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Deepwater Welding and Intervention Technology

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Deepwater Welding and Intervention Technology Underwater Technology Deepwater Welding and Intervention Technology by J. H. NIXON* and I. M. RICHARDSON*t Abstract by SubSea International (OTTO, [1]), Comex (THOR Within the next few years, the offshore oil industry 1 and 2, [2]) and Statoil (PRS, [3]), they have many will be seeking to exploit hydrocarbon reserves at similarities. All use so me form of tractor, controlled depths inaccessible to saturation divers, and at which from the surface, crawling round a track installed the welding processes currently in offshore use will not on the pipe, and upon which is mounted a welding operate. Alternative processes exist for the water depth torch and manipulator, consumable feed unit, and range 500 to 1000 met res, and these have been shown weId pool viewing system. All utilise the GT A welding to be viable in laboratory trials. Further work is process; with filler addition. The task of the saturation required to bring them to full operational status, and diver is to install and remove these systems, and to to integrate them with the wide range of other equip- replace consumable spools and tungsten electrodes as ment required to carry out a complete underwater required. Such systems have been used for the past fabrication procedure without diver intervention. ten years, generally successfully, and are currently Although alternative fabrication techniques exist, it is being considered for operations at shallower depths, generally agreed that if hyperbaric welding can be and for an increasingly wide range of pipeline materials shown to be reliable, and to produce acceptable joints, [4]. it will continue to be used by the offshore industry. Although the pace of development of offshore At present, no facilities exist for hyperbaric welding hydrocarbon reserves has been irregular, and greatly research at depths significantly greater than 1000 influenced by the low price of crude oil, it is generally metres. Cranfield is currently commissioning a 250 bar agreed within the industry that significant reserves research facility, which can be used for undertaking exist in areas where the seabed is at water depths of studies into the performance and properties of arc 1 km and greater [5], and that these will be exploited welding at pressures equivalent to a water depth of during the early years of the next century. In order to 2.5 km. accomplish this, however, it will be necessary to modify current underwater welding practice greatly. Once one considers welding operations at depths t. Introduction significantly greater than 500 m, two problems occur. In general, most of the fabrication and repair of It is generally recognised that the viable limit for underwater pipelines takes place in Continental Shelf saturation diving must be in the 500 to 750 m depth water depths, which are usually less than 200 metres. region. The increasing effects of high press ure nervous For these situations, manual welding techniques are syndrome (HPNS), and the extensive decompression used, with gas tungsten arc welding (GT AW) for root times required, make deep saturation diving progress- welds and hot passes, shielded metal arc welding ively less attractive as depth increases. Additional data (SMA W) being employed to fill the bulk of the on the long-term effects of deep diving, wh ich have weid. led the Norwegian Government to declare operations In the early 1980s, a requirement arose to provide at depths greater than 180 m as especially hazardous, arepair capability for pipelines being laid across the only strengthen this argument (see Table 1, [3]). Norwegian Trench, an area of the North Sea in which In addition, welding process research has shown the water depth can approach 400 metres. It was appreciated that the physical capabilities of saturation Table 1 Depth ranges for diving and welding divers became more limited as depth increased, and Depth msw Diving Welding welding, as a skill requiring long term dexterity and concentration, was particularly vulnerable to this 50 Limit for air diving Manual effect. It seemed unlikely that conventional manual welding welding techniques would produce consistently accept- 180 Norwegian operations able results at the increased depth of the Norwegian hazard limit Trench. 300 Mixed gas saturation For the relatively simple weid geometries required diving GTAW to join pipes, automated orbital welding systems were 400 orbital systems developed. Although several of these exist, developed Limit for 500 saturation *The Marille Techllology Celltre, Cralljield University, diving Limit of GTAW Cralljield, Bedfordshire, MK43 OAL, UK 600 t This paper was presented at all SU T cOllference, SU BTECH Diverless GMAW '95, held 7-9 November 1995, Aberdeen. Proceedings are systems plasma 1000 available.from SUT. (Report Q{meeting Oll p. 28.) 3 Volume 21 Number 3 UndcrII'lItcr Tcchnology that the GT AWare beeomes progressively less stable 3. Alternatives to are welding for deepwater operations as the environmental pressure is inereased, and at If arc welding is not used as the pipe joining technique, about 500 m deep, it will become unacceptably a number of alternatives exist. Several varieties of unstable for operational use. The exact depth will mechanical connector have been developed for trans- depend on a variety of factors, such as the operating mission pipelines, and although bulky and expensive, eurrent, shielding gas composition and weid prepar- could be utilised in deepwater situations. Being purely ation geometry [6]. Shielded metal are welding is not mechanical, their operation is little affected by water espeeially suitable for robotie operation, because of depth. Recent studies have suggested that for maximum thc high lcvels of manipulative dexterity required, and reliability, metal to metal sealing techniques, based on the low metal deposition rate [7]. cold forging technology, should be used [3]. Thus, at depths greater than about 500 m, both the Alternatively, a solid phase welding technique, such capability for diver intervention, and the welding as explosive welding (highenergy bonding) could be processes with which the offshore industry has the used. A complete HEB pipe joining system was most experience, w.illcease to be operable. Alternatives developed by British Underwater Pipelines Engineer- to arc welded fabrieation techniques exist, such as solid ing, of Barrow in Furness, in the late 1970s. This phase welding, mechanical connectors, grouting, and consisted of aseries of engineering packages, deployed the like. However, a general consensus within the by submersible, to cut pipes, remove the weightcoat, industry suggests that if arc welding ean be shown to and prepare the surface of the pipe for bonding. bc viable, it will continue to be used, because of its Although only capable of lap configuration joints, the high joint effieiency, low weight and good operational system was shown to be feasible [10]. The development record. Because of the high probability that so me form of effective arc welding techniques, possibly combined of subsca completion will be used in these water with reservations concerning the use of explosives depths, welding system development to date has adjacent to offshore structures, and the inspection of concentrated on pipeline joining systems, with their the solid phase welds, inhibited further development, simpler and more predictable weid geometry. but the technology could be revived for deepwater operations. Although more speculative, the use of high perform- 2. Non-wclding deepwater intervention teehnology ance adhesive joining techniques might be considered Thc Norwegian objeetive, of minimising all diving for deepwater situations. Adhesives are now utilised operations at depths greater than 180 m, has resulted in many demanding applications, such as the fabrication in an EU R EKA research programme to upgrade the of automotive and aerospace components, and although engineering specification of current diver-deployed the cost of such adhesives is high, the quantities orbital welding systems to enable them to operate required would be relatively smal!. Tt may be that a witllOut diver assistance [8]. Because mechanical hybrid joining technique, using adhesives to enhance systems are relatively depth insensitive, this pro- the performance of a mechanical connector, might gramme will provide valuable experience for the prove effective in these situations. construction of welding systems to operate at greater depths, provided it can be demonstrated that effective 4. Are welding technology for depths of 500 to 1000 m welding processes are available. Over the past few years, research into hyperbaric Even when conducting underwater welding oper- welding technology in the depth range 500 to 1000 m ations, the vast majority of underwater activity is not has been carried out at several research organisations, directly concerned with the welding process. For a principally GKSS and the Universität der Bundeswehr typical pipeline repair operation, the pipe must be in Germany, STNTEF in Norway, and Cranfield located and the worksite surveyed to ensure that the repair equipment can be safely deployed. It may be necessary to uneover buried pipe, or to lift the line sufliciently to permit full equipment access. Any wcightcoat or protectivc covcring must then bc rcmovcd, and thc pipc c\caned and prepared, usually by machining, for the welding operation. Aftcr welding is complete, the weld must be inspected, so me form of protection applied, the equipment recovered to thc surface, and the worksite
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