This paper presents a residual service life assessment of an offshore concrete structure for oil and gas production in the North Sea. After 15 years in operation, plans were initiated for a service life extension from 30 to 50 years, and a condition assessment of the concrete structure above the sea level was carried out. A probabilistic service life model based on Fick’s 2nd law of diffusion was used in the documentation of the residual service life of the concrete structure. The model parameters were adapted the actual structure, where the statistical data for the apparent diffusion coefficients and the surface concentrations were determined from concrete cores drilled out from the concrete above sea level. Using an acceptance limit of 10% probability for corrosion initiation, a service life of 50 years was obtained for the homogeneous, good quality concrete and a concrete cover in accordance with specified requirement of 50 mm + 20/− 0. Further, an updated service life analysis, based on chloride profiles determined 13 years later, indicated an even longer service life.