Pile or anchor

The choice between piles and anchors is only possible for permanent systems. Piles are not a good investment when an anchored entity must be moved. But the choice is often made for piles on emotional grounds; a pile does not drag! However, anchors that are properly pre-tensioned on site will also not drag.

While it is a psychologically loaded subject, experience has shown that the choice between anchor and pile is merely a matter of economics. The required pile weight for a system is equal to the required weight of a Stevpris anchor. Piles cost about 40% of equivalent capability anchors. However, the installation costs for piles are much higher. Piles require a follower and a pile hammer. The installation spread for piles is much more significant; a crane barge with support spread versus the two anchor handling vessels. The weather downtime for a spread involving a crane vessel is much longer than when AHVs are used. To allow drag of the anchors during pretensioning, extra chain length is required. Sometimes the pretension load for piles is much less than for anchors. The survey work for anchors is generally much simpler than for piles. When abandoning a field, anchor removal is much cheaper than removal of installed piles. The choice between piles and anchors strongly depends upon the circumstances. The table L can help in estimating the costs for the two alternatives.

Suction piles are an alternative for drag embedment anchors and piles, also for MODU applications. The advantage is the accurate positioning of the suction piles. The disadvantage is the cost of the pile itself and the cost of the installation. Also many soil types do not allow suction pile applications, whereas drag embedment anchors can be used in any soil type.

53

Introduction

In soil such as sand and medium to hard clay, an anchor with a fluke/shank angle of 32o will give the highest holding power. An anchor with a 50o fluke/shank angle in this soil will not penetrate but will drag along the seabed. If used in mud a 50o fluke/shank angle is appropriate. An anchor with a 32 o fluke/shank angle will penetrate less and generate lower holding capacity in mud(fig. 3-01).

The Stevpris Mk5 anchor has an additional fluke/ shank angle setting of 41o, which can be adopted in certain layered soil conditions (table M).

Changing the fluke/shank angle on the Stevpris Mk3

This can be carried out within half an hour with the

54Stevpris anchor upside down on deck.

Secure the anchor on deck. Connect a tugger wire (C) to the holes (D) on the bottom side of the fluke. Change from mud to sand angle by removing the locking plates and the two rear pins in (B), decrease the fluke/shank angle by hauling the cable (C). Reinstall the pins and locking plates in (A). Seal weld the locking plates, do not weld them to the pins (fig. 3-02).

fluke angle too large in hard soil !

no penetration !

angle setting

* Stevpris Mk5 only

table M

Change from sand to the mud position, increase angle by veering (C), change over pin and locking plates from (A) to (B). No special welding requirements

(fig. 3-03).

Changing the fluke/shank angle on the Stevpris Mk5

Changing the fluke/shank angle on the Stevpris Mk5 anchor is even quicker. No welding required. Veering and hauling (C) to change the fluke/shank angle as above, the pin however remains in (A), the locking plate is secured by means of a cotter pin (fig. 3-04).

55

To connect a swivel to the Stevpris anchor, several different configurations are possible. These are:

to the anchor shackle, for instance the forerunners VA02, VA04 and VA 06 described in the product data section (fig. 3-08).

PL pear link, A common link, B enlarged link, H swivel.

When a chaser is used in combination with the Stevpris and swivel, some of the configurations mentioned above are more suitable than others. In general, swivels are only designed to withstand longitudinal forces, and are usually not designed for use in combination with chasers. The design of the chaser tends to stop it at the swivel. Consequently, there will be high bending forces on the swivel, which can result in damage or even breakage.

Generally, it is best when the swivel is fitted some distance from the anchor when a chaser is used. The chaser can then pass the swivel and stop on the anchor shank. When a load is applied to the chaser, the swivel is only loaded longitudinally. This means that in combination with the use of a chaser, the configuration type III and type IV are preferred.

J C B A

fig. 3-05

J C B A

P L A B H B A

fig. 3-08

damage possible!

NO !

fig. 3-09

When the swivel (or swivel forerunner) is connected to the anchor shackle by means of an end shackle and a chaser is used, the end shackle and the anchor shackle should be connected bow through bow instead of pin through bow as is normal practice. This to minimise the chance of damage to the shackles.

The illustrations fig. 3-09 through fig. 3-14 show how and how not to connect the swivel to the Stevpris anchor when using a chaser.

The best method for chasing with a swivel in the system is to maintain the tension of the anchor line as much as possible during chasing. This will make the chaser pass more easily over the swivel.

57