Reliability-based optimisation (RBO) is a powerful tool for including uncertainties in the optimisation process, in which structural and reliability analyses and optimisation algorithms based on mathematical or evolutionary computation concepts have to be combined effectively. This process is rather complicated and difficult to carry out for large structural systems such as steel offshore structures. In this paper, a calculation system of integrated algorithms for the RBO of the offshore towers is presented. The calculation process is composed of a structural analysis package (SAPOS) based on the finite element method, a reliability analysis program based on the first-order reliability method and an optimisation program based on sequential quadratic programming using the International Mathematics and Statistics Library. In the RBO analysis, multiple limit states based on different criteria are used to check a probable failure condition and to identify the limit state criterion. An offshore jacket-type structure is considered as an example to demonstrate the applicability of the implemented algorithm to realistic structural systems.