Abstract

On December 6, 1996, two seawater lift guides (one at the +19 foot level and one at the -40 foot level on the same caisson) and one firewater caisson guide (at -40 foot level) on the Xijiang 30-2 (XJ30-2) platform offshore South China Sea failed at the connection between the guide bucket and the standoff brace. The failed guides were missing and later retrieved from the seabed. Subsequent inspection detected cracks in other guides supporting the 24-inch caissons on both the XJ30-2 and Xijiang 24–3 (XJ24-3) platforms. Five temporary clamps (two firewater and two seawater on XJ30-2, and one firewater on XJ24-3) were installed to support the caissons at the -40 foot level to allow production to continue during the winter months. Analyses were undertaken to determine the cause of the failures and design of the final repair/strengthening to support the remaining caissons.

Analyses showed that the fatigue failure occurred due to dynamic loading caused by movement of the caissons inside the guides. Initially, wet welding permanent support members at the -40 foot level for the caissons was considered, but detailed FEA analyses of the proposed wet welded doubler plates showed high stresses in the wet welds that could cause future fatigue problems. Permanently installed clamps were designed and installed on all 24-inch caissons at the -40 foot level to replace the original guides. The + 19 guides were also replaced by welding supports between the structure and the caissons. This paper will cover the conclusions drawn from the analyses of the failure, analysis of proposed repair options, the decision making process that led to the final repair solution, and the fabrication and installation of the repairs.

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