Figure 11 Three Accident Rate Models Proposed Curve
But from view point of preventing serious and sudden accidents at rather early stage of rig operation, acute defects as corrosion fatigue should be avoided by proper design, construction, finding and repairing of weld defects and good maintenance during operation.
Figure 12 Top 20 Serious Accidents
6. Conclusion
Conclusions obtained are as following:
1) Mean rig age in service continues to increase or to keep high level till around early stage of 21st century.
2) Accident rate for all kinds of accident type shows a hump at rig age of 10-20 years after rapid decrease from initial high value. This is a characteristic shape for offshore structure accidents and this seems to closely related to rapid decrease of survival rate after rig age of 20 years old.
3) For relation of accident rate and rig age for individual accident type, three model are proposed; in relation to degree of maintenance, "random model" or model A (no relation to maintenance), "bathtub model" or model B (in the case of good maintenance) and "liqueur glass model" or model C (in the case of bad maintenance).
4) Relation between rig age and accident rate of main accident types is discussed based on three models proposed. For the accident type of structural damage, rig age vs. accident curve is decided to be a combination of models B and C (heavily affected by maintenance). For blowout and fire, the curve is combination of models of A, B and C (somewhat affected by maintenance). For both cases of capsize and collision, the curves is simply a model A (not affected by maintenance).
5) Serious accidents from both economical and social point of view have happened relatively young age. Some of them seem to have occurred due to acute type defects like corrosion fatigue or local corrosion. They are also due to bad maintenance at initial stage of service.
6) To prevent the rise of accident rate at rig age of 10-20 years and also happening of serious accidents at rig age of 2-6 years, good maintenance in order to decrease both acute and chronic types of defect or deterioration should be made.
References
1) Offshore Data Service: Historical Offshore Mobile Rig Accidents (1955-present), 1994.
3) Oil Technology Association of Japan: Outline of Rules and Regulations concerning Safety and Environment Protection at the Time of Oil and Gas Development (Part 2), 1988.
4) Japan National Oil Corporation: Report of Piper Alpha Gas Explosion Accident at North Sea, 1990.
5) Robert Bruce Wallace: The Alexander L.Kieland Accident, Proceedings from the first Robert Bruce Wallace Lecture at MIT, PP. 1-20, 1981.
6) OPL: An Introduction to Offshore Maintenance, PP. 25-27, 1991.
7) Takada, M. et al. : The Maintenance of Offshore Structure -On the Present Status-, Proc. 13th Ocean Eng. Symposium, PP. 133-140, 1995.
