Dynamics and Extreme Value Problems for Moored Floating Platforms
This research deals with the dynamic response analyses and extreme value problems of moored floating platforms. It can be divided into three major subjects: first, the analysis of single cables and cable induced mooring damping; second, the dynamic analysis of the moored platforms, with special emphasis on the damping mechanisms and generation of the low-frequency excitation force time series; and third, the extreme wave-frequency responses and the combination of the low-frequency and wave-frequency extreme responses.
The catenary equation or shooting method can be used to obtain an initial estimation of the mooring cable configuration in calm water, after which non-linear static analysis is performed to find the final equilibrium position considering cable weight, current force and seabed friction. Around this equilibrium configuration, either a non-linear time-domain step-by-step integration or a linearized frequency domain analysis can be carried out to determine the response of the cable to various excitations. For the frequency-domain analysis, direct integration and statistic linearization methods are implemented. Example calculations show that the linearized frequency-domain solution and non-linear time-domain solution compare well with each other as well as with model tests and results obtained by others. We improve the quality and extend the applications of Huse's original quasi-static approach of estimating mooring-cable induced damping. By doing so, we could use the quasi-static approach to obtain an estimation of the mooring-cable induced damping that is comparable to the more time-consuming and complex time-domain or frequency-domain approaches.
The dynamic response of moored floating platforms to wave-frequency and low-frequency wave excitations is described. Formulae for low-frequency excitations are presented in both the time-domain and the frequency-domain. Four damping mechanisms for moored platforms are reviewed, with emphasis on mooring-cable induced damping, wave-drift damping and viscous damping, which are important for low-frequency response. The commonly used methods of generating wave-frequency and low-frequency excitation time series are discussed, and a one-dimensional convolution approach is proposed. In this approach, a random signal is passed through a filter, the filter coefficients are determined from the low-frequency excitation spectrum and the random signal follows the theoretical probability density function of low-frequency excitations.
The usual way of estimating the extreme mooring cable tensions is to run many time-domain simulations, which is rather time-consuming. Here however, only one or a few simulations plus the extreme value theory are needed to predict the extreme mooring cable tension. The order statistic theory, the generalised extreme value theory and the peaks-over-threshold methods are applied. A new approach is proposed to estimate the correlation coefficient between the low-frequency and wave-frequency extreme responses. This coefficient can then be used when we estimate the total combined platform motion or mooring cable tension.