If unprotected, seawater lift pump caissons can suffer severe galvanic corrosion. Within 2-5
years holes in the caisson can appear. The corrosion to the steel caissons is caused by the
noble pumps and risers, which normally are made of Ni-Al-bronze, and/or (super) duplex
stainless steel. If the caisson is not protected by a coating, corrosion rates of 2-5 mm/year
are reported. For coated caissons, corrosion rates of more than 10 mm/year have been
seen. This extreme high rate is caused by galvanic current concentration in small coating
defects.
This article will describe how corrosion was stopped on coated caissons on a FPSO. Also
installation of a cathodic protection system for relatively smaller pumps on two platforms in
the Northsea will be discussed. Intensive reference cell measurements confirmed that the
position of the sacrificial anodes is crucial. For this reason an effective way of hanging the
anodes in the annulus of the pump and riser in the caisson was developed. Duplex and
super duplex stainless steels should not be protected at levels more electronegative than -
900 mV Ag/AgCl. This is achieved by a potential control unit on the sacrificial anode
assemblies.