Active Control of Offshore Steel Jacket Platforms

Offshore platforms are extensively used to explore, drill, produce, storage, and transport ocean oil and/or gas resources in different depths. There are several types of offshore platforms, such as self-elevating platforms, gravity platforms, steel jacket platforms, tension leg platforms (TLPs), articulated leg platforms, guyed tower platforms, spar platforms, floating production systems, and very large floating structures. These platforms can be divided into fixed-bottom platforms and buoyant platforms, which have their own particular purposes and different configurations. To meet an increasing demand for marine sources of energy and minerals, in the past several decades, a lot of research effort has been made on offshore platforms. The related investigations are focused mainly on structure design and monitoring, damage detection, fatigue analysis and reliability assessment, mathematical modeling, and analysis of structures. Specifically, offshore platforms, which are located in a very tough ocean environment over a long period of time, are inevitably affected by environmental loading, such as waves, winds, ice, currents, flow, and earthquakes [1, 2]. The environmental loading may lead to excessive vibration of offshore platforms, thereby causing failure of deck facilities, fatigue failure of structures, inefficiency of operation, and even discomfort of crews. Note that reduction of vibration amplitude of an offshore platform by 15% can extend service life over two times and can result in decreasing expenditure on maintenance and inspection of structures [3]. Therefore, it is of great significance to explore proper ways to reduce different types of vibrations of offshore platforms, and comprehensive surveys of vibration control for offshore structures are provided by Kandasamy et al. [4] and Zhang et al. [5].

In this chapter, two dynamic models of offshore platforms and several required lemmas are introduced for investigating active control strategies in this book. In the first dynamic model, only the first dominant vibration mode of an offshore steel jacket platform with an AMD mechanism is taken into account [79]. This model is utilized to design active controllers to attenuate wave-induced vibration of the offshore platform. In the second dynamic model, the first and the second vibration modes of an offshore steel jacket platform subject to an active TMD mechanism are considered [65, 72]. By considering parametric perturbations of the system and external disturbance, several uncertain nonlinear models for the offshore platform are developed. Such models are used to design active controllers to reduce vibration amplitudes of the offshore platform subject to self-excited hydrodynamic forces and/or external disturbance.

This chapter presents an optimal tracking control methodology for an offshore steel jacket platform subject to external wave force. Based on a dynamic model of an offshore steel jacket platform with an AMD mechanism and a linear exogenous system model of the external wave force on the offshore platform, an optimal tracking control scheme with feedforward compensation is proposed to attenuate wave-induced vibration of the offshore platform. A feedforward and feedback optimal tracking controller (FFOTC) can be obtained by solving an algebraic Riccati equation and a Sylvester equation, respectively. It is demonstrated that the wave-induced vibration amplitudes of the offshore platform under the FFOTC are much smaller than the ones under the feedback optimal tracking controller (FOTC) and the feedforward and feedback optimal controller (FFOC). Furthermore, the required control force under the FFOTC is smaller than the ones under the FOTC and the FFOC.

In this chapter, sliding mode H _∞ control for an offshore steel jacket platform subject to nonlinear self-excited wave force and external disturbance is developed. A sliding mode H _∞ controller is designed to reduce oscillation amplitudes of the offshore platform. In the case that the dynamic model of the offshore platform is subject to parameter perturbations, a robust sliding mode H _∞ control scheme is proposed. It is found through simulation results that compared with an H _∞ controller and a sliding mode controller, the sliding mode H _∞ controller requires much less control force; and the oscillation amplitudes of the offshore platform under the sliding mode H _∞ controller are less than those under the sliding mode controller.

This chapter is concerned with active control for an offshore steel jacket platform subject to wave-induced force and parameter perturbations. The offshore steel jacket platform is shown in Fig. 2.​2 [65, 72]. The dynamic model under consideration is given by (2.​44 ). By intentionally introducing a proper time-delay into the control channel, a novel sliding mode control scheme is proposed. This scheme utilizes mixed current and delayed states. It is shown through simulation results that this scheme is more effective in both improving control performance and reducing control force of the offshore platform than some existing ones such as delay-free sliding mode control [99], nonlinear control [65], dynamic output feedback control, and delayed dynamic output feedback control [100]. Furthermore, it is shown that the introduced time-delay in this scheme can take values in different ranges, while the corresponding control performance of the offshore platform is almost at the similar level.

This chapter is concerned with a delayed non-fragile H _∞ control scheme for an offshore steel jacket platform subject to self-excited nonlinear hydrodynamic force and external disturbance. By intentionally introducing a time-delay into the control channel, a delayed robust non-fragile H _∞ controller is designed to reduce the vibration amplitudes of the offshore platform. The positive effects of time-delays on the non-fragile H _∞ control for the offshore platform are investigated. It is shown through simulation results that (i) the proposed delayed non-fragile H _∞ controller is effective to attenuate vibration of the offshore platform, (ii) the control force required by the delayed non-fragile H _∞ controller is smaller than the one by the delay-free non-fragile H _∞ controller, and (iii) the time-delays can be used to improve the control performance of the offshore platform.

This chapter investigates the effect of a time-delay on dynamic output feedback control of an offshore steel jacket platform subject to a nonlinear wave-induced force. First, a conventional dynamic output feedback controller is designed to reduce the internal oscillations of the offshore platform. It is found that the designed controller is of a larger gain in the sense of Euclidean norm, which demands a larger control force. Second, a time-delay is introduced intentionally to design a new dynamic output feedback controller such that (i) the controller is of a small gain in the sense of Euclidean norm and (ii) the internal oscillations of the offshore platform can be dramatically reduced. It is shown through simulation results that purposefully introducing time-delays can be used to improve control performance.

The network-based modeling and active control for an offshore steel jacket platform with an active tuned mass damper mechanism is investigated in this chapter. A network-based state feedback control scheme is developed. Under this scheme, the corresponding closed-loop system is modeled by a system with an artificial interval time-varying delay. Then, a delay-dependent stability criterion for the corresponding closed-loop system is derived. Based on this stability criterion, a sufficient condition on the existence of the network-based controller is obtained. It is found through simulation results that (i) both the oscillation amplitudes of the offshore platform and the required control force under the network-based state feedback controller are smaller than those under the nonlinear controller [65] and the dynamic output feedback controller [100]; (ii) the oscillation amplitudes of the offshore steel jacket platform under the network-based feedback controller are almost the same as the ones under the integral sliding mode controller [99], while the required control force by the former is smaller than the one by the latter.

This chapter investigates the network-based modeling and event-triggered H _∞ reliable control for an offshore structure. First, a network-based model of the offshore structure subject to external wave force and actuator faults is presented. Second, an event-triggering mechanism is proposed such that during the control implementation, only requisite sampled-data is transmitted over networks. Third, an event-triggered H _∞ reliable control problem for the offshore structure is solved by employing the Lyapunov-Krasovskii functional approach, and the desired controller can be derived. It is shown through simulation results that for possible actuator failures, the networked controller is capable of guaranteeing the stability of the offshore structure. In addition, compared with the H _∞ control scheme without network settings, the proposed controller can suppress the vibration of the offshore structure to almost the same level as the H _∞ controller, while the former requires less control cost. Furthermore, under the network-based controller, the communication resources can be saved significantly.