WO2002072415A1

WO2002072415A1 - A method and a means for connecting two floating units

Info

Publication number: WO2002072415A1
Application number: PCT/NO2002/000099
Authority: WO
Inventors: Claes W. Olsen; Svein Børge HELLESMARK
Original assignee: Hitec Marine As
Priority date: 2001-03-09
Filing date: 2002-03-11
Publication date: 2002-09-19
Also published as: EP1377497A1; NO20011232D0

Abstract

A method for connecting two floating units (1) and (2), the lower part (2) being equipped with position keeping means such as a mooring system or thrusters and being capable of transferring position keeping forces according to the environmental loads acting on the upper unit (1) by use of friction pads (5, 6), enabled by establishing a vertical contact force between the two units sufficient to be able to transfer the horizontal loads by use of friction between the friction pads (5, 6) and the upper unit (1).

Description

_ Method and means for connecting two floating units

The present invention is related to a method and a means for connection two units floating in the sea.

Traditionally connection of two floating units in the sea is done by use of ropes, wires, hawsers or by a mechanical system such as hinged rigid yokes or male/female type of connections. Other methods have been proposed without having been implemented, e.g. use of magnets. Connection of two floating units for transfer of mooring forces from one unit to the other is a typical operation, e.g. during offshore loading of crude oil from a loading terminal to a tanker.

The use of mooring methods such as ropes, wires and hawsers is generally a time consuming and risky operation based on manual handling. Mechanical systems will generally require purpose-built units, which thereby will be less cost-efficient and provide less flexibility.

With the invented method and system the above will be avoided, since the connecting operation will be carried out with no manual interaction, and will not be limited to purpose-built units. Figure 1 illustrates schematically a cross section through two floating units connected to each other according to the present invention, figure 2 discloses a top view of the lower unit, figure 3 discloses the two floating units in figure 1 equipped with a thrusters system and figure 4 discloses correspondingly the two units equipped with a morring system. The attachment system or mooring force transfer system according to the present invention uses a combination of buoyancy for the lower unit that will force the unit upwardly towards the upper unit, and a "suction coup" using the hydrostatic pressure at the relevant water depth to create a pressure differential over the suction coup seals.

The lower unit 2 is equipped with buoyancy tanks that when emptied will provide a net buoyancy force in the upwards direction towards the bottom of the upper unit 1 , such that the lower unit 2 will be attached to the upper unit 1.

The lower unit 2 is equipped with seals 3 that will get in contact with the upper unit 1 when the lower unit 2 is forced towards the upper unit 1 by increased buoyancy, where the seals will create closed compartments or cofferdams 4 between the lower unit 2 and the upper unit 1.

The lower unit 2 is equipped with friction pads 5 and 6, typically made of rubber with high friction coefficient, covering the majority of the area surrounded by the seals. The friction pads will make sure that the contact force is equally distributed over the contact area, and provide that horizontal loads can be transferred between the units by friction.

The lower unit 2 will typically be equipped with means for position keeping, such as a mooring system 9 or a thrusters system 8 as disclosed in figures 3 and 4. The attachment system as described herein will be the means for transferring such position keeping forces to the upper unit 1 as this unit is being subject to environmental loads such as current, waves and wind.

The water entrapped in the cofferdams can be pumped out through a water outlet line 10, at the same time as air is being let into the cofferdam through an air inlet line 11, to utilize the hydrostatic pressure at the actual draft. The difference in pressure inside the cofferdam and the hydrostatic pressure outside the cofferdam will force the lower unit 2 towards the upper unit 1 in addition to the buoyancy force provided by the emptied buoyancy tanks.

As an example, at 10 m water depth the hydrostatic pressure will be 10 T/m . With a cofferdam footprint area of 100 m , the lower unit 2 will be forced towards the upper unit 1 by a load of 1 000 T. This load will have no effect on the upper unit 1, since it is the initial buoyancy that will be replaced by the distributed load from the friction pads.

If necessary to obtain higher contact pressure than what the hydrostatic pressure can give, it is possible to shut-off the air inlet line 11, and use the pumps to create vacuum to increase the pressure differential even further.

The number and size of the cofferdams can be varied to fit the relevant purpose and application.

Figure 2 shows shock absorbing fenders 7 that will provide a smooth connection as the two units are brought together.

Claims

P a t e n t C l a i m s

A method for connecting an upper floating unit (1) with a lower floating unit

(2), characterized in the lower unit (2) being equipped with position keeping means such as a mooring system or thrusters and being capable of transferring position keeping forces according to the environmental loads acting on the upper unit (1) by use of friction pads (5, 6) enabled by establishing a vertical contact force between the two units sufficient to be able to transfer the horizontal loads by use of friction between the friction pads (5, 6) and the upper unit (1).

2. A method according to claim 1, characterized in providing a normal force acting from the lower unit (2) on the upper unit (1) due to a net buoyancy of the lower unit (2) in addition to a force established by creating a pressure differential in a closed compartment, i.e. a cofferdam, created by seals (3) when the two units are brought together, where the pressure inside the cofferdams is lower than the hydrostatic pressure outside the cofferdam, thereby replacing the initial buoyancy force on the upper unit (1) with an equally distributed load from friction fenders (5, 6) mounted on top of the lower unit (2).

3. Means comprising a position keeping system for connection to the bottom of an upper floating unit (1) for position keeping purposes of said floating unit (1), characterized in a lower unit (2) comprising buoyancy tanks, buoyancy pumps, top mounted seals (3) that will create cofferdams when connected to an upper floating unit (1), where the cofferdams can be emptied for water and filled with air, where friction fenders (5, 6) are covering the cofferdam (4) footprint area and contact forces are transferred in the vertical direction by an equally distributed load and in the horizontal plane by friction loads.