Reliability Analysis of Jack-up Platforms Based on Fatigue Degradation

Generally, jack-up structures are used for production drilling and exploration of hydrocarbons. The combination of mobility and the behavior as a fixed structure in operational conditions has made it an important structure in the offshore industry over the last 40 years. When a jack-up structure has been in operation for a great part of its original design-life and intention is there to extend the usage of this structure at a specific location, an investigation on fatigue degradation of the structure is an essential factor that has to be carried out before taking any decision. Fatigue is the process of damage accumulation in material due to stress fluctuation caused by variation of loads in service time. The fatigue failure occurs when accumulated damage has exceeded a critical level. In this paper, the remaining fatigue capacity of the jack-up structure is considered as an indicator for adequate use of the structure. It can be specified based on the difference between design-fatigue and fatigue experienced by the structure. The design-fatigue can be determined based on fluctuation of loads during the lifetime of the structure and experienced fatigue is specified by the load conditions, which the structure has experienced during its service time. When the information on the load conditions which the structure has experienced in its service time is available or known precisely, determination of the remaining fatigue capacity could be carried out by using the Palmgren Miner’s rule. In practice, uncertainties are present in loads and characteristics of material. Hence it will be reasonable to determine the remaining fatigue reliability of the structure by the reliability methods. In this paper, based on a crack propagation approach and achieved information from inspection, it is shown that the remaining fatigue reliability of jack-up structures could be determined and updated by using a Bayesian procedure in the duration of the service time. INTRODUCTION Time-varying nature of environmental loads such as wave and wind will cause fluctuation of stresses in structural components of the offshore platforms. The deterioration process, which arises from fluctuation of stress, is known as the fatigue damage and should be evaluated before reaching a critical value. Traditionally, fatigue damage has not been considered as an important problem for jack-up platforms. The main reason was that the most platforms have been designed for areas with low or moderate environment conditions. Recently, by using jack-up platforms in areas with more severe sea conditions or as permanent platforms, the fatigue damage becomes an important factor in evaluating the integrity of the structure [1]. Based on a simple deterministic approach, Connolly [2] has extended a method to determine the remaining fatigue life of tubular joints. In this method, based on fracture mechanics the 1 Copyright © 2002 by ASME remaining fatigue life of tubular joints are determined as the time of propagation of crack from initial value to throughthickness. However, due to uncertainty in characteristics of loads and fatigue, the suitability of deterministic methods will be suspicious and it is reasonable to prefer reliability methods in the analysis, as was also suggested by for instance, Kam and Birkinshaw [3], and Tang and Xi [4], and Tromans and Vandergraaf [5]. In this paper the remaining fatigue reliability of the structures is determined based on the difference between the annual fatigue reliability of the structure during its service time and the annual target reliability. The inspection of the structure during its service time prepares a proper information to update the fatigue reliability. Risk based inspection with an application to pressurized HC production systems is discussed by Hagemeijer and Kerkveld [6]. More details on reliability and inspection updating methods are discussed and numerically illustrated in the subsequent sections. NOMENCLATURE dr(φ) Increment of crack size dN Load cycle