(World Offshore Accident Databank" [2] published by DNV Technica. The published book contains statistical data of accidents of mobile and fixed units from 1970 to 1993. There is no detailed description of each accident, but there are lists of total loss accidents and accidents with several fatalities. The 3rd is a report Outline of Rules and Regulations concerning Safety and Environment Protection at the Time of Oil and Gas Development (Part 2)" by Oil Technology Association of .Japan. This report tried to analyze rig accidents of 1955-1986 based mainly on data published by Offshore Data Service Inc. The 4th is a report of Piper Alpha accident which have occurred in 1988 at the North Sea, and it is edited by .Japan National Oil Corporation [4].
The 5th is also a report of the Alexander L.Kielland accident [5], which have occurred in 1980 at the North Sea.
3, Mean Age of Oil Rigs in Service
In order to analyze offshore accidents from view point of maintenance, data concerning number of annual rig construction, survival rate of rig and rig age distribution are necessary. Figure 1 shows the number of annual rig construction from 1949 to 1991 [7].
Figure 2 shows the survival rate of offshore rig at 199:3 [7].
Figure 1 Number of Rig Construction
Rig age distribution at a given year can be calculated by using data of Figure 1 and by assuming that the survival rate shown in Figure 2 is applicable for all rigs.
Figure 2 Survival Rate of Rig
In the figure, following six types of drilling rig unit
are included;
JU: .Jackup SU: Submersible
JT: . Jacket SS: Semisubmersible
DS: Drill ship DB: Drill Barge
Figure 3 shows the change of mean rig age. Data points in Figure 3 till 1993 are calculated results from rig age distribution as mentioned above and data points after 1994 are estimated values, assuming two cases.
For the case of A: number of rig construction is kept to be same as mean number of previous five years. And for the case B: total number of rigs in service is kept to be same as that of 1993. Assumption A is more realistic nowadays but the assumption B may be realized if oil price goes up rapidly in the very near future. Anyhow, mean rig age continues to going up or to keep high level till at least the early stage of 21st century. So it is estimated that the number of accidents in relation to maintenance condition continues to increase
