bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling Two-Phase Flow With Offshore Applications
Authors: Wemmenhove Rik , Loots Erwin , Luppes Roel , Veldman Arthur E. P.
Published: November 2008

With the trend towards offshore LNG production and offloading, sloshing of LNG in partially filled tanks has become an important research subject for the offshore industry. LNG sloshing may induce impact pressures on the containment system and may affect the motions of the LNG carrier. So far, LNG sloshing has been studied mainly using model experiments with an oscillation tank. However, the development of Navier-Stokes solvers with a detailed handling of the free surface allows the numerical simulation of sloshing. It should be investigated, however, how accurate the results of this type of simulations are for this complex flow problem. The paper first presents the details of the numerical model, an improved Volume Of Fluid (iVOF) method. The program has been developed initially to study the sloshing of liquid fuel in satellites. Later, the numerical model has been used for calculations of green water loading and the analysis of anti-roll tanks, including the coupling with ship motions. Recently, the model has been extended to incorporate two-phase flow. This extension improves its ability to simulate the effect of gas bubbles of different sizes. Gas bubbles are present in virtually all relevant offshore situations; not only at LNG sloshing but also during green water events, bow slamming and water entry. In a two-phase flow model, both the liquid and the gas phase can have their own continuity and momentum equations. The handling of the compressibility of the gas phase is a major issue in the design of a two-phase flow model. However, as a first step in the modeling process, the gas phase is considered as incompressible. For a dambreak experiment, results of the one-phase model, the incompressible two-phase model and model experiment results have been compared. It is shown that the physics are more accurately simulated with the incompressible two-phase model. Furthermore, the paper will show results of the incompressible model for LNG sloshing. The physics of LNG sloshing and several other applications can be approached better by taking the compressibility into account. Therefore, as a second step, a compressible model is currently under construction, involving adiabatic compression of the gas phase.
Sustained Load Cracking of Titanium Alloy Weldments
Authors: Kostrivas Tasos , Smith Lee , Gittos Mike
Published: November 2008

Failure of critical titanium parts, including some offshore components, has drawn attention to delayed cracking in Ti-6Al-4V alloys, but, given good design and alloy variant selection, such failures are avoidable. Delayed cracking, or sustained load cracking (SLC), can occur at low to moderate temperature (approximately: −50 to 200°C), depending on the titanium alloy and condition. Appropriate testing methods are required to generate stress intensity threshold values (KISLC) that can be incorporated into the design of titanium structures and recommendations are needed on the optimum chemistry and microstructure for greatest resistance. In the present work threshold stress intensity factor data (KISLC) were generated for Ti-6Al-4V alloy sheet, forgings, pipe and weldments using two different rising stress intensity factor test methods. It is concluded that material with a beta-annealed microstructure and low oxygen content (i.e. extra-low interstitial material such as ASTM Grades 23 and 29), has high resistance to SLC and that weld metal and transformed heat-affected zone also perform well, before and after postweld heat treatment, provided interstitial element pick-up during welding is prevented. Purchasing material in a general ‘mill annealed’ condition is not recommended without specifying acceptable microstructures. Further refinement of test method is also recommended for defining KISLC.
On the Effects of a Non-Stationary Seabed on the Morison Hydrodynamic Force for Off-Shore Structures
Authors: Olunloyo Vincent O. S. , Osheku Charles A.
Published: November 2008

It is well known that hydrodynamic forces can have significant effect on the dynamic stability and performance of moving offshore structures that are deployed for the exploration and exploitation of seabed geo-resources. This paper presents an integral transform approach for investigating the effects of non-stationary behavior of the seabed on the Morison hydrodynamic force associated with vertically moving submerged and partially buried offshore structures located in the neighborhood of an exploration zone. For this, the fluid-structure-soil dynamic interaction boundary value problem is modeled as a one-degree-of-freedom system, and the geo-mechanical behavior of the seabed is idealized as a spring and visco-elastic damper. In particular, the corresponding fluid velocity potential and acceleration kernels in the Morison force empirical relation as modified by the seabed poro-mechanics are computed and their implications for the design of such offshore structures analyzed.
Real Options Models for Maintenance Decision Making for Offshore Jacket Platforms
Authors: Santa-Cruz Sandra , Heredia-Zavoni Ernesto
Published: November 2008

Real options models are currently available as one of the best tools for the assessment of investment projects. This is so mainly due to the capability of the real options models to: (1) account for uncertainties in financial variables that are crucial to the investment project; and (2) quantify the value of the possibility to make a decision on whether to defer, abandon, expand or reduce the project at one or several points along time. Recently, some researchers have proposed the use of real options models for the assessment of infrastructure projects for hydrocarbon exploitation from an economics point of view. The objective of this work is to develop real options models for decision making regarding inspection, maintenance and decommissioning of offshore facilities taking into account the financial and technical aspects of the project. In all cases it is considered that at some point in the future, within the service lifetime of the structure, the decision maker will have an option to carry or not an inspection, and take or not a maintenance or decommissioning action, which will determine the structural and financial performance of the project for its remaining lifetime. The in-service times with no structural failure and the rehabilitation times are modeled as random variables. The cash flows are modeled as stochastic processes considering interruption of operation due to repairs after failure. Analytical expressions are derived for the computation of structural reliability and availability depending upon maintenance actions. An example is given for a jacket platform subjected to fatigue deterioration and damage. Simple and compound options of maintenance and decommissioning options are analyzed. The value of the project is computed by means of an approach similar to that of Black and Scholes for financial options [2]. The results are compared to those obtained under the traditional Net Present Value approach.
Reduction in Inspection Costs for Dynamic Sensitive Steel Structures by Modal Based Fatigue Monitoring
Authors: Hjelm Henrik , So̸rensen John D. , Brincker Rune , Munch Kasper , Graugaard-Jensen Jesper
Published: November 2008

In risk based inspection planning the number of planned inspections directly depends on the uncertainty on the load history. In this paper it is shown how this uncertainty can be reduced by modal based fatigue monitoring. The technique is based on continuously measuring of the accelerations in few points of the structure. The accelerations are integrated to obtain the displacement and a modal decomposition of the obtained displacements is performed. The relation between the modal coordinates and the stress history in an arbitrary point of the structure is established by a calibrated finite element model. The technique has been applied on a wind loaded lattice pylon structure and compared with strain gauges measurements, showing that the uncertainty on the presented technique is quite small. It is shown that if the technique is applied to an offshore steel structure, then the number of required inspections may be reduced with more than 50%.
A Numerical Lab to Predict the Strength Capacity of Offshore Pipelines
Authors: Bruschi Roberto , Bartolini Lorenzo , Spinazze` Maurizio , Torselletti Enrico , Vitali Luigino
Published: November 2008

In the recent years, the offshore pipeline industry has been under pressure to provide solutions for demanding material and line pipe technology problems, installation technology to safely tackle the ultra-deep waters challenge, quantitative prediction of reliable operating lifetime for pipelines under high pressure/high temperature conditions and remedial measures to tackle considerable geo-morphic and human activity related hazards. Future pipelines are being planned in very difficult environments, i.e. crossing ultra-deep water and difficult geo-seismic-morphic conditions. In these circumstances, it is of crucial importance (1) to adopt advanced design procedure and criteria, possibly based on limit state principles recently implemented in the design codes, and (2) to use advanced engineering tools for predicting the strength capacity and the pipeline behaviour during the installation and operational phase, in order to design the pipeline safely and to assess properly the technic-economical feasibility of the project. This paper discusses the relevant failure modes for offshore pipelines, the FE analysis results relevant to the sectional capacity of thick-walled pipes, and the FE analysis results relevant to the global and local response effect of a pipeline, laid on the sea bottom, and subject to a point-load force.
Propagation of Buckles in Sandwich Pipes Under External Pressure
Authors: Pasqualino I. P. , Lourenc¸o M. I. , Netto T. A.
Published: November 2008

Sandwich pipes have been considered feasible conceptions for ultra deepwater pipelines, since they are capable to work at low temperatures and withstand high hydrostatic pressures. Sandwich pipelines are composed by inner and outer metallic pipes and a suitable core material which must provide high compression strength and good thermal insulation. The aim of this ongoing research is to study the quasi-static propagation of buckles in sandwich pipes. In this paper, a three-dimensional finite element model considering material and geometric nonlinear behavior is presented. The mesh discretization is determined through a detailed mesh sensitivity analysis. Some experiments with small scale models combining aluminum pipes and polypropylene as core material were carried out to calibrate the numerical model. The propagation pressure is evaluated under different bonding conditions between pipe layers.
Fatigue Design Factors and Safety Level Implied in Fatigue Design of Offshore Structures
Authors: Wang Xiaozhi , Cheng Zhan , Wirsching Paul H. , Sun Haihong
Published: November 2008

With the publication of the ABS Guide for Fatigue Assessment of Offshore Structures (2003) and the Commentary to the Guide for the Fatigue Assessment of Offshore Structures (2004), application of the Fatigue Design Factor (FDF) is highlighted in fatigue assessments of offshore structures. Following review of FDF’s in available Rules/Guides from other authorities, FDF’s applied in the ABS Guide is presented and quantified with a corresponding safety level thus helping the user to relate FDF’s to estimated failure probability levels.
Nonlinear Simulation of Multivariate Sea State Time Series
Authors: Monbet Vale´rie , Ailliot Pierre , Prevosto Marc
Published: November 2008

In this paper, three nonlinear methods are described for artificially generating operational sea state histories. In the first method, referred to as Translated Gaussian Process the observed time series is transformed to a process which is supposed to be Gaussian. This Gaussian process is simulated and back transformed. The second method, called Local Grid Bootstrap, consists in a resampling algorithm for Markov chains within which the transition probabilities are estimated locally. The last models is a Markov Switching Autoregressive model which allows in particular to model different weather types.
Time-Variant Ultimate Strength Reliability of FPSOs Including Corrosion and Slamming Effects
Authors: Moatsos Ioannis , Das Purnendu K.
Published: November 2008

This paper discusses the effects of corrosion and slamming on Floating Production Storage and Offloading (FPSO) structures in the North Sea by using a time variant structural reliability based approach. An FPSO structure currently in operation in the North Sea is analysed, an analytical approach is suggested and used to determine the Ultimate Strength of the FPSO structure and compared with other published methods. A non-linear corrosion method is then used to determine the effect of corrosion on the ultimate strength and subsequently in the reliability of the structure. Slamming loads on the midship section of the FPSO are calculated using closed form expressions. The effect of slamming and green water load on the overall vertical bending moment applied on the structure is combined with the vertical wave bending and still water loads using the Ferry Borges- Castanheta method and the SRSS rule, to provide the distribution and variation of a loading component for structural reliability analysis to be performed. Time variant reliability analysis using Monte Carlo Simulation is then used to determine reliability indices and probabilities of failure for the structural system analysed on a component level.
Towards a Method for the Optimisation of Midship Section in Terms of Production Cost in Preliminary Ship Design
Authors: Mermiris George , Moatsos Ioannis , Das P. K. , Winkle I. E.
Published: November 2008

The most closely related attribute to the actual building cost of a new ship is its structural arrangement. Other particulars related to that cost, such as the pay load capacity, the propulsion installation, the endurance of the ship, etc., are more or less defined in the specification document. On the other hand, the structural integrity of the vessel is not in any case directly considered by the ship owner. It is responsibility of the designer to analyse it in depth and within the limits of a given budget. The latter feature directly implies that the strength and production cost of a new vessel are closely related and require special attention in the design stage since this is the time to make substantial changes with the least cost penalty. Considering the above, a method is proposed for the estimation of structural integrity of the hull in terms of production cost. The potential of this method lies in the fact that strength and cost virtues are expressed as functions of the same fundamental variables. As a result the method is particularly useful in the early design stages. Despite the simplistic approach, the foundation is set for a more rational approach of cost analysis in ship design.
Corrosion Fatigue of Welded Stainless Steels for Deepwater Riser Applications
Authors: Woollin P. , Maddox S. J. , Baxter D. J.
Published: November 2008

Steel risers for deepwater offshore oil and gas field developments are subject to seawater on the external surfaces, produced fluids on the internal surfaces and to fatigue loading. This paper reviews current knowledge of the corrosion fatigue behaviour of welded stainless steel for risers and presents results of testing of supermartensitic, duplex and superduplex grades in relevant environments.
Corrosion Fatigue of Welded C-Mn Steel Risers for Deepwater Applications: A State of the Art Review
Authors: Maddox S. J. , Pargeter R. J. , Woollin P.
Published: November 2008

Steel risers for deepwater offshore oil and gas field developments are subject to seawater on the external surfaces, produced fluids on the internal surfaces and to fatigue loading. This paper reviews current knowledge of the fatigue behaviour of welded carbon-manganese steel for risers in relevant environments. A substantial body of data exists relating to the performance of girth welds in seawater with cathodic protection and consequently recent attention has been turned to establishing the fatigue performance in the internal environment, which may contain water, CO2, H2S and chloride and bicarbonate ions.
Vessel Thruster-Thruster Interactions During Azimuthing Operations
Authors: Brown D. T. , Ekstrom L.
Published: November 2008

Flow interaction between thrusters is important, as many offshore vessels are equipped with rotating (azimuthing) thrusters for ease of manoeuvring and dynamic positioning. Accurate prediction of available thrust to resist horizontal environmental forces is essential allowing the realistic station-keeping capability of these vessels to be established. Additionally such thrusters are in many cases used, possibly in conjunction with tunnel thrusters and main propeller to provide propulsive power in transit. This paper presents the influence of two thrusters in close proximity, azimuthing so that flow into one thruster is influenced by the flow through the second thruster. The work extends that previously presented in OMAE 2002, based on a comprehensive experimental study to establish the influence on thruster performance of variations in current inflow velocity, thruster power, and thruster position relative to the hull.
Influence of Pressure in Pipeline Design: Effective Axial Force
Authors: Fyrileiv Olav , Collberg Leif
Published: November 2008

This paper discusses use of the effective axial force concept in offshore pipeline design in general and in DNV codes in particular. The concept of effective axial force or effective tension has been known and used in the pipeline and riser industry for some decades. However, recently a discussion about this was initiated and doubt on how to treat the internal pressure raised. Hopefully this paper will contribute to explain the use of this concept and remove the doubts in the industry, if it exists at all. The concept of effective axial force allows calculation of the global behaviour without considering the effects of internal and/or external pressure in detail. In particular, global buckling, so-called Euler buckling, can be calculated as in air by applying the concept of effective axial force. The effective axial force is also used in the DNV-RP-F105 “Free spanning pipelines” to adjust the natural frequencies of free spans due to the change in geometrical stiffness caused by the axial force and pressure effects. A recent paper claimed, however, that the effect was the opposite of the one given in the DNV-RP-F105 and may cause confusion about what is the appropriate way of handling the pressure effects. It is generally accepted that global buckling of pipelines is governed by the effective axial force. However, in the DNV Pipeline Standard DNV-OS-F101, also the local buckling criterion is expressed by use of the effective axial force concept which easily could be misunderstood. Local buckling is, of course, governed by the local stresses, the true stresses, in the pipe steel wall. Thus, it seems unreasonable to include the effective axial force and not the true axial force as used in the former DNV Pipeline Standard DNV’96. The reason for this is explained in detail in this paper. This paper gives an introduction to the concept of effective axial force. Further it explains how this concept is applied in modern offshore pipeline design. Finally the background for using the effective axial force in some of the DNV pipeline codes is given.
Numerical Simulation of Ringing Response in Marine Platforms
Authors: Han Seon
Published: November 2008

In this paper, ringing phenomenon in a drag dominated marine platform is studied numerically using linear transverse and nonlinear coupled axial and transverse beam models. The effects of inertia, added mass, and relative velocity of the structure on ringing are investigated. It is found that, for the structure considered, inertia and added mass have little effect on ringing, while the relative velocity had a significant effect on its duration. The maximum ringing amplitude predicted by the nonlinear beam model is similar to that predicted by the linear beam model. When a smaller second impact is introduced, ringing stops when the nonlinear beam model is used.
Integrity Assessment and On-Site Repair of a Floating Production Platform
Authors: Marinho Mauro G. , Pope Alexandre M. , Meniconi Luiz Claudio , Alves Luiz Henrique M. , Del Vecchio Cesar
Published: November 2008

Following the warning of a flooded bow horizontal brace of a semi-submersible production platform, an inspection diving team was mobilized and cracks were found at both bow and aft K-joints. Analysis of the service life of the platform, together with the results of structural analysis and local strain measurements, concluded that cracking was caused by fatigue initiated at high stress concentration points on the gusset plates inserted in the tubular joints. As a consequence of the fractured plates other cracks were nucleated close to the intersection lines of the braces that compose the K-joints. Based on this analysis different repair possibilities were proposed. To comply with the production goals of the Business Unit it was decided to repair the platform on-site and in production in agreement with the Classification Society. The proposed repair contemplated the installation of two flanges on the gusset plates between the diagonal braces by underwater wet (UWW) welding. Cracks at the gusset plates were also removed by grinding and wet welding. Defects located at the braces are being monitored and repaired by the installation of backing bars, by wet welding, followed by grinding and welding from the inside. To carry out the job two weld procedures and ten welder-divers were qualified.
Characterisation of the Wave Environment in the Fatigue Analysis of Flexible Risers
Authors: Sheehan John M. , Grealish Frank W. , Smith Russell J. , Harte Annette M.
Published: November 2008

As the offshore industry moves towards deeper water developments and continues to embrace harsh environments, unbonded flexible pipes are increasingly being utilised as a cost effective riser solution. Furthermore, with the advent of issues such as non-pristine annuli environments, the fatigue performance of these flexible risers is becoming a critical issue. This paper presents an overview of the comparisons between deterministic and stochastic global fatigue analysis techniques. Methods used to perform both deterministic and stochastic analyses are outlined, from performing the global analyses to using local models to generate armour wire stresses and subsequent fatigue damage. The paper identifies the key issues in the analysis performed and presents key results and conclusions with regard to the characterisation of the wave environment in the global fatigue analysis of flexible risers.
Dynamic Stress Predictions of Acoustic-Induced Pipe Vibration Failures
Authors: Karczub Denis G. , Fagerlund Allen C.
Published: November 2008

The dynamic stress prediction methodology developed by Norton [1] for broad-band acoustic-induced vibration of piping systems is applied here to the failure data of Carucci and Mueller [2]. Proprietary noise and vibration prediction technologies are used in order to improve the accuracy and robustness of the predictions. This results in generalized dynamic stress predictions that clearly delineate the cases of acoustic fatigue and satisfactory operation documented by Carucci and Mueller, unlike other failure prediction procedures traditionally used by industry. Other advantages of the dynamic stress approach include (i) a theoretically sound approach to account for pipe wall thickness, pipe diameter and internal density; (ii) direct consideration of material fatigue properties and stress concentration effects; and (iii) the potential to evaluate fatigue life for transient blow-down conditions.
Fracture Control — Offshore Pipelines: Current Status of Fracture Assessment for Pipelines, Limitations and the Need for Development
Authors: Bruschi Roberto , Torselletti Enrico , Vitali Luigino , Hauge Mons , Levold Erik
Published: November 2008

Recent development plans envisage the exploitation of very deep offshore reservoirs as well as transport of hydrocarbons at temperature and pressure conditions far more severe than in past projects. Technical feasibility of such projects requires higher material utilisation, and the design guidelines need to be improved to allow for the new design conditions. Fracture assessment methods have been used in the evaluation of pipeline integrity for several years. In particular, the verification of acceptable defect sizes for installation and operational loads are now widely used and assessment methods are referenced in pipeline standards and guidelines. However, design guidelines are still missing the calibrated safety factors and stringent design format required to let the fracture failure mode be consistent with the other failure modes in the pipeline design such as bursting, local buckling and fatigue. The Fracture Control Offshore Pipelines Project is a Joint Industry Research and Development Project, whose objective is to study the behaviour of defected girth welds in pipelines subject to construction and operational loads ever experienced before. Due to the envisaged high loading condition and the high costs of recent offshore pipeline projects it is important, with an accurate defect assessment analysis, to avoid delays caused by unnecessary repairs or failures because of flaws that should have been detected and repaired. The final objective is the development of specific design criteria in the form of a design guideline to be used in the verification and design of offshore pipelines against the fracture/plastic collapse failure of a defected girth weld. The design criteria are based on the application of reliability methods to calibrate the partial safety factors in compliance with the safety philosophy established by DNV OS-F101 and will include the rational application of new NDT techniques. The JI Project is carried over 5 years and has started in 2002. The JI project is sponsored by the industry (BP, ENI Norge, Hydro and Statoil) and by the Norwegian Research Council. This paper describes the current status of existing fracture assessment procedures for pipelines with particular attention to their limitations and the needs for development and a brief overview of the results obtained in the project so far as well as the challenges to be solved in the project.
Continuous Trawl Protection of Heating Cable for Direct Electric Heating of Flowlines
Authors: Ilstad Ha˚var , Sa˚tendal Svein Harald , Melve Bjo̸rn , Bo̸rnes Atle Harald , Henning Lars , Clasen Torunn Lund
Published: November 2008

The following paper describes the experimental and analytical work that was carried out in the development of a continuous external trawling protection for the Direct Electrical Heating cables for the subsea flowlines on the Kristin Field in the North Sea. The developed trawl protection was able to absorb the required 5.0 kJ energy without damage to the cable. The Kristin field is the most extreme HP/HT subsea field developed in Norway to date (170 degC reservoir temperature and 911 barg pressure). This is the first time that a continuous trawl protection is applied to the full length of a flowline. The normal approach would be burying to avoid interaction with fishing activities, however since this is a HP/HT subsea flowline it was necessary with an extra cooling from unburied pipelines to reduce the riser temperatures.
Fracture Control Offshore Pipelines: Advantages of Using Direct Calculations in Fracture Assessments of Pipelines
Authors: Thaulow Christian , Skallerud Bjo̸rn , Jayadevan K. R. , Berg Espen
Published: November 2008

Surface cracks pose major challenges for the structural integrity of pipelines. In fracture assessment programs the use of constraint parameters, such as the T-stress, along with K, J or CTOD are important to account for the limitations of single-parameter fracture mechanics. However, the three-dimensional nature of surface cracks precludes detailed 3-D finite element modeling for routine calculations. Here line-spring/shell-element models are demonstrated to be an efficient and reasonably accurate tool for constraint estimation even under large deformation levels when general yielding prevails in the pipe. Envisaging the potential use of this procedure in fracture analysis of pipelines, a new software, LINKpipe, has been developed. The program has been developed as a part of the Joint Industry project Fracture Control Offshore Pipelines. The objective of this project is to study the behaviour of defected girth welds in pipelines subject to construction and operational loads ever experienced before. The calculations have been performed in close cooperation with the project participants; see presentations of project-colleagues at OMAE 2005: Bruschi et al (2005), O̸stby (2005), Nyhus et al (2005) and Sandvik et al (2005). In this paper the line-spring calculations are compared with 3-D FE calculations and computations according to BS 7910. A pipe geometry, with OD = 400mm, was selected for the comparisons. The line-spring calculations were close to the 3-D calculations, while BS7910 was very conservative for long cracks and unconservative for short cracks. In highly ductile materials, such as pipeline steels, considerably amount of stable crack growth can be tolerated prior to the final failure of the structure. A simple method for simulating ductile tearing in surface cracked pipes with the line-spring model has been developed. A detailed parametric study has been performed to examine the effect of ductile tearing for pipes loaded in tensile, bending and with internal pressure. A significant reduction in deformation capacity from the stationary case is noticed. As the crack depth increases, the effect of ductile tearing becomes more important. And under biaxial loading a significant reduction of the deformation capacity is found as the internal pressure is increased. The development of the line-spring methodology paves the way for a transition from to-days rule-based design to direct calculations.
Damping-Controlled Response of a Truss-Pontoon Semi-Submersible With Heave-Plates
Authors: Srinivasan Nagan , Chakrabarti Subrata , Radha R.
Published: November 2008

Hydrodynamic added mass and damping are old and popular research topics in the field of offshore structures engineering. The concept of added mass has been used very intelligently in the design of modern deepwater floating vessels. The turning point of the conventional Spar to a Truss-Spar is a typical example in which the added mass created by the heave plates in a Truss Spar efficiently reduced the steel weight and the subsequent cost of the Spar hull. However, the damping is not utilized as efficiently as the added mass in the design of the floating offshore platforms. It should be noted that at resonance damping plays an important role in controlling the response amplitude. This resonance is called damping controlled response. An offshore platform efficiently designed to reduce the wave excitation forces and increase the separated-flow damping could qualify as a platform to operate even near resonance. Such design could make this concept cost effective, as well as operationally more productive with minimum downtime. The principal purpose of this paper is to describe an offshore platform design that could face the resonance efficiently. The paper applies the concept of both hydrodynamic added mass and separated-flow damping intelligently in the design of a large floating vessel on column-stabilized principle. The platform is designed to face resonance due to extreme waves and utilizes the damping to control its motion, thereby qualifying its field application. The design is verified and justified with the help of a scaled-model study in a large wave tank. The results are presented herein.
HotPipe JIP: HP/HT Buried Pipelines
Authors: Goplen Stig , Stro̸m Pa˚l , Levold Erik , Mo̸rk Kim J.
Published: November 2008

The HotPipe Project is a Joint Industry Project, whose overall objective is to prepare a DNV Recommended Practice to be used in structural design of high temperature/high pressure pipelines. The developed design criteria are based on the application of structural reliability methods to calibrate the partial safety factors involved. One of the three scenarios covered in this DNV-RP is buried pipes subjected to upheaval buckling which is discussed in this paper. The most significant factor in this scenario is uncertainty in the pipeline configuration and uncertainty in the pipe-soil interaction. The paper presents the background of the proposed soil capacities and the associated uncertainties for both uplift resistance and downward resistance in cohesive and non-cohesive soil. The paper links these soil models with the design requirements to upheaval buckling including: - Functional requirements i.e. survey data accuracy, smoothing of survey data, modeling of the pipeline, design conditions, soil cover etc.; - Trenching technology; - Qualification of the minimum soil cover, natural or artificial, with the aim to guarantee pipeline stability; - Assessment of pipeline response; - Pipe integrity checks and design criteria. The internal confidential project guideline has been completed and is currently in the process of being converted into an official DNV-RP-F110, to be published later this year.
HotPipe JI Project: High Pressure/High Temperature Pipelines Laid on Uneven Seabeds
Authors: Spinazze` Maurizio , Torselletti Enrico , Levold Erik
Published: November 2008

The HotPipe Project is a Joint Industry Research and Development Project, whose overall objective is to prepare a DNV Recommended Practice, to be used in the verification and design of high temperature/high pressure pipelines. The design guideline will cover most practical cases where pipelines are subjected to high internal pressure and high temperature i.e. exposed pipeline laid in a flat or uneven seabottom and buried pipes. The design criteria are based on the application of reliability methods to calibrate the partial safety factors involved. In this paper, the analysis methodology, the design procedures and relevant design criteria/functional checks for pipeline laid on uneven seabed, developed in the HotPipe Joint Industry Project, are discussed.
HotPipe JI Project: Experimental Test and FE Analyses
Authors: Vitali Luigino , Bartolini Lorenzo , Askheim Dag , Peek Ralf , Levold Erik
Published: November 2008

In the last twenty years, experimental tests and FEM-based theoretical studies have been carried out to investigate the buckling mechanisms of thin-walled pipes subject to internal pressure, axial force and bending moment. Unfortunately, these studies do not completely cover the scope relevant for offshore pipelines i.e. outer diameter to thickness ratio lower than 50. In the HotPipe Phase 2 JI Project, full-scale bending tests were performed on pressurized pipes to verify the Finite Element Model predictions from HotPipe Phase 1 of the beneficial effect of internal pressure on the capacity of pipes to undergo large plastic bending deformations without developing local buckling. A total of 4 pipes were tested, the key test parameters being the outer-diameter-to-wall-thickness ratio (seameless pipes with D/t = 25.6, and welded UOE pipes with D/t = 34.2), and the presence of a girth weld in the test section. For comparison a Finite Element Model was developed with shell elements in ABAQUS. The test conditions were matched as closely as possible: this includes the test configuration, the stress-strain curves (i.e. using measured curves as input), and the loading history. The FE results very realistically reproduce the observed failure mechanisms by formation and localization of wrinkles on the compression side of the pipe. Good agreement is also achieved in the moment capacities (with predictions only 2.5 to 8% above measured values), but larger differences arose for the deformation capacity, suggesting that the DNV OS-F101 formulation for the characteristic bending strain (which is based on FE predictions from HotPipe Phase I) may be non-conservative in certain cases.
Fracture Control — Offshore Pipelines: Experimental Studies on the Effect of Crack Depth and Asymmetric Geometries on the Ductile Tearing Resistance
Authors: Nyhus Ba˚rd , O̸stby Erling , Knagenhjelm Hans Olav , Black Scott , Ro̸stadsand Per Arne
Published: November 2008

Engineering critical assessment of offshore pipelines is usually very conservative if standardized single edge notch bend (SENB) specimens are used for the fracture mechanics testing. It is commonly accepted that the fracture toughness is dependent on the geometry constraint at the crack tip. The standardized SENB specimens have a high geometry constraint, and give lower bound fracture toughness for all geometries. For circumferential flaws in pipes the single edge notch tension (SENT) specimens is taken more into use, to establish more correct fracture toughness for the pipe in question. In this paper the effect of crack depth, misalignment and different wall thicknesses in SENT specimens have been studied. In addition the effect of crack depth and internal pressure in pipes have been studied with FE simulations.
Incorporating Human, Management and Organisational Barriers in Risk Assessment of Offshore Marine Operations
Authors: Trbojevic V. M. , Gudmestad O. T.
Published: November 2008

The paper presents an effective way of incorporating the human, organisational and management factors in order to improve integrity of offshore marine operations. The aim of the paper is to establish safety barriers and integrate human, organisational and management factors into these barriers and define performance shaping factors that could erode the barrier function. This can be achieved by using bow tie analysis. Furthermore, the quantification of the risk model, if required, can be achieved by direct application of human reliability methods for operational errors and by defining the failure rate multiplier based on the assessment of management and organisational characteristics.
System Reliability-Based Assessment on Single Point Mooring Jacket Platform
Authors: Jin Wei-Liang , He Yong , Gong Shun-Feng , Song Jian
Published: November 2008

The Single Point Mooring (SPM) jacket platforms have been developed over the last several decades and are widely applied in the exploitation of offshore oil-gas resources. The SPM jacket platforms are continuously subjected to environmental loads due to wave, current, wind and mooring force. As the uncertainty of those loads, the system reliability research of SPM jacket platforms is one of the most important research areas. A methodology of system reliability-based assessment of the SPM jacket platform was introduced in this paper and applied for the assessment of BZ28-1 SPM jacket platform. According to nonlinear mechanical analysis, the mechanics characteristics of SPM jacket platform subjected to extreme loads were analyzed on considering pile-soil-structure nonlinear interaction. Based on the FEM model, with Monte Carlo simulation, the reliability of SPM jacket platform was studied, and the system reliabilities in 20-year-based-period and 50-year-return-period were calculated respectively. The results show that the statistical analysis of parametric variables based on 50-year return period is appropriate, and the mooring forces due to wave are the predominant loads for SPM jacket platforms. The system reliability-based assessment method can provide important and reliable referential suggestions for the design, maintenance and repair of SPM jacket platforms.
A New Conjugated Infinite Element for Modelling Wave Diffraction by Multiple Bodies
Authors: Baghbani A.
Published: November 2008

In this paper a new Infinite Element is presented to solve the problem of two-dimensional linear water wave diffraction by arrays of solid bodies. The method is valid for any number of arrays of arbitrary individual geometries, number and configuration of bodies. Example problems are solved. The results are compared with the analytical solutions for the special case of wave diffraction by arrays of bottom-mounted vertical circular cylinders. The comparison shows a good agreement, thus verifying the accuracy of the infinite elements for water wave diffraction problems.
Hydrogen and Oxygen Pick-Up in Hyperbaric TIG Welding of Supermartensitic 13% Cr Stainless Steel With Matching Filler Wire
Authors: Aune Ragnhild , Knagenhjelm Hans Olav , Ha˚rsvær Ansgar S.
Published: November 2008

The possible sources causing weld metal hydrogen and oxygen pick-up during offshore hyperbaric tie-in TIG welding have been identified and simulated at a hyperbaric chamber pressure of 12 bar, i.e. 110 meter sea depth. The base material was supermartensitic 13% Cr stainless steel. Matching filler wire was used. The weld metal hydrogen and oxygen pick-up from water vapor in the shielding and chamber gases has been investigated by girth welding of pipes. Moist chamber gas seems to have insignificant effect on hydrogen and oxygen pick-up. The largest contribution is from moist shielding gas. Most of the hydrogen content in the supermartensitic welds is diffusible. By applying post-heat, it is possible to reduce the hydrogen content in the weld metal. Microcracks in the cap were observed for welds deposited with 14 mbar water vapor in the shielding gas (117 ppm) and above. Microcracks in the root welds were not observed, which was also confirmed for self restrained cracking tests. During constant load test at 150 MPa simulating hydrostatic testing of pipelines, the microcracks grew, and for the weld deposited with fully moistened shielding gas (175 ppm), a macrocrack appeared in the centre of the capping pass longitudinally to the welding direction. Crack Tip Opening Displacement (CTOD) values based on Single Edge Notched Bend (SENB) testing were all low, and the high constraint of the SENB specimen did not show any effects of hydrogen on fracture toughness. CTOD values based on Single Edge Notched Tension (SENT) testing decreased with increasing shielding gas moisture contents. For specimens without microcracks it was possible to increase the CTOD (SENT) values by post-heat, close to the toughness observed for specimens deposited with dry shielding gas.
Comparison of Formulations for Whitecapping in Mixed Seas
Authors: Lacoin Se´bastien , Hurdle David P. , Van Vledder Gerbrant Ph.
Published: November 2008

Four formulations for dissipation by whitecapping in discrete spectral wave models have been compared, and their applicability in combined swell-sea systems has been investigated. To obtain workable formulations, a sensitivity study has been carried out, followed by a simple calibration. Finally, the effect of a small background swell on the growth of a wind sea has been studied for all formulations.
A 3-Dimensional Continuum ALE Model for Ice Scour: Study of Trench Effects
Authors: Konuk Ibrahim , Yu Shenkai , Gracie Robert
Published: November 2008

A new Arbitrary Lagrangian Eulerian (ALE) Finite Element (FE) model of ice scour was recently developed by the authors. It is based on continuum representation of the soil. It was shown in recent papers that such a model can characterize the mechanics of the ice-soil-pipeline interaction without requiring any of the assumptions that the Winkler models depend on. The model utilizes soil properties obtained by conventional laboratory testing. In a recent paper, this model was used to show that the subscour deformations and ice-soil interaction forces are very sensitive to ice ridge geometries for shallow slope ice features. In this paper, the ALE FE ice scour model is utilized to study the effects of the pipeline trench on the scour process and the forces transmitted to the pipeline. Two different infill soil properties and two different ice ridge geometries are analyzed with a 36 inch diameter pipe buried to in a trench of 1.5 m cover. It is shown that the scour process near and in the trench is significantly different than in the ambient seabed soils and the recognition of this may present some potential advantages for the protection of the pipelines not recognized by the Winkler models. It is also shown that the pipeline loads generated during the scour process are cyclic. They build slowly as the ice moves over the trench and then reverse as the ice ridge moves away from the trench. This is in contrast to monotonic and rapidly growing loads predicted by the Winkler models. The paper shows that the loads transferred to the pipeline depend on the infill soil properties placed in the trench. It is shown that loads experienced by the pipeline are less for the softer infill than stiffer soils.
Float-Off Operation of a Land-Built COT on Dual Transportation Barges: Time- vs. Frequency-Domain Analysis
Authors: Yang Chan K. , Lee D. H. , Kim M. H. , Park B. N. , Yang Y. T. , Yoon K. Y.
Published: November 2008

In this paper, the float-off operation of a land-built crude oil tank (COT) loaded out and towed on launching dual-submersible barges is numerically simulated by a time-domain vessel-mooring-riser coupled dynamic analysis program with multiple floating bodies. The study is particularly focused on the maximum load prediction on connectors and the minimum gap prediction between barges and the COT. In case of simpler modeling, the time-domain simulation results are compared with frequency-domain results. Then, the time-domain model is run for more realistic situation with hawsers and mooring line. In hydrodynamic analysis, the interactions among the 3 floating bodies are fully taken into consideration. In the frequency-domain analysis, the connectors between barges are modeled by equivalent translational and rotational springs, the stiffness of which is estimated using Euler’s beam theory. In order to assess the possible occurrence of contact between COT and barges, the relative motions between barges and the COT at several points of interest were investigated.
Experimental Study on a Self Standing Hybrid Riser System Throughout Tests on a Deep-Sea Model Basin
Authors: Pereira Paulo S. D. , Maeda Katsuya , Morooka Celso K. , Tamura Kenkichi , Itoh Kazuaki
Published: November 2008

The present paper introduces a concept of a hybrid riser, Self Standing Hybrid Riser (SSHR), particularly in development for ultra deepwater around 3000 meters. Main problems faced in the initial definitions for design of such risers in the described scenario are discussed. Unique laboratory test with reduced model carried out in waves, current and riser top oscillation experiments in a very deepwater wave tank is shown. And, experimental results are shown and discussed, especially considering difficulties faced to carry out this experiment. Discussions are addressed in terms of vortex induced vibration (VIV) around buoyancy can, riser line pipe and flexible jumper.
Laboratory Investigation of Environmentally Induced Cracking of API-X70 and X80 Pipeline Steels
Authors: de S. Lima Kellen Rose , de S. Bott Ivani , da C. Ponciano Gomes Jose´ Antoˆnio
Published: November 2008

Carbon steels, used in pipelines for the transport of oil and its derivatives, are frequently exposed to fluids. This can result in stress induced corrosion cracking (SCC) and/or hydrogen embrittlement (HE). The present paper evaluates the susceptibility of pipeline steels (API-X70 and API-X80) to SCC and HE, using a slow strain rate test (SSRT) based on the National Association of Corrosion Engineers’ (NACE) norm and a traditional standard NACE test. The (SSRT) method used, employed a sodium thiosulphate solution to evaluate susceptibility to HE, thereby offering a simpler experimental procedure than the standard NACE test. The results confirm the efficacy of the sodium thiosulphate as an H2S–SCC susceptibility test solution when utilised in SSRT testing. Though no secondary cracks were detected in the materials investigated, both steels were observed to suffer a ductility loss upon exposure to this solution. In NACE type tests, the test pieces were subjected to constant loading at 80% of σy. Fracture did not occur for these samples.