GB2362137A

GB2362137A - Offshore jack-up platform

Info

Publication number: GB2362137A
Application number: GB0011323A
Authority: GB
Inventors: David Bone
Original assignee: Ocean Technologies Ltd
Current assignee: Ocean Technologies Ltd
Priority date: 2000-05-11
Filing date: 2000-05-11
Publication date: 2001-11-14
Anticipated expiration: 2020-05-11
Also published as: GB0011323D0; GB2362137B

Abstract

A shallow water platform comprises a deck structure 1, a plurality of support legs 2 slidably mounted to the deck structure and releasably secured to a foundation and installation template 3, a base structure 4 slidably mounted to the support legs and means for controlling the position of the base structure relative to said support legs and deck structure. The platform can be moved by deballasting the base structure such that it rises up the support legs until contact is made with collars 9 provided on the legs, after which continued deballasting will lift the support legs from the foundation and installation template and facilitate removal of the platform by towing. The foundation and installation template may be inserted into and below the seabed.

Description

1. 2362137 REUSABLE SHALLOW WATER OFFSHORE PLATFORM This invention relates

to offshore shallow water platforms, particularly intended for use in industries associated with exploration and/or production of oil, gas or other resources.

Conventional offshore shallow water platforms comprise a deck structure and a plurality of support legs for supporting the deck structure above sea level. These platforms remain on site during field life and are then removed for decommissioning or possible use elsewhere. The support legs are generally fixed into a gravity base structure, which anchors the platform to the seabed.

In use the platform is floated into position with it's legs in the raised position, whereupon the legs are lowered onto the seabed by ballasting the gravity base. The deck structure is then raised out of the water to a suitable height above sea level. Finally the deck structure is clamped firmly to the legs. Removal is achieved by reversing the above procedure.

A disadvantage of the above-mentioned platform is that the removal procedure involves several support boats, good weather, and is time consuming. Also, once removed the platform is not designed to be easily reusable at the same location. Thus, any impending mobile hazards such as icebergs cannot be avoided. Previous attempts to avoid mobile hazards have been based on excavating very large holes in the seabed to house the structure and have used very large quantities of material.

We have now devised a platform and a means of installing and supporting the same, which alleviates the above problems.

According to the present invention there is provided a shallow water platform comprising a deck structure, a plurality of support legs slidably mounted to the deck structure, a foundation and installation template, a base structure slidably mounted to the support legs, and means for controlling the position of said base structure relative to said support legs and said deck structure thereby enabling quick removal/ reinstallation of the platform.

In use the deck structure is secured to the seabed by connection to a foundation template inserted into and below the seabed. The slidable base provides a means of quickly removing that part of the structure exposed to hazards above the seabed by adding buoyancy to the topsides when raised. The template provides protection for wellheads and trees located beneath.

Also in accordance with this invention, there is provided a method of removing( reinstalling a shallow water platform comprised as above, the removal method comprising deballasting the base structure to raise it up the support legs until contact is made with suitably positioned collars or stop mechanisms whence the additional buoyancy provided will raise the support legs above the seabed (with a jacking system to finther raise the legs if necessary) and facilitate removal by towing, the reinstallation method being the reverse of removal.

A specific embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure I is a side view of a shallow water platform in accordance with this invention; Referring to Figure 1, there is shown a shallow water platform comprising a deck structure 1, a plurality of support legs 2 disposed at respective comers of the deck structure 1, a foundation and Iinstallation template 3 which is secured below the seabed, and a slidably mounted gravity base structure 4.

The foundation and installation template 3 is installed in and below the seabed after excavation of the seabed has been completed. It is secured in position using piles 5, steel skirts, grout or any other suitable method. Preferably the template 3 has a sloping perimeter 6 to aid in the deflection of any mobile hazards such as icebergs. Preferably the template 3 has predrilled/ cast holes 7 for accommodating conductors with well slots. It provides protection for the wellheads and trees (not shown) that are positioned beneath it. Preferably the template 3 will have removable installation guides (not shown) to aid manoeuvre the base 4 into position on it.

The support legs 2 are slidable axially of themselves relative to the deck structure 1. Preferably the support legs 2 are located in the template 3 in flanges 8 which spread the load from the support legs 2. Preferably there are collars/ stop mechanisms 9 suitably positioned on the support legs 2 in order to engage the base 4 when it is raised. The base 4 may also make use of attached stability towers (not shown) which increase the cut water plane area and aid stability during removal of the platform.

The deck structure I and legs 2 are moved relative to each other during installation using a jacking system or strand jacks (not shown). During installation the base 4 is ballasted into position on the template 3. A removable locking mechanism (not shown) can be used to ensure fixity between the support legs 2 and the base 4, To achieve quick removal of the above seabed part of the platform, the base 4 is deballasted causing it to rise up the support legs 2 until contact is made with the collars 9. The additional buoyancy thus applied to the deck structure I and support legs 2 causes the support legs 2 to be raised out of the template 3 and above the seabed. If necessary the relative positions of the deck structure 1, support legs 2, and base 4 can then be adjusted using a jacking system (not shown) so that a stable position for towing is attained. The part of the platform at risk from hazard impact (e.g. icebergs) can then be removed to a safe location using tugs (or/ and aided by outboard motors), the remaining part of the platform being safe below seabed level and protecting the wellheads and trees. The platform can then be quickly reinstalled using the reverse of the above procedure once any hazard danger has passed.

3

Claims

1. A shallow water platform comprising a deck structure, a plurality of support legs slidably mounted to the deck structure, a foundation and installation template, a base structure slidably mounted to the support legs, and means for controlling the position of said base structure relative to said support legs and said deck structure thereby enabling quick removal/ reinstallation of the platform.

2. A shallow water platform as claimed in Claim 1, wherein the support legs are releasably secured to the seabed by connection to a foundation and installation template inserted into and below the seabed.

3. A shallow water platform as claimed in Claim 1 or Claim 2, wherein the base structure can be slid vertically relative to the support legs by addition or removal of ballast material within said base structure.

4. A shallow water platform as claimed in any preceding claim, wherein the support legs are provided with suitably positioned collars or stop mechanisms to limit and control the position attained by the base structure when deballasted.

5. A shallow water platform as claimed in any preceding claim, wherein the base structure has sufficient buoyancy when deballasted to support the deck structure and legs in a stable floating position.

6. A shallow water platform substantially as herein described and illustrated in the accompanying drawing.

7. A method of removing/ reinstalling a shallow water platform comprising a deck structure, a plurality of support legs slidably mounted to the deck structure, a foundation and installation template, a base structure slidably mounted to the support legs, and means for controlling the position of said base structure relative to said support legs and said deck structure thereby enabling quick removal/ reinstallation of the platform, the removal method comprising deballasting the base structure to raise it up the support legs until contact is made with suitably positioned collars or stop mechanisms whence the additional buoyancy provided will raise the support legs above the seabed (with a jacking system to further raise the legs if necessary) and facilitate removal by towing, the reinstallation method being the reverse of removal.

8. A method as claimed in Claim 7, wherein a jacking system is provided to further raise the legs above the seabed to facilitate removal by towing.

9. A method as claimed in Claim 7 or Claim 8, wherein outboard motors are attached to the platform to facilitate rapid removal to a safe location.

10. A method of removing/ reinstalling a shallow water platform, the method being substantially as herein described and illustrated in the accompanying drawing.