WO1994010027A1

WO1994010027A1 - A lifecraft launching assembly

Info

Publication number: WO1994010027A1
Application number: PCT/GB1993/002184
Authority: WO
Inventors: Roger Clark
Original assignee: Roger Clark (Mechanical Engineering) Limited
Priority date: 1992-10-23
Filing date: 1993-10-22
Publication date: 1994-05-11
Also published as: GB9222344D0; AU5340994A

Abstract

A lifecraft launching assembly for launching a lifecraft (5) from a marine structure is disclosed. So that the lifecraft is towed away from a fixed marine structure such as an oil rig, a guide line (2) is anchored to the structure at one end, and anchored to the seabed at the other end so that is inclines down. A tow weight (1) is suspended from the guide line (2) so that it can slide down the line (2). A tow line (6) attaches the lifecraft (5) to thetow weight (1) and is ordinarily clipped to the structure. The tow line (6) is unclipped by the action of lowering a lifecraft (5) so that the tow weight (1) falls down the guide line (2) towing the lifecraft (5) away from the structure and anchoring the lifecraft a safe distance away. Because the guide line (2) is submerged it does not obstruct surface traffic. Because the guide line (2) is a fixed structure there is no time or mechanism required to deploy it and so the launch system is fast, reliable and easy to install and maintain.

Description

A Lifecraft Launching Assembly

The present invention provides an assembly for launching a lifecraft from a fixed marine structure such as an oil rig.

A problem can arise when launching a lifecraft to windward of a marine structure because the lifecraft is blown to leeward onto the structure and this is hazardous, due to the threat of wave action battering the lifecraft and in the case where the structure is an oil or gas rig, there may well be a risk of fire or explosion from the structure.

In previous attempts to alleviate the aforementioned problem assemblies have been proposed in which the lifecraft is suspended from and launched down an inclined guide cable anchored at one end to the sea bed, remote from the structure, and at the other end to the deck of the structure. To avoid the problem of the guide cable causing an obstruction to vessels approaching or departing the structure normally the guide cable is releasably fastened to a submarine part of the structure at a point intermediate the deck and the seabed anchor. However, this arrangement requires some mechanism to release the fastening and to tension the guide cable to deploy it for use. GB-1 297 771 (with respect to which claim 1 is characterised) discloses one prior art assembly of this type wherein the deck end of the guide cable passes over an arrangement of pulleys to suspend a weight from beneath the platform. When the fastening is released the weight falls to tension the cable. In this assembly the pulley and weight mechanism are susceptible to damage from fire and explosion aboard the marine structure and may be difficult to install due to other equipment on the structure. The cable is not instantly ready for use so delaying the launch of the lifecraft. Such a system will also add undesired weight to the structure.

WO 91/03398 discloses an assembly in which a flexible tow cable connects a lifecraft to a buoy. The lifecraft is normally suspended by davits from the deck of the marine structure and the tow line is led via a pulley block anchored to the sea bed through a pulley block on the buoy and the end anchored to the sea bed. The pulley is anchored remote from the structure. To keep the tow line out of the way of surface traffic around the structure the tow line is led vertically down from the lifecraft to a catch anchored to the sea bed and then to the pulley. The catch is such that lowering the lifecraft from the davits releases the tow line from the catch and allows the buoy to float towards the surface. Hence the buoy tows the lifecraft away from the structure as the low line is drawn through the pulley block by the rising buoy. With this system the flexible tow line, the catch, the pulley block and the buoy are all submerged and therefore difficult to install, inspect and repair. The tow line must be flexible and has been found to kink and jam in the pulleys under the influence of currents such as the tide. The tow line is also subject to fretting.

It is an object of the assembly according to the present invention to provide a lifecraft launching facility which is economic and alleviates at least some of the aforementioned technical problems.

Accordingly there is provided a lifecraft launching assembly wherein a lifecraft is towed away from a marine structure by the action of a tow weight falling down an inclined substantially submarine, elongate guide member anchored at one end to the sea bed and at the other end to the structure.

By deploying the guide member below water as a substantially fixed structure, the guide member can be made of relatively thick resilient cable ensuring that the tow weight can slide smoothly down the guide member. Because it is submerged, the guide member is not an obstruction to machinery on the structure or to surface traffic near the structure. The submarine guide member avoids the need for complex and unreliable tensioning deployment mechanisms and this makes installation relatively simple and the apparatus less expensive. Perhaps most importantly, there is no time required for the guide cable to be deployed allowing a lifecraft to be launched without delay. Also the tow line is not subject to fretting and friction.

A flexible tow line may be used to tie the lifecraft to the tow weight. This can be retained in an unobtrusive position, when not in use, by a clip. The clip can advantageously be mounted on the marine structure above the water level where it is relatively easy to mount, inspect and service. Preferable the clip retains the tow line so long as tension is applied to the span of the tow line connecting the clip to the lifecraft. Thus the operation of the clip is simply and reliably effected by lowering the lifecraft and hence relieving the tension. A preferred example of the clip structure comprises a two pronged fork, the forks mounted to extend downwardly and defining a notch between them. A bearing is attached to the tow line and the bearing engaged in the notch. The forks can be of sufficient length to accommodate stretching of the tow line.

In some circumstances it may be convenient to launch several lifecraft down a single guide member. In this case each lifecraft may be provided with a tow line and a tow weight, each tow weight being suspended from a single common guide member. In this form of the launching assembly it will be preferable to provide means to ensure that the lifecraft can only be launched in a safe sequence to avoid fouling one tow weight against another or entangling the tow lines. In another form of the launching assembly several guide members may be anchored to a common sea anchor.

The lifecraft launching assembly according to the present invention is, in relation to the state of the art, quicker and more reliable in operation because there are fewer moving parts and fewer components below sea level. The assembly is also likely to be easier to install and require fewer and less costly components thus encouraging its installation.

A lifecraft launching assembly constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 shows a fixed marine structure equipped with the launching assembly in a standby condition.

Figure 2 shows the assembly with the tow cable clip released by lowering the lifecraft,

Figure 3 shows the lifecraft launched by the assembly and safely anchored remote from the structure,

Figure 4A shows a perspective view of a self releasing catch,

Figure 4B shows a section through the tow line of figure 4A,

Figure 5A is a perspective view showing the tow weight suspended from the cable. Figure 5B shows a detailed perspective view of a towing eye suspending a towing weight from the cable.

The drawings of figures l to 3 show a fixed marine structure such as an oil rig and the lifecraft launching assembly. The assembly consists of a tow weight 1 suspended from an inclined, submarine, elongate guide member provided by a cable 2. The cable 2 is permanently anchored at one end 3, to the sea bed and at the other end 4 to a submarine part of the structure. It may be possible to anchor the end 4 of the cable somewhat above sea level so long as the cable 2 will not then obstruct the traffic of vessels about the structure. The cable 2 is preferably thick enough to be resilient so that kinks will not form in it. The cable 2 might form part of the anchoring for the structure thereby serving a dual purpose.

A lifecraft 5 is suspended in the usual way (e.g. , from davits) from the structure. A tow line 6 connects the lifecraft 5 with the tow weight l. The tow weight 1 is suspended from the cable 2 by a runner or slider such as the towing eye 7 shown in figures 5A and 5B so that it can slide freely down the cable 2. In this case the runner is a simple eye 7 to which the tow line 6 is secured and the towing weight 1 is suspended from the eye 7 by a suspension cable 8. A pulley or roller may be used instead of the eye 7. In the embodiment the tow weight 1 is retained at a position below water level and remote from the structure l by the tow line which is fastened by a clip 9, as shown in figures 4A and 4B, to the structure. The clip 9 comprises a mounting bracket 10, which allows it to be welded or bolted to the structure, and two downwardly inclined forks 11 which define a 'V¹ shaped notch 12. A bearing 13 is secured immovably to the tow line 6 so that the bearing 13 can be received into the 'V sectioned region beneath the forks 11 with the tow line 6 received in the notch 12. In this condition the bearing 12 and consequently the tow line 6 are retained by the clip 9 so long as tension is applied to the span 6A of the tow line 6 extending toward the lifecraft 5. Thus, as the lifecraft 5 is lowered or dropped the clip automatically releases the tow line 6. The clip 9 and bearing 13 are located so that the tow line does not obstruct traffic around the structure.

In the embodiment shown the bearing 13 is ball shaped and has an arcuate passage 13A extending through it to receive the tow line 6. During assembly, a clamp (not shown) and sleeve are threaded onto the line, followed by the bearing 13 and the other sleeve and clamp (not shown) . Thus the position of the bearing 13 can be adjusted to fit in the clip 9 and the clamps locked against the tow line 6.

It will be appreciated that to launch a lifecraft the lifecraft 5 is loaded and either lowered progressively or simply dropped from the structure. By lowering the lifecraft as shown in figure 2 the tension on the tow line strand 6A is relieved and the bearing 13 is pulled out of the clip 9 by the tension applied by the tow weight l. Thus the tow weight 1 is free to slide sown the guide cable 2 thereby towing the lifecraft 5 away from the structure until the weight impacts the sea bed or some other artificial stop or brake as shown in figure 3. At this point the lifecraft 5 is anchored up to about twice the sea depth from the structure, this is some four times what can practically be achieved with the state of the art assemblies discussed previously. The lifecraft 5 can then await rescue at a safe anchorage or the tow line 6 released and the lifecraft propelled further away from the structure.

Claims

1. A lifecraft launching assembly whereby a lifecraft (5) is drawn away from a marine structure via a descending elongate, inclined, guide member (2) characterised in that the guide member (2) is submerged and the lifecraft (5) towed by the action of a tow weight (1) falling down the guide member (2) .

2. An assembly according to claim 1 wherein the tow weight (1) is attached to the lifecraft (5) and suspended from the guide member (2) so that, when the lifecraft (5) is launched the tow weight (1) falls down the guide member (2) and thus tows the lifecraft (5) away from the structure, the elongate guide member (2) being at all times instantly ready for use and sufficiently submerged to avoid obstructing traffic.

3. An assembly according to claim 2 wherein the lifecraft (5) is attached to the tow weight (1) by a tow line (6) .

4. An assembly according to claim 3 wherein the tow line (6) is of sufficient length to allow the weight to fall to the seabed end of the guide member (2) .

5. An assembly according to claim 3 or claim 4 wherein the tow weight (1) is secured ready for use by a clip (9) constructed such that releasing tension applied to the span (6A) of the tow line (6) extending from the clip (9) to the lifecraft (5) releases the tow line (6) to allow the tow weight (1) to fall.

6. An assembly according to claim 5 wherein the clip (9) is mounted on the structure above the waterline for ease of inspection.

7. An assembly according to claim 5 or claim 6 wherein the clip (9) comprises a means defining a downwardly extending notch and a bearing like member (13) fixedly attached to the tow line (2) whereby the tow line (2) can be received through the notch (12) with the bearing like member (13) engaging the means.

8. An assembly according to any one of the preceding claims wherein the guide member (2) is anchored at one upper end, to the structure and at its lowermost end to the seabed.

9. An assembly according to any one of claims 5 to 8 wherein the clip (9) is located so that sufficient of the span of the tow line from the clip (9) to the tow weight (1) is submerged to avoid obstructing traffic when a lifecraft (5) is not being launched.