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

Abstract

THIS PRESENTATION IS SUBJECT TO CORRECTION Offshore Technology Conference on behalf of the American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. (Society of Mining Engineers, The Metallurgical Society and Society of Petroleum Engineers), American Association of Petroleum. Geologists, American Institute of Chemical Engineers, American Society of Civil Engineers, American Society of Mechanical Engineers, Institute of Electrical and Electronics Engineers, Marine Technology Society, Society of Exploration Geophysicists, and Society of Naval Architects and Marine Engineers. This paper was prepared for presentation at the Seventh Annual Offshore Technology Conference to be held in Houston, Tex., May 5–8, 1975. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. Such use of an abstract should contain conspicuous acknowledgment of where and by whom, the paper is presented. presented 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 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 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 pile supported tower was conducted. The tower is a welded steel space frame with four primary legs, braced with horizontal and diagonal members.