Evaluation of corroded chain link for continued use or life extension is a challenging task for the industry. ABS, together with fifteen (15) participating organizations, initiated the Fatigue of Corroded Chains (FoCCs) Joint Industry Project (JIP) in 2016. The objective of the FoCCs JIP is to investigate methodologies for assessing remaining fatigue life of the corroded mooring chain used for floating production systems. The JIP scope includes fatigue testing in labs and finite element analysis (FEA) of corroded chain samples retrieved from six floating production facilities in West Africa and the North Sea. The participating organizations include oil majors, chain manufactures, consulting firms, and classification societies, which represent a pool of broad range of mooring knowledge and experience. Knowledge gained from the JIP will be summarized and used toward the development of guidance notes for assessing fatigue life of corroded mooring chain for the industry. Six sets of mooring chain samples with different corrosion conditions have been collected, cleaned and digitally scanned for fatigue testing and FEA. Procedures for testing and analysis have been developed with the objective of establishing commonly accepted methods. Different FEA procedures have been studied for making a better prediction of stress ranges of the corroded chain links. The findings from the fatigue testing and FEA will be utilized as basis for further development of the methods for fatigue assessment of corroded mooring chain. This paper summarizes the tests and FE analysis work for the selected chain samples. The JIP research work has found that corrosion, either general corrosion or local/pitting corrosion, can significantly reduce the chain fatigue capacity. The location and the geometry of corrosion pits have more impact on fatigue lives than the pit size. The JIP study has shown that FE analysis is an effective tool to capture the hot spot of corroded chain links and can provide insight in their fatigue performance. Different methods on the assessment of the stress range of a hot spot are compared and discussed.
TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract Hurricanes Ivan, Katrina and Rita in 2004 and 2005 resulted in twenty-one MODUs having suffered either complete mooring failures or partial mooring failure. The mooring work group under API SC2 was already actively working on the 3 rd Edition of API RP 2I, In-Service Inspection of Mooring Hardware for Floating Structures [1] prior to these hurricanes. However, after the hurricanes, the API RP 2SK, Design and Analysis of Station Keeping Systems for Floating Structures [2], work group was reactivated to explicitly address this issue. The API RP 2SM, Recommended Practice for Design, Manufacture, Installation and Maintenance of Fiber Ropes for Offshore Moorings [3], work group was also reactivated in early 2006. The need to revise 2SM largely lies in the experience gained since it was first issued. However, the work group addressed numerous items that also benefit moored MODUs, especially since the use of a fiber rope insert mooring system can enhance the MODU mooring performance and reliability.A summary of the major changes to API RP 2SK, 95F, Interim Guidance for Gulf of Mexico (GoM) MODU Mooring Practice -2007 Hurricane Season [4], 2I and 2SM recommended practices will be presented. Highlights will be given to code changes to these Recommended Practices (RP) that enable technology to address reliability and robustness challenges. This information will be helpful for people who analyze, design and install MODU moorings.
Abstract-With the recent reduction in summertime ice cover in the Arctic Ocean, year-long moored measurement programs require detailed information on sea ice thickness and topography data throughout most of the year, as well as ocean wave measurements during summer periods of major sea-ice retreat. This information is required for basic ice covered ocean studies and, increasingly, for addressing important navigation-, offshore structure design/safety-and climate change-issues. Since the early 1990's, upward looking sonar (ULS) instrumentation have been developed and applied to providing under-ice topography data with high horizontal and vertical spatial resolution. The internal recording ULS instruments, or ice profilers, are typically operated from the seafloor on taut line mooring systems. In the winter of 2007-2008, a new generation of ULS instrumentation was field tested, initially in Northumberland
With a goal to improve the overall reliability of moorings used by MODUs (Mobile Offshore Drilling Units), this paper reviews gaps and issues in design standards and operation practices. MODU moorings stay at one location for a short term, compared to tens of years for permanent moorings on production facilities. While the exposure time to the environment is relatively shorter, mobile moorings have been seen to experience a sizable number of failures ironically. Probability of failure has been high on the order of 10−2. Improving reliability of MODU moorings may be achieved through two sides, i.e. better design standards and more rigorous operation practices. On the design side, there appears to be a lack of clear guidance on designing a mobile mooring system to a proper return period. The gap is prominent especially for moorings in tropical cyclone (aka hurricane or typhoon) areas. Current industry codes and standards do not have a clear guidance on what return period shall be used as a minimum to account for the risk associated with close proximity and failure consequence. Some guidance is provided in API RP-2SK, but it is limited to applications in Gulf of Mexico. This paper attempts to close the gap by proposing minimum return periods to be used and requiring a quantitative risk assessment (QRA) to justify the numbers for any region with tropical cyclones. Guidance on performing a QRA is provided, and aspects on how to produce trustworthy results are discussed. On the operation practice side, issues and gaps are identified and reviewed. Often times, MODU moorings do not receive a sufficient amount of attention in system design, deployment, inspection, and equipment maintenance. Common issues are summarized to raise awareness and best practices are presented.
Polyester rope is a visco-elastic material and its stiffness is affected by mean tension, tension amplitude, loading period, and loading history. Qualitatively it may be felt that rope stiffness significantly affects vessel offset, which in turn affects riser performance and cost. However, a systematic investigation of the impact of rope stiffness on riser stress and fatigue life has not been published for a wide variety of design conditions. This paper describes such a study, and provides specific guidance to designers for the prudent selection of a rope stiffness model and values to achieve safe and cost effective riser design. This study investigated the effect of polyester rope stiffness on steel catenary riser (SCR) design for an FPSO sited in a variety of water depths considering environmental conditions representative of West Africa. Static/dynamic stiffness models for polyester rope in the recently issued ABS Guidance Notes on the Application of Fiber Rope for Offshore Mooring [1] were used in a global analysis to provide motion responses for the riser analysis. Time domain riser analysis was performed to obtain maximum riser stress and fatigue damage under various conditions. Based on the results of the investigation of riser and mooring analysis, guidance on the impact of rope stiffness to the riser design has been developed.
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