GB2109036A

GB2109036A - Riser tensioning system

Info

Publication number: GB2109036A
Application number: GB08230852A
Authority: GB
Inventors: Thomas Stewart Taylor
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1981-11-03
Filing date: 1982-10-28
Publication date: 1983-05-25
Also published as: GB2109036B

Abstract

A riser tensioning system for an offshore oil production system comprising a plurality of peripheral risers and a central riser 2, comprises one or more hydropneumatic accumulators 10, operatively connected to a plurality of hydraulic riser tensioners 21 to 24. The latter compensate for movement of the peripheral risers relative to the central riser. <IMAGE>

Description

SPECIFICATION Riser tensioning system This invention relates to a riser tensioning system, more particularly to a riser tensioning system suitable for a plurality of risers located round to a central riser and which are referred to as peripheral risers.

Previously the designs of heave tensioning system for non-integral marine production risers have been complicated and cumbersome especially around the moonpool area of a semi submersible floating production system. One previously proposed production riser tensioning system has individual tensioning wires to each peripheral riser with separate reeved tensioners for the central export riser.

It has been proposed to reduce the number of wire lines to the upper riser connector package by attaching secondary peripheral flowline tensioners to the upper riser connector package. These tensioners have a relatively small stroke, of the order of 1-2 feet and compensate for movements of the peripheral risers relative to the central export riser. Motions of the semi submersible relative to the production riser are taken up by conventional reeved wireline tensioners which have a very long wire line stroke of the order of 50-60 feet.

However, these improvements in production riser tensioning techniques still require individual hydropneumatic accumulators for each peripheral flowline tensioner which have to be attached to the upper riser connector package. Alternatively, individual hydraulic lines have to be hung off from the upper riser connector package and tied back to an accumulator bank positioned at the side of the moonpool.

It is an object of the present invention to provide a secondary tensioning system which will eliminate the need for individual tensioning lines to the peripheral risers and will further reduce the amount of secondary hydropneumatic accumulators.

According to the present invention a riser tensioning system for tensioning a plurality of risers comprises one or more hydropneumatic accumulators, each of which is operatively connected to a plurality of hydraulic riser tensioners.

Conveniently the operative connection is a line through which the hydraulic fluid is conducted.

Conveniently there is located in each line between the accumulator and the hydraulic riser tensioner, an isolation valve and optionally a velocity check valve, the latter being able in the event of hydraulic line breakage to close automatically to isolate the rest of the system.

The invention is illustrated by Figures 1, 2,3 and 4 ofthe accompanying drawings in which Figure 1 is a plan view of a typical production riser showing a central export riser2 and 16 peripheral risers, only two of which are numbered, 4 and 6, located around the export riser 2.

Figures 2 and 3 are schematic views showing one hydropneumatic accumulator (Figure 2) or two hyd ropneumatic accumulators (Figure 3) each operatively connected to a plurality of risertensioners.

Figure 4 is a partial elevation of the system described with reference to Figure 3, illustrating mechanical linkages instead of hydraulic connections.

Referring to Figure 2 four secondary peripheral tensioners 21,22,23 and 24 are connected by lines 14, 16, 18 and 20 to a single hydropneumatic accumulator 10. This accumulator only needs to be sized for one of the largest secondary flowline tensioners and to set the working pressure in the system. As the production riser bends, equal and opposite extensions/retractions will take place in the opposing peripheral risertensioners and theoretically no fluid displacement will take place in the hydropneumatic accumulator 10 no matter how many even number of peripheral risers there are.

Displacement of fluid in say peripheral flowline tensioner 21 will be taken up in cylinder 23 and similarly in all opposing tensioner cylinders with no fluid compression or expansion taking place in the hydropneumatic accumulator. Valves 26,27,28 and 29 are provided to isolate tensioners 21,22,23 or 24 if and when required.

The system shown in Figures 3 and 4 has in addition to the system shown in Figure 2 a secondary or back up accumulator 30, a velocity check valve 32,34,36 and 38 in each line and also a check reset valve 40,42,44 and 46 controlling each line. In the event of a hydraulic line breakage the velocity check valve will automatically close and isolate the rest of the system. In order to reset this check valve, the check reset valve should be opened to balance the pressure across the check which will allow it to open.

Cylinders 21 and 23 operate tensioning links 51 and 53 attached to peripheral riser guide funnels 55 and 57. Similarly cylinders 22 and 24 operate corresponding tensioning links, not shown.

The second accumulator is provided as a back up unit and to assist during running and pulling of the individual peripheral risers. This unit will allow the main accumulator unit to be recharged or maintained without having to close down the whole system. Access points for recharging and draining are provided to enable these maintenance opera tions to be carried out easily.

The advantages of the above described system are: (a) Reduction in the riser tensioner equipment accumulators and pipework when compared with previously proposed systems.

(b) Increase in reliability will result because there are fewer components. The hydropneumatic accu mulator will not be subjected to large pressure variations and will therefore be more reliable.

(c) Maintenance procedures and costs will be reduced because there are fewer components.

(d) Work-over operations will be made easier as the system is simple in operation.

(e) Appreciable cost savings on hardware and spares will be achieved when compared with the other systems described.

Claims

1. A riser tensioning system for tensioning a plurality of risers which system comprises one or more hydropneumatic accumulator(s) which or each which is operatively connected to a plurality of hydraulic riser tensioners.

2. A riser tensioning system according to claim 1 wherein the operative connection is a line through which hydraulic fluid is conducted.

3. A riser tensioning system according to either of the preceding cliamswherein an isolation valve is located in each line between the accumulator and the hydraulic riser tensioner.

4. A riser tensioning system according to any of the preceding claims wherein a velocity check valve is located in each line between the accumulator and the hydraulic riser tensioner.

5. A riser tensioning system as hereinbefore described with reference to the accompanying drawings.