ABSTRACT
In offshore oil spills a very thin oil layer has to be skimmed off from the sea surface. Generally this operation has to be carried out in rough seas, requiring a very accurate separation of oily water from the water surface. This paper presents three different systems for oil recovery in the seaway. All are in service in the Federal Republic of Germany.
The development of spilt-hull multipurpose vessels introduced a new technical concept for oil recovery in the seaway. The ships show good efficiency in open seas up to wave heights of 2 m. The advantage of a split-hull oil combat vessel is its high transit speed. Its operation is favorable in following waves, since the two hinged parts of the hull act as a shielding device.
Also for open sea operations, the Mobil Oil Dike (MOD) has been developed. In this system, 49 m long and 27 m wide, with a draft of 3.2 to 4.4 m, a wave breaking beach is integrated between the two hulls of a catamaran. The vessel moves at an operating speed of 3 to 4 knots, which raises the water level at the beach and forces the surface oil-water layer to rise above the dam crest and splash into the sump. Driven by the pressure head, the mixture proceeds through the settling tanks of the catamaran hulls. The separated water is discharged into the sea through valves in the hull's stern.
For inshore operations a third system has been developed. The Multipurpose Oil Skimming System (MPOSS) is a dock-shaped vessel with two horizontal flap systems that follow the wave profile. The front flap-floater system skims off the oil by controlled undercutting of the water surface. Its motions are damped by a counter-rotating wing, acting as a balanced rudder. After the oil-water layer has passed the intake, it proceeds to the aft separation flap where accurate separation takes place. The overflowing oil proceeds over the flap to the oil sump and is removed by pumps. For offshore missions in coastal waters, this oil skimmer is integrated into a catamaran-type pontoon. The vessel is 33.5 m long, 12 m wide and has a draft of 1.5 m.
All these systems raise considerable hydrodynamic problems as only a thin oil-water layer has to be skimmed off. The motion of the flaps, the deviation of substantial water quantities and the related vortex generation disturb the oil flow at the surface. In waves, breaking phenomena and related forces have been studied. Based on model tests and full scale experience this paper describes some hydrodynamic problems and their solution.