CP MODELLING
The UKCS production industry has a legacy of large old platforms working at the ends of their original design lives with many changes in ownership. Maintaining the structures of these platforms to provide a continued asset condition assurance presents a number of design and assessment challenges. In this particular case a model was developed to verify the design of a retrofit cathodic protection system for a North Sea oil production platform. The retrofit activity was required as the existing cathodic protection system, which included both impressed current and galvanic anodes, was not providing satisfactory protection in some areas. The existing system was near the end of its design life – a significant proportion of the impressed current anodes had failed while many of the galvanic anodes were effectively consumed.
The objective of the retrofit work was to provide sufficient protection levels to minimise corrosion for the remaining few years of platform operation, with minimum installation and operational activity. This required an assessment of the distribution of the existing cathodic protection current across the structure, an assessment of the remaining life of the still working parts of this system and the design of a minimum work programme to achieve sufficient rather than full protection.
The cathodic protection model was developed using an iterative process between a geometrically correct electrical model and a corrosion model to compute the effects of polarisation and determine the steady state distribution across the platform. The model performance was validated using actual site data. This benefited the design process by providing the corrosion potential distribution across the structure prior to retrofit, and at different stages during the two phase application of galvanic anodes. The model enabled the number and location of retrofit anodes to be verified – a process that could not have been undertaken either on site or by using standard design calculations without significant additional installation and inspection costs. The model also allowed for the further progressive failure of the impressed current components and the possible impact on protection levels.
|
