DORMAC :: Cofferdam Repairs - Our patented underwater repair system

Cofferdams Repairs

The Cofferdam has been specially designed, developed and patented by DORMAC to carry out any underwater repair work required on damaged rudders, stern seals, thrusters or any parts of the hull, as though in the dry dock.

Any problems occurring on the rudder, rudder stock / bearings or pintles, shell plating of the hull. Sealing arrangements of the stern seals can be dealt with immediately and the repairs are permanent and timeous. Approved by classification societies, the Cofferdam has already been used successfully many times.

This innovation has been enthusiastically received by the ship owners and managers due to the cost saving and convenience of loading or discharging while the repairs are done.

Spade Type

A cofferdam with the lower locating bracket is pushed under the rudder and pulled up above the waterline using chain blocks.

The pumping out of water forces the cofferdam up until the lower part fits firmly under the rudder.
The water level inside the cofferdam is lowered until the desired level is reached and repairs can start.
On completion of repairs, the cofferdam is flooded and lowered by chain blocks. Once clear of the rudder, it is lifted out of the water by a crane.

Skeg Type

A cofferdam with the 'front door', is of a more complicated nature. The cofferdam is positioned aft of the rudder by a crane and then brought into position by the use of chain blocks.

A diver then riggs and bolts the 'door' into place. Thereafter sealer is inserted around the skeg to ensure adequate dry working conditions. Supporting steel members are fitted to the stern in order to release the upright thrust pressure and prevent it from damaging the skeg.

Hull Type

The hull side cofferdam has the ability to effect permanent repairs without the need to dry-dock the vessel, it has been designed to replace, under the waterline, shell plating on most vessels.

The cofferdam operates by forming a watertight pocket on the side of the vessel in order to affect permanent repairs.

The cofferdam has up to this point carried out various repairs. The largest being a, 12 meter x 3 meter 20 mm steel plate, 11 meters below the waterline.

Stern Tube Type

The cofferdam is fitted in two sections around the stern tube with the use of a shore-side crane and the assistance of the DORMAC diving division.

A sealing method developed by DORMAC is used to create a watertight seal around the tail shaft.

The cofferdam is then pumped dry and inspected by DORMAC engineers, classification society surveyors and owners representatives. The stern seal box is opened, inspected and cleaned.

The damaged seals are removed and new seals are then fitted by means of bonding. The seal box is closed / reassembled and stern seal pressure tested. The rope guard is then welded back into position in a dry environment. The cofferdam is then flooded and removed.

Bow Thruster

Large vessels usually have one or more tunnels built into the bow below the waterline. An Impeller in the tunnel can create thrust in either direction which makes the ship turn.

These bow thrusters may allow the ship to dock without the assistance of tugboats, saving the costs of such service.

Ships equipped with bow thrusters have a sign above the waterline over each thruster on both sides.

Flexi Dam

This light weight flexible cofferdam was developed by DORMAC'S diving division. This cofferdam is our newest design and has been used on damaged rudders so far. The reasons for this concept, is to tackle problems we have when working outside of our home base.

Especially in the hard to work in ports in Africa where mobilization is difficult , cranage a problem to hire and the local work shops are usually limited in the size of steel works projects that they can handle.

Most African ports have their own advantages which would benefit this mobile light weight cofferdam. These being where ports have sheltered bay's or inner anchorages and the works can start straight away from a barge or a launch.

This type of cofferdam can be used at anchorage or in river ports where the conventional cofferdams would be impractical due to currents.

 

The types of routine maintenance and permanent repair jobs that are ideal for this cofferdam are:

• Lower pintle bush replacements
• Pintle nut removal
• Grinding out and re-welding of cracks in the rudder plating
• Inspections

Due to the simple concept and design we have found that we are less affected by surface weather conditions and therefore able to cut down the time to fit and remove the cofferdam, this allowing for less off hire time for the vessel. We are also able to work with a smaller crane, at restricted berths where in the past we had to use a floating crane and can work where the vessel is at deeper draft while working cargo.