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Results
Advances in Low Cost, Quantitative Inspection Techniques for Floating Systems
Guest, B. (BHP Billiton) | Carra, C. (AMOG Consulting Inc) | Wales, S. (AMOG Consulting Inc) | Potts, A. E. (AMOG Consulting Inc) | Kilner, A. A. (AMOG Consulting Inc) | Barrow, S. (Intervention Engineering) | Pudney, S. (Intervention Engineering)
Abstract With the challenges faced by operations in a continuing low oil price environment and the continuing high failure rate of offshore moorings, the value derived from traditional mooring inspection strategies needs to be reassessed. This paper outlines a significantly improved approach to mooring and riser system inspection based on the acquisition of quantitative condition data using low cost/low risk inspection techniques. Specifically, this paper addresses the ability to perform specific, detailed, quantitative inspections using standard inspection tools mobilized to and launched from an offshore facility. By modifying inspection strategies to target specific areas of high risk and known degradation mechanisms, a much-improved understanding of the current condition of the system is gained and appropriate integrity management strategies developed based on the observed and measured rate of component degradation. In addition, the ability to respond rapidly to event-based forms of degradation is significantly improved. This approach will be demonstrated by a case study of the inspection of a known area of risk that is traditionally difficult to inspect and interpret - mooring chain links within a hawse pipe. High definition imagery of elements within the hawse pipe of an FPSO turret mooring system were acquired and then post processed into highly accurate (ยฑ1 mm) 3D chain link data, following which detailed numerical analysis of the remaining link capacity for worn links was performed. Previously acquired diver measured data is compared against 3D scan data and the improvements in the accuracy of the estimate of the remaining strength on the worn links is demonstrated. Based on the understanding of the remaining capacity and rate of degradation, future inspection intervals can be set on an informed basis, such that the rate of volumetric loss can be verified by measurement, thereby confirming the rate of degradation in link capacity. A cost-benefit analysis highlights the improvements in overall lifecycle costs that these quantitative, facility-based, inspection techniques can provide. This paper provides a demonstration of the ability to perform detailed quantitative surveys and utilize the acquired information to assess both current and future condition of safety critical elements. The paper demonstrates improvements in inspection strategy that are consistent with the new approaches to mooring system integrity management that are currently being finalized (e.g. the new API-RP-2MIM).
Abstract This paper outlines a methodology for developing consistent robustness criteria; accounting for multiple hazards that may occur during long Return Period (RP) events, and accounting for other degradation mechanisms unrelated to long RP events but, based on experience, were root causes of many failures. The "if in doubt, make it stout," philosophy is critically examined against a higher fidelity approach, utilizing new knowledge of reported causes of failure to achieve better design. With uncertainty when designing FPSO mooring systems, regulatory responses focus on increasing line numbers and capacity (e.g. increasing the Factor of Safety (FOS) from 1.67 to 2.20, or consideration of longer RPs for design.) This design basis response presumes failures primarily occur due to overloading events. Instead of arbitrarily increasing RPs or the FOS, a more robust methodology is developed that utilizes new knowledge of root causes in higher fidelity time domain numerical modelling to better understand true mechanisms of failure, and design accordingly. These higher fidelity methods take account of system response complexities not usually accounted for in conventional accepted design analysis approaches. An alternate risk based methodology is described which is aligned with System Safety Engineering philosophies, whereby a probabilistic approach is used to ensure overall probability of failure is less than 1/10,000. The approach considers multiple mechanism of failure, arising from both high RP storm events and operational motions, in conjunction with corrosion wear rates as observed from recent studies (e.g. SCORCH JIP). Observations are drawn from mooring failure investigations and on-going in-field mooring monitoring campaigns. The results of recent studies are discussed, revealing that half of the failures investigated were due to time dependent degradation from corrosion and/or fatigue. Where multi-line failures have occurred, the root cause is almost exclusively due to common mode degradation resulting in multiple lines experiencing significant loss of capacity, whereby un-zipping is not due to overload, but stochastic undetected or unremedied loss of capacity. Thus, arbitrarily increasing design storm severity and FOS may not be an informed way of addressing actual failure modes, but a blunt instrument that may introduce unintended consequences and design spirals with new failure mechanisms introduced by pushing the limits of component sizing and manufacturing capabilities. A more rational robustness criteria is developed, that aims to ensure that designers understand their system to a higher level of fidelity. There is no presumption that a tension mechanism of failure during a high RP storm event is governing, but rather, robustness is evaluated using a reliability based approach. This accounts for corrosion, wear, bird caging, out-of-plane bending, and other phenomena discovered in recent studies of actual failures, ultimately delivering a much higher level of design robustness without unnecessarily increasing strength of the system's resistance to an irrelevant failure mode.
- Asia (0.68)
- Europe (0.68)
- North America > United States > California (0.46)
- North America > United States > Texas (0.28)
Abstract The first phase of the Chain FEARS (Finite Element Analysis of Residual Strength) Joint Industry Project (JIP) aimed to develop guidance for the determination of a rational discard criteria for mooring chains subject to severe pitting corrosion which, based on current code requirements, would otherwise require immediate removal and replacement. Critical to the ability to evaluate the residual strength of a degraded chain, is to have an accurate estimate of the chain in its as-new condition, providing a benchmark for any loss in strength associated with severe corrosion or wear. A Finite Element Analyses (FEA) residual capacity assessment method was developed and correlated against available break strength test data of degraded links as part of the JIP. FEA were conducted of as-new chain links in accordance with this validated methods, which when compared with current Class Rules specified Minimum Break Load (MBL) demonstrated a significant disparity between Predicted Break Load (PBL) and the codified MBL, particularly for larger chain sizes. It was identified, consequent on this inconsistency in MBL with respect to chain size that inconsistency in proof loading would also result for chains of larger size. Consequently further investigations were carried out with the objective of:Determining an alternative formulation for the break strength of studlink and studless chain, thereby allowing comparison with the current Class Rules break strength formulation. Assessing the extent and effect of inconsistencies in the Conventional Proof Load Formulation Establishing the origin and the technical basis for the Conventional MBL formulae. Assessing the validity of the Code MBL with respect to Actual Break Load (ABL) of common studlink mooring chain and the implications of using an alternative Amended Formulation for as-new chain strength. The basis for the determination of the current Class Rules on chain strength is established and in doing so serves to explain the a significant disparity between PBL and the codified MBL that is evident particularly for larger chain sizes. An alternative formulation for the break strength of as-new studlink and studless chain is presented. It is recommended that further investigation be conducted to establish an alternative proof load formulation optimized for fatigue endurance.
Abstract As part of improvements to the integrity management practices for two in-service FPSO mooring systems, detailed reviews of existing video inspection data were undertaken. These reviews identified separate high frequency mooring system dynamic responses in frequency ranges higher than typical wave frequency responses. In relatively benign conditions associated with ROV-based inspection activities, torsional vibrations in deep water wire rope sections were observed. In a moderate water depth system, vortex induced vibration (VIV) excitation of a wire rope segment, leading to high frequency oscillations of chain links within the chain hawse tubes on a turret moored system, were also observed. These oscillation frequencies were typically an order of magnitude higher than those arising from wave frequency motions. The implication of these observed high frequency motions was that there were potential fatigue and wear degradation mechanisms that had not been accounted for in the mooring system design. A review of the observed high frequency response mechanisms - torsional vibrations and VIV excitation of mooring line segments - was performed followed by an assessment of the implications on mooring system design and ongoing integrity. The theoretical foundations for the two mechanisms were identified, and the frequency and amplitude of the resultant motions and component stress ranges evaluated. The degradation effects of the two mechanisms were then assessed in terms of both the core mechanisms and the child degradation processes they may initiate (e.g. wire rope segment VIV leading to increased chain wear). A review of different mooring system configurations and water depths was performed to identify the propensity for these mechanisms to occur and their potential for adverse impacts on mooring line integrity were evaluated. For each mechanism, the key influence parameters on the observed high frequency response were identified. The potential impacts on overall mooring system integrity were evaluated, along with the implications for the inspection strategies to verify the ongoing condition of the mooring components. These mechanisms show that mooring system designers must be aware of the dynamic responses that can be excited with typical mooring systems. Without paying due attention to these potential effects at the design stage and during operation, the degradation mechanisms that may be initiated by these effects may be detrimental to the overall long-term integrity of the mooring system.
- Europe (0.46)
- North America > United States > Texas (0.30)
Abstract Drilling risers are regularly deployed in depths beyond 5000 ft with large sections covered in buoyancy. The smooth cylindrical shape of buoyancy modules can result in significant VIV response, causing overall amplification of drag experienced by the riser. In turn, operations can be suspended due to drag effects on top and bottom angles; high current speeds can lead to a halt in operations, even complete disconnection and retrieval of the riser string. Although suppression technologies exist to reduce VIV response (helical strakes/fairings), these have not been integrated into the manufacture of buoyancy modules for practical reasons (stackability, ease of manufacturing/assembly/installation). Innovative Longitudinally Grooved Suppression (LGS) technology has recently been developed and tested with two main bodies of work undertaken; model testing and full scale numerical modelling. This paper covers the methodology and results of full-scale OrcaFlex/SHEAR7 simulation work performed on drilling risers in realistic current profiles for the Gulf of Mexico (GoM), concluding with the impact this technology could have in reducing costs by increased operability for drilling risers. LGS technology provides significant reduction in VIV response whilst maintaining similar buoyancy to conventional modules. Significant reduction in VIV response has multiple benefits: less drag, reduced fatigue damage, and an increase in maximum currents drilling risers can be operated in - without any moving parts or extra deployment installation time. High Reynolds Number model testing results showed LGS technology behaved favorably in observable repeatable conditions, whilst the full scale numerical modeling demonstrated how these parameters measured in model testing can be used to perform numerical modelling of full scale offshore drilling risers in a variety of water depths and conditions; improving operability and riser fatigue life. Operability during GoM eddy current events was calculated to increase 30% annually, equating to estimated annual savings of over $10M, with potential to save similar amounts within a short period in the case of extreme current events. By applying LGS technology, significant financial savings can be achieved through increased operability and increased fatigue lifetimes.
- Overview (0.34)
- Research Report > New Finding (0.34)
Abstract As part of a study conducted by Esso Australia investigating means of reducing the high cost of construction of submarine pipelines in Bass Strait, flexible risers connecting bottom-towed steel pipelines and the topsides of freed platforms were identified as a potential means of providing significant cost savings. To assess the concept, an extensive program of dynamic analyses and state model tests in a large wave-current flume were undertaken. This paper presents the findings of these analytical and test studies. Additional dynamic analyses were performed to assess the sensitivity of flexible riser performance to various changes in hydromechanics and other design parameters. A number of recommendations were developed for design criteria and analytical parameters for flexible riser systems, not addressed in current codes of practice for such systems. Introduction The relative remoteness of Australia from major centres of offshore oil and gas activity presents a number of problems to the viability of potential offshore field developments, notably the cost of mobilization of specialist construction vessels, such as derrick and pipelaying barges, pipe reeling vessels and diving support vessels (DSVS). In recent years Esso Australia Ltd. (EAL) has used innovative engineering and new construction techniques, minimizing or avoiding the use of such vessels altogether, to make substantial reductions in the cost of development of offshore structures and facilities in Bass Strait. Most future field developments in Bass Strait involve much smaller reservoirs than in the past, such that the high cost of conventionally installed pipelines, which has References, tables and figures at end of paper been identified as a major component of the total cost of such projects, would either preclude development or would make it a marginal proposition. In view of these considerations, EAL initiated the "Pipeline Cost Reduction Study" (PCRS), investigating novel pipelaying procedures to identify cost effective means of installing pipeline links to new marginal field developments. One of the innovations investigated in the PCRS was the use of single-catenary flexible risers as tie-ins to fixed platforms from on-bottom steel pipelines. This paper describes wave tank testing and dynamic numerical analysis of this concept, and discusses design criteria and analytical parameters for use in the design of such flexible riser systems. The following section introduces the PCRS. Section 3 develops the criteria used for the design of flexible riser systems. Section 4 describes the dynamic numerical analysis and the use of FLEXCOM-3D, while Section 5 describes the physical model studies including the effect of spectral versus regular waves and the influence of the structure. Section 6 discusses the design parameters obtained and compares the capabilities of numerical and physical models this discussion is summarised in Section 7 Conclusions. The Pipeline Cost Reduction Study and Its Application The PCRS identified a number of alternative methods of construction and new pipeline configurations which might offer substantial savings in pipeline cost. The most promising concepts were reviewed and the technical feasibility of specific aspects which offered the possibility of significant reductions in the cost of installation of the pipelines for a number of future projects were established.
- Oceania > Australia > Tasmania > Bass Strait (0.64)
- North America > United States > Texas (0.28)