QTF-derived stochastic load characterisation of buoyancy-decentralised TLP tendons for ULS assessment

• The buoyancy-decentralisation in the design of modularised tension leg platforms is analysed. • The ULS is followed up to evaluate the performance of tendons and slackline occurrence. • The second-order potential flow model is used to involve the QTF in the hydrodynamic load. • Diagonal decentralisation leads to a slight decrease in the amplitude of QTF derivatives. • Uniform decentralisation leads to better performance of tendons under ULS load cases. Toward designing modularised lightweight Tension Leg Platforms (TLP), the impact of buoyancy decentralisation on the station-keeping system is investigated under the Ultimate Limit State (ULS). Following the former study, two types of buoyancy decentralisation, which cause the lower tension loads under linear hydrodynamic loads, are evaluated under realistic loading conditions. Decentralisation design strategies result in the uniform and diagonal distribution of floater modules positioned at the farthest feasible distance from the centre of the designated footprint. Combinations of environmental loads, such as wave, wind and current, are outlined to address the ULS and evaluate slackline occurrence along tendons. By gazing over the existing offshore renewable energy sites, such as wind farms across the European riparian, the characteristic environmental data is extracted from the existing met-ocean database. Corresponding to 100-year wave characteristics, a second-order potential flow model is deployed to determine the second-order hydrodynamic loads and the dynamic responses of the floater and the tendons. Meanwhile, the impact of the buoyancy decentralisations on contortions of nonlinear wave force components out of the sum and difference pairs of frequencies is pursued. Indeed, the Quadratic Transfer Function (QTF) tensor, which is a function of floater geometry, evolves due to the buoyancy decentralisation. Obtained dynamic tensions for both decentralisation approaches are statistically processed to determine tension loads characteristics. Eventually, the slackline occurrence is evaluated using the involvement of the robust safety factors in tension load characteristics. The uniform decentralisation appears more effective in enhancing the reliability of the station-keeping system under specified load combinations.