Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
1999-11-22
2001-06-12
Lillis, Eileen D. (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S196000, C405S195100, C114S296000
Reexamination Certificate
active
06244786
ABSTRACT:
The invention relates to a method for offshore load transfer operations, wherein a floater is taken to a transfer site at sea and loads are transferred between the transfer site and the floater, the floater being anchored to the seabed with anchors prior to the load transfer.
The invention also relates to a floater having adjustable buoyancy for offshore transport, installation and removal of structural elements.
The invention has been especially developed in connection with the need for the mounting and dismantling of large structural elements offshore, especially in connection with the removal of existing fixed platform installations at sea.
Basically, the floater may have any form that is adapted to or suitable for offshore transport, installation and removal of structural elements. The floater may thus, for example, be of the type taught in NO 135056, which illustrates and describes a derrick barge, or it may be a catamaran-like floater, for example, as taught in U.S. Pat. No. 3,078,680. Other embodiments of suitable floaters can be found in EP 000 462 A1, FR 247 992, NO 160424 and NO 171495.
Mounting and dismantling of large structural elements offshore requires an accurate, careful load transfer, both when retrieving the load and when putting it in place.
One of the main objects of the present invention is to permit an accurate, careful load transfer of this kind in a favourable and expedient manner.
Briefly, the inventive idea is that a floater is used which is provided with storage and deployment apparatus for suction anchors. The suction anchors are placed in the seabed without the use of support vessels and are attached to the floater by means of adjustable tethers or tendons.
Once the suction anchors have been placed in the seabed, the individual tethers are adjusted to attenuate relative motions between floater and seabed and to attenuate the rolling motion of the floater during a load transfer. A major advantage of the invention is that the floater can be swiftly anchored as soon as it has reached its destination, and can be kept anchored by means of the adjustable tethers attached to the suction anchors. Once the load transfer operation has been completed, the suction anchors are released from the seabed and taken up. Optionally, the suction anchors can remain on the seabed, only the tethers being disconnected.
According to the invention a method is thus proposed for offshore load transfer operations, wherein a floater is taken to a transfer site at sea and a load is transferred between the transfer site and the floater, the floater being anchored to the seabed by means of anchors prior to the load transfer, and what characterises the method according to the invention is that the anchoring is carried out by placing suction anchors connected to the floater via attached adjustable tethers in the seabed directly from the floater, and by adjusting the individual tethers to attenuate relative motion between floater and seabed and to attenuate rolling motions of the floater during a load transfer.
According to the invention a floater having adjustable buoyancy is also proposed for offshore transport, installation and removal of structural elements, which floater according to the invention is characterised in that it is equipped with suction anchors with adjustable tethers attached thereto, so arranged that during load transfer operations involving the structural elements they can be placed in the seabed in such manner that relative motions between the floater and the seabed and rolling motions of the floater can be attenuated by adjusting the attached adjustable tethers.
According to the invention, the floater may to advantage have suction anchors which are attached to a respective pump for the provision and maintenance of a negative pressure in the suction anchor.
It would be particularly advantageous if the suction anchors were stored on board the floater above the waterline.
An advantageous embodiment of a floater is one which includes two parallel groups of elements, each group including a pontoon having a horizontal axis and, projecting upwards from the pontoon, columns which at the upper ends thereof are connected to a horizontal beam, and the groups are fixedly interconnected at one end to form a floater that is U-shaped in plan view. According to the invention, a floater of this type may be characterised in that in the area at the comers of the U and at the end of each leg of the U there is provided a respective storage/deployment apparatus for a suction anchor.
Each pontoon may advantageously be attached to one or more propulsion units, it being an advantage that the pontoons should have a respective bottom which is drawn up at each end in order thereby to make room for the respective propulsion unit so that it does not project down below the bottom.
As propulsion unit, it may be especially advantageous to use a Voith propeller.
Advantageously, the fixed connection between the groups below the waterline may consist of a crossbeam having a wing profile between the two pontoons.
Above the waterline, the horizontal beams may advantageously be interconnected by a crossbeam extending between two beam ends and together with the horizontal beams form a U-shaped box structure.
It is especially advantageous if the horizontal beams include gravity-discharge ballast tanks.
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Lagman Frederick L.
Lillis Eileen D.
Lipsitz Barry R.
Master Marine AS
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