Structural Risk Analysis Of FPSO During Lifetime

With the increasing demand on energy resource stimulated by fast development of economy, offshore structures are widely used to explore ocean oil and gas resource. Accompanying with considerable benefits from exploitation, the problems of risk and cost relevant to offshore structures are becoming more and more prominent, which include the exorbitant cost of design, construction, service, maintenance and failure, and sequential environmental pollution, fatal loss and social impact caused by accident. The risk of offshore structure is always concerned by people.FPSO (Floating Production, Storage and Offloading unit) is one of the main floating platforms positioned by mooring system. During its service, FPSO has to endure fatigue and corrosion damage under severe sea state. Possible accidents, such as collision, fire and explosion, etc., will affect structure adversely. The serious consequence aroused by the failure of FPSO by any possibility makes the risk level focused. Therefore, it is necessary to evaluate the structural risk of FPSO during its lifetime, which is also the research project concerned by offshore industry recently.The structural risks of FPSO during lifetime include many aspects. Some key points and those problems neglected before are selected to be studied in order to learn characteristics of structural risk of FPSO generally and get useful results for further research and control on risk of FPSO during its lifetime, which benefits to secure the safety of offshore industry. The summary of main research works and results are listed below:Firstly, the possible risks of FPSO during lifetime are analyzed briefly and the necessary and meaning of evaluating them are introduced. The latest domestic and international progresses are summarized according to different aspects of risk analysis of FPSO.Considering the corrosion difference between FPSO and merchant ships, proper corrosion model and database are selected to calculate the ultimate strength of FPSO based on different corrosion level. Time-variant probabilities of failure of FPSO hull girder in mooring condition are calculated based on limit state function. The available service years corresponding to different corrosion level are worked out to reflect the influence of corrosion on FPSO structural safety, which emphasize the importance of controlling corrosion to maintain the risk level of FPSO. The sensitivity of arrival rate of a single loading condition and environmental factor are analyzed. It is obvious that the environmental factor has great effect on the structural risk of FPSO.Taking damage of collision and corrosion into account together, the structural risk of damaged FPSO hull girder are evaluated when collision takes place at different service year. The loads of damaged FPSO still follow former long-term distribution within short period after collision because the time of collision occurrence is considered as a factor in this study. Therefore, the loads combination of damaged FPSO hull girder which is still in mooring condition after collision is adjusted. The probabilities of failure of FPSO hull girder are calculated separately based on different locations of damage due to collision, which are mid section of FPSO and other sections. The results show the necessary of evaluating all possible consequences, which can be caused by damage of each ballast tank. The results also show the difference of structural risk levels after collision induced by different corrosion model and database clearly. Combined with residual strength index, risk acceptance criteria of FPSO hull girder, which is still in mooring condition within short period after collision, is presented based on calculated probability of failure.Taking some factors into account, such as towing speed, sea state, FPSO structure condition (including corrosion state and hull integrity), etc., the structural risk of FPSO is analyzed when it encounters a short-term storm under towing condition. Three possible structure conditions of towed FSPO are considered, which are new-built intact condition, retirement condition and collision damage condition, respectively. Time of retirement and collision occurrence is taken into account when calculating probabilities of failure under towing condition, which can reflect the structural risk levels of FPSO during lifetime more generally since corrosion damage effect is introduced into calculation. It can be found that the significant wave height is the most prominent factor to probabilities of failure under towing condition. Acceptable maximum significant wave heights satisfying risk acceptance criteria are derived according to different situations, and are taken as decision-making index to estimate structural risk level of FPSO hull girder under specific towing condition.Since inspection result can update prior distribution, risk based inspection and maintenance theory is applied on FPSO corrosion damage control and cost optimization. Hull girder plates at different location are selected to make inspection schedule according to various combinations of corrosion extents, inspection levels and failure criteria. Then corresponding total expected costs, failure costs, inspections costs and maintenance costs are calculated based on demand of cost optimization. After comparing result of costs, it can be found that the variation of total expected costs is too complicated to be summarized as uniform rule to describe all structure components. It is caused by many factors, such as component location, corrosion extent, failure criteria, inspection level and maintenance criteria, etc. This phenomenon proves the necessary of inspection and maintenance study on different FPSO components. Obviously, deterioration of corrosion extent has unfavorable effect on costs. It is reasonable to bring design cost into cost optimization of whole schedule in further research by choosing different anti-corrosion methods at design stage.Further optimization on FPSO inspection and maintenance schedule is carried out. Improved model for selecting inspection time is presented to derive congregation of optional inspection time. And costs corresponding to new model are calculated, consequently. It can be seen that the inspection time achieved from new model has lower costs than former schedule, which is benefit to avoiding unnecessary expenditure. Based on derived optimized inspection time, optimized risk acceptance criteria are analyzed according to different conditions and the differences of costs distinguished by inspection level and corrosion extent are also discussed. The difference of costs induced by dimension of defects detected on various components in industrial application is analyzed, which proves the practicability of risk based inspection and maintenance method. The costs rise with increase of dimension of defects, which emphasizes the importance of controlling corrosion to reduce costs again. The data of costs shows the probable uneconomical situation of FPSO during its overage service, which is helpful to make maintenance or retirement decision for owners when FPSO exceeds its designed service life.