GB2191230A

GB2191230A - Flexible riser system

Info

Publication number: GB2191230A
Application number: GB08613690A
Authority: GB
Inventors: Keith Shotbolt
Original assignee: Bechtel Ltd
Current assignee: Bechtel Ltd
Priority date: 1986-06-05
Filing date: 1986-06-05
Publication date: 1987-12-09
Also published as: GB8627392D0; GB8613690D0; GB8627696D0; GB8700099D0; GB8709301D0

Abstract

A flexible riser 10 has a lower connector 11 which is moved laterally over the seabed by a pull-in sled 7 under a mid-water buoy 3. After connecting to a connection point 17 on subsea equipment 1, the riser 10 is attached at a point part way along its length to buoy 3. This is done prior to construction vessel 13 laying the riser over the buoy and connecting the upper connector to a production vessel positioned over the subsea equipment. <IMAGE>

Description

SPECIFICATION Flexible riser system This invention relates to flexible risers (or control umbilicals) extending from subsea oil or gas production equipment positioned on the seabed, over a mid-water buoy and up to a floating vessel on the surface of the sea.

Previous designs and configurations for flexible risers have been described by Coflexip in their brochure Dynamic Flexible Risers printed in September 1985. These include five configurations named as follows: (a) free-hanging (b) lazy S (c) steep S (d) lazy wave (e) steep wave.

It is likely that many semi-submersible floating production systems will in future have at least some of their production wells positioned directly beneath the vessel for two reasons: -produced fluids can flow more readily without suffering from the severe slugging, hydrate, wax and emulsion problems associated with subsea flowlines.

-workover and other subsea equipment maintenance can be performed directly without requiring a second vessel.

The steep S and steep wave configurations, which are designed to be used when wells are beneath the vessel, are difficult to install and introduce more connection points at the riser base. It is desired therefore to reduce the number of connections to a minimum and to make installation of mid-water buoyancy and multiple flexible risers as simple as possible. It may be necessary to perform the installation without using divers.

According to the present invention there is provided a flexible riser system comprising: a tethered mid-water buoy with at least one tether extending up from a base; a flexible riser pipe for forming a connection between subsea production equipment on the seabed and a floating vessel on the surface of the sea, having a lower connector at one end and an upper connector at the other, means for laterally moving the lower connector across the seabed from a point on one side of a vertically downward projection of the buoy, passing under the buoy to a connection point on the opposite side of said projection; and means to attach a point of the riser pipe which is part way along its length to the midwater buoy such that some of the wieght of the riser pipe is transferred to the buoy before the upper section of the riser pipe is laid over the buoy.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 Shows the riser being unreeled and suspended vertically from a construction vessel, prior to landing the lower connector into a sled.

Figure 2 shows the riser after pulling the sled from the buoy base and making the connection to the subsea production equipment, at the time during installation when the riser is about to be attached to the mid-water buoy.

Figure 3 shows one method of attaching the riser to the buoy and transferring some of the riser weight to the buoy before laying the riser over the buoy.

Figure 4 shows the riser after laying the upper section over the buoy and attaching the upper connector to the production vessel.

Figure 5 shows an isometric view of an entire nine riser system.

Referring to Fig. 1, the subsea production equipment 1 is shown positioned on the seabed and a production vessel 2 is floating directly above. A mid-water buoy 3 is tethered to a seabed base 4 by a main tether 5 and may include a shorter subsidiary tether 6 to partially deflect the buoy. The base 4 must be accurately positioned relative to the subsea production equipment 1 and the connection point 17. This may be done by a sonar positioning system which should be able to work to a tolerance of +1- 1 metre, or preferably better, over a distance of 100 metres. Alternatively the base 4 can be positioned relative to the subsea equipment 1 by using a seabed spacing frame or mudline cover (not shown) which could also be used to provide a stable working surface between the base 4 and subsea equipment 1 in soft soil areas.

A receptacle sled positoned on the base 4 comprises a sled 7 and a receptacle 8 mounted on sled 7 by a pivot 9. Items 3,4,5,6,7,8 and 9 could be installed simultaneously on the seabed in a single lift by a crane vessel.

A flexible pipe 10 with a lower connector 11 is lowered vertically from a reel 12 on a construction vessel 13. A remotely-operated vehicle (ROV) 14 is used to monitor the approach of connector 11 towards receptacle 8 and vessel 13 manoeuvres, possibly using dynamic-positioning, to enable connector 11 to enter receptacle 8 as the connector is lowered. Riser pipes having multiple bores must be correctly oriented, possible by incorporating a helical cam (not shown) in the receptacle 8 and a cam follower on the connector 11.

Alternatively, the sled 7 could be lowered down from vessel 13 with the connector 11, but the sled would require orientation as it approached base 4.

A second ROV 15 launched from vessel 2 has theability to attach a pulling line 16 to sled 7 and then the land on the subsea equipment 1 and pull the sled 7 towards the subsea equipment connection point 17.

Fig. 2 shows the sled 7 after being pulled into the subsea equipment 1 and connection having been made between the lower riser connector 11 and the connection point 17.

ROV 14 enables visual monitoring as the vessel 13 manoeuvres to position the riser 10 adjacent to the mid-water buoy 3 prior to attachment. One method of ensuring that the free-hanging catenary 18 between the midwater buoy 3 and the seabed is shaped as required would be to clamp the flexible pipe at a marked position to the buoy 3.

An alternative method, which is shown in Fig. 3, is to provide a short steel spool 19 part-way along flexible pipe 10. The spool 19 has an enlarged portion 20 which can nestle into a carrier 21 in a similar way to the upset end of drill pipe into elevators. The carrier 21 may be suspended on chain 22.

When the attachment has been made, some of the weight of the riser pipe 10 can be carried by the buoy 3. The construction vessel 13 can then move towards the production vessel 2 and more flexible pipe 10 can be unwound from the reel 12 so that pipe can be laid across and into the cradle 23 without slipping.

Fig. 4 shows the upper riser connector 24 being pulled in to make connection with the production vessel at a connection point 25, the pipe length being such as to form a catenary 26.

If more than one flexible riser pipe is to be connected to the upper connector 24, all the flexible pipes would first be connected to the seabed equipment 1 and attached to the buoy 3. Then all the pipes would be laid over the buoy 3 together prior to attachment to the upper connector 24.

Fig4. 5 is an isometric view showing a cylindrical buoy 3 carrying eight riser pipes (or umbilicals) 10. An export riser 27 could be installed in a lazy S configuration away from the base 4.

As mentioned previously, part of the means to laterally move the lower connector across the seabed (i.e. the sled) could be attached to the lower connector before unreeling the riser from the construction vessel. Fig. 6 shows means of assisting orientation of the lower connector and integral sled as it approaches the base.

Orientation of the lower connector 11 with integral sled 7a (see Fig. 6) on the base 4 is assisted by channels or grooves 28 with define the correct direction to the subsea equipment 1 and connection point 17. A bump or slight rise 29 in each channel guide will assist in restraining movement of the nose 30 of the sled 7a along the channel selected as the support vessel 13 manoeuvres away from the production vessel 2. The sled 7a and riser 10 will then lay away from the subsea equipment to ensure that as extra flexible riser is unreeled at the surface, it takes up the correct position for pull-in of the sled and lower connector.

Unreeling the sled together with the lower connector from the surface allows later recovery of a faulty riser and installation of a replacement riser by the same method.

Claims

1. A flexible riser system for under sea oil or gas production, wherein means are provided to attach a part of a flexible riser pipe to a mid-water support positioned intermediately beween the sea bed and the surface such that during installation the weight of the lower portion of the riser pipe extending from the sea bed to the mid-water support is transferred to the said support before the upper portion of the riser pipe which extends towards the surface from the said support is laid over that support.

2. A riser system according to claim 1 wherein a lower portion of the riser pipe which extends downwardly from the said support passes under the said support.

3. A system according to claim 2 wherein the flexible riser pipe forms a connection leading from sub-sea production equipment on the sea bed, the flexible riser pipe has a lower connector at its lower end and means are provided for laterally moving the lower connector across the sea bed from a point on one side of a notional vertical projected downward from the mid-water support, thereby passing the lower connector under the support to a connection point of the said production equipment, which point is on the opposite side of the said notional vertical.

4. A system according to claim 1 wherein a lower section of the riser pipe which extends downwardly from the said support is attached to or is part of a line which extends away form the said support in an opposite direction to the upper portion of the said riser pipe.

5. A system according to claim 4 wherein the lower portion of the flexible riser pipe extends as a J-shaped catenary between the sea bed and the mid-water support.

6. A flexible riser system according to any one of the preceding claims wherein the said mid-water support is a buoy tethered by means of at least one tether to a base on the sea bed.

7. A system according to any one of the claims 1 to 5 wherein the mid-water support rests on the sea bed.

8. A system according to any one of the preceding claims wherein the upper portion of the flexible riser pipe which is laid over the mid-water support forms a U-shaped catenary between the mid-water support and the sea surface.

9. A system according to any one of the preceding claims wherein the mid-water support includes spaced apart projecting members to support a clamp attached around the flexible riser pipe with the lower portion of the riser pipe below the said clamp passing between said protruding members and with a portion of the riser pipe below the said clamp resting against the said mid-water support.

10. A method of laying a flexible riser system comprising attaching a part of a flexible riser pipe to a mid-water support to support the weight of the portion of the pipe below that support before laying the upper portion of the pipe over that support.

11. A method according to claim 10 wherein the lower end of the lower portion and the upper end of the upper portion are both positioned at one lateral side of a notional vertical through the mid-water support.

12. A method according to claim 11 which includes drawing the lower end across on the sea bed to the one lateral side from the other.