Digital-Control-Based Approximation of Optimal Wave Disturbances Attenuation for Nonlinear Offshore Platforms

Abstract

Abstract:The irregular wave disturbance attenuation problem for jacket-type offshore platforms involving the nonlinear characteristics is studied. The main contribution is that a digital-control-based approximation of optimal wave disturbances attenuation controller (AOWDAC) is proposed based on iteration control theory, which consists of a feedback item of offshore state, a feedforward item of wave force and a nonlinear compensated component with iterative sequences. More specifically, by discussing the discrete model of nonlinear offshore platform subject to wave forces generated from the Joint North Sea Wave Project (JONSWAP) wave spectrum and linearized wave theory, the original wave disturbances attenuation problem is formulated as the nonlinear two-point-boundary-value (TPBV) problem. By introducing two vector sequences of system states and nonlinear compensated item, the solution of introduced nonlinear TPBV problem is obtained. Then, a numerical algorithm is designed to realize the feasibility of AOWDAC based on the deviation of performance index between the adjacent iteration processes. Finally, applied the proposed AOWDAC to a jacket-type offshore platform in Bohai Bay, the vibration amplitudes of the displacement and the velocity, and the required energy consumption can be reduced significantly.

Simulation and Experimental Studies of a Multi-Tubular Floating Sea Wave Damper

“…These areas are directly related to many fields of human activity, such as industry, trade, agriculture, fishing and tourism. As a rule, hydrotechnical facilities are structures that are subjected to complex mechanical loadings [2][3][4][5][6][7][8], among which of primary importance are the hydrotechnical forces associated with water movement [9,10]. The most important factors that cause water movement in coastal zones are surface waves and wave-driven currents [11][12][13].…”

“…The vertical element (8) is a tube closed at both ends which constitutes a natural float with a specific buoyancy force. On each vertical tubular element (8), in its top and bottom part, there is a clamp (7) which is a three-sectional bracket connected by means of bolts. Each of the three elements of the bracket is equipped with a clamp to which the cut-off top of the rigid spacer (9) is attached using two bolts.…”

“…The spacers (9) combine the vertical elements (8) in their upper and lower parts, forming two rigid trusses in two planes. In one row of the vertical tubular elements (8) three top ends of the rods (6) are attached to the bottom of the clamps (7), and each of the lower ends of the rods (6) are fastened to the anchoring element (5) located on the seabed (4). Figure 5B present a top view of the device.…”