• States. Therefore, mandatory vessel retirements are determined by MARPOL and not by the Oil Pollution Act of 1990 (P.L. 101-380) (OPA 90), which bans non-double-hull tankers from U.S. waters no later than 2015 but does not preclude them from trading with other nations.
• Because the committee is examining the impact of mandated retirements only, economic or other types of retirements not driven by MARPOL 13G are excluded.
• MARPOL retires tankers based on delivery date. The Clarkson database used by the committee gives the year, but not the month, of delivery. Therefore, in the year of mandatory retirement, half of the vessel's deadweight is deleted from the supply. The full deadweight is deleted from all subsequent years.
• Tankers without double hulls that comply with MARPOL are retired at 30 years of age. (Double-hull tankers are unaffected by MARPOL retirement provisions.) MARPOL tankers are identified as those built from 1983 onward, plus those identified in Clarkson's database as having double sides or double bottoms, or having been built after 1979 and having segregated ballast tanks.
• All pre-MARPOL tankers will use hydrostatically balanced loading (HBL) in order to trade to 30 years of age. HBL results in an 8 percent loss in cargo carrying capacity for all pre-MARPOL tankers from age 25 through 30.
• Because of the OPA 90 lightering exemption, no tankers of more than 120,000 DWT are excluded from trade to the United States until 2015.
• Because of the current tight market for chemical tankers, these vessels are excluded from the supply of product tankers.
• Oil-bulk-ore carriers are identified separately.
• All government-owned military supply tankers, specialty tankers, and gas tankers are excluded from the supply.
• The deadweight of newbuildings is added to the supply based on the month and year of delivery. The full deadweight is included in all subsequent years. When delivery month is not specified, delivery is assumed to be at the beginning of the fourth quarter (i.e., 25 percent of the vessel's capacity is added to the supply in the year of delivery).

The committee's supply analysis is based on tanker fleet statistics (including orders) as of October 1, 1995.