Piggy-backing

Introduction

Piggy-back is the practice of using two or more anchors in order to obtain holding power greater than can be achieved with one only. Piggy-backing is used when anchors are employed with insufficient holding capacity. This can be caused by improper design for the particular environment or insufficient anchor size.

In some soil conditions, the use of two smaller anchors in piggy-back can offer an advantage over the use of one larger anchor. This can be the case when the anchor has to hold in a certain layer and holding capacity in the underlying layer is uncertain.

Considerations to remember on piggy-backing:

• Installing a piggy-back system is more costly than

74the installation of a single anchor.

•If the mooring line of the second anchor is connected to the rear of the first anchor, the stability, penetration and holding capacity of the first anchor may be less than is the case for a single anchor. The force from the second anchor may tend to pull the fluke of the first anchor closed (hinging type anchors).

•If the piggy-back anchor is connected to the first anchor by means of a chaser, the chaser may obstruct penetration of the first anchor.

•Both anchors must be exactly in line with the mooring line load. The lead anchor may become unstable if a lateral load is applied.

•Two hinging anchors in piggy-back do not provide 2 times but only 1 to 1.6 times the individual holding capacity of the two anchors, for reasons described in second point above.

•If the first anchor is not influenced by the pull from the second anchor, and the second anchor (fixed fluke/shank type anchors) is connected at 3 to 4 shank lengths distance from the first anchor, the holding capacity of the 2 anchors may be up to 2.5 times the holding capacity of the individual anchors, due to the extra penetration of the second anchor.

Piggy-back methods

Piggy-backing involving hinging anchors

Since there is little difference between handling one hinging anchor or two, the first method is described with a Stevin anchor (hinging) in combination with a Stevpris anchor (non-hinging).

Here, the Stevpris is main anchor and the Stevin is back-up. This is the best solution when using a fixed shank anchor as the fluke of the Stevpris anchor can not be pulled closed. The pendant line is connected to the padeye near the anchor shackle so performance is not reduced.

Note: if the piggy-back anchor can not be laid in line with the mooring load, the piggy-back anchor makes the main anchor unstable. In such a case the Stevpris can better be placed as the second anchor.

pendant line between the two anchors should be wire rope, to promote penetration and obtain better holding capacity (fig. 3-60).

The installation procedure is described as follows:

•Pay out the main anchor as usual.

•Tension the mooring line until the anchor slips.

•Connect the second anchor to the pendant line.

•Bring the anchor to its location.

•Lower the piggy-back anchor and tension the mooring line again.

•Provide the pendant of the second anchor with a buoy for easy retrieval.

fig. 3-60

Piggy-back methods

Piggy-backing by using a chaser

Sometimes chasers are used to connect the piggyback anchor to the first anchor (fig. 3-63), although a pendant line connected directly to the padeye behind the main anchor shackle of the first anchor is prefered.

The installation procedure described for two Stevpris anchors is also applicable when a chaser is used for the connection.

During the deployment of the piggy-back combination, care must be taken that anchors are installed in line with the load.

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fig. 3-63