GB1567628A - Offshoe oil production platforms - Google Patents

Description

(54) OFFSHORE OIL PRODUCTION PLATFORMS (71) We, FOSTER WHEELER POWER PRODUCTS LIMITED, a British Company of Greater London House, Hampstead Road, London NW1 7QN, do hereby declare this invention for which we pray that a Patent may be granted to us, and the method by which is to be performed, to be particularly described in and by the following statement: - This invention relates to the construction of platform structures which are to be positioned at sea or in water with the platform held above the water level by legs which project up from a gravity base which sits on the sea floor. In particular the invention relates to offshore oil production platforms e.g. those for use in the North Sea.

These offshore oil production platforms are usually massive and they comprise a platform or deck on which the oil production equipment and accommodation for the operators is carried, the deck often having at least two levels. This deck is supported by legs or columns of concrete, usually two or three, which extend down to the sea floor. At their lower ends the columns are attached to a heavy or gravity base usually in the form of a number of concrete tanks into which the oil is pumped for storage after it has been freed of highly volatile components.

In the past these platform structures have been made by constructing the base which comprises the oil tanks and the first few feet of the concrete columns on dry lands close to the sea at a level below sea level. Once this construction is complete, the area surrounding the base is flooded.

The empty base will then float and is towed to a site for forming the concrete columns.

The formaion of the concrete columns is continued, the column being progressively formed by means of a concrete slip casting technique.

The deck to be attached to the top of the column is made separately from the base and column and is usually built close to shore with its opposite edges resting upon two floating pontoons. While the base and columns are being formed, therefore, the deck can be built and all its equipment assembled close to short in a harbour where full testing and commissioning facilities are available.

Once the deck is complete and commissioned and the columns have been built both the deck and the columns and base are towed to an area of deep water. The buoyancy in the base and columns is adjusted so that the latter sink to a level such that the tops of the columns are just above water level. The deck is towed over the top of the columns, positioned accurately and the buoyancy of the columns and base increased slightly so that the columns rise and engage the underside of the deck. They are then joined finally to the deck and thereafter the buoyancy of the base is increased such that the deck is raised well clear of its supporting pontoons which then can be towed away and then used to support a new deck to be built.

After final commissioning, the resulting production platform is ready to be towed to its resting position at sea and upon arrival there the ballast in the base is adjusted such that the base sinks to the sea floor and sits thereon to anchor the production platform in place.

In order to construct production plat forms in this way it is vital that deep water be available at a reasonably sheltered location. Thus because the columns are often as high as 600 feet there must be at least this depth of water available for the base and legs to be submerged and brought beneath the floating deck. Also, the final positioning of the deck on the legs is an operation which requires calm water to ensure accurate positioning and no damage to the columns or deck. While the Norwegian fiords provide an excellent location where deep water and a sheltered location are both available, around the coast-line of the United Kingdom there are few if any comparable locations.Thus the deep water required is generally not available close to shore and so the possibility of joining the separate deck and columns is entirely subject to non-predictable weather conditions and indeed the occurrence of sufficiently calm days offshore would be very rare.

This is a reason why it has proved almost impossible to complete the joining of pre-fabricated decks and columns and bases in United Kingdom waters. It is therefore an object of the present invention to provide a method of constructing production platforms which does not require the availability of deep water close to shore.

According to the invention there is provided a method of making an offshore oil production platform in which a separately prepared production deck having a number of jack-up legs and their operating apparatus, is positioned over a separately prepared gravity base structure which is floating in water at a sheltered site and which has provision for attachment of the jack-up legs, the jack-up legs are attached to the base structure, the production desk is progressively jacked-up on the jack-up legs while at the same time the permanent concrete columns of the platform are progressively built up from the base structure, the deck is joined to the tops of the permanent concrete columns and the jack-up legs removed.

By following the above procedure it is not necessary to have very deep water beneath the column and base while the former are being cast. As a result, there are numerous places around the coast-line of the United Kingdom which are sufficiently well sheltered for the deck to be floated over the base and attached to the jack-up legs and thereafter the column construction to be continued relatively unhampered by the weather but which are sufficiently deep to be close to the shore for ready access for materials and for shelter from the weather.

The invention also provides an offshore oil production platform comprising a production deck which has a number of jack up legs and their associated operating apparatus and whose fabrication has been largely completed apart from the regions which accommodate the jack-up legs and their associated operation apparatus and a gravity base structure which has incorporated permanently in it the lower ends of the jack-up legs.

The jack-up legs and their operation are items of equipment which can be entirely conventional. Such jack-up legs are widely used in for example seaborne oil rigs which are towed to drilling sites offshore and whose legs are then thrust downwards so that the buoyant drilling platform is hoisted above sea level and the legs stand on the sea floor. After completing a drilling operation the legs are retracted and the oil rig can be towed to another site. These legs are generally of a size such that they can readily support the massive weight of the largely completed oil production deck and their number, usually two or three, is chosen so as to be capable of supporting this weight.

The lower ends of the jack-up legs will for stability normally be embedded in part of the base. Theses lower ends need not be removed after the removal of the major portion of the jack-up legs.

The construction of an oil production platform in accordance with the invention will now be described by way of example, with reference to the accompanying drawings in which the seven Figures, Figures 1 to 7, represent the successive stages in the construction and emplacement of the platform.

The deck 10 shown in Figure 1 is formed in an entirely conventional fashion. It is fabricated possibly ashore and then placed in position spanning a number of floating pontoons 12. The deck 10 can have two levels as shown. Once in position on its pontoons 12 it is completed as far as possible at the dock side by installing modules or sub-modules of the production equipment and other portions such as living quarters. It is also commissioned as far as possible at the dock side.

The only significant departure required from conventional deck formation is that provision has to be made for jack-up legs 20 (see Figure 3) to pass through the deck and for the associated jacking-up apparatus to be accommodated. Accordingly this provision is made and the jack-up apparatus positioned and so it is not possible at this stage to complete and commission those areas of the deck which are to accommodate the jack-up legs and the jacking apparatus.

In the particular embodiment shown in the drawing there are are three jack-up legs and eventually there will be three concrete supporting columns or legs 22 (see Figure 5). In plan view -the jack-up legs 20 and concrete columns 22 are positioned at the corners of an imaginary hexagon with the three jack-up legs and the three columns at diametrically opposite corners of that hexagon.

Separately the gravity base structure 14 is prepared in conventional fashion on dry land. It consists of a number of cylindrical concrete tanks positioned adjacent one another in honeycomb fashion. The size of these tanks and their number is determined by the total weight of the deck and the oil storage capacity required. Into three of these tanks are incorporated the lower lengths 16 of the conventional jackup legs and spud cans. The area surrounding the base structure is then flooded and the latter can be floated out.

At the same time the deck 10 is towed to a position where there is a sufficient depth of water from the base structure to be submerged. The base structure as shown in Figure 3 is floated to the same spot and is then submerged sufficiently as shown in Figure 3 for it to pass beneath and be positioned beneath the deck 10. The jackup legs 20 of the deck are then lowered to engage the lengths 16 incorporated in the base 14, the lengths 16 and the legs 20 are then joined in conventional fashion by welding. The jack-up legs then raise the deck 10 clear of the pontoons 12 which can be removed.

The formation of the concrete columns 22 is then begun in conventional fashion by slip casting, the rate of slip casting and the rate of jacking up being matched to maintain the slip casting area of the columns 22 just below the level of the deck. As and when required, fresh lengths for the legs 20 are added in conventional fashion. Depending upon the buoyancy of the base structure 14 it may be necessary to begin the casting of a short initial part of the columns 22 on shore during the construction of the base structure or as shown in the drawings the casting can be begun once the base structure supports the deck.

The slip casting operation is continued until the legs 22 have the desired height as shown in Figure 5. The tops of the columns 22 are then completed in conventional fashion and joined to the deck 10 after the deck has been lowered onto the tops of the legs by slightly lowering the jack-up legs. Once the tops of the columns 22 and the deck have been united the jack-up legs can be raised and progressively dismantled in conventional fashion. The lower ends of the jack-up legs 22 are severed at or near the top of the base structure 14. As shown in Figure 5 the left hand jack-up leg is progressively being removed.

Once the jack-up legs have been completely removed and the jacking-up apparatus removed those regions of the deck are then finished and commissioned and the completed oil production platform 30 is ready to be towed out to its final resting position. The platform 30 is shown ready to be towed out to its final resting position.

The platform 30 is shown ready for towing out to sea in Figure 6. Once the production platform 30 arrives at its desired location, the buoyancy of the base structure 16 is adjusted so that the platform sinks to leave the base structure resting on the sea bed 32. The towing out and positioning of the production platform does not differ from conventionally produced platforms.

As will be appreciated the manner of construction according to the invention has the advantage that during the construction of the platform deep sheltered water is not required. Instead one merely needs a depth of water corresponding to the height of the base structure 14 together with sufficient further depth to provide the buoyancy to support the completed platform as shown in Figure 5.

WHAT WE CLAIM IS: - 1. A method of making an offshore oil production platform in which a separately prepared production deck having a number of jack-up legs and their operating apparatus is positioned over a separately prepared gravity base structure which is floating in water at a sheltered side and which has provision for attachment of the jack-up legs, the jack-up legs are attached to the base structure the production deck is progressively jacked-up on the jack-up legs while at the same time the permanent concrete columns of the platform are progressively built up from the base structure, the deck is joined to the tops of the permanent concrete columns and the jack-up legs removed.

2. A method as claimed in Claim 1 in which the permanent concrete columns are formed by slip casting.

3. A method as claimed in Claim 1 or Claim 2 in which lower ends for the jackup legs are embedded in part of the base structure.

4. A method as claimed in any preceding claim in which the production deck, apart from regions for accommodating the jackup legs and their operating apparatus, is completed and commissioned before being positioned over the gravity base structure.

5. A method as claimed in any preceding claim in which the production deck has been built spanning spaced apart barges or pontoons and is positioned over the floating gravity base while supported by those barges.

6. A method for making an offshore oil production platform, substantially as herein described with reference to the accompanying drawings.

7. An offshore oil production platform comprising a production deck which has a number of jack-up legs and their associated operating apparatus and whose fabrication

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. is prepared in conventional fashion on dry land. It consists of a number of cylindrical concrete tanks positioned adjacent one another in honeycomb fashion. The size of these tanks and their number is determined by the total weight of the deck and the oil storage capacity required. Into three of these tanks are incorporated the lower lengths 16 of the conventional jackup legs and spud cans. The area surrounding the base structure is then flooded and the latter can be floated out. At the same time the deck 10 is towed to a position where there is a sufficient depth of water from the base structure to be submerged. The base structure as shown in Figure 3 is floated to the same spot and is then submerged sufficiently as shown in Figure 3 for it to pass beneath and be positioned beneath the deck 10. The jackup legs 20 of the deck are then lowered to engage the lengths 16 incorporated in the base 14, the lengths 16 and the legs 20 are then joined in conventional fashion by welding. The jack-up legs then raise the deck 10 clear of the pontoons 12 which can be removed. The formation of the concrete columns 22 is then begun in conventional fashion by slip casting, the rate of slip casting and the rate of jacking up being matched to maintain the slip casting area of the columns 22 just below the level of the deck. As and when required, fresh lengths for the legs 20 are added in conventional fashion. Depending upon the buoyancy of the base structure 14 it may be necessary to begin the casting of a short initial part of the columns 22 on shore during the construction of the base structure or as shown in the drawings the casting can be begun once the base structure supports the deck. The slip casting operation is continued until the legs 22 have the desired height as shown in Figure 5. The tops of the columns 22 are then completed in conventional fashion and joined to the deck 10 after the deck has been lowered onto the tops of the legs by slightly lowering the jack-up legs. Once the tops of the columns 22 and the deck have been united the jack-up legs can be raised and progressively dismantled in conventional fashion. The lower ends of the jack-up legs 22 are severed at or near the top of the base structure 14. As shown in Figure 5 the left hand jack-up leg is progressively being removed. Once the jack-up legs have been completely removed and the jacking-up apparatus removed those regions of the deck are then finished and commissioned and the completed oil production platform 30 is ready to be towed out to its final resting position. The platform 30 is shown ready to be towed out to its final resting position. The platform 30 is shown ready for towing out to sea in Figure 6. Once the production platform 30 arrives at its desired location, the buoyancy of the base structure 16 is adjusted so that the platform sinks to leave the base structure resting on the sea bed 32. The towing out and positioning of the production platform does not differ from conventionally produced platforms. As will be appreciated the manner of construction according to the invention has the advantage that during the construction of the platform deep sheltered water is not required. Instead one merely needs a depth of water corresponding to the height of the base structure 14 together with sufficient further depth to provide the buoyancy to support the completed platform as shown in Figure 5. WHAT WE CLAIM IS: -

1. A method of making an offshore oil production platform in which a separately prepared production deck having a number of jack-up legs and their operating apparatus is positioned over a separately prepared gravity base structure which is floating in water at a sheltered side and which has provision for attachment of the jack-up legs, the jack-up legs are attached to the base structure the production deck is progressively jacked-up on the jack-up legs while at the same time the permanent concrete columns of the platform are progressively built up from the base structure, the deck is joined to the tops of the permanent concrete columns and the jack-up legs removed.

2. A method as claimed in Claim 1 in which the permanent concrete columns are formed by slip casting.

3. A method as claimed in Claim 1 or Claim 2 in which lower ends for the jackup legs are embedded in part of the base structure.

4. A method as claimed in any preceding claim in which the production deck, apart from regions for accommodating the jackup legs and their operating apparatus, is completed and commissioned before being positioned over the gravity base structure.

5. A method as claimed in any preceding claim in which the production deck has been built spanning spaced apart barges or pontoons and is positioned over the floating gravity base while supported by those barges.

6. A method for making an offshore oil production platform, substantially as herein described with reference to the accompanying drawings.

7. An offshore oil production platform comprising a production deck which has a number of jack-up legs and their associated operating apparatus and whose fabrication

has been largely completed apart from the region which accommodate the jack-up legs and their associated operation apparatus and a gravity base structure which has incorporated permanently in it the lower ends of the jack-up legs.

8. An offshore oil production platform as claimed in Claim 7 which is supported on spaced-apart barges or pontoons.

9. An offshore oil production platform, substantially as herein described with reference to the accompanying drawings.

10. A method as claimed in any of claims 1 to 5, which uses an offshore oil production platform as claimed in any of claims 7 to 9.

11. An offshore oil production platform which has been made by a method as claimed in any of claims 1 to 6 and 10.