In connection with underwater installations of heavier modules and tools in already installed structures, there is often a need for damping systems in order to prevent large dynamic impact forces due to heave movement of the installation ship. For intervention tools the requirement is normally that it must be damped from a landing speed of 1.8 m/sec to 0.1 m/sec, while for large structures which are installed with a good heave compensating system on the crane, the requirement is typically that it must be damped from 0.5 m/sec to 0.1 m/sec.

A problem with earlier damping cylinders, which generally were of the hydraulic type, was that is was difficult to make the piston return quickly enough out again in order to remain in contact with the substructure when the crane hook was subjected to a heave movement in the last phase of the lowering. This could give rise to several and possibly strong impacts before the structure came to rest on the substructure. WO 2009/011596, which is hereby incorporated by reference, shows a damping cylinder which solves this problem and concurrently provides a shock absorber which is simple and reasonable to produce and, additionally, has a quick and reliable function. Such cylinders are built in different dimensions, the largest having a weight of the piston and piston rod of 700 kg or more. During transportation to the installation site, the piston and piston rod are fixed in a retracted position and must therefore be released in order to assume their stand-by position before the structure, for instance a heavy module, is lowered. This release causes the piston and piston rod to fall freely until the piston stops with a violent and potentially destructive impact against the bottom of the cylinder. The present invention aims at dampening such impacts, and this is obtained according to the invention by a shock absorber as recited in claim 1.

Advantageous embodiments of the invention are defined in the dependent claims.

For better understanding of the invention it will be described more closely with reference to the exemplifying embodiment illustrated in the appended drawings, where all the figures 1-3 show an axial section through a shock absorber according to the invention.

The shock absorber shown in the drawings has a cylinder 1 and a piston 2 slidably arranged therein. The piston has a piston rod 3, which is solid and in the example shown has a diameter which is somewhat larger than half the diameter of the piston 2. The piston 2 with the piston rod 3 therefore has a relatively substantial weight.

The piston has an upper portion 4 having a full diameter and a lower portion with a depending annular portion or skirt 5 surrounding the piston rod 3. A number of axial through-going holes 6 are present in the upper portion of the piston. These holes 6 have together a relatively large flow area. At the top, the holes 6 are covered by a valve body 7, which is constituted by an annular disk which is axially movable within limits along a guide 8 having a stopper collar 9. The disk has two 0- rings 10 and 11 in order to seal against the piston 2 during an upward movement of the piston.

At the top, the cylinder 1 is closed by an upper cover 12 which is attached to the cylinder 1 by means of a weld 13.

At the bottom, the cylinder 1 has a threaded end piece 14 which forms a guide for the piston rod 3. The bottom side of the end piece 14 is provided with a

substantially annular plate 15, which contains a locking pin 16 which cooperates with a semi-circular groove 17 at the lower end of the piston rod 3. The locking pin 16 has a semi-circular portion where it crosses the semi-circular groove 17 of the piston rod, so that by rotating the locking pin 16, it can be brought retain the piston rod during transport and release it during commissioning on the installation site. At the bottom the cylinder 1 is provided with a number of inlet openings 19 which are in flow connection with the through-going holes 6 in the piston 2. The chamber 20 formed inside the cylinder above the piston 2 in the lower position thereof and the upper cover 13, is provided with a plurality of exit openings 21, shown here in one axially extending row. The openings 21 have decreasing diameter in the upward direction, so that when the piston moves past the openings, the flow area of the remaining openings will be reduced in an approximately exponential manner towards an asymptotic value determined by the uppermost exit opening 20 and a ventilation opening 22 through the cover 12. The end piece 14 of the cylinder is provided with a cavity 23 in the form of a recess or pocket, which will receive the depending portion of the skirt 5 on the piston when the piston is situated in its lowest position, with the piston rod extending completely out of the cylinder. In the illustrated exemplifying embodiment the annular portion 5 has a straight cylindrical form, and the cavity 23 has a conical downwardly tapering outer delimitation. The annular portion 5 fits tightly to the piston rod 3, and the inner delimitation of the cavity 23 is formed by the piston rod 3. Other embodiments are conceivable, e.g. the skirt 5 may be tapering in the direction downwards and the pocket 23 can be straight cylindrical. Besides, the skirt 5 may be arranged at a distance from the piston rod 3, so that the inner delimitation of the pocket 23 no longer will be the piston rod.

In the illustrated exemplifying embodiment the shock absorber is provided with a casing 24, also called a bucket, which is used to fill the shock absorber and form a water reservoir for testing the shock absorber on land and in readying the shock absorber from transport position to stand-by position. Figure 1 shows the shock absorber in a position for readying with a suitable water level 25 in the casing 24 and cylinder 1.

When the shock absorber is made ready for use, the piston rod 3 is initially retracted and locked in the position shown in Figure 1, while the cylinder 1 and the casing 24 are partly filled with water up to a suitable level 25. By means of an operating device (not shown) the locking pin 16 is turned a quarter of a revolution counter clockwise, alternatively three quarters of a revolution clockwise, so that the lower end of the piston rod is released. The piston rod 3 with the piston 2 will fall freely until the piston hits the water surface 25. However, this will not provide sufficient breaking because the water under the piston will be forced out through the holes 6 in the piston and the openings 19 lowest in the cylinder 1. A stronger breaking begins when the skirt 5 penetrates into the pocket 23, because the water in the pocket will have to flow out through a gap between the front edge of the skirt and the slightly conical wall of the pocket. This gap becomes gradually narrower as the skirt penetrates into the pocket, thereby providing an approximately even retardation of the piston and piston rod. Figure 2 shows the situation when the skirt has penetrated about half-way into the pocket, while Figure 3 shows the position of the parts at completion of the operation. The water levels have been left out in Figures 2 and 3. During lowering of the structure to the sea floor, the piston 2 with the piston rod 3 will assume the position shown in Figure 3. Sea water will flow into the cylinder chamber 20 through the openings 19 and piston holes 6 passed the valve body 7.

When the structure suspended in a heave compensated crane on a surface vessel has come sufficiently close to its landing spot, the piston rod 3 will impact against its intended substructure and force the piston 2 upwards in the cylinder chamber 20. The valve body 7 will keep the piston closed, forcing the water on the top side thereof to exit through the exit openings or nozzles 21, thus providing the piston with the desired piston force. As the piston moves into the chamber 20 and passes the holes 21 in succession, the available flow area through the nozzles is reduced, so that the flow resistance would increase if the penetrating velocity of the piston 2 were constant. However, the penetration velocity will decrease due to the slowing of the structure, so that by suitable sizing of the nozzles 21, the penetration force will stay substantially constant during the entire penetration. If the structure should be subjected to a heave movement before it has arrived at its final destination, the cylinder 1 of the shock absorber will move upwards together with the structure. However, the piston rod will stay in contact with the substructure because the inlet openings 19 and holes or channels 6 in the piston 2 have a sufficiently large flow area for the volume in the chamber 20 above the piston to be refilled so quickly that a sufficiently large lifting force on the piston for the piston rod to move up from the substructure will not occur. It will be

understood that only a small force will be necessary to lift the valve body 7, so that the valve body will not create any noticeable flow resistance either. Therefore, the shock absorber will be ready to perform its impact dampening function once more when the structure moves downwards again.

It will be understood that the invention is not limited to the exemplifying embodiment described above, but may be modified and varied by the skilled person within the scope of the following claims.