City Research Online

Abstract

A total of eight tubular joints made from a 590 MPa yield strength steel used for Jack Up leg construction have been successfully fatigue tested. Specimens were tested at constant amplitude using out of plane bending in both an air environment and in seawater. Seawater specimens were cathodically overprotected at a potential of-1000 mV (vs Ag/ AgCl). In addition to the endurance life of the specimens, the crack depth was monitored.

Two methods of welding tubular joints have been identified as being used in Jack up construction: flux cored arc welding (FCAW), where the beads are laid perpendicular to the weld toe in a weave action; and shielded metal arc welding (SMAW) beads are laid in a direction parallel to the weld toe as stringer beads. The quality of the weave welds was inferior to that of the string welds. The fatigue properties of both methods have been investigated.

Results show that current fatigue guidelines for the endurance life of offshore grade steels of yield strength less than 400 MPa provide a conservative estimate for the endurance life of both air and cathodically protected test specimens regardless of the weld technique. However there is evidence to suggest that the weave welding technique may give improved fatigue endurance performance compared to the string welding technique.

An empirical fracture mechanics methodology has been derived to estimate the crack growth rate of both the weave and string welded specimens. A difference in the empirical stress intensity factor for the weave welded specimens compared to the string welded specimens has been noted. The string welded specimen Y value showed close agreement with empirical Y value formulae derived at University College London.

Crack growth rates under conditions of cathodic overprotection were comparable to crack growth rates of steels with a yield strength less than 400 MPa under conditions of optimum cathodic protection, however crack growth rates were noticeably higher for the string welded specimens than for the weave welded specimens under comparable loading conditions.

Crack growth paths for the test specimens were quite different from that normally associated with cracks in unstiffened tubular joints. This has implications for the in service inspection, residual strength and stiffness of the joints. The difference in the crack path was particularly noticeable with the string welded specimens. Manganese Sulphide inclusions were observed in the chord material, these may have influenced the crack path.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Departments:	School of Science & Technology > Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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