INTRODUCTION
Historically, cathodic protection (CP) surveys of offshore platforms have been comprised of a number of steel to seawater potential measurements recorded at selected points on or around the structure. Little or no emphasis has been placed on evaluation of the resilience of the CP system; i.e. how hard is the system working, how efficiently, and how much longer will it continue to provide efficient protection. The new approach, which has been used by some operators since 1989 in the Gulf of Mexico, addresses some of these areas without significant increase in the time required to perform the inspection.
In the Gulf of Mexico and in many other offshore producing areas, government regulations require that CP systems on offshore platforms are surveyed periodically to verify the adequacy of protective potentials?. In the Gulf of Mexico, most operators perform an annual drop cell survey and a diver or Remotely Operated Vehicle (ROV) subsea inspection every three to five years, These surveys are usually performed as a part of a larger scope of work which involves other structural inspections and non-destructive testing.
Drop Cell Surveys
The drop cell survey is performed from the surface and requires that a reference electrode is lowered through the water column adjacent to the platform structure. Readings are recorded at predetermined depth intervals at various locations around the structure. This method is quick and inexpensive, it will give some indication of the surface potentials on the structure. They will not be accurate due to the fact that the reference electrode could be remote from the structure, however, the readings obtained do give a good general indication as to the protective status of the structure. Potentials recorded are generally optimistic so far as cathode potential is concerned; errors typically range between (-)0.02 and (-)0.06 Volts vs Ag/AgCl, depending on sea conditions, water depth and the protective level of the structure. It is fair to say that if a well conducted drop cell survey indicates marginal protective levels, then inmost cases, a detailed survey would reveal areas which were below protected criteria. Figure 1. shows a typical silver/silver chloride drop cell. Figure 2. shows a typical survey report from such a survey.
SURVEY EQUIPMENT & PERSONNEL
It is virtually impossible to perform inaccurate CP survey with the wrong equipment or personnel. An inaccurate survey could lead to serious consequences with either optimistic or pessimistic inaccuracy, The writer?s company has experienced this first hand, having arrived offshore with a dive boat loaded with new anodes only to find that the original CP survey was flawed and that there was no need for a retrofit. Similarly an optimistic survey could result in corrosion damage to the structure, The most common error sources for diver/ROV assisted surveys are:
· Inexperienced Survey Personnel
· Poor Quality Silver Chloride Reference Electrodes
· Damage to Electrode Lead Wire Insulation from Barnacles
· Poor Reference Ground Connection
· Data Transfer Error
