Mooring components

A typical mooring system can be divided in three different components, the mooring line, the connectors and the anchor point.

Mooring line

Chain

The most common product used for mooring lines is chain which is available in different diameters and grades. Two different designs of chain are used frequently, studlink and studless chain. The studlink chain is most commonly used for moorings that have to be reset numerous times during their lifetime, for instance semi-submersibles, while studless link chain is often used for permanent moorings (FPSOs, buoys, FSOs). A chain mooring line can be terminated in either a common link or an end link (fig. 1-03).

Wire rope

When compared to chain, wire rope has a lower weight than chain, for the same breaking load and a higher elasticity. Common wire ropes used in offshore mooring lines are six strand and spiral strand. The wire rope is terminated with a socket (for instance open spelter, closed spelter, CR) for connection to the other components in the mooring system. Generally wire rope is more prone to damage and corrosion than chain (fig. 1-04).

Synthetic fibre rope

A recent development is the use of synthetic fibre ropes as mooring line. Typical materials that can be used are polyester and high modulus polyethylene (Dyneema). The major advantage of synthetic fibre ropes is the light weight of the material and the high elasticity. The synthetic fibre rope is generally terminated with a special spool and shackle for connection to the other components in the mooring system.

fig. 1-03

fig. 1-04

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Connectors

Shackles

The shackle is a connector that is very common in the offshore industry. It consists of a bow, which is closed by a pin. Many different types of shackles are available, depending on the application. The shackle can be used in both temporary and permanent moorings

(fig. 1-05).

Connecting link kenter type

The connecting link kenter type is most commonly used for the connection of two pieces of chain mooring line, where the terminations of the two pieces have the same dimensions. The connecting link kenter type has the same outside length as a chain

12link of the same diameter. Generally connecting links kenter type are not used in permanent mooring

systems, as they have a shorter fatigue life than the chain (fig. 1-06).

Connecting link pear shaped

The pear shaped connecting link is similar to the connecting link kenter type, except that it is used for the connection of two pieces of mooring line with terminations that have different dimensions. Like the connecting link kenter type, the pear shaped connecting links are not used in permanent mooring systems

(fig. 1-07).

Connecting link c type

Like the connecting link kenter type, the connecting link c type is used for the connection of two pieces of mooring line with terminations that have the same dimensions. The major difference between the kenter type and the c type is the way that the connector is opened and closed. This connector is generally not used in permanent moorings (fig. 1-08).

fig. 1-05

fig. 1-06

fig. 1-07

fig. 1-08

Swivels

A swivel is used in a mooring system, generally of a temporary type, to relieve the twist and torque that builds up in the mooring line. The swivel is often placed a few links from the anchor point, although it can also be placed between a section of chain and a section of wire rope. There are many different types of swivels available, although a disadvantage of most common swivels is that they may not function while under load, which is caused by high friction inside the turning mechanism. A new development is swivels that are capable of swivelling under load, due to special bearing surfaces inside the mechanism (fig. 1-09).

Anchoring point

Dead weight

The dead weight is probably the oldest anchor in existence. The holding capacity is generated by the weight of the material used and partly by the friction between the dead weight and the seabed. Common materials in use today for dead weights are steel and concrete (fig. 1-10).

Drag embedment anchor

This is the most popular type of anchoring point available today. The drag embedment anchor has been designed to penetrate into the seabed, either partly of fully. The holding capacity of the drag embedment anchor is generated by the resistance of the soil in front of the anchor. The drag embedment anchor is very well suited for resisting large horizontal loads, but not for large vertical loads although there are some drag embedment anchors available on the market today that can resist significant vertical loads

(fig. 1-11).

fig. 1-09

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fig. 1-10

fig. 1-11

Pile

The pile is a hollow steel pipe that is installed into the seabed by means of a piling hammer or vibrator. The holding capacity of the pile is generated by the friction of the soil along the pile and lateral soil resistance. Generally the pile has to be installed at great depth below seabed to obtain the required holding capacity. The pile is capable of resisting both horizontal and vertical loads (fig. 1-12).

Suction anchor

Like the pile, the suction anchor is a hollow steel pipe, although the diameter of the pipe is much larger than that of the pile. The suction anchor is forced into the seabed by means of a pump connected to the top of the pipe, creating a pressure difference. When

14pressure inside the pipe is lower than outside, the pipe is sucked into the seabed. After installation the

pump is removed. The holding capacity of the suction anchor is generated by the friction of the soil along the suction anchor and lateral soil resistance. The suction anchor is capable of withstanding both horizontal and vertical loads (fig. 1-13).

Vertical load anchor

A new development is the vertical load anchor (VLA). The vertical load anchor is installed like a conventional drag embedment anchor, but penetrates much deeper. When the anchor mode is changed from the installation mode to the vertical (normal) loading mode, the anchor can withstand both horizontal and vertical loads (fig. 1-14).

fig. 1-12

fig. 1-13

fig. 1-14