To ensure that oil rigs, tankers and other commercial ships are in safe operating condition, governments around the world, including the U.S., often rely on inspections by private firms that are hired and paid by the vessels' owners.

But how much confidence should the world have in the maritime watchdogs?

The Deepwater Horizon catastrophe, which claimed 11 lives and fouled the Gulf of Mexico, has revealed that the mobile oil rig leased by BP had a host of maintenance problems.

Case in point: In April, the company that owned the rig gave parts of two cranes its worst rating, indicating that they did not work or should be removed from service.

Although there is no reason to think the cranes had anything to do with the explosion that destroyed the rig on April 20, records of the Deepwater Horizon's maintenance and inspection history open a window on possible weaknesses in the little-known industry that oversees the world's commercial fleets.

Owners of vessels flagged in the United States generally have a choice. To fulfill federal requirements, they can let the Coast Guard conduct primary inspections, or they can enlist one of several private firms authorized to do the work in place of the Coast Guard.

When it comes to vessels flagged by other nations -- such as the Deepwater Horizon, which is registered in the Marshall Islands -- the Coast Guard depends largely on the oversight of foreign governments, which essentially might outsource the job to the same firms.

The fact that the inspection firms, known as "classification societies," are hired and paid by the vessels' owners appears to present "a built-in conflict," said Steve Gordon, a Houston maritime lawyer at the firm Gordon, Elias & Seely. Its clients include eight surviving members of the Deepwater Horizon crew.

Such "class hopping" became a cause of concern in the 1990s with ship owners "trying to avoid doing repairs," said Colin Wright, an officer at the International Association of Classification Societies.

The Deepwater Horizon was inspected by the Houston-based American Bureau of Shipping, or ABS, one of the biggest inspection firms.

ABS certified in a Feb. 22 report focusing exclusively on the four cranes that they "were thoroughly examined by a competent person and that no defects affecting ... their safe working condition were found."

Less than two months later, a different assessment commissioned for internal use by Transocean concluded that some of the cranes "were in bad condition."

Transocean commissioned the report by the firm Modu Spec as a routine part of its maintenance program, Transocean spokesman Brian J. Kennedy said.

Using ModuSpec's work, Transocean compiled an April 12 rig assessment that listed 10 features of the cranes as being both "critical items that may lead to loss of life, a serious injury or environmental damage" and in "Level 1" condition -- the worst of four categories.

Stewart Wade, ABS's vice president for external affairs, declined to comment on the ModuSpec assessments.