Durable Reliability of Jack-up Platforms. The impact of Fatigue, Fracture and Effect of Extreme Environmental Loads on the Structural Reliability

A variety of factors is governing the operational conditions of jack-up platforms. The platforms are moved to different locations, which are causing changes in water depth, environmental and seabed conditions, drilling depths, payload etc. Insight should be gained in how the different factors affect the lifetime in order to be able to make a reliable prediction of the safety. The structure of the jack-up platform, usually consisting of three legs, which are supporting the platform containing living quarters, installations for power generation, machinery and equipment for drilling or production facilities, differs from a fixed platform. Aspects as the need for more flexibility and the development of marginal fields, initiate also a trend for jack-up platforms to operate in deeper water. The subject of this research project is durable reliability of jack-up platforms with the aim to explore the possibilities for the extension of the life-time. Aspects as fatigue, fracture and failure due to extreme environmental loads are investigated. The uncertainties in loads, material characteristics and structural modelling are investigated. A deterministic approach cannot properly take these uncertainties into account. For the reliability calculations, a method, based on refined stress states, is presented. The fracture mode is of importance when extreme environmental loads are combined with an existing crack in a structural element, which may be due to the fabrication process or fatigue. To estimate the statistical information about the crack a method based on the Monte Carlo simulation technique is presented. This method uses a fatigue crack growth formulation to specify the crack size. For the ultimate failure mode under extreme environmental loads the axial and bending stresses are combined to establish the time history of the usage factor. Due to the redundancy of the structure the system effect may be important. For this purpose a method is presented to specify the limit state function for sequences of failure leading to structural collapse and the failure probabilities of these sequences are calculated. The branch and bound technique is used to establish the branch tree to identify the important failure sequences. Then the system failure probability is calculated by combining the most important sequences leading to structural collapse. For this purpose The First Order Multi Normal approach (FOMN) and bound techniques are used. For a case study, the 'Neka' platforms which was chosen in this thesis, the failure probability of the structural system in the combination of fracture and fatigue failure modes proved to be the most important. The results show significant system effects. The conclusion may be drawn that this type of structural failure deserves significant attention during the ageing of jack-up platforms. This research work does not claim to have solved all issues related to the structural reliability of a jack-up platform. However, a number of valuable stepping-stones have been developed as a basis for further research.Design, Engineering and Productio