Author(s)

R. Jayalekshmi, R. Sundaravadivelu & V. G. Idichandy

Abstract

The exploration and production of hydrocarbons from marginal fields located in isolated ultra deep-water locations is an area of active research. Among the deep-water production platforms the tension leg platform (TLP) has proven itself to be a reliable platform choice for deep-water oil and gas developments. A riser is a unique element connecting the deck with sub sea wells and is critical to safe field operations. Though investigations reported on the riser behavior due to platform motions are many, the reported literature on the effect of riser dynamics on TLP behavior is very few. In deeper waters, risers and tethers contribute significant inertia and damping due to their longer lengths, larger sizes and heavier weights. This paper investigates the effect of riser dynamics on the response characteristics of deep water TLPs using a nonlinear finite element analysis program. To account for the nonlinearities associated with waves of large steepness, a Hybrid wave force model is proposed, which is a combination of the relative velocity model of the Morison equation and the higher order terms in the FNV model developed by Faltinsen et al. (Non linear wave loads on a slender vertical cylinder. Journal of Fluid Mechanics, 289, 179–198, 1995.) Current force is also included along with the wave forces. Parametric studies have been conducted for TLPs in two water depths; 900 m and 1800 m and wave steepness (H/L) ratios of 0.025, 0.05, 0.075 and 0.1. Results are reported in the form of power spectral density functions, mean and dynamic responses. The natural periods of TLPs are also estimated and compared with published results. The tether tension is found to increase due to inclusion of risers. The surge response is found to decrease when risers are included. Keywords: deep water, riser dynamics, tension leg platform, nonlinear finite element analysis, hybrid wave force model, Morison model, wave steepness.

Keywords

deep water, riser dynamics, tension leg platform, nonlinear finite element analysis, hybrid wave force model, Morison model, wave steepness.