Hydraulic and earth engineering – Marine structure or fabrication thereof – Floatable to site and supported by marine floor
Reexamination Certificate
2002-02-11
2003-11-18
Lee, Jong-Suk (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
Floatable to site and supported by marine floor
C405S196000, C405S206000, C403S013000, C114S125000, C114S259000
Reexamination Certificate
active
06648553
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for removing a platform topsides from a substructure by a floatable, ballastable transporter floating in the sea, in which the transporter is possibly moving due to motions of the sea. The invention further relates to a load transfer unit of a floatable, ballastable transporter, for transferring load from a platform topsides to the transporter during removal of the topsides from a substructure, which removal is carried out by ballasting the transporter, locating the transporter underneath the topsides and deballasting the transporter, causing load transfer units of the transporter to contact and lift the topsides off the substructure. The invention also relates to a combination of a load transfer unit and a lift-bracket attached to an underside of the topsides.
DESCRIPTION OF BACKGROUND ART
An offshore platform consists of a substructure made from steel or concrete and a topsides comprising one or more decks, production equipment and other facilities which are required to exploit a subsea hydrocarbon reserve. Offshore platforms can also be used for other purposes, e.g. injecting water or gas in the reservoirs, or as living quarters for offshore personnel. When their life-time are ended, the offshore platforms should from an environmental point of view be removed.
Normally, due to the great size of an offshore platform, the topsides will have to be removed first, followed by a removal of the substructure. The topsides may be removed by the method of the invention and similar methods, in which a transporter is ballasted, located underneath the topsides and deballasted to thereby mate with the topsides and lift the topsides off the substructure. In this patent application “transporter” shall mean any kind of transporter, including barges and ships, capable of carrying out such a removal of a topsides.
Topsides' and transporters are heavy structures, and the transfer of the weight of a topsides from the substructure to the transporter is related to various technical problems. One problem is to align several contact points of the transporter with corresponding contact points of the topsides. Another problem is impacts which will normally occur when the transporter contacts the topsides. A further problem is that complete simultaneous and even contact of several contact points are practically difficult to achieve, which may cause an uneven load distribution. In isolation or together these problems may cause damage both to the transporter and the topsides. To solve these problems both the transporter and the topsides may be provided with some kind of load transfer structures.
Many places, like in the North Sea, there is heavy seas, and almost always the transporter will be moving in the sea prior to the mating with the topsides. If no precautions are made, the motions of the transporter strongly worsen the above discussed problems. Elliptical horizontal motions induced by the combined effect of yaw, surge and sway will worsen the problem of aligning the contacts points of the transporter with the contact points of the topsides. Elliptical vertical motions induced by the combined effect of heave, pitch and roll will cause the transporter to repeatingly contact and lose contact with the topsides during the deballasting of the transporter, causing a number of impacts. This will last until the transporter is deballasted so much that the buoyancy of the transporter is big enough to maintain the contact during a downwardly directed heave movement. Rotating motion, known as roll, pitch and yaw, will worsen the problem of uneven contact between several contact points.
When transferring a heavy topsides from a substructure to a transporter when the transporter is moving, it is required first to establish contact between the topsides and the transporter, and then, when the contact is established, carry out the actual transfer of the weight of the topsides and lift the topsides off the substructure. For simplicity the first phase is called the contact phase, while the second phase is called the lift-off phase. During the contact phase, to avoid that the great mass of the moving transporter creates forces which cause damage, the connection between the topsides and the transporter must be flexible. During the lift-off phase, however, to ensure a stable lift-off, the connection between the topsides and the transporter must be stiff.
WO 99/06270 describes a transporter comprising pontoons which define a moonpool with an open side for the location of a substructure, and a topsides load transfer structure above the pontoons. The transporter can be ballasted and then moved into a position in which it is located underneath the topsides and embraces the substructure, i.e. the substructure is in the moonpool. The transporter can then be deballasted, and the topsides load transfer structure contacts the underside of the topsides and lifts the topsides off the substructure. The transporter can then transport the topsides to a receiver, e.g. a construction yard. The load transfer structure of WO 99/06270 is static, and is not able to provide a solution to the problems of transferring the weight of a topsides from the substructure to the transporter if the sea is in motion.
U.S. Pat. No. 4,607,982 discloses an installation of a platform topsides upon a previously installed substructure. The topsides is mounted on a barge positionable between legs of the substructure. The barge can be ballasted and lowered a sufficient distance to allow leg elements of the topsides to contact and mate with leg support elements of the substructure. Impacts between the structures during the mating operation are absorbed by resilient neoprene pads carried by the leg supports. After the platform structure rests upon the leg supports, the barge is removed, and the platform structure is lowered and levelled by draining a select volume of sand from the leg supports. The system involves a complex use of valved tubing and instrumentation to control the levelling and lowering.
U.S. Pat. No. 5,219,451 discloses a barge for locating a topsides on a previously installed substructure. The topsides is located on the barge, and the barge is positioned between legs of the substructure and ballasted until the topsides is in engagement with the substructure. The legs of the substructure have sand jacks and shock-absorbers made from elastomeric members. The weight of the topsides is transferred to the substructure through the shock-absorbers. The sand-jacks are then used to lower the topsides into final steel-to-steel contact with the legs of the substructure. The barge also carries a topsides support structure comprising sand jacks with shock-absorbers. These sand-jacks are used to lower the topsides support structure from the topsides after the topsides have mated with the substructure.
U.S. Pat. No. 6,027,287 describes a load transfer system for placing a load on a barge onto a substructure, comprising a main transfer connector with a plurality of hydraulic jacks and a secondary transfer connector with a hydraulic lifting jack and a sand hopper with an opening valve for rapid flow-out of sand, the sand hopper having a top on which the hydraulic lifting jack is placed.
In the load transfer structures of U.S. Pat. No. 4,607,982 and U.S. Pat. No. 5,219,451 having sand jacks and neoprene pads or other resilient members, these members are shock-absorbers. All the load of the topsides is transferred through the shock-absorbers, and depending of the characteristics of the shock-absorbers, a gradually stiffer connection between the topsides and the transporter is established. The subsequent lowering of the sand jacks is for levelling and lowering the topsides. Thus the transfer of the topsides with these load transfer structures does not provide a contact phase with a flexible connection between the topsides and the transporter, followed by a lift-off phase with a stiff connection between the topsides and the transporter. These load transfer structures will be suitable for calm seas in wh
Lee Jong-Suk
Marine Shuttle Operations AS
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