By Yereth Rosen

(Reuters) - A German cargo shipper targeted last year by Somali pirates will pay a $1.2 million penalty and be barred from U.S. ports for five years as punishment for oil-dumping violations off the Alaska coast and elsewhere in the open ocean, U.S. federal officials said on Friday.

Nimmrich & Prahm Bereederung and Nimmrich & Prahm Reederei, the operator and owner of the Susan K, pleaded guilty on Friday to obstructing justice and concealing the practice of dumping oily bilge water at sea, said federal prosecutors in Alaska.

The companies entered their pleas and were sentenced in the U.S. District Court, Southern District of Texas, in Houston, where the Alaska and Texas cases were consolidated, Assistant U.S. Attorney Kevin Feldis said.

The ship involved in the environmental crimes was hijacked by Somali pirates last year off the coast of Oman. The ship and its crew were freed after the owners paid a ransom that one of the pirates claimed totaled $5.7 million.

The environmental violations occurred after that ordeal, according to court documents. Oil dumping in Alaska waters took place sometime between June 2011 and January 27, 2012, the documents said.

The Susan K crew involved in the oil-dumping violations was not the same crew that was taken hostage by the Somali pirates but a "replacement crew," Feldis said.

That crew, on instructions of the chief engineer, rigged a bypass hose that dumped oily bilge water directly into the sea instead of channeling it into an onboard oil-water separator that is required under federal and international maritime law, the court documents said.

The chief engineer tried to hide the practice with sham reports presented to U.S. Coast Guard inspectors in Seward, Alaska, and later in Houston, Feldis said. "The oil-record book had been intentionally falsified in order to conceal the illegal dumping of oily bilge at sea," he said.

The chief engineer also lied to the Coast Guard during a subsequent Houston inspection, Feldis said.

The companies pleaded guilty to a single oil-dumping count and two obstruction counts, according to court records.

Low-level crew members tipped off prosecutors to the oil-dumping violations, Feldis said. The three whistleblowers are entitled to $67,000 each for their actions, he said.

The chief engineer previously pleaded guilty in the case and was sentenced to a year's probation and a $1,000 fine.

Brian McCarthy, the attorney for the Susan K's owner and its operator, was not immediately available for comment.

Federal officials in Alaska have prosecuted other shippers for illegal use of bypass hoses to dump oily bilge water at sea.

Federal and international law requires documentation of waste practices and discharge of oily waste into approved onshore treatment facilities, but many shippers seek to cut costs by ignoring that mandate, Feldis said. "This is a worldwide problem," he said.

(Reporting by Yareth Rosen in Anchorage, Alaska; Editing by Bill Rigby and Carol Bishopric)