bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of a Cost-Benefit Model for Inspection of Offshore Jacket Structures in Mexico
Authors: Ortega Estrada Ce´sar E. , De Le´on Escobedo David
Published: January 2009

Planning of inspections is an important task for optimal maintenance of a jacket platform system, and the current practice has been based mostly on judgment and experience. Given the limited resources available for maintenance, there is a growing interest of PEMEX in the optimization of inspection strategies. In order to make easier and more practical the determination of the inspection strategies, in this work, it is assumed that inspections are performed at a constant time interval. A strategy is formulated so that the elements with larger failure probability have priority over other components in the inspection schedule. In this paper, the areas most prone to be damaged are identified, the frequency of damage as a function of its depth in a platform is studied, the levels of damage on joints and elements are identified to define the levels of repair, and the importance of each element is selected according to its capacity to contribute on the base shear resistance of the platform. In addition, a fatigue analysis of the platform is performed to identify the joints most prone to be cracked by fatigue, and to estimate the evolution of damage, where limits in measures for crack magnitude are found and a uniform probability distribution for crack magnitudes on joints are proposed. Several inspection intervals are proposed, their corresponding life-cycle costs are estimated and the optimal schedule is chosen so that the expected life-cycle cost is minimized. The expected costs associated with the no failure event include the costs of inspection and repair whereas the expected failure costs involve the potential consequences of damaged equipment, cost associated with the pollution by crude oil, injuries, cost of avoiding the loss of life, deferred production and indirect loss. In this paper, indirect losses are estimated through an assessment based on the concepts of the Leontief’s input-output model. In addition, relationships between each item costs and the global damage index are proposed. As before mentioned, the optimal inspection schedule corresponds to the minimum life-cycle cost.
Spectral Evolution and Wave Age Analysis of an Exceptional Storm Off the Mediterranean Spanish Coast
Authors: Sanchez Rodolfo Bolan˜os , Arcilla Agusti´n Sa´nchez , Gomez Jesu´s , Sairouni Abdel
Published: January 2009

A severe storm arose in the North-western Mediterranean during November 2001, causing severe damage to the coast (including loss of beaches, structural damage to harbours and also loss of human life). This paper focuses on the storm description and the corresponding spectral analysis of buoy data recorded during the storm and wind data collected from meteorological stations along the Spanish coast. Additionally it shows wave hindcast for the event and the implications of the spectral characteristics for wind wave prediction in the Mediterranean. The buoys recorded wave heights (Hs) of up to 6 m and periods (Tz) of 10 s. The direction (θp) was recorded as mainly easterly during the storm peaks and north-westerly during calm conditions. The changes of wave-age during the development of the storm have not shown correlation with wave Hs prediction errors. The recorded directional spectra present bimodal features in frequency and direction related to wind variability. The directional spectrum predicted did not reproduced such bimodal features. It is suggested to consider the bimodal characteristics and/or spectral width of recorded data for a better wave-age description in the Catalan coast.
Experimental Study on the Dynamic Response of a Moored Floating Wave Energy Device
Authors: Rapaka E. Vijayakrishna , Natarajan R. , Neelamani S.
Published: January 2009

A detailed experimental investigation conducted on a moored Oscillating Water Column (OWC) wave energy device has been reported in this paper. The experiments were conducted on 1:20 scale model of the wave energy device, which was moored to the bed using 6 mooring lines in a 2m wide (deep and shallow water) wave flume at Ocean Engineering Department, IITM, Chennai. A range of hydrodynamic parameters with different damping ratio of the OWC chamber at scope 4 (length of the mooring line/depth of water) for a constant water depth was used. The effect of non-dimensionalized parameters like non-dimensionlized wave frequency parameter (ω2B/2g) and device breadth to wave length ratio (B/L) on the mooring force and on the efficiency of the wave energy device has been studied. The motion responses and mooring forces were measured and the test results are analysed and presented with discussions in this paper.
On the Efficiency and Transmission Characteristics of a 1:20 Model Fixed Wave Energy Device
Authors: Rapaka E. Vijayakrishna , Neelamani S. , Natarajan R.
Published: January 2009

Wave transmission and pneumatic efficiency of an oscillating water column (OWC) type wave energy device resting on group of piles is investigated using physical model study. The caisson blocks 45% of the water depth. The co-efficient of transmission of the device varies from 0.1 to 0.4 for B/L range of 0.1 to 0.7, where ‘B’ is the width of the caisson in the direction of wave propagation and ‘L’ is the wavelength. The pneumatic efficiency varies from 20% to 50% with an average value of 0.35. The results of the present study can be used in the design of OWC caisson used for both wave energy conversion and breakwater in deeper water.
Multi-Layer Modeling of Wave Groups From Deep to Shallow Water
Authors: Lynett Patrick , Liu Philip L.-F. , Hwung Hwung-Hweng , Ching Wen-Son
Published: January 2009

A set of model equations for water wave propagation is derived by piecewise integration of the primitive equations of motion through N arbitrary layers. Within each layer, an independent velocity profile is determined. With N separate velocity profiles, matched at the interfaces of the layers, the resulting set of equations have N+1 free parameters, allowing for an optimization with known analytical properties of water waves. The optimized two-layer model equations show good linear wave characteristics up to kh ≈8, while the second-order nonlinear behavior is well captured to kh ≈6. The three-layer model shows good linear accuracy to kh ≈14, and the four layer to kh ≈20. A numerical algorithm for solving the model equations is developed and tested against nonlinear deep-water wave-group experiments, where the kh of the carrier wave in deep water is around 6. The experiments are set up such that the wave groups, initially in deep water, propagate up a constant slope until reaching shallow water. The overall comparison between the multi-layer model and the experiment is quite good, indicating that the multi-layer theory has good nonlinear, as well has linear, accuracy for deep-water waves.
Evaluation of Ductile Cracking Criterion for Grade X100 Linepipe and Relationship to Large Scale Fracture Behavior
Authors: Ishikawa Nobuyuki , Endo Shigeru , Glover Alan , Horsley David , Toyoda Masao
Published: January 2009

Recent developments in the manufacturing process of steel plate for high strength linepipe have enabled superior toughness to prevent brittle fracture of the pipe body. Techniques for non-destructive inspection have also improved, and large flaws that could lead to brittle fracture are highly unlikely in recent high strength pipelines. However, large amounts of plastic deformation can be expected in seismic or permafrost regions. Prevention of ductile fracture of the pipe body or weldment therefore becomes a key issue in defining the tensile strain limit. Ductile fracture is considered to occur by growth and coalescence of voids, and is affected by stress triaxiality and plastic straining at the cracked region. Although many studies have been carried out to evaluate ductile cracking criteria, its transferability to large-scale fracture behavior has not been thoroughly investigated. In this study, ductile cracking of high strength linepipe steels, Grade X80 and X100, was investigated. Notched round bar specimens with different notch root radii were tested to determine the precise conditions for initiation of ductile fracture. Stress and strain conditions at the notch regions were evaluated by FE analysis, and the “critical equivalent plastic strain” was defined at conditions corresponding to ductile fracture initiation in the experimental small specimen tests. Ductile crack initiation behavior was also determined for wide plate test specimens by making close observations of the notch root area. 3-D FE analysis of the wide plate tensile test showed that the equivalent plastic strain at the point of ductile fracture initiation was in close agreement with that in the notched round bas specimen. Thus, the “critical equivalent plastic strain,” determined by small notched round bar specimens, can be considered as a transferable criterion to predict large-scale fracture behavior in wide plate tests. Concepts of strain based design in terms of preventing ductile failure from a surface flaw by applying critical strain to cracking were also discussed in this paper. Results were compared to conventional grade linepipe steels and structural steels, showing that recent high strength linepipe steels have higher resistance to ductile cracking than conventional structural steels. In addition, 3-D FE analyses were used in a parametric study to determine the effects of Y/T and uniform strain on the onset of ductile cracking behaviour. The results of these analyses show the relative importance of materials properties on the resistance to ductile cracking.
Drag Force Reduction and VIV Suppression of Circular Cylinders Using Radial Water Jets
Authors: Skaugset Kjetil B. , Larsen Carl M.
Published: January 2009

Deep-water oilfield developments demand accurate predictions of vortex induced vibrations (VIV) of risers and free span pipelines subjected to ocean current. In order to prolong operational life of such structures, VIV suppression devices such as helical strakes or shrouds are often employed. Such devices will, however, imply certain disadvantages such as drag amplification and increased operational costs. Therefore the quest for effective suppression devices with a minimum of such drawbacks is still ongoing. The present paper presents a novel approach for VIV suppression based on radial water jets from prescribed patterns of circular openings in the cylinder wall. Jet flow will introduce a disturbance that will change VIV amplitudes. The alternation of the flow pattern must be understood to have both 2-dimensional (2-D) and 3-D effects. 2-D effects will influence the local pressure on the cylinder surface by altering the separation point as well as creating a general disturbance to the flow, while the 3-D effects involve changes in correlation of the vortex shedding process along the span of the cylinder. Results will be presented from experiments in a towing tank testing four 2m long spring supported cylinders with diameter 0.1m and three different patterns of radial water jets. One cylinder has three straight rows of holes at angular positions 0° and ±120° with respect to the ambient flow. The second has two straight rows of holes at positions ±120° with respect to the ambient flow, while the last has one straight line of holes blowing directly upstream. Since the first can be said to consist of the two latter, comparing them to each other gives valuable information in order to understand the physics of the first. The volume flow rate and reduced velocity have been varied in the tests. Oscillation amplitudes, frequencies, added mass and drag force coefficients are presented and compared to a smooth cylinder.
FPSO-Shuttle Tanker Collision Risk Reduction
Authors: Chen Haibo , Moan Torgeir
Published: January 2009

To reduce the occurrence of tanker drive-off, and to reduce the probability of recovery failure should drive-off happen, are two basic principles to reduce FPSO and DP shuttle tanker collision in tandem offloading. This paper focuses on measures to reduce recovery failure. Previous analyses revealed that recovery failure probability is high due to lack of time for DP operator to initiate the recovery action in tanker drive-off scenario. To increase time window and to reduce operator reaction time are therefore two main principles of recovery failure reduction. Substantial increase of the separation distance between FPSO and tanker, and possible reduction of the main propeller(s) forward thrust that can be used by tanker DP system are two measures to increase the time window. Effective reduction of operator reaction time can be achieved by a number of measures, i.e. training, procedure, crew resource management, and automation support, which are designed to improve early detection as well as effectively reduce operator’s time in diagnosis and situation awareness.
Effect of Truncated Demand and Capacity Distributions on Reliability of Pipelines
Authors: Nakat Ziad , Bea Robert
Published: January 2009

Reliability of pipelines has been considered assuming different probability continuous distributions of demand and capacities. These distributions in reality can be truncated at the tails by physical constraints such as pressure relief valves (demand truncation), and hydro-testing (capacity truncation). This paper describes the effect of truncations on the reliability of pipelines. The effect of truncation by relief valves on the demand distribution is studied first; the effect of truncation by hydro-testing on the capacity distribution is studied second; and last, the combined effect of truncation on demand and capacity is studied. A comparison and analysis of results is presented to assess the importance of truncated distributions of demand and capacity on the reliability of pipelines. The results show that truncated distributions can have large effects on the reliability and should be accounted for, since they can alter significantly inspection and management policies.
On the Extreme Analysis Applied to Moored Floating Platforms
Authors: Va´zquez-Herna´ndez Alberto Omar , Sudati Sagrilo Lui´s Volnei , Ellwanger Gilberto Bruno
Published: January 2009

This paper presents an insight into some different methods for prediction of extreme response values of slender structures belonging to moored floating units. The methods investigated are: Weibull-tail fitting, SRSS, SRSS modified, LS and LS modified (API). All these methods are based on a single time series sample of the response parameter. The statistical uncertainty associated to each method is assessed by means of analyzing several independent simulations of the response parameter. Numerical examples include the top tension response analysis of two mooring lines: one connected to an FPSO in deepwater and the other to an FPSO in shallow water.
Flow Assurance of Deepwater Oil and Gas Production: A Review
Authors: Su Jian
Published: January 2009

Flow assurance is essential for economic and reliable production of oil and gas in deep water environment. The present paper discusses the complex physical phenomena involved in deep water production and the challenging engineering problems of flow assurance; and reviews recent works to understand the processes and tackle the problems. The following topics are discussed: flow regime transition, pressure drop, flow pattern, slug flow and severe slugging, transient multiphase flow, thermal insulation, insulation materials, active heating and wax deposition.
The Role of ECA in the Development of Dynamic Riser Systems
Authors: Campbell Mike , Jones Stephen , Korloo Jafar
Published: January 2009

An Engineering Critical Assessment (ECA) is a procedure that may be used to define and characterise critical flaw sizes from which acceptance criteria for non-destructive examination (NDE) can be developed or confirmed. Dynamic riser systems are fatigue sensitive structures that are commonly regarded as one of the most challenging aspects of a deepwater development. An ECA provides the facility to increase confidence as it develops an understanding of the margin of safety afforded by the derived acceptance criteria based upon variations in extreme and long-term loads, material properties such as fracture toughness, quality standards for welding, control of stress concentration factors and the nature of potential failure. The paper will demonstrate the objectives, adopted methodology and explore the conclusions of an ECA with the aid of examples.
Influence of Ultra-Short Mooring Radius in Torsional Behavior of FPSO-Turret in Ultra Deepwater
Authors: Va´zquez-Herna´ndez Alberto Omar , Ellwanger Gilberto Bruno , Prates de Lima Edison Castro , de Albuquerque Carlos Frederico Alves
Published: January 2009

This paper discusses the effect of the torsional (yaw) stiffness of the mooring system of an FPSO turret in its mooring radius. This study was developed to determine the minimum mooring radius of a FPSOs turret mooring system in ultra-deep waters (3,000 meters) in Campos Basin, offshore Brazil. The paper shows that the mooring system torsional stiffness can limit the minimum mooring radius.
The Interaction Between Steep Waves and a Vertical, Surface-Piercing Column
Authors: Sheikh Rizwan , Swan Chris
Published: January 2009

The paper describes new laboratory observations concerning the interaction between a series of steep incident waves and a vertical, surface-piercing, column. The motivation for the study arose as a result of wave impact damage sustained to the undersides of several concrete gravity-based structures in the northern North Sea. Earlier work, Swan et al. [1], demonstrated that in the case of multiple column structures, the individual diameters of which lie outside the typical (linear) diffraction regime, there exists a new and previously unexpected mechanism leading to the scattering of high-frequency waves. Although the implications of this effect was carefully documented, not least because it explained the occurrence of wave impacts in relatively moderate seas, its physical origins remained unclear. In particular, it was uncertain whether this type of scattering would be observed in the case of a single column, or whether it results from the transmission of wave modes trapped between the legs of a multiple column structure. In the case of a single column, if the diameter, D, is such that the flow lies within the drag-inertia regime, D/λ < 0.2, where λ is the corresponding wavelength, linear diffraction theory suggests there will be little or no scattered wave energy. The present laboratory observations demonstrate that this is not, in fact, the case. If the incident waves are steep, a strong and apparently localised interaction is clearly observed at the water surface. This, in turn, leads to the scattering of high-frequency waves. Although these waves are relatively small in amplitude, their subsequent interaction with other steep incident waves takes the form of a classic long-wave short-wave interaction and can produce a significant increase in the maximum crest elevation relative to those recorded in the absence of the structure. The present paper will demonstrate that the scattering of these high-frequency waves, and their subsequent nonlinear interaction with other incident waves, has significant implications for the specification of an effective air-gap and hence for the setting of deck elevations.
Measuring Ice Loading on Podded Propellers Systems
Authors: Akıntu¨rk Ayhan , Jones Stephen , Moores Corwyn , Bell John
Published: January 2009

Currently, work is ongoing to update all of the regulations governing the design of vessels for arctic navigation including the propulsion systems by the International Association of Classification Societies. However, there is a lack of information as to the loads occurring on podded propeller systems operating in ice. Hence, this study aims to close the knowledge gap for this type of propulsion systems. This paper describes the experimental setup designed and built at the National Research Council of Canada Institute for Marine Dynamics (NRC-IMD), for measuring ice loads on such a system. The experimental system is designed to measure the loads on three levels: 1– loads on an individual blade (six components), 2– loads on the propeller shaft (thrust and torque) and, the fore and aft bearings which support the propeller shaft (six components), 3– global forces and moments on the (propeller + pod + strut) system (six components).
Non-Linear Space-Time Evolution of Wave Groups With a High Crest
Authors: Arena Felice , Fedele Francesco
Published: January 2009

The theory of quasi-determinism, for the mechanics of linear three-dimensional waves, was obtained by Boccotti in the eighties. The first formulation of the theory deals with the largest crest amplitude; the second formulation deals with the largest wave height. The theory was verified in the nineties with some small-scale field experiments. In this paper the first formulation of Boccotti’s theory, valid for the space-time domain, is extended to the second order. The analytical expressions of the non-linear free surface displacement and velocity potential are obtained. Therefore the space-time evolution of a wave group, to the second-order in a Stokes expansion, when a very large crest occurs at a fixed time and location, is investigated. Finally the second-order probability of exceedance of the crest amplitude is obtained, as a function of two deterministic parameters.
Oceanographic Data for the Design of Maritime Structures Under Cyclone Conditions in the Bay of Campeche, Mexico
Authors: Govaere G. , Silva R. , Mendoza E. , Martinez E.
Published: January 2009

In this paper we present a methodology for the evaluation of significant wave heights, significant wave periods, maximum winds and minimum pressures using a modification of the original Hydromet-Rankin Vortex Model, Bretchneider (1990) and Holland (1980). The cyclone model is compared to the data series of 29 cyclones recorded by NOAA National Data Buoy Center bouys. Both models present very good results. For the analysis, the database uses 53 years of records, 541 hurricanes on the Atlantic Ocean. For the extreme analyses of wind and wave heights on the Mexican coasts, maps of the location and scale parameters used in the Gumbel cumulative distribution function and numerical results for 50 and 100 years return period are provided.
ASTRA: An Automatic Spading Tool for the Remote Application at Abyssal Depths
Authors: Clauss Gu¨nther F. , Gerber Hans W. , Hippe Carsten
Published: January 2009

The future approach of the European marine research programme focuses on the establishment of multi-disciplinary networks including existing systems, and the development of new technologies for efficient installation and near real time communication. Within this framework the European Community funds the networks ORION (Ocean Research by Integrated Observation Networks - GEOSTAR 3) and ESONET 2 (European Seafloor Observatory NETwork) with several European partner institutions involved in marine research. Key technologies for deep-sea research have been developed in the frame of the GEOSTAR project (GEophysical and Oceanographic STation for Abyssal Research - Deep-Sea Mission). The concept comprises: • the deep-sea benthic observatory for geophysical and oceanographic purposes with its unique Data Acquisition and Control System (DACS), • the innovative underwater communication system as near real time interface, and • the deployment and recovery vehicle MODUS (MObile Docker for Underwater Sciences) for precise operations with heavy payloads. The know-how and existing equipment establishes a good basis for extensions and further developments to be used for network projects. ASTRA — an Automated Spading Tool for Remote Applications at abyssal depths — is one of these new concepts. This tool — integrated into the GEOSTAR-Bottom Station (BS) (also called node) and deployed by MODUS — will bury an Ocean Bottom Seismometer (OBS) into the sub-sea soil. The OBS will be laterally displaced to the node and connected by an umbilical. At the end of a long-term mission the node will be recovered by MODUS and the OBS is pulled out of the ground hanging underneath the node. Both components, node and OBS, will be recovered by the surface research vessel. Equipment and procedures developed within the GEOSTAR project have been focused on providing a long-term stationing on the ocean floor to perform scientific measurements. The ASTRA concept takes a next step and aims for the interaction with its environment. Based on the proven and reliable combination of the deep sea shuttle MODUS for heavy load transportation and the payload Bottom Station as the carrier of equipment the new module ASTRA will extend the spectrum of scientific operating possibilities in the deep sea adding engineering services. An iterative process with an integrated design application of a 3D-CAD-system, FEM structural analysis and Multibody Simulation (MBS) characterizes the development phase of ASTRA. Using virtual mock-up critical aspects of handling and operation can be identified, and in consequence more easily avoided. Simulation results are validated by experimental investigations. Operations with the ASTRA prototype and the complete network-system will be performed in the Tyrrhenian Sea in late summer 2003.
Comparison of Full-Scale Measurements With Calculated Motion Characteristics of a West of Africa FPSO
Authors: van Dijk Radboud R. T. , Quiniou-Ramus Vale´rie , Le-Marechal Guillaume
Published: January 2009

Full-scale measurements of the motions of the Girassol FPSO are available over a one-year period. During this same period also vessel draft, wave height and wave direction have been recorded. This allows the calculation of motion transfer functions as function of the FPSO loading condition. A comparison has been made between the measured motion transfer functions and the calculated motion transfer functions based on diffraction theory, taking into account the measured wave spreading. Viscous roll damping has been added in the calculation to obtain maximum agreement between measured and calculated RAO’s. The results show that wave spreading is a very important factor on how the vessel behaves and should be taken into account when evaluating measured full-scale motion RAO’s. This paper addresses the full-scale monitoring campaign in some detail and the related issues. Furthermore it describes the technique how measured wave spreading is taken into account in the calculation of theoretical motion RAO’s. It is shown that the agreement between calculated and measured RAO’s is greatly improved by the use of measured wave spreading in the calculation. The levels of viscous roll damping found in this tuning process can be described as function of loading condition and sea state. The viscous roll damping found in this process also shows good agreement to model test results performed earlier on this same FPSO.
Application of Equivalent Damage Concept to Evaluation of Ductile Cracking for Linepipe Under Large Scale Seismic Loading
Authors: Ohata Mitsuru , Toyoda Masao
Published: January 2009

A large scale seismic loading sometimes produces local buckling in onshore or offshore linepipe and subsequent loading can lead to ductile cracking followed by ductile failure. It is important to assess the ductile crack initiation of linepipe subjected to a large scale cyclic straining induced by seismic loading for safety assessment of linepipe. This paper is mainly paid attention to the applicability of the damage concept proposed by authors for evaluation of ductile cracking of steel pipe under large scale cyclic loading. The damage concept is based on the “two-parameter criterion”, using the effective plastic strain, which is taken into account mechanical and microstructural aspects of Bauschinger effect of steel. The transferability of small scale tensile test results to the assessment of ductile crack initiation of steel pipe under seismic loading by using the effective damage concept is verified by conducting cyclic bending tests for straight pipe with initial deflection. The effective damage strain under cyclic loading, which is derived from the evolution of back stress, was calculated by FE-analysis employing a combined (isotropic/kinematic) hardening material model. It is found that the critical safety assessment of ductile crack initiation can be conducted based on the strain-based criterion in accordance with the proposed damage concept.
Effect of Chloride and Partial Pressure of H2S on the SSC Susceptibility of Martensitic SS
Authors: Rogne Trond , Svenning Morten
Published: January 2009

Two weldable martensitic SS grades (1.5Mo and 2.5Mo) have been welded using matching consumables. Both seam welds and girth welds have been undertaken testing with respect to sensitivity towards sulphide stress cracking (SSC). The effect of chloride concentration, either simulating condensed water situation or simulated formation water situation, and H2S partial pressure have been investigated. Based on these studies application limits are evaluated. Under condensed water conditions the critical partial pressure of H2S increased 1–2 decades when the NaCl concentration was reduced from 1000 to 100 ppm. At 100 ppm of NaCl the critical partial pressure of H2S was >100 mbar. The same situation was observed under formation water conditions. The data also show a large increase in the SSC resistance when the seam welds were grit blasted prior to pickling and passivation.
MOLDI™: A Fluid Permeation Model to Calculate the Annulus Composition in Flexible Pipes — Validation With Medium Scale Tests, Full Scale Tests and Field Cases
Authors: Taravel-Condat Carol , Guichard Marc , Martin Joseph
Published: January 2009

In flexible pipes, the steel wires in the annular space can sometimes be in contact with a corrosive environment containing condensed water or seawater and acid gases (CO2 and H2S), coming from the bore by permeation through internal polymer sheaths. In order to choose the most suitable steel wires to avoid hydrogen embrittlement and to evaluate their resistance to corrosion and corrosion fatigue, it is necessary to know the annulus composition. A finite element model named MOLDI™ has been developed in order to precisely calculate this environment. This model is composed of two major modules: The first one describes the time dependent permeation of fluids through the pipe layers. Concentration and pressure versus time are calculated assuming Fick and Henry’s laws. The shielding effect of steel wires on the plastic sheaths and the temperature gradient through the structure are taken into account. The second module uses a thermodynamic flash algorithm to calculate the phase equilibrium in the annulus, including water condensation. In order to validate this model, a permeation testing program has been initiated in 1997. It is based on medium and full scale permeation tests and field cases. For each test, the pressure build-up and the composition inside the annulus are measured and compared with the ones calculated with MOLDI™. Medium scale prototypes consist in a polymer tube surrounded by a steel part simulating the annulus geometry of a flexible pipe (shielding, free volume). One of the prototypes has been specially designed in order to be able to maintain a chosen temperature gradient between the bore and the external side. The full scale permeation tests are conducted on a piece of manufactured flexible pipe. At the beginning of the test, the bore is filled up with a pure gas or with a fluid mixture (gas–water–crude oil) under pressure and temperature. The permeation rate from the bore to the annulus is determined by recording the pressure build-up in the annulus versus time. The gas composition in the annulus is measured using a Gaseous Chromatography device. Field cases have also been used to validate MOLDI™. To do that, the annulus of flexible risers has been monitored on site and the measurements compared with MOLDI™ simulations. The validations conducted at the time being, using different polymer sheath materials and fluid mixtures, have shown that our permeation model predictions are in good agreement with experimental results. Consequently, MOLDI™ is considered as validated and is now used to calculate the annulus composition from the field conditions.
Flow Induced Vibrations in Flexible Pipes
Authors: Lo̸tveit Svein Are , Eklund Trond , Olsen Gudmund Per , Pollestad Anne
Published: January 2009

Flexible pipes for large volume gas export and gas injection have been installed on several platforms offshore Europe. In some of these applications high frequency vibrations have been reported. Onset of the vibrations is related to flow rate. The vibration frequency is typically above 150Hz and structures with stresses above the fatigue limit will rapidly fail in fatigue. This paper covers the following: • Results from a measurement program performed at the Norsk Hydro Troll C platform. • Theoretical studies related to gas flow in flexible pipes. • Actions taken to control the flow induced vibrations at Troll C.
Dynamic Compression of Rigid and Flexible Risers: Part I — Experimental Results
Authors: Fujarra Andre´ L. C. , Simos Alexandre N. , Yamamoto Newton Y.
Published: January 2009

Dynamic compression is a critical issue for the viability of submerged lines used in offshore applications, especially for deepwater operations. The subject has been addressed both numerically and analytically. However, few experimental data exist in literature for validation purposes. The aim of this first paper is to present experimental results on the dynamic compression of rigid and flexible risers, obtained in towing-tank tests. Two small-scale models have been built, one emulating the dynamic behavior of a steel catenary riser (SCR) and the other corresponding to a much more flexible case. Uniform circular motion has been applied to the top of the line, representing the floating system oscillation. Four different amplitudes have been considered, each one of them with five different frequencies. The influence of current velocity has also been evaluated. Tension has been measured at the top. In this work the small-scale models and experimental setup are described and some comprehensive results are presented and discussed. In a companion paper, comparisons between theoretical (numerical and analytical) and experimental results are presented.
Wave Damping by Single and Double Chamber Breakwaters
Authors: Ramakrishnan Balaii , Vallam Sundar
Published: January 2009

The performance of horizontal slotted single and double screens in reducing the reflection and dynamic pressures on a vertical wall positioned on its lee ward side has been investigated through an experimental program. The wave screens consisted of equally spaced circular wave intercepting elements. The porosity of the screens was dictated by the clear spacing between elements. Four different screen porosity combinations were adopted for the study. The effect of an additional screen on the reflection from the wall and the dynamic pressures on the vertical wall have been compared with that due to the presence of a single screen by maintaining a same range of relative water depth, d/Lp. The details of models, instrumentation, analysis of the data and the results are presented and discussed in this paper.
Numerical Simulation of Artificial Purification System by Using Hydrostatic and FULL-3D Combined Model
Authors: Kinoshita Tsuguki , Tabeta Shigeru , Fujino Masataka
Published: January 2009

Ohmura bay is a typical enclosed estuary located in Kyushu, Japan. In the summer season, strong stratification is formed which brings oxygen-dificient water mass in the bottom layer. For the purpose of restoring water quality in the bay, field experiment of an artificial purification system was carried out. In the experiment, a diffusion pump was installed on the bottom of the bay. The instrument draws in the surface water of lower density and rich oxygen, mixes it with the bottom water of higher density and poor oxygen, and diffuses the mixed water upward. The mixed water is expected to spread along the isopycnic as density current, which will cause resolution of anoxic water in the bottom layer and promote the circulation of nutrients. However, it cannot be said the experiment was successful, and detail analysis by numerical simulation should be necessary in order to design more effective purification system. Most of ocean models employ the hydrostatic approximation because the horizontal scale is usually much larger than the vertical scale in oceanic phenomena. In the hydrostatic approximation, dynamic pressure is neglected and momentum equation of vertical direction need not to be solved. But in the present case, around the purification system, hydrodynamic pressure is not negligible and momentum equation of vertical direction must to be solved (called FULL-3D here). In FULL-3D calculation the time of calculation is much longer compared with using hydrostatic approximation. It is almost impossible to calculate the flow of the whole Ohmura bay by FULL-3D approach. The authors developed a new type of ocean model for multi-scale analysis, which conducts hydrostatic analysis for phenomena in wide area and FULL-3D analysis for the detail flow around the interesting object simultaneously. In order to connect the hydrostatic region and FULL-3D region, nested grid system is employed. Using this combined system, the effect of purification system to the whole bay will be investigated accurately.
Uncertainty Modeling and Probabilities in Engineering Decision Analysis
Authors: Faber Michael Havbro
Published: January 2009

The present paper initially gives a basic introduction on the interpretation of uncertainty and probability in engineering decision analysis and explains how in some cases uncertainties may change type depending on the “scale” of the applied modeling and as a function of time. Thereafter it is attempted to identify and outline the generic character of different engineering decision problems and to categorize these as prior, posterior and pre-posterior decision problems in accordance with the Bayesian decision theory. Finally an input is given to an ongoing discussion concerning the correctness and consistency of the uncertainty modeling applied in most recent reliability updating analysis for structural re-qualification and inspection and maintenance planning. To this end an outline is given in regard to the appropriate uncertainty treatment in the probabilistic modeling for different types of decision problems.
Minimum Wall Thickness Requirements for Ultra Deep-Water Pipelines
Authors: Torselletti Enrico , Vitali Luigino , Bruschi Roberto , Collberg Leif
Published: January 2009

The offshore pipeline industry is planning new gas trunklines at water depth ever reached before (up to 3500 m). In such conditions, external hydrostatic pressure becomes the dominating loading condition for the pipeline design. In particular, pipe geometric imperfections as the cross section ovality, combined load effects as axial and bending loads superimposed to the external pressure, material properties as compressive yield strength in the circumferential direction and across the wall thickness etc., significantly interfere in the definition of the demanding, in such projects, minimum wall thickness requirements. This paper discusses the findings of a series of ultra deep-water studies carried out in the framework of Snamprogetti corporate R&D. In particular, the pipe sectional capacity, required to sustain design loads, is analysed in relation to: • The fabrication technology i.e. the effect of cold expansion/compression (UOE/UOC) of TMCP plates on the mechanical and geometrical pipe characteristics; • The line pipe material i.e. the effect of the shape of the actual stress-strain curve and the Y/T ratio on the sectional performance, under combined loads; • The load combination i.e. the effect of the axial force and bending moment on the limit capacity against collapse and ovalisation buckling failure modes, under the considerable external pressure. International design guidelines are analysed in this respect, and experimental findings are compared with the ones from the application of proposed limit state equations and from dedicated FE simulations.
Bending Capacity of Pipes Subject to Point Loads
Authors: Vitali Luigino , Torselletti Enrico , Spinazze` Maurizio , Bruschi Roberto , Brunetto Luca
Published: January 2009

Large bending moments may develop on free pipeline lengths in the proximity of pipe sections subject to a local force. Sometimes the local force is such as to cause a partial loss of the sectional strength capacity of the pipe. This is the case of a pipeline plastically bent over the stinger of an S-lay barge, or of a pipeline laid on the sea bottom and hooked by an anchor or trawling gear, or of a pipe subject to cold bending when it is made to cope with sharp bottom roughness etc. In such conditions, the limit bending capacity of the pipe section, subject to local load effects, is significantly influenced. This aspect is not covered by international design codes and the scope of this paper is to show that, in some circumstances, it must be taken into due account. In this paper: • The relevant literature as concerns experimental tests, interpretative models, analysis methodologies and design approaches, is reviewed; • The FE model and post-processing, purpose-developed to investigate the interaction between local and global effect, are discussed; • The findings of FE analyses, in particular the effect of load combination, load history, pipe geometric characteristics and loading — magnitude and shape of the contact area, are presented. It is concluded that the limit bending capacity reduces significantly when local effects are such as to develop stresses on the pipe wall that affect the activation of the sectional buckling mechanism.
Risk Based Acceptance Criteria for Joints Subject to Fatigue Deterioration
Authors: Straub Daniel , Faber Michael Havbro
Published: January 2009

Different approaches to determine the acceptance criteria for fatigue induced failure of structural systems and components are discussed and compared. The considered approaches take basis in either optimization (societal cost-benefit analysis) or are derived from past and actual practice or codes (revealed preferences). The system acceptance criteria are expressed in terms of the maximal acceptable annual probability of collapse due to fatigue failure. Acceptance criteria for the individual fatigue failure modes are then derived using a simplified system reliability model. The consequence of fatigue failure of the individual joints is related to the overall system by evaluating the change in system reliability given fatigue failure. This is facilitated by the use of a simple indicator, the Residual Influence Factor. The acceptance criteria is thus formulated as a function of the system redundancy and complexity. In addition, the effect of dependencies in the structure on the acceptance criteria are investigated. Finally an example is presented where the optimal allocation of the risk to different welded joints in a jacket structure is performed by consideration of the necessary maintenance efforts.
A Study on Structural Design of Huge Barge Structures Based on Collapsing Behavior and Reliability Analysis
Authors: Katsura Shinji , Okada Hiroo , Masaoka Koji , Tsubogo Takashi
Published: January 2009

This paper deals with the limit state and reliability analysis of huge barge structures (HBS) which are supposed floating refuse storage and incineration plant based on collapsing behavior analysis in irregular waves as a part of studies on structural reliability-based design methods. First, a limit state and reliability analysis method is shortly presented for the buckling and ultimate collapse strength of deck, bulkhead and bottom panels of HBS. Next, a simplified method is briefly introduced for collapsing behavior and reliability analysis of HBS under extreme sea loads by using a developed system combined with a finite element method and plastic node method using hexahedral element models. Moreover, a simplified estimation method is shortly introduced for the probabilistic load model considering the hydro-elastic response of the structure in irregular waves. Finally, dominant limit state modes of 1,000m-class HBS under combined loads with bending moment, shearing force and lateral pressure are obtained by applying the above methods. Then, the features of the collapsing behavior and reliability level are investigated by using above calculation results. Effects of statistical values such as reduction of thickness due to corrosion, yield stress and design parameters are also investigated using sensitivity analysis.
Experimental Study on the Effect of Light-Weight Backfill for Enhancement of Earthquake Resistance of Buried Pipelines
Authors: Sakanoue Takashi , Yoshizaki Koji
Published: January 2009

In this paper, the effect of light-weight backfill was evaluated for reducing the soil-pipelines interaction. To investigate the effect of light-weight backfill, full-scale experiments were conducted. 100-mm-diameter pipeline was buried in the ground assuming a pipeline buried under roads. Then, the pipeline was pushed into the ground horizontally with a hydraulic jack and the reaction force was measured. The result showed that light-weight backfill had significant effect on the soil-pipelines interaction. In the case when EPS (Expand Poly Styrene) or EGW (Expanded Glass Waste) was used for backfill, the peak value of measured reaction force was less than that of the case when compacted sand was used for backfill. As a result, using light-weight for backfill in area where PGD is expected, the effect of reducing the soil-pipelines interaction and the earthquake-resistance can be increased.
Research on Characteristics of Responses on Floating Structures Due to Various Tsunami Wave Profiles
Authors: Masuda Koichi , Ikoma Tomoki , Uchida Maki , Kobayashi Akio
Published: January 2009

It is the objective of this research to develop practical estimation methodologies for responses of and the mooring force on a floating structure in shallow seas and to clarify the characteristics of responses induced by various tsunami wave profiles. In the present paper, the estimation methodology is introduced. The Tsunami propagation is numerically simulated from the hypocenter to shallow sea after an initial Tsunami elevation around sea area above the hypocenter is predicted by using Manshinha-Smylie’s method. The numerical simulation is carried out two-dimensionally. It is assumed that the floating structure is moored by catenary chains in shallow sea around Japanese Island. The two-dimensional boundary integral method is applied to a computation of the Tsunami propagations on a floating structure. The convolution integral method is applied to the prediction of the motion response with the catenary mooring tethers. In the present paper, some tools of parts of system are verified. The analysis method for the Tsunami exciting forces based on the 2-D BEM simulation program code is verified by comparing with the experimental results and another simulation results. The characteristics of responses due to the difference of Tsunami wave profile are discussed.