Ice loads on offshore structures: The transition from creep to fracture

Abstract During landfast winter ice conditions an offshore structure experiences loads due to static wind-driven sea ice. The load is generally not applied suddenly, but gradually increases as winds build up during the course of minutes or hours. Typical stress-rates for a structure 100 m wide are 0.02 kPa s −1 and extreme stress-rates may be 1–5 kPa s −1 . Laboratory investigations by Sinha (1983) enable us to predict the onset of crack formation in ice as stress increases, using a delayed elastic strain criterion (Sinha, 1982). If we apply this criterion to realistic offshore cases, we find that first cracks in ice should typically occur at 0.4–0.7 MPa and in extreme conditions may occur at 0.8–1.8 MPa. We speculate that complete failure of the ice should occur soon afterwards. This would explain why existing measurements of winter ice forces on offshore structures seldom exceed 1 MPa. We cannot, however, exclude the possibility that higher stress-rates and higher stresses could, under different conditions, be achieved.