Cathodic Protection Of Fourteen Offshore Drilling Platforms^★

Over one-half million square feet of submerged bare steel surfaces on fourteen drilling platforms and four accompanying LST drilling tenders in the Gulf of Mexico have been placed under cathodic protection with magnesium anodes during the past fifteen months. Present practice is to use a high current density of short duration furnished by magnesium ribbon (Galvo-line) to deposit a calcareous coating on the surfaces to be protected. This polarization treatment effectively reduces current requirements for protection to a value approaching three milli-amperes per square foot, and therefore a relatively small "permanent" installation of 51 -lb. magnesium anodes serves to maintain both the coating and protective potentials upon dissolution of the Galvo-line. In general, a sufficient number of anodes is installed to provide protective potentials of —0.78 v minimum, referred to a saturated calomel electrode adjacent to the surface being tested, with anode outputs uniformly adjusted to project a life of two to three years for the protective system. Methods of installing the magnesium anodes and Galvo-line are discussed and illustrated. Current and potential data obtained during the course of a typical installation are presented, and experiences and results obtained during installations on several different types of structures are discussed. In an effort to obtain on-location data showing the differences in corrosion rates for protected and unprotected steel pilings in the splash, tidal and sub-tidal zones, corrosion test racks have been mounted on one cathodically protected producing platform off the Louisiana coast.