IE43330B1 - Platform for marine work - Google Patents

Description

The present invention relates to a platform for ntarine work, having a buoyant foundation member which can be lowered into the sea bed, and from which support legs extend upwardly, and a floatable deck which embraces the legs. In another aspect it relates to a method of varying the separation of the deck and foundation member of such a platform.

A platform of this kind is known whose legs comprise steel lattice towers adapted to be pushed telescopically one within the other, the lower elements being anchored to a float ι able foundation menber, and the upper elements being adapted to be secured to a buoyant deck which embraces the towers.

To seat the platform on the seabed, the foundation menber is flooded and, lowered with the aid of cables connected to winches on the floating deck.

This known platform cannot have relatively large dimensions and is unsuitable for use in considerable depths of water, such as 30 m or more. For greater water depths, there have recently been constructed heavy-weight platforms, the foundation members and supports of which are made from materials such as reinforced or prestressed concrete which are resistant to seawater and sufficiently stable to take up the considerable dynamic forces resulting from sea motion and wave action, and to carry a large deck having a high useful load in spaced relationship over the water surface. In this connection, one endeavours to achieve the smallest possible draft of the platform when in its float-out state, in order - 3 that it may be manufactured in shallow waters and floated through shallow waters to the site of erection. During such a journey, the platform should preferably be capable of carrying a large useful load, so as to be able to carry on deck all the necessary machines and equipment for the work to be performed at sea, for example, drilling for oil. Separate transport to the site and assembly there of such items would be costly and inconvenient owing to dependence on weather conditions.

With a buoyant deck and a floating foundation member, it becomes possible in a heavy-weight platform to carry high loads thereon, so that all the essential machines and devices, such as derricks, cranes, pipes and pumping installations can be assembled at the site of manufacture in the vicinity of the shore. When a platform is floating with a heavily laden deck and with its support legs projecting high into the air, its centre of gravity is generally located above its centre of buoyancy, but the inherent stability of the foundation member projecting together with the deck above the water surface adequately enables the platform to be transported reliably to the work site. There, however, when the foundation member is being lowered on to the sea bed, the platform passes through a critical phase in its stability.

When the foundation member passes completely under water, its stabilising effect on the platform is lost. Since the centre of gravity of the entire system is located above the centre of buoyancy, there is a risk of the platform capsizing if lateral disturbing forces, such as wind and swell, act on the platform during lowering of the foundation member. In this case, the deck floating on the surface is not able to adequately stabilize the foundation member, since the cables - 4 42330 connecting the foundation member to the deck can become slack intermittently, owing to deck oscillations caused by the swell, and they are not then able to transfer to the foundation member the restoring moment of the deck. Additionally, the periodic oscillations of the deck produced by the swell set up considerable mass forces in the cables, which may readily result in rupture thereof.

It is the object of the invention to obivate these difficulties and to so design the platform that it is able to float reliably with a laden deck during transport to the work site, and to be lowered to the sea bed, at the work site, with little danger of capsizing.

According to the present invention, there is provided a platform for marine work including: a floatable foundation > member; support legs attached to the foundation member and extending upwardly thereof; ties extending from the foundation member to the upper ends of the support legs; a floatable deck embracing the support legs; and tractive devices arranged on the deck for engaging the ties and effecting relative move) ment of the deck and the ties whereby the separation of the deck and the foundation member is variable, in order to lower the foundation member from a position adjacent the deck at the sea surface and to raise the deck above the sea surface, said ties being prestressed to such an extent that they substantially never become tensionless in any part as a consequence of wave action during such variation of the separation of the deck and the foundation member. In another aspect the invention provides a method of varying the separation of a floatable deck and a floatable foundation member of a platform according to the first aspect of the invention said method comprising prestressing said ties and effecting relative movement 3 330 - 5 between said deck and ties by means, of said tractive devices, said prestressing of the ties being such that the ties substantially never become slack in any part owing to wave action.

This design has the advantage that the deck remains firmly connected to the foundation member in every position, by means of tensioned ties. The foundation member has great inertia due to its mass, and also resists sudden vertical movement due to its large area (viscous drag). Once it has been lowered beneath the sea surface, out of the influence of waves and swell, it acts to stabilise the deck. For example, when the deck passes into a wave trough, it tends to fall. However, movement towards the foundation member is restrained by tension in the sections of the ties located between the deck and the upper ends of the legs. At the same time, the tension in the lower tie sections between the deck and the foundation member is reduced. However, with normal wave sizes this tension is not reduced to zero, so the ties remain taut, and the deck and foundation member remain virtually rigidly connected as the trough gives way to a wavecrest.

When a wave crest impinges on the platform, it tends to raise the deck, but this is restrained by tension in the lower sections of the ties. Since the deck and foundation member are connected by ties which do not slacken, the inertia of the foundation member prevents substantial riding up and down or rocking of the deck.

Even if external forces such as wind and swell, do displace the platform out of its vertical position during lowering of the foundation member, the floating deck, then dips deeper into the water on one side under the influence of the disturbing forces, so that the buoyancy generates an erecting moment opposing the capsizing moment, and which acts - 6 as a couple on the foundation member, on the immersed side directly via the lower sections of the ties, and on the other, emerged, side via the upper sections of the ties and the support legs.

Due to their pretensioning, individual ties are prevented from alternately becoming slack and retensioned and thereby subjected to high impulsive stresses, which might result in a considerable degree of deformation and fatigue. Additionally, there is thereby prevented impulse loading of the raising devices and the points of attachment of the ties, which is particularly dangerous, since it may result in sudden failure.

The ties should be prestressed to such an extent that none of their sections can become untensiohed because of the effect on the floating deck of the highest expected waves. Variations in tension produced by the motion of the sea then lead only to an increase or reduction of the pre-existing tensions in the ties, there occurring simultaneously in the lower and upper section of each tie a change in the tension which, however, never reaches zero.

Since the ties extend through from the foundation member to the upper end of the support legs they can, in combination with the lowering devices, be directly employed both for setting down the foundation member on the sea bed and also for raising the deck into its working position above the water surface.

The ties may be cables roven on pulley blocks arranged on the deck and/or on tha foundation member and/or at the upper end of the supports,one end of the cables being wound on winches arranged on the deck. The high tensions required for vertical movement of the foundation member or the deck can in - 7 43330 this way be distributed to a plurality of cables, and controlled by the winches. Each tie is secured at one end to the foundation member, and at the other end to the upper end of one of the support legs, and may extend through the deck.

When either lowering the foundation member or lifting the deck the same winches may operate on the same cables, without it being necessary to interchange the cables or to reverse the direction of rotation of the winches.

Alternatively, the raising and lowering devices may also be hydraulic climber jacks which grasp the ties. A climber jacks is a hydraulic lifting device having two cable clamps, a first one of which is connected to the deck, the second being connected to a piston which is vertically movable relative to the deck. For example, to raise the deck, the first clamp grips the cable, and the second is raised by means of the piston, and then caused to grip the cable. The first clamp is released, and the piston is retracted, whereby the deck is raised. The first clamp then grips the cable again, the second clamp is released, and the cycle can be repeated.

When such jacks are used, pre-stressed steel rod may be employed as ties.

An embodiment of the present invention will now be described in detail, with reference to the accompanying drawings, in which:Figure 1 is an elevation view partially in section, of a platform embodying the invention in its float out state; Figure 2 shows the platform of Figure 1 during lowering of the foundation member; Figure 3 is an elevation view partially in section of the platform with the foundation member set down on the sea bed; - 8 Figure 4 shows the platform of Figure 3, in the working position with the deck raised; Figure 5 is a diagram corresponding to Figure 2 and showing the stressing caused by wave action; Figure 6 is a diagram corresponding to Figure 5, illustra ting the stressing caused by lateral forces; Figure 7 is a larger scale diagram of part of Figure 2 showing the arrangement of the ties and one type of raising and lowering device; and ) Figure 8 shows on a larger scale a climber jack.

The drawings illustrate a platform 10 for marine work, designed as a centre-of-gravity platform and usable, for example, as a drilling island for oil-drilling in deep water. The platform 10 comprises a buoyant foundation member 11 made from reinforced or prestressed concrete, and having an internal space 12 sub-divided into a plurality of cells, which can receive ballast 13 in the form of sand, mud or like dense material and be flooded and again pumped out. It can also be employed for the storage of oil. Extending upwards from . the foundation member 11 are four support legs 14 which are anchored therein and which are made from prestressed concrete or steel and are preferably tubular, they are guided for displacement in the vertical direction through a buoyant deck 15. The deck 15 preferably comprises a steel box-form platform carrying a derrick 16 and other devices (not shown) for performing the work to be carried out on location. The support legs 14 extend through the deck 15 in suitable apertures 17, in which the supports 14 are laterally guided on lowering the foundation member 11 on to the sea bed 18, and via which horizontal forces are transferred from the deck 15 to the legs 14, and vice versa. 433 3 0 - 9 The deck 15 is connected with the foundation member 11 by ties 19 extending through suitable ducts 20 in the deck 15 and extending the entire length of the legs 14. Each tie 19 is secured at one of its ends 21 to the foundation member 11, and at its other end 22 to the upper end 23 of a leg 14 at a carrier head 24 which, during float out, lowering of the foundation member, and pulling up of the deck on the legs 14, is secured to a leg 14.

Associated with each of the braces 19 is at least one raising and lowering device 26 arranged on the surface 25 of the deck 15. The device 26 displaces the deck 15 along the ties 19.

In the float out state shown in Figure 1, the deck 15 bears on the buoyant foundation member 11, and is secured to the latter via the ties 19, by which the foundation member 11 is pulled towards the deck 15, with the aid of the lowering devices 26. The space 12 in the foundation member 11 is completely pumped out, and the platform floats on the water surface 28 and is towed by tugs 29,to the work site.

Having arrived at the work site (Figure 2) the foundation member 11 is released from the deck 15 and flooded to such an extent that its weight, and the weight of the support legs 14 secured to it, reaches or somewhat exceeds the buoyance of these components. The foundation member 11 is then suspended by the lower sections 19b of the ties 19, from the devices 26 continuously and uniformly lower the foundation member 11 until it reaches the sea bed 18.

During lowering the deck 15 floats on the water surface 28 and the ties 19, each of which extends over the entire length of the support legs 14, travel through the devices 26. Meanwhile, the tugs 29 hold the platform at the correct - 10 position, so that it cannot drift because of currents, wind or motion of the sea.

Since, at sea even in good weather there are normally wind and swell, it is necessary to allow for the effect of waves on the deck whilst the foundation member, 11, after reaching a predetermined depth, has been withdrawn from the influence of the swell. The waves tend to raise and lower the floating deck 15. This is resisted by the inertia of the foundation member 11 so that extra tensions are introduced alternately into the lower sections 19b and the upper sections 19a of the ties 19, and other sections in each case experiencing reduced tensions. If, for example, a wave crest 30 tends to raise the deck 15, the tension in the Upper sections 19a of the ties 19 is reduced, whereas the tension in the lower sections 19b is increased (see Figure 5). If, on the other hand, the deck 15 is urged downwards on encountering a trough 31, then the distribution of tensions is the reverse of that just described. In either case, the inertia of the member 11 is transmitted through the ties 19 to the deck 15, which is ) therefore stabilised, and moves little.

This alternating loading causes large changes of stress in the ties 19 which must be prestressed to such an extent that none of their lower sections 19b and upper sections 19a located above and below the deck 15 become tensionless under the influence of the highest waves 30 and 31 to be expected on the floating deck 15. Thus, the ties 19 are not subjected to impulsive loading, but only to swell stressing i.e. the tensile stressing effect in them becomes gradually larger or smaller depending on the motion of the sea, but does not suddenly increase from zero to a higher value or decrease therefrom to zero. - 11 Figure 6 shows a situation during lowering of the platform when lateral disturbing forces, such as wind and swell, act to tilt the platform. Since the centre of gravity G is initially above the centre of buoyancy A, the tilting tendency increases. With increasing tipping, however, more of the deck 15 is submerged on one side than at the opposite side, and the buoyancy forces on the more submerged side become greater than on the opposite side. This produces a restoring force which is transferred, as a couple, via the lower sections 19b of the ties 19 to the support legs 14 on the more submerged side, and via the upper section 19a of the braces 19 to the support legs 14 which are on the opposite sides. The legs transmit the couple to the foundation member 11, so that the system is once again able to right itself. Thus the deck 15 thereby makes possible stabilization of the platform against capsizing during lowering of the foundation member 11. Here again, the prestressed ties 19 are subjected only to swell tension variation, resulting from the gradual inclination during swaying of the platform.

For lowering the foundation member 11, and subsequent lifting of the deck 15 out of the water, there may be employed winches 33 (shown purely diagrammatically in Figure 7) arranged on the deck 15, the ties 19 in this case being suitably highstrength steel ropes wound over a plurality of pulley blocks 34 and 35, some of the pulley blocks 34 being secured to the carrier head 24 of the supports 14 or to the upper side 36 of the foundation member 11, and the others 35 to the upper side 25 or to the under side 37 of the deck 15.

Instead of steel cables, prestressed steel rods may be used as the ties 19. In this case, it is expedient to employ as raising and lowering devices hydraulic climber jacks 38 - 12 /13 3 3 0 (one of which is shown schematically in Figure 8) which could also be employed with steel ropes. Each jack 38 comprises one or more hydraulic cylinders 39 carrying a lower clamp 40 by means of which it is able to secure itself to a tie 19. Arranged on a piston 41 of each cylinder 39 is an upper tie clamp 42 which is also able to grasp the tie 19. The hydraulic climber jack 38 operates in a housing 43 which is fast with the deck 15 and through which extends the tie 19.

Referring to Figure 8, initially the climber jack 38 is 0 firmly clamped by its lower clamp 40 to the tie 19. As the pistons 41 extend, the upper clamp 42 slides loosely along the tie 19 and pushes the housing 43 upwards, and with it the deck 15. As soon as the pistons 41 have extended completely, the clamp 42 grasps the tie 19, and holds up the housing 43 and the deck 15, while the lower clamp 40 is opened, and the pistons 41 are retracted into the cylinders 39, the clamp 40 sliding upwards a predetermined distance along the tie 19.

The clamp 40 is then clamped firmly on the tie 19, the upper clamp 42 is opened, and the procedure just described recom) mences.

When lowering the deck or lifting the foundation member, the same procedure takes place in reverse.

After the foundation member 11 has been lowered, it is completely flooded, and the deck is, with the aid of the raising and lowering devices 25, raised out of the water and pulled along the support legs 14, until it reaches the position shown in Figure 4.

When this state has been reached, the carrier head 24 on the support legs 14 can be removed, together with the ties 19, if other devices are provided for securing and connecting the deck 15 to the support legs 14. With the deck lifted out - 13 of the water, the platform is stable, and little affected by wind and waves.

The invention is not restricted to the embodiment shown. For example, instead of ropes or rods, bundles of rods may be used for the ties. The ties may be treated to protect them against corrosion. Instead of the raising and lowering devices described, any other suitable devices may be employed.

Claims (9)

1. CLAIMS:1. A platform for marine work including: a floatable foundation member,- support legs attached to the foundation member and extending upwardly thereof; ties extending from the foundation member to the upper ends of the support legs; a floatable deck embracing the support legs; and tractive devices arranged on the deck for engaging the ties and effecting relative movement of the deck and the ties whereby the separation of the deck and the foundation member is variable, in order to lower the foundation member from, a position adjacent the deck at the sea surface and to raise the deck above the sea surface, said ties being prestressed to such an extent that they substantially never become tensionless in any part as a consequence of wave action during such variation of the separation of the deck and the foundation member.

2. A platform according to claim 1, wherein the ties comprise cables which are reeved through pulleys arranged on the deck and/or the foundation member and/or the upper end of the supports, and which travel over winches arranged on the deck.

3. A platform according to claim 1 or 2, wherein each tie is secured at one end to the foundation member, and at the other end to the upper end of one of the supports, and extends through the deck.

4. A platform according to any one of the preceding claims, wherein the ties comprise steel rods.

5. A platform according to any one of the preceding claims, wherein the tractive devices are hydraulic climber jacks which grasp the ties.

6. A platform according to any one of the preceding - 15 claims, wherein the deck and the foundation member are clampable together for floating out by means of the lowering devices and ties.

7. A platform for marine work, constructed and arranged 5 to operate substantially as herein described, with reference to and as illustrated in the accompanying drawings.

8. A method of varying the separation of a floatable deck and a floatable foundation member of a platform for marine work having a floatable foundation member; support 10 legs attached to the foundation member and extending upwardly thereof; ties extending from the foundation member to the upper ends of the support legs; a floatable deck embracing the support legs; and tractive devices arranged on the deck for engaging the ties, said method comprising prestressing said 15 ties and effecting relative movement between said deck and ties by means of said tractive devices, said prestressing of the ties being such that the ties substantially never become slack in any part owing to wave action.

9. A method of varying the separation of a floatable 20 deck and a floatable foundation member of a platform for marine work substantially as described herein with reference to and as illustrated in the accompanying drawings.