Fire from a spill of liquefied natural gas on water would be "beyond the capability of existing firefighting methods to extinguish," according to a comprehensive study of the volatile fuel.

The study, to be issued Monday and done by the congressional Office of Technology Assessment, says that any evaluation of the safety of liquefied natural gas (LNG) and its transportation system is largely speculation, due to the lack of experience and the immense quantities to be transported. Congress might want to correct that by drawing some guidelines, the report suggests.

Although not necessarily the most dangerous substance transported in cargo ships today, plans and projects presently under way will likely mean a sizable increase in the amounts being transported, the study says. LNG presently accounts for 1/20 of 1 per cent of the nation's gas consumption, but that could rise to between 5 and 15 per cent by 1985, the study concludes.

The gas is liquefied by cooling it to 260 degrees Fahrenheit, which reduces it to 1/600th of its gaseous volume. It is then loaded aboard ships designed to maintain it at that temperature.

Critics of LNG have worried that the huge insulated tankers might be "floating bombs" that could cause massive fires in the event of collision or large-scale spills in the terminal areas. One of those terminals is the $350 million Cove Point complex in the Chesapeake Bay, scheduled to begin operations in January.

Although researchers disagree on just what would happen in a spill over water, the study noted, "it is genrally agreed that if the vapor from a large LNG spill ignites, it would be beyond the capability of existing firefighting methods to extinguish it. Therefore, the key to reducing the hazard of an LNG fire is a strong prevention program.

In addition to cleaning up the areas of overlapping responsibility among agencies, the study says, Congress might want to consider tanker operating procedures. The only existing LNG import terminal in this country is in Everett, Mass., and all traffic in Boston harbor is stopped when LNG ships arrive every 20 days. Such ships are expected to arrive at Cove Point every two or three days and will have to mix with other bay traffic.

The report cited chlorine, acids, gasoline and liquefied petroleum gas (propane) as hazardous cargos already in common traffic that ought also to be considered in regulations for LNG. The following nine areas were suggested for congressional proposal:

Tanker design and construction is currently variable. There have been no serious accidents involving any of the 32 existing LNG tankers, but another 47 much larger ones are under construction. "It would be prudent to consider the need for limits on either tank sizes or total ship sizes," the study said.

As tankers age, Congress might consider the need for maintenance, inspection and foreign vessel evaluation standards for all hazardous cargo ships, the report said.

Tanker operating regulations may need revision. Congress might examine crew training requirements. Coast Guard training courses and adoption of traffic control standards, the report said.

Terminal operation regulations are difficult to enforce. Rules ought to allow flexibility in dealing with possible spills and will have to vary for each site, the study said. Congress should note the need for guidelines for inspection and enforcement, since "at the present time there is no mechanism for enforcing these orders."

The decision-making process is overlapping and confused. "There are currently no national policies for LNG that could be used as a basis for consistent Federal Power Commission decisions," the study said. WInning approval for a project can take three years, involve 130 or more permits and $5 million to $8 million in advance paperwork, all of which increases consumer costs and reduces the amount of ordinary citizen participation, the study said.

Safety research has not been definitive. Previous experiments have been difficult to compare with each other and have varied in their conclusions, but a proposed $50 million study of LNG over five years by the Energy Research and Development Administration might still not be definitive, the report said. Test spills should be done under model and natural safety conditions.

Facility siting criteria are nonexistent. "Either a standard site screening process . . . or a set of uniform siting criteria could be developed," the report said, involving minimum overall standards, national planning and specific local site evaluation. Special consideration should be given to offshore facility planning and to deciding whether "remote siting" advantages outweighed drawbacks in availability of firefighting equipment and ease of access.

Liability for accidents is uncertain. Maritime laws don't deal with new complex patterns of vessel ownership or dangerous cargo that could come up, while state-federal jurisdiction is equally cloudy, the report said.

LNG supply may not always be reliable. Both major exporters, Algeria and Indonesia, are members of Organization of Petroleum Exporting Countries (OPEC), which cut off U.S. oil supplies in 1973.

Although LNG supplies were not affected, future politically motivated disruption of supply "is at least plausible" and could have severe impact on eight states that will be getting as much as one-quarter of their natural gas from imported LNG by 1985 - Alabama, California, Georgia, Michigan, New York, Ohio, Pennsylvania and South Carolina.