Discussion. Graving Dock at Nigg Bay for Offshore Structures.

Abstract

The site, which both above and below ground suited the task of constructing and launching a heavy steel oil rig, was well selected, but not until late 1971. The first floatout was successfully accomplished 23 months later. The work included the excavation of a dock the size of six football pitches, in waterlogged sand, closing it against the sea with a removable concrete caisson 125 m long and 15 m high, building a rolling mill, and manufacturing the frame members of the oil rig which were erected by a 600 ton and 250 ton crane in combination-a great achievement.71. I would like to ask the Authors if they would use dewatering, or other means on a similar occasion to stabilize sand banks? How close were the banks to becoming landslides of 'porridge' after the flooding and emptying of the dock? What reserves of pumps and well-points and supervision were necessary for safety? What was the cost per day to keep the system in operation? The cost of the dock gates was about onethird of the price quoted by conventional manufacturers. Do the Authors see a future for removable reinforced concrete caissons as dock gates?72. I should now like to draw attention to a very minor feature of the work, but one which could be significant as the first example of a new generation of fenders.73. The dock gate when floated into the Cromarty Firth was moored to dolphins protected by nylon rope fenders. Fig. 10 shows a model of a proposal for a nylon or polypropylene rope fender suitable for platform jetties. It is suspended by a system of ropes from three points, is self-aligning, and can be pressed by a ship in any direction. Fig. 11 shows a proposal for giving a high degree of protection to vulnerable concrete columns supporting oil rig platforms. The lattice system of ropes is prestressed like the spokes of a bicycle wheel. M r A. M . M u i rWood, Sir William Halcrow & Partners The geology is briefly described as having a thickness of Devonian sandstone beneath superficial deposits. It was presumably from site investigation that it was known that no groundwater was likely to be found in the Devonian stratum. Was this a surprise; was it not expected to be jointed; or was there sufficient siltstone and mudstone in it satisfactorily to have sealed off any joints and thus prevent it acting as a by-pass to the dewatering system? 75. The Devonian layer is mentioned as being 11 m below ground level at the north end of the dock and 38 m at the south end. Fig. 5 shows the gates with the sill founded on sandstone; this must have been at a level not lower than about -14 OD. Figures related to datum would give more information about the dip of the sandstone than those given in the Paper, which are related to original ground level.