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ABSTRACT Crack coalescence by stress-corrosion cracking (SCC) can lead to cracks that eventually are large enough to be fracture-critical (unstable). Up to that point continuing coalescence produces ever longer cracks, which because of coalescence experience significant jumps in their average crack growth rate. Therefore, coalescence can be an important aspect of the SCC failure process. This paper develops a criterion to determine when the conditions between two crack tips, expressed in terms of the linear-elastic fracture mechanics stress-intensity factor, K, will support the onset of coalescence. Whether the loading remains sufficient to support the continuation of the coalescence process can likewise be determined with this criterion. This criterion is based on satisfying the condition between the two tips that K KCoalesce, where KCoalesce is a measure of the material's resistance to cracking due to either SCC or fracture. This criterion is illustrated by its application to field data for the coalescence of SCC that resulted in a leak in a gas-transmission pipeline. INTRODUCTION Stress-corrosion cracking (SCC) occurs occasionally on the outside surface of gas-transmission pipelines. Where failure occurs, this stress-corrosion process produces an axial flaw that can involve the coalescence of many crack segments, which often have similar depths. Coalescence can lead to crack sizes that do not support ductile tearing but can continue to grow stably by SCC until the wall is penetrated (i.e., a leak occurs). This coalescence, which occurs at a rate determined by the kinetics of SCC, begins when the adjacent crack tips sense one another. As shown by Stoneseifer et al (1993), crack tips tend to grow toward each other for interacting colinear crack pairs and away from each other for coparallel pairs.
- North America > United States (0.89)
- North America > Canada (0.89)
ABSTRACT Tubular joints of offshore structure are subject to cyclic loading of wave during the service life of the platform. This normally lead to fatigue failure, crack growth and fracture of the joints. All these failures will lead to reduction of platform reliability for safe operations. Operating oil company spent millions of dollar to carry out underwater inspection of tubular joints of offshore structures. Inspections are normally carried out in a fixed interval time period (i.e. 4, 5 years and etc.) determined by either company standard practice or international codes. The interval period may be increased or reduced if proper analyses of the joints reliability can be systematically established. This paper outlines the analytical basis and the calculation method used in the derivation of reliability indices, fracture assessment and the development of an outline inspection schedule for critical joints of offshore platform. INTRODUCTION The emphasis of this paper is on the computation of the reliability index of steel tubular connections in plain cylindrical joint-cans (i.e. no ring stiffening present). There are two broad categories of joint that are considered:Where a joint is intact, the reliability index and an outline inspection schedule may be developed on the basis of the S-N spectral fatigue analysis. Where a joint is reported as cracked, the remaining life of the joint will be assessed by using fracture mechanics methods. Overview of Analysis Procedure The reliability index of a structural connection at a joint is a statistical measure of the acceptability of that connection for service in a marine environment. The calculation of the reliability index for joints in an existing structure involves a lengthy procedure. The procedure takes account of: ·The wave-loading environment for the structural location and orientation. · The predicted fatigue life of the joint
- Well Completion > Hydraulic Fracturing (1.00)
- Well Drilling > Casing and Cementing > Casing design (0.34)
ABSTRACT The work performed was an investigation of stable crack growth failure assessment of cracked tubular joints similar to those used in offshore structures. Three full scale experiments were performed where tubular Xjoints with initial cracks developed by fatigue loading, were loaded till significant ductile crack growth occurred. The crack growth in the experiments at different load levels were beach marked and compared to crack growths predicted by Finite Element Analyses (FEA). The applied crack driving force in terms of the J-integral was used for the crack growth analyses. Failure assessment procedures such as the PD6493 and the CEGB R6 were adopted to predict failure/crack growth and compared to the experimental results and FEA results illustrating the validity of the different failure assessment procedures for cracked tubular joints. INTRODUCTION The aim of the project was to provide recommendations and gain understanding of structure integrity assessment of tubular joints containing cracks by adopting Failure Assessment Diagram (FAD) based concepts. The following activities were performed: Fracture testing of three large scale tubular X-joint specimens with fatigue precracks was carried out. Stable ductile crack growths during tests were measured. Limit loads were determined for all tests and strains and displacements were measured at a number of locations, on the specimens. Material characterisation for the chosen steel was conducted and both true stress-strain relation and fracture toughness data were determined. Finite Element (FE) models were developed to determine quantifies needed for evaluation of FAD based approaches. The FE results were checked against the test data. Different FAD concepts were evaluated and recommendations and comment on FAD for tubular joints were worked out. BACKGROUND TO FAILURE ASSESSMENT DIAGRAMS Fracture mechanics based assessment procedures such as PD6493/1/and the CEGB procedure 121, known as the R6 procedure, are based on the Failure Assessment Diagram (FAD) concept.
UOE-Formed Tendon Pipes For Deep Water Oil & Gas Exploration In the Gulf of Mexico
Ikeda, Tomoaki (Sumitomo Metal Industries, Ltd.) | Ohnishi, Kazushi (Sumitomo Metal Industries, Ltd.) | Yamamoto, Akio (Sumitomo Metal Industries, Ltd.) | Nagase, Makoto (Sumitomo Metal Industries, Ltd.) | Takeuchi, lzumi (Sumitomo Metal Industries, Ltd.) | Smith, James D. (Shell Offshore Inc.)
This paper describes the development, pre-qualification testing and production properties of heavy wall-thickness TLP tendon pipes for the TLP projects in the Gulf of Mexico, X-60, 26" O.D. x 1.300"W.T. (AUGER), X-70, 28"O.D.X1.200"W.T. (MARS, RAM/POWELL), X-65, 32"O.D. X 1.500"W.T. (URSA). The AUGER TLP, installed in 2,860 ft of water in 1994, was the first step-wise development of deep water Gulf of Mexico discoveries. This success in subsea field exploitation accelerated the consecutive deep water TLP construction, MARS, RAM/POWELL, and URSA, was based on the lessons obtained. The thick-walled tendon pipes were also developed in this stream with the TLP construction projects. The tendon pipes for AUGER TLP was the first thread, manufactured by UOE process using proprietary TMCP-practiced steel plate with double-submerged arc seam welding (DSAW). The development of tendon pipes for MARS, RAM/POWELL, and URSA TLP's went on stream. Although the steel grade or wall thickness was increased more than for AUGER tendons, its excellent weldability with low carbon equivalent had to be maintained in order to keep the fabrication cost equivalent to the application of no-preheat easy welding methods. From these points of view, the same control ranges in chemical compositions were applied for all grades and thicknesses but with some modification in the TMCP parameters and reinforcement in the UOE press machine for higher grade and heavier wall thickness. The evaluation test results and production history show the superb weldability and excellent fracture toughness for every tendon pipe and within tight production tolerance. This also enabled the rapid consecutive construction TLP's in the Gulf of Mexico setting world records. For the success of all these TLP construction projects were a result of the concentrated efforts of TLP design engineers, construction fabrication contractor, and the steel maker.
- North America > United States (1.00)
- North America > Mexico (1.00)
- Materials > Metals & Mining > Steel (1.00)
- Energy > Oil & Gas (1.00)
ABSTRACT Consideration of hummock impact is a complex problem in designing oil and gas structures. Results of field studies of ice features serve as a basis for selecting design parameters. Data of hummock structure studies are further applied to develop "hummocks - oil and gas structure interaction" models. This paper describes studies of both internal and external parameters of ice features on the northern Sakhalin offshore. Based on the obtained data, design parameters for hummocks have been developed, which are necessary for adopting optimum technical solutions to provide for serviceability of future hydraulic structures. INTRODUCTION The analysis of the status of oil and gas field development in the Arctic seas indicates that offshore field development rates depend to a great extent on ice conditions and the level of their investigation. Data on ice conditions is required to ensure serviceability of hydraulic structures. Hummocks and grounded hummocks will be the most dangerous for hydraulic structures on the northern Sakhalin offshore. Consideration for ice hummock impact is a complex problem in designing oil and gas facilities. Design parameters are selected based on data obtained as a result of field studies of ice features. Data on hummock structure is further used to derive hummock - oil and gas structure interaction models. Basically, the available models describe hummock geometry, without representing the internal structure of a hummock. This is attributed to limited possibilities in obtaining data on internal structure of hummocks. Ice hummocks are known to have heterogeneous vertical structure. Brash ice which forms due to hummocking by gravity and buoyancy moves towards the center of a hummock and fills voids between ice blocks. Therefore, porosity for middle part of a hummock (with a center being at the sea level) is lower than that for lower and upper parts of a hummock (Grishhenko, 1988).
- Asia > Russia > Far Eastern Federal District > Sakhalin Island > Sea of Okhotsk > East Sakhalin - Central Sea of Okhotsk Basin > Odoptu License Block > Odoptu License Block > Odoptu Field (0.99)
- North America > Canada > Quebec > Arctic Platform (0.98)
- North America > Canada > Nunavut > Arctic Platform (0.98)
- Well Completion > Hydraulic Fracturing (0.48)
- Facilities Design, Construction and Operation > Processing Systems and Design (0.34)
ABSTRACT A thermodynamic formulation for pack ice is presented that includes a means of relating stresses in floes to thermally-induced strain rates. The formulation combines some basic thermodynamics as well as a mechanics component which includes viscous creep and mechanically induced strain rates due to typical floes being too large to bend or twist in response to spatially varying strain rates. Moreover, the rheology quantifies the impact of existing cracks in the floe on the overall strain rate in the floe and the resulting stress state. A paradigm is put forward which specifies tensile fracturing and the resulting stress relief as being proportional to the amount of existing cracks and some measure of the tensile strength of the ice in the floe. This allows the thermomechanical equations to be used for both first-year and multi-year floes by differentiating the two by the extent of existing cracks. INTRODUCTION In addition to the body of work on the thermodynamics of sea ice, a great deal of work has been performed dealing with the mechanics of pack ice. The two topics are closely related in that thermal forcing can cause significant stresses and fracturing in pack ice. In this work, an attempt is made to provide a thermo-mechanical formulation that combines the more basic thermodynamic processes in a floe with a rheology relating thermally-induced strain rates to the stress state within the floe. Crossing over from pure thermodynamics to thermomechanics is complicated by the fact that we do not have a definitive relationship between the stressing and fracturing of pack ice. In addition, observations of thermo-mechanical processes in first-year and multi-year floes are quite different even for identical thermal forcing. Here we will provide the basic formulation for the thermodynamics and then concentrate on the details of the mechanics.
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics (1.00)
ABSTRACT By analyzing the test data of the fatigue crack propagation in offshore structural steel A537 at room and lower temperatures, a pivot point on the FCP diagram is presented, from which a relationship of m- ln C is derived, where m and C are the constants in Pans Equation. The derived m-In C relationship for steel A537 is identified with a group of reports on the exist of experimental m- In C relationship from literature. From the discussion and assessment on the m-InC relationship, a critical stress intensity factor range named ∆Kpc is proposed. Hence, ∆Kth, ∆Kpc and ∆Kfc represent different fatigue crack propagation stages, and AK m can be used as a low-temperature fatigue safety index that is very important in monitoring the safe propagation of fatigue cracks and also important in evaluating the low temperature properties of materials. 1. INTRODUCTION The offshore petroleum platforms in Bohai Gulf, China, experience low temperatures that fall to as low as 238K for three months in winter. Cracks in the structures have to withstand low temperature fatigue and unstable propagation of fatigue cracks or brittle fracture would occur most possibly. Under ice loading, the shortest estimated fatigue life of an offshore platform by use of S- N curves is only 2 years. The study of low temperature fatigue of offshore structures under ice loading is getting more and more urgent with the large scale exploitation of offshore oil in Bohai Gulf. Fatigue tests on the offshore structural steels are the basic part of this study. Steel A537 is the main structural steel of the offshore tubular joint platforms in Bohai Oil Field and has to be tested to determine if this steel can meet with the platform's withstanding both low temperatures and ice loadings.
- North America > United States (0.47)
- Asia > China (0.35)
ABSTRACT Transverse corrosion fatigue tests were performed on a carbon-fiber epoxy-matrix composite in air, simulated seawater, and distilled water environments. Crack growth rates were determined for ambient and elevated pressure (up to 4000 psi) fluid environments and were compared to baseline values for the composite tested in air. In addition, the effects of water chemistry and loading frequency on fatigue crack propagation were investigated. Finally, composite specimens saturated with simulated seawater were fatigued in order to determine the effect that prior moisture content has on crack growth rates of specimens tested in either air or immersed environments. INTRODUCTION In the coming years, exploration for offshore oil deposits will employ the use of tension-leg platforms (TLP) to drill at ocean depths of 10,000 feet where the hydrostatic pressure reaches 4400 psi. The high hydrostatic pressure, corrosive seawater, and cyclic loading induced by wave motion or ocean currents present a challenging environment for TLP components. A TLP is weight critical, and the potential for mass and cost savings afforded through the use of carbon or glass-reinforced epoxy composites make these composites ideal candidate materials for TLP components such as platform facilities, risers, and tethers (Kim, Hahn, and Williams, 1988). However, the effect of long term high pressure seawater exposure on these materials must be understood if they are to be used in structures with design-lives of 20 or more years. Previous research has identified both beneficial and deleterious effects of seawater and hydrostatic pressure on composite materials. These effects are dependent not only on the choice of the fiber and matrix composite system but also upon the composite laminate layup. In addition, the composite response to seawater and pressure exposure is highly dependent upon the mode of deformation imposed during testing.
- North America > United States > Texas (0.29)
- North America > United States > Connecticut (0.28)