Dry Habitat Connections And Repairs Of Subsea Pipe-Line And Structures

Abstract

Abstract Research into hyperbaric welding (welding in dry habitats at the ambient pressure) was initiated by Comex several years ago. Since 1975, when the first site weld was executed, some 36 subsea connections and repairs have been completed at depths down to 165 m. In addition, the development work has allowed welding procedures to be qualified at 300 m depth. To-date, Comex has designed and built 6 substantial welding spreads consisting of a habitat and pipe alignment capabilities. These range from a large 220 t combined habitat and alignment frame, used for mid-line connections, to a simple 12 t habitat for pipe-line repair. In addition, several "consumable" pipe-line repair. In addition, several "consumable" habitats which can easily be fabricated on site have also been built for repair purposes as well as a unique inflatable, rubber habitat for repair and replacement of platform risers. Introduction A certain number of subsea welds had been made before 1970 but it was the exploitation of offshore oil and gas fields which gave the necessary impetus to develop underwater welding and repair techniques. Some of the welding techniques which could be found at that time included wet welding, locally shielded welding and dry hyperbaric welding. Wet welding has been used and is still used to a limited extent for certain temporary/cosmetic repairs attachment of minor fittings, etc .. However, this technique suffers from a number of major drawbacks.–Reduced arc visibility, making manipulation the welding arc and weld pool impossible.–High levels of porosity and lack of fusion.–Rapid quench rates and high hydrogen contents resulting in poor weld metal properties. In order to have welds which meet the specifications normally used in offshore construction (API 1104, ASME or DNV rules etc ..) then it is essential to isolate water from the work piece and welding arc by construction of a suitable dry work area or dry habitat. Preparatory work on hyperbaric welding was started by Comex in 1972 with the objective of completing welded pipe-line connections at 100–150 m in the North Sea. In 1976, 4 subsea tie-ins were made. More recently, the experience gained, equipment and technique developed have been applied and developed for use at shallower depths where the same high standard of welding can be achieved using simpler and cheaper diving spreads and equipment. Since 1976, a further 23 pipe-line tie-ins and repairs have been completed at depths from 20 to 165 m plus several structural repairs at depths from the plus several structural repairs at depths from the splash zone down to 30 m. All of these by hyperbaric welding, at the ambient pressure either in a dry breathable atmosphere or by welding in the dry and the diver retaining his mask. DEVELOPMENT OF WELDING TECHNIQUES Since 1973, Comex has continuously undertaken research and development in hyperbaric welding with the result that procedures for butt and fillet welding have been fully qualified down to 300 m.This research has been in 2 parts:–Small scale tests.–Manned hyperbaric welding trials and qualifications. The small scale tests are used to evaluate the welding process, parameters, deposition and arc characteristics, and weld metal properties at different pressures. These are carried out in a small instrumental pressures. These are carried out in a small instrumental pressure vessel, fitted with a movable work table at pressure vessel, fitted with a movable work table at the Institute Soudure in Paris (Fig. 1).