GB2024292A - Barriers for preventing marine collisions - Google Patents

Abstract

To protect coastlines, offshore structures or other marine structures from being struck by drifting ships, a protective barrier comprises a plurality of lines 6 which are supported adjacent the water surface by floats 3 and 7 and some of the floats 7 are moored to the seabed by anchor lines 4 of synthetic fibres. Preferred arrangements of the barrier and its moorings are described. <IMAGE>

Description

SPECIFICATION An arrangement for providing protection against collisions with drifting floating bodies The present invention relates to protection against collisions with drifting floating bodies, and more particularly to an arrangement for safeguarding permanent or temporary, fixed or floating offshore installations against ships in distress or out of control colliding therewith. The arrangement can also apply to the protection of some coasts, particularly those on which oil tankers are liable to be shipwrecked or to the protection of vulnerable port installations.

An arrangement for protecting structures at sea in shallow water against heavy-tonnage ships colliding into them has already been proposed, said arrangement being a massive structure (sea wall made of rocks, ballasted caissons, etc.) but this is assuming that the ship goes aground thereon or strikes it and is damaged thereby.

Preferred embodiments of the invention provide an arrangement which protects, in particular, structures at sea against the largest of ships in distress which are liable to collide therewith while ensuring that such ships are recoverable without being damaged and while safeguarding the protection system, said arrangement being immediately ready to repeat its performance and also being advantageously adaptable to the great depths of water. During operation, the arrangement must therefore allow the development of a shock-absorbing power which is sufficient to cancel the energy accumulated by the moving floating body, so that its travel will be stopped at a reasonable distance from the structure to be protected while keeping a comforable safety factor for the structure. Clearly small scale versions of the invention may suffice in special circumstances.

The invention provides an arrangement for providing protection against collisions with drifting floating bodies, the arrangement including a floating screen formed by protection lines carried by floats, some of said floats being connected to the sea bed by anchor lines designed and disposed to hold the screen in a position for preventing a collision and for damping the energy accumulated by the floating body in motion which strikes said screen without damaging said body, said anchor lines performing their essential shock-absorbing function by at least some of them being designed on the basis of synthetic fibre cables.

Further, the arrangement according to the invention can also have at least one of the following characteristics: - the ends of the synthetic fibres cables are connected to chains which connect them to the floats and to the sea bed, the length of the connecting chains between the floats and the upper ends of the synthetic fibres cables being greater than the draught of the largest ships to be stopped, so as to prevent damage to said cables when there is a collision.

- the floating screen is substantially polygonal and surrounds the components which are to be protected against a collision, the tops of the polygonal perimeter being occupied by main floats connected to the sea bed by main anchor lines.

- the polygonal perimeter is closed and includes a protected access passage in the form of two non-connected sides of said perimeter overlapping each other.

- polygonal perimeter is open, the end vertices being occupied by main floats connected to the sea bed simultaneously by main anchor lines and by auxiliary anchor lines which hold the screen in position; advantageously, the two anchor line systems associated with the end vertices of the polygonal perimeter are spaced out substantially symmetrically with respect to the last corresponding protection line of said perimeter.

- the main anchor lines are spaced out substantially on the bisectors of the angles at the vertices of the poligonal perimeter from the associated main floats.

- the floating screen has a substantially rectilinear shape, at least the ends of the rectilinear perimeter being occupied by main floats connected to the sea bed by anchor lines; then, the anchor systems are spaced out substantially symmetrically and orthogonally on either side of the screen, so as to delimit a channel, or, even, the ends of the rectilinear perimeter are occupied by main floats connected to the sea bed simultaneously by main anchor lines and by anxiliary anchor lines holding the screen in position, in which case it is advantageous for the two anchor line systems associated with the end of the rectilinear perimeter to be spaced out substantially symmetrically with respect to the line of said perimeter.

- the auxiliary anchor lines hold the screen in position by pre-tensioning the connected system; advantageously, the pretensioning is provided by mechanical means such as winches, disposed on the end main floats of the screen so as to allow the protection lines of said screen to be submerged at any moment to a depth of less than the draught of the smallest floating body to be immobilized; and - the floats have sufficient reserve buoyancy for them to remain at the surface due to the maximum vertical forces provided for the operation of the device.

Other characteristics and advantages of the invention will become more clearly apparent from the following description given by way of non-limiting example, with reference to the figures of the accompanying drawings, in which: - Figure lisa plan of a polygonal protection system in accordance with the invention, in the centre of which there is shown, by way of an example, an oil drilling structure to be protected; - Figure 2 is a cross-section of the system in Figure 1; - Figure 3 is a detail of an example of the connection of the various components of the system and more particularly of the main anchor lines and floats; - Figure 4 shows the position of a system for tensioning the protection screen and the connection of the end main floats to the assocated anchor systems;; - Figure 5 shows an arrangement in which shock-absorbtion by cables is replaced by a submergible float system; - Figure 6 shows the same principle, but the float is replaced therein by a mooring block; - Figure 7 is a plan of a rectilinearvariant; and - Figure 8 is a cross-section of the system in Figure 7.

Figures 1 and 2 relate to the protection of a weight structure 1 in particular a weight structure for oil production against collisions with drifting bodies and in particular with ships, by providing, in accordance with the invention, an arrangement which comprises a floating screen formed by protection lines supported by floats, some of said floats being connected to the sea bed by anchor lines designed and disposed so as to hold the arrangement for preventing collisions and to absorb the energy accumulated by a moving floating body which strikes said arrangement without damaging said body, said anchoring lines performing their essential shock-aborbing function by virtue of at least some of them being designed on the basis of synthetic fibre cables.

Here, a substantially polygonal screen 2 surrounds the structure to be protected against collision. The vertices of the perimeter are occupied by main floats 3 connected to the sea bed by main anchor lines 4. It should be observed that it is perfectly possible to provide a floating screen with a closed polygonal perimeter, said screen then advantageously including a protected access passage in the form of two non-connected sides of said perimeter overlapping each other (not shown). The end vertices of the polygonal perimeter are occupied by main floats 3' connected to the sea bed both by main anchor lines (like the floats 3) and by auxiliary anchor lines 5 which hold the arrangement in place.Thus, the floating screen has a plurality (here, five) of protection lines 6 which join two adjacent main floats together, these lines being formed by sections which are connected together by auxiliary floats 7 between which they hang in festoons.

The spacing of the anchor lines must be chosen so as to keep the arrangement balanced and to take part as much as possible in the absorption of the energy accumulated by a moving floating body which happens to strike said arrangement in any way. The spacing illustrated in Figure 1 is given by way of a preferential embodiement: the main anchor lines 4 are spaced out substantially on the bisector of the angles at the vertices of the polygonal perimeter starting from the associated main floats 3 and the systems of anchor lines provided at the ends of said perimeter (main lines 4 and auxiliary lines 5) are spaced out substantially symmetrically with respect to the last corresponding protection line 6.

A more detailed description of a very important feature of the invention, namely, the constitution of the anchor lines designed with a view to performing the essential shock-absorber function in optimum conditions will be given further on, since it is preferable first to describe the operation of the arrangement in accordance with the invention briefly.

The arrangement provided, which is advantageously adaptable to the deepest depths of water, is such that a floating drifting body 8 can collide with any point of the screen 2. When there is a collision, the device operates as follows: - in a first stage, the floating body 8 to be immobilized deforms one side of the screen and takes it along with it until it begins to generate traction in the anchor lines on the deformed side; - in a second stage, the anchor lines stretch due to their particular and original design up to a certain percentage of their length, said percentage corresponding to the absortion of the energy developed by the floating body which stops at a reasonable distance from the structure to be protected; and - in a third stage, the floating body is pushed back in a direction which moves it away from the structure to be protected by the effect of the release of the energy accumulated in the anchor lines.

Therefore, the floating body breaks its movements by collidng against the screen and deforming it, benefiting from the extension possible of the main anchor lines under the tractive force imposed by the energy of the moving body. The limit of the travel of a floating body which collides against any point of the screen is shown by the curve 9 (Figure 1).

Let us return to the constitution of the fundamental components of the arrangement in accordance with the invention and firstly to the main constitution of the protection lines of the floating screen: the choice depends of the amount of protection to be provided, the general dimensioning being subject to the principle in accordance with which the installation or the structure to be protected always remains at a reasonnable distance from the limit of the travel of any point of the screen in the maximum shock-absorbing condition.

In Figure 3, the main anchor lines 4 are fixed firstly to the associated main float 3 and secondly are anchored to the sea bed 10 by conventional means: the piles illustrated here are driven into the ground, but othertype of anchoring means could be used, depending ontthe kind of ground, e.g. special anchors or mooring blocks. The anchor lines are designed as a cable 12 of synthetic fibres whose ends are connected to chains 13, and 14 which link said cable respectively to the associated main float 3 and to the sea bed 10; the length of the chain 13 which forms the upper end of the anchor line is preferably greater than the draught of the heaviest ships to be stopped, so as to prevent damage to the cable 12 during operation.

The cables 12 are advantageously made of braided synthetic fibres because this provides excellent mechanical qualities, combining load capacity with a high modulus of elasticity: preferably, the extension of the cables will not exceed the threshold of 25% corresponding to half their breaking stress, it being understood that the size (length and cross-section) will be calculated as a function of the size and speed of the floating body which is to be stopped and therefore as a function of the energy to be damped.

By way of example, a braided nylon cable with a circumference of about 60 cm is able to withstand 750 tonnes before breaking and to be stretched by 25% at half the maximum load. The energy which such a cable 380 m long, (bent double, but applying a safety factor of 2) is E1 = 750tx2x380mx25% = 35,625tom 2 x2 Again by way of example, the energy developped by a floating body of "D" tonnes displacement drifting at a speed of 2 knots, i.e. 1 metre per second is: E1 = D/g v2 = 1/20 "D" t.m.

The mass which can be stopped at that speed by such a cable on a run of 95 m is demonstrated by the following equation: E1 = E2 35,225 t.m. = ȯ"D"t.m.

whence = 35,225 x 20 = 704,500t.

which is the effective moving mass of a ship of 500,000 t displacement.

In these conditions, an anchoring system with 8 active cables bent double, such as that illustrated in Figures 1,2 and 3, by way of an example, can stop a 400,000 t ship sailing at a speed of 6 knots or a floating body of 22,800,000 t (including its mass of moving water) drifting at 1 knot. This also corresponds to an iceberg of 15 million tonnes or to a 25 hectare surface ice-floe with a draught of 60 m.

Generally, anchors, piles, mooring blocks, chains and connections units are preferably calculated to withstand twice the maximum forces generated by the floating body whose travel is to be stopped.

Likewise, the main floats 3 and the auxiliary floats 3' are advantageously made of reinforced concrete filled with synthetic foam and have a buoyancy reserve such that they remain at the surface in spite of the maximum vertical forces exerted thereon by the operation of the screen.

It is advantageous to provide for the screen at rest to be held in position by the auxiliary anchor lines 5 in such a way that a certain pre-tension is induced in the system from the end main floats 3': indeed, the tensioning of said system must be calculated as a function of the weight and of the shape of the screen protection lines so that their maximum depression always remains less than the draught of the smallest floating body to be immobilized, whatever the condition of the sea may be. The system can be pre-tensioned by means of a tensioning device 15 (or of winches) disposed on each of the main floats 3' : therefore, in figure 4, the upper end of the auxiliary anchor line 5 which, like the main anchor lines 4, is constituted by a cable 16 made of synthetic fibres and chains (only the upper chain 17 being shown here), is inserted in the tensioning device 15.

The anchor cables, whose resilience and strength from an excellent damping system, can be replaced by equivalent mechanical systems such as anchor lines with hanging masses or with submergible intermediate floats designed in such a way that the weight of the hanging masses or the buoyancy reaction of the floats cause, by their movement, the required damping.

Figures 5 and 6 illustrate these two ways of modifying the previously illustrated synthetic fibre cable damping principle by an equivalent mechanical system.

In the first case (figure 5), an anchor line 18 carries a submergible intermediate float 19 which is pulled down under the surface under the effect of the load of the anchor line (maximum position illustrated by a dashed line). The damping of the system is a function of the buoyancy reserve F of the float 19 and of its travel C. Therefore, damping which is comparable to that of the example chosen for a nylon cable with a circumference of about 60 cm and a length of about 380 m leads to a float whose characteristics are given by the following equation: 35,625t.m. = Ft x Cm which shows that an immersion of 50 m leads to envisaging a submergible float with a buoyancy reserve of 713 m3, this corresponding to a sphere with a radius of 6 m.

In the second case (Figure 6), the submergible float 19 is replaced by a mooring block 20 whose action is identical, providing its apparent weight in water is equal to the buoyancy reserve of the float 19 and that its travel C is comparable.

Lastly, Figures 7 and 8 illustrate a variant of the device in accordance with the invention. In said variant, the floating screen 2 has a substantially rectilinear shape: such a variant can be very advantageous, for example, to delimit a channel effectively. The component parts are the same for this variant as for the preceding example described in detail hereinabove. In both examples, identical reference symbols have been used for clearness' sake. it should be observed that the anchor connection systems are spaced out in a substantially symmetrical way (and orthogonally for the main floats which are not related to the ends of the rectilinear perimeter) on either side of the screen.

It is self-evident that the invention, whose many advantages have been set forth and described and whose applications to very varied types of protection, is in not way limited to the examples which have been given by way of illustration, but includes all variants which resume with equivalent means the characteristics of the invention such as claimed.

Claims (15)

1. An arrangement for providing protection against collisions with drifting floating bodies, the arrangement including a floating screen formed by protection lines carried by floats, some of said floats being connected to the sea bed by anchor lines designed and disposed to hold the screen in a position for preventing a collision and for damping the energy accumulated by the floating body in motion which strikes said screen without damaging said body, said anchor lines performing their essential shock-absorbing function by at least some of them being designed on the basis of synthetic fibre cables.

2. An arrangement according to Claim 1, wherein the ends of the synthetic fibre cables are connected to chains which connect them to the floats and to the sea bed, the length of the connecting chains between the floats and the upper ends of the synthetic fibre cables being greater than the draught of the largest ships to be stopped, so as to prevent damage to said cables when there is a collision.

3. An arrangement according to Claim 1 or 2, wherein the floating screen is substantially polygonal and surrounds the components which are to be protected against a collision, the tops of the polygonal perimeter being occupied by main floats connected to the sea bed by main anchor lines.

4. An arrangement according to Claim 3, wherein the polygonal perimeter is closed and includes a protected access passage in the form of two non-connected sides of said perimeter overlapping each other.

5. An arrangement according to Claim 3, wherein the polygonal perimeter is open, the end vertices being occupied by main floats connected to the sea bed simultaneously by main anchor lines and by auxiliary anchor lines which hold the screen in position.

6. An arrangement according to Claim 5, wherein the two anchor lines systems associated with the end vertices of the polygonal perimeter are spaced out substantially symmetrically with respect to the last corresponding protection line of said perimeter.

7. An arrangement according to Claim 3,4,5 or 6, wherein the main anchor lines are spaced out substantially on the bisectors of the angles at the vertices of the polygonal perimeter from the associated main floats.

8. An arrangement according to Claim 1 or 2, wherein the floating screen has a substantially rectilinear shape, at least the ends of the rectilinear perimeter being occupied by main floats connected to the sea bed by anchor lines.

9. An arrangement according to Claim 8, wherein the anchor systems are spaced out substantially symmetrically and orthogonally on either side of the screen, so as to delimit a channel.

10. An arrangement according to Claim 8, wherein the ends of the rectilinear perimeter are occupied by main floats connected to the sea bed simultaneously by main anchor lines and by auxiliary anchor lines holding the screen in position.

11. An arrangement according to Claim 10, wherein the two anchor line systems associated with the ends of the rectilinear perimeter are spaced out substantially symmetrically with respect to the line of said perimeter.

12. An arrangement according to any Claim 5 or 6, wherein the auxiliary anchor lines hold the screen in position by pre-tensioning the connected system.

13. An arrangement according to Claim 12, wherein the pretensioning is provided by mechanical means such as winches, disposed on the end main floats of the screen so as to allow the protection lines of said screen to be submerged at any moment to a depth of less than the draught of the smallest floating body to be immobilized.

14. An arrangement according to any one of Claims 1 to 13, wherein the floats have sufficient reserve buoyancy for them to remain at the surface in spite of the maximum vertical forces provided for the operation of the device.

15. An arrangement for providing protection against collisions with drifting floating bodies, substantially as herein described with reference to and as illustrated in the accompanying drawings.