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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.

Introduction

Following Lord Cullen's Public Inquiry into the 1988 Piper Alpha disaster, in 1992 the UK introduced a requirement for offshore installations in its sector of the North Sea to submit safety cases for acceptance by the Offshore Safety Division of the UK Health and Safety Executive. Amongst the details to be included in the safe cases are:

(a) a demonstration that all hazards with the potential to cause a major accident have been identified,

(b) a demonstration that risks have been evaluated and measures have been, or will be, taken to reduce the risks to persons affected by those hazards to the lowest level that is reasonably practicable.

Examination of the way in which these requirements were being met with respect to workover/wirelining issues in a number of safety cases revealed concerns in that:

(a) the issue of workover/wirelining activities had not been specifically considered within many safety cases. Although there are many different tasks that can be carried out under the general description of workover/wirelining, there had often been no attempt to examine such tasks individually and to determine what could go wrong, its significance, its likelihood and whether there were possible remedial measures,

(b) where separate workover/wirelining risk figures had been calculated, the estimates often were for very low risk levels which it was claimed justified the area not being examined in further detail. Well Operations specialists in OSD were not convinced that this necessarily gave a reliable or realistic picture,

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