Offshore platforms are used to exploit oil and gas reserves that exist below a seabed. A well is drilled into the seabed in a potential oil or gas field and, if sufficient oil or gas is found, exploitation of the well may commence. A tree is coupled to the wellhead, the tree comprising safety valves and distribution connections. The tree may be located on the seabed, or on a platform above sea level installed above the wellhead. Oil and gas extracted from the well pass up to the platform and may be processed before being piped to storage on or offshore.

Installing a platform is a complex and expensive operation usually requiring the hiring of specialist installation vessels that have cranes or hoists with the required reach and lift capacity. Typically platforms are only installed over wells that have a comparatively high expected output as this helps to justify the high capital cost of installation. For wells that have a low expected output the tree is often installed on the seabed and the oil or gas pumped to a nearby platform for distribution. This can increase the useful life of a platform and therefore further help to justify the capital cost of platform installation. However, sub-sea trees suffer from comparatively high intervention costs in the event of a malfunction as divers and sub-sea vessels are usually required to perform the required intervention.

It is an obj ect of the present invention to provide a method of constructing an offshore platform and an offshore platform that address some of the above issues.

According to the invention there is provided a method of constructing an offshore platform, the platform comprising at least one pile and a topside, the pile comprising at least two pile sections, one of the pile sections having a sleeve portion attached thereto and the other pile section including a joining portion, the method comprising the steps of; a. driving a first pile section into a seabed in the desired location for the platform such that a root portion of the first pile section extends into the seabed and a stem portion of first pile section extends substantially vertically upwards from the seabed to a level below a Lowest Astronomical Tide level; b. locating a second pile section above the first pile section such that the sleeve portion substantially surrounds the joining portion such that the second pile section extends substantially parallel to the first pile section ; c. inserting a grout between the joining portion and the sleeve portion to form a grouted connection to secure the two pile sections together; and d. supporting the topside using the pile.

Building the platform in sections in this way reduces the weight and overall dimensions of each section of the platform and the result is that there are an increased number of vessels that could potentially install each

section and this reduces the installation cost as there is increased competition. Each section is smaller and so is more easily manoeuvrable and can more easily be arranged as required and this is likely to reduce the installation time.

By having a first pile section that extends into the seabed and substantially vertically upwards to a level below the Lowest Astronomical Tide (LAT) level the section that is driven into the seabed can be free of non-essential attached apparatus. Any apparatus required can be attached to the second or other section of the pile. This reduces the likelihood of damage to the attached apparatus during the driving operation.

A grouted connection is a rapid and reliable way to secure two sections of pile to one another and does not require sub-sea operations to be carried out. The grout is preferably pumped from above sea level through a pipe into the space between the sleeve portion and the joining portion and allowed to cure to secure the two sections together. It should be understood that other methods of inserting the grout between the two sections could be employed.

The term grout is used herein to refer to an adhesive that may be pumped into the gap between the sleeve portion and the joining portion and allowed to set to secure the two portions to one another.

The use of a joining portion and a sleeve portion that substantially surrounds the joining portion facilitates

the alignment of the sections of the pile.

It is preferred that the sleeve portion is attached to the second pile section as this allows a feed pipe for the grout to be attached to the second pile section which reduces the likelihood of damage to the feed pipe during positioning and also during driving of the first pile section. The second pile section may also comprise an integral conduit means for guiding grout between the joining portion and the sleeve The topside may be attached to the second pile section before the second pile section is installed, or it may be installed afterwards. If the topside is installed after the installation of the second pile section then it is preferred that the topside is secured to the second pile section using a second grouted connection. If this is the case, it is preferred that the topside includes a second sleeve portion.

This invention is particularly applicable to a platform which is not normally manned and consequently has a minimal topside having facilities for workers only suitable for use in rare circumstances including a blast resistant temporary refuge. It is therefore preferred that the platform comprises only one pile comprising at least two pile sections and a topside.

The pile is preferably substantially hollow as this allows conduits and pipes to be located within the pile which provides protection for those pipes and conduits within the pile. A hollow pile also enables the pile to be

installed over an existing wellhead, or for the well to be drilled through the pile once the platform has been installed.

The present invention is particularly intended for a gas platform which includes a riser passing away from the platform, a conduit for passing gas from the wellhead to the topside and a tree on the topside connecting the conduit and the riser. It is preferred that the pile is substantially hollow and the conduit is located within the pile. It is also preferred that the riser passes through the hollow interior of at least some of the second pile section.

The invention also provides an offshore platform comprising a pile embedded in a seabed and extending to a level above sea level, the pile having a substantially hollow interior, the platform further comprising a topside supported by said pile, a tree located on said topside, a riser and a conduit connected to said tree, the riser for passing fluid from the platform and the conduit for passing fluid from a sub-sea wellhead to the tree, wherein the conduit and riser are located within the hollow interior of the pile in a region of the pile vulnerable to ship impact such that the riser and conduit are substantially protected from ship impact.

An offshore platform according to the invention is preferably a not normally manned gas platform comprising only one pile which is most preferably formed by the method of the present invention as described above. It should be understood that an offshore platform according to the invention may be constructed by a method according to the invention.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a platform constructed according to the invention; Figure 2 shows a different platform constructed according to the invention.

Figure 1 shows a not normally manned gas platform 1. The platform 1 includes one substantially hollow pile 2 and a topside 4. The pile 2 comprises two sections, a first pile section 6 and a second pile section 8. The first pile section 6 includes a joining portion 10 and the second pile section 8 includes a sleeve portion 12. The sleeve portion 12 is secured over the joining portion 10 using a grout 14 between the portions 10,12.

To construct the platform 1, a root portion 18 of the first pile section 6 is driven into a seabed 16 to a depth of about 25 m below an upper level 17 of the seabed 16. A stem portion 20 extends substantially vertically upwards from the seabed 16 to a level below a Lowest Astronomical Tide (LAT) level 22.

The sleeve portion 12 of the second pile section 8 is then located over the joining portion 10 of the first pile section 6 and the grout 14 is forced into the joint between the portions 10,12 to secure the first pile section 6 to the second pile section 8. This forms a grouted connection 24.

The topside 4 is attached to the second pile section 8 before the second pile section 8 is installed onto the first pile section 6.

A wellhead 26 is located within the hollow pile 2 and a conduit 28 passes within the pile to the topside 4. The conduit 28 is connected to a tree 30 on the topside 4 that connects the conduit 28 to a riser 32. The riser 32 passes down through the interior of the second pile section 8 from the tree 30 until sleeve portion 12 is reached, at which point the riser passes out through the pile 2 and to the seabed 16. In this way, the riser and conduit are protected within the pile at levels above the Lowest Astronomical Tide Level 22 and the riser 32 does not. pass though the first pile section 6 and so is protected from damage during the driving operation. It should be noted that the conduit 28 will typically be a substantially rigid vertical pipe, although it may be possible for the conduit to be flexible or take a different form.

The topside includes a helideck 34 at the top and a temporary blast refuge 36 for workers that have to stay on the platform overnight for any reason.

Figure 2 shows a different platform 101 constructed according to the invention. The platform 101 includes a single pile 102 that comprises three pile sections, a first pile section 6, a second pile section 108 and a third pile section 38 to which the topside 4 is attached.

As with the platform 1 of Figure 1, the first and second pile sections are joined using a grouted connection 24.

The second and third pile sections 108,38 are also joined using a grouted connection 124 with the third pile section 38 comprising a sleeve portion 112 and the second pile section 108 comprising a joining portion 110. In the platform 101, each sleeve portion 12,112 includes a conduit means 40 through which grout 14 may be forced and guided into the gap between the joining portions 10,110 and the sleeve portions 12,112 to form the connections 24,124.

It should be understood that the invention has been described above by way of example only and that modifications in detail may be made without departing from the scope of the invention as described in the claims.