Truss spar CP modeling

Predicting remaining CP life

A detailed, time-resolved cathodic protection model of a truss spar in 4,500 feet of water in the Gulf of Mexico was constructed from provided CAD files, drawings, documents and photographs. Metocean data was used where available and inspection data was carefully screened for quality, favoring only consistent measurements with a higher degree of confidence. Model parameters of anode closed-circuit potential and anode capacity were calibrated using both historical contact potential measurements and anode wastage.

The model predicts that the rate of depolarization will rapidly increase beginning next year. -800mV criteria will be crossed in two years in some regions and will be largely noticeable in three years. Active corrosion is expected to occur in four years when potentials in some locations fall below -760 mV.

The original CP design life was 20 years. It was recommended that the structure should receive a cathodic protection retrofit as soon as possible to avoid corrosion within the next few years. The model constructed during this scope may be used to support that effort in the following ways:

1. Evaluate proposed retrofit designs;

2. Optimize the arrangement of retrofit anodes to reduce cost and improve performance (multi-objective optimization);

3. Design alternative sacrificial anode or impressed current retrofits;

4. Update depolarization predictions post-retrofit.