Offshore Platforms: Observed Behavior and Comparison With Theory
Authors: Ruhl J.A. 
Published: December 2007

The measured static and dynamic response characteristics of four deep-water, jacket-type platforms are described. Fundamental periods and damping are determined by both free-vibration tests and spectral analysis of random response data. Spectra of both deck motions and wave heights are presented for one platform. Measured response amplitudes are compared presented for one platform. Measured response amplitudes are compared with theoretical predictions.

Introduction

Need for Comparisons
Until recently, the design of offshore structures emphasized static response to extreme waves. Because structures are designed for deeper water and more hostile environments, dynamic response (to both wave and earthquakes) and fatigue caused by many small waves have become additional important considerations. New questions arise, therefore, regarding our ability to predict the response of platforms to small waves and to perform predict the response of platforms to small waves and to perform dynamic response analysis. Concern about the response to small waves results from both the sensitivity of fatigue life to stress range and the general emphasis on the extreme wave condition in past wave-force research. With regard to dynamic analysis, testing the ability of modeling techniques to predict natural periods and to gather information on damping is appropriate. The natural period comparison provides a check on mass and stiffness modeling, and, to perform proper parameter variations, designers must have some knowledge of the accuracy of the natural period estimates. Fixed platforms with long natural periods (more than 4 seconds) will respond dynamically to the small waves causing most fatigue damage. Near-resonance damping controls the response level; therefore, measured damping values must be obtained on typical offshore platforms.

Objective and Scope of Tests
Several limited-scope, field-measurement and data-analysis programs were conducted by Shell Oil Co. to gain a better understanding of static response to small waves and dynamic response parameters.
The test's objectives were (1) to measure platform natural periods and to compare them with theoretical predictions, (2) to measure platform damping, and (3) to predictions, (2) to measure platform damping, and (3) to measure platform motions and wave heights for small sea states simultaneously and to compare the measured behavior with theoretical predictions. Data pertinent to Objectives 1 and 2 were obtained for four platforms; a pilot study was conducted at one platform to meet Objective 3.
Platform periods were determined by both free-vibration response to boat impacts and spectral analysis of response motions to ambient sea conditions. These data were used subsequently to estimate damping by the log-decrement and other spectral methods. To meet Objective 3, displacements were measured using an inertial displacement meter. Data were compared with theoretical predictions for several sea states with maximum wave predictions for several sea states with maximum wave heights up to 13 ft. Theoretical predictions of platform response were conducted using a three-dimensional model of the platform and the regular Airy wave theory. Future plans include testing wave theories incorporating the spreading of wave energy, with the amount of spreading determined from measured water-particle velocities.
JPT
P. 638