A marine drilling riser system is used in offshore exploration as a conduit connecting the drilling vessel with the subsea well. It is a complex structural subsea piping system commonly constructed by 75-90-feet long joints, typically sequentially assembled until they reach the wellhead, sometimes at water depth exceeding 10,000 feet.

In a recertification project of a riser system meant to ensure compliance with regulatory requirements, inspection findings strongly indicated that the system had been exposed to an accelerated corrosion process. Corrosion rates for carbon steel in a seawater submerged application are normally measured to 0.1-0.4 mm per year. The inspection data showed localized corrosion rates exceeding 4 mm per year. Thirty riser joints were completely disassembled and inspected. However, 65 riser joints were inaccessible as they were located offshore and already in service. To quantify the operational risks and estimate the probability of non-compliance with the governing code, it became urgently necessary to extrapolate the corrosion data from the 30 inspected units to the inaccessible 65 units.

Data distributions from the sample of 30 riser joints was used to run Monte Carlo simulations, using transfer equations modelled through a Fast Flexible Filling Design DOE in which the responses were generated through deterministic computer simulations. While the results of the simulations showed that the risk of non-compliance was unacceptable if the system was utilized to its design limits, even a slight reduction of the pressure level in the pipes reduced the risk of non-compliance to acceptable levels.