Life Extension of Ageing FPSO and SPM Calm Buoy Mooring Chains Under Corrosion Effects – Part 2A (Published)
This study unveils the recent challenges that mooring chains used for station keeping of floating assets face while in operational marine environment. Considering the extremely harsh operating environment, the mooring chains on floating production, storage and offloading (FPSO) and single point mooring calm buoy (SPM) will definitely undergo degradation and deterioration over time. Although, general/uniform and pitting corrosion has been implicated as major cause of mooring chain degradation. Ageing and age related challenges such as pits, wear and corrosion wastages across the mooring chains intergrip, side and straight bars were key considerations for life extension evaluations since it weakens the strength of the chains. The biological phenomenon that causes pits on mooring chains were highlighted. Regrettably, the scarcity of real time corrosion wastage data has made researchers to adopt data from laboratory in corrosion coupon for the purpose of study. Although, laboratory data are usually under control, and hence researchers are looking for more realistic data from marine corrosion environment. It was observed that corrosion wastages are known to alter the geometry of mooring chains but not the chemical and mechanical properties. It was observed that real time corrosion wastage data from a floating asset in marine environment is key to drive life extension decision making. However, it was observed that there are still limited information as regards the type of data to measure, their collation, analysis and interpretations towards the ultimate goal of mooring chains mitigation strategy for longevity.
Keywords: FPSO/SPM calm buoy, MIC, Re-evaluation, corrosion wastage, life extension, mitigation strategies, mooring chains, mooring longevity
Life Extension of Ageing FPSO and SPM Calm Buoy Mooring Chains Under Corrosion Effects – Part 2B. (Published)
Mooring chains replacement and life extension campaigns is trending in offshore West Africa and the rest of the world is not left out. Mooring chains being a critical component of floating production, storage and offloading (FPSO) and single point mooring (SPM) calm buoy for the purpose of station keeping. However, considering the extremely harsh environmental conditions the mooring chain is subjected to, the mooring chains will definitely undergo corrosion wastages over time. Although, general/uniform and pitting corrosion has been implicated by several researchers as major cause of mooring chain degradation and deterioration among other new discoveries. For example, the activities of microbial organisms referred to as microbial induced corrosion (MIC) has been identified and implicated in most cases as the key cause of pitting corrosion on mooring chains and its mechanism is highlighted on this paper together with other mooring chains challenges and mitigation strategies which are beneficial to mooring and marine engineers. Ageing and age related challenges such as wear and corrosion wastages across the mooring chains intergrip, side and straight bars were key considerations for life extension evaluations. It was remarked that marine corrosion wastage on mooring chains do not alter their chemical and mechanical properties but only changes their geometry. Thus, the remaining thickness is a measure of the remaining strength. It was observed that most design codes for mooring chains in temperate or tropical regions like Gulf of Guinea (GoG) are based on the code for North Sea. It was also noticed that temperature, dissolved nitrogen, and current velocity effects mooring chains corrosion wastage. Thus, the higher the temperature especially in GoG the higher the rate of corrosion wastage in the splash zone. Thus, usually higher than the immersed zones. It was also observed that the scarcity of real marine environment data made most researchers to adopt data from corrosion coupon in laboratory for the purpose of studies. However, there are still limited information as regards the type of data to measure, their collation, analysis and interpretations. While class society guidance notes appeared large to manage, this paper presents in a nutshell the relevant information that will serve as a guide to mooring, marine and subsea engineers involved in asset integrity management of FPSO and SPM calm buoy mooring chains/lines to have a first-hand knowledge of the data required for mooring performance evaluation and ultimately mooring chains life extension studies. The paper also, presents some new generations issues and mitigation strategy towards mooring chains longevity.
Keywords: FPSO/SPM calm buoy, MIC, Pitting corrosion, life extension, mitigation strategies, mooring chains
Life Extension of Ageing FPSO and SPM Calm Buoy Hulls and Deck Plates Under Corrosion Effects – Part 1B (Published)
Corrosion wastages occurs in the form of pitting corrosion or uniform/general corrosion. However, it was observed that previous academic studies relied on data obtained from shuttle tankers and imposed such to FPSO for the purpose of studies. While other scholars took data obtained from laboratory in corrosion coupon for the purpose of research. The state-of-the art is the use of data obtained from FPSO and SPM calm buoy in real marine environment operational location. It was revealed that most of the offshore structural design codes for floating assets in tropical waters or temperate region were still based on the codes for North Sea installations, even when both were known to have obvious different environmental conditions. These data are a true representation of the state of marine corrosion wastages associated with the floating assets. The data obtained from laboratory is usually simulated and controlled while that of marine environment occurs naturally. It was also observed that the validity of data suitable for life extension studies is placed at 18 months by some class societies, while some oil majors allows data obtained within 60 months and provided that such data will be scrutinized by relevant subject matter expert. Usually a minimum of five points thickness gauging and maximum of eight-point thickness measurements where there is marginal wastage is required per strake of a square metre. It was also observed that the collection, collation, analysis and interpretation of these data drives the technical aspect of the intelligent business decision aimed at making life extension repairs decisions of FPSO and SPM calm buoy. Although, it was remarked that corrosion wastages do not alter the chemical and mechanical properties of steel but only changes their geometry. Thus, the remaining thickness is a representation of the remaining strength of the steel plate. It was also remarked that these repairs are done according to class society requirements and supervisions from material selection, verifications and repairs. Steel plate grade, heat number, attending surveyors work order number, mill test certificate and reports signed at steel yard during manufacturing testing by the surveyor are some of the requirements. Others are the steel plates welding processes, sighting of procured steel plates and welders’ certificates at repair sites. Thus, steel plates repairs on classed vessels must be done with classed steel plates of the same grade and thickness to the badly corroded one being renewed. Although, in the absence of the same thickness, a higher thickness can be used but must be approved by the class society covering such asset. Regrettably, steel plates of different grades mean that they have constituents’ elements percentage composition and hence microstructure which can pose a threat of galvanic corrosion if applied in such marine environment.
Keywords: certificate of class, classed steel plates, classed vessels, corrosion wastages, crop and renew, end of design life, life extension, mitigation strategies