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ABSTRACT

This paper describes application of simulation to a specific case in which significant corrosion (in the form of perforations –aligned with uncoated welds) had been detected in a lined CS seawater lift pump (SWLP) caisson. The coated CRA pump column and SDSS pump bowls were fitted with galvanic anodes. The holes had then subsequently been trepanned and covered with SS plates with sealing gasket. However, the holes had continued to grow. Fatigue assessment indicated the need to restrict hole size to less than 350mm. During withdrawal of the SWLP column for maintenance, very poor coating condition was found.

Modelling was used to inform the retrofit CP design, and to determine the limiting size of perforation of the caisson. It was selected to assist understanding of the behaviour of different materials (under high flow rate condition); restrictions to current flow in the narrow annular gap between pump and caisson; a combination of discrete and uniform coating damage areas; complex structural geometries, and used survey measurements to capture the effects of the external CP system (fitted to the associated jacket). The models demonstrated that the edges of the holes could be protected and holes would reach a maximum size that was less than the structurally critical level. The model was used to establish the likely corrosion rate on the exposed steel area at the welds, and the effectiveness of alternative CP designs (and coating re-conditioning strategies).

Key words: Cathodic Protection System, pump caisson, CRA, Super Duplex stainless steel, BEM

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