ABSTRACT

Within the next few years, the offshore oil industry will being seeking to exploit hydrocarbon reserves at depths inaccessible to saturation divers, and at which the welding processes currently in offshore use will not operate. Alternative processes exist for the water depth range 500 to 1000 metres, and these have been shown to be viable in laboratory trials. Further work is required to bring them to full operational status, and to integrate them with the wide range of other equipment required to carry out complete underwater fabrication procedure without diver intervention. Although alternative fabrication techniques exist, it is generally agreed that if hyperbaric welding can be shown to be reliable, and to produce acceptable joints, it will continue to be used by the offshore industry.

At present, no facilities exist for hyperbaric welding research at depths significantly greater than 1000 metres. Cranfield is currently commissioning a 250bar research facility, which is capable of undertaking studies into the performance and properties of arc welding at pressures equivalent to a water depth of 2.5km.

INTRODUCTION

In general, most of the fabrication and repair of underwater pipelines takes place in Continental Shelf water depths, which are usually less than 200 metres. For these situations, manual welding techniques are used, with gas tungsten arc welding (GTAW) for root welds and hot passes, shielded metal arc welding (SMAW) being employed to fill the bulk of the weld.

In the early 1980's, a requirement arose to provide a repair capability for pipelines being laid across the Norwegian Trench, an area of the North Sea in which the depth can approach 400 metres. It was appreciated that the physical capabilities of saturation divers became more limited as depth increased, and welding, as a skill requiring long term dexterity and concentration, was particularly vulnerable to this effect. It seemed unlikely that conventional manual welding techniques would produce consistently acceptable results at the increased depth of the Norwegian Trench. For the relatively simple weld geometries required to join pipes, automated orbital welding systems were developed. Although several of these exist, developed by Sub Sea international (OTTO, Lyons and Middleton, 1984), Comex (THOR 1 and 2, Blight and Baylot, 197) and Statoil (PRS, Styve and Andersen, 1994), they have many similarities. All use some form of tractor, controlled from the surface, crawling round a track installed on the pipe, and upon which is mounted a welding torch and manipulator, consumable fees unit, and weld pool viewing system. All utilize the GTA welding process, with filler addition. The task of the saturation driver is to install and remove these systems, and to replace consumable spools and tungsten electrodes as required. Such systems have been used for the past ten years, generally successfully, and are currently being considered for operations at shallower depths, and for an increasingly wide range of pipeline materials (Malone and Ralston, 1992).

Although the pace of development of offshore hydrocarbon reserves has been irregular, and greatly influenced by the low price of crude oil, it is generally agreed

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