Figure 5 Accident Rate (Structural Damage)
Figure 6 Accident Rate (Blowout and Fire)
Figure 7 Accident Rate (Capsize)
Figure 8 Accident Rate (Collision)
to directly discuss the relation between rig age and type of accident. It is because one accident is usually classified into two or more types of accident so that there rarely exists an accident which can be classified into one type. For example CA can happen as a result of ST or BL (blowout) or TO (towing accident) or independently. So here we propose rather theoretical three models of typical relationship between rig age and accident rate as following:
1) In the case that accidents occur with no relation to maintenance or to rig age as shown by vague curve A in Figure 11. This is called "Random Model".
2) So called bathtub curve relation between operating hour (rig age) and failure rate (accident rate) for the case of good maintenance. See Figure 10 [6] and curve B in Figure 11. This is called "Bathtub Model".
3) In the case of not appropriate maintenance as shown by curve C in-Figure 11. This C curve means that accidents due to bad maintenance begin to increase from relatively early stage of operation. This curve C is called "liqueur glass model" due to its shape.
Accident rate curve due to ST (Figure 5) can be considered to be a combination of bathtub curve B and liqueur glass curve C. Accident rate shows a hump between rig age of 10 and 20 years old and then it decreases. In Figure 2, survival rate of rig begins to decrease sharply at around rig age of 20. This means a sharp decrease in number of rig and it is estimated that this comes from a fact that not a small number of rigs are forced to retire due to accident at around age of 10 to 20 years old. It is because rigs of accident at age after "infant mortality stage" (Figure 10) and also due to bad maintenance are difficult to be reused by repairing due to accumulated characteristics of defects in structure which have caused accident.
Figure 6 is for BL (blowout) and Fl (fire). Curve in this
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