A modal approach to identify fatigue damage in threaded connections of large scale tubular structures

Tubular structural elements with diameters in the range from 200 mm to 1400 mm are widely used in onshore and off-shore constructions. Typical examples include (drilling) platforms, bridges, and pipelines. In most cases several tubular elements (made of steel) are combined by means of weld joints. The current welding procedures for high strength steel are very time-consuming and thus constitute an important part of the total construction cost. In some applications, the need is inherent for pipes being frequently coupled and uncoupled as in the case of drill pipes and tension-leg platforms. Therefore, the application of threaded connections is considered as a valuable alternative and their design as an important challenge, especially the proof of sufficient fatigue strength. In the case of drill strings, fatigue failure is caused by cyclic load during drilling operations. In other applications, dynamic loads can be caused by environmental and operational conditions, e.g. wind, waves, vortex induced vibrations, internal pressure changes, etc. In this context, fatigue damage identification at an early stage plays an important role on the integrity of the structure. The present work aims at assessing progressive damage in threaded connections subjected to cyclically varying loads by means of vibration measurements. For this purpose, a four-point bending fatigue test with standard connection has been setup [1, 2] and a damage assessment approach has been developed based on dynamic system identification. As damage in a threaded connector is a local phenomenon, which may not significantly influence the lower frequencies or the global response, it is essential to gather accurate information about natural frequencies and mode shapes of many vibration modes [3]. The progressing damage will be compared with the forward predictions of a physical model that is able to predict the stiffness degradation as a function of an arbitrary, but known, load history [4].