Accident statistics of offshore structure are analyzed from view point of maintenance. At first, the tendency of mean age of offshore structures in the future is estimated, assuming that number of offshore structure construction is kept to be as it is now or total number of offshore structure is to be constant. And it is estimated that the tendency of increase of mean structure age is kept to be same as it is at least for a while. And the age distribution of offshore structures at each year is estimated on the data of survival rate at 1993. The relation between structure age (Aa) and accident rate (Ar) is plotted for all accidents and also for main types of accident. These Aa-Ar relations are analyzed and discussed based on the three basic and typical Aa-Ar relation models proposed by authors. They are corresponding to the following three situations
1) Accidents occur with no relation to maintenance conditions.
2) Accidents occur according to so-called "bathtub curve" of failure rate vs. service period for the case of good maintenance.
3) Accidents occur with close relation to bad maintenance.
These analyses show the importance of good maintenance for the avoidance of offshore accidents.
1. Introduction
Mean age of offshore structures is going up rapidly in these days. It is because that those structures which have been massively built till 1983 are still used and those built after 1984 have been very small in number. This situation is considered to continue, while the circumstance surrounding fossil energy supply and consumption remains to be unchanged. Therefore it is naturally concluded that good maintenance is strongly requested, in order to keep the safety of offshore structures.
Offshore structure accident data are usually classified into some groups according to type of accidents. And the types of accident include blowout, collision, fire, leakage, structural damage and so on, but there are no direct indication of the relation between the virtual cause of accidents and the manner of maintenance. So we examined the relation between the accident ratio of individual accident type and the structure age. It was because that accidents due to bad maintenance might have close relationship with structure age. And we tried to find out basic types of accident rate and structure age curves in relation to maintenance level.
2. Accident Data Used
In order to discuss the offshore accidents from view point of maintenance, we used following five data and references. The 1st is the "Historical Offshore Rig Accidents" (1955-Present) [1] published by Offshore Data Service, Inc. It is a worldwide listing of all major mobile rig accidents from 1955 to 1994 and is listed chronologically by accident year. Information of the listing includes name, owner, type, location and description at the time of accident. The 2nd is "WOAD"
