GB2531750A - Subsea foundation installation system - Google Patents

Abstract

A subsea foundation installation system i.e. jackets for tidal turbines. The system comprises a buoyant platform 1 and a plurality of hollow preferably metal supports 3. The hollow supports are spaced in a predetermined spacing about the platform and are adapted to raise the platform above the water level and to function as conductor tubes for receiving and transferring drilling equipment 7 and piles 8 to the seabed. The platform preferably comprises a modular construction with at least some modules having through passages which match the spacing of the pile guides in a subsea footprint. The platform may comprise a slot 4 allowing the subsea structure 5 to extend above the platform. A later embodiment relates to a method of installing said subsea foundations.

Description

SUBSEA FOUNDATION INSTALLATION SYSTEM

This invention relates to a system and method of installation of a subsea or surface piercing support structure, such as a jacket structure, in particular for tidal turbines.

In the offshore oil and gas industry, a jacket is a steel frame which supports a topside platform. The jacket is typically mounted on piles extending into the seabed by as much as 1/5 of the length of the depth of water in which the jacket is to be mounted. Installing the piles and the jacket requires specialist equipment. In general, oil and gas platforms are not installed directly in high current flows, so the time available for the installation is more dependent on having periods of settled weather, than on the state of the tide. However, operations for installation and pile anchoring of subsea jacket structures for tidal turbines in a tidal race are generally limited to slack water. The daily rate for the specialist crane barge required to lift and position the structures is high and makes the installations uneconomical unless done in large numbers.

In accordance with a first aspect of the present invention, a subsea foundation installation system comprises a buoyant platform; and a plurality of hollow supports, spaced in a predetermined spacing about the platform, the hollow supports being adapted to raise the platform above water level and to function as conductor tubes for receiving and transferring drilling equipment and piles to the seabed.

Preferably, the hollow supports comprise metal cylinders.

Preferably, the buoyant platform comprises a modular construction, at least some modules comprising through passages, wherein the number of through passages is at least equal to the number of hollow supports.

Preferably, the spacing of the through passages is chosen to match a spacing of pile guides in a subsea structure footprint.

Preferably, the system further comprises one of a subsea structure for installation, or a drilling template.

Preferably, the system further comprises a slot in the platform, allowing the subsea structure to extend above the platform.

Preferably, the hollow supports further comprise couplings at their ends remote from the platform, for coupling to the subsea structure or drilling template.

In accordance with a second aspect of the present invention, a method of installation of subsea foundations using a system comprising a buoyant platform and a plurality of hollow supports comprises setting a spacing of the plurality of hollow supports; lowering the hollow supports to the seabed; jacking-up the platform on the supports; feeding drilling equipment through one of the hollow supports and drilling a foundation hole in the seabed; removing the drilling equipment from the support and feeding a pile through the hollow support to the drilled hole in the seabed; grouting the pile in place; repeating the drilling and piling for each of the plurality of hollow supports where a foundation is required; lowering the platform to water level; and raising the hollow supports.

Preferably, the method further comprises coupling one of a support spacing template, or a subsea structure to ends of the hollow supports remote from the platform before lowering the hollow supports to the seabed.

Preferably, the step of setting the spacing of the hollow support comprises arranging modules of the platform with through passages at locations corresponding to a spacing of pile guides in a footprint of a subsea structure.

An example of a subsea foundation installation system and method according to the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates one example of an installation system according to the present invention, in transportation mode; Figure 2 illustrates the system of Fig. 1, in piling mode; Figure 3 illustrates a subsea structure after installation using the system of Fig.1; Figure 4 illustrates an example of an installation system according to the present invention, using a template to provide foundations for a subsea structure, in piling mode; Figure 5 illustrates foundations installed using the system of Fig.4; and, Figure 6 is a flow diagram of a method of installation of a subsea structure using a system according to the present invention.

Typical practice for the installation of subsea jacket structures involves lowering the structure (or a template matching the pile arrangement of the structure) onto the seabed in the required location using a crane barge with a sufficiently large crane to lift the structure. In addition, a jack-up-barge is needed to drill the foundations for the structure. The jack-up barge is set up next to the structure that has been lowered into place by the crane barge. The jack-up barge has a number of tubular legs which are extended down to the seabed and then the barge is jacked-up out of the water, thereby providing a stable platform for drilling, free from the effect of the rise and fall of the sea level. The jack-up barge has its own crane and lifts a drill conductor tube into position on a side closest the place where drilling is to take place and lowers this down and locates it in guide sleeves of the jacket structure (or the holes in the template). The conductor tubes are supported at their upper end by a clamping arrangement mounted on the jack-up barge, or alternatively by a temporary structure on the surface piercing jacket structure. Once in place the conductor tube serves as a guide for seabed drilling and piling operations, with the drill passing through the tube and the guide sleeves, or template holes, into the seabed.

The present invention reduces the number of vessels required by enabling both the transport and installation, as well as the drilling of the foundations, to be carried out by the same vessel. Furthermore, as this one vessel carries out both the drilling and the installation in jack-up mode, then it is not limited to installation operations only at slack water, increasing the utilisation rates of the barge.

An example of an installation system according to the present invention is illustrated in Fig.l. The system is designed to take both foundation templates and subsea structures, of which part may have significantly greater height than the width of its footprint. The example of Figs. 1 to 3 is described with respect to using the subsea structure. The system comprises a vessel or buoyant platform, which is able to jack itself up to provide a stable platform for installation of foundations for a subsea structure. A plan view of a suitable buoyant platform 1, in this case a jack-up barge, shows four equally spaced openings 2, which jack-up legs 3 are able to move up and down through. The arrangement of the jack up legs does not have to be exactly as shown, but for efficiency, it is best that the legs 3 of the jack-up barge are arranged such that they match the relative locations of guide sleeves, which receive the foundations, on the jacket structure to be installed. The vessel may be constructed in a modular form, in order to be able to adapt the arrangement of the jack-up legs on the barge to the guide sleeve of the structure or foundation being installed. Alternatively, a bespoke jack-up barge may be designed for the jacket being installed.

A slot 4 is provided in the body of the vessel or floating platform 1 of sufficient width to allow a central extending part 5 of the structure to slide in, whilst a base 6 of the structure is below the platform. The base 5 is attached via its support legs 7 to the jack-up legs 3 in their fully raised position. Unlike conventional jack up barges, the jack up legs and support legs are both completely hollow, for example hollow cylinders of steel, or other suitable material, open at both ends. The attachment may be done ashore before launching the vessel, or by floating the structure into place and sliding the hollow jack-up legs 3 over the support legs 6 of the structure and fixing the structure for relocation with the barge.

With the jack-up legs 3 and the jacket structure 5, 6 fully raised, the jack-up barge transports the structure to the required location at sea and when in position, as shown in Fig.2, the jack-up legs 3 are lowered until the support legs of the jacket structure touch the seabed. The jack-up barge then continues to be jacked up on the legs and supports until the barge 1 has been raised sufficiently far above the water level that wave action does not affect it unduly. The load of the barge is now taken down through its legs 3 and through the guide sleeves, or support legs 6, of the structure into the seabed. This ensures that the barge is clear of the seawater, both wave action and rise and fall of tide, and also provides extra weight to pin the structure 5, 6 to the seabed, ready for drilling.

Once the barge has been jacked up, the hollow jack-up legs act as conductor tubes for drilling and piling, in combination with the guide sleeves of the structure. A drill 7 is lowered into a first one of the jack-up legs 2, and drills into the seabed for the foundations. When the drilled hole is sufficiently deep, the drill is removed and a pile is inserted through the conductor tube. This is repeated for each of the legs where a foundation is required. The position of the piles is constrained by the structure at the bottom and the barge at the top. When drilling and piling have been completed, the barge 1 is lowered back into the sea and the jack-up legs are then disconnected from the foundation and raised, as shown in Fig.3, ready for the jack-up barge to return to shore.

The jacket structure has been securely anchored to the seabed without the need for more than one vessel, or a heavy lift crane. The barge 1 may still have its own crane (not shown) to move the drilling equipment 7 from one leg 3 to the next and to lift the piles off the deck and lower them into the conductor tubes.

Figure 4 illustrates an alternative embodiment in which the vessel is used to install foundations in the seabed, ready for subsea equipment, or a subsea structure, that is brought along later and lowered into position. This reduces the load carrying capacity required of the vessel, as all the foundations can be put in place before the tidal turbines are installed. This may be beneficial in terms of installing multiple subsea structures in a short timescale, as there is no delay waiting for the drilling and piling to be completed for one piece of subsea equipment before the next one can be collected and delivered to the already prepared foundations.

In this embodiment, rather than using the guide sleeves of the structure itself to provide the spacing at the seabed end, a template is used. The correct template is chosen, with openings as required by the layout of the structure for which the foundations are being made. The template is mounted beneath the vessel 1 and carried to the site where the foundations are required. As can be seen from the plan view of Fig.4, the vessel does not need to have the slot 4 that is present in the earlier examples because the template is carried entirely beneath the vessel. On arrival at the site, the jack-up legs 3 are lowered and as before, the platform 1 is jacked up above the level of the water to provide a stable platform for drilling. A drill 7 is lowered into a first one of the jack-up legs 2 and passes through the hollow jack-up leg and the opening in the template 9, so that drilling into the seabed for the foundations can be carried out. When the drilled hole is sufficiently deep, the drill is removed and a pile is inserted through the conductor tube and grouted into place in the hole. This is repeated for each pile, using each of the support legs as conductor tubes for the drill, pile and grouting. In some case, the platform may have more legs than the number of guide sleeves that the structure to be installed has, in which case, a foundation is only constructed using the legs corresponding to the guide sleeves present in the structure to be installed. As shown in Fig.5, when the piles have been fitted in each of the required locations, the barge is lowered down the jack-up legs, so that it is floating again. Then the jack-up legs 3 are raised up, so that all except a section on which the template 9 is mounted are above the vessel. The template is suspended beneath the vessel on this lower section of the jack-up legs and the barge is able to move on to the next location to fit another set of foundations, or to return to shore.

The method of the present invention is summarised in Fig.6. For installing foundations for a subsea structure, such as a jacket for a tidal turbine, a suitable spacing of the jack-up legs of the vessel is set, or a suitable template for the foundation is chosen, or the foundations are constructed with the subsea structure in situ. For the second and third options, a base of the structure, or the template, is mounted 10 to the lower end of jack-up legs of a vessel and carried 11 beneath the vessel to the site where the foundations are to be installed. Once on site, the jack-up legs, together with the structure or template, if used, are lowered 12 to the seabed and the vessel is jacked up out of the water. On the now stable platform of the jacked-up vessel, a drill is lowered 13 down one of the legs to the seabed and a hole for a foundation is drilled 14, in the seabed. The drill is removed from the leg and a pile is lowered down 15 into the hole and grouted into place. The drilling and piling steps inside each leg are repeated for each foundation according to the layout of the foot of the structure, or the layout of the template. When all the required foundations have been installed, the vessel is lowered down 16 the jack-up legs back to the water level and the jack-up legs, with the template, if used, are raised, leaving the structure, or the piles, in place on the seabed. With the jack-up legs mainly out of the water, the vessel can then return to collect the next structure or move on 17 to the next site for installing foundations using the template.

As indicated above, although, the examples of Figs. 1 to 5 show the use of either the guide sleeves or the template as the basis for the spacing of each foundation, the same effect can be achieved by spacing the jack-up leg openings in the vessel according to the required spacing for the structure for which the foundations are being constructed. Thus, a modular platform, with interchangeable units to accommodate a range of different positions for each jack-up leg would mean that the changes for different structures are made by lifting new modules into place and use of a template, or the structure for drilling is optional. The advantage of using the structure or template is that the drill conductor tubes are constrained in the correct position at both seabed and barge level, improving accuracy of drilling and piling. Using the structure combines both the foundation construction and the installation of the structure in a single operation, whereas just using a modular platform is the most flexible option, so the preferred method depends upon customer requirements.

The present invention connects the legs of the jack-up barge to the guide sleeves of the subsea structure, or to the drilling template, or sets the spacing of the jack-up legs on the vessel, so that the jack up legs act as the drill conductor tubes for guiding of drilling and piling operations, as well as carrying and lowering of the jacket structure, or template to the seabed and jacking the vessel clear of the water. There are advantages in this combination of functions, including removing the need for a separate crane barge for lowering the structure/template to the seabed and removing the need for separate drill conductor tubes and their set-up and movement from one position to another on the sides of the jack-up barge. In addition, the weight of the jack-up barge provides extra load to pin the structure to the seabed, removing the need for any additional ballasting of the structure whilst the foundations are being constructed. As the drilling platform and jack-up leg conductor tubes are fixed in place at the start, the set up time prior to drilling each foundation is reduced.

Claims (10)

1. CLAIMS1. A subsea foundation installation system comprising a buoyant platform; and a plurality of hollow supports, spaced in a predetermined spacing about the platform, the hollow supports being adapted to raise the platform above water level and to function as conductor tubes for receiving and transferring drilling equipment and piles to the seabed.
2. 2. A system according to claim 1, wherein the hollow supports comprise metal cylinders.
3. 3. A system according to claim 1 or claim 2, wherein the buoyant platform comprises a modular construction, at least some modules comprising through passages, wherein the number of through passages is at least equal to the number of hollow supports.
4. 4. A system according to claim 3, wherein the spacing of the through passages is chosen to match a spacing of pile guides in a subsea structure footprint.
5. 5. A system according to any preceding claim, the system further comprising one of a subsea structure for installation, or a drilling template.
6. 6. A system according to claim 5, further comprising a slot in the platform, allowing the subsea structure to extend above the platform.
7. 7. A system according to any preceding claim, wherein the hollow supports further comprise couplings at their ends remote from the platform, for coupling to the subsea structure or drilling template.
8. 8. A method of installation of subsea foundations using a system comprising a buoyant platform and a plurality of hollow supports, the method comprising setting a spacing of the plurality of hollow supports; lowering the hollow supports to the seabed; jacking-up the platform on the supports; feeding drilling equipment through one of the hollow supports and drilling a foundation hole in the seabed; removing the drilling equipment from the support and feeding a pile through the hollow support to the drilled hole in the seabed; grouting the pile in place; repeating the drilling and piling for each of the plurality of hollow supports where a foundation is required; lowering the platform to water level; and raising the hollow supports.
9. 9. A method according to claim 8, wherein the method further comprises coupling one of a support spacing template, or a subsea structure to ends of the hollow supports remote from the platform before lowering the hollow supports to the seabed.
10. 10. A method according to claim 9, wherein the step of setting the spacing of the hollow support comprises arranging modules of the platform with through passages at locations corresponding to a spacing of pile guides in a footprint of a subsea structure.