GB2199004A - Marine survival system - Google Patents

Abstract

On launch of a lifecraft (14) from a marine platform (10) or other vessel, an electrically powered tug (20) connected to the lifecraft (14) by stowed cable (22) aligns the lifecraft (14) away from the platform (10) and tows it away from the platform (10) in a predetermined attitude and track. In this way, the lifecraft (14) is controlled in the hazardous initial period between launch and start of its own powered movement. The tug (20) is advantageously stowed and launched bows-out and has station-keeping means responsive to deviation of the towline (22) from the tug fore-and-aft line to correct the tug helm so that the tug (20) not only follows a predetermined compass heading but follows a predetermined track relative to the lifecraft (14). A portion of the towline (22) is stored in retractable payout means biased towards towline retraction so that the towline (22) is paid out as the tug (20) is lowered and retracts into the tug (20) during initial movement of the lifecraft (14) with the tug (20) still in the water and acting as a sea anchor. <IMAGE>

Description

MARINE SURVIVAL SYSTEM The invention relates to a marine survival system in @hich a lifeboat, rescue craft or like small boat is launched from a ship, floating or fixed shore marine platform or other vessel by which it is carried.

aunching a survival craft fron an offshore platform for emergency escape can be in the face of a combination of adverse circumstances, e.g. with the platform listing, in the car; into dense smoke, into fire on the sea, into a toxic or flammable gas cloud, into high winds or against adverse tides. It is therefore desirable to launch the survival craft positively away from the platform and in a bows-out heading.

One method that has been suggested is to launch the craft from a downwardly inclined chute extending partway from the platform to the sea, the remainder of the distance being traversed in free fall. Although this system can launch a boat away from a platform at a desirable heading and at a useful speed, it is expensive to install and subjects the passengers to high g-loads.

In patent specification no. GB-A-2142310 there is described a method that Involves providing a flexible boom mounted on a hinge secured to the parent vessel adjacent to the stowed survival craft. When not in use the boom is secured In some way so as not to interfere with the working of the parent vessel. A so-called "tag-line" is attached at one end to the outboard end of the boom and is secured at its other end to the bow of the survival craft.

In operation all personnel embark into the survival craft wearing lifejackets and secure themselves with seatbelts.

All doors and hatches are closed and the engine is started. owering is actlvated from within the craft, and as it starts to descend the tension thus induced in the tagline causes the boom to hinge outwards and downwards to take up an approximate horizontal position pointing away from the platform. As the craft continues to descend further the tagllne causes the boom to flex downward lie a giant fishing rod until the craft is waterborne.At this point the engine throttle ray be opened to full ahead, the lifting hooks released, and the tagline continues to exert a pull on the bow of the craft In an upward ana outward dIrectIon until the craft has travelled the length of the booz away from the platform. Only then, when the bow of the craft is directly under the tip of the boom, s the tag Ine released, by which time the propulsion unit will have attained its maximum thrust and the craft will have reached a speed In excess of six knots.

The above launching system is not suitable for all situations and in particular it is difficult to retro-fit to existing lifecraft installations and it is an object of this invention to provide a compact launching system that can be fitted inexpensively to both new and existing Installations.

US Patent No. 3980038 describes a tug for keeping a hose and mooring line extending from a single buoy mooring terminal in a desired direction but the tug relies on signals from the buoy to maintain its direction. Such an arrangement cannot be accepted for a survival craft which is required to operate Independently of power on the vessel from which it Is launched.

Our patent application GB-A-2169247 provides a marine survival system for a vessel (Including a marine platform) comprising: a lifecraft; an unmanned tug; a tow'ire connected between the bows of the lifecraft and a centre of neutral steer of the tug; a first launching means by which the lifecraft Is stowed on the vessel, said first launching means including releasable falls and braked winch means controlling pay-out of the falls so that the lifecraft is lowered slowly to the sea; a second launching means by which the tug is stowed on the vessel, said second launching means including a release permittIng the tug to drop to the sea; autopilot means n said tug operatively connected to a rudder of the tug to angle the rudder as called for by a predetermined compass heading; a sensor in the tug responsive directly or indirectly to the direction of pull in the towline to change state depending upon the magnitude and direction of the deviation of said pull from the fore and aft direction of the tug; a trim control of the autopi-lot means responsive to the state of the sensor to adjust the effective compass heading to maintain the tug on a predetermined track relatIve to te lifecraft: and release means operably connected to the winch means and to the tug release so that on actuation of the release means the tug falls into the sea and deploys to the extremity of the towline on the predetermined heading and track as the lifecraft is slowly lowered to he sea any on release of the falls tows the lifecraft away from the vessel.

According to a preferred aspect of the invention, It is advantageous for a tug storing a section of towline to be able to pay it out during raunch of the tug and to recover it by Internal biasing means so that the tug acts as a sea anchor with the towllne fully deployed and recovers towline as the llfecraft begins to move away from the vessel from which it is launched.

The invention therefore provides an unmanned tug for incorporation into a marine survival system which when dropped into the water deploys at an end of a towline or a predetermined heading and track relative to a lifecraft, said tug having autopilot means thereIn operatively connected to a rudder of the tug to angle the rudder as called for by a predetermined compass heading, a sensor in the tug responsive to a direction of pull 1 in the towline to change state depending upon the magnitude and direction of a deviation of said pull from a fore and aft direction of the tug, a trim control of the autopilot means responsive to the state of the sensor to adjust an effective compass heading to maintain the tug on the predetermi-ned track relatIve to the lifecraft, means in the tug for paying out and recovering a portion of the towline and means biasing the towline payout means in a recovery direction while towline is paid out to maintain a tension therein whIch s less than the pull of the tug when the tug is still in the water so that in use of the tug the towline Is initially paid out and as the survIval craft starts to move towards the tug said portion of the towline is recovered into the tug with the tug substantially still in the water.

The towllne is preferably stowed In the tug between sheaves of first and second sheave blocks one of which is movable towards and away from the other under the control of means biasing the biocks apart. n one form the bissing means is a ram naving a rod connected to the first sheave block for movement towards and away from the second sheave block and the ram is connected to hydraulic accumulator means to supply fluid to the accumulator means as the rod 15 extended, the accumulator means biasing the rod to return into the ram.In a second form which is preferred on the ground of simplicity resilient means such as shock cord or a tension spring Is provided between the first sheave block and a fixed structure in the tug bracing the first and second sheaves apart.

An embodiment of the invention Will now be described, by way of example only, with reference to the accompanying drawings. in which: Figures 1a to 1d represent successive stages in the tug-assisted launch of a lifecraft according to the invention and Figure 1e represents the start of the recoverIng of the lifecraft by a rescue ship; Figure 2 -s a diagram of the tug and lifecraft stowed prior to launch showing cables that provide for simultaneous launch of the tug and initiation of lifecraft descent Figures 8, 4 and 5 are for reference purposes and are respectively a diagrammatic side elevation, top plan and rear elevation of a tug as described in GB-A-2169247:: Figure 6 is a block diagram of station keeping means fitted in the tug of Figures 3 to 5 and to the tug showr.

in Figures 8 to 10; Figure 7 is a diagram showing the effects of the tug proceeding on a correct course but off track; Figures 8, 9 and 10 are respectively a centre line sectional elevation, an underside view and a section on the line A-A of Figure 8 of a tug forming part of a punching system according to the invent ion; F igures 11 and 12 are respectIvely longitudInal and transvese sectIons of a damped pendulum unit fitted into the tug of Figures 8 to 10; and Figure 13 is a diagrammatIc centre line sectional elevation of a second form of z tug formIng part of a aunching system according to the Invention.

In Figures 1a to 1d a marine platform 10 has a first davit structure 12 from which a lifecraft 14 is supported by means of falls 16 controlled by winch means 17 (Figure 2). The lifecaft 14' may be carried in an alongside attitude with respect to the platform 10 as shown or it may be carrIed in a bows-out attitude. A second davit strucrure i8 adjacent the davlt structure 12 carries G tug 2C In a bowe-out attitude with respect to the platform 10.

The tug 20 may alternatively be stowed In a downwardly and outwardly inclined launching chute located adjacent the davit structure 12. During launch passengers enter the craft 14 and start the launch procedure by releasing a brake 19 In the wInch means 17 by pulling on cord 21 attached to brake actuation and release lever 23 after which the craft 14 begins a controlled descent. As the brake 19 is released the movement of lever 23 is transmitted via cord 25 to release mechanism 27 holding the tug 20 In position below the second davlt structure 18, and the release mechanism 27 drops the tug 20 Into the sea on a bows-out heading with respect to the platform 10.

The tug motor starts on release and the tug 20 proceeds away from the platf-orm 10 on a predetermined heading maintained by station keeping means therein. A towing cable 22 deploys from the tug and is permanently connected to the bows of the craft 14, the condition where the craft 14 has been partly lowered being as shown in Figure ib.

In Figure ic the craft 14 has been fully lowered and pulled by te tug 2C Into a oows-out heading, after which the falls 16 are released and the tug 20 pulls the craft 14 away from the platform 10 (Figure 1d). A motor in the craft 14 is started when the craft 14 Is afloat and as the craft 14 gathers seed the tug 20 is no longer needed.

The towline 22 may be held permanently in a release hook at the bows of the craft 14 while the tug 20 and craft 14 are stowed. and when the tug 2C is no longer recurred the release hock may be operated either manually from within a passenger compartment of the craft 14 or by means of a timer to ship the tug 20 after which the craft 14 proceeds under its own power. Advantageously. however, the tug 20 is retained and towed by the craft 14 after its motor has ceased to operate, the tug 20 then acting as a buoy marking the end of a deployed towline 22 that can easIly De picked U, ov a rescue ship 24 as in Figure C.

A structure of a known version of the tug 20 is shown in more detail in Figures 3 to 5. A casing or body 26 of generally cylindrical section is divided at transverse bulkheads 28, 30, 32 into a bow compartment 34, a forward compartment 36, an aft compartment 38 and a stern compartment 40. The bow compartment 34 has a suItable length such as 40 metres of tow rope or cable 22, most of which is held in serpentine manner in stowage tubes 44, but the last 15 metres of which are held on a tension reel 46.Accordingly the rope 22 will pay out freely fro the compartment 34 as the tug deploys and the tension reel 46 serves to apply the towing load on the rope 22 gradually rather than wit a sudden snook. The rote 22 emerges via a bows aperture 42 which Is of rounded section as shown so that the rope 22 does not cat or fret. The forward compartment 36 contaIns high discharge rate batteries 43 which may be silver zinc batteries and should be capable of giving a current of the order 300 amps at 100V for a period of 10 minutes. The forward bulkhead 28 carries an autopilot u-: t 50 which is connected by leads 52 to a towing rope angle sensor 54 mounted to the middle bulkhead 30.A motor 56 is mounted in the bulkhead 30 and drives a gearbox 58 containIng four bevel gears that serve to drive a pair of coaxIal contrarotating output shafts 60 that pass through stern gland 62 on aft bulkhead 32 and through nozzle-defining partition 64 In flooded stern compartment 40 where they drIve twin contra-rotating propellers 66, 68. The gearbox 58 may be dispensed with if the motor 56 is itself In two contra- rotatIng parts like the motors of many torpedoes.

Aithough the use of contra-rotating propellers is preferred on the ground of efficiency, a single propeller can be used provided that adequate thrust is obtainable from it. Inlets 70 in the lower sides of the casing of compartment 40 allow water to flow to the propellers 66, 68. The use of a nozzle or jet in which ducted propellers 66, 68 run facilitates station keeping while the tug 20 Is not movlng and maximises pull with the tug stalled so that she full power cf the tug Is available to tow the lifecraft 14 on launch thereof. A rudder 72 is pivoted aft of propeller 66 with its angular position being set by servo 74 controlled from the autopilot 50.A recovery handle 76 of inverted V-shape is attached to the casing in register with forward compartment 36.

A V-shaped keel 79 is attached along the underside of the casing 26 and may contain the position servo 74 for the rudder 72. leads (not shown) from the autopilot 50 to the servo 74. and ballast if required. A flotation jacket 78 generally of inverted U-profile Is secure; above and along the casing 26 and is made of expanded plastics material with a soft core of flexible polyethylene foam skinned with polyurethane. The volume and density of the flotation jacket is selected so that the tug 20 has a suitable low waterline as shown in Figure 5 to minimise the effects of wave action and the combination of the buoyant flotation jacket 78 and the keel 79 gives good stability and self-righting properties. Furthermore, the resIlIence of the jacket 78 makes the tug 20 selffendered.Extending aft fro the bows aperture 42 Is a vertical channel 81 through the jacket 78 which serves to lead the towing cable 22 to an aft-facing steerable fairlead 83 positioned at or as ciose as possible to the centre of neutral steer. The falrlead 83 releaseably retains the cable 22 while responding to the angle that aft portion 22a thereof makes with the fore and aft direction of the tug 20. Thus the fair eat a3 may have retaining means such as a pair of split rings that remain closed until opened by a trigger mechanism controlled by a timer, or the fairlead may be Jettisoned after a predetermined time. A spindle 85 operably connects the fairlead 83 with the angle sensor 54.

Referring now to Figures 6 and 7, the autopilot 50 receives input signals from the tow line sensor 54 and from magnetic or gyroscopic compass means 84. Thus In Figure ib, before the boat 14 is afloat the tug 20 heads away from the platform 10 at a predetermined compass heading set in response to compass 84, but when the boat is afloat (FIgures lc and 7), If the aft towllne portion 22a deviates from the fore-and-aft direction as indicated by fairlead 83 and sensor 54 a helm correction signal is fed to autopilot 50 to bring the boat 14 bac onto the correct track.In Figure 7 the dotted lines show the tug 20 on the correct course but on the wrong track as indicated by an angle-D between the cable 22 and the fore and aft direction. The action of the sensor 54 and autopilot 50 is to reduce angle D to a minimum, bringing tug 20 onto the correct track which is indicated by solid lines. It will be appreciated that the above launching system is compact but meets the operational requirement of guiding the lifecraft 14 away from the platform 10 on a predetermined safe heading in the period immediately after launch.

A form of the tug 20 according to the invention, but arranged as described in Figures 6 and 7 is shown in Figures 8 to 10 and comprises a casing 26 in a flotation jacket 78 as previously described. A forward compartment 36 of the casing 26 behind a bulkhead 28 houses an autopilot unit 50 a roll pendulum unit 90 for providing an input signal to the autopilot 50 and steering servos 74 connected by cables 2 to the rudder 72. A bulkhead 94 divides the forward compartment 36 from aft compartment 38 which houses a 100 volt 65 ampere-hour battery 48.A bulkhead 96 divides aft compartment 38 from motor compartment 98 which houses 8 50 HP contra-rotating motor 56' which drives propellers 66, 68 via a pair cf concentric output shafts 60 as previously described. The keel 100 houses a mechanism that is capable of paying out and recovering a predetermined length (suitably 60 feet) of the towIng cable 22. The rope 22 passes from a falrlead 102 at the cer.tre of neutral steer of the tug 20 forwardly over upper and lower bow guide sheaves 104. 106 to a pair of lead sheaves 108, 110 and thence to an end sheave 112 of a 6-sheave block 114 which is mounted on rod 118 of an hydraulic ram 120 in the keel 100 located approximately under bulkhead 98. The cable 22 passes over the sheaves of the six sheave block 114 and over the sheaves of a fixed five sheave block 116 located under the bows compartment 34.Between the sheaves 104, 106 the cable 22 runs vertically, the lead sheaves 108, 110 deviate the cable 22 in a horizontal plane rom the longitudinal centre line of the tug 10 to align the caDe 22 with end sheave 112 of the movable block 114. Between the sheaves of blocks li4, 116 the cable 22 is stowed n vertical loops.The rod 118 has a travel of five feet as Indicated by the arrow 122, the aft position of the block 114, corresponding to stowage of the tow rope 22, being shown by the dotted c-rcle. With this travel the sI:; loops of cable 22 In the keel 100 can pay out and recover 60 feet of cable.The rod side of ram 120 is connected by line 125 to a pair of hydraulic accumulators 124 in the bows compartment 34 with the combined working -volume of accumulators 124 preferably being more than twice the working volume of the ram 120. Typically the accumulators i24 are pressurised to 2000 psi and have a combined workIng volume of 10 gallons whereas the ram 120 has a working volume of about 3 litres. With this arrangement tension in rod 118 and hence the tension In towing cable 22 varies only slightly through the strcke of ram 120.

With this arrangement when the tug 20 is stowed on its davit 18 the rod 118 of the ram 120 is retracted and the 60 foot recoverable section of towing cable 22 is stowed in the keel 120 of the tug. A further length of cable 22 interconnects the tug 20 with the survival craft 14, that further cable being of length at least equal to the distance t-hat tug 20 has to fall before it hits the water (Figure la-Figure 1b) and is stowed at any convenient location where It is freed and deployed automatically as the tug 20 falls to the water.As the craft 14 reaches its lowered position (Figure 1c) the tug 20 has trvelied to the end of the free length of cable 22 and a further distance corresponding to deployment of the stowed 60 foot length of cable 22, the cable pulling block 114 forward and extending rod 118 with fluid being fed into hydraulic accumulators 124. Tension n the cable 22 pulls the craft 14 towards a correct heading (Figure 1c) As the falls are released and in the initial 60 feet of travel of the craft 14 the tug 20 is substantially still in the water and acts as a sea anchor. the travel of the craft 114 towards the tug 120 allowing the deployable portion of the cable 22 to be rewound onto the sheaves of blocks 114, 116 as rod 118 retracts, hydraulic fluid returning from accumultors 124 to ram 120.To enable this deployment and recovery to happen an appropriate balance is required between the forward pull at line 22 from the action of motor 56' and propellers 66, 68 which may be of the order of 500 ibs with the tug 20 still in the water and the pull In the deplorable section of the towline exerted through ram i20 which is somewhat less typically of the order of 460 lbs.The reason for using the tug 20 as a sea anchor and recovering a deployable portion of cable 22 during the first part of travel of craft 14 from the platform 10 is that the thrust from propellers 66.. 68 is the greatest when the tug 20 is still in the water and falls off rapidly as the tug begins to move forward and with the tug 20 still the whole of the available towing force is applied in line 22 to accelerate the craft awav from the platform 10.

It will be noted that the cable 22 passes through a simple fairlead 102 as opposed to the steerable fairlead 83 of the earlier embodiment. Deviation of the cable e 22 from the fore end aft direct ion of the tug 20 gives rise to a roll to port or starboard depenalno cn the angle D (Figure 7) which is a permanent component superimposed on the roll imparted to tug 20 by wave act Ion. The permanent component of tug roll is sensed by pendulum 130 of the roll pendulum unit 90 whose motion is damped by oil in pendulum casing 132.The autopilot 50 has a trim control 134 which is a potentiometer whose shaft 135 s rotatable clockwise or anti-clockwise to cause servo 74 to angle rudder 72 to port or starboard in addition to the rudder angle called for by tbe compass heading. The pendulum 130 is pivoted on a shaft 136 which Is coupled to the potentiometer shaft 135. Since the towing cable 22 ieaves the tug 20 at fairlead 102 which Is located above the roll centre the force In the towing cable 22 will roll the tug 20 clockwise or anticlcockwise as the tug tracks to port or starboard of a bearing set on the auto-pilot 50 from the stowed position from which the tug 20 was launched.

An internal compass in autopilot 50 maintains the tug on course and the pendulum 130 and trim control 134 keep the tug 20 on track while the lifecraft is being lowered but is not released from the falls.

Various modifications may be made to the embodiment described above without departing from the invention, the scope of which is defined in the appended claims. For exampie, as see in Figure ;3. the re 120 and accumulator 124 may be replaced by simple "shock cords" 140 of elastomeric material fixed between sheave 114 and an anchorage 142 to the aft end of tug 120, which cords are in tension and pull the sheave 1;4 away from the sheave 116. With this arrangement the sheaves 114, 116 need not be located in the keel of the tug and could be relocated to the top of the tug as shown.The shock cords 140 comprise strands of elastomeric material in a braided casing and are typically of diameter about 14 mm, a doubled loop when fully extended typically giving about 120 lbs pull but taking a strain of up to 4 tons through the outer braiding. The tug may have a standard commercial marine type solid state autopilot fitted with a fluxgate compass and an automatic tracking device.

The sequence of events in launching the unmanual tug or so-called "Power DolphIn" is as follows: 1. Initiation of the lowering of the survival craft automatically activates the Power Dolphin!s jet power unit which as seen In Figure 13 has a single propeller 143, and nozzle 145 driven by a rearwardly and upwardly inclined shaft from motor 144 and at the same time releases the Power Dolphin from its downwardly and outwardly inclined launching frame. The Power Dolphin free falls to the water ahead of the survival craft, which continues to lower at its controlled speed. During the Power Dolphin's descent, the tow line 22 from the Power Dolphin to the bow of the survival craft pays out from non-tangle stowage tubes.

2. The Power Dolphin drops at the same angle as when stowed in its launching frame throughout its fall (approximately 400). On impact with the water, because of its angle of entry, the angle of the bow and the angle of the jet power unit 144 (Figure 13) the Power Dolphin follows e very shallow under-water trajectory and quickly re-surfaces assisted by its buoyancy fenderlng 78. It then proceeds away from the installation at high speed on a predetermined track controlled by the automatic pilot 50.

3. Th-e tow line 22 continues to pav out until the Power Dolphin is apporox 1.5 times the distance the survival craft was stowed above the waterline. At this point, tension will commence being applied to the tow line from the bow of the survival craft, which will not yet have reached the water. This tens-ion operates the Power Dolphìn's reeling gear 114, 116, pulling out further line until the ;et thrust and the reeling gear tension are equal sex at 600 Kg. At this point, the Power Dolphin's forward movement will stop and the pull exerted by the reeling gear turns the survival craft to face away from the installation.

-. As soon as the survival craf Is waterborne and Its lifting hooks released, the continuing action of the Power Dolphin's reeling gear 114, 116 accelerates It away from the installation, assisted by increasing thrust from its own power unit. As the survival craft's speed increases, the Power Dolphin then also gathers s?eed ano continues until its batteries are exhausted (approximately 6 minutes from being launched).

The launching frame which slopes downwardly end outwardly at 400 can either be bolted or welded to the installation's deck, close to the survival craft. The Power Dolphin can then be lifted and its trunnion support rollers positioned on an inclined launching ramp of the launchIng frame. The Power Dolphin is supp - ied as a completely operational unit with its batteries fully charged. The automatic pilot 50 is set to the desired course. The tow ine 22 is stowed in stowage tubes, with the ends connected to the Power Dolphin and the bow of the survival craft.At the sa""..e time, the tow line is also engaged to the release mechanism which is contained within one of the stowage tubes, so that, when the survival craft starts to descend and the tow line commences to pay out, the Power Dolphin is mechanically released and the jet power unit switched on. A power cable from the installation is connected to the Power Dolphin's umbilical power inlet via a watertight, non-spark, quick release plug ano. socket, which disengages when the Power Dolphin is launched.The power source should be connected at all times, so that the battery c-harge is maintained. A green light on an indicator panel indicates full charge and a red licht warns of any defect. Provision is made to brlefly run the motor and Jet power unit for periodic testing whilst the Power Dolphin is in its launching frame.

When the survival craft is successfully evacuated from the installation, the helmsman can eIther: a. Release the towline and proceed under the survival craft's own power, or o. If the helmsman wishes to heave to because of extreme weather conditions or to await rescue, the tow line not released. Because of the Power Dolphin's weight and low freeboard, it is subject to very little drift caused by w,nd forces, whilst the opposlte applies to the survival craft. A survival craft without power would stay downwind of the Power Dolphin n at a distance determined by the length of the tow line. Keeping the tow line attached has the advantage of assisting rescue by ship.A large recovery ring on the stern of the Power Dolphin facilitates recovery and it can be hoisted on board the ship and the survival craft towed to safety w-th the reeling gear absorbing the snatch loads.

To ensure the efficient use of the Power Dolphin system ano for safety reasons. it Is recommended that all survival craft be equipped with "On Lcad" release gear and a means of releasing the tow line from within the craft.

The principal advantages of the tug or Power Dolphin are: (a) Reasonable cost and low weight.

(b) Of robust. watertight construction with simple.

regular maintenance procedures.

(c) Occupies minimal deck space.

(d) Compact deck stowage does not present a hazard to helicopters or ships.

(e) nstallation's deck only has to support the weight of the Power Dolphin and its launch cradle. There are no stresses requIrIng structural reinforcement.

(f) Can be retrofitted to all types of existing Offshore Installations.

(g) Operational from any height and angles of heel up to 20 .

(h) Fully automatic operation. from commencement of descent of survival craft to final clearance away from the InstallatIon.

(i) survival craft restrained from being washed under the InstallatIon.

(j) Positive towing force from the Power Dolphin turns the survival craft to face away from the installation during its descent and when launched, tows it away from the installation.

(k) A pre-set automatic pilot in the Power Dolphin ensures that the survival craft is oulded on a true track away from the InstallatIon.

(1) Because the tow line is almost horizontal when the survival craft is waterborne, maximum towing force is attained.

(m) when the survival craft is clear of the Installation and the Power Dolphin's batteries exhausted, the craft can remain connected to the Power Dolphin by the tow iine. The Power Dolphin can be readily recovered by a s-hip and the surviva; craft towed to safety.

(n) Because of low positive buoyancy, the Power Dolphin has a very low freeboard and its track is therefore hardly affected by wind forces.

(o) The configuration of the buoyancy fendering ensures that the Power DolphIn is self-rightIng.

(p) The survival craft can be operated with or without the Power Dolphin.

(q) Crew training can take place at all times, without operating the Power Dolphin.

(r) The Power Dolphin can be recovered and re-used.

Claims (9)

CLAIMS:

1. A- unmanned tug for incorporation Into a marine survival system whIch when dropped In to the water deploys at an end of a tolle on a predetermined heading and track relative to a lifecraft said tug having autopilot means there In operatively connected to a rudder of the tug to angle the rudder as called for by a predetermined compass heading. a sensor in the tug responsive to a direction o' cull in the towline to change state depending upon the magnitude and direction of tt deviation of said pull from a fore and aft direction of the tug.. a trim control of the autopilot means responsive to the state of the sensor to adjust an effectIve compass heading to maintain the tug on the predetermined track relative to the lifecraft, means ir. the tug for paying out and recovering E portion of the towllne and means biasing the towllne payout means in a recovery direction while towline is paid out to maintain a tension therein which is less than the pull of the tug when the tug is still in the water so that in use of the tug the towline is initially paid out and as the survival craft starts to move towards the tug said portion of the towline is recovered into the tug wlth the tug substantially stil; in the water.

2. A tug according to Claim 1, wherein the towline is stowed in the tug between sheaves of first and second sheave blocks one of which is movable towards and away from the other under the control of means biasing the blocks apart.

3. A tug accordIng to claIm 2 whereIn the biasing means Is either (a) a ram that has a rod connected to the first sheave block for movement towards and away from the second sheave block, the ram being connected to hydraulic accumulator means to supply fluid to the accumulator means as the rod is extended, the hydraulic accumulator means biasing the rod to return into the ram or (b) is a multiplicity of elastomeric cords connected between the first sheave block and an anchorage in the hull of the tug.

4. A tug according to Claim 2 or 3, wherein the towline leaves the tug at a point above a roll centre of the tug so that pull in the towline rolls the tug clockwise or anticlockwise as the towline passes to starboard or port of the fore and aft direct Ion of the tug, damped pendulum means in the tug senses the roll of the tug and the; trim control responds to the angular positIon of the damped penduum means.

5. A tug according to Claim 4, wherein batteries- motor propellers and duct are housed in a tubular body running the length of the tug and the tug is fitted wiith a flotation jacket of expanded plastics material- also serving as a fender for the tug.

6. A tug according to Claim 5, wherein the first and second sheave blocks and the ram are located in a keel of the tug.

A A tug accordIng to Claim 6, wherein the towline passes from a fair lead at the centre of neutral steer of the tug into the keel of the tug via first and second sheaves, at the bows of the tug spaced vertically apart and third and fourth sheaves in the eel fcc displacing the towline across the tug into alignment with an end sheave of the first sheave block.

8. A marine survival system for a vessel (including a marine platform) comprising: a lifecraft; an unmanned tug as claimed in any of Claims ; to 7.

sad tug having a bow and stern between which is defined a fore and aft direction of said tug; a towline connecting the bows of the lifecraft to the centre of neutral steer of the tug; a first launching means by which the lifecraft is stowed on the vessel, said first launching means including releasable falls and braked wlnch means controlling payout of the falls so that the lifecraft is lowered slowly to the sea: a second launching means by which the tug is stowed on the vessel, said second launching means including a release permitting the tug to drop to the sea; and release means operably connected to the winch means and to the tug release so that on actuation of the release means the tug falls into the sea and deploys to the extremity of the towline on the predetermined heading and track as the lifecraft is slowly lowered to the sea and on release of the falls tows the lifecraft away from the vessel.

9. A system according to Claim 8, wherein the tug is stowed on the platform and is launched therefrom in a bows-out attitude.