This invention relates to multiple riser connection systems for flexible risers extending

from a floating production vessel.

Floating oil and gas production vessels are generally based on a semisubmersible hull or a ship-shaped hull structure. Multiple flexible risers are used to connect multiple subsea wellheads located on the seabed to processing facilities situated on the deck of the floating vessel. When connecting to a semisubmersible structure the riser connectors may be arranged along a pontoon (submerged when the vessel is in normal producing mode), cross-brace (submerged) or side of the deck (above- water). The multiple connectors are usually arranged along a straight line. When connecting to a ship-shaped structure moored in a calm climate the riser connectors will be arranged in-line along a side of the deck. However, in areas subject to high winds from many directions, a ship-shaped structure must be moored to allow it to "weather-vane" or to align itself with the wind and wave direction. A common mooring

for a ship-shaped oil and gas production system in a rough weather area comprises an internal or external turret set in bearings to allow the hull to align with the weather, thus reducing mooring loads and vessel motions. In this case the risers connect to the turret and are usually arranged in a circular pattern.

Normally the risers are installed individually, and if a fault develops in one riser it is desirable to be able to remove it without interfering with production from the other risers. Two other events, however, need to be considered with respect to disconnecting the entire riser system. The first event is a requirement to move the vessel to a sheltered location for

maintenance and/or modification of the topside and/or hull. The second event is the result of

a decision to be taken after failure of a single mooring line during a severe storm. In this case, the Production System Manager is either satisfied that no further mooring lines will fail and that the remaining lines will suffice to position the vessel within the position limits of the riser system, or he will prepare for the possibility that mooring failure will progress and allow the

vessel to move beyond the position limits of the riser system. Disconnection of all the risers (typically around 20) within a short time, i.e. a few seconds, may lead to loss of control of position of the released ends and possible entanglement of the risers.

It is desired therefore, after disconnection of multiple flexible risers within such a short time, to maintain the released ends of the risers in a controlled spaced relationship, to prevent the released ends hitting each other and the seabed, and to ease recovery of the multiple risers

for re-connection.

According to the invention therefore there is provided a riser connector assembly for a plurality of risers attached to a floating production vessel, comprising clamps means for holding and retaining the ends of a plurality of risers with the ends of the risers spaced and

arranged to link with mating inlets on the production vessel, and buoyancy means for supporting the assembly when released with attached risers at a location below the surface but above the seabed.

The present invention is concerned to allow initial individual installation of multiple flexible risers to a floating vessel, subsequent individual removal of a faulty riser, and

maintenance of the positional relationship of released ends of risers following their disconnect

in a short time (usually between 5 days and 5 seconds) and during re-connection. The connection assembly may be in the form of a beam (for in-line positioning) or hub structure

(for circular positioning), located in or close to a horizontal plane. Buoyancy modules (one per riser and planned riser) can be attached which incorporate a part-circular recess to accommodate the outside diameter of a close to vertical flexible riser end-fitting. After

installing an individual riser, a cover can be attached to each buoyancy module. The cover may include a part-circular ledge detent which can mate with an undersurface of the riser end fitting. The beam or hub structure preferably includes means for attachment of pulling lines to ease simultaneous recovery of all riser end-fittings to the vessel during re-connection.

The buoyancy of the termination can be inherent in the structure of the termination, can be attached to the termination, can be created at the time of disconnection of the termination or can be a combination of these. Thus an inflatable buoyancy bag together with means for inflation can form part of the termination. Thus, rather like an air safety bag in a car, the buoyancy required can be provided at the time of disconnection and this can be triggered automatically upon a "panic" or quick release of all of the risers.

Existing flexible riser end-fitting buoys do not maintain position of one end-fitting relative to the others and do not assist simultaneous recovery of all end-fittings.

Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is an isometric view of an entire floating production system showing multiple risers, which can be spaced at approximately 1 to 2 metres, from or to multiple wells;

Figure 2 is an isometric view showing a beam-type linking structure and multiple flexible riser ends prior to re-connection to a semisubmersible vessel pontoon after maintenance or emergency-disconnect;

Figure 3 is a section through the linking structure showing a buoyancy module, riser end-fitting and cover; and

Figure 4 is an isometric view of a hub-type linking structure under the turret of a turret-moored ship-shaped vessel.

Referring now to Figure 1, multiple flexible risers 1 are shown connected to a semisubmersible vessel pontoon 2 with the connectors 3 in an in-line relationship.

In Figure 2, multiple flexible risers 1 are shown positioned for re-connection to the semisubmersible vessel pontoon 2. A beam-type linking structure 4 has pad-eyes 5 which allow attachment of recovery lines 6 which run through guides 7 and bring the riser ends into

correct positional engagement with vessel connectors 3.

Figure 3 shows a section through the beam-type linking structure 4 with a buoyancy module 8 attached to it by a retainer 9 and prevented from rotation relative to the beam by key 10. An end-fitting 1 1 of a flexible riser 1 is set in a part-circular recess 12 of the buoyancy module 8. Cover 13 is attached to buoyancy module 8 by bolts 14 and the end- fitting 1 1 can be either clamped between the cover 13 and module 8 or it can remain freely located with around 10mm radial clearance. Cover 13 has a detent ledge 15 which engages the underside of the end-fitting 1 1 but remains clear of the riser bend restrictor 16. The end- fitting 1 1 has an upwardly-protruding section 17 designed for engagement with a connector 3. The riser end-fittings will be typically spaced at around 1 metre so that each buoyancy

module 8 will be close to 1 metre across.

During initial riser installation the linking structure with buoyancy modules attached will be held in place by lines 6 close to the pontoon. It may be advantageous to perform riser connections above water with the semisubmersible vessel in a de-ballasted condition. Riser

connector inspection and maintenance will be easier if the connectors are above sea level, at least when the vessel is de-ballasted. Individual risers will be pulled into position until the protruding sections 17 are in pressure tight engagement with their corresponding connectors.

Covers 13 will then be added. Prior to adding cover 13, end-fitting 1 1 can move axially upwards or downwards relative to buoyancy module 8. When cover 13 is attached to module

8, the ledge detent 15 prevents the high density end-fitting 1 1 moving axially downwards relative to the lower density buoyancy module 8.

If a multiple disconnect is required, the connectors will be released and the linking structure 4 can be lowered by lines 6 or allowed to free fall. The riser end-fittings 1 1 will remain in correct spacing relationship and the buoyant assembly of linking structure 4, buoyancy modules 8, end-fittings 11 and covers 13 will sink to the level where the buoyancy

is balanced by the reduced catenary weight of the flexible risers. This situation is shown in broken lines in Figure 1. It would be advantageous for this neutral position to be between 50 and 100 metres below sea surface, i.e. below the majority of wave action.

Figure 4 shows a circular linking structure 18 with buoyancy modules 19 arranged

around its outer rim. In this case the riser connectors may be located at the top of guide tubes 20 set in the turret 21 of the ship-shaped vessel 22. On release, the end-fittings (not shown) are lowered or fall down the tubes until they are caught by the covers 23 of buoyancy modules 19, which are positioned clear of mooring lines 24. When all end-fittings are resting in the modules, the circular linking structure 18 can be either lowered on lines or allowed to free fall until the overall buoyancy of the assembly of structure, buoyancy modules and end- fittings is balanced by the reduced length of risers hanging from the buoyant assembly. The end-fittings will be held in correct positional relationship in preparation for re-connection.