Reliability Management of Surface-Controlled Subsurface Safety Valves for the TOGI Project
Authors: Moines E. , Iversen M. 
Published: October 2008

This paper describes the combined efforts taken by Norsk Hydro and SINTEF on obtaining higher reliability of Surface Controlled Subsurface Safety Valves (SCSSV) for the TOGI Project.

Due to the severe consequences of system failures, reliability management has been fully implemented on the TOGI subsea production system.

The methodology used combines reliability theory with operating experience. Databases with relevant field data, design review techniques, valve prototype testing and review of test results are other elements of the reliability management for the SCSSVs on TOGI.

Although this specific application has been on SCSSVs, the methodology presented in this paper can be easily applied for systems where the consequences of failure will be severe.

1. INTRODUCTION

TOGI is an acronym for the Troll - Oseberg Gas Injection Project. The objective of this development is to export gas from the Troll field for injection into the oil producing Oseberg reservoir. By injecting Troll gas instead of water into the Oseberg reservoir, Norsk Hydro will be able to recover an additional 12 million SCM oil. When Oseberg starts producing gas, approximately 70% of the injected gas will be recovered.

The TOGI Project is developed under Troll Unit with Norsk Hydro Production a.s. as the operator for development and operation of the installations. The partners are Statoil, Saga Petroleum a.s., A/S Norske Shell, Conoco Norway Inc., Total Marine Norsk A.S. and Elf Aquitaine Norge A/S.

The TOGI production system will consist of a subsea station at a water depth of 303 metres, with 5 gas producers and one spare slot. A 48 kilometre export pipeline and service line ties the subsea station to the Oseberg Field Centrewith support functions.

The subsea station at the Troll end will be the first subsea production unit at such a great water depth on the Norwegian continental shelf. The station consists of components which can be installed, disconnected and brought to surface without the assistance of divers. Adjustments and maintenance which cannot be controlled remotely from Oseberg will be executed using ROVs supported by a surface vessel. A schematic presentation of the TOGI Project is shown in Fig. 1.

Requirements for high safety and production regularity were defined for the TOGI subsea production system. To meet these requirements, high reliability of vital components was necessary. A comprehensive overview of TOGI reliability requirements is provided in /1/.

The consequences of a SCSSV failure on a TOGI well would be a decrease in the amount of gas available for injection, which in the worst case could lead to a decline in Oseberg oil production rates. A costly workover operation would also be required. The direct cost of a workover at the subsea station is estimated at NOK 80 million (USD 12 million).

Recent statistics clearly show that SCSSVs is the primary cause of workover operations initiated by failure of the completion equipment. The failure rate ranking is shown in Fig. 2. This ranking is based on a review of open reliability data sources, Norsk Hydro in house experience and data from other operating companies.