IE48433B1 - An arrangement for providing protection against collisions with drifting floating bodies - 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.

Description

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 nas already been proposed, said arrangement being a massive structure (sea walls 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 comfortable safety factor for the structure. Clearly small scale versions of the invention may suffice in special circdmstances.

The invention provides a barrier arrangement for keeping moving floating bodies out of a stretch of water to be protected, the barrier canprising a floating screen formed by protection lines supported byfloats, and anchor lines connecting seme of the floats to the water bed, the anchor lines being disposed to retain the screen in place under normal conditions, and, in the event of a moving floating body colliding with the screen being designed and disposed to absorb the kinetic energy of the said moving floating body without causing damage thereto, wherein at least seme of the anchor lines, denoted main anchor lines, are constituted over a majority of their length by cable of resiliently extensible synthetic fibres with relatively short lengths of chain attached to each end of the resilient cable, said lengths of chain serving to connect the anchor lines to the floats and to the water bed, and the lengths of chain at the upper ends of the anchor lines being lcng enough to prevent the resilient portions of the anchor lines frcm being danaged by caning into direct contact with floating bodies of the deepest expected draught.

Further, the arrangement according to the invention may also have at least one of the follcwing characteristics: - the floating screen is substantially polygonal and surrounds the components which are to be protected against a collision, the vertices 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-oannected 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 poligcnal perimeter fran the associated main floats. 8 4 3 3 1C 2C 3C - 4 - 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 auxiliary 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 pre-tensioning 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 1 is a 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 associated anchor systems; - Figure 5 is a plan of a rectilinear variant;and - Figure 6 is a cross-section of the system in Figure 5.

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-absorbing 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 8 ί 33 - 6 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' con5 nected 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 for10 med 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 : - 7 - in a first stage, the floating body 3 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 colliding 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 on 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 reasonable distance from the limit of the travel of any 3C 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 other types of anchoring means could be used, depending on the kind of ground, e.g. 8 13 3 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 ; the length of the chain 13 which forms the upper end of the anchor line is preferably greater than the draught of the exoected heaviest ships to be stopped, so as to prevent damage to the cable 12 during operation.

The cables 12 are advantageously made of braided 10 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 = 750 b x 2 x 380 m x 25 7. = 35,625 t.m x2 Again by way of example, the energy developped by 25 a floating body of D tonnes displacement drifting at a speed of 2 knots, i.e. 1 metre per second is : 2- D V2 _ 1 „D„ t.m. - 9 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 : Eq = E2 = 35,225 t.m. = |θ D t.m. whence D = 35,225 x 20 = 704,500 t 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, 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 connection 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 Frora 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 end main floats 3' : therefore, in figure 4, the upper end of the auxiliary anchor line 5 which, like the main anchor lines 4, is cons4 8 4 3 3 - 10 tituted 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.

Lastly, Figures 5 and 6 illustrate a variant of the 5 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 recti15 linear perimeter) on either side of the screen.

Claims (14)

1. A barrier arrangement for keeping moving floating bodies out of a stretch of water to be protected, the barrier comprising a floating screen formed by protection lines supported byfloats, and anchor lines connecting some of the floats to the water bed, the anchor lines being disposed to retain the screen in place under normal conditions, and, in the event of a moving floating body colliding with the screen being designed and disposed to absorb the kinetic energy of the said moving floating body without causing damage thereto, wherein at least some of the anchor lines, denoted main anchor lines, are constituted over a majority of their length by cable of resiliently extensible synthetic fibres with relatively short lengths of chain attached to each end of the resilient cable, said lengths of chain serving to connect the anchor lines to the floats and to the water bed, and the lengths of chain at the upper ends of the anchor lines being long enough to prevent the resilient portions of the anchor lines from being damaged by coming into direct contact with floating bodies of the deepest expected draught.

2. An arrangement according to claim 1, wherein the floating screen is substantially polygonal and surrounds the stretch of water to be protected, the vertices of the polygonal perimeter being occupied by means floats connected to the sea bed by main anchor lines.

3. An arrangement according to claim 2, 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.

4. 8 A 3 3 - 12 4/ An arrangement according to claim 2, 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 auxi5 liary anchor lines which hold the screen in position.

5. / An arrangement according to claim -4, wherein the two anchor line systems associated with the end vertices of the polygonal perimeter are spaced out substantially symmetrically with respect to the last 1C corresponding protection line of said perimeter.

6. / An arrangement according to claim 2, 3, 4 or 5, v/herein the main anchor lines are spaced out substantially on the bisectors of the angles at the vertices of the polygonal perimeter from the associated 15 main floats.

7. / An arrangement according to claim 1, wherein the floating screen has a substantially rectilinear shape, at least the ends of the rectilinear perimeter being occupied by main floats connec20 ted to the sea bed by anchor lines.

8. / An arrangement according to claim 7, wherein the anchor systems are spaced out substantially symmetrically and orthogonally on either side of the screen, so as to delimit a channel. 25

9. / An arrangement according to claim 7, 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. - 13 484 3 3

10. An arrangement according to claim 9, 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. 5

11. An arrangement according to claim 1, wherein the floating screen has a protected side and an unprotected side from which collisions are expected and wherein the main anchor lines are disposed on the unprotected side while auxiliary anchor lines are disposed on the protected side, 10 the lengths of the anchor lines being such that under normal conditions the floating barrier is held in place by opposing forces between the main and the auxiliary anchor lines.

12. An arrangement according to claim 11, wherein the pretensioning provided by mechanical means such as winches, 15 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.

13. An arrangement according to any one of claims 1 to 12, 20 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.

14. An arrangement for providing protection against moving floating bodies, substantially as herein described with refer25 ence to and as illustrated in the accompanying drawings.