Flexibility Monitoring of Offshore Jacket Platforms

Abstract

ABSTRACT The new concept of Flexibility Monitoring, emphasizing the detection of fundamental vibration mode shapes, is an outgrowth of a general study of vibration monitoring for offshore oil and gas platforms. Its theoretical promise has been borne out in practice over the past several years in laboratory and field tests. The method is useful for underwater damage detection and, more generally, for identification of flexibility parameters of the structural dynamic mathematical model. Blind failure conditions involving removal of a K-brace section and foundation stiffness change were correctly identified in a government run laboratory test program. The brace removal was located in the correct bay and face of the model platform. Follow-on special testing of the model further demonstrated the theoretical expectation that diagonal severance is clearly distinguishable from both foundation flexibility and deck mass changes. It was shown that a very simple mathematical model provides similar sensitivity trends to those observed experimentally. Field experiments on the Shell Cognac and Chevron Garden Banks platforms have indicated that the necessary high quality data can be obtained from ambient acceleration measurements at underwater corner positions of a working platform. Continued development is warranted. Directions for future work are outlined. INTRODUCTION With Flexibility Monitoring, diagnosis of potential failure is based primarily on detecting the deflection shapes of the three fundamental vibration modes (two sway modes and one torsion). Advantage is taken of the predominant shear beam behavior of a steel jacket platform and the approximate equivalence between the fundamental mode shapes and the static deflection shapes due to corresponding deck loading. The concept can be effective for examining changes in directional shear flexibilities of individual jacket framing bays and of the foundation. An important consequence is that mass changes are deemphasized. The original "global mode monitoring" concept, with its emphasis on frequency shifts of the lower modes, typically provides less capability to detect and locate failure, because it is less sensitive to jacket damage (for reasons of high overall redundancy) and is a much poorer discriminator of the cause of change: jacket flexibility, foundation flexibility, or mass. l Field application of Flexibility Monitoring is visualized as involving temporary deployments of accelerometer packages placed down special, above water accessible, instrument chutes. The chutes are located on corner legs of a platformand extend down to the foundation. Ambient vibration data are acquired from a sequence of package positionings at the upper and lower levels of the jacket bays simultaneously with acquisition of deck corner accelerations. The acquisition system is optimized to detect the fundamental modes from ambient vibration, generally in the frequency range of 1/4 to I Hz. Optimized experimental apparatus and procedures are required to yield accurate Flexibility Parameters. These key parameters are equal to average horizontal deflection across each bay, normalized to the corresponding horizontal deflection in the above water portion of the platform. With Xi denoting the average deflection of level i, the Flexibility Parameter depicted in Fig. I is defined as:(Equation Available In Full Paper)