Offshore Tanker Terminal Berthing Problems

Introduction Offshore tanker terminals in the form of single-point moorings (SPM's) have been in use for a number of years where adequate deep water shore berths are impractical. Typically, the SPM may be 6 km offshore, supplied by an undersea pipeline from a shore pumping station. In general, such installations are in relatively sheltered areas and are not subject to severe weather conditions. The advent of offshore exploration and oil recovery has led to the need to provide offshore terminals in the open sea, subject to extreme weather conditions, particularly in the North Sea and the Arctic. Manned and unmanned SPM's of varying complexity have been deployed successfully offshore and a later concept that is becoming popular is to moor a ship offshore, which combines the functions of production, storage, and tanker loading. Securing the tanker to the terminal has always presented a problem. The earlier inshore moorings were picked up with the aid of a work boat, but this method was difficult and dangerous in wind strengths above Force 4 and use of a work boat specifically for this purpose represented an additional cost to the operation. The alternative method developed to enable a tanker to self-moor was to suspend a heavy grapnel secured to a winch from the bow fairland and to steer a collision course for a floating messenger line attached to the mooring bridle. This method is attractive in that the only additional equipment required on the tanker is a special grapnel suitable for engaging the floating messenger, but it suffers from a number of operational difficulties. The position of the floating messenger is controlled entirely by surface currents, and is not responsive to wind strength or direction. In the inshore mooring case, particularly at slack water, the messenger can lie in the water in an extremely complex pattern, so that it involves tanker heading changes of as much as 270 ° in order to achieve the desired collision. This calls for exceptional skill in handling the tanker, which may be a VLCC in ballast, as the vessel has to be maneuvered at slow speed crosswind, the messenger line will be out of sight from the bridge, and there is the attendant danger of fouling the ship's propeller. The problem is aggravated further if the operation has to be carried out at night. In the offshore case, conditions of wind Force 8 and 6-m seas have to be considered. In these conditions, steerage way must be maintained on the tanker (typically 1.0 knot) and the ship's head must be held within 10 ° of the wind to maintain steering control. The use of a bow thruster will assist the operation, but they are expensive and relatively ineffective in these conditions and certainly not powerful enough to hold the shi'ps head against any significant crosswind component. When a collision with the messenger has been achieved, the height of the seas makes the grapnel 'fishing' exercise difficult, and the messenger tends to be washed ahead of the stem by the confused seas, particularly when the ship is in ballast and has a bulbous bow. The Ti-Fast System Flight Refuelling Ltd. was presented with this problem and proposed a solution that has proved effective in the comparable problem of connecting a hose between aircraft in flight. The solution was to pick up the floating messenger line with a grapnel line throwing gun on the tanker. In this case, instead of steering a collision course for the messenger, it would be left to one side of the ship and the powered grapnel line would reach out and bring the messenger to the ship. This involved a much less exacting steering requirement on the tanker, which could approach head to wind and pick up the messenger even if it lay in the water in a complex pattern. At night the deck officer stationed in the bows would be in the best position to see the messenger line and engage it with the grapnel when it came within range. Sea state would not reduce the effectiveness of the operation.