PROCEDURES FOR COMPONENT REPLACEMENT

Many of these specialized corrective actions require a detailed knowledge of the main principles, rules and safety procedures for marine engineering work, as well as knowledge of some of the techniques used in operating specialist equipment and materials. The main problem that will be faced when a mooring grid component is replaced is that all components are under very strong load, mainly owing to line tension and drag forces.

If any of these elements are detached, the integrity of the mooring grid system will be compromised. The loose ends of the disconnection point will be pulled apart by the opposing forces and may be difficult to reattach.
Furthermore, in order to unscrew a shackle and/or loosen a knot, the tension on the shackle and/or knot has to be initially reduced. This is accomplished by the use of a block and tackle and an auxiliary line that temporarily takes the strain off the components to be changed, and prevents the two detached components from spreading apart.

Note: The techniques described below may be changed and/or integrated according to: (i) the competence and experience of operators; (ii) the availability of tools; and (iii) the hydrodynamic characteristics of the site.

Anchor repositioning and mooring line tightening

If the anchor has been turned upside down or dragged across the sea bed, it is necessary to re-set the anchor correctly. An anchor that is not properly positioned will lose its holding capacity.
To carry out a re-setting operation, the trip line (also known as the crown line or the heel line) must be accessible and securely attached to the back of the anchor. If there is no trip line, it should be re-attached.
First, the service boat recovers the anchor's trip line from the surface floats and connects it to a V-shaped bridle line.
Initially, the anchor must be dragged towards the cage so that it breaks free completely from the sea bed, usually 20-30 m. This is necessary so that there is a larger manoeuvring margin during the tightening phase, which permits better repositioning of the anchor.
Then, after the anchor is free, the boat moves away from the cage with its bow pointing away gradually tighten the mooring. The trip line is released when the grid system is correctly tensioned (surface buoys are correctly aligned and scuba divers verify the correct tensioning) and the anchor should re-set into the sea bed. If not, the action is repeated.

Replacement of a grid-line-to-corner-plate shackle

The service boat, equipped with a hydraulic line-hauler, is moored onto the buoy above the plate where the shackle has to be replaced.
An auxiliary line is threaded through the centre of the plate, passing through a temporarily installed shackle, and is then attached with a "stopper knot" on the line to be disconnected (Figure 43). The other end of the auxiliary line is wound onto the winch on the vessel.
When the winch is engaged, the line will be pulled up, which loosens the tension on the line between the plate and the stopper knot. The diver can then unscrew and replace the worn-out shackle. The auxiliary line is then slowly released, so that the grid line returns to its original tension. The stopper knot is untied, and the auxiliary line is retrieved on board.
Note: If the grid system is not under high tension, the auxiliary rope may also be pulled up by a lift bag operated by a diver; the lift bag applies a direct force upwards.

Replacement of buoy-to-corner-plate shackle

The same general principles apply for replacing the buoy-to-corner-plate shackle as described in the above paragraph. In this case, the operation is easier because the tension is less, the shackle is smaller and the applied force is only vertical.
The buoys are connected to the grid plates via a length of chain. This chain is fixed to the buoy and to the plate through two shackles, one between the chain and the buoy, and the other between the chain and the corner-plate.
To replace the shackle between the chain and corner-plate (Figure 44), a diver ties one end of an auxiliary line (using a bowline knot) onto a grid-line shackle on the same corner-plate. The other end of the auxiliary rope is wound onto the boat's winch. When the winch is started, an upwards force is applied onto the corner-plate, lifting the corner-plate and relieving the tension on the chain. The diver is then able to unscrew the worn-out shackle and replace it. Once this shackle has been replaced, the auxiliary line is let out, the plate drops back to its original position, and the chain will be tensioned again. The bowline knot onto the shackle is then untied and the line is recovered to the vessel.

Replacement of a shackle between chain and buoy

A diver secures an auxiliary line onto the chain one metre below the buoy. The auxiliary line is then connected to the boat's winch, and the chain is raised as much as necessary to reduce the tension on the shackle to be replaced. The buoy is secured to the boat, and the shackle is replaced. The auxiliary line is then carefully released and recovered.

Buoy replacement

Using a bowline knot, a diver secures an auxiliary line onto a line's shackle, attached to the same chain-plate (see Figure 44). The other end of the auxiliary line is wound onto the boat's winch. The plate is lifted up as the winch is engaged, thereby loosening the chain. At this point, the diver is able to unscrew the old shackle. The damaged buoy is lifted aboard the vessel, and replaced with a new one. The auxiliary rope is released and recovered.

Replacement of a bridle-line shackle

To replace the shackle connecting the bridle line to the corner-plate, it is only necessary to untie the knot on the cage collar, on the opposite end of the bridle line. A diver is able to accomplish this work easily, and can replace a worn-out shackle.

Grid line replacement

Worn grid lines must be replaced as soon as possible. A grid failure will apply an excessive load on the plastic collar of the pen as a result of the diversion of the load onto the bridle lines.
To replace one of the grid lines between two grid-plates, the boat moors onto the buoy above one of the two plates concerned. A diver passes an end of the auxiliary line through a temporary shackle on the corner-plate and secures the end of the line onto the opposite plate.
When the boat's winch is engaged, the two plates will be pulled closer together. As the two plates approach each other, the tension is released from the grid line to be replaced and it becomes loose. Divers can then unscrew both of the shackles on the plates on either end of the grid line, and retrieve the worn rope.
Note: A hand-lever hoist, also called a come-along winch (Plate 83), can replace the boat winch in reducing tension on the components to be replaced. The hoist can be securely fixed over or around the point where the tension is to be reduced. Hand-lever hoists usually require more manual labour, and make it longer to complete a task, but it can save the use of the boat or can be used when a boat with a hydraulic winch is not available. The hoist should be stored immersed in a bucket of gasoline or light oil to prevent it from rusting when not in use (the lubricant should be washed off before using the hoist).

Cleaning the nets