Federal law says U.S. merchant rips have to have U.S. crews, and the lost of a typical U.S. crew of 32 board a modern oil tanker is about 1.7 million a year, one New Year tanker owners' association has calculated.

All Italian tanker crew of the same size costs only $600,000 a year; Italian wages are lower, benefit fewer. A Greek crew can be assembled for an even lower amount, perhaps $325,000, and a Chinese crew for under $300,000.

A tanker owner can thus save more than $1 million a year by registering his vessel in some country other than the United States.

That compelling bit of international arithmetic is the major explanation or a strange maritime pheomenon that has been frequently in the news in recent weeks - the vast merchant fleet of the tiny nation of Liberia in Equatorial West Africa.

It is one of the major reasons why so many of the tankers carrying oil to this country fly the Liberian, Panamanian or other foreign flags.

There has been a spate of accidents and oil spills from these foreign-flag tankers in or near U.S. waters, beginning Dec. 15 when the Liberian-registered Argo Merchant ran aground 27 miles off Nantucket and eventually spilled 7.6 million gallons of oil into the ocean.

Two days later another Liberian tanker blew up in Los Angeles harbor, leaving nine dead and 50 injured.

Since then, two Liberian tankers have run aground in the Delaware River near Philadelphia, one spilling 133,000 gallons of oil; another Liberian tanker has run aground off Puerto Rico; still another has spilled 2,000 gallons of oil in the Thames River near Groton, Conn., and a Panamanian tanker, the Grand Zenith, has disappeared with a crew of 38 and 8.2 million gallons of oil in heavy seas off Nova Scotia.

And the accidents continue: at 1 a.m. Friday the loaded Liberian tanker Barcola ran aground off Port Arthur, Tex., and at midnight on Friday an explosion injured several crewmen and knocked out the navigational equipment of the empty Liberian tanker Mary Ann 300 miles east of Norfolk.

It is easy to conclude from all this that the problem lies with Liberia, that the Liberian merchant marine must be the bilge of the world's tankers and seamen. But the problem is more complicated.

Ships flying the Liberian flag are seldom owned or manned by Liberians. Their money and men come from every sea faring nation on th globe.

What Liberia's "flag of convenience" represents is a stable government, with a stable urrency, and mortgage and tax laws that are favorable to the industry - a combination not all that common.

Thus, the fleet is almost impossible to categorize. Although it includes its share of aging rust-buckets, it also includes some of the world's most modern tankers - many of the tankers owned by the major oil companies, for example.

The Liberian fleet as a whole has nowhere near the world's worst accident and spill rates. Almost all maritime experts agree, for example, that Liberia's tankers as a group are far safer than Panama's.A study by the U.S. Coast Guard suggests that Liberian tankers also spill a smaller percentage of the oil they carry than do Italian, Greek and even U.S. tankers.

The basic oil-spill problem is simply that more oil is being moved today by sea. World tanker tonnage has increased four fold in 12 years. The more oil being moved, the greater then chance that some will spill.

One expert, Arthur McKenzie, director of the Tanker Advisory Center in New York, a ship-buyers' information service, says 104 tanker accidents occurred world worldwide in the first nine months of 1976, of which 21 resulted in substantial spills. The total spilled was more than 60 million gallons. The number of accidents has been higher in previous years. It was over 900 in 1973, for example. But the amount of oil spilled in 1976 was a record, McKenzie said, partly because tankers are being built bigger year by year, so a single accident can produce a larger spill.

At the start of last year, the Commerce Department says, there were 5,311 tankers in the world, with a total bulk of [WORD ILLEGIBLE] million tons.

Nearly a fifth of these tankers - 1.014 of them, representing almost a third of the tonnage, because they include so many of the newer, larger vessels - were registered in Liberia. This nominally Liberian tanker, fleet was the world's largest by far followed by the fleets of Japan (331 tankers), Russia (462), Britain (459), Greece (345), Norway (332), the United States (250), Panama (231) and Italy (236).

The 250 U.S.-registered ships are almost all involved in coastal trade.

U.S. companies or individuals owned about a fourth of the Liberian tankers; Greek and Japanese interests also owned substantial numbers.

There is nothing new about using flags of convenience in ocean commerce. American history, is dotted with examples.

Phillip Loree, chairman of the Federation of American Controlled Shipping, an organization of major U.S. companies with ships registered in Liberia and Panama, says flags of convenience were used in the 1920s to escape the dry hand of Prohibition Liquor could not lawfully be served on U.S. soil nor on U.S. ships - so entrepreneurs re-registered ships under foreign flags, took customers cut beyond the three-mile limit and sold then all they could hold.

Flags of convenience were also just before World War II by President Roosevelt to circumvent the Neutrality Act, which forbade using U.S. ships to carry certain goods to belligerents.

Roosevelt managed to send supplies to allied nations anyway, by having ships re-registered under the Panamanian and Honduran flags.

Liberia did not become a favored flag of convenience until after World War II.

American owners obtain two important economic advantages in registering their vessels under foreign flags.

First, it relieves them of such costly requirements as hiring only U.S. crews - a restriction Congress has imposed in U.S. vessels at the behest of the maritime unions.

Second, they are able through various sections of the tax code to defer, reduce and sometimes entirely escape U.S. income taxes on their "foreign" shipping profits.

Certain other characteristics have lifted Liberia to first place among flag of convenience countries. Strangely enough, its mortgage laws are important, Loree says. Tankers are built with borrowed money; the tankers themselves are generally used to secure the loans; and the leaders want the tankers registered in countries whose laws will give them clear title to the vessels if the original owners default.

Whether low safety standards are also a factor in Liberia's popularity as a flag-of-convenience country is a subject of some debate. On the one hand, the news of the last few days indicates that Liberia's standards are low enough to permit broken-down ships to keep on sailing. On the other hand, many maritime experts say Liberia's other flag-of-convenience countries, although they are not as high as U.S. standards, particularly as to manning qualifications, for officer and crew.

Congress in 1972 empowered the Coast Guard to impose unilateral safetystandards and to bar a tanker from U.S. waters if it doesn't comply. The Coast Guard so far has declined to do much of this, but pressure from Democrats and environmentalists will make it an issue in Congress again this year. CAPTION: Picture 1, Black crude oil seeps out of Liberian-registered tanker that ran aground near Philadelphia Dec. 27,; Picture 2, Nntucket student rescues [WORD ILLEGIBLE] with oily feathers after Liberian Argo Merchant breakup Dec. 15. AP