GB2311496A

GB2311496A - Tailbuoy for towing in water behind a seismic survey vessel

Info

Publication number: GB2311496A
Application number: GB9606522A
Authority: GB
Inventors: Michael Souter
Original assignee: MICHAEL SOUTER
Current assignee: MICHAEL SOUTER
Priority date: 1996-03-28
Filing date: 1996-03-28
Publication date: 1997-10-01
Also published as: GB9606522D0

Description

IMPROVEMENTS IN OR BELATING TO TAILBUOYS This invention relates to tailbuoys.

Tailbuoys are buoyant structures used by seismic survey vessels. That is to say ships which tow seismic streamers used to record acoustic energy returning from the seabed, as a result of noise being directed towards the seabed in a controlled manner.

Tailbuoys are attached to the streamers so as to indicate streamer position to the towing ship. Monitoring of streamer position is important as it is essential that the streamers are kept separate and do not tangle with each other.

According to the present invention, a tailbuoy for towing in water behind a seismic survey vessel by means of a tow line, comprises a buoyant hull defining a fore to aft tunnel through which the water flows as the tailbuoy is towed, and means for adjusting the relative directions of tunnel water flow and the tow line, whereby lateral forces are exerted by water pressure on said hull.

The tow line may conveniently comprise a seismic streamer.

The adjustment means may comprise a pair of cables extending from the tailbuoy to the tow line in a converging manner, and cable adjustment means for varying the effective length of each cable.

The cable adjustment means may be radio-controlled, and may comprise linear actuators.

The cross-sectional area of the tunnel is preferably relatively large at the entry end of the tunnel and relatively small at the exit end of thereof.

The tailbuoy is preferably provided with at least one of:a) self-righting means, b) electrical power supply and/or generating means, c) radar beam reflector means, d) strobe light means, and e) water depth sensing means.

The tailbuoy is preferably of GRP (glass reinforced plastics) and KEVLAR construction.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a semi-diagrammatic plan view of a seismic survey vessel towing an array of seismic streamers, plus tailbuoys, Figure 2 is a side view of a tailbuoy, Figures 3 and 4 are respectively plan and front end views thereof, and Figure 5 is a fragmentary plan view of a tailbuoy tow.

With reference to Figure 1, a seismic survey vessel 1 is shown towing a plurality of seismic streamers 2a - 2d by way of cables 3 and paravanes 4 secured thereto. Towing speed is about 3.5 to 5.0 knots. The towing direction is indicated by arrow 5. The vessel 1 also tows, by way of the streamers 2a - 2d, tailbuoys 6, one tailbuoy 6 being connected to one streamer. The streamers may vary in length, the longest being of about 6000 metres.

Figures 2, 3 4 and 5 together illustrate a tailbuoy 6 which, as shown in Figure 5, is connected to the vessel 1 by a tow line, which comprises a seismic streamer 2. The water line of the tailbuoy 6 is indicated by reference numeral 7.

The tailbuoy 6, which is of GRP/KEVLARs construction, comprises a buoyant hull 10 defining a fore to aft, open-ended tunnel 11 disposed on the central longitudinal axis 30 of the hull. As shown in Figure 2, the cross-sectional area of the tunnel 11 is relatively large at the entry end 13 of the tunnel, and relatively small at the exit end 14 thereof. Tapering of the tunnel 11 in this manner provides directional stability and aids overall craft stability as well.

The hull 10 carries a pair of laterally-spaced bilge keels 15 adjacent its stern, and a single, central keel 16 adjacent its bow. When on the deck of the seismic vessel 1, the tailbuoy is supported by the keels 15, 16. See dotted line 17 of Figure 2, which represents the said deck.

The entry end 13 of the tunnel 11 is covered by a protective grill structure (not shown).

The hull 10 also carries a cabin 20 which supports a retractable mast assembly 21 carrying a white strobe light unit 22 and a radar beam reflector unit 23. Electric batteries are stowed in side hatches 24. The batteries provide electrical power for the strobe unit 22. The batteries are recharged by a paddle wheel generator 25, the lower part of which is disposed in the stern end of the tunnel 11. The generator 25 is rotatable about a laterallyextending axis 26. A hatch cover 27 provides access to the interior of the cabin 20. The output of the strobe light unit 22 is sufficient for it to be easily seen from the vessel 1.

The tailbuoy 6 is provided with self-righting means (not shown) and with water depth sensing means in the form of a retractable transducer or transponder unit 28 (Figure 2). The unit 28 is used to sense the position of any cable that may be present in the water below the tailbuoy 6.

The unit 28 and mast assembly 21 are lifted and lowered by associated hydraulic actuators (not shown), operable by radio control signals transmitted by the seismic vessel 1.

With reference to Figures 3 and 5, the tailbuoy 6 is also provided with means 40 for adjusting the relative (and opposed) directions of tunnel water flow, represented by arrow 41 and the tow line 2, represented by arrow 5, whereby lateral forces are exerted by water pressure on the hull 10. In other words, the means 40 are operable to adjust misalignment of the arrows 5 and 41 from the longitudinal centre line 30.

The means 40 comprise a pair of cables 42, 43 extending forwardly and in convergent manner, from points disposed equi-distantly on opposite sides of the longitudinal axis 30, to a towing connection 46 secured to streamer 2.

The rearward or aft ends of the cables 42, 43 are secured to cable adjustment means comprising linear actuators 48, 49 operable by a control unit 50. In turn, the control unit 50 is operable by GPS (Global Positioning System) signals transmitted by satellite and/or the seismic survey vessel 1.

In operation, should the tailbuoy 6 tend to deviate from a straight path while being towed, ie if arrows 5 and 41 (Figure 5) become misaligned, the deviation can be corrected by the control unit 50 operating actuator 48 or 49, so as to vary the effective length of the associated cable 42 or 43, whereby a restoring force is exerted by water pressure on the hull tunnel 11.

Thus if the effective length of cable 42 is reduced, the tailbuoy 6 will tend to veer to port, (as viewed in Figure 3), and if the effective length of the cable 43 is reduced instead, the tailbuoy 6 will tend to veer to starboard.

Deviation changes may also be detected visually and corrected by transmitting a radio signal to the control unit 50. For example, if the distance between the strobe lights of two adjacent tailbuoys, viewed from the vessel 1 changes substantially. Alternatively, or in addition, deviation changes may be detected by radar.

The tailbuoys 6 possess advantages evident when launched from the stern of the seismic survey vessel 1 as they can be controlled in such a manner, using their control units 50, so as not to be affected unduly by the wake of the vessel whereby collisions between tailbuoys could otherwise occur.

Claims

1. A tailbuoy for towing in water behind a seismic survey vessel by means of a tow line, comprising a buoyant hull defining a fore to aft tunnel through which the water flows as the tailbuoy is towed, and means for adjusting the relative directions of tunnel water flow and the tow line, whereby lateral forces are exerted by water pressure on said hull.

2. A tailbuoy as claimed in claim 1, wherein the tow line comprises a seismic streamer.

3. A tailbuoy as claimed in claim 1 or 2, wherein the adjustment means comprise a pair of cables extending from the tailbuoy to the tow line in a converging manner, and cable adjustment means for varying the effective length of each cable.

4. A tailbuoy as claimed in any one of claims 1 to 3, wherein the cable adjustment means comprise linear actuators.

5. A tailbuoy as claimed in claim 3 or 4, wherein the cable adjustment means are radio-controlled.

6. A tailbuoy as claimed in any one of claims 1 to 5, wherein the crosssectional area of the tunnel is relatively large at the entry end of the tunnel and relatively small at the exit end of thereof.

7. A tailbuoy as claimed in any one of claims 1 to 6, provided with at least one of:a) self-righting means, b) electrical power supply means, c) electrical/power generating means, d) radar beam reflector means, e) strobe light means, and f) water depth sensing means.

8. A tailbuoy as claimed in claim 7, which is provided with electrical power generating means, said means being operable by water flow through said tunnel.

9. A tailbuoy as claimed in claim 8, wherein said electrical power generating means comprise a paddle wheel generator.

10. A tailbuoy as claimed in any one of claims 1 to 9, which is of GRP (glass reinforced plastics) and KEVLAR construction.

11. A tailbuoy as claimed in any one of claims 1 to 10, wherein the cable adjustment means employ (GPS) Global Positioning System signals.

12. A tailbuoy, substantially as hereinbefore described, with reference to the accompanying drawings.

13. The combination of a seismic survey vessel, and a plurality of seismic streamers and tailbuoys towable thereby, each tailbuoy being of the formed claimed in any one of claims 1 to 12.