GB2109311A - Work platform - Google Patents

Abstract

A stable offshore platform 10 for surface or undersea work is provided by connecting a semi-submersible trailer unit with a workboat. Preferably, two columns 14 of the trailer are joined by a beam member 16 or truss, and this assembly is linked to the workboat via a truss-like trailer tongue 17 with a ball swivel joint 19. When in transit, the semi-submersible unit is de-ballasted to a shallow draft condition and towed by the workboat. The columns each have a pontoon 11, the pontoons optimally having a ballast connection between them. <IMAGE>

Description

SPECIFICATION Work platform Many thousands of vessels form a fleet of marine transportation boats serving companies working in offshore waters throughout the world. The vast majority of these vessels support offshore oil and gas exploration and production activities. Most of these vessels are classed as tugboats, supply boats, or crew boats, and have displacement type hulls. A common problem with such hulls is that it is difficult for men to perform useful work from the decks thereof due to waves which wet the decks and which cause excessive roll and pitch motions of the vessels. Weather in the Gulf of Mexico is occasionally severe enough to hinder or delay operations, but weather in the North Sea is so severe as to have a significant impact on operations from such vessels.

Transfer of men and equipment to offshore platforms, handling of anchor buoys, and deployment of men and equipment subsea are examples of operations wherein a safe and stable work platform is needed.

A potential solution to this problem is to build and use larger vessels. This slightly improves the situation, but at a significantly increased cost, and generally it is not considered an economically feasible approach.

Another potential solution is to build a fleet of small to medium sized, column-stabilized, semisubmersible vessels like the larger semisubmersible vessels used for drilling operations.

These are stable work platforms having long natural periods of response in the order of 20 to 40 seconds.

By contrast, a 60 m supply boat has natural periods of from 6 to 9 seconds, which is quite close to the period range of highest wave energy. While such semi-submersibles would significantly reduce waveinduced motions and deck wetness, it would be at an excessive cost of retiring the conventional fleet and building a new and more expensive fleet.

The present invention generally pertains to a work platform including at least two connected semisubmersible flotation means, means for pivoting the flotation means, and connecting means extending between the flotation means and the pivoting means.

In a preferred embodiment the flotation means is two connected pontoons, and the pivoting means is a ball swivel joint.

More generally, the present invention pertains to a stable offshore platform for surface or subsea work which is a semi-submersible unit attachable to a workboat or other support means. Pontoons of the semi-submersible unit are connected to the work boat or other support means via a gooseneck truss, trailer tongue or the like, and a ball swivel joint or the equivalent. When not in use, or in transit, the semi-submersible unit may be deballasted to ride high in the water and permit entrance to shallow harbours, or it may be hoisted over the workboat or other vessel or support means.

The apparatus according to the invention will be described with reference to the drawings, wherein: Figure 1 discloses a side view of an embodiment of the apparatus according to the invention.

Figure 2 provides a plan view of the embodiment of Figure 1.

Figure 3 provides a view from astern, of the embodiment of Figure 1.

Figure 4 discloses a modification of the embodiment of Figure 3.

The present invention provides a new solution to problems of offshore handling and transfer operations in rough weather. Generally, the invention is a semi-submersible, gooseneck style trailer/work platform which is towable by a conventional vessel such as a supply boat, or by other means. The trailer/work platform is pivotable from a raised or hoisted transit mode to a lowered or partially submerged working mode. With the floating semi-submersible trailer work platform, workmen can safely climb aboard from a supply boat, rig or other vessel and do useful work on a stable, dry deck. Further, using the present invention, men and equipment can be easily and safely transferred subsea or upward to an offshore platform or larger floating vessel.

As shown in the drawings the semi-submersible trailer/work platform 10 includes pontoons 11 and 12 which may be in a lowered position for work use or in a partially raised position (as shown in phantom) for transit. Alternatively, the pontoons 11 and 12 may be hoisted over a vessel 21 or other floating support means. Pontoons 11 and 12 are at the ends oftwovertical columns 13 and 14which in turn are connected by slanted cross beams 15 and 16 which abut pivoting framework 17. Avertical beam 18 extends downwardly from the opposite end of framework 17 to a ball swivel joint 19, or the like, which in turn can be moved forward or aft, or even released aft in an emergency. If the trailer/work platform 10 is released, buoyancy of the beam 18 provides sufficient flotation to prevent the trailer/ work platform 10 from overturning.The buoyancy may be obtained by means of a water-tight compartment 9 tankage, foam, or any other suitable means.

For best motion response with platform 10 in a working mode, ball swivel joint 19 is located at the centre of pitch of the vessel 21 or other floating support means to which it is attached. For storm survival, ball swivel joint 19 can be moved aftto minimize the risk of the underside of framework 17 contacting the stern of vessel 21 while the vessel is pitching in severe waves. Alternatively, or in addition, the trailor/work platform 10 can be deballasted to a light condition as shown in phantom in Figure 1 for storm survival. Low tension tieback ropes, chains or cables 22 and 23 are used to ensure that the trailer/work platform 10 does not jackknife into contact with the vessel 21 or other floating support means.

The trailer/work platform 10 can take many forms, depending on the specific application. Thus, it can have an aft deck 24 to support workmen or it can be used for off-loading to platforms. Crane 25 may be used for off-loading to platforms, etc. not having cranes, but it generally is preferred to use other cranes when available. The trailer/work platform 10 can have various other lifting means such as gantry hoist 28 for lifting loads from supply vessel deck 26 to work platform 24 or to move equipment and/or personnel subsea. Also, the trailer/work platform 10 can be used for submarine operations by having special launch/recovery equipment 29. Although submarines can be launched from the platform in various ways, it is preferred to launch from a duct 32 between pontoons 11 and 12.This facilitates getting the submarine through the wave/air interface, hence the cylindrical duct 32 to guide the submarine in and out of the water. Alternatively, an elevator may be used to lift and lower the submarine. Although it is prudent to keep the trailer/work platform 10 simple, thrusters 30 can be placed in pontoons 11 and 12 and extended downward for operation to aid in offloading operations, then retracted when not in use.

Just like other semi-submersibles, the trailer/work platform 10 can be towed out in a shallow draft, low ballast condition and then be ballasted down at the work site to perform required work. By towing the trailer/work platform 10 in a lightened condition, the reduction in tow vessel speed due to the trailer is normally small, and shallow harbours can be used.

In the lighted mode, the trailer/work platform 10 has the performance of a displacement type catamaran.

Once the trailer/work platform is ballasted down with equipment weight and/or by water 31 in the pontoons 11 and 12, the favourable long period semi-submersible characteristics above mentioned appear.

Afloodable chamber 33 may be attached to pontoons 11 and 12 by beams 34 and 35 and hinges 36 in order to provide added stability to the semisubmersible. The amount of ballast in chamber 33 is adjustable by changing water level 37. This chamber provides more righting moment in roll of the semi-submersible. Alternatively, a dead weight 50 may be suspended by cables 51 and 52 from pontoons 11 and 12 as shown in Figure 4.

The present invention was tested at 1/50th scale in a wave tank, using a simulated 2000-ton prototype supply vessel and a simulated 350-ton trailer/work platform, the trailer/work platform was calm and stable in 12 and 25 feet (3.7 and 7.7 m) waves with wave lengths 20 times the wave height while the bow moored vessel was excessively pitching and rolling. Natural periods for prototype roll and heave for the trailer/work platform were in the 15 to 35 second range. For this combination of trailer/work platform and vessel, useful work could be done in wave heights to about 20 feet (6.2 m). The vessel/ trailer/work platform combination tracked well under tow in both light and heavy ballast conditions. Based on estimations, light ballast condition would only reduce the normal vessel transit speed by no more than one knot.

Discussion of the prior art: U.S. patent No. 3,323,478 discloses a method for joining two vessels of conventional shape using spreader bars and tie lines, with a truss work between the vessels supporting a drilling rig. The joined vessels are like a hinged catamaran and not like a semi-submersible vessel as used with the present invention and as shown in The Technology of Offshore Drilling, Completion and Production by ETA Offshore Seminars, Inc., 1976, page 14. In the present invention the trailer is at the stern of the towing vessel, as opposed to alongside the vessel.

Also, the present invention is structurally joined to the towing vessel with a single ball swivel joint and not the complicated means shown in U.S. patent No.

3,323,478.

The Technology of Offshore Drilling (above cited, page 14) shows a typical semi-submersible vessel commonly seen in rough weather areas like the North Sea as well as in more moderate weather climates. These vessels are free standing, either self-propelled and positioned with thrusters or nonself propelled and moored in position to do work. If the semi-submersible is not self propelled, it is towed to its work location by tow vessels and the semi-submersible vessel. Thus, the semisubmersible is handled like a conventional barge. By comparison with a free standing semi-submersible, the semi-submersible of the present invention has one corner supported by a structural ball swivel joint on the deck of a tow vessel.So, in effect, the semi-submersible of the present invention is like a trailer at the rear of the two vessel and is markedly dissimilar to a free standing semi-submersible.

Manned Submersibles by R. Frank Busby, 1976, page 611, Office of the Oceanographer of the U.S.

Navy, shows how a spar buoy assembly can be connected alongside a floating vessel by way of a single-degree-of-freedom hinge (like a door hinge).

This assembly then can be used to support operations of a subsea nature. By comparison with the present invention, (a) the spar buoy assembly is positioned alongside the vessel and not behind it; (b) the spar buoy assembly has no submerged pontoon below water to permit heavier lifts; (c) the hinge point of the spar buoy assembly is well above the vessel deck so that roll motion of the vessel is amplified into a sizeable traverse motion and rotation of the spar buoy assembly; (d) the hinge of the spar buoy assembly is a simple on-degree-offreedom hinge and not a ball swivel hinge having three degrees of freedom as in the present and (e) there is only one flotation means (a spar buoy) whereas a minimum of two flotation means (columns plus pontoons) is utilized by the present invention. Hence, there are few similarities between the present invention and the spar buoy assembly.

Claims (1)

1. A work platform comprising at least two connected semi-submersible flotation means, means for pivoting the flotation means, and connecting means extending between the flotation means and the pivoting means.

2. The platform of claim 1, wherein the pivoting means comprises a swivel joint.

3. The platform of claim 1, wherein the flotation means comprises pontoons.

4. The platform of claim 1, wherein the connecting means comprises at least one truss.

5. The platform of claim 1, wherein the pivoting means is attached to floating support means.

6. The platform of claim 5, wherein the support means is a vessel.

7. The platform of claim 1,wherein the pivoting means is attached to non-floating support means.

8. The platform of claim 7, wherein the support means is a dock.

9. The platform of claim 2, including tie lines connecting each flotation means to means supporting the pivoting means.

10. The platform of claim 1, wherein the swivel joint is mounted on a floating vessel at least near the centre of pitch on the deck of the vessel.

12. The platform of claim 1, wherein supply moving means is operatively engaged with the connecting means to move supplies between means supporting the pivoting means and the area about the flotation means.

13. The platform of claim 1, wherein thrusters are positioned within and beneath the pontoons.

14. The platform of claim 2, wherein the swivel joint is releasably mounted on moving means.

15. The platform of claim 14, wherein the moving means is a track extending longitudinally on support means.

16. the platform of claim 14, wherein the moving means is a slot extending longitudinally on support means.

17. The platform of claim 1, including ballast means deployable from the flotation means.

18. The platform of claim 17, wherein the ballast means is a floodable chamber.

19. The platform of claim 17, wherein the ballast means is a floodable chamber.

19. The platform of claim 1, including a deck.

20. The platform of claim 1, including a crane.

21. The platform of claim 1, including a launch/ recovery means for a submarine.

22. The platform of claim 1,wherein the connect- ing means includes flotation means.