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Abstract

Industry practice for the design and operation of oil and gas producing wells incorporates a two-barrier policy, and defines an integrity envelope for environmental protection and safe operations.  The standard practice when removing or undertaking major operations on the Xmas tree is to set one or more plugs in the production tubing to maintain two barriers between the reservoir and the environment.

This paper assesses the relative risk of using a Down Hole Safety Valve (DHSV) as a qualified barrier during such operations.  Quantitative Risk Assessment (QRA) has been used in this study to understand the implications of the change in working practices.

QRA has been used to assess the risks of an uncontrolled flow of fluid from the reservoir when using a qualified DHSV as a barrier as compared to the standard practice of setting one or more wireline plugs.  Three representative cases have been considered in this study:

Case 1 examines changing valves downstream of the master valve on a sub sea well.

Cases 2 and 3 focus on platform wells.

Case 2: changing valves downstream of the lower master valve, and

Case 3: removing the Xmas tree.

The study shows that the major risk when using a wireline set plug as a downhole barrier arises from the blowout risk associated with wireline operations. Use of a DHSV as a qualified barrier eliminates the wireline blowout risk, and reduces the overall risk during Xmas tree operations. Hence, the study recommends the use of a qualified DHSV as a barrier, subject to a risk assessment on a case by case basis.

Introduction

Industry practice regarding the design and operation of oil and gas producing wells incorporates a two-barrier policy for safe operation and environmental protection.  At all times there must be two independent barriers in each flow path between the reservoir and the surface.  A barrier in this context is any system that can be used to contain well fluids within the well envelope; examples are a bridge plug or cement, the Xmas tree or blowout preventer (BOP).  A fluid (e.g. drilling, completion or workover fluid) will normally be considered to constitute a barrier provided that the level of the fluid can be monitored and maintained.

Standard practice when removing or undertaking major operations on the Xmas tree has been to set one or more plugs within the production tubing to maintain two barriers between the reservoir and the environment.  Typically, any change to these normal procedures and standards requires a quantitative risk assessment as part of the dispensation process.

This study was undertaken to assess the relative risk of using the downhole safety valve (normally a Tubing Retrievable Safety Valve, TRSV) as a qualified barrier during such operations.  The initial application was the replacement of a failed production wing valve on a subsea gas producer.  Two further cases were assessed for platform wells; a similar valve changeout downstream of the master valve and complete removal of the Xmas tree.  In each case the risks were assessed when using a DHSV as a barrier for operations on the Xmas tree as opposed to performing interventions to set wireline plugs.

The methodology used was to evaluate and quantify the risk of a loss of containment occurring for each scenario using Fault Tree Analysis (FTA).

UK Legislation

Legislation regarding barrier policy differs greatly between countries, with some having adopted very prescriptive approaches.  In the UK, Health & Safety legislation for the offshore industry does not prohibit the use of a DHSV as a barrier.  The regulations use a goal-setting framework and do not provide a prescriptive guide to the practical application of tools and techniques.  

However, it is a requirement of the regulations and standards that well operators and contractors must state their isolations standards for hydrocarbon containment and pressure systems and ensure that these are met in all failure scenarios.

The Wells Design and Construction Regulations [DCR]¹

Regulation 13(1) requires well-operators to ensure that each well is so designed ……….[and] operated, maintained …….. [so that] so far as is reasonably practicable, there can be no unplanned escape of fluids from the well; and risks to the health and safety of persons from it or anything in it, or in strata to which it is connected, are as low as is reasonable practicable.