A Novel Concept for a Deformable Joint in Offshore Structures

Abstract

A new semi-rigid beam-to-column, as well as bracing-to-frame, connection has been developed. In addition to its application in building structures, due to its special features and characteristics, it can best lend itself to offshore jackets, where fracture caused by fatigue has always been a major concern. The adverse effects of welding, material embrittlement, residual stresses, etc., pronounced by cyclic loading on such structures, can be overcome by using this new connection. Moreover, it can be easily used in the structure of various modules used in the topside of platforms and FPSOs. The particular feature of this connection is its geometry, which is so devised that it allows much higher rotations to be delivered than those normally used. Furthermore, it can work in a ‘sacrificial’ capacity with the ability to ‘contain’ the damage, preventing it from propagating through the jacket/frame members. This, together with its ‘replaceability,’ minimizes the cost of repair, if the structure is subjected to severe loading conditions, such as earthquakes, blast, or ship/ice impact. Being a separate entity, it is of a limited size, and can be easily housed in a heat treatment oven to be annealed and thereby to remove the adverse effects of welding. These features eliminate the need for taking additional measures to prevent the development of cracks in such joints, and can better guarantee the safety of the entire structure. Due to the use of high energy absorbing elements within this connection, it can dissipate high amounts of energy prior to its failure. The normal dilemma of not being able to provide high strength (moment capacity) together with large rotational capacity, presented by almost all conventional joints, is overcome in this new design. These two important features can be adjusted to the demand of the structure quite independently. A comprehensive experimental study of the comprising energy-dissipating elements showed the crucial role of annealing in eliminating the embrittlement effects of welding on the connections, in particular the developed cracks. Finally, the behaviour of two single-bay, single-storey, building frames, fitted with this connection, subjected to the ground acceleration records of four major earthquakes, was studied numerically. Various dynamic response parameters of these frames were worked out and compared with those of their rigidly-connected, as well as hinged-connected, counterparts.