(Updates with excerpt from lawsuit in third paragraph.)

July 13 (Bloomberg) -- Ameron International Corp. was sued by an investor claiming a $772 million buyout offer from oilfield-equipment provider National Oilwell Varco Inc. undervalues the stock.

Directors of Pasadena, California-based Ameron, which makes pipe systems, have a duty to get the best price, plaintiff Alan Kahn said in a Delaware Chancery Court complaint made public today in Wilmington. National Oilwell bid $85 a share. Analysts have suggested they may be worth more than $100 each, Kahn said.

Ameron directors “have failed to take adequate measures” to protect shareholders’ investments and “embarked on a process that avoided competitive bidding,” Kahn said.

National Oilwell, based in Houston, said July 5 it would buy Ameron to gain access to its fiberglass-pipe business.

Gary Wagner, chief financial officer for Ameron, didn’t immediately return a call seeking comment on the lawsuit.

Ameron fell 1 cent to $85.30 in New York Stock Exchange composite trading at 12:02 p.m.

The case is Kahn v. Ameron, CA6660, Delaware Chancery Court (Wilmington).

--Editors: Charles Carter, Stephen Farr.

To contact the reporter on this story: Phil Milford in Wilmington, Delaware, at pmilford@bloomberg.net

To contact the editor responsible for this story: Michael Hytha at mhytha@bloomberg.net.