Structural Integrity Evaluation Of A Fixed Platform Using Vibration Criteria
Authors: Wojnarowski Matias E. , Stiansen Stanley G. , Reddy Neil E. 
Published: October 2008

A rational evaluation of the structural integrity of an offshore structure based on vibration calculations and on-board vibration measurements is performed for the USCG Ambrose Tower, a four-legged fixed platform at the entrance to New York Harbor. A mathematical model of the structure is analyzed by the SAP IV computer program, taking into consideration the effects of mass, entrained water, soil support conditions, equipment, etc. The various parameters affecting the lower-mode vibration characteristics of the structure are considered separately and their quantitative effect on the natural frequencies is evaluated. The theoretical analysis provides a convenient way to incorporate changes of structural, inertial and foundation properties, together with their effect on the vibratory response of the structure. Results of computer calculations are presented to illustrate this capability.

Vibration measurements taken on the tower are recorded and processed through a spectrum analyzer to establish correlation between calculated and measured results. This method of structural integrity evaluation by means of vibration characteristics, applicable to. many types of offshore structures, can reduce costly, inconvenient inspections and eliminate sources of unexpected catastrophic failure through early damage detection and repair.

INTRODUCTION

The structural integrity of offshore structures has been an overall concern of designers, builders, operators and underwriters. The increase in the number of these structures, their growth in both size and cost, as well as their location in deeper and less accessible waters, have only compounded the problem.

Several years ago, H.M. Tiedemann & Co. proposed using the vibration characteristics of offshore structures for an indication of structural damage [1]. This approach had been successfully used to perform seismic damage surveys of buildings. The goal is the eventual replacement of the long, expensive and depth limited system of diver inspection with vibration calculations and measurements that can predict the failure of structural members at a fraction of the cost. The vibration characteristics approach is based on the fact that each offshore structure, regardless of type, has natural modes of vibration that are continually excited by the wind and wave forces of the environment. These modes are dependent on the characteristics of each structure and not on the excitation; they are peculiar to each structure and do not change unless the structure or the masses on it change.. The natural mode response to ambient external excitation can be calculated and can also be measured with small portable accelerometers.

The method provides a way to confirm possible structural damage, especially damage below the waterline which is often difficult to detect. It is also particularly effective in detecting microcracks in concrete in structures such as reinforced concrete platforms. These microcracks are otherwise undetectable, and can lead to structural failure.

The method can also be advantageously used to detect major structural damage which has occurred frequently during transportation of the platform to its site. Vibration signatures obtained before and after the towing operation will give an indication of such damage.