Analysis of Quadratic Transfer Functions in Operability Studies for Heavy Lift Operations

Abstract

Current industry standards for vessel motion analysis commonly considers only 1st order diffraction analysis. This assumes that external excitation only occurs at the incident wave frequencies and thus natural modes outside of the wave frequency range would not be excited. However, in some cases, research have shown that 2nd order effects could still induce resonances despite being outside of the wave frequency range. The lifting of a monopile by an offshore heavy-lift vessel (HLV) could be one such case where, due to an upwards shift of the vertical center of gravity (VCG) as the monopile is lifted, the natural frequency of roll falls below and outside of the incident wave frequency range.

This thesis presents a calculation and analysis of the effects of including quadratic transfer functions (QTFs) in a heavy-lift analysis. In the beginning of the thesis, a comparison of QTFs is performed between two different software NEMOH and Orcawave for the simple geometry of a floating cylinder. This is done to validate the calculation results of Orcawave with NEMOH, whose calculation results have been verified in some publications. Then, mesh studies are performed on the vessel lid, vessel hull, and free surface to study their effects and convergence.

The first part of the main work of the thesis covers radiation-diffraction analyses performed on a heavy-lift vessel using Orcawave to obtain, analyse, and compare the 1st and 2nd order transfer functions of two loading conditions. An attempt to compare 1st and 2nd order loads for a given sea state in presented, as well as a rough attempt to compute and compare the 1st and 2nd order significant roll motions in a given range of incoming wave periods.

The second part of the main work of the thesis uses Orcaflex to perform time-domain simulations of the heavy lift. This allows modelling of flexible and multi-bodies and solves the complete equation of motion including both 1st and 2nd order excitations naturally. Separate simulations omitting and including QTFs are performed. The resulting motion response is analysed in both time and frequency domain to obtain the most probable maximum (MPM) roll motions and identify excitation of the natural modes. Two operability studies omitting and including QTFs are then carried out to obtain limiting wave heights by comparing the MPM roll motions against allowable limits. The results are analysed and compared to observe the effect of QTFs on operability of the heavy-lift.