GB2246104A

GB2246104A - Support buoy

Info

Publication number: GB2246104A
Application number: GB9114976A
Authority: GB
Inventors: George Henry Vernon Owen
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-07-17
Filing date: 1991-07-10
Publication date: 1992-01-22
Anticipated expiration: 2011-07-10
Also published as: GB2246104B; GB9114976D0; GB9015713D0

Abstract

A buoy for supporting a payload comprising buoyancy (101) for supporting the payload e.g. of seismic equipment, a submerged propeller propulsion unit (105) for propelling the buoy in a determined direction and tethering points (113, 117) on the buoyancy via which the buoy can be tethered against the propulsive force of the propulsion means. The buoyancy may be of annular or of dish form (as shown) and the propulsion unit comprise a remotely operated hydraulic motor operable via an umbilical 109. A crane secured to the craft from which the buoy is deployed suspends the umbilical. A pivoted wing 116 beneath the unit urges the buoy to tauten tethering bridles 118, 119. <IMAGE>

Description

Support Buoy The present invention relates to a support buoy, particularly though not exclusively for supporting a seismic source.

Offshore seismic surveying is mainly carried out in two ways - using the "surface" and "borehole" methods. In the surface method, a streamer of hydrophones is towed astern of a seismic recording vessel for detecting reflections from the the strata below the seabed, which reflections are of signals originating from seismic sources streamed off the quarters of the vessel. In some varieties of the borehole method, a geophone is lowered down a borehole to receive signals from the source positioned by a vessel.

Potentially this latter form of surveying is the more accurate, because there are less variables. The position of the geophone with respect to the strata of interest is precisely known, in that the geographic position of the well, its borehole's deviation from the vertical and the distance down the borehole to which the geophone is lowered are all accurately known. Therefore it is important that the seismic source's position also be accurately known. For this an accurate knowledge of the position of the seismic source relative to the vessel is required. Such knowledge is easier if the source is close to the vessel. However, positioning the source too close to the vessel damages the vessel, due to the repeated discharge of the source. The source - of its nature - creates high energy acoustic waves.

Streaming of the source at a specified distance behind the vessel cannot be relied upon, because the vessel may need to be static, whilst the geophone is adjusted in its position down the borehole.

The object of the invention is to provide a support buoy for a seismic source, which can be deployed at a determined position relative to the vessel.

A buoy for supporting a payload according to the invention comprises buoyancy for supporting the payload, a submerged propeller propulsion unit for propelling the buoy in a determined direction and at least one tethering point on the buoyancy via which the buoy can be tethered against the propulsive force of the propulsion means.

In use, the buoy is tethered to a deployment craft by lines included in the tethering means, with the propulsion means propelling the buoy away from the vessel so that the lines are taut. If as is usual at least two lines of known length - one to the vessel's bow and the other to the vessel's stern or a point aft of midships - are used, the buoy will take up a known position with respect to the vessel. Then provided the vessel's position and heading are accurately known with respect to a reference, the position of the buoy and its supported payload - usually a seismic source - will also be accurately known.

In development of the invention, water jet propulsion of the buoy was tested. It was found that in order to provide sufficient power to maintain the buoy's position with respect to the deployment craft during walkaway surveys, such powerful jet propulsion was required that there was a danger of the buoy being projected through the air should it be thrown clear of the sea by a wave for instance. This risk is not present with propeller drive of the buoy, because if the buoy should be thrown in the air, the propeller's propulsive effect is destroyed. This renders the propeller drive inherently more safe. Further, in order to provide a sufficient water flow to water jets, a very large umbilical would be required, with its attendant weight, drag and cost problems.

Preferably, the tethering means includes a forward tethering point, an aft tethering point, at least one bridle line attached to the forward tethering point and at least one bridle line attached to the aft tethering point, the forward and aft tethering points being spaced to determine the orientation of the buoyancy. A further, common tethering point, a further forward bridle line and a further aft bridle line may be included, the respective forward and aft bridle lines being connected together and continued as one to be made fast at their ends remote from the buoy at forward and aft positions, the two further bridle lines being connected to the buoyancy at the common tethering point. The common tethering point is preferably beneath the propulsion unit, which is conveniently mounted below the buoyancy.

The propulsion means may comprise a single fixed direction propulsion unit or a single variable direction unit or indeed a pair of fixed direction units. Preferably the propulsion unit comprises a remotely powered motor and a propeller. The propeller may be shrouded. The motor is preferably hydraulically powered. For this, the buoy can include an umbilical for conveying pressure hydraulic fluid to the motor. Conveniently, the umbilical is reinforced for lifting of the buoy. In the preferred embodiment, the umbilical passes through a central aperture in the buoyancy to the motor. The central aperture has a curved surface having a radius of curvature no less than the minimum bend radius of the umbilical.

Preferably the buoyancy is circular in plan and is provided with a peripheral elastomeric fender.

In accordance with an important optional feature, the buoy includes means for hydrodynamically reacting water flow past the buoy to urge the buoy in use in a direction to tauten tethering line(s) attached to the tethering point(s).

Preferably, the hydrodynamic reaction means is a wing both adapted to feather to current of a direction tending to slacken the tethering lines(s) and adapted with the buoyancy to be stopped from feathering to current tending to tauten the tethering line(s). The wing can be pivoted for feathering about an axis extending generally vertically in the buoy when deployed. Preferably, the wing is so stopped from feathering to a tautening current as to be able to lie to either side of a central line of thrust of the propulsion unit. Conveniently, the angular freedom of the wing is restricted to less than 1800, whereby an appreciably tautening effect is experienced by the wing for currents substantially at right angles to the line of thrust of the propulsion unit. The wing can be arranged beneath the propulsion unit.

Normally the buoy will be in combination with a crane adapted to be secured on a craft from which the buoy is to be deployed and to suspend the umbilical substantially throughout its extent from the remote end of a jib of the crane to the buoy. The remote end of the jib may be provided with a fairlead surface for the umbilical having a radius of curvature no less than the minimum bend radius of the umbilical. In the preferred embodiment, the crane has a skid for deck mounting and an umbilical winch mounted on the skid and having hydraulic line connections for supplying hydraulic fluid to the umbilical. The skid is provided with ballast tanks able to be flooded for securing the crane to the deck of the craft.

The buoy may include a winch for supporting the seismic source at a determined depth below itself.

To help understanding of the invention, two specific embodiments thereof will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a perspective view of a support buoy of the invention; Figure 2 is a plan view of a modified buoy deployed in use; Figure 3 is a diagrammatic cross-sectional view of another support buoy of the invention; Figure 4 is a diagrammatic underneath view of the buoy of Figure 3; Figure 5 is a diagrammatic plan view of a craft having the buoy of Figure 3 deployed from it; and Figure 6 is a diagrammatic athwartships view of the craft shown in Figure 5.

Referring first to Figure 1, the buoy B comprises a primary floatation chamber 1, suitably of galvanised plate steel, to which an upper replaceable ring 1' (shown broken away) is attached to match the buoyancy of the buoy to the weight of the payload 2. A winch 3 for supporting the payload 2 is provided on the chamber 1. Beneath the chamber 1, a pair of thrusters 4,5 are provided. They are set at right angles to each other. At the centre line of each thruster tethering points 6,7 are provided, fixed to the thrusters' housings. Lines 8,9 are secured to the points 6,7 for tethering the buoy. Positioning of the points 6,7 on the line of action of the thrusters avoids any resultant capsizing moment from relative displacement of the axes of action of the thrusters 4,5 and the tethering lines 8,9.

One of the lines supports an umbilical 10 providing electrical power to a hydraulic power pack 11 for power the thrusters 4,5 and the winch 3. In Figure 1 the payload 2 is a seismic source supported on a line 13 from the winch 3.

As additionally shown in Figure 1, the buoy has a hydraulic oil tank 14 and a lifting eye 15 for launch of the buoy.

Referring next to Figure 2, the buoy B is shown deployed from a vessel V with the tethering lines 8,9 secured to the bow and stern of the vessel. With the thrusters acting approximately along the lines, which are of known length, they are kept taut. Thus the position of the buoy is determined with respect to the vessel by the geometry of the triangle formed by the lines and the length of the vessel, that is to say the buoy is at a fixed position and distance D from the side of the vessel V.

The vessel may be steaming at 5 knots over the sea bed against a 3 knot tide. To aid the thrusters in holding the lines taut, the buoy is fitted with an asymmetric scoop 21, facing forwards on the side of the buoy away from the tethering points 6,7. Water flowing into the scoop tends to move the buoy away from the vessel. The relative spacing in the buoy of the tethering points resists twisting of the buoy by the action of the scoop.

Referring now to Figures 3 to 6, a second buoy in accordance with the invention will now be described. It comprises a circular floatation dish 101, which has a shallow aspect ratio. This shape provides even resistance to tidal flow from any quarter. The dish is of glass reinforced plastics material with a plastics foam core. It has a peripheral, elastomeric fender 102 and a flared, central aperture 103. Bolted to a reinforced region 104 of the underside of the dish across the aperture 103 is a propulsion unit 105. This has a hydraulic motor housed in a casing 106 and driving a shrouded propeller 107. The unit is arranged to draw water from the housing side of the propeller 107 and displace it in the general direction of the axis 108 of the propeller, that is to say the line of thrust of the unit.An umbilical 109 for supplying and withdrawing hydraulic oil under pressure to and from the propulsion unit passes through the central aperture 103.

The flared upper surface 110 of the aperture has a radius of curvature which is at least as great as the minimum bend radius of the umbilical. The umbilical is reinforced and made fast to the propulsion unit whereby the entire buoy can be lifted by the umbilical.

Beneath the propulsion unit is fixed a stop plate 111 and a post 112. The latter extends down to a payload attachment ring 113. The stop plate 111 is triangulated by three struts 114 extending to the plate from the reinforced region 104 of the dish. The stop plate extends laterally with respect to the axis 108 and has two stops 115 extending down from its underside. Mounted for free pivoting on the post 112 is a wing 116. It is free to pivot between the stops, that is to say 650 on either side of the axis 108.

The dish is equipped with two upper bridle attachment or tethering rings 117 at its periphery at the level of the elastomeric fender 102. These are positioned at approximately 650 to either side of the axis 108 and are referred to as the forward and aft rings, with reference to forward and aft bridles 118,119 attached in use to forward and aft positions 120,121 on a craft from which the buoy is deployed. Each bridle is in the form of a Y, having upper "arm" portions 118Ull9u lower "arm" portions 1181,1191 and remote legs 118r'119r The free ends the upper arm portions 118u,ll9u are connected to the forward and aft rings 117.

The free ends of the lower arm portions 1181,119l are connected to the payload support ring 113. The free ends of the remote legs 118r'119r are connected to forward and aft positions 120,121 on the craft C from which the buoy is deployed. This arrangement operates such that when the propulsion unit is working, the buoy is thrust from the craft until the bridles are taut in the position as shown in Figure 5. Since the propulsion unit is positioned both vertically and horizontally between the rings 113,117, they and the length of the bridles' arms determine the orientation of the float, namely with the post upright.

When a tidal current is flowing from the buoy to the craft, the wing 116 feathers to the current. When the current is flowing from the craft to the buoy, the wing is held against one of the stops 115. When the craft is steaming ahead, the current will be across the axis 108.

The wing will stop against the after stop 115. The angle of incidence of the current on the wing is approximately 1550, with the result that the current's reaction on the wing urges it and the buoy away from the craft. This is of particular significance in enabling the propulsion unit to be less powerful than it would otherwise have to be to maintain the bridles taut at all times.

Referring in more detail to Figures 5 and 6, the buoy is deployed from the craft C via crane 130. The crane has a jib 131 adjustably supported on a skid 131. The skid carries ballast tanks 132, which when filled with water secure the skid in place on the deck of the craft. The skid carries a winch 133 for the umbilical 109, the winch being equipped with rotating connections allowing the supply of pressure fluid to the umbilical from a pump separately mounted on the deck. At the end of the jib is provided a fairlead 134 having a radius of curvature not less than the minimum bend radius of the umbilical.

Not only does the crane enable the buoy to be deployed from the craft, but also it holds the umbilical clear of the sea for most of its length between the crane and the buoy.

Figure 6 shows a payload 140 supported via the ring 113.

Claims

CLAIMS:

1. A buoy for supporting a payload comprising buoyancy for supporting the payload, a submerged propeller propulsion unit for propelling the buoy in a determined direction and at least one tethering point on the buoyancy via which the buoy can be tethered against the propulsive force of the propulsion means.

2. A buoy as claimed in claim 1, including a forward tethering point, an aft tethering point, at least one bridle line attached to the forward tethering point and at least one bridle line attached to the aft tethering point, the forward and aft tethering points being spaced to determine the orientation of the buoyancy.

3. A buoy as claimed in claim 2, including a further, common tethering point, a further forward bridle line and a further aft bridle line, the respective forward and aft bridle lines being connected together and continued as one to be made fast at their ends remote from the buoy at forward and aft positions, the two further bridle lines being connected to the buoyancy at the common tethering point.

4. A buoy as claimed in claim 3, wherein the common tethering point is beneath the propulsion unit.

5. A buoy as claimed in any preceding claim, wherein the propulsion unit is mounted below the buoyancy.

6. A buoy as claimed in any preceding claim, wherein the propulsion unit comprises a remotely powered motor and a propeller.

7. A buoy as claimed in claim 6, wherein the propeller is shrouded.

8. A buoy as claimed in claim 6 or claim 7, wherein the motor is hydraulically powered.

9. A buoy as claimed in claim 8, including an umbilical for conveying pressure hydraulic fluid to the motor.

10. A buoy as claimed in claim 9, wherein the umbilical is reinforced for lifting of the buoy.

11. A buoy as claimed in claim 9 or claim 10, wherein the umbilical passes through a central aperture in the buoyancy to the motor.

12. A buoy as claimed in claim 11, wherein the central aperture in the buoyancy has a curved surface having a radius of curvature no less than the minimum bend radius of the umbilical.

13. A buoy as claimed in any preceding claim, wherein the buoyancy is circular in plan.

14. A buoy as claimed in any preceding claim, wherein the buoyancy is provided with a peripheral elastomeric fender.

15. A buoy as claimed in any preceding claim, including means for hydrodynamically reacting water flow past the buoy in use to urge the buoy in a direction to tauten tethering line(s) attached to the tethering point(s).

16. A buoy as claimed in claim 15, wherein the hydrodynamic reaction means is a wing both adapted to feather to current Of a direction tending to slacken the tethering lines(s) and adapted with the buoyancy to be stopped from feathering to current tending to tauten the tethering line(s).

17. A buoy as claimed in claim 16, wherein the wing is pivoted for feathering about an axis extending generally vertically in the buoy when deployed.

18. A buoy as claimed in claim 17, wherein the wing is so stopped from feathering to a tautening current as to be able to lie to either side of a central line of thrust of the propulsion unit.

19. A buoy as claimed in claim 18, wherein the angular freedom of the wing is restricted to less than 1800, whereby an appreciably tautening effect is experienced by the wing for currents substantially at right angles to the line of thrust of the propulsion unit.

20. A buoy as claimed in claim 17, claim 18 or claim 19, wherein the wing is arranged beneath the propulsion unit.

21. A buoy as claimed in any preceding claim, including a winch for supporting the seismic source at a determined depth below itself.

22. A buoy as claimed in claim 9 or anyone of claims 10 to 21 as appendant to claim 9, in combination with a crane adapted to be secured on a craft from which the buoy is to be deployed and to suspend the umbilical substantially throughout its extent from the remote end of a jib of the crane to the buoy.

23. The combination as claimed in claim 22, wherein the remote end of the jib is provided with a fairlead surface for the umbilical having a radius of curvature no less than the minimum bend radius of the umbilical.

24. The combination as claimed in claim 22 or claim 23, wherein the crane has a skid for deck mounting and an umbilical winch mounted on the skid and having hydraulic line connections for supplying hydraulic fluid to the umbilical.

25. The combination as claimed in claim 24, wherein the skid is provided with ballast tanks able to be flooded for securing the crane to the deck of the craft.

26. A support buoy substantially as hereinbefore described with reference to the accompanying drawings.

27. The combination of the support buoy and crane substantially as hereinbefore described with reference to the accompanying drawings.