ABSTRACT

A routine underwater visual inspection of a semisubmersible operating in a relatively mild environment reported a 210 mm (8.27 in.) long crack in a 9 ft diameter tubular. This member is a main load carrying tubular in a horizontal framing plan having limited structural redundancy. A procedure was developed to calculate margins of safety against ductile tearing that accounts for the strain-hardening behavior of materials. The method is founded upon the construction of a failure assessment diagram (FAD). These diagrams have been used by the nuclear power industry for thick wall tubulars acting under plane strain conditions. Modifications are made to these procedures to reflect the low strength and high toughness materials used in thin wall tubulars by the offshore industry. The procedure is illustrated with a worked example. Results show that failure assessment curves based upon linear elastic fracture mechanics are very inaccurate and wi11 provide overestimated margins of safety against ductile tearing. Failure assessment curves recommended by certain nuclear power industry guidelines are inaccurate, due to their inability to account for strain-hardening and actual geometry of the structure containing the crack.

INTRODUCTION

The object of this paper is to present a simple engineering procedure that may be used to establish margins of safety against ductile tearing of thin wall tubulars containing circumferential through-wall cracks. The onset of ductile tearing is important in establishing the end of stable fatigue crack growth, as described by the Paris Law. The calculation of a margin of safety against ductile tearing permits an estimation of the load carrying reserve capacity of a cracked member. This simple engineering procedure recognizes the highly ductile nature of offshore steels. It is valuable in establishing the urgency of costly repairs and in determining adequate safety margins.

BACKGROUND

A routine underwater visual inspection of a semisubmersible discovered a 210 mm (8.27 in.) long crack on the outer surface of a member in a horizontal framing plan of the semisubmersible Figure 1. The horizontal plan constitutes the more critical structural members of the vessel by providing the means of holding the caissons together. Essentially, they are tension members under loading of topside weight and service loads. Mooring and wave loads acting on the vessel increase the tension further. These latter loads, which are cyclic, influence the fatigue life. Based on damaged condition strength analyses of the semi submersible, failure of this member can lead to immediate and serious consequences, due to the 1imited structural redundancy of the framing plan. The crack was located on the aft side of the 9 ft diameter × 0.75 in. thick tubular, as illustrated in Figure 2. The crack was monitored a total of four times over a period of approximately eight weeks using visual, UT, and EMD means. It was determined that it was a circumferential surface crack. Its size increased to 257 mm (10.12 in.) long and 12 mm (0.47 in.) deep by the time of the last inspection. It is necessary to analyze the static and cyclic forces acting in the cracked member to determine crack behavior.

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