10400 CP Designs-Problematic Tanker Ship CP Designs for Offshore Facilities and Interference

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Product Number: 51300-10400-SG

ISBN: 10400 2010 CP

Author: Svenn Magne Wigen, Lene Marita Green and Harald Osvoll

Publication Date: 2010

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Over several years tankers have been converted to offshore oil and gas producers, FPSOs. Tankers are often equipped with Impressed Current Cathodic Protection with few anodes based on regularly dry docking for maintenance and re-coating. In most cases these CP systems remain installed, or a similar “marginally type” are designed by the shipyard. In addition to the ICCP, several sections, such as sea chests and turret/moon pools are protected by galvanic CP systems. Experiences have shown that the galvanic CP system will override the ICCP systems the first years in operation and actually protect the entire hull if the standard recommended settings for the ICCP are followed. This may result in insufficient capacity on the galvanic CP system for its dedicated areas after years in service. Further, experiences have shown that, even if the ICCP as per current capacity is sufficient to protect the hull, the CP at the latter stage of life will not fulfil polarisation requirements. The paper will show how Boundary Element Method (BEM) modelling is useful to optimise the ICCP settings to work with galvanic CP systems, and further, to verify how a typical tanker ship CP design is not adequate for an offshore production facility.

Keywords; Impressed Current Cathodic Protection, “tanker CP design”, FPSO hulls, Boundary Element Method CP modelling