ANCHOR CHAIN

Abstract

The invention relates to the field of anchor chains, and more particularly an anchor chain (1) comprising a first plurality of links (3) and, at the end of the chain, a second plurality of links (4) comprising at least three successive links (4) in each of which at least one property selected from its bar diameter and the elastic limit of its material is substantially greater than in each of the links (3) of the first plurality, in order to improve the ability of the chain to withstand fatigue by bending of the links (4) in the second plurality.

Claims

1. An anchor chain comprising a first plurality of links and, at an end of the chain, a second plurality of links comprising at least three successive links in each of which an elastic limit of its material is substantially greater than in each of the links of the first plurality of links.

2. The anchor chain according to claim 1, wherein a bar diameter of each of the links of the second plurality of links is substantially greater than a bar diameter of each of the links of the first plurality of links.

3. The anchor chain according to claim 2, wherein the bar diameter of each of the links of the second plurality of links is at least 1.2 times the bar diameter of each of the links of the first plurality of links.

4. (canceled)

5. The anchor chain according to claim 4, wherein the elastic limit of the material of each of the links of the second plurality of links is at least 1.2 times the elastic limit of the material of each of the links of the first plurality of links.

6. The anchor chain according to claim 1, further including at least one intermediate link between said first plurality of links and said at least three successive links of the second plurality of links, said at least one intermediate link differing from each of the links of the first plurality of links and of the second plurality of links.

7. The anchor chain according to claim 6, wherein said at least one intermediate link presents a bar diameter substantially greater than a bar diameter of each of said first plurality of links, but less than a bar diameter of each of the links of the second plurality of links.

8. The anchor chain according to claim 7, including at least one first intermediate link adjacent to said first plurality of links, and at least one second intermediate link adjacent to said at least three successive links of the second plurality of links and presenting a bar diameter that is substantially greater than a bar diameter of the first intermediate link.

9. The anchor chain according to claim 1, wherein said second plurality of links comprises at least three successive links at a first end of the anchor chain and at least three other successive links at a second end of the anchor chain, opposite from said first end.

10. A floating body having at least one anchor chain comprising a first plurality of links and, at an end of the chain, a second plurality of links comprising at least three successive links in each of which an elastic limit of its material is substantially greater than in each of the links of the first plurality of links.

11. The floating body according to claim 10, in the form of a floating platform.

12. The floating body according to claim 10, supporting at least one device for generating electricity.

13. The floating body according to claim 12, wherein said device for generating electricity is a wind turbine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention can be well understood and its advantages appear better on reading the following detailed description of two embodiments shown as nonlimiting examples. The description refers to the accompanying drawings, in which:

[0023] FIG. 1 shows a prior art anchor chain segment;

[0024] FIGS. 2A to 2B show two different views of a first link of the FIG. 1 chain;

[0025] FIGS. 3A to 3B show two different views of a second link of the FIG. 1 chain;

[0026] FIGS. 4A to 4B show two different views of a third link of the FIG. 1 chain;

[0027] FIG. 5 shows how a bending moment is generated at the end of the FIG. 1 chain;

[0028] FIG. 6 shows an anchor chain segment in a first embodiment;

[0029] FIG. 7 shows an anchor chain segment in a second embodiment;

[0030] FIG. 8 shows diagrammatically a floating platform supporting a wind turbine and connected to a plurality of anchor points by anchor chains; and

[0031] FIG. 9 shows diagrammatically a marine turbine with positive buoyancy that is held submerged below the water surface by anchor chains connecting it to a plurality of anchor points.

DETAILED DESCRIPTION OF THE INVENTION

[0032] FIG. 1 shows a prior art anchor chain 101 in compliance with the API Spec 2F standard of the American Petroleum Institute. This anchor chain 101, which is connected at one end to an anchor shackle 102, comprises a plurality of successive common links 103, and between these common links 103 and the shackle 102, a standard enlarged link 104, and an end link 105.

[0033] As shown in FIGS. 2A and 2B, each common link 103 presents a bar diameter d, a width equal to 3.35 times d, and a length equal to six times d. The standard enlarged link 104, which in the chain 101 is adjacent to said plurality of successive common links 103, and which is shown in detail in FIGS. 3A and 3B, presents a bar diameter d1 equal to 1.1 times the bar diameter d of the common links 103, a width equal to 3.35 times d1, and a length equal to six times d1. Finally, the end link 105, which is interposed between the standard enlarged link 104 and the anchor shackle 102, resents a bar diameter equal to 1.2 times d, a width equal to 4 times d, and a length equal to 6.75 times d, as shown in FIGS. 4A and 4B.

[0034] Although the standard enlarged link 104 and the end link 105 are of diameter greater than the diameter of the common links 103, some of the common links are close enough to the end of the chain 101 to be affected by bending moments in response to lateral movements of the floating body secured to the anchor shackle 102.

[0035] FIG. 5 shows the generation of such a bending moment M.sub.F when the force F transmitted by the link 102 to the end link 105 is not in alignment with the main direction X of the chain 101, and when surface imperfections of the links 105, 104 and the tension between them prevent them from turning freely relative to one another. The force F is thus resolved into a tension force F.sub.X in alignment with the main direction X of the chain 101, and a lateral force F.sub.Y perpendicular thereto and generating, in the end link 105, a bending moment M.sub.F increasing towards the enlarged standard link 104. In the end link 104, this bending moment M.sub.F leads to additional stress o perpendicular to the cross-section of the end link 104. As explained in the summary of the invention, in the long term, these additional stresses, which are variable, can lead to fracture by fatigue.

[0036] In order to avoid that, in a first embodiment as shown in FIG. 6, an anchor chain 1 comprises, between two ends connected to anchor shackles 2, a first plurality of successive common links 3 and a second plurality of enlarged end links 4 comprising at least three successive end links 4 at each end of the chain 1. More specifically, in the embodiment shown, this second plurality of end links 4 comprises four end links 4 at each end of the chain 1. In order to ensure a transition that is gradual between the common links 3 and the end links 4, the chain 1 also includes, at each end, a first intermediate link 5 adjacent to the common links 3, and a second intermediate link 6 adjacent to the end links 4.

[0037] In this chain 1, each of the common links 3 presents a bar diameter D, the first intermediate link 5 presents a bar diameter substantially greater than the bar diameter D of the common links 3, e.g. 1.1 times D, the second intermediate link 6 presents a bar diameter substantially greater than the bar diameter of the first intermediate link 5, e.g. 1.2 times D, and each of the end links 4 presents a bar diameter D.sub.end substantially greater than the bar diameter of the second intermediate link 6, e.g. 1.3 times D. In the embodiment shown, all of the links 3, 4, 5, and 6 also have the same diameter/width/length ratios of 1:3.35:6.

[0038] Also, the end links 4 present not only a bar diameter that is substantially greater than the bar diameter of the other links 3, 5, and 6, but they are made of a material that also presents an elastic limit in tension that is substantially greater than that of the common links 3, e.g. 20% greater.

[0039] Because of their larger diameter and their higher elastic limit in tension, the end links 4 are substantially less sensitive to lateral forces than are the common links 3. For a given lateral force, the bending moment M.sub.extrem transmitted between two such end links 4 may be defined by the following formula:

M.sub.extrem=M.sub.courant.Math.K.sub.mat.Math.K.sub.diam.Math.D.sub.extrem/D

where M.sub.courant corresponds to the bending moment that would be transmitted between two links of dimensions and material identical to those of the common links 3 taking the place of the end links 4, K.sub.mat corresponds to an improvement coefficient due to the increase in the elastic limit, and K.sub.diam corresponds to an improvement coefficient due to the increase in the diameter of the bar. The ratio D/D.sub.extrem corresponds to the increase in the lever arm because of the increase in the diameter of the bar and the increase in the other dimensions of the link.

[0040] An increase of 20% in the elastic range of the material of the end links 4 relative to the test load at which the chain 1 is to be tested, which corresponds to 70% of the breaking load of the weakest links in the chain 1, i.e. the common links 3, enables the contact area affected by strength tests to be reduced, thereby resulting in a reduction in the stress concentration factor (SCF) of SCF.sub.courant=1.25 for a link having the elastic limit of the common links 3, to SCF.sub.extrem=1.06 for the end links 4. The coefficient K.sub.mat may be calculated using the following formula:

K.sub.mat=1−SCF.sub.extrem/SCF.sub.courant

which, with the above mentioned values, results in K.sub.mat=0.85.

[0041] Furthermore, the greater diameter of the end links 4 also leads to a reduction in the areas of contact between adjacent links damaged by the load test, which reduction is to be seen in the coefficient K.sub.diam, which, in the embodiment shown, may be 0.95.

[0042] Above all, for a given bending moment being transmitted between adjacent links, the bending stress is inversely proportional to the cube of the bar diameter of the links.

[0043] Consequently, if σ.sub.xx, extrem represents the stress induced by the bending moment M.sub.extrem in such an end link 4, and σ.sub.xx, courant represents the stress that would be induced by the bending moment M.sub.courant in a common link 3, then the relationship between these two stresses can be expressed by the following formula:

σ.sub.xx, extrem=σ.sub.xx, courant.Math.K.sub.mat.Math.K.sub.diam.Math.(D/D.sub.extrem).sup.2

which, with the above mentioned values, results in

σ.sub.xx, extrem=0.48.Math.σ.sub.xx, courant.

[0044] With these values, the stresses induced by bending at the head end of the chain are thus reduced by more than half, which represents the lifetime of the chain 1 being multiplied by a ratio of (1/0.48).sup.3=9.04.

[0045] Although in this first embodiment the links 3, 4, 5, and 6 are not stud links, the same principle is equally applicable to stud links, such as the links in the chain of the second embodiment shown in FIG. 7. Apart from the transverse studs 7 in the links, all of the elements are equivalent to elements of the chain in the embodiment shown in FIG. 6, and they are thus given the same reference numbers. Furthermore, although in both embodiments shown, the anchor chains are provided with end links and enlarged intermediate links at both ends of the chain, it is equally possible to provide them at only one end of the chain.

[0046] Because of their increased resistance to fatigue, these chains are particularly applicable to long-term anchoring of floating bodies, such as a floating platform 8 supporting a wind turbine for generating electricity, as shown in FIG. 8, or an under-sea marine turbine 9, as shown in FIG. 9.

[0047] Although the present invention is described with reference to a specific embodiment, it is clear that various modifications and changes may be made to these embodiments without going beyond the general ambit of the invention as defined by the claims. Also, individual characteristics of the various embodiments described may be combined in additional embodiments. Consequently, the description and the drawings should be considered in a sense that is illustrative rather than restrictive.