Oil drilling platforms are extremely expensive structures which require a great deal of inspection and maintenance if they are to remain in a safe condition. Initially it was not thought that fouling would be a great problem and few precautions were taken against it. However, two problems have become apparent...Although the rigs are normally stationary there can be a considerable tidal flow of water past them and this can be greatly increased under storm conditions. In some waters fouling has been found to be very extensive, especially around the tidal splash zone and this can increase the drag sufficiently to affect rig stability. The pounding of wind and sea on these rigs causes high alternating stresses which can initiate failures in structural members and it is therefore essential to maintain a regular programme of inspection on legs, bracing struts and the nodes that join them. Only after fouling has been removed can inspection for excessive corrosion or cracks be undertaken.
Biofouling not only makes inspection more difficult but it actually causes anticorrosive coatings to fail, especially in the splash-zone where barnacle shells often cut right through to the steel. This can result in expensive steel replacement for the rig owner. Conventional antifouling paints may be applied during rig construction but they have a limited life, which can be as short as 18 months. After this short period many rigs, especially production platforms, cannot be docked even though they suffer from serious fouling and exploration rigs have to be taken out of service for maintenance that Thorn-D® could make unnecessary. Periodic repainting of the accessible splash zone may preserve the upper parts of the rig legs but this is an extremely expensive undertaking which is not always successful. The lower part of the splash zone is not usually accessible for repainting due to very infrequent calm low tides. Current practice is to add 12 to 16 mm extra to the plate thickness as a corrosion allowance for a reasonable life.