The creation of a large network of submarine cables and oil and gas pipelines, together with their associated sea bottom structures, has given rise to the use of a wide range of subsea vehicles which work on these installations. The most convenient way of launching and recovering such vehicles from their support ship is generally considered to be by means of an apparatus known as an A frame.

A known A frame is shown schematically in Figure 1, and is described in more detail in European'Patent 0165284.

Referring to Figure 1, a ship 1 lifts or lowers a load 2, for example a subsea plough, via a lift rope 3 running over a pulley 4 to a winch 5. The pulley 4 is carried by an A frame 6 comprising a portal frame 7 having a pair of legs 8 and a cross beam 9, the legs 8 being mounted to respective pivots 10 to enable the portal frame 7 to pivot about an axis 11. The A frame 6 also includes a pair of hydraulic cylinders 12 mounted between the legs 8 and supports 13 for controlling movement of the portal frame 7 about pivot axis 11.

As shown in Figure 2, which shows a side elevation of the arrangement shown in Figure 1, the load 2 exerts a turning moment on the A frame 6 about pivot axis 11, this turning moment being resisted by the moment exerted in the opposite sense by the hydraulic cylinders 12. The moment exerted by the load 2 is applied to the A frame 6 by the lift rope 3 passing around the pulley 4, the lift rope 3 exerting forces on both sides of the pulley 4. These forces have a resultant P which has a moment arm x about the pivot axis 11, and the hydraulic cylinders 12 exert a force R which has a moment arm y about the pivot axis 11.

Figure 3 is a diagram showing curve X, the variation of the moment Px exerted by the maximum load which can be handled by the apparatus, with the angle a of the portal frame 6 relative to the supports 13 (see Figure 2), and curve Y, the variation with angle a of the maximum moment Ry exerted by the hydraulic cylinders 12. In particular, the moment exerted by the load 2 is a maximum where the A frame 6 is fully pivoted outwards of the ship 1, and a minimum when the portal frame 6 is near its upright position. The moment exerted by the hydraulic cylinders 12, on the other hand, is a maximum near the upright position of the portal frame 6, where the load moment is a minimum, and a minimum at the two extremes where the moment exerted by the load 2 is a maximum.

As a result of this mismatch, the prior'art load handling apparatus described with reference to Figures 1 to 3 suffers from the disadvantage that the two hydraulic cylinders 12 can only provide sufficient turning moment between points A and B shown on Figure 3. As a result, this simple type of A frame in practice typically only operates through a total angle of 80° to 90°, and is also very inefficient in that it has excessive turning moment through most of its working arc, as a result of which poor use is made of the work capacity of the hydraulic cylinders 12. This is particularly disadvantageous since the hydraulic cylinders 12 used on such offshore load handling apparatus are of such a size that they are very expensive.

Figure 4 shows a side elevational view of another known A frame which attempts to overcome this problem and can rotate through 160° (from a = 10° to a = 170°). Such load handling apparatus are particularly popular for handling cable ploughs to enable a telecommunications cable to be buried at the same time that it is laid on the seabed.

Referring in detail to Figure 4, in which parts common to the apparatus described with reference to Figures 1 to 3 are denoted by like reference numerals but increased by 100, the schematic side elevation view of the A frame apparatus is shown as it would be used in a plough handling system in which a ship 101 pulls a plough (not shown) along the seabed. In pulling the plough along the seabed, the portal frame 106 is fully inboard with angle a approximately 10°, and tow rope 103, which also serves as the lift rope, runs above the deck of the ship 101 and over an overboarding sheave 120.

In order to retrieve the plough, the plough is lifted off the seabed with the portal frame 106 in this position, and the portal frame 106 is maintained in this position until the plough has been lifted up to just below the ship 101. In this way, the A frame 106 is protected from shock loads imposed by the motion of the ship 101 during the lifting and lowering periods (which may be up to 2 hours long) and the large load often required to break the plough out from the seabed.

As the plough approaches the ship 101, the portal frame 107 is driven outwards of the ship, lifting up the tow rope 103 until it is fully outboard. At this point, the portal frame 107 is low down near the roll axis of the ship 101, which minimises sideways movement of the pulley 104. The load can then be brought out of the water into prompt engagement with a restraining apparatus (not shown) suspended from the cross beam 109 without being freely suspended for sufficient time to pick up any swinging movement. The portal frame 107 is then swung inboard until the angle a is about 45°, when the load is lowered on to the deck of the ship 101. This type of A frame enables a plough to be deployed and recovered without any manual intervention, which is clearly of considerable advantage when such activities are attempted in rough weather.

Furthermore, there is no need for a separate lift winch and lift rope, the tow winch and tow rope being used for both towing and lifting.

The large angle of movement of the portal frame 107 is achieved by the use of two further hydraulic cylinders 121 in addition to the lower hydraulic cylinders 112. The additional hydraulic cylinders 121 are each attached at one end to a pivot 122 at an upper part of a vertical post 123 extending upwards from the deck of the ship 101 in line with the main pivot axis 111 of the A frame 106. The other ends of the cylinders 121 are each attached at a pivot 124 on the outside of the legs 108 of the portal frame 107. It can therefore be seen that when the legs 108 of the portal frame 107 are vertical, the hydraulic cylinders 121 are also vertical and exert generally zero moment on the portal frame 107 about pivot axis 111. The moment exerted is generally the force S multiplied by the moment arm z as shown in Figure 4.

Figure 5 is a graph corresponding to Figure 3 showing the moment Px about pivot axis 111 exerted by the maximum load, and the moment Ry exerted by the lower hydraulic cylinders 112 in the apparatus of Figure 4. Figure 5 also shows the moment Sz about pivot axis 111 exerted by the two upper hydraulic cylinders 121, as well as the sum (curve Z) of the moments exerted by the two pairs of hydraulic cylinders 112,121.

It is clear from Figure 5 that in the arrangement of Figure 4, there is adequate moment to drive the portal frame 107 through 180°. Consequently, the four cylinder A frame device described with reference to Figures 4 and 5 is presently used to handle a substantial proportion of the subsea telecommunications ploughs in existence, as well as systems involving seabed tractors.

However, this known four cylinder A frame apparatus suffers from the disadvantage that because the large hydraulic cylinders used for such apparatus are very expensive, and four such cylinders are required, the system is very expensive to produce.

Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.

According to the present invention, there is provided a load handling apparatus comprising: a support frame including first and second pivotable members pivotably mounted in use about a pivot axis to a base portion and connected to each other in use by a support member for supporting a load, wherein said support frame is adapted to pivot about said pivot axis between first and second orientations to transfer a load respectively between a first position, in which the load is supported adjacent said base portion, and a second position; and a plurality of linear actuators acting in use between said support frame and said base portion for producing a turning moment to pivot said support frame between said first and second orientations, wherein the turning moment produced by each said linear actuator is zero at a respective orientation of said support frame between said first and second orientations, and said linear actuators do not produce a zero total turning moment at any orientation of the support frame between said first and second orientations.

The present invention utilises the surprising discovery that it is possible to construct a load handling apparatus in a non- symmetrical manner, so that for example an apparatus having two actuators can be constructed, but because of the non- symmetrical arrangement of the actuators, one produces a non- zero moment when the other produces a zero turning moment.

Hitherto, there has been a strong prejudice in the art against constructing load handling apparatus in which the turning moment is not produced symmetrically on both sides of the apparatus.

By arranging each linear actuator to produce a zero turning moment at a respective orientation of the support frame between the first and second orientations, this provides the advantage that the moment produced by each individual actuator can be arranged to have greater magnitude at or near to the first and second orientations. This in turn reduces the mismatch between the load and actuator characteristics, thus enabling more efficient use of the linear actuators. As a result, the cost of manufacture of the apparatus can be reduced by comparison with the prior art, since expensive linear actuators having a moment producing capacity far in excess of that required do not need to be provided. In addition, in the case of an apparatus having only two actuators, the angular range over which a load handling apparatus equipped with only two actuators can effectively operate is increased.

Each said actuator may be connected between a respective first pivot on said first or second pivotable member and a respective second pivot on said base portion.

In a preferred embodiment, said second pivots are angularly offset relative to each other in planes transverse to the pivot axis.

Preferably, the position of at least one said first pivot is adjustable.

This provides the advantage of enabling the actuator pivot positions to be chosen most efficiently for different load arrangements.

The position of at least one said second pivot may be adjustable.

Each said actuator may be a hydraulic actuator.

The apparatus may further comprise hydraulic drive means for driving said hydraulic actuators such that hydraulic fluid is supplied at substantially the same pressure to each said actuator independently of the speed of movement of said actuators.

Preferably, the support member is adapted to transfer torque between said first and second pivotable members.

This provides the advantage of compensating for imbalance in the torque exerted by said actuators on the first and second pivotable members.

The support member may be of substantially circular transverse cross-section.

The apparatus preferably further comprises restraining means for limiting the extent of movement of said support frame about said pivot axis in at least one direction.

In the case of an apparatus equipped with only two actuators, this provides the advantage of avoiding a situation in which only one actuator is acting on the support frame.

The restraining means preferably comprises a flexible member extending between said first and/or second pivotable member and an attachment point fixed relative to the base portion.

Alternatively, or in addition, the restraining means may comprise means for restricting the extent of operation of at least one of said actuators.

The apparatus preferably further comprises switching means adapted to operate at different positions of the movement of the support frame to reverse the direction of motion of each said actuator.

The apparatus may comprise two said actuators.

In a preferred embodiment, the apparatus is adapted to transfer a load between said first position in which the load is supported on a ship and said second position in which the load is adjacent the ship.

A preferred embodiment of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which Figure 6 is a schematic perspective view of a load handling apparatus embodying the present invention; Figure 7 is a detailed elevational view of the A frame apparatus shown in Figure 6; and Figure 8 is a graph showing the turning moments exerted by the load and hydraulic cylinders of the apparatus of Figures 6 and 7 about the pivot point.

Referring to Figure 6, in which parts common to the arrangement of Figure 4 are denoted by like reference numerals but increased by 100, an A frame 206 is shown including a portal frame 207 comprising legs 208 and cross member 209, the portal frame 207 being pivotable about a pivot axis 211.

The pivot axis 211 passes through a pair of attachment members 230,231 having pivots 232 and 233 to which the ends of respective linear actuators in the formof hydraulic cylinders 221 are attached. It can be seen from Figure 6 that pivot 232 on attachment member 230 lies generally vertically above the pivot 210, whereas pivot 233 on attachment member 231 is displaced from a line 235 extending vertically through pivot 210. The opposite ends of the hydraulic cylinders 221 are attached to the legs 208 at respective pivots 234.

As shown in more detail in Figure 7, the pivot 233 on attachment member 231 is angularly offset by approximately 30° in an anticlockwise sense as shown in the Figure relative to the pivot 232 vertically above the pivot axis 211 on attachment member 210. As a result, the hydraulic cylinders 221 shown in Figures 6 and 7 are throughout most of the process of pivoting portal frame 207 about pivot axis 211 at different points in their respective strokes, as a result of which the turning moment applied to the portal frame 207 about pivot axis 211 by each hydraulic cylinder 221 will generally be different.

Referring now to Figure 8, the hydraulic cylinder 221 attached to pivot 232 will produce substantially zero turning moment about pivot axis 211 when angle a is approximately 90° (curve A), whereas the other hydraulic cylinder 221 connected to pivot 233 will produce substantially zero turning moment when the angle a is 120° (curve B). As a result, the total turning moment (curve Z in Figure 8) applied by hydraulic cylinders 221 is such that at no point in the travel of the portal frame 207 about pivot axis 211 is zero turning moment applied to the portal frame 207 simultaneously by both cylinders 221, with the consequence that a zero total turning moment is never applied by the cylinders 221. This results in the two hydraulic cylinders 221 being able to efficiently drive the portal frame throughout the desired range of movement of approximately 180°.

Because the hydraulic cylinders 221 produce generally zero turning moment at the 90° and 120° positions of the portal frame 207 respectively, the load is supported by only one hydraulic cylinder 221 at each of those positions. However, the load is applied through the pulley 204 which is suspended at the mid point of cross beam 209. Consequently, the load needs to be supported equally on each leg 208 of the portal frame 207.

The required moment therefore needs to be transferred across the cross beam 209 from the hydraulic cylinder 221 exerting a non zero moment, with the consequence that the cross beam 209 is designed to carry more torque than would be the case in a prior art arrangement. It has been found that it is particularly advantageous if the cross beam 209 is a tube of generally circular cross-section.

The movement of the hydraulic cylinders 221 is controlled by reversing the flow of hydraulic fluid into the cylinders 221 when they reach their shortest length in order to effect a smooth forward or backwards movement. This can be achieved by a variety of conventional control means (not shown) which will be familiar to persons skilled in the art. Such control means operate by taking a signal that indicates the position of the portal frame 207 and using it to drive a hydraulic changeover valve.

In the fully outboard position of the portal frame 207, only one of the two cylinders 221 will be fully extended. This is the position in which the greatest snatch load can be applied to the A frame system 206.

At this position, it is advantageous to provide a mechanical restraint (not shown) on one side of the portal frame to limit the extent of movement of the portal frame 207. The restraint can be a flexible member, preferably a wire rope, interposed between the pivot 234 at which the hydraulic cylinder 221 is attached to the leg 208 and extending to a point generally vertically above the pivot 210 of the portal frame 207.

Alternatively, or in addition, the outboard excursion of the portal frame 207 can be restricted to, say, 10° above the position where the shorter hydraulic cylinder 221 is fully extended. This can be achieved by means of a similar control means (not shown) to that used to reverse the direction of travel of the hydraulic cylinder 221. Any shock loads are then carried in a more symmetrical manner through hydraulic pressure on both sides of the portal frame 207.

It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. For example, more than two linear actuators, and linear actuators of different types, may be used. Furthermore, the A frame need not be used on a ship, but could also be used in any other suitable load handling environment. Furthermore, the position of pivots 232,233 on attachment members 230,231 may be adjustable, as may the position of pivots 234 on legs 208 in order to enable the A frame to be adjusted to best suit the particular load being handled.