bluuebunny/crossref_metadata_2025 huggingface dataset:
Using In-Line Inspection to Address Deformations Containing Near-Neutral pH Stress Corrosion Cracking
Authors: Johnston Dennis C. , Hrncir Thomas G.
Published: March 2009

Marathon Ashland Pipe Line LLC (MAPL) experienced a pipeline release on January 27, 2000 from cracking in a bottom-side shallow deformation. The crack that caused the release was determined to have propagated due to corrosion fatigue that progressed from an area of near-neutral pH stress corrosion cracks. A combination of a transverse field magnetic flux inspection (TFI) tool and a slope-deformation tool were used to inspect the entire pipeline segment for additional injurious deformations that could contain cracks. The TFI tool data were used to differentiate deformations that contained linear indications (typically cracks or corrosion) from deformations that did not. The slope-deformation tool data were used to measure the deformation magnitudes and shapes and to assist with locating the (TFI) deformations that were to be excavated. The pipe conditions necessary for these types of cracking to occur are discussed as well as the findings of the in-line inspection and remediation program. Prior to this release, it was perceived within the industry that constrained shallow deformations were not likely to fail catastrophically. The failure mechanism, particularly from constrained deformations, was normally as a leak. The near-neutral pH stress corrosion cracking phenomenon within deformations was first thought to be a unique event. Based on the investigation program conducted by MAPL, this phenomenon was identified elsewhere within the pipeline system. Based on MAPL’s investigation, pipelines susceptible to or containing slight deformations (typically high D/t ratios) in areas with groundwater containing high levels of dissolved carbon dioxide and coated with materials prone to shielding cathodic protection may be particularly susceptible to the deformation near-neutral pH stress corrosion cracking phenomenon.
Casing Treatment of Various Types in Compressor Stage: Parametric Representation
Authors: Nezym Vitaliy Yu.
Published: March 2009

Applying of casing treatment in compressor has only some examples of positive influence on compressor efficiency simultaneously with very significant increase of stable operation range. It was performed the parametric investigation of entire annular recesses (EAR) of various configuration and dimension in casing above compressor rotor blades. It was tested rotor with various tip clearances and two groups of rotor with EAR (without and with penetration of rotor blade tips to the casing). The results of 20 tested devices are presented in the proposed paper. The obtained results are analyzed in connection with relative tip clearance and EAR relative volume influence on compressor rotor efficiency. The conclusions permit to wait some positive EAR influence on rotor efficiency due to useful choice of EAR configuration and dimensions.
Long Seam Welds in Gas and Liquids Pipelines and Near-Neutral pH Stress Corrosion Cracking and Corrosion Fatigue
Authors: Eadie Reg , Hung Larry W. , Sutherby Robert , Roy George , Shen Guowu , Luo Jingli , Chen Weixing , Hamre´ Ted C. , King Fraser , Jack Tom
Published: March 2009

Near-neutral pH SCC and corrosion fatigue at the toe of the longitudinal weld has been responsible for several pipeline failures in Canada. In this paper examples of such failures and other in-service toe cracks are discussed. The results of an investigation into several factors affecting near-neutral pH SCC at the toe of the longitudinal weld are discussed and summarized. Four different pipes were evaluated including both double submerged arc welded (DSAW) pipe and electrical resistance welded (ERW) pipe. The properties considered were 1) residual stresses near the toe of the weld as determined by hole drilling and slitting, 2) stress-raising characteristics of the weld shape as determined by elastic finite element analysis and 3) electrochemical properties near the toe of the weld as determined by the scanning reference electrode technique. For one line pipe we also evaluated the room-temperature creep characteristics near the weld with and without cyclic loading. The implications of these results for near-neutral pH SCC and corrosion fatigue are discussed. It is concluded that high stresses originating from the stress-multiplying effects of the weld crown contributed significantly to the failures.
Reliability-Based Limit States Design for Onshore Pipelines
Authors: Nessim Maher , Zimmerman Tom , Glover Alan , McLamb Martin , Rothwell Brian , Zhou Joe
Published: March 2009

The traditional approach to pipelines design is to select a wall thickness that maintains the hoop stress below the yield strength multiplied by a safety factor. The main design condition implied by this approach is yielding (and by extension burst) of the defect-free pipe. Failure statistics show that this failure mode is virtually impossible as the majority of failures occur due to equipment impact and various types of defects such as corrosion and cracks. Recent investigations show that these failure causes are much more sensitive to wall thickness than to steel grade. As a consequence, current design methods produce variable levels of safety for different pipelines — small-diameter, low-pressure pipelines for example have been shown to have higher failure risks due to mechanical damage than large-diameter, high-pressure pipelines. In addition, the current design approach has been shown to have limited ability to deal with new design parameters, such high steel grades, and unique loading conditions such as frost heave and thaw settlement. The paper shows how these limitations can be addressed by adopting a reliability-based limit states design approach. In this approach, a pipeline is designed to maintain a specified reliability level with respect to its actual expected failure mechanisms (known as limit states). Implementation involves identifying all relevant limit states, selecting target reliability levels that take into account the severity of the failure consequences, and developing a set of design conditions that meet the target reliability levels. The advantages of this approach include lower overall cost for the same safety level, more consistent safety across the range of design parameters, and a built-in ability to address new design situations. Obstacles to its application for onshore pipelines include lack of familiarity with reliability-based approaches and their benefits and lack of consensus on how to define reliability targets. The paper gives an overview of the reliability-based design approach and demonstrates its application using an example involving design for mechanical damage.
Pipeline Girth Weld Residual Stresses and the Effects of Hydrotesting
Authors: Chauhan Vinod , Feng Zhili
Published: March 2009

Welding residual stresses are an important consideration in the fracture mechanics based fitness-for-purpose (FFP) assessment of steel structures. Reliable predictions of structural integrity can only be made provided that welding residual stresses are adequately accounted for. In the majority of cases, their magnitude is not known and can vary widely. In the absence of detailed information, it is common practice to assume that the welding residual stress is tensile, uniform through the thickness and of yield strength magnitude. However, this assumption will often lead to conservative fracture assessments which may lead to the conclusion that a weld repair is necessary when in practice the structure is safe to continue operation. In this paper, an integrated thermal-metallurgical-mechanical finite element (FE) model is described which simulates the formation of residual stresses at pipeline girth welds. The simulation takes into account detailed variations of the microstructure in the weld and heat affected zone (HAZ) in order to predict residual stress levels. Results of the FE analysis were validated with measurements of the microhardness and surface residual stresses using the air abrasive center hole drilling method. Sensitivity of residual stress levels to steel strength level, pipe wall thickness and pipe misalignment is discussed. The effects of hydrotesting and the alleviation of welding residual stresses are also described.
Blockage Remediation in Deep Water Pipelines
Authors: Kuchpil Ca´ssio , Gonc¸alves Marcelo A. L. , Ferreira Antoˆnio C. P. , Albernaz Roberto S. , Camerini Cla´udio S. , Marquez Luiz C. C.
Published: March 2009

Flow assurance is an important issue in the design and operation of production systems in deep waters. The implementation of prevention and remediation methods is necessary mainly due to the low temperatures, high production pressures, long tie-ins and oils prone to organic deposit formation. Despite the development and improvement of these prevention and remediation techniques, failures or exceptional operational conditions can lead to the complete blockage of the submarine flowlines, risers or equipment. Although the complete blockage is not frequent, the related production losses generally are high; furthermore, the technical difficulties and the costs involved in the removal of blockages can be high. The steps to the remediation of subsea blockages are the localization, identification and removal methods. Due to the variety of problems, the different subsea layouts and surface facilities, it is not possible to have a general recipe for all problems. This paper presents some blockage remediation cases, including the localization and blockage remediation methods. The blockage localization methods used for the blockage removal cases described in this paper are the following: a) the echo of pressure pulses reflected at the blockage and b) a tool that detects the pipeline diameter variation with the pipeline pressure variations. The field results for these methods and the pros and cons of the methods are discussed. The remediation methods described are the following: external heating, internal intervention and exothermal chemical reaction using gravity.
Natural Hazard and Risk Management for Pipelines
Authors: Savigny K. Wayne , Porter Michael , Chen Joyce , Yaremko Eugene , Reed Michael , Urquhart Glenn
Published: March 2009

Pipeline systems must contend with many hazards, of which ground movements such as landslides and washouts represent one type. Under the broader umbrella term, natural hazards, individual ground movement threats can be subdivided into geotechnical and hydrotechnical hazards. A four-phase natural hazard and risk management system (NHRM) is being developed. Although research and development are ongoing, implementation over the past seven years spans approximately 25,000 km of main-line pipeline in North and South America. It complies with CSA requirements for ‘hazard identification’ as well as current standard-of-care guidelines related to case-law in Canada. It is designed as a simple yet reproducible methodology that can be operated by pipeline companies, particularly their field staff. The first two phases of hazard identification/assessment are described here with reference to a recent study of hydrotechnical hazards along the Trans Mountain Pipe Line Co. Ltd. main line from Hinton, Alberta to Kamloops, British Columbia in the mountains of western Canada. The relative hazard ratings generated by the Phase I and II methodology can be integrated into existing risk management methodologies used in the industry. Alternatively, the risk assessment and risk management methodology of the NHRM system can be used as outlined in this paper.
Structural Integrity Assessments of Pipelines Using Crack Growth Resistance Curves
Authors: Hippert Eduardo , Dotta Fernando , Ruggieri Claudio
Published: March 2009

This study explores further extension of the computational cell methodology to model Mode I crack extension in high strength pipeline and structural steels. First, validation analyses of the cell methodology described in this study compare predictions of crack growth response with experimentally measured R-curves for a standard DIN StE 460 steel. Next, laboratory testing of an API 5L X70 steel at room temperature using standard, deep crack C(T) specimens provides the crack growth resistance curve to calibrate the micromechanics cell parameters for the material. The cell model incorporating the calibrated material-specific parameters is then applied to predict the burst pressure of a thin-walled gas pipeline containing longitudinal cracks with varying crack depth to thickness ratios (a/t). The plane-strain analyses reported here demonstrate the capability of the computational cell approach to simulate ductile crack growth and to predict the burst pressure of thin-walled tubular structures containing crack-like defects.
Risk-Based Integrity Project Ranking
Authors: Misewicz David , Smith Anita Cuevas , Nessim Maher , Playdon David
Published: March 2009

Kinder Morgan, Inc. (KMI) is one of the largest midstream energy companies in North America, operating more than 30,000 miles of natural gas and product pipelines. Major interstate natural gas pipeline assets include Natural Gas Pipeline Company of America (NGPL), Kinder Morgan Interstate Gas Transmission, L.L.C., TransColorado and Trailblazer. NGPL transports up to 5.7 billion cubic feet (Bcf)/day) of natural gas through 10,000 miles of pipeline and has 210 Bcf of working gas storage. Other gas pipeline operations in intrastate service include Kinder Morgan Texas Pipeline, L.P., Kinder Morgan Tejas Pipeline, L.P., Northern Gas Company and Rocky Mountain Natural Gas Company. KMI affiliates also own and operate significant liquid pipelines, CO2 pipelines, bulk and liquids terminals, natural gas retail distribution and electric generation. KMI has a long history of performance under a risk based Integrity Management Program. Integrity maintenance projects carried out in a given year are selected from a list of proposals submitted by individual pipeline operations managers. A variety of integrity project proposals are received for specific pipeline segments each year, including replacements, in-line inspections and hydrostatic tests. KMI’s Risk Engineering group performs a risk-based evaluation of the projects proposed in any given year to identify the most cost effective collection of projects that provide the greatest level of risk reduction. The approach is based on a benefit cost ratio, defined as the expected risk reduction in dollars per mile over the project useful life, divided by the total project cost. Risk reduction is estimated using a quantitative risk analysis approach in which the failure rate reduction achieved by carrying out a given project is multiplied by the expected failure costs. The project ranking provides a useful guide for selecting projects that fit within the maintenance budget while providing the greatest risk reduction. The benefit cost results can also be used as a tool to justify the maintenance budget. This paper describes the quantitative risk evaluation approach and demonstrates its benefits, which include substantial potential savings and a convincing case to support the decisions made.
Computational Pipeline Monitoring in Challenging Situations
Authors: Short James E.
Published: March 2009

This paper introduces a new, active methodology to modeling and leak detection intended to mitigate the effects of data uncertainty in such challenging situations, and presents three case studies. The American Petroleum Institute (API) has coined the phrase Computational Pipeline Monitoring (CPM) to encompass several methods of leak detection. The use of real-time transient hydraulic simulation tools, driven by data gathered by a Supervisory Control and Data Acquisition (SCADA) system, is one form of CPM system. Such real-time simulations impose SCADA-gathered data (typically pressures, flows, temperatures) onto a characterization of the pipeline (the model) and the fluids in the system. In a tuned CPM system, if the SCADA-gathered data cannot be successfully imposed on the model without transgressing the laws of fluid mechanics, this signifies a pipeline anomaly, which may be a release. However, in reality, many pipeline hydraulic anomalies are due to changing uncertainties in the data presented to the model and if annunciated to the pipeline operators would constitute a “false leak alarm.” While they typically are not large enough to compromise pipeline operations, uncertainties abound in the SCADA-gathered data. Even were the SCADA-gathered pressure and temperature data to contain no uncertainty, the fluid properties might not be sufficiently characterized for the simulation to accurately calculate how the fluid behaves under pressure and/or temperature changes. Measurement failure further complicates the task of the CPM application, as does slack line flow. Uncertainty in the CPM-driving data is not constant, it is ever-changing with variations in the pipeline flow rate, the characterization of the fluids in the line, and the quality of the individual measurement data, to mention only a few. CPM systems use a variety of methodologies to vary their sensitivity according to the uncertainty in the data used for their calculations. However, in general terms, the more uncertainty there is in the data, the lower the resulting system sensitivity becomes. Active features in a CPM leak detection system can mitigate the performance degradation due to varying data uncertainty.
Pipeline Girth Weld Inspections Using Ultrasonic Phased Arrays
Authors: Moles Michael , Dube´ Noe¨l , Ginzel Ed
Published: March 2009

Mechanized ultrasonics is rapidly replacing radiography worldwide for gas pipeline girth weld inspections. Compared with radiography, mechanized ultrasonics is more reliable, faster, has better detectability for critical Lack of Fusion defects, and poses no safety hazard. Phased arrays present major improvements over conventional multiprobe ultrasonics, both for onshore and for offshore use. Probe pans are lighter and smaller, permitting less cutback; scans are quicker due to the smaller probe pan; phased arrays are considerably more flexible for changes in pipe dimensions or weld profiles, and for different scan patterns; phased arrays have great potential for the future, such as compensating for variations in pipe wall thickness or temperature. This paper describes the evaluation of the PipeWIZARD ultrasonic phased array system for girth weld inspections, using standard ASTM E-1961 Mechanized Ultrasonics set-ups. Some typical results will be shown. The paper will describe the latest phased array UT results, plus developments like automated set-ups and improved imaging using an increased number of zones. A brief comparison of E-1961 and the recent API 1104 19th Edition codes will be made, plus a summary of approvals and track record to date.
Offshore Installation of Spool Pieces
Authors: Reinholdtsen Svein-Arne , Sandvik Peter Christian , Hanson Tor David
Published: March 2009

This article presents a numerical model for dynamic analysis of an offshore installation of a spool piece. A spool piece is a pipe element used for connection between a pre-laid pipeline and a structure on the seabed. It is often designed with several knees, to accommodate thermal expansion. The spool piece can be lowered through the splash zone with a close to horizontal orientation, or it can be tilted during the splash zone crossing, to control the flooding and to reduce the wave loads. The spool piece is modelled as several interconnected stiff segments, which can have any orientation. The calculated forces include the distributed weight and buoyancy, and also hydrodynamic loads from regular or irregular waves, either if the segment is fully submerged or partly out of water. Concentrated masses, representing connectors, may be included in the spool piece model. The model is demonstrated through a few selected examples including comparisons with field observations. The calculated maximum forces, which will be used for deriving operation limits, are compared to results from simpler and currently used methods of analysis.
An Experimental Investigation of Wave Steepness and Cylinder Slenderness Effects on Wave Run-Up
Authors: Morris-Thomas Michael , Thiagarajan Krish , Krokstad Jo̸rgen
Published: March 2009

This paper details an experimental investigation of wave run-up on a fixed vertical surface piercing circular cylinder. The study focuses on two important parameters, wave steepness and body slenderness, which are shown to influence the wave run-up. The wave steepness, kA, is varied from 0.041–0.284, and the body slenderness parameter, ka, is varied from 0.208–1.386. The zero-, first- and second-harmonic components of the wave run-up are compared with frequency based numerical predictions of the free-surface elevation by a commercially available boundary element program, WAMIT. The comparison illustrates the importance zero- and higher-harmonic contributions to the wave run-up. In particular we show that the linear diffraction prediction of the first-harmonic component is reasonable, however, the trends exhibited by the zero- and second-harmonics are not captured well by perturbation theory. Using a regression analysis, involving a separation of ka and kA dependence, the importance of higher-order wave steepness effects on wave run-up is demonstrated.
Frequency and Time Domain Analysis of Vortex Induced Vibrations for Free Span Pipelines
Authors: Larsen Carl M. , Koushan Kamran , Passano Elizabeth
Published: March 2009

The present paper will discuss various models for calculation of vortex induced vibrations (VIV) of free span pipelines, and present a new strategy for such analyses. Applications of traditional models are presented and their limitations discussed. The new approach is based on the combination of an empirical linear frequency domain model, and a non-linear time domain structural model. The first step is to carry out the VIV analysis according to linear response theory, and next introduce the calculated hydrodynamic forces to the non-linear structural model. The benefit from using the non-linear model is to describe stresses at the shoulders more accurately, which is important since fatigue damage in many cases will be largest in this area. The conclusion is that the interaction between pipe and seafloor is crucial for accurate stress prediction, and that a non-linear time domain model will give the most accurate result.
Dynamic Response of Shallow Water Monopod Platforms
Authors: Pilotto Micaela , Ronalds Beverley F. , Stocker Roman
Published: March 2009

This paper describes a systematic desktop study of the non-linear dynamic behavior of monopod platforms. The aim of this work is to highlight some important factors in the dynamics of minimum structures in shallow water. The analysis is performed in the time domain with regular wave loading. The non-linearities are due to the wave theory (Stream function of 8th order), to the shallow water environment and to the drag-dominated situation. Idealizations of two braced monopod configurations are compared with the simpler and more commonly studied unbraced monopod. Aspects highlighted for each configuration include the effect of wave period and top mass on the dynamic amplification factor. In particular, the analysis focuses on the highly non-linear behavior in the wave zone. The results show that braced monopods are dynamically more sensitive than unbraced monopods. In particular, braced monopods exhibit more energy at higher harmonics in the quasi-static response. This yields a consistently stronger dynamic response even if the wave period and the natural period of the structure are very different. The importance of the mass at the top of the structure in the dynamic response and in particular its role in increasing the dynamic amplification factor up the water column are highlighted.
Riser Angle Based Position Reference System
Authors: So̸rensen Asgeir J. , Leira Bernt J. , Larsen Carl M. , Ho̸iskar Paul Erik
Published: March 2009

The requirements to reliability and accuracy have resulted in requirements for DP systems to class notations compliant to the International Maritime Organization (IMO) [7] equipment class 2 or 3. For both class notations it is required 3 position reference systems, whereof at least 2 systems should be based on different principles for the class notation to be maintained. Installation of satellite based systems and hydro-acoustic systems normally accomplish this. However, both of these systems are subject to degradation or drop-outs. In areas where frequent degradation or fall-out is encountered, loss of class will result in decreased drilling time, as operation will have to come to a halt and preparations for riser-disconnection may have to be initiated. This has motivated the development of a third position reference system based on measurement of riser angle offsets. In this paper the design principles for such a riser angle position reference system are presented.
Calibration of a Flexible Pipe Tensile Armour Stress Model Based on Fibre Optic Monitoring
Authors: Sævik Svein , Igland Ragnar T.
Published: March 2009

The present paper addresses calibration of a flexible pipe tensile armour stress model based on fibre optic monitoring. A full-scale flexible pipe was instrumented with fibreoptic Bragg grating (FBG) strain sensors in the inner tensile armour, enabling measuring the axial stress due to friction effects and the bending stresses due to bending about the strong axis of the tensile armour tendon. The measurements were used to study the dynamic stress as a function of global loading providing data for model calibration. A brief description is given of the applied sensor technology, how the sensor was integrated in a flexible riser, the test set-up and the test program. The paper focus on how these data were used and which parameters that are critical in order to provide a model that gives a best fit estimate of the dynamic stress distribution. A simple theory of shear interaction is presented. The theoretical model was implemented into the computer code BFLEX and comparisons made in terms of stress history plots including both the tested and predicted values.
Lazy-Wave Steel Rigid Risers for Turret-Moored FPSO
Authors: Lima Torres Ana Lu´cia F. , Gonzalez Enrique Casaprima , de Siqueira Marcos Queija , Silva Dantas Claudio Marcio , Mourelle Marcio Martins , Correia da Silva Renato Marques
Published: March 2009

The free-hanging SCR (Steel Catenary Riser) was adopted by Petrobras as a cost-effective alternative for oil and gas export lines on deepwater fields, where large diameter flexible risers present technical and economic limitations. It is considered an available technology for semi-submersible application. There was interest in applying SCR’s attached to FPSO (Floating, Production, Storage and Offloading) units due to the trend of using these units for exploration and production in Brazilian deep waters. This alternative has to be carefully studied due to the high offsets and heave motions imposed by the vessel on the top of the riser. This work presents the approach and methodology adopted in Petrobras to study the structural integrity and feasibility of a lazy-wave SCR attached to a bow turret-moored FPSO at a water depth of 1290 m. The analysis was performed using the Petrobras’s in-house computer codes ANFLEX and POSFAL developed and implemented as part of projects from CENPES with “COPPE/UFRJ - The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”. For VIV (Vortex Induced Vibration) fatigue damage calculation SHEAR7 was used.
Robust Reliability Assessment for Offshore Minimum Structures Under Dynamic Loads
Authors: Xu Liangbin , Chen Guoming
Published: March 2009

The offshore minimum structures are widely applied in the development of margin oilfield because of their simplicity in fabrication, low initial investment and fast-track schedule. However, they would suffer large dynamic response under exciting loads such as seismic and ice loads, which might lower their service safety. The paper is focused on robust reliability assessment for the offshore minimum structures under dynamic loads by considering first-exceeding failure mode, fatigue failure mode. The robust reliability of offshore structure is a measure of its resistance to the uncertainties, and it suits very much to the condition that the information and data are scarce. The multi-level fortification for ice-resistant offshore minimum structures is presented in this paper, that is, the structure should not be defective under normal ice condition, repairable under heavy ice condition and not collapse under the heaviest ice condition. A numeric calculation method of robust reliability and several robust reliability dynamic models for offshore minimum structures are put forward in this paper, in which plastic collapse, fatigue, and fracture are dealt with. The interaction between the different failure modes under these loads is also considered in the paper. Based on research work mentioned above, the comprehensive safety assessment for the offshore minimum structures would be more easily realized under dynamic loads such as ice loads, seismic loads. Finally, the example is given.
Local Scour and Protection of F3 Offshore GBS Platform
Authors: Bos Klaas Jan , Chen Zhiwen , Verheij Henk J. , Onderwater Martijn , Visser Mees
Published: March 2009

In 1992 a GBS (Gravity Based Structure) was placed in the F3 block in the Dutch sector of the North Sea. Gabion mattresses around the GBS were applied to prevent scour. The potential scour and the required dimensions of the gabion mattresses were determined before the installation in small-scale physical model tests. After the installation of the GBS, annual surveys have been carried out to provide information about the scour development around the GBS and the status of the protection system. This paper presents field observations of local scour development around the F3 GBS platform and an evaluation of the performance of the scour protection system. The objectives were to assess the maximum scour around the GBS and to determine a cost-effective survey strategy. To assess the maximum scour around the GBS, metocean conditions for the F3 GBS location were hindcasted for the period between the platform installation and the latest survey. The observed scour depths were analysed and compared with predictions both from the model tests and from existing empirical formulas. The results showed that the predicted scour depths based on the model tests agree reasonably well with the field observations. The empirical formulas give larger scour depths than the surveys. Based on the field observations in combination with predictions of the models, the maximum scour depths under the design extreme environmental conditions have been determined. To determine a cost effective survey strategy, the behaviour and performance of the gabion mattresses were evaluated based on the annual surveys and video inspections. It was concluded that the protection system has functioned well and no damage was observed. On the basis of the maximum expected scour and the performance of the gabion mattresses, it was considered that the survey frequency may be reduced. Surveys and inspections may be carried out every three years instead of every year, and after a severe storm.
Hydroelastic Responses of Pontoon and Semi-Submersible Types of Very Large Floating Structure in Regular Head Waves
Authors: Phan T. S. , Temarel P.
Published: March 2009

The symmetric dynamic behaviour of two types of Very Large Floating Structure (VLFS) is investigated. The structures are of pontoon (or mat like) and semi-submersible type and have the same beam, length and displacement. The responses for these stationary and free-floating structures in regular head waves are investigated using the three-dimensional hydroelasticity theory, applicable to structures with arbitrary shape. The “dry analysis” is carried out by discretising the structures using beam and shell finite elements, as appropriate. The solution of the fluid-structure interaction problem is achieved through a pulsating source distribution whereby the mean wetted surface of either structure is discretised using four-cornered panels. The symmetric dynamic characteristics of both structures are compared, both in vacuo (e.g. natural frequencies and mode shapes) and in water (e.g. generalised added mass and hydrodynamic damping). Predicted responses such as vertical deflections and direct stresses, in regular head waves, are also discussed and compared.
U.S Navy Practical Experiences in Cable Laying
Authors: Racette Russell A. , Yinger Peter B.
Published: March 2009

Two recent United States Navy projects incorporated cable-laying operations to construct and replace existing Underwater Tracking Ranges. Many constraints involving environmental concerns, budget reductions, etc., have required the United States Navy to be creative in planning and executing cable laying operations. The ship selections marginally meet the maneuverability design requirements without training. The cable types selected are expected to require minimal design and be a commercial off the shelf item. Environmental concerns and their mitigation have been challenging. Successful projects in the Bahamas and United States Virgin Islands were performed yielding similar but varying results. The majority of the resources were common excluding the installation ship. The Bahamas project was performed without incident. The Virgin Islands project encountered a cable run away in the deployment machinery. Results of the post analysis compare the two projects. This quantifiable data has improved the planning of upcoming projects.
Optimum Reactive Control and Control by Latching of a Wave-Absorbing Semisubmerged Heaving Sphere
Authors: Hals Jo̸rgen , Bjarte-Larsson Torkel , Falnes Johannes
Published: March 2009

The theoretical potential for maximising energy output of wave-energy converters by means of optimum control is quantified for a heaving semi-submerged sphere on deep water. The heave amplitude is constrained to not exceed 0.6 units of the radius. Sinusoidal incident waves of amplitudes up to 3 m and period in the range of 6 s to 12 s are considered, when the sphere radius is 5 m. Optimum reactive control, contrary to sub-optimal latching phase control, requires ability for reversing the energy flow through the power take-off machinery. Computed results show that, for a typical wave of 0.5 m amplitude and 9 s period, the maximum absorbed power is 24, 137, and 172 kW for the cases of no phase control (passive system), sub-optimal latching control, and ideally optimal reactive control, respectively. The ratio between the maximum/minimum instantaneous power and the average absorbed power is 2/0, 4.1/0 and 11.0/-9.0, respectively, for the three different strategies.
Hydrodynamic Characteristics of Submerged Impermeable and Seaside Perforated Semicircular Breakwaters
Authors: Dhinakaran Govindasamy , Sundar Vallam , Sundaravadivelu Renganathan , Graw Kai-Uwe
Published: March 2009

The semicircular breakwater (SBW) is one of the varieties of breakwaters, which has emerged recently. The world’s first semicircular breakwater has been constructed at Miyazaki port in Japan. The dynamic pressures due to regular waves exerted on a SBW with 7% of its seaside surface area with perforations were measured. The variations of dimensionless pressures for three different water depth (hw) to total height of the model (ht), ratios are presented and discussed in this paper. In addition, the reflection and transmission characteristics of the perforated SBW as a function of scattering parameter, ka is reported. The results on the above stated variables for a seaside perforated SBW are compared with the results for an impermeable SBW.
Reliability Based Design Criteria for Installation of Pipelines in the Bay of Campeche, Mexico: Part 2
Authors: Bea Robert , Xu Tao , Heredia-Zavoni Ernesto , Lara Leonel , Burbano Rommel
Published: March 2009

Studies have been performed to propose reliability based design criteria for the installation of pipelines in the Bay of Campeche, Mexico. This paper summarizes formulations that were used to characterize the important Ultimate Limit State capacities of the pipelines during the installation period (collapse, bending, tension, combined, and propagating buckling). A large database of laboratory and numerical analysis ‘tests’ (more than 2,000 results) to determine pipeline capacities was assembled to help evaluate the Biases (ratio of measured/predicted capacities) in the analytical methods used to determine pipeline capacities. Given the formulations, target reliabilities, and installation demand characterizations summarized in a companion paper (Part 1), installation design criteria were developed for both Working Stress Design and Load and Resistance Factor Design formats.
Truss Spar Structural Design for West Africa Environment
Authors: Wang J. , Luo Y. H. , Lu R.
Published: March 2009

As the Truss Spar gains greater recognition by the offshore industry, it is expected that more Truss Spars will be built for deepwater oil and gas development not only for the Gulf of Mexico, but also for offshore West Africa, Brazil, and other deepwater fields. The offshore West Africa environment is known to have long-period swells with period in excess of 20 seconds although its 100-year extreme wave height and wind speed are significantly lower than those for the Gulf of Mexico. The long-period swells present unique challenges for structural design of the Truss Spar. In particularly, heave motions and structural fatigue caused by the swells can be significant and must be addressed early in the design process. A comparative study between a Truss Spar and a Classic Spar on global motions is provided in this paper. Inplace strength and fatigue performance is investigated using a generic Truss Spar model for both the Gulf of Mexico and West Africa environments. Motion RAOs, loads, structural strength and fatigue results are presented to illustrate the findings.
Evaluation of Welding Residual Stresses in Power Plant Facilities by Using a Newly Developed Indentation Technique
Authors: Jang Jae-il , Son Dongil , Choi Yeol , Lee Yun-Hee , Ji Won-Jae , Kwon Dongil , Park Sang-Ki
Published: March 2009

It is well known that residual stress is one of the important problems in welding design/fabrications and sound maintenance of welded structures. Thus, the demand for quantitative evaluation of welding residual stress has been increased. However, conventional non-destructive techniques for welding residual stress measurement have many difficulties in in-field applications according to poor repeatability, large scatter of obtained data, complex procedures, inaccurate results, and etc. To overcome these difficulties, a newly developed indentation technique was proposed in this study, and applied to evaluate the welding residual stress in electric power plant facilities. By comparing with the stress values obtained from the destructive saw-cutting test, it could be concluded that the new indentation technique is very useful for quantitative/non-destructive evaluation of welding residual stresses in industrial fields such as power plant facilities.
Analysis of Field Data From the Performance of Offshore Pipelines (POP) Project
Authors: Bea Robert , Smith Charles , Smith Bob , Rosenmoeller Johannes , Beuker Thomas , Brown Bryce
Published: March 2009

The Performance of Offshore Pipelines (POP) joint industry – government agency sponsored project was conceived to test pipelines in the field to allow verification of procedures used to analyze their potential loss of containment characteristics. This paper summarizes a series of analyses performed to predict the loss of containment (LOC) characteristics of one pipeline in the Gulf of Mexico. The oil pipeline tested had been in service for 22 years and was scheduled for removal. The pipeline was in-line instrumented, and then hydro-tested to failure. The failure section and other sections of the pipeline that had indicated significant corrosion features were retrieved and the geometric and material properties of the failure section and the other sections determined. LOC pressure forecasts were done in three stages: 1)before field testing, 2)after in-line instrumentation was performed and the data analyzed, and 3)after geometry measurements and materials testing. The LOC pressure and location determined during the field test were not released to the analysts until after all of the forecasts were completed and documented. This paper summarizes the results from the analyses of the field and laboratory test results to forecast the LOC pressure and compares the forecasts with the hydro-test results.
Reliability Analysis of Jack-Up Platforms Based on Fatigue Degradation
Authors: Shabakhty Naser , van Gelder Pieter , Boonstra Hotze
Published: March 2009

Generally, jack-up structures are used for production drilling and exploration of hydrocarbons. The combination of mobility and the behavior as a fixed structure in operational conditions has made it an important structure in the offshore industry over the last 40 years. When a jack-up structure has been in operation for a great part of its original design-life and intention is there to extend the usage of this structure at a specific location, an investigation on fatigue degradation of the structure is an essential factor that has to be carried out before taking any decision. Fatigue is the process of damage accumulation in material due to stress fluctuation caused by variation of loads in service time. The fatigue failure occurs when accumulated damage has exceeded a critical level. In this paper, the remaining fatigue capacity of the jack-up structure is considered as an indicator for adequate use of the structure. It can be specified based on the difference between design-fatigue and fatigue experienced by the structure. The design-fatigue can be determined based on fluctuation of loads during the lifetime of the structure and experienced fatigue is specified by the load conditions, which the structure has experienced during its service time. When the information on the load conditions which the structure has experienced in its service time is available or known precisely, determination of the remaining fatigue capacity could be carried out by using the Palmgren–Miner’s rule. In practice, uncertainties are present in loads and characteristics of material. Hence it will be reasonable to determine the remaining fatigue reliability of the structure by the reliability methods. In this paper, based on a crack propagation approach and achieved information from inspection, it is shown that the remaining fatigue reliability of jack-up structures could be determined and updated by using a Bayesian procedure in the duration of the service time.
Fatigue Safety Factors for Deepwater Risers
Authors: Stahl B. , Banon H.
Published: March 2009

Fatigue life is governed by a number of variables that are highly uncertain. The safety factor on fatigue life is used in a deterministic way to account for the estimated fatigue damage uncertainty. High uncertainties lead to high fatigue safety factors, and vice versa. Evaluation of the uncertainties in the variables governing fatigue design provides a grip on what the safety factor should be. This paper addresses riser fatigue using a fatigue reliability model that is relatively simple but still captures the important elements of the fatigue problem. The bias and uncertainty of stress range are extremely important parameters in design against fatigue. This is due not only to the fact that these parameters are highly uncertain, but also to the fact that they are greatly amplified in the fatigue damage equation by the ‘slope’ m of the S-N curve. The Palmgren-Miner fatigue damage index and the intercept value of the S-N curve are additional important variables in fatigue design. A model for combining wave-induced and vortex-induced vibration (VIV) is introduced together with the best available data and reference to industry work in this technology area. A recently completed joint industry project on riser reliability provides good calibration points for the critical fatigue reliability variables. Reliability and sensitivity studies are performed to demonstrate the effect of the uncertainty parameters. An approach to selecting deterministic fatigue design factors that yield specified reliability targets is developed and illustrated. The study provides a rational approach to selecting safety factors for design of deepwater risers, taking into account both wave and VIV-induced fatigue damage.
The Heave Response of a Central Spar Fish Cage
Authors: Fredriksson David W. , Swift M. Robinson , Irish James D. , Celikkol Barbaros
Published: March 2009

As the aquaculture industry considers moving into the open ocean, understanding the dynamic response of fish containment structures becomes critical. Identification of possible resonant conditions and motion characteristics is necessary for system structural integrity and maximizing fish survivability. In this study, heave (vertical motion) free release tests of a central spar fish cage were conducted using a combination of physical and finite element models and field observations. These tests were performed to investigate the added mass, damping ratio and natural period of the system in the vertical direction. The test results were analyzed considering both linear and nonlinear damping. The comparison of these tests show that (1) the damped natural period of this fish cage is longer than 20 seconds, (2) the numerical model underestimates the damping and the cage oscillates longer and at a higher frequency than observed with the field tests and (3) the physical model is nearly critically damped near equilibrium due to Reynolds number effects at the model scale.
Time-Domain Investigation of a Semisubmersible in Rogue Waves
Authors: Clauss Gu¨nther F. , Schmittner Christian , Stutz Katja
Published: March 2009

Heave, pitch and roll motions as well as airgap are key characteristics of semisubmersibles in extreme seas which are defined by Ultimate Limit State design conditions (ULS) with a specified 100-year design wave height Hs and peak period Tp. The increasing number of reported rogue waves with unexpected large wave heights (Hmax/Hs > 2), crest heights (ζmax//Hmax > 0.6), wave steepness and group patterns (e.g. Three Sisters) may suggest a reconsideration of design codes by implementing an Accidental Limit State (ALS) with a return period of 104 years. For investigating the consequences of specific extreme sea conditions this paper analyses the seakeeping behaviour of a semisubmersible in a reported rogue wave, the Draupner New Year Wave embedded in irregular sea states. The numerical time-domain invegstigation using a panel method and potential theory is compared to frequency-domain results. In particular, the characteristics of the embedded rogue wave is varied to analyse the dynamic response of the semisubmersible in extreme wave sequences For validation, the selected sea condition is generated in a physical wave tank, and the sea-keeping behaviour of the semisubmersible is evaluated at model scale. In conclusion, the results deomstrate the consequences of rogue wave impacts, with respect to the relevance of present design methods and safety standards.