In particular, the invention relates to a docking arrangement for installation at a shallow water site (e. g. in water depths from 15 to 30m). The arrangement is capable of locating and supporting a tanker type vessel at that site, for the purpose of processing and storeing fluid hydrocarbons. Advantageously, the docking arrangement has provision for the mooring alongside of another vessel to offload at least some of the fluid hydrocarbons. Conveniently, the docking arrangement may have provision for storeing at least some of the products.

Background of the Invention Heretofore it has been known to have Floating Production Storage and Offloading facilities (FPSOs) to operate offshore. These FPSOs have typically been converted from Very Large Crude Carrier (VLCC) type tankers. The FPSO has received fluid hydrocarbons directly from an subsea oil/gas field. The hydrocarbons have been subjected to primary processing on the FPSO at or near the oil/gas field, and the products have been removed by pipeline or shuttle tanker. The products so removed can be transferred to onshore processing facilities.

Mooring the FPSO so that it can remain safely on field in extreme weather conditions can create problems. (A mooring arrangement for an FPSO is shown diagramatically in Figure 2 of PCT Application No W088/08806.) It has been proposed to moor the FPSOs at or near shore, so that the products can be transmitted directly to onshore processing facilities. This may be advantageous where the onshore processing facilities are themselves situated at remote sites. However, the safe mooring of the FPSO at a near shore site may present difficulties, particularly where extreme weather conditions (e. g. cyclones) are expected.

If transportation of products from an FPSO is to be effected in a succession of shuttle tankers, a further problem arises in mooring the shuttle tankers safely near to the FPSO.

(Typical shuttle tanker mooring arrangements are shown at the top of page 47 of"Offshore Engineer"for October 1999) Thus there is a requirement for a docking arrangement for a vessel adapted to process hydrocarbons, so that the vessel can be safely founded at a remote shallow water site.

Disclosure of the Invention The invention provides a semi permanent docking arrangement for a flat bottomed production and storage vessel, the arrangement comprising a submersible elongate base slab with a flat upper surface upon which a flat bottomed vessel may be set, and a generally flat lower surface to be grounded on a shallow seabed, in which there is means to stabilize the base slab on the seabed and means to locate the flat bottomed vessel directly or indirectly on the base slab.

It is preferred that there are at least two columns upstanding from the base slab so that the flat bottomed vessel can be secured to at least two of the columns.

It is further preferred that at least two of the columns are adapted to act as mooring dolphins, so that another vessel (e. g. an offtake tanker) can be moored alongside and spaced apart from the flat bottomed vessel.

It is still further preferred that there is an array of two rows of columns, and the rows of columns are disposed so that the flat bottomed vessel can be located on the base slab and secured between the rows of columns.

It is preferred that at least the base slab has capacity to store fluid products to be transferred between the flat bottomed vessel and another vessel.

It is preferred that the base slab is capable of floating for movement between its construction yard and a remote shallow water site at which it may be required to be grounded.

The generally flat lower surface of the base slab may have downwardly extending skirts which are capable of projecting into the seabed; and there may be provision for piles to be driven through the base slab into the seabed.

It may be preferred that there is provision for grouting space between the generally flat lower surface of the base slab and the seabed.

In one form the base slab is cellular. In this last mentioned form at least one of the cells is be upwardly open so that solid ballast may be deposited therein.

The docking arrangement described above may be used in combination with a generally similar docking arrangement, so the two arrangements can be set on the seabed adjacent to each other in end to end relationship, and one flat bottomed vessel can be set on the two adjacent base slabs.

It is preferred that the base slab is arranged to act as a prepared seabed foundation upon which the vessel is to be located, whereby to stabilise the vessel against environmental loads e. g. arising from cyclones.

The invention includes a docking arrangement as described above in combination with a flat bottomed vessel having facilities to process and store fluid hydrocarbons, the vessel being located on the base slab.

Brief Description of the Drawings A specific embodiment of the invention and two variants thereof will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a perspective view of a docking arrangement; Figure 2 is a perspective view of that docking arrangement with a production and storage vessel being located thereon; Figure 3 is a cross section of the docking arrangement shown in Figures 1 and 2, with another vessel moored alongside; Figure 4 is a cross section of a second docking arrangement; and Figure 5 is a cross section of a third docking arrangement.

Description of the Specific Embodiments As shown in Figure 1, a docking arrangement 10 has a base slab 11. The base slab 11 is of elongate rectangular planform and has a flat upper surface 12. Within the base slab there are longitudinal and transverse bulkheads 14 and 15 respectively. The docking arrangement has an array of upstanding columns 16 disposed around its periphery. The columns 16 are disposed in rows along each of the longer sides of the base slab 11.

The docking arrangement 10 may be constructed of steel or concrete at any convenient yard. The docking arrangement is self buoyant, so that it may be floated from the yard to a remote shallow water site. The docking arrangement has watertight internal compartments.

When empty, these contribute to the buoyancy of the docking arrangement. These compartments may also be adapted for the storage of hydrocarbon product. Ballasting of the compartments stabilises the base slab on the seabed.

The base slab 11 may have skirts (not shown) extending downwardly from its undersurface. Advantageously the skirts surround the periphery of the base slab. These skirts are configured to penetrate into the seabed at the shallow water site.

The base slab 11 may also have provision for piles (not shown) to be driven downwardly through it into the seabed.

The skirts and/or piles stabilise the base slab 11 on the seabed.

The base slab 11 is conveniently of cellular construction. Walls of the cells may be formed of the bulkheads 14 and 15. Some of the cells may be open topped, so that they may be at least partially filled with solid ballast. Some of the cells may be closed to form the buoyancy/storage compartments referred to above. The optional skirts and piles may be arranged to extend down from or through the walls or bulkheads.

When complete, the docking arrangement is floated to its intended site. At that site it is ballasted down to rest on the seabed. The columns 16 give the docking arrangement stability during submergence of the base slab 11. Thus the docking arrangement is self installing.

(Alternatively, the docking arrangement may be prefabricated elsewhere and assembled at the intended site.) If necessary, the seabed may have been roughly leveled before the docking arrangement is ballaste down. Any remaining space between the underside of the base slab and the seabed can be filled with grout for uniform bearing pressure.

When the docking arrangement 10 is securely founded on the seabed, a production and storage vessel 17 is floated over the base slab 11 between the rows of columns 16, as shown in Figure 2. The production and storage vessel has a flat bottomed tanker type hull. The vessel 17 is ballaste down to rest on the upper surface of the base slab 11, and is fixedly located on the docking arrangement. Pipelines (not shown) are arranged to transmit hydrocarbon fluids from subsea wells to the vessel 17 for primary processing.

On the side of the docking arrangement shown in figure 2, the columns 16 have fenders 18, so that the columns can act as mooring dolphins.

When the vessel 17 is located on the docking arrangement 10, production can take place. Well fluids (hydrocarbons) are separated into products, e. g. Liquefied Natural Gas or Liquefied Petroleum Gas (LNG or LPG) by facilities 20 on the deck of vessel 17. These products can be stored either in the vessel 17 or in compartments within the base slab 11.

When significant volumes of products have accumulated, an offtake tanker 19 is brought alongside the fenders 18, and is secured to the dolphins. Product (e. g. LPG) is transferred from the vessel 17 into the offtake tanker 19 by an offloading arm 21.

When an appropriate amount of product has been transferred, the offtake tanker 19 is released, and can steam away to an onshore terminal.

In some circumstances, it may be appropriate for two docking arrangements to be placed adjacent to each other in end to end relationship, and for one production and storage vessel to be located on both of them.

The docking arrangement may be designed specifically to support a particular production and storage vessel. The cross section of Figure 4 shows a docking arrangement 30 designed to support a production and storage vessel 37. In this case the longitudinal bulkheads 34 of the docking arrangement are disposed to lie directly beneath the bulkheads of the vessel 37.

This docking arrangement is made of reinforced concrete, but it could be made of steel.

Alternatively, the docking arrangement may be a standard barge of suitable draft. The cross section of Figure 5 shows a docking arrangement 40 in the form of a submerged barge.

In this case the barge is sufficiently robust to support a vessel 47, notwithstanding that the bulkheads 44 of the barge do not align with the bulkheads of the vessel 47. The barge is constructed of steel, using conventional shipbuilding techniques.

The framing of vessel 37/47 shown in Figures 4 and 5 is that of a standard design, to which no claim is made.

Avantages of the Invention The vessel 17/37/47 is located on the docking arrangement 10/30/40 for the life of an offshore oil/gas field. This may extend over several years. Thus the combination of docking arrangement and vessel can be regarded as semi-permanent. However, at the end of the life of the field, the vessel may be floated off to another site.