Calculated Dynamic Response of an Articulated Tower in Waves—Comparison With Model Tests

Abstract

Calculated dynamic response of an articulated tower in waves is compared with model tests. The theory used is based on Morison’s equation and linear wave theory and requires specified hydrodynamic force coefficients. Calculations are done with three different sets of coefficients. Firstly, coefficients are assumed not to vary with wave period. Secondly, they are selected from experimental data of oscillating flow past stationary cylinders. Thirdly, they are based on calculations using diffraction theory. Added mass and inertia coefficients have a predominant effect on calculated response, drag coefficients have almost no effect. Calculated tower top motion and horizontal force at the universal joint correlate well for all three sets of coefficients, indicating that hydrodynamic coefficients for normal flow are reasonably well selected and need not be specified with undue precision. In contrast, hydrodynamic coefficients for axial flow need to be chosen carefully. Calculated vertical force at the joint, using initially specified axial flow coefficients, correlates poorly with measurements. Correlation is greatly improved using reduced coefficients for axial flow. Calculated response is reasonably linear with wave height. Spectral analysis techniques are used to determine statistical measures for three irregular seastates. Agreement with corresponding model test results is satisfactory.