In the offshore oil and gas production industry it is frequently necessary to connect a chain or the like to a floating single point mooring structure or vessel, such as a tanker loading buoy, floating production vessel, spar buoy, semi-submersible etc).

These chain connections are often used to connect anchor lines but could also be used to connect flexible or steel riser pipes, flowlines or umbilicals, as for example in UK Patent Application 9806521.2.

A major problem with these prior art chain connections is that there is only one axis about which the attachment means of each chain is able to rotate.

When the floating single point mooring structure or vessel rotates in response to, for example, waves approaching from a particular direction, any attachment means which has its axis of rotation parallel to the axis of motion of the floating single point mooring structure or vessel can also rotate.

However, any attachment means which has its axis of rotation deviating from the axis of motion of the floating single point mooring structure or vessel cannot rotate sufficiently and as a result a large amount of wear can occur in the chains. Particularly in very deep water when large chain pretensions are used, this can generate unacceptable wear conditions on the chain.

The present invention provides apparatus for connecting a chain to the non-rotatable part of a floating single point mooring structure which defines a first axis about which the rotatable part of the structure can rotate, comprising a securement member for receiving and securing the end of a chain to the single point mooring structure and being pivotally attached to the single point mooring structure for rotation about a second axis which is perpendicular to the first axis, and about a third axis which is perpendicular to the second axis and which is movable relative to the first axis.

Preferably, the first axis is substantially vertical in use and the second axis is substantially horizontally in use.

In the preferred embodiment, the securement member comprises a tube receiving an end portion of the chain and a gripping device to secure the end portion in the tube.

The tube may be curved and the lower end of the tube may be splayed.

Preferably, the tube is secured to an arm and the arm is pivotally attached to the single point mooring structure.

In addition, a link is preferably pivotally mounted on the single point mooring structure by a first pin and the arm is pivotally mounted on the link by a second pin perpendicular to the first pin, whereby the arm and link are together rotatable about the first pin relative to the single point mooring

structure and the arm is rotatable about the second pin relative to the single point mooring structure and the link.

The longitudinal axis of the arm may be substantially aligned with the axis of the opening at a lower end of the tube and may intersect the second axis and be perpendicular to the third axis.

The third axis may intersect the second axis.

The invention will now be described in detail, by way of example only, with reference to the accompanying drawings in which: FIGURE 1 is a schematic diagram of a turret buoy; FIGURE 2 is a schematic diagram of a turntable buoy; FIGURE 3 is a schematic diagram of an F (P) SO internal turret; FIGURE 4 is a schematic diagram of an F (P) SO external turret; FIGURE 5 is a schematic diagram of a prior art arrangement showing a turret buoy and risers suspended by riser hang-off legs; FIGURE 6 is a schematic diagram of a turret buoy incorporating a chain attachment apparatus in accordance with one embodiment of the present invention; FIGURE 7a is an enlarged view of the chain attachment apparatus shown in Figure 6; FIGURE 7b is an enlarged view of the apparatus shown in Figure 7a in the direction of arrow X; and FIGURE 8 is a side view of a second embodiment of chain attachment apparatus in accordance with the

present invention.

A floating single point mooring is composed of a rotatable member and a non-rotatable member, connected to each other so that the vertical centreline of the non-rotatable member coincides with the vertical axis of rotation of the rotatable member. The non- rotatable member is kept in position by means of anchor legs. The non-rotatable member also facilitates the entry of submarine flexible or steel riser pipes, flowlines or umbilicals. The anchor legs can be composed of steel or man-made fibre wire, cable or chain. Often the top section connected to the non- rotatable member of the floating single point mooring is a chain.

One example of a floating single point mooring is a turret buoy as illustrated in Figure 1, in which a turret T is rotatably mounted in a floating buoy B.

The turret T is anchored by chains C and receives riser R. The buoy B can rotate about the turret T.

A second example of a floating single point mooring is a turntable buoy as illustrated in Figure 2. Here a floating buoy B in anchored by chains C and receives riser R. A turntable TT is rotatably mounted on top of the buoy B and includes a fluid swivel S for receiving the end of the riser R.

A third example of a floating single point mooring is a turret moored floating production and offloading unit. In this case a turret T is rotatably mounted to a floating vessel V, either internally as in Figure 3, or externally as in Figure 4. The turret is anchored by chains C and receives riser R. The

vessel V can rotate about the turret T.

Figure 5 shows a typical prior art chain attachment arrangement applied to a turret buoy. The rotatable member is the buoy body 1 and the non- rotatable member is the turret 2 with a spider 3 underneath. The turret 2 is anchored to the seabed by anchor legs 4 attached to the spider 3. The rotatable buoy body 1 allows the attachment of vessels such as tankers 5. The buoy body 1 and turret 2 with spider 3 therefore provide a single point mooring structure about which the vessel 5 can freely weathervane in response to wind, current and waves.

A product transfer system 6 allows products to be transferred between the vessel 5 and the subsea riser 7. The subsea riser 7, in the form of a steel or flexible pipe, flowline or umbilical may be supported from the spider 3 by means of a riser hang-off leg 8 which may be a chain, a steel wire or man-made fibre wire. The attachment to the spider 3 is typically in the form of a known chain attachment apparatus, such as a conventional articulated chain stopper 9 which allows rotation relative to the non-rotatable turret 2 and spider 3 about a single pivot axis only. Such a device may also be used to connect the anchor legs 4 to the spider 3.

A first embodiment of the present invention is illustrated in Figure 6. In this example a single point mooring structure in the form of a turret buoy is fitted with a chain attachment apparatus in accordance with one embodiment of the present invention. As described above, the buoy body 1 can rotate relative to the turret 2 and spider 3 about a

first vertical axis V in the direction of arrows A.

As best seen in the enlarged views of Figures 7a and 7b, the top end of each anchor leg 4, or of the riser hang-off leg 8, is attached to the non-rotatable spider 3 by securement means 10. The securement means 10 is made up of a chain receiving tube 11 and an arm 12.

The chain receiving tube 11 receives the top end portion of the chain 4. The tube 11 may be curved so as to redirect the chain 4 from the angled orientation in which it enters the tube 11 into a substantially vertical position. In this way an upper part of the chain 4, for example during the initial anchoring procedure, can be winched in and out from the floating single point mooring structure. When the single point mooring structure has been suitably positioned, a conventional chain gripper 13 mounted on top of the tube 11 is closed, securing the chain 4 and allowing the portion of the chain 4 extending above the tube 11 to be severed.

The lower end of the chain receiving tube 11 may be splayed as illustrated to facilitate entry of the chain 4 into the tube 11.

The chain receiving tube 11 is rigidly secured to an arm 12. The arm 12 is pivotally attached to the spider 3 for rotation about two mutually perpendicular axes. Specifically, a link 14 is pivotally attached to the spider 3 by a first pivot pin 15, which is substantially horizontal. The arm 12 is pivotally attached to the link 15 by a second pivot pin 16 oriented at 90° to the first pivot pin 15.

In this way, the arm 12 and the link 14 are together able to pivot about a second axis H, defined by the first pivot pin 15, relative to the spider 3.

This pivoting movement is illustrated by arrows B.

The arm 12 is also able to pivot about a third axis M, defined by the second pivot pin 16, relative to the link 14 and the spider 3. This movement is illustrated by arrows C.

The second axis H and the third axis M are perpendicular to one another, the second axis H being substantially horizontal. Thus, the second axis H is fixed perpendicular to the first axis V about which the rotatable part of the floating single point mooring structure, in this case the buoy body 1, rotates. However, the angle of the third axis M relative to the first axis V varies depending upon the pivotal movement of the arm 12 and link 14 about the second axis H.

As best seen in Figure 7a, the longitudinal axis L of the arm 12 preferably intersects the second axis H. In addition, the longitudinal axis L is preferably coincident with the axis of the opening to the lower end of the chain receiving tube 11 and substantially coincident with the axis of the chain 4 as it enters the chain receiving tube 11.

In this way, the chain attachment apparatus 10 is able to accommodate movement of the whole single point mooring structure irrespective of the direction of the waves, wind and/or current acting on it and excessive wear on the chain is avoided.

It will be apparent to those skilled in the art that variations and modifications may be made to the exact configuration described and illustrated without departing from the scope of the invention. The invention may of course be used to connect a cable rather than a chain and the precise configuration of the chain receiving tube 11, the arm 12 and its pivotal attachments can be altered provided the required freedom of movement is achieved.

For example, a second embodiment of chain attachment apparatus is illustrated in Figure 8, which uses like reference numerals to Figures 7a and 7b.

Here, the chain receiving tube 11 is straight and has its longitudinal axis coincident with that of the arm 12. In addition, the third axis M intersects the second axis H.