Want To Make Tree Operations Safer? Why Not Use the DHSV as a Barrier?

摘要

Industry practice for the design and operation of oil and gasrnproducing wells incorporates a two-barrier policy, and definesrnan integrity envelope for environmental protection and safernoperations. The standard practice when removing orrnundertaking major operations on the Xmas tree is to set one orrnmore plugs in the production tubing to maintain two barriersrnbetween the reservoir and the environment.rnThis paper assesses the relative risk of using a Down HolernSafety Valve (DHSV) as a qualified barrier during suchrnoperations. Quantitative Risk Assessment (QRA) has beenrnused in this study to understand the implications of the changernin working practices.rnQRA has been used to assess the risks of an uncontrolledrnflow of fluid from the reservoir when using a qualified DHSVrnas a barrier as compared to the standard practice of setting onernor more wireline plugs. Three representative cases have beenrnconsidered in this study:rnCase 1 examines changing valves downstream of the masterrnvalve on a sub sea well.rnCases 2 and 3 focus on platform wells.rnCase 2: changing valves downstream of the lower masterrnvalve, andrnCase 3: removing the Xmas tree.rnThe study shows that the major risk when using a wirelinernset plug as a downhole barrier arises from the blowout riskrnassociated with wireline operations. Use of a DHSV as arnqualified barrier eliminates the wireline blowout risk, andrnreduces the overall risk during Xmas tree operations. Hence,rnthe study recommends the use of a qualified DHSV as arnbarrier, subject to a risk assessment on a case by case basis.