GB2286166A - Anchoring arrangement - Google Patents

Abstract

A catenary anchoring arrangement for mooring a vessel, e.g. a multiline weathervaning system, provides 3 or more approximately equally spaced groups (13) of anchoring chains or cables (10) extending in a catenary manner from the vessel. Each group (13) comprises at least 2 individual chains or cables (10) which extend substantially parallel to one another. The invention seeks to optimise load sharing between adjacent anchor legs. <IMAGE>

Description

ANCHORING ARRANGEMENT Catenary anchoring systems are employed worldwide in the mooring of vessels. These systems range from a simple single line, such as when anchoring a ship temporarily by means of its own anchor, to complex multiline Single Point Mooring systems, allowing vessels to weathervane around such a system in response to wind, wave and current action.

Such multiline weathervaning systeming is described in eg. US 3735435 which shows a catenary anchorleg mooring (CALM). This usually consists of a floating buoy fitted with a turntable and anchored to the seabed by 6 or 8 chains. In recent years there weathervaning systems have also been integrated in the vessel itself as described in eg. US 4654015. Here a '2CALM" is secured to the bow of a vessel, which effectively becomes the turntable.

Traditionally, anchoring systems based on a weathervaning principle employ a number of anchorlegs, ie. chains or cables, which are equally spaced.

The number of anchor legs to be employed in any case is based on the load imposed by the vessel on the anchor legs and on the requirement to meet certain factors of safety eg. if one anchorlegs is broken.

The fact that the anchorlegs are equi-spaced is usually based on the fact that weather may approach such an anchored vessel from each compass direction.

Equally spaced anchor patterns have the drawback that load sharing between adjacent chains is relatively low because adjacent chains are subject to less excursion along their plane of operation than the lead chain (ie. the chain whose plane of operation corresponds to the direction of the excursion) and due to the highly non-linear force-excursion relationship of catenary systems. The lead chain therefore carries perhaps 70-80t of the total mooring load by itself.

It is an objective of the present invention to optimize the load sharing between the various anchor chains.

This is achieved by way of the following: Selecting the minimum number of anchorlegs that will provide a determinate position of the anchored object. This number can not be less than three and is preferably not more than three; and Making up each anchorleg of two or more individual chains or cables such that in the event of failure of one such individual chain, the overall determinate position of anchored object is not compromised.

Thus, the present invention provides an anchoring arrangement comprising a plurality of anchor chains or cables extending in a catenary manner from a common mounting point to seabed anchors wherein the anchor chains or cables are arranged in 3 or more groups generally equally angled from one another and wherein each groups comprises at least 2 individual anchor chains or cables which are substantially parallel to each other or only slightly divergent.

The invention will now be described in detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a typical anchor pattern such as that, for example employed in Single Point Mooring Systems; Figure 2 shows one embodiment of anchor pattern in accordance with the present invention; Figure 3 shows the typical catenary behavior of an individual anchor chain in any typical anchor pattern; Figure 4 shows the load-extension curve for both anchor patterns whereby the centre of each anchor pattern is given an excursion along the line marked E in Figures 1 and 2.

Figure 1 shows a typical anchor pattern used in Single Point Mooring Systems. It comprises 6 individual anchor chains (10) extending equally spaced from one anchor from a common mounting point on the vessel. As described above this arrangement gives little load sharing between anchor chains as the vessel (11) moves in the direction shown by arrow E and the lead chain (12) carries the majority of the mooring load.

The embodiment of the present invention shown in Figure 2 also comprises 6 individual anchor chains (10). However, here they are grouped in pairs to form 3 anchor chain groups (B). The 3 groups (13) are positioned substantially equally spaced from one another ie. at approximately 1200 to each other. In this case, when the anchored vessel moves in the direction of arrow E load sharing between the 3 anchor chain groups is significantly increased. Thus, a greater force is required to move the vessel a given excurion distance as shown in Figure 4.

From Figure 4 it can be seen that, for a given excursion (e), the force required to produce that excursion is greatest for an anchor pattern in accordance with the present invention as shown in Figure 2. This is turn means that by grouping individual anchor chains in 3 groups, spaced at 1200 to each other, a larger vessel can be moored in a given environment.

Conversely, less material ie. a smaller anchor weight and/or a smaller or shorter anchor chain, would be required for the anchoring of ships in a given environment when compared to traditional anchoring systems.

Claims (2)

CLAIMS:

1. An anchoring arrangement comprising a plurality of anchor chains or cables extending in a catenary manner from a common mounting point to seabed anchors wherein the anchor chains or cables are arranged in 3 or more groups generally equally angled from one another and wherein each groups comprises at least 2 individual anchor chains or cables which are substantially parallel to each other or only slightly divergent.

2. An anchoring arrangement substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.