ABSTRACT

Compliant offshore platforms deployed for drilling and production are compatible with their intended function. However, large displacements pose operational and structural stability challenges under extreme environmental conditions. The key to the safe topside operation is, therefore, to minimize the response in flexible modes of operation using appropriate control mechanisms. This paper discusses the surge response control of a Tension leg platform using a tuned mass damper (TMD). TMD, consisting of spring-mass-dashpot, helps to control the surge response, being the predominant degree of freedom exhibiting large displacement under unidirectional waves. Response control in surge motion is assessed for different ratios of the mass of the TMD to that of the TLP. TMD is positioned such that the mass center of the primary and secondary system is colinear, concurrent and coplanar. TMD is enabled only with surge motion, which is used to control the platform surge motion by tuning their frequency ratios. The fender elements of TMD are oriented in such a manner to ensure an effective transfer of the restoring and damping forces to the primary TLP structure. Numerical studies are carried out on a TLP, with and without TMD and the surge responses are compared to assess the effectiveness of the control. Results show that the RMS value of surge response is effectively reduced for a mass ratio of 0.3. It also showed the maximum reduction in the surge amplitude, about 30%; higher mass ratios did not show a higher reduction. The response reduction is essentially due to the phase shift between the nature of the responses with and without TMD. Power spectral density plots showed a significant reduction in the energy content in the presence of TMD, confirming the proof of concept.

INTRODUCTION

In the historical development of offshore drilling and exploration platforms, compliant platforms are one of the milestones. Their compliance in the horizontal plane, resulting in the large surge, sway, and yaw period, helps reduce the wave load action on the platform. TLP is form-dominant and hybrid by its design as there exists a combination of stiff and flexible motion; heave, roll, and pitch are stiff, while surge, sway, and yaw are flexible Abou-Rayan et al., 2013; Chandrasekaran and Jain, 2002a; 2002b). The hydrodynamic stability of the platform is derived from the dynamic equilibrium between its weight, buoyancy, and pretension in the tethers. The buoyancy force induced by the large water plane area exceeds its weight, counter-balanced by the initial pretension in the tethers (Chandrasekaran et al., 2006). Even though the TLP possesses high operational stability, large surge displacement remains an unaddressed challenge (Chandrasekaran and Nagavinothini, 2020; Chandrasekaran et al., 2007). Controlling the response can improve the performance of TLP, especially in harsh environmental conditions (Chandrasekaran and Yuvraj, 2013; Chandrasekaran et al., 2015). Many researchers over the past decades have studied the dynamic response of tension leg platforms (Chandrasekaran et al., 2011; Nordgren, 1987). The linear response analysis, presented by Yoneya and Yoshida (1984), helps evaluate response motions, tension variations of tendons and structural member forces in a simplified manner. The spectral analysis confirmed the high-frequency response in stiff degrees of freedom, exhibiting large damping ratios (Nordgren, 1987). Effective design methods, such as response-based design, can help reduce TLP hull size and tendon payloads (Chen and Dagang, 2017).

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