Numerical and Experimental Study on Dynamic Response Mitigation of Tension Leg Platform Using Tuned Mass Damper

_ Response amplitude mitigation of the offshore structures like tension leg platform(TLP) is important since these structures are always exposed to environmental loads such as waves, and in the case of TLP, reduction in response amplitude of platform causes reduction in stress range in tendons; this would increase the fatigue life of tendons, and therefore, increases the structural safety. Also providing stable conditions for machinery and crew increases the efficiency and functionality of the platform. This article thus aims to investigate the possibility and effectiveness of applying tuned mass damper (TMD) as a passive structural control system to suppress the surge motion of TLP that is exposed to wave load. Both numerical and experimental studies were carried out to assess the performance of the TMD. A close agreement is obtained between the numerical simulations and experimental results. The results of numerical and experimental investigations in this study indicate that applying the TMD, tuned to the surge natural frequency of the platform or frequencies close to the surge natural frequency of the platform, doesn’t have efficiency in reducing the surge responses of TLP in the range of probable waves in seas and oceans. Introduction Tension leg platform (TLP) is one type of the offshore structures used for exploitation of petroleum from deep seas and oceans. This is a type of positively buoyant floating platform, which means that the buoyancy caused by the hull of the TLP in operational draught is more than the total weight of the platform. This excess buoyancy is used to impose initial pretension in straight vertical mooring lines called tendons. TLP is moored to the seabed using the group of tendons at its every corner. One of the features of tendons is that this type of mooring lines have high axial stiffness. Because of that, tendons restrict the vertical motion (heave) and vertical in-plane rotations (pitch and roll) of the platform, but show flexibility in horizontal motions (surge and sway) and horizontal in-plane rotation (yaw), therefore, TLP can be considered laterally compliant because of significant range of displacements in these degrees of freedom. </jats:sec

Conceptual study on dynamic responses of semi-submersible platforms

Hydrodynamic analysis of semi-submersible offshore structures has been the topic of several numerical and experimental research studies conducted in the past. In this work, the effect of structure alignment and mooring lines on the dynamic behaviour of semi-submersible platforms was taken into account. The contribution of pitch and roll into heave motion of side-points was also investigated. Mooring lines are connected to semi-submersible platforms at the sides; therefore, the total displacement of side-points is related to the total displacement of mooring lines and total tension. A three-dimensional boundary element model of Amirkabir platform in the Caspian Sea was created using the boundary element method and the resulting response amplitude operators (RAOs) were obtained. A conceptual study on RAOs was conducted, which showed that structure alignment with respect to wave direction is highly important for a range of wave periods. In addition, in surge and sway, the effect of mooring lines is significant in particular ranges of wave periods. The contributions of roll and pitch to the side-points of semi-submersible platforms are also considerable in a range of periods. </jats:p