Shell Expro operates a 36-inch oil pipeline which runs from Cormorant Alpha to Sullom Voe, through Yell Sound in the Shetlands. Yell Sound is noted for its complex tidal regime with frequent occurrences of disturbed and very fast-moving water masses.

Access to the pipeline is required for inspection purposes and for maintenance/repair if inspection should reveal the need for that to be carried out. As the area is largely sheltered form stray wave action, access to the pipeline is determined by the strength of the tidal currents.

Extensive measuring programmes carried out for the northern part of Yell Sound (1976–1978) and the southeastern parts (1982) have revealed the presence of considerable variations (surges) in current speeds which could not be explained as part of the ordinary tidal regime. It was suggested that the surges reflect the appearance of gyres (vortices) in the area, but until recently it was not possible to test this theory nor to assess the spatial extent of the surges or to identify the tracks along which the gyres move.

In order to obtain an improved understanding of the complex current pattern in the area around the Brent System Pipeline, Shell Expro commissioned the Danish Hydraulis Institute (DHI) to set up a computer based model of the Shetland area. The results of the study will be used also for planning and scheduling diving work in Yell Sound and to assist in the establishment of Pipeline Maintenance Systems.

This chapter describes how the model was established using Systems 21, DHL's numerical modelling systems for two-dimensional, nearly horizontal flow.

The model was validated using measurements form five different locations in the 1982 survey. The results confirm the gyre theory and show the spatial extent of the gyres as well as the tracks followed by them. Finally, a program (for an IBM PC) which makes use of the model results to predict current velocities along the pipeline, and the amount of time available for diving, is described.

Fig 1 Brent and Ninian systems pipeline routes to Sullom Voe oil terminal: existing facilities (available in full paper)

INTRODUCTION

Shell UK Exploration and Production (Shell Expro) on behalf of a consortium of 17 participants, operates the Brent System Pipeline which runs from Cormorant Alpha platform to Sullom Voe in the Shetkand Isles (see Fig 1). The pipeline is 914 mm (36 in) in diameter and carries live crude from a number of fields in the Northern sector of the United Kingdom's North Sea continental shelf, for separation at Sullow Voe and re-export via tanker.

The approaches to Yell Sound, from the north and from the east, both act like funnels, with the width of the channel and its depth both reducing as the centre of Yell Sound is approached (see Fig 2 and 3). The reducing cross-sectional area gives rise to very high current speeds; surface currents in the region of five knots have been measured and higher currents have been predicted in the once-per-100 years condition.

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