New Safety Valve for Subsea Installations

Abstract

Abstract A new safety valve was required by North Sea Operators for- their prospective sub-sea completions. The valve would have to combine compatibility with low result subsea wellhead controls with high reliability. A design team was organized consisting of engineers from an operator and a manufacturer. The project began in 1975. Four years of testing have provided data on the operation of this safety valve. The first two valves were installed in a North Sea Sub Sea Completion in the Fall of 1980. This paper discusses the design parameters and features of the safety system as well as the performance of the system in tests and In service. Introduction In 1975 Shell International Petroleum Maatschappij B. V. (SIPM) in the Hague, Holland was looking for a reliable deep set surface controlled, tubing retrievable, subsurface safety valve (SC-TRSSV) for installation in a dual 3-1/2 inch TFL-completion for a new subsea production wellhead referred to as the Insert Tree project. The attention was focused on Otis who presented at that time their below mud line wellhead (BMW) study which had evolved around the Otis Tubing Hanger Safety System and incorporated SC-TRSSV's. In the same year Shell Expro, operating in the UK sector of the North Sea, had developed an interest in SC-TRSSV's for their underwater development scheme for the Cormorant field. As a result of this both SIPM and Shell Expro ordered (different) balance line SC-TRSSV's from Otis for in-house testing. Tests carried out at the E&P laboratory in Holland demonstrated that fail safe closing of either valve under the test conditions, i.e. 3000 ft installation depth and 5,000 psi internal pressure could not be achieved. In addition, the leakage rate after 1000 opening and closing cycles was approximately five times that permitted by the API-14A. These disappointing test results and the pressing need for a reliable safety valve for both the Insert Tree project and the Central Cormorant subsea development scheme, led to the decision to initiate a joint program for the development of a SC-TRSSV which would comply with the requirements for a dual 3-1/2 inch underwater TFL-completion. Throughout the design phase regular meetings were held between Shell and Otis with continuous Input of ideas from both sides. This finally led to a valve which incorporated both the operators experience and the manufacturers engineering skills. The valve has unique features such as a second balance chamber to facilitate fail safe closing under all circumstances, a lock-out/isolation sleeve, a sliding sleeve which accommodates a TFL back-up valve and a carefully selected sealing arrangement for all static and dynamic seals. During the extensive testing phase both in Holland and at the Becton TFL test site, some minor modifications proved to be necessary. Finally, after evaluating all test results, the valve was accepted for offshore service. Two of the requirements for the safety system for the Insert Tree changed with the effect that a single line valve had to be installed.