Marine Operations For Platform Maintenance And Repair

Abstract This paper is concerned with the history of the development of submarine and diving support vessels in the North Sea and an analysis of future trends based on various companies experience of different support vehicles. Specifically it examines various features of the support vessel in relation to its economical viability and reaches the conclusion that there is room for a variety of different types of vessel and that a dominant type is unlikely to emerge. Introduction Any subsea maintenance task is impossible without surface support and while subsea platform maintenance may increasingly be undertaken from the platform itself, nevertheless the majority of work in the North Sea will be supported from vessels. These vessels may be submarine support vessels (although strictly speaking these vessels are as yet used little on platform work), pure diving vessels, or vessels with a combined diving/supply vessel, diving/firefighting or diving/rescue role. They come in all shapes and sizes from 1,000 tons to 9,000 tons displacement, from less than £1m. To over £30m, capital cost. At the last count, there were 24 such vessels working in the North Sea and this number is increasing monthly. Presently some of these are involved on construction, some on inspection and some on remedial work but increasingly the latter two roles will predominate. History Diving support vessels originate from three sources:-U.S. and Royal Navy diving vessels;Stern trawlers;Traditional offshore service vessels. The first type of vessel gave rise to a number of ideas such as the moonpool, rather than to a vessel type; the second to most of the submersible and some diving support vessels and the third source has given rise to a number of the more successful diving and submersible support vessels. Submersible Support VICKERS VENTURER, (an ex-stern trawler) was the first North Sea subsea support vessel. She came into service in 1969 and was closely followed by VICKERS VIKING, VISCOUNT, VANGUARD and VOYAGER. All these later vessels were converted stern trawlers of about 270' LOA whose suitability for conversion to a submersible support role was dictated by their relative cheapness for their overall length, their seaworthiness and freeboard, their large accommodation and workshop space, their generally full bodied sterns (to give adequate buoyancy for lifting submersibles) and their powering arrangement (either diesel electric or CP direct drive diesel, both of which are suitable for the slow towing speed required for submersible recovery). Stern trawler The stern trawler design has also given rise to three diving support vessels, all of which have been fitted with moonpools, four point mooring and in two cases, with dynamic positioning. The same characteristics which made them suitable as submersible support craft also made them useful for diving support, although their usefulness as maintenance vessels is somewhat limited by their lack of beam which limits their overside lifting capacity. However, the first conversions from stern trawler to diving support vessels did not take place until 1975 which is jumping the gun slightly as far as pure diving vessels are concerned. Submersible Support VICKERS VENTURER, (an ex-stern trawler) was the first North Sea subsea support vessel. She came into service in 1969 and was closely followed by VICKERS VIKING, VISCOUNT, VANGUARD and VOYAGER. All these later vessels were converted stern trawlers of about 270' LOA whose suitability for conversion to a submersible support role was dictated by their relative cheapness for their overall length, their seaworthiness and freeboard, their large accommodation and workshop space, their generally full bodied sterns (to give adequate buoyancy for lifting submersibles) and their powering arrangement (either diesel electric or CP direct drive diesel, both of which are suitable for the slow towing speed required for submersible recovery). Stern trawler The stern trawler design has also given rise to three diving support vessels, all of which have been fitted with moonpools, four point mooring and in two cases, with dynamic positioning. The same characteristics which made them suitable as submersible support craft also made them useful for diving support, although their usefulness as maintenance vessels is somewhat limited by their lack of beam which limits their overside lifting capacity. However, the first conversions from stern trawler to diving support vessels did not take place until 1975 which is jumping the gun slightly as far as pure diving vessels are concerned.