Development of a Reliable Gas Injection Operation for the North Sea's Largest-Capacity Production Platform, Statfjord A
Authors: Hancock W.P. 
Published: December 2007

Hancock, W.P.; Mobil Exploration Norway Inc.

The energy crisis of 1973 highlighted the need to conserve valuable associated natural gas from producing formations in certain remote locations where the flaring of gas previously had been tolerated because of the low gas price. Reliable gas injection therefore became an essential requirement to reduce gas flaring. However, reinjection of gas at high pressure represents a great challenge to both equipment manufacturer and pressure represents a great challenge to both equipment manufacturer and field operator, especially from a remote North Sea platform. The paper describes how reliable gas injection was achieved from the Statfjord A platform. Careful equipment selection, planning, commissioning, debugging, maintenance, and operation were all important factors in achieving the successful outcome.

Introduction

Reliable gas compression, particularly high-pressure gas injection, has proved extremely difficult to attain on offshore installations in various proved extremely difficult to attain on offshore installations in various parts of the world, sometimes taking years to achieve satisfactory parts of the world, sometimes taking years to achieve satisfactory performance. For the Stat fjord offshore field it was essential to achieve performance. For the Stat fjord offshore field it was essential to achieve reliable gas injection as soon as possible after production startup to minimize flaring of valuable gas and optimize production rates. This paper describes how sound planning, careful precommissioning, and computer simulation of potential failure modes were used to help to achieve this goal. Nevertheless, the achievement of reliable gas injection still proved a difficult process, and several unforeseen problems were encountered that took considerable time and effort to resolve. The most serious problems encountered with the primary gas compression trains werethe inability to handle the heavier gases during startup andcontamination of the seal oil.

For the injection compressors (the final stage of compression), the principal problems were (1) control panels, (2) lubrication, and (3) short lifetimes of the packing and piston panels, (2) lubrication, and (3) short lifetimes of the packing and piston rings. The Stat fjord field is the largest oilfield discovered in the North Sea. It lies in about 145 m (475 ft) of water about 180 km (110 miles) northwest of Bergen and straddles the Norway/U.K. median line. Total field reserves are about 4.7 × 10 8 Mg (470 million metric tons) [570 × 10 6 m3 (3.6 billion bbl)] oil and 70 billion std m3 (2,470 Bscf) gas, of which 84% lies in the Norwegian and 16% in the British sector. Mobil Exploration Norway Inc. is operator on behalf of the Stat fjord Unit* for development and production of the field. Development started in 1974 and will be completed in three phases by the late 1980's. The first phase of development, Stat fjord A, is a concrete platform with an articulated loading platform (ALP) for direct loading of crude into tankers. The platform itself weighs 6.5 × 10 5 Mg (650,000 metric tons) and has 19 concrete cells lying on the seabed. Of these, three are elongated into three concrete shafts that support the steel deck; the remainder are used for storage. The steel deck has an operating weight of about 5.0 × 10 4 Mg (50,000 metric tons) and includes drilling and production facilities and accommodations for 200 people. production facilities and accommodations for 200 people. JPT

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