Hydraulic and earth engineering – Marine structure or fabrication thereof – With anchoring of structure to marine floor
Reexamination Certificate
1999-08-04
2001-10-09
Bagnell, David (Department: 3673)
Hydraulic and earth engineering
Marine structure or fabrication thereof
With anchoring of structure to marine floor
C405S195100, C405S204000
Reexamination Certificate
active
06299385
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to support platforms for offshore drilling and production, and more particularly to a mini-jacket support that can be installed with a minimal amount of lift equipment. Still more particularly, the present invention relates to a mini-jacket that can be supported solely by a caisson during installation of the supporting piles.
BACKGROUND OF THE INVENTION
In an offshore installation where a conventional jacket is used to support a well, the jacket itself rests on the ocean bottom. In order to keep the jacket from sinking into the ocean bottom, it is common to affix a plurality of large, flat, rigid plates, known as mudmats, to the bottom of the jacket. The mudmats rest on the sea floor and support the weight of the jacket until the piles are installed. The mudmats are designed to temporarily distribute the weight of the jacket and prevent it from sinking into the floor until the installation of the piles and the connection of the piles to the pile sleeves of the conventional jacket is completed. Once the piles have been installed and connected to the jacket, the weight of the jacket is carried by the piles and the mudmats become redundant.
The size of the mudmats required depends on the soil conditions at the ocean floor. In instances where the ocean floor is soft, the mudmats required are enormous in size and weight. Furthermore, in instances where the soil is extremely soft, besides large mudmats, a lift vessel is required to hold the conventional jacket so that it does not sink and remains level while piles are being installed. In addition, the pile installation procedure requires its own lift vessel. Therefore, two lift vessels may be required for pile installation. The first holds the jacket while the piles are being installed and connected to the pile sleeves of the jacket, and the second performs the pile installation.
Because the cost of providing any piece of equipment offshore is relatively high, it is desired to provide a technique for installing a jacket using a caisson without requiring either the extended use of jackup construction platform or large mudmats that are currently required.
In another scenario, a single well may have already been drilled by driving a caisson into the ocean floor. As disclosed in U.S. Pat. No. 5,012,875, a structural frame can be installed and connected to the caisson. The frame includes a plurality of pile sleeves, through which piles are driven into the ocean floor. The objective is to laterally brace the caisson and use the caisson to support a deck in order to support equipment needed to drill and produce a well. In other configurations, disclosed in U.S. Pat. Nos. 4,842,446 and 5,051,036, both the piles and the caisson support the deck on the top. In each of these inventions, the idea is to strengthen the caisson so that it can be used to support the deck. The caisson is pre-existing in all these cases and is attached to the side of the structural frame.
In these other configurations, the structural frame is attached to one side of the caisson, before the piles are installed, the weight of the frame tends to bend the caisson somewhat. It is usually necessary to provide a lift vessel to support the opposite side of the frame so as to maintain it level until the piles have been installed. Hence, it is desirable to provide a system for supporting a jacket during pile installation without incurring the expense of multiple lift vessels or extended lift vessel usage.
SUMMARY OF THE INVENTION
The present invention is a specially configured structural frame, or mini-jacket, that is installed using a caisson. The caisson is positioned within the perimeter of the mini-jacket. The height of the mini-jacket is less than the water depth at the point of installation, so the mini-jacket does not rest on the sea floor in its installed position and has a unique method for installation. Instead of being held by a lift vessel during pile installation, the weight of the mini-jacket is entirely supported by the caisson itself. The caisson can be either pre-existing in the field or specifically installed for this purpose. In instances where caisson does not exist at a site, a caisson is first installed using a lift vessel or a drilling rig in a manner well known to the art.
In a preferred embodiment, the mini-jacket comprises a structural frame having pile sleeves in its corners and a caisson sleeve located within its perimeter. During installation, the mini-jacket is lifted and lowered so that its caisson sleeve slides over the caisson. The caisson sleeve preferably has weight bearing plates or a similar means for engaging the caisson at its top. The weight bearing plates rest on the top of the caisson, thereby transferring the weight of the mini-jacket to the caisson. The caisson supports the weight of the mini-jacket during the pile installation, thereby eliminating the need for extended use of one or more lift vessels. With the mini-jacket resting on the caisson, the piles are driven through the pile sleeves of the mini jacket and the piles are connected to the pile sleeves in a conventional manner. The deck is installed on the piles. After the installation is complete, the wells may be drilled through the caisson, and additional wells may subsequently be added. The provisions for the support of the additional wells are preferably incorporated into the design of the mini-jacket.
By employing this method, the installation is simplified and its cost is reduced, as no lift vessel is required to hold the mini-jacket during pile installation. Similarly, the present invention eliminates the need to use mud-mats. The resulting mini-jacket is lighter than conventional jackets. The term mini-jacket is therefore used herein to distinguish from the conventional jackets that extend to the ocean floor. This invention has particular utility in instances where the exact water depth is not known. Since the mini-jacket does not rest on the ocean bottom, it is not necessary to know the exact water depth. Similarly, the mini-jacket design can be standardized such that the same mini-jacket can be used in a large range of water depths.
Another application of this invention is in mudslide areas. In mudslide areas the mud at the ocean bottom moves. If a conventional jacket that extends to the ocean bottom is used, the mudslide can generate large forces on the conventional jacket. In such cases, the conventional jacket needs to be stronger and should have piles driven deeper to tie it to the ocean floor. The use of the mini-jacket avoids the large mudslide forces, thereby allowing a lighter structure.
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Bagnell David
Kreck John
Paragon Engineering Services Incorporated
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