Detection of Structural Failure on Fixed Platforms By Measurement of Dynamic Response

Abstract

ABSTRACT A technique is described that can be used to detect subsurface structural failure by detecting changes in the natural frequencies of the structure. The experimental techniques are discussed, as well as the parameters which determine the minimum detectable damage level. Statistical Energy Analysis is introduced as a method for predicting the dynamic response of a wide variety of fixed and floating offshore structures to random wave forces. INTRODUCTION As offshore platforms are erected in progressively deeper waters, the problem of on site inspection of subsurface structural members becomes increasingly difficult and expensive. The current use of divers is hampered by poor visibility, poor lighting and hazardous conditions. These obstacles worsen rapidly with increase in depth. In addition, marine growth and corrosion may conceal structural defects. The inadequacy of current inspection techniques is amplified by the frequent reports of total or near total loss of platforms. The inspection technique described here requires periodic measurement of selected natural frequencies that show direct response to wind and waves. Wind and wave force spectra are sufficiently broad band random excitation to drive most offshore structures at one or more of their natural frequencies. Accelerometers can be used to measure the platform response, and from the records the natural frequencies can be determined. Such determinations might be made on a semiannual or annual basis. A detected shift in natural frequency between successive measurements would indicate a change in the mass or stiffness of the structure. A reduction in stiffness implies failure in the structural members and joints, or in the supporting bottom conditions. In some circumstances the measurements might be helpful in determining the location of the failure. At the very least the periodic inspection can be used as the "go" or "no go" decision maker for a much more expensive diver survey. A detailed study of one offshore pile supported tower' was conducted. The tower is a welded steel space frame with four primary legs, braced with horizontal and diagonal members. It is fixed to driven steel piles and stands 150 feet above the mudline in 70 feet of water and weighs approximately 600 tons. The tower serves as a manned United States Coast Guard Light Station near the Massachusetts coast. Though small by most standards, its limited size made an in depth survey possible. A computer simulation of the Buzzard Bay Light Station was prepared using the M.I.T. ICES-STRUDL 11(1) structures program. A systematic study of the effect of structural damage was conducted and the results compared to the accuracy of experimental determinations of natural frequency. It was sound that except for a few of the most insignificant members, the determination of natural frequency was accurate enough to detect failure of individual subsurface members. In some circumstances it would even be possible to isolate the general location of the break. It was also determined that widespread corrosion would cause a detectable change.