Reliability Analysis of Alternative Gas Well Safety Valve Completions

Abstract

Using conventional reliability analysis techniques, a gas well completion was examined to determine the best subsurface safety valve installation for maximum well safety. Consideration was given to a well's production, wireline and workover activity, and use was made of reliability data based upon 3600 gaswell years of experience in the Netherlands and a worldwide survey of offshore blowouts. A deep set (packer depth), surface controlled subsurface safety valve completion was found to be the most reliable completion, whilst wireline activity posed the dominant risk in well safety. Introduction The oil production industry has not readily applied the technique of reliability analysis to its operations. Papers were published in the early 1970's on the application of reliability engineering to offshore production systems (Refs. 1 and 2), but qualitative evaluation of production systems (Refs. 1 and 2), but qualitative evaluation of reliability was considered to be more applicable than quantitative evaluation methods (Ref. 3). Recently a number of studies of basic well systems have been published (Refs. 4, 5 and 6), and this paper is intended to extend those works to determine the optimum location for the downhole safety valve to maximise well safety. Reliability Analysis and Event-Consequence Analysis techniques are described by Howey and Gaarder (Ref. 2). Reliability analysis incorporates:–a definition of the system (and alternatives),–a definition of the failure modes,–construction of a model to describe the failure relations,–the use of component reliability data,–the determination of the probability of failure. The event-consequence approach incorporates the above but additionally identifies the (monetary) consequences and determines an expected value of the system. Whilst Event-Consequence Analysis is the more thorough approach, reliability analysis may be adopted to determine the most reliable system. This simplification is justified where a failure would be classified as catastrophic and the achievement of high reliability is governed only by the advance of technology.