case is considered to be a combination of accident rate models of A, B and C. There is a hump between rig age of 10 and 19 years old but the distribution of accident rate as a whole is somewhat vague compared to the case of ST (Figure 5). And the accident rate is very small for rigs older than 24 years old. This fact means that blowout and / or fire accidents can be partly avoided by good maintenance but not perfectly.
Curves for CA (Figure 7) and CL (Figure 8) are estimated to be composed almost of random model (curve A in Figure 11).Accident rate level itself is low compared to that of ST or BL/F1 and a scatter band is very wide. This means that accident type of CA or CL can be difficult to be avoided by means of good maintenance.
Figure 9 Number of Accident for Accident Type
Figure 10 Bathtub Curve
5. Relationship between Serious Accidents and Structure Age
Seriousness of accident is possible to be counted by its cost of damage (from economical side of view) or number of fatalities (from social side of view).
Top 20 serious accidents are selected by their cost of damage for a period 1955-1994[1] and also by their number of fatalities for a period 1970-1993 [2] and these 40 accidents are shown against rig age in Figure 12. The most big fatality accident, which is plotted at rig age of 13 years old is the case of Piper Alpha [4] with fatalities of 167. This accident is concluded to be due to big explosion, but the big number of fatalities is considered to be largely due to bad maintenance of equipments for emergency, such as emergency pump, emergency power unit, emergency communication system, life raft and so on. Except this Piper Alpha case, in Figure 12, almost all serious accidents from economical and social point of view seem to occur at rig age of 2-6 years old. In these, accident of Alexander L.Kieland [5],123 persons killed at this second big fatality accident, is plotted at rig age of 4 years old. The cause of this accident is already concluded to be due to a defect in weld bead connecting hydrophone to a brace and overlook of it at construction stage and during operation. Usually corrosion fatigue crack growth in seawater is very fast and a crack can be a critical size of a structure within several years.
Alexander L.Kielland accident was the case of this category. So there is a possibility that some of the serious accident in Figure 12 at rig age of 2-6 years old may be due to structural defects and oversight at construction stage and fast corrosion fatigue growth to critical one, and also due to fast growth of local corrosion to critical size of corrosion hole.
It is necessary to understand that there are two kind of defects due to bad maintenance, which lead structure to accident. One is acute one as corrosion fatigue and the other is chronic one as deterioration of material or general corrosion. From general view of maintenance, chronic one should be avoided by good maintenance.
