ACTIVE CONTROL FOR ENHANCING FATIGUE LIFE OF TLP PLATFORMS AND TETHERS Rosane Martins Alves Engineering Graphics Department Poli -Federal University of Rio de Janeiro Rio de Janeiro, Brazil Rosane@deg.ee.ufrj.br Ronaldo Carvalho Battista Civil Engineering Program COPPE- Federal University of Rio de Janeiro Rio de Janeiro, Brazil Battista@labest.coc.ufrj.br Carl Horst Albrecht Engineering Graphics Department Poli-Federal University of Rio de Janeiro Rio de Janeiro, Brazil Carl@deg.ee.ufrj.br Abstract. This paper is concerned with the dynamics of a tension leg platform (TLP) under environment load effects. In deep water scenarios, large heave amplitudes caused by dynamic loads are considered as one of the most deleterious effects to the structural safety. Here it is shown that active control systems may be installed inside the hull of a TLP to attenuate dynamic amplitudes in heave motion which leads to a significant reduction of the stress levels in tendons and their links and anchorages, minimizing fatigue problems and increasing tether’s service life, improving the production system performance. The control devices may be located inside the columns. The uncontrolled and controlled dynamic behaviors of a TLP prototype are investigated by using simplified mathematical model where the TLP is considered as a tridimensional rigid body, with six degrees of freedom and the tendons as elastic springs. The dynamic behavior is described by nonlinear coupled second order differential equations of motion by using the Hamilton Principle. The numerical results lead to the conclusion that active systems are effective in reducing and controlling the heave displacement amplitudes, for a given low mass ratio, and consequently the stress variations in tendons and risers of a TLP. Keywords. Offshore Structure, Active Control, Fatigue Analysis 1. Introduction Tension Leg Platforms (TLP) have been considered the most promising hydroelastic systems intended for deep water oil exploitation, especially because of its economical viability. Like any other floating offshore structure, the TLP displays large response amplitudes in heave motions to wave disturbances, which vary in severity with sea/wind conditions. These large heave amplitudes present a serious drawback to the required dynamic behaviour and in service life of both prestressed tethers and hanging risers because early fatigue cracks may develop in these structural components. The active control of heave motions of these huge compliant offshore structure is made feasible by a logical control applied to servo-hydraulic/pneumatic actuators that accelerate reaction masses counteracting the TLP’s floating hull movements. Other conceptions of active and semi-active control of floating structures may be found in the literature (Hrovat, 1983; Reinhorn, 1987; Sirlin, 1980). The active system optimal control laws keep response amplitudes under pre-defined performance measures. Under severe environmental forces, the structural components of uncontrolled systems may develop inelastic deformations in a manner for dissipating energy. Alternatively, the response amplitudes may be attenuated by an active control device, through which they may be continually monitorated and corrected by added inertia forces. The actuator stroke and the developed hydraulic pressure are among the most important factors to be considered in the design of active control systems (Alves, 1997; Battista, 1993). Control systems application to attenuate the heave motion in TLP’s may be more efficient and less costly than pneumatic/telescopics devices. The TLP heave motion control leads to a significant reduction of the stress levels in risers tendons and their links and anchorages, besides minimising fatigue problems, then improving the production system performance and increasing tether’s service life. The uncontrolled and controlled TLP dynamic behaviour was investigated by using simplified mathematical models for the hydroelastic structure under irregular wave loads. The goal of this paper is to demonstrate analytically the