Robust design of offshore wind turbine foundations requires that the ultimate capacity is acceptable. In addition the accumulated deformations over 25 years of operational life, typically 108 cycles, must also be limited. This paper outlines a numerical method for modelling the response of monopiles to monotonic and cyclic lateral loading to very large cycle numbers, by combining the PISA design model (e.g. Byrne et al., 2019a) and the HARM framework (e.g. Abadie et al., 2017b; Houlsby et al., 2017). The approach is used to determine the in-service performance for a typical offshore monopile, embedded in a layered soil, and subjected to a load signal representing a 25 years of service loading, followed by a severe storm. The model captures the pile response to 3x108 cycles in 30 minutes computational time on a standard lap-top computer. The results identify the effect of ratcheting on the foundation response.