After ending their service lifetime, offshore structures will be decommissioned and subject to decision of removal/final disposal. At this cold phase, it is required that the structures shall still maintain the minimum safety requirement for removable. How to minimize the total maintenance costs of offshore structures at cold phase is the concern of operators. In this paper, a methodology is presented on assessment of offshore structures at cold phase considering the effect of inspection based on system reliability method. The effect of inspection updating on the assessment of offshore structures is investigated systematically. In particular, the effect of inspection on the reliability of both inspected and un-inspected joints is assessed. The influence of inspection of certain joints on the system reliability is studied and the effect of system-based updating policy on fatigue reliability is investigated. A numerical example is given to demonstrate the use of proposed method to assess offshore structures at cold phase efficiently and economically. It is believed that the maintenance costs could be reduced significantly by applying the proposed methodology.
Offshore structures are designed for a certain length of service lifetime. After ending their service lifetime, the structures will be decommissioned and subject to decision of removal/final disposal. There is a time interval between platform abandonment and final disposal, whilst the structure is still located in-situ without presence of hydrocarbons, this time period is called "cold phase". To prevent economic loss and to remain disposal flexibility, it is still very necessary to maintain the minimum structural integrity cost effectively. How to optimize condition monitoring of offshore structures at cold phase and fulfil the minimum safety requirement are the major concerns of operators. For this reason, the offshore structures need to be assessed based on their actual condition depending upon the inspection results.
