GB2117714A - Improvements in offshore mooring systems - Google Patents

Abstract

A single point mooring buoy of the kind having a rotatable top-side structure and a fixed lower structure with a telemetry system for transmitting data to a ship and controlling valves on the lower structure. The top-side platform 7 is mounted in bearings 4 on the fixed mast 15 carried by the lower part 1, and within the superstructure of the platform is provided a cavity 16 which is of toroidal form and closed by sliding rubber seals 17 which serve to provide some protection from sea-water. The aerials for a second telemetry system comprise firstly an annular lossy feeder 18 mounted on a ring-shaped support 19 carried by the lower structure 1 and connected with a transmitter/ receiver on the part 1 and a short antenna element 20 connected through feeder 21 to the transmitter/ receiver on platform 7. The feeder 18 lies around the bearing mast and the element is always within close proximity to same, regardless of the rotational position. <IMAGE>

Description

SPECIFICATION Improvements in offshore mooring systems This invention relates to improvements in offshore mooring systems, especially single point mooring systems, used as unloading or loading terminals for tankers.

Single point mooring buoys or towers are known, these having a turntable or rotatable arm structure to which a ship is secured by a hawser, this forming a single attachment whereby the ship may be moored by its bow in such a manner that it may freely swing around the buoy or tower. Product transfer pipes also connect with the ship from the turntable or arm and a complex coupling is used in the turntable bearing assembly of the buoy or tower to pass the product through to the main body of the bouy or tower from the ship. It is also known to provide instrumentation on the buoy or tower by way of a telemetry system to provide monitoring of the mooring loads and other functions as well as control of product valves by using simplex, semi-duplex or duplex transmission systems.

Generally radio telemetry is used for digital transmission of coded signals in both directions.

Radio telemetry systems may be used for control and monitoring of a wide range of functions on the buoy or tower structure but in practice problems occur in the transmission of data between the rotatable turntable or arm and the fixed structure of the buoy or tower.

There are a number of arrangements known whereby electrical signals may be passed between fixed rotating structures but such systems, e.g. slip rings, prove undesirable because of electrical noise generated, difficult maintenance in the conditions prevailing and unreliability. It is also difficult if not impossible to rectro-fit slip ring assemblies.

It is an object of this invention to provide a buoy or tower incorporating a data transmission system which may reliably control onbuoy or tower operations and monitor loads and systems status at various parts of the structure.

According to this invention there is provided an offshore single point mooring buoy or tower incorporating a rotatable structure, a fixed structure and a telemetry system for transmission of data between the buoy or tower and a remote point, wherein the telemetry system includes means for transmission of data between the fixed and rotatable structures, said means being located about the bearing assembly carrying the rotatable structure and being a 'wireless' transmission system.

The wireless transmission system serves to provide coupling across the gap between the fixed and rotatable structures of the buoy or tower. The transmission system will preferably be a radio system with respective radiation emitting and receiving aerials located on the fixed structure and the rotatable structure and arranged to be maintained in effective communication for any position of rotation. Means other than radio may be used such as inductive or optical systems. When a radio system is used the two aerials, one on the fixed structure and one on the rotatable structure, are configured so that one aerial or aerial system provides a reasonably uniform field about the bearing with the other aerial or aerial system being constrained to move within the uniform field.Preferably one aerial comprises a loop lying in the rotational plane of the top-side rotatable structure the other aerial being a pick-up probe arranged to move in close proximity to the loop.

The loop aerial may comprise a lossy feeder system and this arrangement provides substantial immunity to sea-water penetration in the vicinity of the aerials as well as avoiding substantial standing waves in the cavity containing the aerials.

The invention is applicable to buoys having a top-side rotatable structure and a lower side fixed structure or arrangements having a floating hull which is rotatable and mounted on a fixed shaft moored in position.

In a preferred embodiment of the invention the loop aerial is positioned within an annular or toroidal cavity provided between the rotatable and fixed parts of the buoy or tower structure.

One of the structures preferably has two or more aerials each being associated with a respective signal demodulator the outputs of which are compared the output having the least error at any given instant being selected as the signal transmission path. The two aerials may comprise stub aerials or pick-up probes. Where the data transmission is digital then the outputs from the two aerials can be monitored and the error determined by geometric parity detection. Other methods are possible for determining whch aerial or channel has the least error.

The telemetry system will normaliy be located on the rotatable structure and provides duplex transmission whereby various parameters may be monitored and sent to the remote location and control of valves on the buoy or tower may be effected from the remote location. Such valves can be on the lower part of the buoy or tower or operatively coupled therewith and controlled through a second duplex data transmission system linked through the aerials.

Further and preferred features of this invention are described in conjunction with the accompanying drawings showing an embodiment by way of illustrative example only. In the drawings:- Figure 1 shows a plan view of a buoy structure, Figure 2 shows a section on A-A of Fig. 1, Figure 3 shows a detail view of the on-buoy transmission means, and Figure 4 shows a schematic view of the telemetry transmission system overall.

Referring to Figs. 1 and 2 these show a single point mooring buoy for offshore use by a tanker for loading or discharging an oil cargo. The buoy comprises a lower structure 1 which is secured to the sea-bed through chains at 2 and which has provision for transporting oil through piping 3 to the sea-bed via flexible pipes. The piping system includes valves and in addition sea-bed valves are provided. These parts are not shown nor described in detail as they form part of the known art.

Centrally disposed on the structure 1 is a bearing assembly 4 which provides for communication between the piping 3 of the lower structure and piping 5 of a top-side structure 6 which is rotatably mounted on the bearing.

The top structure 6 comprises a turntable platform 7 with an arm 8 proving connection for a hawser and an arm 9 providing connection for the product pipes 10 to the moored vessel.

The platform also includes a control station 11. The mooring is effected through load sensors 1 2 the output signals of which are fed to a radio telemetry system housed at station 11 and by which data on loads is transmitted to shore and also the ship's bridge for monitoring. A number of on-buoy systems may also be monitored and operated, for example the status of the various valves and their control, such as the valves 1 3. It is also desirable to effect similar control and monitoring in respect of the lower structure 1 and sea-bed systems but to effect this it is necessary to provide connection across the bearing assembly 4 whilst still allowing 360 degree unimpeded rotation of the platform.

In this invention this is achieved by a second radio telemetry system arranged to pass signals between the platform 7 and lower structure 1 and positioned in the vicinity of the bearing at 14. Around the bearing a toroidal cavity exists and into which the aerials of the second telemetry system may be located. One specific arrangement is shown in Fig. 3 as example.

Referring to Fig. 3 the platform 7 is mounted in bearings 4 on the fixed mast 1 5 carried by the lower part 1 and within the superstructure of the platform is provided a cavity 1 6 which is of toroidal form and closed by sliding rubber seals 1 7 which serve to provide some protection from sea-water. The aerials for the second telemetry system comprise firstly an annular lossy feeder 1 8 mounted on a ring-shaped support 1 9 carried by the lower structure 1 and connected with a transmitter/receiver on the part 1 and a short antenna element 20 connected through feeder 21 to the transmitter/receiver in the station 11.The feeder 1 8 lies around the bearing mast 1 5 and the element is always within close proximity to same regardless of the rotational position. The lossy feeder is preferred as the radiation characteristics are largely unaffected by environmental conditions other forms of transmission coupling can, however, be used. The close proximity requires only small transmitter powers to be used typically in the milliwatt range whilst maintaining reliable communication and the field strength at any point is largely unaffected by standing waves. The feeder 1 8 can be terminated or left open. Each buoy part is provided with a separate battery power supply.

Fig. 4 shows schematically the telemetry system comprising a transmitter/receiver 40 at station 1 2 operating in duplex mode with a remote station, e.g. shore based. The frequency employed is at U.H.F. and encoding/ decoding is effected at a terminal unit 41 again at the station 1 2 on the platform part.

This topside installation is supplied from battery 42. The received signal is used to modulate a second transmitter/receiver 43 which is coupled with the aerial 20. This duplex signal is passed through the element 20 and feeder aerial 1 8 to a transmitter/receiver 44 in the lower part 1 which is associated with the encoding/decoding terminal unit 45 through which valves 48 may be operated by command system 46 and signal inputs reporting status may be fed-back through unit 47. A separate battery 49 is used to power the system in the lower buoy part 1.

The encoded signals are transmitted back to transmitter/receiver 43 via transmitter/receiver 44 and subsequently decoded for impression onto the transmitted signal from 40.

This arrangement ensures that reliable control and monitoring is provided at all times and all conditions between the upper station and hence the shore base and the lower structure of the buoy.

Data is preferably transmitted as F.S.K. on the carrier frequencies with the bandwidth being some 3 KHz.

Remote control of valve systems or other devices is generally effected by a hydraulic power pack energised from compressed air or gas stored in cylinders. One power pack would be positioned within-the hull of the buoy and the other on the turntable structure.

The telemetry system described functions by closing a relay on command which in turn controls the state of a solenoid valve admittirig or venting high pressure hydraulic fluid from the appropriate control line.

In a second arrangement (not illustrated) multiple null points in the radiation field pattern from the aerials can be reduced even further than that provided by the aforementioned system of leaky feeder and stub aerial.

In this second arrangement two stub aerials are used instead of, or in conjunction with a loop, with each stub aerial being connected with a separate demodulator the digital signals outputs from which are compared and the error rates monitored using for example geometric parity detection methods. If the error rate at the particular aerial being used rises above that of the other aerial then transmission is automatically switched to the second path. This system has been found to avoid problems which might arise from the field distribution producing deep nulls which could result in temporary errors in the transmitted data. The two aerials are arranged so that neither has coincidence or null points within the radiation field in the cavity.

Claims (16)

1. An offshore single point mooring buoy or tower incorporating a rotatable structure, a fixed structure and a telemetry system for transmission of data between the buoy or tower and a remote point, wherein the telemetry system includes means for transmission of data between the fixed and rotatable structures, said means being located about the bearing assembly carrying the rotatable structure and being a 'wireless' transmission system.

2. A buoy in accordance with Claim 1, wherein the transmission system comprises a radio transmission system.

3. A buoy in accordance with Claim 1, wherein the transmission system comprises an inductive system.

4. A buoy in accordance with Claim 1, wherein the transmission system comprises an optical system.

5. A buoy in accordance with Claim 1 or 2, wherein the transmission system is a radio system with respective radiation emitting and receiving aerials located on the fixed structure and the rotatable structure and arranged to be maintained in effective communication for any position of rotation.

6. A buoy in accordance with Claim 5, wherein the two aerials, one on the fixed structure and one on the rotatable structure, are configured so that one aerial or aerial system provides a reasonably uniform field about the bearing with the other aerial or aerial system being constrained to move within the uniform field.

7. A buoy in accordance with Claim 5 or 6, wherein one aerial comprises a loop lying in the rotational plane of the top-side rotatable structure, the other aerial being a pick-up probe arranged to move in close proximity to the loop.

8. A buoy in accordance with Claim 7, wherein the loop aerial comprises a lossy feeder.

9. A buoy in accordance with Claim 7 or 8, wherein the loop aerial is positioned within an annular or a toroidal cavity provided between the rotatable and fixed parts of the buoy or tower structure.

10. A buoy in accordance with any preceding Claim 5 to 9, wherein one of the structures carries two or more earials each being associated with a respective signal demodulator the outputs of which are compared.

11. A buoy in accordance with Claim 10, wherein the output having least error at any one time is selected as the signal transmission path.

1 2. A buoy in accordance with Claim 10 or 11, wherein the two aerials comprise stub aerials or pick-up probes.

1 3. A buoy in accordance with any preceding Claim 10 to 12, wherein the transmission system is digital the outputs of the two aerials being demodulated and the error rate monitored using a geometric parity detection method.

14. A buoy in accordance with any preceding Claim, wherein the telemetry system is located on the rotatable structure and provides duplex, semi-duplex or simplex transmission, various parameters being monitored and sent to a remote location and control of valves on the buoy or tower being effected from the remote location.

1 5. A buoy in accordance with Claim 14, wherein the valves are on the lower part of the bouy or tower or operatively coupled therewith, and controlled through a second duplex, semi-duplex or simplex data transmission system linked through the aerials.

16. An offshore single point mooring buoy system substantially as described herein and exemplified and with reference to the drawings.

1 7. A method of transmission of telemetry data using a buoy constructed and arranged to operate in accordance with any preceding Claim.