S Nallayarasu

Spar platforms are used for drilling, production, storage and offloading of oil in deep water. Spar with its deep draft and large inertia experience low heave and pitch motions in operating conditions. However, the heave motions can be large when encountered by long-period swells or near resonant period. The heave motion of the Spar platforms can be reduced by decreasing the water plane area, increasing the draft, the added mass and/or damping. Various alternatives to reduce the heave motion in long-period swells have been in the fore front of research for the last two decades. An alternate hull form of a shape similar to a buoy with deep draft has been proposed in this study. The buoy form Spar is a cylindrical floating vessel with curved surface near the water plane. The present study focus on the efficiency of the buoy form Spar in reducing the heave motion and increasing the heave natural period. A classic Spar of 31 m diameter and deep draft buoy form Spars with 25 and 20 m diameter at the water plane area has been considered. The moon pool diameter of 12.5 m and the displacement of 63,205 tonnes are maintained for all Spars. The experimental investigations are conducted using 1:100 scale models in the wave flume. The natural period and the damping ratio for the heave and pitch motions were obtained by conducting free decay tests. Numerical simulations have been carried out using panel method. Based on the study, it is concluded that the reduction in water plane area is effective in reducing the hydrodynamic response of the buoy form Spar and increase in the heave natural period is noted.

Hydrodynamic response of spar with appendages such as heave plate has been investigated in the past, mostly attached at the bottom of the spar. The effect of geometry and appendages on the hydrodynamic response of spar has been investigated in this article. A curved neck form with a heave plate near the free surface is proposed as an energy dissipation device for both heave and pitch responses. Numerical simulation using Computational Fluid Dynamics (CFD) is used for capturing the flow around the curved neck with heave plate and corresponding damping characteristics. CFD free decay simulations have been carried out to obtain heave and pitch damping and were noted to be higher than the conventional spar with heave plate at the bottom. Comparison of the proposed geometry and heave plate at the free surface with a conventional heave plate at the bottom of the spar has been made, and significant changes to the response and hydrodynamic characteristics have been noted. It is observed that the buoy form spar combined with the heave plate located near the surface (within 10% of the draft) helps dissipate energy and thus reduce the heave response.

Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave response by increasing heave damping and added mass. Conventional practice is to use a linear damping ratio, which is typically obtained from free heave decay tests or Computational Fluid Dynamics (CFD) simulations. However, with the addition of heave plates, the system damping becomes nonlinear. Understanding such nonlinear damping behavior of spar with circular heave plates in various configurations could be useful in the design of heave compensation devices. Experimental and numerical investigation on heave damping and added mass of a scaled model of spar with a variety of heave plate configurations have been carried out using free heave decay for a range of initial heave displacements. Applicability of the linear and quadratic damping models have been assessed for all configurations. The effects of parameters such as heave plate diameter, location of heave plate above the keel, and spacing between two plates on damping and added mass have also been studied. Flow fields obtained from numerical simulations are presented, and their implication on damping discussed.

Hydrodynamic response characteristics of classic and other Spar geometries have been investigated in the past using experimental and numerical studies in regular waves. The response of such floating system in regular waves has been reasonably established. However, in real sea condition, the existence of multiple wave frequencies in a sea state may alter the response, when the excitation frequency has the component of resonant frequency of the hull. The wave energy at the resonant frequency of the design wave spectrum is generally small, but may lead to large motion like Mathieu-type instability caused by the heave–pitch coupling effects, when the pitch/heave natural frequency ratios are critical (0.5, 1.0, 2.0, etc.). Various alternatives to reduce the heave motion of the Spar platform such as decreasing the water plane area, increasing the draft, added mass and/or damping have been in the fore front of recent research. Buoy form Spar proposed in this paper has a reduced diameter near the water plane. The motion response of such Spar in regular waves has been presented in another paper published in this journal; the heave response under random waves is presented in this paper. A classic Spar, with constant hull cross section diameter of 31 m and buoy form Spars with reduced water plane diameter of 25 and 20 m with draft of 12.5 m and displacement of 63,200 tons were considered for the study. The experimental and numerical investigations were conducted on 1:100 scale models. The pitch/heave natural frequency ratios of CLS-31, BFS-25 and BFS-20 Spars are 0.5, 0.667 and 1.0, respectively. CLS-31 and BFS-20 are vulnerable to Mathieu-type instability, since their pitch/heave natural frequency ratios are 0.5 and 1.0. Based on the study, it is concluded that the reduction in water plane area reduces the response in random waves and reduces the susceptibility to Mathieu instability.