GB2491546A

GB2491546A - Installing large taut moored buoyant systems

Info

Publication number: GB2491546A
Application number: GB1003558.2A
Authority: GB
Inventors: David Bone
Original assignee: Tamacrest Ltd
Current assignee: Tamacrest Ltd
Priority date: 2010-03-04
Filing date: 2010-03-04
Publication date: 2012-12-12
Anticipated expiration: 2030-03-04
Also published as: GB2491546B; GB201003558D0

Abstract

A gravity base 1 and a taut moored buoyant structure 2 are connected by one or more pre-attached flexible connectors 5 with calculated pre-determined lengths and are floated to an installation site, the gravity base 1 is then ballasted to the seabed whilst the buoyant structure 2 remains attached to the base 1 by the flexible connectors 5 and is thus pulled into an upright semi-submerged position close to the desired installed draught held by tension in the connectors 5. Flexible connectors 5 may be replaced following installation by permanent moorings (10 fig 3) and the final position of the buoyant structure 2 can be finely adjusted using hydraulically extendable devices (9 fig. 4) pre-attached to each flexible connector 10. The buoyant structure 2 can be supported by the gravity base 1 during tow out (fig. 5) or the two can be towed some distance apart. The gravity base 1 may include storage capacity for fluids and the buoyant structure 2 may be for use in the offshore oil, gas or energy industries.

Description

SELF-iNSTALLING INSTALLATION SCHEME FOR LARGE BUOYANT SYSTEMS This invention relates to the installation of large buoyant systems designed for use in the offshore oil, gas and energy industries.

Current installation techniques are carried out in a series of unrelated activities whereby individual components of the system to be installed are separately installed. Additionally large specialised equipment is usually required to aid installation. This may take the form of offshore crane-ships or vessels with high capacity winch-down capability (say of the order of several hundred tonnes).

The process described herein avoids the need for highly specialised and expensive vessels or equipment and utilises the mass of a gravity base (or associated storage unit) to act as an installation aid for the surface buoyant unit.

According to the present invention there is provided an installation method for a gravity base and a taut moored buoyant structure, the method comprising of floating the combined gravity base and buoyant structure, towing the combination to a site location, then ballasting the gravity base to the seabed, during ballasting the buoyant structure remains connected to the gravity base by one or more pre-attached flexible connectors with calculated predetermined lengths such that as the gravity base approaches and then reaches the seabed said buoyant structure is pulled down further in the water to an upright semi-submerged position close to the desired draught, the buoyant structure then being held in said position due to the tension acting in said flexible connectors.

Some or all of the flexible connectors can also act as the permanent moorings for the buoyant structure, or can be used for installation alone. Subsequently, one or more of the flexible connectors can be disconnected thereby leaving the remaining connectors to maintain the buoyant structure at or close to the desired draught relative to sea level and to provide stability to the buoyant structure during subsequent operation.

In one embodiment the gravity base includes storage capacity for fluids.

In one embodiment the gravity base is combined with the buoyant structure by use of one or more flexible connectors attached between the two with the gravity base and buoyant structure each floating some distance apart during tow out.

In one embodiment of the method a single flexible connector is connected between the gravity base and the buoyant structure and is attached prior to installation, and arranged such that submergence of the gravity base to the seabed causes the buoyant structure to further partially submerge and assume an upright semi-submerged position close to the desired draught.

In the case where a single flexible connector only is used for installation, then further flexible connectors required for stability during subsequent operation are added following the submergence of the gravity base to the seabed. The additional flexible connectors are connected between the gravity base and the buoyant structure using a remotely operated vehicle (ROy), then the single connector used for installation is detached by the ROV.

In another embodiment of the method the buoyant structure is supported by the gravity base during tow out, the gravity base is attached to the buoyant structure by one or more flexible connectors attached prior to installation and arranged such that submergence of the gravity base to the seabed causes the buoyant structure to further partially submerge and assume an upright semi-submerged position close to the desired draught.

In the case where the buoyant structure is supported by the gravity base during tow out, then for installation during submergence of the gravity base a central flexible connector between the gravity base and the buoyant structure is fed out of the buoyant structure using either a line winch or a winch. Alternatively, all the flexible connectors are used to tower the gravity base with synchronous strand jacks.

Preferably, a hydraulically extendable device is attached prior to installation onto each of the flexible connectors such that the final position of the buoyant structure relative to the sea level can be finely adjusted by operation of each of said hydraulically extendable devices if required.

Also, if required the applied forces resisting uplift due to buoyancy can be transferred to adjacent flexible connectors.

Preferably, each hydraulically extendable device is normally locked to its minimum length but can be extended by the action of releasing hydraulic fluid.

Preferably, the hydraulically extendable device consists of two concentric cylinders with one slidably mounted around the other such that they are capable of sliding motion relative to one another but are ultimately restricted by stop mechanisms at opposing ends.

The buoyant structure can be towed in either an upright or near horizontal attitude.

Prior to installation the buoyant structure is ballasted so that it assumes an upright attitude.

There are three options for ballasting of the buoyant structure which are as follows; Buoyant structure ballast option I is achieved by adding ballast to the lower inside part of the buoyant structure.

Buoyant structure ballast option 2 uses one or more ballastable sinkers which are attached to the underside of the buoyant structure and have capacity for additional ballast.

Buoyant structure ballast option 3 uses temporary ballast containers fixed externally to the outside of the buoyant structure near to the bottom.

Specific embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: -Figure 1 shows in plan view a layout of a gravity storage base and buoyant structure prior to commencement of installation.

Figure 2 shows the gravity storage base partially submerged during installation.

Figure 3 shows the gravity base fully submerged and the buoyant structure connected to it with long-term flexible connectors fitted and adjusted ready for operation.

Figure 4 shows in cross section a hydraulically extendable device used for final adjustment of the flexible connector lengths.

Figure 5 shows the option of the gravity storage base supporting the buoyant structure during tow out.

Referring to Figure 1, the Installation system comprises of a gravity storage base 1, a buoyant structure 2, three installation vessels 3, a control vessel 4, an installation wire (flexible connector) 5, installation buoyancy aids 6, pre-laid moorings 7, and descent control lines 8.

Also included are hydraulically extendable devices 9, as shown in Figure 4, and long-term flexible connectors 10 shown in Figure 3.

The installation is completed in six stages (with reference to Figures 1 to 4), as following; Stage I (reference Figure 1), the structures are positioned ready for installation with the gravity storage base 1 floating above the desired location. The buoyant structure 2 is ballasted down into an upright semi-submerged position providing enough stability for installation operations.

The installation vessels 3 are connected to pre-laid moorings 7 consisting of typically a mooring buoy, a clump weight, and an anchor. These vessels 3 are initially positioned about 100 to 200m from the gravity storage base 1 to allow vertical control of the gravity storage base 1, but are subsequently moved outwards to allow for greater horizontal control. Alternatively, dynamic position control vessels could be used without pre-laid moorings.

Stage 2 (reference Figure 2), the gravity storage base us ballasted with sea-water until it is submerged below the water line.

Stage 3 (reference Figure 2), a controlled descent of the gravity storage base 1 is implemented using minimal additional ballast. The position of the gravity storage base 1 is controlled using the installation vessel descent control lines 8.

Stage 4, the gravity storage base 1 is held close to but off of the seabed using the buoyant resistance of the buoyant structure 2.

Stage 5, the exact location of the gravity storage base 1 is confirmed and it is then further ballasted to the seabed.

Stage 6 (reference Figure 3), further flexible connectors 10 as required for long-term use are attached between the buoyant structure 2 and the gravity storage base 1 using an ROV. These long-term flexible connectors 10 will allow subsequent stability during operation and extreme weather conditions for the buoyant structure 2. The installation wire 5 is then loosened off and removed.

Stage 7 (reference Figure 4), the final position of the buoyant structure 2 is finely adjusted using the hydraulically extendable devices 9 which have been pre-attached (not shown) to each flexible connector 10. The hydraulic fluid (hatched area) II is released into the hydraulic system (not shown) allowing adjustment as indicated by arrow 12 until the desired length is obtained.

Referring to Figure 5, an alternative method is shown for the tow out of the gravity storage base I and buoyant structure 2. The buoyant structure 2 is supported by the gravity storage base I and connected to it with flexible connectors including a central flexible connector (not shown).

Claims

CLAIMSI. An installation method for a gravity base and a taut moored buoyant structure, the method comprising of floating the combined gravity base and buoyant structure, towing the combination to a site location, then ballasting the gravity base to the seabed, during ballasting the buoyant structure remains connected to the gravity base by one or more pre-attached flexible connectors with calculated predetermined lengths such that as the gravity base approaches and then reaches the seabed said buoyant structure is pulled down further in the water to an upright semi-submerged position close to the desired draught, the buoyant structure then being held in said position due to the tension acting in said flexible connectors.
2. An installation method for a gravity base and a taut moored buoyant structure as claimed in Claim 1, wherein some or all of the flexible connectors can also act as the permanent moorings for the buoyant structure, or can be used for installation alone.
3. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein subsequent to installation, one or more of the flexible connectors can be disconnected thereby leaving the remaining connectors to maintain the buoyant structure at or close to the desired draught relative to sea level and to provide stability to the buoyant structure during subsequent operation.
4. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein the gravity base includes storage capacity for fluids.
5. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein the gravity base is combined with the buoyant structure by use of one or more flexible connectors attached between the two with the gravity base and buoyant structure each floating some distance apart during tow out.
6. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein a single flexible connector is connected between the gravity base and the buoyant structure and is attached prior to installation, and arranged such that submergence of the gravity base to the seabed causes the buoyant structure to further partially submerge and assume an upright semi-submerged position close to the desired draught, then further flexible connectors required for stability during subsequent operation are added following the submergence of the gravity base to the seabed.
7. An installation method for a gravity base and a taut moored buoyant structure as claimed in Claim 6, wherein the further flexible connectors are connected between the gravity base and the buoyant structure using a remotely operated vehicle (ROy), then the single connector used for installation is detached by the ROV.
8. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein the buoyant structure is supported by the gravity base during tow out, the gravity base is attached to the buoyant structure by one or more flexible connectors attached prior to installation and arranged such that submergence of the gravity base to the seabed causes the buoyant structure to further partially submerge and assume an upright semi-submerged position close to the desired draught.
9. An installation method for a gravity base and a taut moored buoyant structure as claimed in Claim 8, wherein for installation purposes during submergence of the gravity base a central flexible connector between the gravity base and the buoyant structure is fed out of the buoyant structure using either a line winch or a winch.
10. An installation method for a gravity base and a taut moored buoyant structure as claimed in Claim 8, wherein all the flexible connectors are used to lower the gravity base with synchronous strand jacks.
11. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein a hydraulically extendable device is attached prior to installation onto each of the flexible connectors such that the final position of the buoyant structure relative to the sea level can be finely adjusted by operation of each of said hydraulically extendable devices if required, and also if required the applied forces resisting uplift due to buoyancy can be transferred to adjacent flexible connectors.
12. An installation method for a gravity base and a taut moored buoyant structure as claimed in Claim 11, wherein each hydraulically extendable device is normally locked to its minimum length but can be extended by the action of releasing hydraulic fluid.
13. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein ballasting is achieved by adding ballast to the lower inside part of the buoyant structure.
14. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein ballasting is achieved by using one or more ballastable sinkers which are attached to the underside of the buoyant structure and have capacity for additional ballast.
15. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein ballasting is achieved by using one or more temporary ballast containers fixed externally to the outside of the buoyant structure near to the bottom.
16. An installation method for a gravity base and a taut moored buoyant structure as claimed in any preceding claim, wherein marine vessels are used with control lines attached to the gravity base to aid vertical and horizontal control of the gravity base during ballasting operations.
17. An installation method for a gravity base and a taut moored buoyant structure substantially as herein described and illustrated in the accompanying drawings.