediment-pipe interaction. Among them we find all the models which permit prediction of the dynamics of two-phase flows, like the Level Set and the SPH methods. We here focus on evaluation of the performances of a hydro-morpho-dynamic model, based on the former approach, for use in pipe-soil interaction calculations. Limits and potentials of this rather innovative approach are being investigated using as test cases both well controlled experimental benchmarks and also realistic flow conditions.
Nonlinear Wave Scattering From a Single Surface-Piercing Column Comparison With Second-Order Theory
Authors: Masterton Stephen , Swan Chris , van Zutphen Hermione
Published: May 2009

This paper concerns nonlinear wave scattering from large surface-piercing columns. The motivation behind this study has principally arisen due to a number of operational problems observed on large volume structures, particularly, the loss of effective air-gap. The present study builds on recent observations of nonlinear wave scattering fully reported in (1). These observations are significant since they arise in a flow regime where no significant scattering is expected. The present paper extends these observations by considering two larger, practically important, column sizes; the largest lying just outside the linear diffraction regime, D/λ⩾0.2 (where D is the column diameter and λ is the incident wavelength). Experimental observations are compared to second-order theory calculated using an industry standard panel method, WAMIT. Across the full range of column size the presence of the unexpected nonlinear scattering mechanisms reported by (1) is clearly demonstrated. However, their significance reduces with increasing column size. As a consequence the second-order prediction is shown to improve for the larger column sizes and for the largest cases the comparison with the measured data demonstrates excellent agreement. This latter aspect being important considering the steepness of the incident wave (Ak = 0.3) where typically the convergence of a truncated diffraction solution may be questioned.
Analysis of Coupled Hull: Cold Water Pipe Mooring for Offshore Desalination Plant
Authors: Subbaiah A. Venkata , Sundaravadivelu R. , Subramanian V. Anantha
Published: May 2009

The design of a 0.25 Million Litres per Day (MLD) low pressure desalination plant in 150m water depth using a Tension Leg Counterweight Platform (TLCP) consisting of a 10m diameter hull, 1m diameter cold water pipe with counterweight and vertical taut mooring has been carried out. A 1:50 scale model of the TLCP is tested in the 30m × 30m × 3m wave basin in Department of Ocean Engineering, IIT Madras. The tensions in mooring lines are measured using load cells. The coupled analysis of the TLCP is studied using the Nonlinear Analysis of Offshore Structures (NAOS) program developed at IIT MADRAS. The measured tension response of mooring line compare well with numerical results. The Surge, Heave, Pitch and Tension response amplitude operators for the TLCP obtained from numerical study are observed to be within the permissible limits for the operational wave condition whereas the TLCP performs satisfactorily for the survival wave condition. The numerical and model studies are carried out for regular waves. The effect of counterweight and pretension on platform motions is studied in this paper.
Linear and Nonlinear Approach of Hydro-Pneumatic Tensioner Modeling for Spar Global Performance
Authors: Yang Chan K. , Tahar Arcandra , Kim M. H.
Published: May 2009

This paper presents a numerical model of top tension risers (TTRs) with hydro-pneumatic tensioner for Spar application in the Gulf of Mexico environment. Nonlinearity of the stiffness and friction characteristics of the tensioner combined with stick-slip behavior of riser keel joint is investigated. The relationship between tensions and strokes for hydro-pneumatic tensioner is based on the ideal gas equation where the isotropic gas constant can be varied to achieve an optimum stroke design based on tensioner stiffness. Challenges of modeling the coupling effects in the finite element (FE) method between the tensioner and hull motion are also presented. This new FE model is implemented into CHARM3D, a fully coupled time domain analysis program of floating bodies. The effect of nonlinearity of tensioner curve, tensioner friction and riser keel friction is intensively investigated. The resultant global motion, TTR stroke and tensions are systematically compared with those of a simple engineering approach in which the nonlinear coupling effect is captured by linearization.
Investigation and Prediction of Wave Impact Loads on Ship Appendage Shapes
Authors: Fullerton Anne M. , Fu Thomas C. , Hess David E.
Published: May 2009

Navy fleet problems with damage to hatches and other appendages after operation in high sea states suggest that wave impact loads may be greater than the current design guidelines of 1000 pounds per square foot (48 kilopascal) (Ship Specification Section 100, General Requirements for Hull Structure and Guidance Manual for Temporary Alterations, NAVSEA S9070-AA-MME-010/SSN, SSBN). These large impact forces not only cause damage to ships and ship structures, they can also endanger the ship’s crew. To design robust marine structures, accurate estimates of all encountered loads are necessary, including the wave impact forces, which are complex and involve wave breaking, making them difficult to estimate numerically. An experiment to investigate wave impact loads was performed at the Naval Surface Warfare Center, Carderock Division in 2005. During this experiment, the horizontal and vertical loads of regular, non-breaking waves on a 12 inch (0.305 m) square plate and a 19.75 inch (0.5 m) diameter horizontal cylinder were measured while varying incident wave height, wavelength, wave steepness, plate angle and immersion level of the plate and cylinder. Wave heights of up to 1.5 feet (0.46 m) were tested, with wavelenghs of up to 30 feet (9.1 m). In all cases, the horizontal wave impact force increased with wave steepness. For some angles, the horizontal wave impact force increased with greater submergence. A feed-forward neural network (FFNN) developed by Applied Simulation Technologies was used to predict the horizontal forces measured during the experiment based on the values of wave height, wavelength, wave steepness, plate angle and immersion level of the plate and cyclinder. A FFNN is a computational method used to develop nonlinear equation systems that use input variables to predict output variables. Predictions of forces from the FFNN compare well with the experimental data, and may be useful in future design of ships and ship structures.
Dynamic Behavior of a Semi-Submersible Platform Coupled With Drilling Riser During Re-Entry Operation in Ultra-Deep Water
Authors: Yamamoto Marcio , Morooka Celso K. , Ueno Seiya
Published: May 2009

The present study deals with the dynamics of a semi-submersible platform coupled with a free-hanging vertical rigid riser during the re-entry operation in ultra-deep water. The reentry operation occurs after the drilling phases are accomplished which only use the bare drillstring. The re-entry operation entails positioning the subsurface Blow-Out-Preventer (BOP) above the wellhead and installing it to the wellhead. During this operation, the BOP is suspended by the drilling riser. The placement and installation is time consuming and requires a great amount of accuracy. One challenge is the effects of platform motions on the installation procedure. The present work was carried out with the aim to understand the influence of platform motions during the re-entry operation in ultra-deep waters. Another benefit of this study is the knowledge of the effect of platform motions on the drilling riser and BOP which leads to the possibility of drilling more than one well without the need to raise the BOP to the surface. This could greatly reduce the amount of time needed to drill subsurface wells which in turn, reduces overall operational costs. A numerical simulation in time domain has been carried out using a non-linear model for the platform dynamics. The dynamics of a free hanging drilling riser and a dynamic positioning system (DPS) of the semi-submersible platform are included in the numerical model. Simulation results in time domain of the platform displacements with DPS and riser displacement are shown. A discussion of riser displacement and DPS control is also included.
Observations of Breaking and Grouping of Sea Waves
Authors: Yu Dingyong , Liu Huaxing , Li Jing
Published: May 2009

Breaking and grouping of surface waves are observed at Bohai Sea. The measurement system mainly consists of two hydrophones and a pressure sensor underwater. The broadband hydrophones are able to track breaking waves by passive detection of naturally generated ambient sound of breaking. The breaking criterion in terms of Sound Spectrum Level (SSL) is chosen to identify the breaking waves. The simultaneous wave elevation signals are measured with the pressure sensor, to get the wave group information. Special emphasis is put on the portable data acquisition system based on virtual instrumentation, which makes the in situ measurement convenient and user-friendly. By use of the field data, a wave age dependent model for wave breaking probability is reanalysed, and the result shows a good agreement; moreover, the relationship between breaking and grouping is tentatively investigated, the results are in agreement with the earlier findings that breaking was correlated strongly with group structure and occurred most commonly in the center of a group.
A Novel Approach to Calibrating Wavemakers and Generating Wavemaker Transfer Functions
Authors: Fu Thomas C. , Fullerton Anne M. , Rice James R. , Fu Thomas K. S. , Hess David E.
Published: May 2009

Limitations in our current knowledge of a ship in extreme wave conditions have been illuminated by the work being done by the US Navy on advanced hull-forms. Stochastic approaches to these phenomena are insufficient, and deterministic testing of these events must be performed if the physics is to be understood. Although modern computer controlled wavemakers provide the ability to generate regular sine waves, long crested multi-spectral waves, mixed seas of almost any sea spectra, and even “freak” waves, all of these systems require the wavemakers be tuned to the specific facility and that transfer functions between wavemaker settings and the generated wave be calculated. This tuning is performed to compensate for the facilities geometry, wave absorbers (beaches), etc, as well as to aid the researcher in using the wavemaker system. The Maneuvering and Seakeeping (MASK) basin at the Naval Surface Warfare Center, Carderock Division (NSWCCD) is a large rectangular basin, measuring 240 feet (73.2 m.) by 360 feet (109.7 m.). Two adjacent walls of the MASK are equipped with pneumatic wavemakers, while the other two banks are equipped with wave absorbing beaches. This paper describes the development of a feed-forward neural network model of the MASK wavemakers and demonstrates the utility of this approach in calibrating wavemakers and generating wavemaker transfer functions.
Second Order Loads on LNG Terminals in Multi-Directional Sea in Water of Finite Depth
Authors: Rezende Fla´via , Li Xin , Chen Xiao-Bo
Published: May 2009

Large LNG terminals are designed to be installed in an offshore area approximate to harbors where the water is of finite depth and waves are multi-directional. The terminal can be of a barge type LNG/FRSU including accommodations, gas preconditioning and liquefied plant, a number of storage tanks and offloading facilities. It serves also as a support to moor a LNG carrier during offloading operations. In the design of such mooring system of LNG/FRSU and LNG carriers in a zone of shallow water, one key issue is the accurate simulation of low-frequency motions of the system to which the second-order wave loading is well known as the main source of excitation. The computation of second-order wave loading in multi-directional waves of finite waterdepth is considered here. New formulations obtained recently in [1] for the computation of second-order loads in mono-directional waves are extended to the case of multi-directional waves. Both the classical near-field formulation and the new middle-field formulation developed in [1] are used and numerical results are compared. Unlike the usual near-field formulation giving results of second-order loads with poor convergence, the middle-field formulation provides an accurate method for the computation of vertical components.
Ormen Lange Pipelines: The Design Process
Authors: Marthinsen Tom , Halvorsen Vidar Henrik , Paulsen Gunnar
Published: May 2009

The paper presents an overview of the Ormen Lange pipeline engineering process from the early design studies until the presently ongoing activities of preparing for operation. In particular, ways of handling the following topics that presented unusual challenges are addressed: • Pipeline routing and installation engineering. • Landfall design. • Free span design. • Seabed preparation techniques and tools. • Optimal combination of dredging and subsea rock installation. • Pipeline installation with tight laying tolerances. • Expansion design on extremely uneven seabed with soft soil conditions. • Loads from heavy trawl gear.
Investigation Into the Sensitivity of the Dynamic Hook Load During Subsea Deployment of a Suction Can
Authors: Ireland J. , Macfarlane G. , Drobyshevski Y.
Published: May 2009

Suction cans are commonly used as foundations of fixed offshore structures, subsea equipment, and anchors of mooring lines. During the offshore installation phase, when a suction can is submerged, it attracts large heave added mass, which may be an order of magnitude higher than the mass of the can in air. Due to motions of an installation vessel the dynamic hook load may significantly exceed the submerged weight of the can. The dynamic hook load must be accurately predicted, as it governs selection of the vessel, lifting gear and rigging, and defines the allowable installation sea state. The objective of this paper is to examine the sensitivity of the dynamic hook load to hydrodynamic properties of the suction can, in particular its heave added mass and damping. This research is motivated by the lack of data on such properties, which are usually estimated by simplified methods with some engineering judgement and assumptions. A single degree of freedom system is considered and the frequency domain spectral analysis is used, which employs the stochastic linearization of the nonlinear damping component. The added mass and damping of a 6-meter diameter suction can of dimensions typical for Australian North West Shelf developments have been determined by testing a 1:10 model in the 4.1 m deep basin of the Australian Maritime College. Free decay tests were conducted at several frequencies and the added mass, linear and nonlinear damping components determined. The effect of open hatches on the hydrodynamic properties was examined by fitting the model with hatches of various diameters, with up to 4.8% of the relative area open. Results of the tests demonstrate that the added mass and damping are higher, when compared with estimates based on empirical data for non-oscillatory flow. Within the Keulegan-Carpenter number range of 0.1–1.0, open hatches impact significantly on the added mass and produce additional damping, which is found to be linear with the heave velocity. Results of the tests and their interpretation are discussed. Sensitivity analysis shows that if the model test results are used in the dynamic lift analysis for an installation vessel and sea states considered, the predicted hook load is generally less than its values obtained by using simplified estimates. In particular, the increase in linear damping due to open hatches is responsible for up to 20% reduction in the dynamic hook load, with 2.4% of the relative top area open.
Application of WES2808 to Brittle Fracture Assessment for High Strength Gas Pipelines
Authors: Kubo Takahiro , Igi Satoshi , Handa Tsunehisa , Suzuki Nobuhisa , Toyoda Masao , Ohata Mitsuru , Minami Fumiyoshi
Published: May 2009

This paper presents the results of a preliminary study to establish an assessing method for the tensile strain limit against brittle fracture of pressurized gas pipelines subjected to axial tensile deformation. The basis of the assessment method is the Japan Welding Engineering Society standard WES2808–2003. WES2808 provides a procedure for evaluating the fracture limit using the CTOD design curve relating flaw size, applied strain and fracture mechanical parameter (CTOD). The main characteristics of the method are a consideration of the deterioration of the fracture toughness of material resulting from large cyclic and dynamic straining, a correction of CTOD fracture toughness for constraint loss in structural components in large scale yielding, and an estimation of critical CTOD value from Charpy test results. Modifications of the procedure to enable evaluation of the fracture properties of high strength gas pipelines under biaxial loading conditions are studied.
Small Scale Model Tests on Subgouge Soil Deformations
Authors: Vikse Normann , Gudmestad Ove T. , Nystro̸m Per R. , Liferov Pavel
Published: May 2009

There are several challenges related to Arctic offshore oil and gas field developments. Among these is the design of pipelines transporting hydrocarbons in the field or to shore. Special considerations must be carried out to take into account the Arctic conditions. One of the most critical concerns for pipelines in shallow waters is the possible damage due to ice ridge gouging. The ice ridge keels can in some areas of the Arctic be as deep as up to 30 meters, ref. Gudmestad et al., (1999), and may damage the pipeline even if that is trenched below the mudline. Laboratory small scale tests executed to establish qualitative figures of the soil and buried pipe behavior below the gouge are reported in this paper. The tests were conducted at 1g and aimed to visualize the effects of gouging on the soil below and the buried pipe segment. A box (115 × 50 × 40 cm) filled with different types of soils (sand and silty sand) was used for the tests. The model ice keel was made of a steel plate hanging from a wooden frame and mounted on top of the box such that only horizontal translation was allowed. For several of the tests, pipe segments were embedded into the soil. The performed tests indicated the presence of subgouge soil deformations and their dependence of several gouge-related parameters. It was observed that the pipes experienced cyclic movements, being first dragged forward-downwards as the model keel approached the pipeline and then re-bounding when the keel passed over. The tests showed that the maximum pipe displacement decreases exponentially with the pipe burial depth. Naturally formed soil mound in front of the keel during gouging showed to influence vertical pipeline displacement. Furthermore, horizontal pipe movements were larger at lower attack angles of the ice keel. The authors are by no means the first to ponder the subject of subgouge soil deformations and pipeline response. Many researchers have reported their studies before; see e.g. Woodworth-Lynas et al. (1996), Nixon et al. (1996), Yang and Poorooshasb (1997), Phillips et al. (2005) and Konuk et al. (2006). The authors are also fully aware of the challenges related to scaled testing of the soil-structure and do not assume the obtained results to be scaled up. Nevertheless, the reader may find some observations to be of interest while evaluating the present (disputed) approach to the problem.
Experimental Setup: Low Speed Permanent Magnet Generator for Marine Current Power Conversion
Authors: Yuen Katarina , Nilsson Karin , Grabbe Ma˚rten , Leijon Mats
Published: May 2009

Marine currents, e.g. tidal currents, ocean currents and unregulated water courses are characterized by a fairly steady or regular flow and pose a significant potential for electricity generation. An ongoing research project at Uppsala University is looking into an energy conversion system with a vertical axis turbine and a permanent magnet direct drive generator placed directly in the water flow, i.e. without using a dam. This choice of technology is intended to provide a simple and robust system with low maintenance needs and minimal environmental impact. During 2006 an experimental setup has been built. It consists of a frequency converter, a motor and a gearbox to drive the generator, the generator itself, and a resistive load to consume the generated power. The generator is two meters in diameter and is built on an elevated structure over the motor and gearbox. A stainless steel structure supports the cable wound stator. The permanent NdFeB magnets are fastened in milled grooves in the rotor. The experimental setup will be used for verification of a simulation and optimization program.
Optimized Solutions to Control Lateral Buckling of Pipelines With Snaked-Lay: Theoretical and Numerical Studies
Authors: Guan Jiong , Nystro̸m Per R. , Hansen Hans F.
Published: May 2009

Due to the offshore development moving to a more hostile environment, thermal buckling becomes an important issue needed to be considered for HPHT (high-pressure high temperature) subsea pipelines design. In order to control the lateral buckling, the snaked lay method is investigated theoretically and numerically. The buckling mechanisms of the curved beam are studied with methods considering the finite strain and simplified straight beam theory, respectively. The differences between the two methods are found to be negligible when the radius of curve is larger than a certain value. Detailed FE analysis results are given for the buckle behavior of a snaked-lay pipeline. The method to control the subsea pipeline lateral buckling is discussed and followed by a design example.
A Practical Approach to Risk Based Assessment and Maintenance Optimisation of Offshore Wind Farms
Authors: Bharadwaj Ujjwal R. , Speck Julian B. , Ablitt Chris J.
Published: May 2009

Offshore wind farm managers are under increasing pressure to minimise life cycle costs whilst maintaining reliability or availability targets, and to operate within safety regulation. This paper presents a risk based decision-making methodology for undertaking run-repair-replace decisions with the ultimate aim of maximising the Net Present Value (NPV) of the investment in maintenance. The paper presents the methodology developed for the risk based life management of Offshore Wind farms under the remit of the CORLEX (Cost Reduction and Life Extension of Offshore Wind Farms) project funded by DTI (Department of Trade and Industry, UK) Technology Programme on Renewable Energy. Unlike traditional approaches to decision-making that consider either the probability of failure of a component or the consequence of failure in isolation, a risk-based approach considers both these aspects in combination to arrive at an optimal solution. The paper builds a basic Qualitative Risk Analysis methodology to highlight high-risk components that are then investigated further by a Quantitative Risk Analysis. The risk is now quantified in monetary terms and the time of action — replacement or maintenance — indicated by the model is such that the NPV of the action is maximized. The methodology is demonstrated by considering offshore wind turbine tower as the critical component and corrosion as the damage mechanism.
Three Dimensional Wake Structures of Flow Around an Oscillating Circular Cylinder
Authors: Gioria Rafael S. , Carmo Bruno S. , Meneghini Julio R.
Published: May 2009

Direct numerical simulationsthe three-dimensional flow around an oscillating circular cylinder are carried out. Imposed body oscillations are realized for low amplitude of oscillation, A/D = 0.4 and for high amplitudes, A/D = 1.0. As the intention is to analyze the amplitude influence in the wake dynamics, the frequency of oscillation is fixed and chosen to be inside the lock-in region, 0.95fs, where fs is the shedding frequency of fixed cylinder. The three-dimensional wake characteristics of the oscillatory body simulations are compared to the fixed body. Floquet stability analysis of two-dimensional oscillatory flow is carried out to complete the investigation and consistently analyze the three-dimensional flow results. The different unstable modes are identified for each of the cases, and they are found to depend basically on the vortex patterns.
Numerical Investigation of Vortex-Induced Vibration of a Marine SCR
Authors: de Lima Alessandro A. , Meneghini Julio R. , Mourelle Marcio , Casaprima Enrique , Flatschart Ricardo B.
Published: May 2009

In this paper the dynamic response and fatigue analysis of a marine SCR (Steel Catenary Riser) due to vortex shedding is numerically investigated. The riser is divided in two-dimensional sections along the riser length. The discrete vortex method (DVM) is employed for the assessment of the hydrodynamic forces acting on these two-dimensional sections. The hydrodynamic sections are solved independently, and the coupling among the sections is taken into account by the solution of the structure in the time domain by the finite element method implemented in ANFLEX code [1]. Parallel processing is employed to improve the performance of the method. A master-slave approach via MPI (Message Passing Interface) is used to exploit the parallelism of the present code. The riser sections are equally divided among the nodes of the cluster. Each node solves the hydrodynamic sections assigned to it. The forces acting on the sections are then passed to the master processor, which is responsible for the calculation of the displacement of the whole structure. The time histories of stress are employed to evaluate the damage as well as the life expectancy of the structure by the rainflow method to count the cycles in the dynamic response.
Pipeline Subsea Repair
Authors: Heier Espen , Mellem Tore
Published: May 2009

Pipeline repairs at large water depths represent challenges for both for mechanical repair systems and for welding systems. This paper presents the main results from a Joint Industry Project (JIP) launched in late 2005 that was focused on the following main topics: • Revision of the Recommended Practice for “Mechanical pipeline couplings” [2] in order to include requirements for clamp repairs, hot-tapping and the pipeline’s response to isolation plug forces. • Methods to qualify remotely operated subsea hyperbaric welding in a dry habitat without performing NDT. At present such welding technology is being developed to join pipelines though the use of a sleeve and two fillet welds, and for the seal welds of hot-tapping pipeline branches.
Viscous Flow Computations on a Smooth Cylinders: A Detailed Numerical Study With Validation
Authors: Vaz Guilherme , Mabilat Christophe , van der Wal Remmelt , Gallagher Paul
Published: May 2009

The objective of this paper is to investigate several numerical and modelling features that the CFD community is currently using to compute the flow around a fixed smooth circular cylinder. Two high Reynolds numbers, 9 × 104 and 5 × 105, are chosen which are in the so called drag-crisis region. Using a viscous flow solver, these features are assessed in terms of quality by comparing the numerical results with experimental data. The study involves grid sensitivity, time step sensitivity, the use of different turbulence models, three-dimensional effects, and a RANS/DES (Reynolds Averaged Navier Stokes, Detached Eddy Simulation) comparison. The resulting drag forces and Strouhal numbers are compared with experimental data of different sources. Major flow features such as velocity and vorticity fields are presented. One of the main conclusions of the present study is that all models predict forces which are far from the experimental values, particularly for the higher Reynolds numbers in the drag-crisis region. Three-dimensional and unsteadiness effects are present, but are only fully captured by sophisticated turbulence models or by DES. DES seems to be the key to better solve the flow problem and obtain better agreement with experimental data. However, its considerable computational demands still do not allow to use it for engineering design purposes.
Residual Stress Shakedown in Typical Weld Joints and Its Effect on Fatigue of FPSOs
Authors: Li Liangbi , Moan Torgeir , Zhang Bin
Published: May 2009

Structural members of FPSO hulls often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause shakedown of the residual stresses. Only a few papers seem to deal with appropriate procedures for fatigue analysis by considering the combined effect of variable amplitude cyclic loading with shakedown of residual stresses. Hence, the fatigue behaviour of welded joints in some experiments could not be explained reasonably well. In this paper, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. The proposed fatigue procedure was validated against the experimental results. Therefore, the modified fatigue procedure could be applied to welded joints under arbitrary cyclic loading while accounting for shakedown of residual stresses.
Numerical Simulation of Sloshing in LNG Tanks With a Compressible Two-Phase Model
Authors: Wemmenhove Rik , Luppes Roel , Veldman Arthur E. P. , Bunnik Tim
Published: May 2009

The study of liquid dynamics in LNG tanks is getting more and more important with the actual trend of LNG tankers sailing with partially filled tanks. The effect of sloshing liquid in the tanks on pressure levels at the tank walls and on the overall ship motion indicates the relevance of an accurate simulation of the fluid behaviour. This paper presents the simulation of sloshing LNG by a compressible two-phase model and the validation of the numerical model on model-scale sloshing experiments. The details of the numerical model, an improved Volume Of Fluid (iVOF) method, are presented in the paper. The program has been developed initially to study the sloshing of liquid fuel in spacecraft. The micro-gravity environment requires a very accurate and robust description of the free surface. Later, the numerical model has been used for calculations for different offshore applications, including green water loading. The model has been extended to take two-phase flow effects into account. These effects are particularly important for sloshing in tanks. The complex mixture of the liquid and gas phase around the free surface imposes a challenge to numerical simulation. The two-phase flow effects (air entrapment and entrainment) are strongly affected by both the filling ratio of the tank and the irregular motion of the tank in typical offshore conditions. The velocity field and pressure distribution around the interface of air and LNG, being continuous across the free surface, requires special attention. By using a newly-developed gravity-consistent discretisation, spurious velocities at the free surface are prevented. The equation of state applied in the compressible cells in the flow domain induces the need to keep track on the pressure distribution in both phases, as the gas density is directly coupled to the gas pressure. The numerical model is validated on a 1:10 model-scale sloshing model experiment. The paper shows the results of this validation for different filling ratios and for different types of motion of the sloshing tank.
Dynamics and Stability of a Fluid Conveying Vertical Beam
Authors: Olunloyo Vincent O. S. , Oyediran Ayo A. , Osheku Charles A. , Adewale Ajayi , Ajayi Arinola B.
Published: May 2009

Throughout the oil industry, vertical pipes are used to convey crude oil either offshore from the bottom of the sea or onshore from the depths under the soil. These pipes, otherwise called risers, are attached to a platform at one end and buried under the sea bed or in the ground at the other end. The stability of these pipes is the subject of this investigation. The history of such analysis dates back over five decades when the vibration and stability of fluid conveying Trans-Arabian pipeline network was first studied; albeit for an on-shore environment. For that case, it was found that instability of the flow can be induced by vibration and that if such a horizontal conveyance pipe is supported at both ends, it bows out and buckles when the flow velocity of the conveyed fluid exceeds a critical value. Because of the industrial relevance of such conveyance networks, the problem has continued to generate interest over the years and especially now that deep waters offshore exploration is assuming increased importance in the Oil and Gas sector. When dealing with the stability of these pipes, most workers usually assume the Euler-Bernoulli hypothesis, which requires that plane sections perpendicular to the axis of the beam remain plane and perpendicular both before and after deformation. This essentially means that the deformation of such sections is neglected. However, the Timoshenko hypothesis accounts for such deformation by including transverse shear which is usually neglected. In this paper, the energy method is invoked to derive the governing equations including the effects of external temperature variation along the length of the pre-stressed and pressurized pipe. The stability of such pipes under plug flow model is thereafter presented.
Concerning the Transverse and Longitudinal Vibrations of a Fluid Conveying Beam and the Pipe Walking Phenomenon
Authors: Olunloyo Vincent O. S. , Oyediran Ayo A. , Adewale Ajayi , Adelaja Adekunle O. , Osheku Charles A.
Published: May 2009

In oil and gas installations, whether on-shore or off-shore, pipes are the primary vessel for the conveyance of either crude or products from one location to another. Under use, the pipes are subjected to both internal and external temperature fluctuations while repeated operational start-up and shut-down procedures triggers vibrations of these pipes, propagates internal waves and results in finite and irreversible longitudinal extension of the pipe over time. This longitudinal extension which is sometimes accompanied by pipe buckling is known as ratcheting and has also been described by some as pipe walking. In view of the complicated and intractable nature of the problem, most attempts to study the behavior of these pipes have been limited to the analysis of some reduced problem based on heuristic arguments and idealizations. Within this context, the transverse vibration and stability of such pipes have been studied while the problem of undamped clamped-pinned pipe conveying fluid has also been tackled numerically. Keiper and Metrikine [2004] however pointed out that such numerical schemes sometimes lead to disputed or controversial results. More importantly, the coupling between the transverse vibration and longitudinal motion has been largely ignored or neglected altogether by most writers. The objective of this paper is to formally derive the governing equations of Euler-Bernoulli beam capturing various effects including temperature variations (within and without), Coriolis acceleration, transverse acceleration, pre-stress, pressurization, rotatory inertia, and cross-sectional area change. In particular, it is shown that the latter effect is what causes pipe walking phenomenon. Most of the other effects were either earlier accounted for by Semler, et al [1994] or recently captured by Reddy and Wang [2004]. Nonetheless earlier contributions neglected the effect of the cross-sectional area change completely, thereby omitting the pipe walking phenomenon. Simple examples are considered to demonstrate the importance of these terms.
A Non-Linear Potential Model to Predict Large-Amplitudes-Motions: Application to the SEAREV Wave Energy Converter
Authors: Gilloteaux J.-C. , Babarit A. , Ducrozet G. , Durand M. , Cle´ment A. H.
Published: May 2009

A time-domain non-linear potential-flow model has been developed in order to predict the large-amplitude motions of the SEAREV device in a prescribed or arbitrary sea state. The floating body three-dimensional, undergoing six degree-of-freedom motions. The fluid is considered homogeneous, incompressible, inviscid and with an irrotational flow. Surface tension is not taken into account and the depth is considered infinite. The incident wave field is given by a higher-order spectral method. This spectral method permits the simulation of very steep wave field.
Simulation of Offshore Wind Turbine Response for Extreme Limit States
Authors: Agarwal P. , Manuel L.
Published: May 2009

When interest is in estimating long-term design loads for an offshore wind turbine using simulation, statistical extrapolation is the method of choice. While the method itself is rather well-established, simulation effort can be intractable if uncertainty in predicted extreme loads and efficiency in the selected extrapolation procedure are not specifically addressed. Our aim in this study is to address these questions in predicting blade and tower extreme loads based on stochastic response simulations of a 5 MW offshore turbine. We illustrate the use of the peak-over-threshold method to predict long-term extreme loads. To derive these long-term loads, we employ an efficient inverse reliability approach which is shown to predict reasonably accurate long-term loads when compared to the more expensive direct integration of conditional load distributions for different environmental (wind and wave) conditions. Fundamental to the inverse reliability approach is the issue of whether turbine response variability conditional on environmental conditions is modeled in detail or whether only gross conditional statistics of this conditional response are included. We derive design loads for both these cases, and demonstrate that careful inclusion of response variability not only greatly influences long-term design load predictions but it also identifies different design environmental conditions that bring about these long-term loads compared to when response variability is only approximately modeled. As we shall see, for this turbine, a major source of response variability for both the blade and tower arises from blade pitch control actions due to which a large number of simulations is required to obtain stable distribution tails for the turbine loads studied.
Empirical Wind Turbine Load Distributions Using Field Data
Authors: Agarwal P. , Manuel L.
Published: May 2009

In the design of land-based or offshore wind turbines for ultimate limit states, long-term loads associated with return periods on the order of the service life (20 years, usually) must be estimated. This requires statistical extrapolation from turbine loads data that may be obtained by simulation or by field tests. The present study illustrates such extrapolation that uses field data from the Blyth offshore wind farm in the United Kingdom, where a 2MW wind turbine was instrumented, and environment and loads data were recorded. From this measurement campaign, the loads data available are in two different formats: as ten-minute statistics (referred to as “summary” data) and as full time series (referred to as “campaign” data). The characteristics of the site and environment and, hence, of the turbine response as well are strikingly different for wind regimes associated with onshore winds (winds from sea to land) and offshore winds (those from land to sea). The loads data (here, only the mudline bending moment is studied) at the Blyth site are hence separated depending on wind regime. By integrating load distributions conditional on the environment with the relative likelihood of the different environmental conditions, long-term loads associated with specified return periods can be derived. This is achieved here using the peak-over-threshold method based on campaign data but derived long-term loads are compared with similar estimates based on the summary data. Offshore winds are seen to govern the long-term loads at the site. Though the influence of wave heights on turbine long-term loads is smaller than that of wind speed, there is possible resonance of tower dynamics induced by the waves; still, to first order, it is largely the wind speed and turbulence intensity that control the design loads. Predicted design loads based on the campaign data are close to those based on the summary data discussed in a separate study.
Advances in Deepwater Steel Catenary Riser Technology State-of-the-Art: Part I — Design
Authors: Song Ruxin , Stanton Paul
Published: May 2009

Steel Catenary Riser (SCR) concept offers great advantages over others and has been widely deployed worldwide. The first deepwater SCR was installed in the Gulf of Mexico in 1994. Since then, more than 100 SCRs have been installed for many types of deepwater floaters (SPAR, TLP, SEMI, and FPSO) in the deepwater fields of West of Africa, Gulf of Mexico, and Offshore Brazil. This paper presents the state-of-the-art of the design methodology of deepwater SCRs. First of all, the design procedure is discussed and is also illustrated in a flowchart. Material selection is discussed in terms of weldability, corrosion resistance, and effect on riser performance. Different wall thickness sizing criteria and design codes are compared. The three most commonly used types of SCR hang off system (flex joint, stress joint, and pull tube) are presented and their application limitations are discussed. Strakes and fairings are discussed and compared as the vortex induced vibration (VIV) suppression devices. Focus is given to the design of SCR global configuration and riser routing. Effect of different floaters on the global configuration design is discussed and illustrated through examples. Thermal performance requirements versus riser global response are traded off. Corrosion, thermal insulation, and anti-abrasion coating materials available for deepwater SCRs are summarized. SCR cathodic protection design methodology is summarized and a design guideline is given. The number one challenge of deepwater SCR design is fatigue. Selection of SN curve, effect of sweet and sour service on fatigue performance, stress concentration factor (SCF) calculation, full scale fatigue testing requirements, application of fracture mechanics to engineering criticality assessment (ECA) is discussed. Fatigue mitigation design is also explored supported by examples. Design of the SCR subsea interface to flowline and pipeline is presented.