AU2011101650A4

AU2011101650A4 - Self installing tension leg platform and subsea storage tank

Info

Publication number: AU2011101650A4
Application number: AU2011101650A
Authority: AU
Inventors: Neil Alexander Keron
Original assignee: KERON NEIL
Current assignee: Keron Neil Alexander Dr
Priority date: 2011-12-16
Filing date: 2011-12-16
Publication date: 2012-01-19
Anticipated expiration: 2019-12-16

Abstract

SELF INSTALLING TENSION LEG PLATFORM AND SUBSEA STORAGE An offshore platform for producing oil or gas with integral storage of a tradeable quantity of crude oil the whole of which can be installed without offshore crane barge or other specialist construction equipment. The TLP platform is stable in its free floating condition. o ~~ Figure 2

Description

[1] Australia Patents Act 1990 Complete Specification Innovation Patent SELF INSTALLING TENSION LEG PLATFORM AND SUBSEA STORAGE TANK The following statement is a full description of this invention including the best method of performing it known to me [2] SELF INSTALLING TENSION LEG PLATFORM AND SUBSEA STORAGE TANK PRIOR ART Tension leg platforms have been known for some years. They all require to be installed with expensive offshore crane barges. Tendons are floated to site and upended. Mini TLPs have been used for a number of years but are not stable in the event of tendon failure. These also require to be transported to the offshore site on loaded out barges and installed with heavy lift barge. Subsea tanks have also been known for some years. Most these support a fixed superstructure which provides water plane area to give stability during lowering of the tank to the seabed. Only BP Harding utilised a tank installed with no water plane area. This is difficult because when submerged, the tank is unstable. Distributed buoyancy cables were used but are difficult to manage at an open water location making the operation very weather sensitive. Topsides are usually installed after substructure is complete using heavy lift crane barge which is also weather sensitive often resulting in costly waiting on weather time and/or costly project delays if either crane barge or topsides are delayed. DESCRIPTION The offshore oil industry frequently makes use of tension leg platforms. These comprise a buoyant hull tethered to the seabed with a plurality of tendons. The tendons are of a length such that they pull the hull deeper in to the water than its free floating draft thereby removing the heave motion that is induced in a free floating body under the influence of waves.

[3] According to the present invention there is herein described a design of a tension leg platform and a method of installing a tension leg platform and a subsea oil storage tank. The invention may be better understood with reference to the illustrations of embodiments of the invention which: Figure 1 is an isometric view of the tension leg platform and subsea tank installed in its operating location. Figure 2 is an elevation of the tension leg platform and subsea tank installed in its operating location. Figure 3 is an elevation of the hull and topsides and a plan of the topsides, hull and subsea tank. Figures 4a to 4d are a series of elevations of the tank, hull and topsides depicting the construction, assembly and installation process. Figure 5 is an elevation of the connection between tendon wires and hull pontoons detailing the wire rope clamps and tension equalising beam. The entire assembly comprises the subsea tank 1, the hull 2 and topsides 3 supporting various process equipment, a plurality of tendons 4 connecting the hull to the tank and a plurality of piles 5 to fix the tank to the seabed, the method comprising the steps of: Constructing tank, hull and topsides separately at a construction site(s) near to the ocean; Launching the tank into the ocean when construction is complete. Figure 4a steps 2 & 3 Towing the tank to the site where the hull has been constructed and ballasting the tank until its free board is equal to the height of the wharf whereon the hull has been constructed. Figure 4a steps 4 & 5 Loading the hull onto the top of the tank by means of winches mounted on the outboard edge of the tank. Figure 4a step 6.

[4] Fixing the hull to the tank by means of wire rope tendons attached to the top of the tank, passed through attachment guides 7 in the four outer corners of the hull pontoons Figure 5, tensioning them and clamping them with hydraulic remotely operated cable grips 8. Installing piles 5 in the pile guides 6 while the tank is within reach of an onshore crane. Figure 4b step 7. Loading the completed topsides on to a transport cargo barge by skidding the topsides from wharf to barge. Figure 4b step 10. Towing the assembled tank and hull to sheltered water of depth greater than the combined height of the tank and hull. Ballasting the tank and hull until the top of the hull is a few metres above the water line. Figure 4b step 9. Towing the topsides barge to alongside the almost submerged hull. Connecting temporary skid beams 9 from the ends of the hull cap beams 10 to the transport barge skid beams. Welding the connection between topside deck beams and the hull cap beams. Deballasting the tank to lift the topsides a few metres clear of the waterline. Fit temporary winches 11 to the topsides and connect the wire rope tendons to them. Figure 4c step 12. Tow the whole assembly to the desired offshore location. Ballast the hull to maintain it at its operating draft. Release the cable grips connecting the hull tightly to the tank and using a constant tension setting for the temporary topsides winches, increase the ballast in the subsea tank until its negative buoyancy is a few tens of tonnes. Lower the tank through the water by paying out on the winches. Since the tension in the cables remains constant, the hull remains at the pre-determined draft.

[5] When the tank reaches the seabed, the tank is completely filled with ballast water to create significant on-bottom weight while the piles are installed. The piles are driven in to the seabed and the annulus between each pile and its guide is filled with grout to create a permanent connection between pile and sleeve. When the grout has set to give a full strength connection, the hull is de ballasted and the tendons restrain the hull at its operating draft thereby creating several thousand tonnes of tension in the tendons. The operating draft is chosen such that when the trough of the design wave passes there is still sufficient buoyancy in the hull to maintain a positive tension in the tendons to avoid the tendons going slack at any time thereby eliminating snatch loads. The strength of the tendons is chosen to ensure adequate reserve strength when the crest of the design wave passes creating the maximum hull displacement and hence maximum tension in the tendons. The tendon tension creates a horizontal restoring force whenever the hull is displaced horizontally due to the action of wind, waves and current. The tension equalising beam 12 ensures that the tensions in each pair of tendons are always equal thereby reducing maximum stress levels and reducing the chance of failure. The dimensions of the hull pontoons and column are chosen such that the TLP is hydrostatically stable in the free floating condition preventing capsize in the event of total tendon failure. The dimensions of the hull and its operating draft are such that tendon tension remains positive even when the trough of a 100 year return period wave is passing. The hull columns are located at the mid points of the pontoon sides rather than at the corners reducing the span between columns and hence the dimensions of cap beams required to support the topsides.

Claims

1. A method of installing a tension leg platform and subsea oil tank, the method comprising the steps of: Constructing tank, hull and topsides separately at a construction site near to the ocean; Launching the tank in to ocean when construction is complete. Ballasting the tank until its free board is equal to the height of the wharf whereon the hull has been constructed. Loading the hull on top of the tank by means of winches mounted on the outboard edge of the tank Fixing the hull to the tank by means of wire rope tethers attached to the top of the tank, passed through attachment guides 7 in the 4 outer corners of the hull pontoons Figure 5, tensioning them and clamping them with hydraulic remotely operated cable grips. Installing piles in the pile guides while the tank is within reach of an onshore crane. Towing the assembled tank and hull to sheltered water of depth greater than the combined height of the tank and hull. Ballasting the tank and hull until the top of the hull columns is a few metres above the water line. Loading the topsides on to a transport cargo barge by skidding the topsides from wharf to barge. Towing the barge to alongside the almost submerged hull. 2 Connecting temporary skid beams from the ends of the hull cap beams to the barge skid beams. Skidding the topsides from the barge to the hull and welding the connection between topside deck beams and the hull cap beams. Deballasting the tank to lift the topsides a few metres clear of the waterline. Fitting temporary winches to the topsides and connecting the wire rope tendons to them. Towing the whole assembly to the desired offshore location. Ballasting the hull to maintain it at its operating draft. Releasing the cable grips connecting the hull tightly to the tank and using a constant tension setting for the temporary topsides winches, increasing the ballast in the subsea tank until its negative buoyancy is a few tens of tonnes. Lowering the tank through the water by paying out on the winches. Since the tension in the cables remains constant, the hull remains at the pre-determined draft. When the tank reaches the seabed, the tank is completely filled with ballast water to create significant on-bottom weight while the piles are installed. The piles are driven in to the seabed and the annulus between each pile and its guide is filled with grout to create a permanent connection between pile and sleeve. When the grout has set to give a full strength connection, the hull is de ballasted and the tendons restrain the hull at its operating draft thereby creating several thousand tonnes of tension in the tendons. The tension equalising beam 12 ensures that the tensions in each pair of tendons are always equal thereby reducing maximum stress levels and reducing the chance of failure.

2. A tension leg platform and subsea oil tank as claimed in claim 1, wherein the TLP hull design comprises pontoons of tubular cross section forming a square ring and the columns are set at the midpoint of each side rather than 3 the corners thereby reducing the size of cap beams required to support the topsides but providing a wide span for the tendon attachment points.

3. A tension leg platform and subsea oil tank as claimed in claims 1 & 2, wherein the tension leg platform is hydrostatically stable in its free floating condition.

4. A tension leg platform and subsea oil tank as claimed in claims 1, 2 & 3, wherein the design allows installation without use of offshore crane barge or diving operations or other specialist construction vessels.

5 A tension leg platform and subsea oil storage tank as claimed in claims 1, 2, 3, & 4 and substantially as herein before described with reference to Figures 1 to 5 of the accompanying drawings. DR NEIL ALEXANDER KERON 16TH DECEMBER 2011