also on economic trends, administrative policy of her owner, renovated means of transportation, etc. which are changeable and unstable. And, life span of hull structures of a ship depends not only on their strength and quality as built but also on sea conditions during voyage, quality and timing of maintenance, dynamic and chemical actions of cargoes and ballast water on them, etc. throughout her life. Moreover, as explained below, most ships are sold to continue operations.
Accordingly, it is unrealistic and not required to exactly estimate the life span of a ship at the time of her building. A ship is usually designed and built in vague estimation of her life span on the basis of experience and is operated as long as possible with allopathic maintenance. Thus, multifarious conditions of ship lives result in unstable life spans of ships.
Fig.l shows the distribution of the existing large bulk carriers with deadweight 40,000 tons and over by the country of builders and by the 5 year period of their birth, which are picked up from "REGISTER OF SHIPS 1995-1996" published by Lloyds Register of Shipping, and FIG.2 is prepared for the existing large oil tankers with deadweight 40,000 tons and over in the same manner as FIG.2. Considering the time of its publication, it can be considered that the graphs show the data as of the begining of 1995 and, for example, the ages of the ships built for the period from 1975 to 1979 are between 15 and 19.
These graphs carry suggestions as follows.
(1) In each period, about half the number of the large bulk carriers were built in Japan. It is generally considered that the older a ship is, the more possibly substandard her condition is. In this sence, half the number of the candidates for subsatandard ships were built in Japan.
(2) It is well known in shipping market that there are many ships scrapped before the age of 20 years. Especially these several years, through Enhanced Hull Survey by the member classification societies of IACS and Port State Control upon the entry to ports, the elimination of substandard ships have been promoted. But, there are many bulk carriers and oil tankers over 19 years old still being operated, most of which may be kept healthy. Thus, ships vary in their lives.
(3) There exist an opinion that large bulkers and oil tankers over 15 years old should be regarded as substandard ones. If they are defined as substandard ones by their ages, the life expectancy of ships may become younger due to negligence of proper maintenance for some years before the time limit reflecting shipowners' intentions to save costs. It should be recognized that the life of a ship may be remarkably prolonged with proper maintenance of her hull structures in time.
