This paper presents a comprehensive dynamic analysis of a marine spar platform with various mooring system configurations. From a practical viewpoint, the mooring system configuration is managed by reel-motor devices that change cable lengths while keeping all cables under tension. The spar platform is anchored to the seabed by twelve mooring cables (in six cable bundle arrangements), and the domain that the cable-driven spar platform can be within is called the platform effective area. The analysis is based on a global frame of reference at the seabed and a local frame of reference at the platform center of gravity. Under the context of rigid body dynamics, the averaged values of the mooring cable tension are calculated through the use of a second norm measure. The platform dynamic response under unidirectional harmonic water waves and changeable submerged depths is investigated over the entire spar platform effective area. The minimum platform natural frequency at each location within the effective area is used as a measure of the platform degree of rigidity.
Spar floating marine platforms are often used for offshore operations such as oil and gas exploration and production and wind energy harvesting. A spar platform consists of a floating structure that is connected to a heavyweight spar and anchored to the seabed by a cable-based mooring system. The first spar platform in the oil industry was installed in the North Sea in the 1970s and used for oil storage and offloading (Bax and de Werk, 1974; Van Santen and de Werk, 1976). While the floating platform is exposed to the environmental loads, the mooring system has the objective of retaining the floating structures’ location. Commonly used mooring systems consist of three cables (Karimirad and Moan, 2012; Jeon et al., 2013; Muliawan, Karimirad and Moan, 2013; Muliawan et al., 2013; Si et al., 2014; Kim et al., 2014; Yu et al., 2015), four cables (Downie et al., 2000; Chen et al., 2001; Sethuraman and Venugopal, 2013), nine cables (Zhang et al., 2007; Zhang et al., 2008; Montasir and Kurian, 2011; Montasir et al., 2015), and twelve cables (Wang et al., 2008; Yang et al., 2012).
A number of researchers analyzed the dynamic response of spar platforms through the use of different numerical and experimental techniques. The spar platform motion was investigated by Ran et al. (1996) through the use of a higher-order boundary element method, and they compared their numerical results with the measurement data, showing good agreement. Jha et al. (1997) obtained an analytical prediction for wave drift damping and viscous forces that influence the dynamic response of spar platforms. The effect of nonlinear sea waves on the dynamic response of a spar platform was investigated by Anam and Roesset (2002) through the use of the hybrid wave, stretching, and extrapolation models. Using Morison’s equation, Anam et al. (2003) studied the differences between the time domain analysis and frequency domain analysis in predicting the spar platform slow drift response.