SeaDragon to Use Cathelco Jotun Corrosion Protection on Second Hull

The second semisubmersible rig to be built for SeaDragon Offshore will have a hull corrosion protection system designed and supplied by Cathelco Jotun.

The SeaDragon Offshore project involves the construction of two hulls which will be built in Russian yards and fitted out at Teesside in the United Kingdom. The first of the semisubmersibles is already under construction at the Sevmash yard for completion in 2008. The second hull is to be built at the Vyborg yard and delivered to Teesside in 2009.

Cathelco Jotun is supplying a comprehensive impressed current cathodic protection system for the SDO II semisubmersible. Both of the pontoons will have two transformer/ rectifiers serving a number of hull mounted anodes and reference cells to achieve the optimum current distribution.

In addition, the vertical bracing will be protected by sacrificial anodes, creating a 'hybrid' system which will provide total hull protection in the toughest operating conditions.

Numerous factors have to be taken into consideration in the design of an ICCP system of this type. One of the main considerations is that the vessel has a variable draught, approximately 35,700 tonnes in transit and 54,600 in operation. In these circumstances, the use of a combination of sacrificial and ICCP anodes gives the flexibility to provide the most effective form of corrosion protection.

As rigs of this type drydock infrequently, Cathelco Jotun are providing 'diver change' anodes and reference cells which can, if necessary, be replaced while the semi-submersible is in operation. They will also be supplying sacrificial anodes to control corrosion in the ballast water tanks and seachests.

Impressed current cathodic protection (ICCP) systems are designed to eliminate corrosion on the wetted surface of the hull by emitting a more powerful current which suppresses the natural corrosive activity.

They consist of a control panel (transformer/rectifier) and an arrangement of hull mounted anodes and reference cells. In operation, the reference cells detect the electrical potential at the hull/seawater interface and send a signal to the control panel which automatically raises or lowers the output to the anodes. In this way, the hull receives the optimum level of protection at all times.

Cathelco have a record of more than 6,000 ICCP installations on ships, rigs and FPSOs operating worldwide. They also supply marine pipework anti-fouling systems to protect seawater life pumps against bio-fouling.