GB2159481A - Method of lifting an object from a surface exterior to a vessel - Google Patents

Abstract

A method is provided of using a vessel provided with lifting means, to effect or assist the lifting of an object from a surface exterior to the vessel. In one mode, the method comprises rapidly increasing tie buoyancy of the vessel by supplying air or other gas to at least one buoyancy chamber (1) thereof. In another mode, the method comprises producing a first alteration in the buoyancy of the vessel in a first sense prior to commencing lifting, by altering the pressure of air or other gas in at least one buoyancy chamber (1) opened to allow water to enter therein, and producing a second alteration in the buoyancy of the vessel in the opposite sense by altering the pressure in the said at least one buoyancy chamber (1), the second alteration being rapid compared to the first alteration and serving to effect or assist the said lifting. <IMAGE>

Description

SPECIFICATION Method of lifting an object from a surface exterior to a vessel This invention relates to a method of lifting an object from a surface exterior to a vessel, and to a vessel having lifting means.

U.K. Patent Specification GB 2091192A (of which the inventor is also the inventor of the present invention) discloses a vessel provided with a motion suppression system to reduce the effect of waves on the vessel, and, in particular, to provide for a reduction at least in the heave motion of the vessel. The specification describes two versions of the system, namely an unpowered version and a powered version. The powered version, although capable of effecting a greater degree of motion suppressing, requires at least one compressor as a source of compressed air, and the simple unpowered version is therefore preferred. The unpowered version will now be summarised briefly.

The motion suppression system is based around a system of chambers which are open at their bottom to the sea and pass through the still water line of the vessel. The water level inside the chambers is at still water level in calm water. A volume of air can be trapped above this internal water level by shutting a valve connecting the air volume to atmosphere.

The action of waves on the underside of the chamber compresses and rarifies the trapped air above and below the atmospheric pressure and serves to generate an oscillating force on the vessel which is out of phase with and thus tending to cancel the predominant wave forces on the vessel.

The net effect of the chambers is, therefore, to substantially reduce wave-induced forces and motions of the vessel. In normal operations, the motion suppression chamber valves are closed.

The motion suppression system is shown in Figures 1 to 3 of the accompanying drawings, in which: Figure Th shows a side elevation of the motion suppression system as applied to a semisubmersible; Figure ib is a view taken on line X-X in Figure 1a; Figures 2a, 2b and 2c are a side elevation, a plan view and an end elevation of the system as applied to a monohull; Figure 3a is a side elevation of an externally mounted buoyancy chamber; Figure 3b is a view taken on line Z-Z in Figure 3a.

Figure la shows motion suppression chambers 1 mounted on vertical columns of a semisubmersible 2. As shown in Figure 1b the motion suppression chambers can be separated by vertical partitions into segments T1, T2, T3 and T4. In the installation of the systems on a monohull vessel, shown in Figures 2a, 2b and 2c, the still water surface is denoted by A with the water level inside the chambers being marked by T, the motion suppression chambers 3 are mounted on the bow and stern as well as on both beams of the monohull vessel 4.

The chambers are separated by partitions 5, and are open to the sea at their base 6.

Figure 3a shows an externally mounted form of chamber 8, the chamber 8 being mounted on a semisubmersible column 7. It is open to the sea at its base 9, with the internal and external water levels being denoted by B and A respectively. The chamber can be partitioned by vertical walls 10. A horizontal annular bulkhead 11 traps a volume of air above the internal water level. A valve 12 can be maintained in the open or shut position to respectively switch off or activate the motion suppression function. The chambers for a monohull vessel shown in Figure 2 have a similar construction to that shown in Figure 3, and may be internally mounted (as shown in Figure 2) or externally mounted (as shown in Figure 3).

For a semisubmersible vessel, the motion suppression operates on the basis of the fact that ocean wave induced oscillating forces on the chambers are out of phase with conventional wave induced forces on the vessel. Thus wave forces on the chambers tend to cancel out wave forces on the rest of the vessel, thereby reducing the waveinduced heave, roll and pitch motions of the vessel. The chambers also introduce a small additional reduction in motions due to the presence of a damping contribution in the chamber water column motion mode due to gravity wave radiation from the chambers.

For a monohull vessel, reductions in motions are primarily achieved by modifying the natural periods of the vessel in the valve open and valve shut configurations such that by choosing an appropriate valve setting, i.e. appropriately selecting those valves which are to be open and those which are to be closed, wave induced motions of the vessel due to the prevailing wave period range can be minimised. This is particularly true for roll motions of monohull vessels which are reduced substantially due to the suppression system. Heave and pitch motions are also reduced, but to a lesser degree.

The present inventor has now appreciated that the mean pressure in the motion suppression chambers due to vessel motion and wave action, with the valves closed, only influences the motion performance to a small extent. Thus, the mean pressure in the chambers can be at, above or below atmospheric pressure without markedly affecting the extent of motion suppression obtained.

This fact is of significance for the present invention, as will become apparent from what is said below.

According to the present invention there is provided a method of using a vessel provided with lifting means, to effect or assist the lifting of an object from a surface exterior to the vessel, the method comprising rapidly increasing the buoyancy of the vessel by supplying air or other gas to at least one buoyancy chamber thereof.

The invention further provides a vessel having lifting means for lifting of an object from a surface exterior to the vessel, the lifting means including vided a tyre for vehicle wheels, comprising in the crown zone, a tread-band that is axially extended from one sidewall to the other of said tyre, and between which there is impressed a pattern that comprises a plurality of circumferential zig-zag grooves delimiting a plurality of ribs, the two axially outer ribs being circumferentially continuous, with at least one axially inner rib being subdivided into a series of circumferential blocks separated from one another by a continuous transverse lamella or slot that extends from one side of the rib to the other and opens out into both the grooves that delimit said rib, characterized by the fact that the circumferential zig-zag line that delimits each rib is constituted by the periodic succession of one long side - inclined with respect to the circumferential direction of said tyre, and at least one short side - of a length not greater than 35% of said long side and disposed substantially perpendicular to the latter, and that the lamella or slot separating the two contiguous blocks from each other, has a broken line lay-out that is constituted by three successive segments - the central straight segment extending between two points, at least one of which lies outside of the circumferential interval delimited by the pair of axial straight lines that pass through the points where said slot opens into said grooves.

In one embodiment of the tyre described in the preceding paragraph, the circumferential zig-zag line that delimits each rib may be constituted by the periodic succession of one "long" side and three "short" sides, said lamella or slot extending axially between the two vertices respectively created by the meeting of two short-sides on each sidewall, said long-side being inclined with respect to the circumferential direction of the tyre according to an angle of not greater than 30 , the angle formed between two consecutive sides of said circumferential zig-zag line being comprised between 80"-100".

In combination with said zig-zag development, the central segment of the broken-line constituting said lamella or slot may have a length of not less than 25% of the axial development. Preferably, the two end segments of said broken:line constitute the extension of one of the said short-sides, while the angle formed by two consecutive segments of said broken-line is at least equal to 30 .

As for the dimensions of said lamellae or slots, their depth may be at least equal to 60% of the said circumferential grooves, while their width may be such as not to exceed the value of 3 mm. However, the segments of the broken-line constituting said lamella or slot may also not be of the same width.

Some embodiments (given by way of example only) of a tyre according to the present invention will not be more particularly described with reference to the accompanying diagrammatic drawings, in which: Figure 1 illustrates, in a general way, the principle underlying the invention; and Figure 2 illustrates one embodiment for the tread pattern of a tyre according to the invention, which has been further improved, and to particular advantage.

Referring to Figure 1, this illustrates a limited portion of the circumferential development of the tread pattern in question, which is therefore repeated, cyclically and identically, around the entire circumference of the tyre.

This pattern comprises three circumferentially extending grooves 1 which delimit two more or less central ribs 2 which are substantially of the same width. There are also two shoulder ribs 3 in an axially outer position, which have their respective axially inner walls shaped according to a zig-zag configuration and substantially parallel to the axially outer walls of the respective adjacent ribs 2.

Said zig-zag configuration is constituted by the periodic succession of a side "a" which we shall define as being "long", and a side "b" which we shall define as being "short" because said short-side has a length not exceeding 35% of the length of the long-side. Moreover, the ribs 2 are subdivided into a plurality of circumferentially disposed blocks, created by a series of transverse lamellae or slots 4 (hereafter simply called slots); each slot is constituted by three segments "c", "d", "e", of which the two lateral segments "c" and "e" depart from the metting-points A and B, respectively, between the iong-side and the short-side, in the zone where a convex angle lies, corresponding to the interior of the rib.

It can be noted that this type of transverse slot 4 which is disposed in the manner indicated and which is coupled with the particular type of zig-zag profile of the ribs creates an indenting or interlocking between two contiguous or circumferentially adjacent blocks which is such as quite easily to cope with or absorb the transverse thrusts (namely, those thrusts which are directed truly or substantially axially of the tyre on the wheel) and to impede any relative movement whatsoever between said contiguous blocks in the direction of said thrusts.

In the version shown in Figure 1, each slot 4 connects two vertices, on opposed sides of the ribs, which are circumferentially closer together. If from each of these vertices, we were now to draw lines transversely of the tread pattern and therefore parallel to the tyre axis (respectively "r" and "s"), said two lines would delimit a circumferential area "M" on the rib (shown by hatching lines between the lines "r" and "s" in the Figure) having a circumferential length or development "x". It is to be noted that the central segment "d" of the slot 4 extends between two points P and Q of which the point P is external to said hatched area; consequently, the circumferential development P-Q is such as to guarantee the effect of a reciprocal locking of the contiguous blocks.

This idea or principle underlying the Applicants' solution of the problem has been further developed and elaborated in the tread pattern shown in Figure 2 that illustrates one generally preferred embodiment of a tyre according to the invention. Said tread pattern comprises four circumferential grooves 5 which together delimit three circumferential and more or less central ribs 6 which are all substantially of the same width. There are also two shoulder ribs 7, in axially outermost positions, whose axially inner walls are shaped in a zig-zag configuration and are clearance in a short space of time typically 9 to 45 seconds. The rapid trim system can provide a rapid draught and trim or heel change which will rapidly lift and lever the weight through the lift off clearance.

The need for direct use of compressed air or vacuum can be reduced (with consequent power and equipment savings) if the concept of preloading is used. If the required rapid draught, trim and heel changes and the necessary chamber water level movements above or below still water are known before the rapid trim system is operated, chamber water level changes equal to those required, but of opposite sense, can be preset by smaller compressor and vacuum pumps over a longer period of time prior to rapid trim system operation. The air pressures in the chambers will now be above or below atmospheric pressure. The out of trim movements or draught change induced during the preloading can be compensated for by a water ballast system, if necessary.

Now, at the time that a rapid trim change is required, the pressure preloading in each chamber can be released to atmosphere at the appropriate rate in order to induce the necessary draught, trim or heel changes. The pressure release can be performed through the same valves as those used for the motion suppression system (denoted by 12 in Figure 3).

Application of the preloading concept does substantially reduce compressor and vacuum pump power and weight requirements. There are attendant disadvantages associated with the fact that the preloading procedure requires time prior to application of the rapid trim system and assumes that the required ballast system condition (or chamber water levels) at the final stage of its application are known.

Claims (14)

1. A method of using a vessel provided with lifting means, to effect or assist the lifting of an object from a surface exterior to the vessel, the method comprising rapidly increasing the buoyancy of the vessel by supplying air or other gas to at least one buoyancy chamber thereof.

2. A method of using a vessel provided with lifting means, to effect or assist the lifting of an object from a surface exterior to the vessel, the method comprising producing a first alteration in the buoyancy of the vessel in a first sense prior to commencing lifting, by altering the pressure of air or other gas in at least one buoyancy chamber opened to allow water to enter therein, and producing a second alteration in the buoyancy of the vessel in the opposite sense by altering the pressure in the said at least one buoyancy chamber, the second alteration being rapid compared to the first alteration and serving to effect or assist the said lifting.

3. A method according to claim 1 or 2, wherein the lifting means comprises a crane.

4. A method according to any preceding claim, wherein the or each change in buoyancy induces a pitching moment.

5. A method according to any preceding claim, wherein the or each change in buoyancy induces a rolling moment.

6. A method according to any one of claims 1 to 3, wherein the or each change in buoyancy induces a pure heaving force.

7. A method of using a vessel, substantially as herein described with reference to Figures 4 and 5 of the accompanying drawings.

8. A vessel having lifting means for lifting an object from a surface exterior to the vessel, the lifting means including means for rapidly increasing the buoyancy of the vessel by supplying air or other gas to at least one buoyancy chamber thereof.

9. A vessel having stabilizing means for alleviating at least heave, comprising a chamber disposed to lie at least partly beneath the surface of the water when the vessel is immersed or semi-immersed in water, the chamber being open to allow water to enter, means for detecting wave-induced motion of the vessel and for producing a signal in accordance therewith, and valve means selectively movable in response to the said signal to a first position in which said chamber is connected to atmosphere to permit air to enter or leave said chamber in response to the lowering or rising of the water level in said chamber due to wave action, and a second position in which air is not permitted to enter or leave said chamber, thereby to alleviate at least the heave motion of the vessel, the vessel further comprising lifting means for lifting an object from a surface exterior to the vessel, the lifting means including means for rapidly increasing the buoyancy of the vessel by supplying air or other gas to the said chamber or at least one of the chambers.

10. A vessel according to claim 8 or 9, wherein the lifting means comprises a crane.

11. A vessel according to any one of claims 8 to 10, wherein the buoyancy increasing means are such as to permit the generation of a pitching moment.

12. A vessel according to any one of claims 8 to 11, wherein the buoyancy increasing means are such as to permit the generation of a rolling moment.

13. A vessel according to any one of claims 8 to 12, wherein the buoyancy increasing means are such as to permit the generation of a pure heaving force.

14. A vessel having lifting means, substantially as herein described with reference to Figures 4 and 5 of the accompanying drawings.