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COMPARATIVE RESPONSE OF DOUBLE HINGED ALP USING AIRY’S AND STOKES’ WAVE THEORIES

Abstract

Articulated loading platform (ALP) is one of the compliant offshore structures that are economically attractive especially as loading and mooring terminal in deep waters. These platforms are light in weight than conventional fixed platforms. An Articulated tower is a linear structure, flexibly connected to the sea bed through a universal joint and held vertically by the buoyant forces acting on it. The tower does not resist forces in bending due to wind, waves and currents rather; these forces are countered via a large buoyancy force. In this paper, dynamic analysis of the tower under regular waves has been carried out without current forces. The nonlinear governing equations of motion are derived using Lagrangian approach. Nonlinear effects due to variable submergence, buoyancy, added mass, instantaneous position of the tower and relative-velocity squared drag force are considered in the analysis. The equation of motion has been solved in time domain using NewMark’s-β integration scheme. Modified Morison equation is used to model the fluid forces as these equations account for non-linearities associated with vortex shedding effects accurately in comparison to standard Morison equation. Analytical studies are conducted to compare the response of double hinged articulated tower under regular waves using Airy’s wave theory evaluated with Chakrabarti’s modification and that obtained by using Stokes’ fifth order nonlinear wave theory. Stokes fifth order non-linear theory agrees closely in deep and intermediate water and it is found that for higher waves the difference in the values of responses obtained by Airy’s and Stokes’ are lesser while the difference is significantly higher for smaller waves. Results show that the deck displacement response as well as hinge rotation and hinge shear obtained using Stokes’ theory are lesser than that obtained using the Airy’s theory.

Moazzam Aslam, Nazrul Islam, Mohd Moonis Zaheer, Mehtab Alam

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