GB2501459A

GB2501459A - Platform removal and transportation system comprising flotation and stabilization units

Info

Publication number: GB2501459A
Application number: GB1204007.7A
Authority: GB
Inventors: Malcolm Lee
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-03-07
Filing date: 2012-03-07
Publication date: 2013-10-30
Anticipated expiration: 2032-03-07
Also published as: GB2501459B; GB201204007D0

Abstract

The invention relates to an offshore platform lifting and transportation system for jacket type platforms which incorporate a flotation unit 1 and a stabilisation unit 2 which can be assembled in halves or sections around the platform to lift the complete platform after cutting of the jacket legs 6. Stability and structural integrity of the system are further provided by vertical columns and bracings 3 attached to the upper part of the platform with the capability to allow connection and vertical adjustment between the flotation and stabilization units by sliding or geared connections. The stabilisation unit is maintained at sea water level during lifting and transportation using tow lines 4 and a suitable towing vessel 5.

Description

The conventional technology for removal of offshore jacket type platforms is with crane vessels or derrick barges with the platforms being disassembled in the reverse sequence to their original offshore installation. This starts with the removal of the upper structure of the platform and finally removal of the jacket itself in sections or as one, down to sea bed level The removed sections must then be sea-fastened to the derrick or transport barges in preparation for transport back to a suitable onshore location. They are then removed for further dismantling for scrap or, where possible, recycling. This procedure using derrick barges is normally costly due to the limitations of the cranes dependancv on suitable weather conditions for lifting and that removal must be done sequentially to allow access and preparation prior to each lifting operation. This conventional technology has been improved by the construction and use of larger derrick barges with heavier lift capacity and therefore are less sensitive to weather conditions and sea state. This would normally help in reducing the number of lifting operations by allowing a larger and heavier section to be lifted. However, this usually involves extensive structural modification if the original lifting points cannot be utilized for a heavier lift.

Other technology is based on removing the upper part of the platform using ships or barges in a catamaran arrangement but this limited to removing the upper platform structure and then the support jacket in separate operations.

This invention enables the in-situ removal and transportation of the complete platform as one unit enabling the platform to be moved to another offshore location or an inshore facility of suitable draft for further modification or dismantling. It also designed to permit the system itself to be extended for the lifting and transportation of other platforms of a differing dimension and weight.

A further key embodiment of the invention is that the system provides the means whereby the removed platform can be adapted for use as a floating platform.. These may used for the development of smaller, marginal oil or gas fields or for alternative uses such as a platform for wind turbines.

The invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows the system attached to a typical jacket type platform Figure 2 shows the system in plan form with the lower submerged flotation unit being positioned for assembly around the jacket.

Figure 3 shows the system and platform after lifting to a stable draught and ready for tow from location. Figure 4 shows the connection of the two halves of the flotation unit after mating with the tensioned Macalloy bars I or similar and the shear keys 2 to facilitate positioning and provide shear * strength to the joint.

" Figure 5 shows suggested details for the pre-installed leg brackets which comprise a split collar which is bolted around the jacket legs with guide plates 1 preventing lateral excursion of the platform after cuthng and quick release or explosive bolts 2 which secures the platform until cutting of all legs is complete and ensures separation and lifting of the platform takes place uniformly and simultaneously. * .**

In figure 1, the submerged flotation unit 1 is connected to the vertical columns 2 which are further * : connectedvia a rack and pinion gear or sliding connection in sealed openings in the upper stabilisation unit 3. This allows the vertical distance between the tower flotation unit 1 and the upper stabilising unit 3 to be varied to maintain the stabilization unit at the sea water surface level at all stages of the lifting and transportation operations and while the lower flotation unit is ballasted or de-ballasted for lifting or transportation of the platform.

The horizontal bracings 4 are attached between the top of the vertical columns and the upper part of the platform structure. These bracings maintain the lateral structural rigidity of the platform and system during lifting and transportation.

The flotation unit 1 is designed to be assembled in sections in modular form to allow assembly around the jacket and for the diameter of the unit to be varied for other differing size platforms. The individual sections are also divided with internal walls to form individual cells with interconnecting pipework to allow water ballast trimming of the flotation unit and platform during assembly, lifting and transportation and to reduce the free water surface.effect from the ballast water. The stability of the system is enhanced by having as low a centre of gravity as possible and therefore it is likely the unit would be constructed in reinforced concrete and, for economic design and in order to resist higher submerged pressures, post or pre-stressed concrete may also be used.

The upper stabilisation unit 3 is designed with the same modular section principle for assembly around the platform and further extension but as it remains at sea water surface level, will not be submerged and requires no ballasting system. It is expected to be constructed in steel as this is lighter than concrete and assists in maintaining a low centre of gravity for the combined platform and system. The design further includes provision for the depth or draught of the flotation unit 1, together with the platform, columns 2 and bracings 4 to be varied while the stabilisation unit 3 is maintained at the sea water surface level.

This is achieved with the vertical columns having a fixed connection to the lower flotation unit I and a geared or sliding connection with the upper stabilisation unit 3. This maintains the overall rigidity of the system and sea water surface level.

The connection of the flotation unit 1 and the platform is via pre-installed brackets 5 attached to the tubular jacket legs. Recessed lifting points within the internal wall of the flotation unit engage the brackets during de-ballasting.

In figure 2 the flotation unit 1 is shown in the submerged position in two halves being assembled around the platform jacket. This is facilitated by tugger winches and cables 5 to draw the two halves together and anchor cables 6 to assist in securing the flotation unit in position. The winches can be : submersible types* located on the flotation unit or on the platform deck with suitable reeving blocks * * fixed to the lower part of the jacket structure. The anchor cables S are also operated by this system to *:*e manoeuvre the two halves of the flotation unit 1 together. These are finally connected with tensioned Macalloy bars or similar and the mating surfaces incorporating interlocking shear keys to form a strong connection. The anchor and tugger winches are then used to assist in positioning the assembled flotation unit I to engage the lifting brackets 5 as the system is deballasteci and lifts the platform. A * .*. further innovation to assist in positioning the flotation unit is the use of air bags 6 to cushion the impact from the flotation unit 1 against the platform legs.

The stabilization unit 3 is also assembled and positioned in two halves around the platform using winches. After the vertical columns 2 and bracings 4 are installed and connected the flotation unit is de-ballasted into position and contact with the leg brackets 5 to provide positive bouyancy. The load on the individual leg brackets is determined by strain gauges and equalised using the de-ballasting system. With the flotation system providing neutral to positive bouyancy, the jacket legs are then cut by thermic lance or high pressure abrasive jet. The final stages of cutting cariled out with a quick release or explosive bolt connection in place and guide plates to prevent excursion of the buoyant platform and flotation system when separation takes place. The quick release or explosive bolts provides simultaneous separation of the jacket legs allowing the platform to be lifted by the flotation system. The system and platform are then de-ballasted to a stable draft and prepared for transportation.

In figure 3, the system has lifted the complete platform after de-ballasting of the flotation unit 1 and culling of the jacket support legs 6. The flotation unit I and stabilization unit 2 together with the columns and bracings 3 are positioned at a stable draft for towing using attached tow lines 4 and suitable towing vessel 5. *e * * * * * *

****** * * ** .* **** * *

S

S..... * .

Claims

CLAIMS1. An offshore platform lifting and transportation system comprising a flotation unit and stabilisation unit designed to be assembled around a jacket supported platform and interconnected by a plurality of columns which are connected to the upper part of the plafform with cross bracings.
2. A system according to claim 1. wherein the flotation unit can lift the platform via pre-installed brackets attached to the jacket legs.
3. A system according to claims 1 & 2 wherein the flotation unit and stabilisation are interconnected via vertical columns which allow the draught to be varied.
4. A system according to claim 3 wherein the connection to the stabilisation unit is maintained via a geared or sliding connection.
5. A system according to any preceding claim wherein the attached system provides the means to allow the platform to be transported to a new location.
6. A system according to any previous claim wherein the attached system provides the means for the platform to be used in a floating mode of operation.