Requalification of an Unmanned Jacket Structure Using Reliability Methods

Abstract

ABSTRACT A reliability assessment of an existing jacket structure exposedto reservoir subsidence is considered in this paper. Due to reduced air gap the probability that a wave will hit the deck is increasing and this is crucial concerning the future safety for an existing structure. A consideration of the safety level as a function of subsidence is therefore established in order to identify when the safety level drops below an acceptable safetylevel and corrective actions must be taken. Reliability methods is used to describe the uncertainties involved in the problem and then used to make a cost optimal decision. INTRODUCTION Offshore structures do usually have a large reserve capacity. This reserve capacity is often measured in terms of a Reserve Strength Ratio (RSR) defined as the ratio between the load level causing global failure of the structure and the 100-year design environmental load. For a jacket structure in the North Sea, the RSR value is typically in the range of 2-4. This means that a very large wave is required in order to fail the jacket structure. It is therefore often claimed that the most likely failure mode for offshore structure is failure due to a wave hitting the deck. Large reservoir subsidence will reduce the available air gap of existing platforms considerably compared to what was used in the design load calculations. Loads caused by a wave crest hitting the deck will be very large. and deck mpact is therefore likely to be crucial regarding the structural safety of platform under such conditions. If the code requirement to available air gap can not be met after a number of years in operation, e.g. due to subsidence. it must be documented that the platform has a sufficient capacity to resist the loads which can occur if a wave crest hits the deck structure. By using reliability analysis. the following important questions can be addressed:Is the present safety level acceptable?When are actions necessary in order to maintain an acceptable safety level?Which actions are most cost effective? One advantage by a reliability approach is that it is possible to utilise a realistic description of the uncertain parameters. This is not possible by a standard code check. The approach presented in this paper involves the following steps:Probabilistic description of the severe environmental conditions.Estimation of the loads caused by waves hitting the deck structure.Assessment of the platform capacity using a non-linear structural analysis. The results of these steps are then build into a reliability formulation of the problem. and the annual failure probability for different subsidence levels are estimated. An unmanned jacket structure is considered in this study. If a structural collapse only have economical impact. the information about failure probability can be used to do a cost optimal decision regarding corrective actions. The example platform is a four legged jacket structure with K-brace configuration and located in a water depth of about 70 m.