Aquaculture Fish Cage Mooring System

Moorings are required to keep the cages in a fixed position and to reduce the transfer of excessive forces generated by wind, currents and waves to cages. The mooring system for a fish farm consists primarily of chains, ropes, floats and anchors, and in addition, several smaller components like shackles, connection plates, rings etc. 

Net pens have been moored individually or within a group, frequently referred to as a flotilla. Mooring net pens individually is achieved by using 3-4 mooring lines that connect the surface collar to the seabed. However, the most common mooring strategy is to use a submerged grid system, with anchor lines arranged in a catenary shape to secure a group of net pens on a site lease. 

Specifics of submerged grid systems from the surface collar to the seabed include: 

Bridles: Bridles secure the net pen collar to the submerged grid system. Each bridle extends from a submerged grid plate up to the surface collar where the bridle is tied to hold the net pen spatially. At least two bridle lines extend from each grid plate to the surface collar so that the environmental load transferring to the surface collar is diminished at each point source where the bridles are tied.

Compensator buoy: A length of chain/rope also extends from the top of the submerged grid plate to a surface compensator buoy located directly above the grid plate at the water surface. The primary purpose of this intermediate buoy is to support the weight of the mooring grid. Other purposes might include site marking in some jurisdictions. 

Grid cell: Four grid plates/compensator buoys mark the four corners of a mooring grid cell. Submerged grid lines connect adjacent grid plates to form the perimeter of each grid cell. Grid lines must be sized to absorb the loads transferred along the submerged grid mooring system and deployed at a depth that must not interfere with site vessel traffic that frequently enters the site to address various tasks required to operate the site successfully. 

Tension member: Each submerged grid plate is connected through the water column downwards to the seabed along at least one anchor line. However, multiple anchor lines from specific grid plates can be expected on the grid corners and on grid sides that experience high energy. Typically synthetic rope is used to extend the anchor lines through the water column but its use terminates prior to reaching the seabed to prevent chafing and subsequent failure of the mooring system. At this depth, heavy chain is shackled to the anchor line rope and continues along the seabed for a length typically of at least 15-30 m before reaching the anchor line terminus. Catenary arrangements in marine applications work best with heavy anchor lines as the resulting slack line catenary curve provides a low angle of pull on the anchor (the heavy chain is on the seabed) thereby increasing the force required before the anchor drags and fails.

1. Anchors: Different types of anchors are suited to various types of seabed. Our supplied Double Fluke Stingray Anchors are used for sandy, medium clay and silt bottoms. It is the most commonly used anchors in farm mooring.

2. Ground Chain: The ground chain, connecting the anchor and the mooring rope is primarily used to provide weight to the mooring line, in order to keep the angle between seafloor and the mooring line within the desired range, between 9° and 12°.

3. Shackles: Shackles are used to connect mooring ropes, chains and anchors. The SWL is usually used to identify the size of shackles. Shackles can be D Type and Omega Type. Omega shaped shackles are the most common used, because they can accommodate a greater number of connections.

4. Ropes and Thimbles: Ropes are the main components of the mooring system, and are used for both the mooring lines and grid system lines. Thimbles are generally made of hot-dip galvanized steel, and are used to reinforce the rope loop (eye splice) where it is connected to metal equipment (shackles, rings, etc.), and thus where it is subjected to heavy abrasion. Thimbles can markedly reduce wear on the rope. Thimbles used for moorings are usually tube type and open type.

5. Steel Rings: Hot-dip galvanized steel rings are used at intersections in the mooring system. Oval shaped are inserted into steel thimbles in order to avoid using larger more expensive elements, like shackles.

6. Connection Plate: Mooring plates are the main connecting points of the whole grid structure where all components come together and are locked with shackles: the grid system, the mooring lines, the buoy chains and the cage bridles. Plates are usually  roundshaped, with sufficient holes for the shackle pins to be inserted.