Localized corrosion was seen during normal underwater inspections of several Exxon platforms offshore Louisiana. Although observed damage was not severe enough on any platform to impair structural integrity, this study was initiated to determine when such corrosion occurred, whether observed damage was a continuing problem, and finally, what could be done to minimize such damage. Our studies indicated that the platform was not continuing to corrode, so our current investigation was directed to the offshore construction of the facility. Even though normal offshore welding practices, which depend on hull to- platform wire connections, were believed satisfactory to minimize corrosive effects of current flow, an investigation was done to verify that normal practices were adequate. Our, investigation, which included pictures, potential measurements, sample' analysis, and current measurements revealed that the standard welding procedure of hull-to platform grounding was inadequate to completely stop corrosion from stray current flow. However, we determined that corrosion can be mitigated by connecting the platform to the welding machine return lead and insulating the complete return circuit from the barge.
In July, 1974, normally scheduled underwater corrosion inspections on some of Exxon's Gulf of Mexico platforms showed unexplained metal loss in weld areas. These were all relatively new structures. Although observed metal loss was not severe, if it were a continuing problem such damage could eventually impair structural integrity; therefore, the problem was studied. We found that the platform was not continuing to corrode so we directed the study to the offshore construction phase of the facilities. The most probable cause was stray current corrosion which has long been associated with improper welding techniques.
Before continuing the discussion of our study, I will digress to review some well known facts about stray current effects and some accepted welding practices which were believed to eliminate this type of corrosion during platform construction. Although stray current effects seen on submerged members of platforms are similar to other corrosion in form, stray current effects differ from all other forms of corrosion in that the current which causes the corrosion has a source external to the platform. During installation, the external sources are welding machines located on the derrick barge with positive electrode leads connected to the platform at points where welding is under way. With inadequately isolated negative returns, some current conducted by positive leads to the platform returns to the derrick barge hull through seawater. Platform areas discharging current to water are anodic. At these anodic areas, oxygen and chlorine are evolved and metal goes into solution. The principle reaction is metal dissolution followed by combination with dissolved oxygen to form hydrous ferric oxide (FeO(OH)).
Previously accepted offshore welding practices include precautions to minimize stray current effects.
