As the production of the North Sea oil and gas is moving out into deeper water and more adverse areas, the structures that have to be erected are increasing rapidly in dimensions and weights. Consequently the size of the employed floating cranes has to go up. The paper describes the typical design of the first 2000-ton crane that was built to handle heavy loads in unfavorable working conditions without concessions to safety.
BP announced the development of the Forties Field by means of four gigantic production platforms in 1971. As soon as the design of these platforms became known the IHC management realized the need for installation tools with larger capacities than were available. IHC cranes have been working in the North Sea from as early as Ig68. From the experience gained by operations of these 800 and 1200-ton cranes a long list of design considerations was distilled. After part of the installation contract was awarded to Heerema Engineering it soon turned out that even a 1200-ton crane would not nearly be sufficient, so a 1600-ton revolving / 2000-ton fixed position crane design was initiated. The Heerema group was one of the very first offshore installation contractors who recognized the necessity for 2000-ton crane barges. This was one of the reasons why the group purchased the tanker forebody "Veedol" in 1972 for conversion into a 2000-ton crane Qarge. The conversion, done by Boele's Shipyard, Bolnes, The Netherlands, started early 1973.
After the crane foundation had been finished IHC Gusto started the erection of the crane and by April 1974 the W.S. "Thor" was ready for operation.
The barge is a converted tanker of which only the forebody, 550 feet long, was used. The following major components were added to the ship:
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Blisters for widening the forebody to nearly 130'
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Accommodation for 180 men.
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A crane foundation.
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A complete new engine room, with a total installed 8000 HP.
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A mooring system with 8 anchors, each weighing 30,000 lbs, with anchor winches having a 120-ton pull and 320-ton holding power (anchor lines are 6000 ft. of 2¾" diameter).
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Further miscellaneous activities included many details, like a helideck for a S-61 and S-65, ample underdeck storage, new fuel tanks, water tanks etc. The total steel weight involved was approximately 3500 tons. The deck equipment of the "Thor" includes Menck MRBS 7000 pile driving hammers with a blow energy of 630,000 ft lbs, a deck crane with 165 tons capacity, welding and diving equipment and further tools required for offshore installation work. See fig. 1. The crane is located at the aft end of the ship so that load handling both over the stern and port and starboard side is possible. The crane foundation carries the tub collar with the slewing rails, the slewing rack and the center pin.
