Compliant Tower Hydrodynamic Loading and Response Measurements
Authors: Cuffe P.O. , Finn L.D. D. , Lambrakos K.F. 
Published: October 2008

Laboratory data collected from tests with models of the Compliant Piled Tower (CPT) and the Lena guyed tower are compared to analytical predictions for forces, motions, and near field cinematic velocities. Predictions from conventional methods of analysis, as well as from procedures that account for the effects of member shielding and wakes, are presented.

The comparisons indicate the importance and quantify the contribution of current shielding and wake effects to compliant tower forces and motions, and show the existence of a low frequency component in the motions. Due to shielding, the current velocities inside the structures were only about half of the far field velocity, and current induced forces were only 45 to 65 percent of the predicted forces for the CPT (depending on tower orientation). These current reduction factors, when used in wave plus current cases, adequately predict forces and motions. Test results also indicate that the large low frequency response calculated by conventional procedures is significantly over predicted. A major part of this over-prediction can be removed by accounting for current shielding and wake effects. Good comparisons between the forces from the Lena model and Lena field measurements lend support to the findings for the CPT.

INTRODUCTION

Analysis procedures used to design steel piled jackets typically calculate the water particle kinematics that exist in the free field (1. e. in the absence of the structure) and then, for the calculation of the forces on each platform member, assume that these kinematics are unaltered in the presence of the platform. The objective of the research effort discussed herein was to determine the nature of the flow-structure interaction and how this interaction affects the hydrodynamic forces and dynamic response of the CPT, and to develop procedures that can predict the forces and response.

Recent research efforts have shown that modifications to the conventional analysis procedures are required to accurately predict the dynamic response of compliant towers. Data taken from the Lena guyed tower revealed that the current induced forces were only about 20 percent of those predicted, and the predicted motion contained a significant low frequency component that was not present in the measured motion [1,2]. Previous analytical studies have indicated that the response of a deep water compliant tower contains a large low frequency component which can be appreciably reduced by considering current shielding and wake effects.

To identify and quantify the effect that flow-structure interaction has on the forces and dynamic response of compliant towers, a series of tests were performed with a 1:47.5 and a 1:91 scale model of a 2500 ft Compliant Piled Tower (CPT) and a 1: 48 scale model of the Lena guyed tower. The tests included variations in sway period, member roughness, structure heading, and current and wave conditions. Total model forces, motions (displacements), and accelerations, as well as wave heights and flow velocities were measured. The following sections will discuss the model test setup conditions, the methods used to analyze the data, and the test results under two major topics, hydrodynamic loadings and tower motions.