Scour in the North Sea
Authors: Watson T.N. 
Published: December 2007

Watson, T.N., SPE-AIME, Amoco (U.K.) Exploration Co.

If you ail from a feeble foundation and your jack-up rig is getting the shakes, maybe you've got scour. Here are cures for the ailment from practitioners who have handled some of the toughest of cases in the meanest of places-the North Sea.

Introduction

The North Sea is well known for the numerous environmental problems that face the oil industry. One such problem is that of seabed erosion, or scour, caused by tidal currents that occur around the increasing number of offshore structures, pipelines, and jack-up drilling rigs in the Southern North pipelines, and jack-up drilling rigs in the Southern North Sea gas fields. The problem has been apparent in the more developed gas fields of the U.K. sector where the seabed is predominantly sandy. Scouring around platform legs and pipelines occurs in these areas owing to the combination of shallow water depths-usually less than 130 ft. and the fast tidal currents, normally running between 1 1/2 and 3 knots. The structures also lie in an area noted for mobile sand waves and banks, which can be up to 60 ft high and 20 miles long. Amoco (U.K.) Exploration Co. produces gas from six fixed drilling and production platform complexes and has two terminal platforms in the Leman and Indefatigable fields. Each two-platform complex generally has fourteen 40-in.-diameter legs with piles driven to depths between 50 and 300 ft, piles driven to depths between 50 and 300 ft, depending on soil conditions. The legs are braced laterally, with the lowermost brace on the sea bottom. This is taken as the datum line when depths of erosion are measured. Gas is fed from the production platforms by one or more vertical risers to production platforms by one or more vertical risers to 20- or 30-in. diameter buried pipelines. A pipeline system more than 100 miles long interconnects the platforms and delivers gas to a shore terminal. platforms and delivers gas to a shore terminal. With this platform and riser complex installed, it has been essential for our company to consider and field test a variety of methods for controlling and preventing scour.

Formation of Scour

Considerable progress has been made during recent years toward understanding the mechanism of seabed scour around offshore platforms. However, much is still to be learned about the North Sea, where the complex environmental conditions are not easily reproduced in laboratory test tanks. From Amoco's experience, there are two recognizable types of scour that appear to occur simultaneously around North Sea offshore structures located on an unconsolidated seabed. The first type takes the shape of an inverted cone and is localized around legs and risers where they enter the seabed (Fig. 1). This is commonly seen on a smaller scale around beach piers. The seabed directly beneath the platform is lowered as the individual scour holes link platform is lowered as the individual scour holes link up. Before piling is installed, the seabed (composed of sand or loosely compacted alluvial material) is at a constant level, and erosion and deposition of material are in equilibrium. After installation, the current flow is split and deflected around the leg, producing a vortex in which the downward velocity producing a vortex in which the downward velocity component erodes a hole upstream of the leg. This is shown in Fig. 2. As the tidal currents rotate, the familiar cone-shaped hole is created.

JPT

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