Risk Assessment of Hydrocarbon Releases during Workover and Wireline Operations on Completed Wells on Offshore Platforms

Abstract

Abstract Safety Regulations in the UK require operators to demonstrate that the risks to workers from well operations on offshore platforms have been reduced to the lowest level that is reasonably practical. Each offshore platform in the UK has a safety case which includes a quantified risk assessment of well operation hazards with the potential to cause a major accident. Assessment of the safety cases by the UK Health and Safety Executive (HSE) showed that few operators have specifically analysed risks resulting from well workover or wireline operations, regarding such risks as subsumed within general blowout risks. Where workover/wirelining have been considered separately, the risk estimates have been determined from average worldwide historical hydrocarbon release frequencies and not by analysis of particular operations being carried out on a specific platform. The prevalent conclusion from such risk analyses is that the contribution of workover/wireline risks to the total platform risk levels is very small. In order to evaluate whether such a conclusion is necessarily valid in all cases, HSE undertook a study to establish a method for estimating workover/wirelining risks for a number of different types of platform. This paper describes the method used by HSE to assess the risk of hydrocarbon releases during workover and wireline operations. The workscope was restricted to completed wells on offshore platforms. The method is based on a detailed analysis of all the different potential failure modes associated with the various well intervention operations carried out on these platforms. The failure mode analysis results are then used to estimate the significance of these risks relative to risks from other activities on these platforms and to assess where the use of average historical release frequencies could be justified and where additional analyses are necessary. The study also provides a mechanism for identifying those workover/wirelining operations which are predicted to give rise to the highest risks and for which risk control measures should be reviewed very closely.