The Western Leg Gas Gathering Pipeline
Authors: Pullin Kenneth , Daniels Myer 
Published: December 2007

This paper describes Shell/Esso's Western Leg gas pipeline, the first system to be built in the east pipeline, the first system to be built in the east Shetland area (Scotland) to gather marginal quantities of associated gas from several oil fields. It provides a means of transporting this gas to shore provides a means of transporting this gas to shore through the 36-in. FLAGS pipeline. The paper describes how small quantities of gas can be transported, the major pipeline-design features, the installation procedures, and the planned commissioning technique.

Introduction

Increasing concern for the conservation of world energy sources and their prudent use has focused the attention of regulatory authorities and the general public on the practice of flaring associated gas during public on the practice of flaring associated gas during production of liquid petroleum. In the case of many production of liquid petroleum. In the case of many offshore fields, the quantities of associated gas (while significant especially during the first years of oil production) are insufficient to justify economically production) are insufficient to justify economically an independent pipeline system to shore. In the East Shetland basin, several such fields had been developed for oil production on the basis of flaring associated gas surplus to platform fuel requirements. In 1978 Shell/Esso, developers of the Brent field and constructors of the FLAGS 36-in. gas line from the Brent field to St. Fergus on the Scottish mainland, considered the possibility of gathering the relatively small amounts of gas from other producers and shipping it ashore through the FLAGS system. From the outset, it was obvious that the economics of a gathering system would be marginal since quantities were small and peak plateaus brief. A possible longer-term asset is the availability of the pipeline to supply fuel gas to a platform when its own fuel gas production falls below its needs. production falls below its needs.A review of the circumstances under which potential sources of gas could be brought into a potential sources of gas could be brought into a gathering system produced, among others, the following major constraints.To be economically viable, the capital cost must be kept to an absolute minimum.Recompression before entering the FLAGS pipeline was not feasible. pipeline was not feasible.The maximum operating pressure, determined by available platform riser and flanges, would be 2,160 psig (ANSI Class 900 flange limits).The Brent A platform was the preferred entry point to the FLAGS pipeline. point to the FLAGS pipeline.Not all potential participants would be ready to ship gas at the same time. Provision must be made to commission connections with the main system.

The planned sources from which to gather surplus gas were five known fields in the east Shetland area that were either in the course of production or planned for early development-i.e., Comorant, planned for early development-i.e., Comorant, Heather, North Comorant, Ninian, and Northwest Hutton. The geographic disposition of these fields and the lack of availability of suitable risers on existing platforms precluded a pipeline system which "hedge-hopped" from platform to platform with above-water connections. Subsea connections between pipelines was the only alternative and was accepted in principle, although there was no precedent in the northern North Sea or in water precedent in the northern North Sea or in water depths of 150 m. After negotiations with the various block holders and a study of flow patterns and construction cost estimates, the route of the system finally agreed to by the U.K. DOE was as shown in Fig. 1.

JPT

P. 569