Marine Transport Lines Inc. has agreed to sell 50 percent of a tanker operating subsidiary to P&O Bulk Shipping Ltd. for 11.2 million ($18.4 million), the company said Thursday.

Proceeds from the sale of Rowbotham Tankships Ltd. will be used to retire debt associated with the takeover of MTL last year.Rowbotham is one of the largest operators of product tankers in North Europe and has a fleet of 21 vessels.

Richard T. du Moulin, chairman of Secaucus, N.J.-based MTL, said the company has been looking for a buyer for half of Rowbotham since MTL underwent a leveraged buy-out last November.

''The money we are receiving from both companies is being used to repay the debt we used to buy the company," Mr. du Moulin said. "They knock out about all our acquisition debt. It makes things much more comfortable here."

He said such a move would provide growth opportunities for Rowbotham. Along with the sale of part ownership in a 90,000-ton tanker to Norwegian interests, the Rowbotham sale will permit MTL to retire almost all of its acquisition costs.

The board of Rowbotham will be composed of representatives of MTL, P&O and Rowbotham, according to Mr. du Moulin. The companies said in a statement that Karl Timmermann, managing director of P&O Bulk Shipping, would serve as chairman of Rowbotham. Albert Shaw will remain as managing director.

Mr. du Moulin said the sale would provide support for the growth of Rowbotham. "It never had strong support to grow. Now we're going to give it strong support."

He added that he viewed developments in Europe as favorable for Rowbotham. "The room for growth in Eastern Europe and the Continent is very large. All the developments in Europe are very good for what we want to do."

Analyst reaction to the sale was positive. "It's a real coup for them," said James L. Winchester, shipping analyst with Mabon, Nugent & Co., a New York brokerage house. "They've been trying to find a buyer for them since they took over MTL last summer."

''They had quite a bit of debt to pay down. So they were looking for a way to sell some assets of the company. That's a problem with a lot of companies today: They're over-leveraged," said Mr. Winchester.

He added that the deal was as favorable for P&O Bulk Shipping as it was for MTL. "It gives them a foothold and a piece of an interesting niche in the tanker business."

P&O Bulk Shipping's parent is the Peninsular & Oriental Steam Navigation Co., London.

P&O Bulk Shipping operates a fleet of 10 vessels, including six bulk ships and three oil tankers.