Hydraulic and earth engineering – Marine structure or fabrication thereof – Floatable to site and supported by marine floor
Reexamination Certificate
2002-10-15
2004-05-18
Will, Thomas B. (Department: 3671)
Hydraulic and earth engineering
Marine structure or fabrication thereof
Floatable to site and supported by marine floor
C405S204000
Reexamination Certificate
active
06736571
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a method of removing a deck from an offshore structure and to a vessel suitable for use in such a method.
There are many structures, for example in the North Sea, that have been built and installed on the seabed for purposes connected with the offshore oil and gas industries. Such structures commonly comprise a supporting framework, usually referred to as a jacket, which stands on the seabed and extends up to a height above sea level, and a superstructure, often referred to as a deck, supported above sea level on the jacket. A jacket typically comprises a plurality of legs extending upwardly from the seabed to the top of the jacket and diagonal and cross bracing that together hold the legs against relative lateral movement; the vertical load carried by the jacket is borne principally by the legs. The nature of the deck is dependent upon the purpose of the structure. For example, it would commonly comprise principally a drilling rig but might consist exclusively of accommodation for workers on an adjacent rig. During installation, the jacket is commonly located in position on the seabed first and the deck thereafter placed on top of the jacket. The deck may be built as a single unit onshore, taken out to sea and placed on top of the jacket, or it may be built as a number of separate modules that are taken separately to the jacket and assembled only as they are placed on the jacket. Modules can also be added to a deck that has previously been placed on a jacket at a later stage to enhance or alter the capabilities of the deck.
It will be appreciated that the form of superstructure and the form of the supporting structure vary considerably from one structure to another and the terms “deck” and “jacket” as used herein need to be understood as correspondingly broad.
(2) Description of Related Art
As environmental considerations assume greater importance, so the need increases for satisfactory methods of removing a deck from a jacket of an offshore structure after the useful life of the structure is over. One way that may be adopted is to use a vessel with a large crane to lift the deck from the jacket and place it on a barge. Many other options have, however, also been proposed and in some cases also used in practice; in some of these options a floating vessel, which in plan view is generally U shape, is moved up to the structure with the opposite limbs of the “U” on opposite sides of the structure and some system, which may be a ballasting or a jacking system, used to lift the deck clear of the jacket.
In practice, however, it has proved difficult to provide a method of removing a deck from an offshore structure that (1) is able to remove a relatively large and heavy deck, (2) is able to bring the deck inshore all the way to a yard and (3) does not require a very great investment in equipment.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of removing a deck from an offshore structure that overcomes at least partly some or all of the difficulties referred to.
According to the invention there is provided a method of removing a deck from an offshore structure including a deck supported on a jacket, the method including the following steps:
a) positioning a floating vessel around the jacket with respective parts of the vessel on opposite sides of the jacket and trusses extending between the opposite parts of the vessel,
b) engaging parts of the trusses with legs of the jacket,
c) securing the trusses to the jacket,
d) relieving the load carried by portions of the legs of the jacket,
e) cutting through the portions of the,legs of the jacket to divide the jacket into a lower part and an upper part carrying the deck, the trusses being secured to the upper part of the jacket,
f) transferring the weight of the upper part of the jacket and of the deck via the trusses to the floating vessel, and
g) removing the floating vessel, with the trusses, the upper part of the jacket and the deck supported thereon, from the vicinity of the lower part of the jacket.
In the method just defined the steps are set out in one particular order which is the preferred order, but it should be understood that it is within the scope of the invention to make some alterations to the order. For example, it is within the scope of the invention for the step of relieving the load carried by portions of the legs of the jacket to be carried out before the trusses are secured to the jacket.
By cutting through the jacket below its top and then removing the deck by supporting the uppermost part of the jacket, rather than trying to remove the deck from the top of the jacket, a more reliable support of the deck is ensured: by continuing to support the deck through the jacket the nature of the support for the deck itself remains unchanged and therefore there can be confidence of adequate support for the deck; if the deck were removed from the jacket, however, the nature of its support would almost inevitably change and therefore there could be less confidence that its support would be satisfactory, especially in the case of a deck of modular construction and/or a deck to which structural alterations had been made subsequent to installation. Where reference is made herein to “cutting” a leg it should be understood that the term should not be regarded as restricted to any particular method of creating a separation of upper and lower parts of a leg; methods that may be employed and are to be regarded as cutting include, for example, a shearing action, application of heat and explosive methods.
Usually it will be advantageous for the trusses to be secured to the jacket above sea level. Also it will usually be advantageous for the legs to be cut above sea level.
The floating vessel preferably includes two barges, which may or may not be identical, for positioning on opposite sides of the jacket. The barges preferably are able to be separated and be used for another purpose as individual barges. Constructing the vessel in this way enables the cost of the vessel to be reduced.
Preferably the barges are connected together side-by-side with a space therebetween, by front and rear trusses, which preferably are detachably connected to the barges.
One of the trusses may be retractable, preferably by being separated into two parts, to leave an open-ended space between the barges. In such a case, step (a) of positioning the floating vessel preferably includes the sequential steps of retracting the retractable truss, positioning the vessel around the jacket with the barges on opposite sides of the jacket and the jacket positioned within the open-ended space, and returning the retracted truss to a position in which it extends across the gap between the barges on the opposite side of the jacket from the other truss.
As an alternative to the procedure described immediately above, step (a) of positioning the floating vessel around the jacket may include the following steps:
i) positioning the vessel adjacent to the jacket with one of the trusses immediately adjacent to the jacket,
ii) releasing the truss that is immediately adjacent to the jacket from the vessel, and
iii) moving the vessel away from the jacket and then to an opposite side of the jacket and repositioning the vessel around the jacket with respective parts of the vessel on first and second opposite sides of the jacket and the trusses extending between the opposite parts of the vessel on third and fourth opposite sides of the jacket.
The truss that is immediately adjacent to the jacket is preferably mounted on a buoyancy unit which supports at least most of the weight of the truss when the truss is released from the vessel. That avoids the need to have the offshore structure supporting the weight of the truss at this stage.
Preferably the step of releasing the truss that is immediately adjacent to the jacket from the vessel includes the step of adding ballast to the vessel to lower it.
Preferably the vessel is moved to the opposite side of the jacket and reposition
McCarthy Vincent George
McGuire William
O'Donnell Hugh James
Frommer & Lawrence & Haug LLP
Mayo Tara L.
Saipem U.K. Limited
Santucci Ronald R.
Will Thomas B.
LandOfFree
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