ABSTRACT

Statoil is operator of the major part of the gas transport network in the Norwegian part of the North Sea, exercised on behalf of the owner constellations. Several reservoirs involved with associated sales contracts and transport agreements invoke commercial issues together with technical limitations of operation. In order to operate this transport network at high capacity utilization combined with ultimate regularity of the gas deliveries Statoil has established an inclined organization and implemented a sophisticated computer based information and control system. When installed in 1993, this system represented state-of-the-art technology. However, experience has revealed that reliable operations of the system demand involvement of operators capable of bridging the physical and the generic systems. Facing the altered operational complexity in 1996 with the Troll gas on-stream, the system has proven its reliability and usefulness accommodating models that closely correlate the operational conditions.

INTRODUCTION

Since 1985, Statoil has constructed and put into operation approximately 1,725 miles (2,775 km) of large size steel pipelines in the North Sea. The major part of these pipelines are in gas service, some 265 miles (425 km) are in service for crude oil and condensate products. Towards year 2000 another 1,750 miles (2,815 km) of pipelines will be constructed and put into operation. Development of the Norwegian part of the North Sea was initially concentrated around oil reservoirs producing associated gas. However, the Heimdal and Frigg gas fields were also developed. Signing of the Troll Gas Sales Agreement (TGSA) in 1986 introduced a new era in Norwegian gas industry. Large volumes of gas were sold to a buyer consortium on the Continent, commencing with gas deliveries from the Sleipner-East field in October 1993. According to these commitments the plateau deliveries will be 1.6 tcf/yr (45 BCM/yr). However, total yearly gas export commitment from Norway between year 2000 and 2009 is 2.2 tcf (62 BCM). Initially, gas deliveries under TGSA are provided by Zeepipe-I, the worlds largest offshore gas pipeline system, constructed along with the Sleipner field production facilities. In May 1996, the giant Troll field with estimated 46 tcf (1,300 BCM)recoverable gas reserves was put into operation. Implementation of new technology was mandatory to realize this $4.5 billion project. With a daily production capacity of 3.5 bcf (100 MSm3/d) the gas deliveries to the Continent have been significantly increased. Specific to the Norwegian transport network for gas is the long distances of pipelines without any intermediate system control. Zeepipe-I, 506 miles (814 km) long, is operated without any intermediate compression or control. This is basically due to the relative deep waters in the North-Sea and the associated cost of platform installation involved. Consequently, the pipelines are operated at high pressures to gain capacity. This is illustrated by the two export pipelines for Troll gas, which have a design pressure of 2770 psig (191 barg) at Kollsnes, the Troll gas onshore terminal.

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