In response to “a series of serious accidents and near-hits within the global organization,” Transocean, the world’s largest offshore drilling company, commissioned the risk management company Lloyd’s Register to investigate its Houston headquarters and three other gulf rigs besides the Deepwater Horizon to assess its safety culture.

The confidential internal reports, obtained by The New York Times, offer an unusually candid view of safety and maintenance concerns within the world’s largest offshore drilling company, and they indicate that the problems highlighted in earlier reports provided to The Times about the Deepwater Horizon were not limited to that rig, which exploded on April 20, leading to an oil spill that is estimated to have poured at least four million barrels of oil into the gulf.

Transocean has 14 rigs now operating in the Gulf of Mexico, and 139 worldwide, and these documents raise concerns about locations beyond Deepwater Horizon, especially the three additional gulf rigs that were recently investigated. In fact, one of those rigs is being leased by BP to drill one of the two relief wells near the Deepwater Horizon site.

The new documents also shed light on one of the lingering mysteries of the disaster: why the rig sank. They indicate that there were problems with the Deepwater Horizon’s ballast system that was responsible for keeping the rig afloat and stable. If the rig had not sunk, the leak might not have occurred. Federal investigators have questioned whether deferred maintenance and other factors had played a role in the sinking of the rig.

A previous set of worker-safety reports provided to The Times were specific to the Deepwater Horizon. The new documents draw from analyses of three other rigs in the gulf and attempt to provide an overview for the entire North American division of the Transocean fleet.

The safety concerns cited in the company’s assessment of its North American division are supplemented by newly released internal reports concerning the Deepwater Horizon’s equipment. These equipment reports identify dozens of deficiencies, including some relating to the rig’s blowout preventer, and some that are categorized as “critical equipment items that may lead to loss of life, serious injury or environmental damage as a result of inadequate use and/or failure of equipment.”

“Without a doubt, previous incidents and near-hits experienced throughout the organization were as a result of multiple causes and many contributory factors,” said the summary report, which gave an overview of the company’s North American Division and draws from investigations of Transocean’s Marianas, Discoverer Clear Leader, GSF Development Driller II and Deepwater Horizon drilling rigs.

This is not the first report of the Deepwater Horizon experiencing problems with its ballast system. In May 2008, Transocean was forced to evacuate more than 70 workers after problems with the ballast system flooded part of the rig, causing it to list to its side, federal records show.

A lack of hands-on experience for workers and managers has contributed to safety concerns at the company, and a stifling bureaucracy imposed by onshore management has led to widespread resentment among rig workers, the investigators found.

Nearly 40 percent of workers interviewed on the four rigs said that past problems were typically investigated by company officials strictly to attribute blame.

“It ticks me off when someone fails or has an incident; they focus on the paper rather than the process that was gone through,” said a worker from the Discoverer Clear Leader.

Another worker on Transocean’s Marianas rig said that the safety manual seemed to be “written for the courtroom, not the oil field.”

The reports are likely to broaden the discussion of blame for the April 20 explosion, which killed 11 workers. BP, which was leasing the Deepwater Horizon from Transocean at the time of the explosion, has been under the harshest glare for its role, but the Justice Department has said that its criminal investigation of the disaster will look at the role of the many companies involved.

About 43 percent of workers on the four rigs expressed fears of reprisals for reporting problems, the documents said. About 54 percent of Deepwater Horizon workers cited these fears, while about 61 percent of workers on the Marianas did so.

Some workers said the company was systematically deferring maintenance to save money.

“This rig is getting $550,000 per day; unless it’s a sink that needs fixing it isn’t getting fixed,” said a worker from the Marianas about the maintenance concerns. “They won’t send the rig to the shipyard for major refurb that is required in certain areas.”

The investigators who visited the four rigs in March concluded that many crew members and front-line supervisors were too readily promoted without sufficient on-the-job experience to appreciate the hazards. “Front-line crews are potentially working with a mind-set that they believe they are fully aware of all the hazards when it is highly likely that they are not,” the investigators said, adding that the workload, and thus the risks, on the rigs was increasing.

After reviewing the new documents, Senator Patty Murray, Democrat of Washington and the chairwoman of the Subcommittee on Employment and Workplace Safety, voiced concern.

“These documents are more evidence that despite the growing count of worker deaths and safety violations, the oil and gas industry still just doesn’t get it,” she said. “They need to change their worker-safety culture, and I am pretty sure we can’t count on them to do it by themselves.”

She added, “The oil and gas industry is not the same as a mom-and-pop grocery, and they can’t be treated the same.”

Lou Colasuonno, a spokesman for Transocean, wrote in an e-mail that the company was committed to safety and maintenance and that it proactively commissioned independent employee surveys and rig condition assessments.

“Reading the complete reports makes it abundantly clear that that both studies were positive and were designed specifically to identify strengths and weaknesses — a critical step in evaluating performance,” he wrote.

“Overall maintenance on the Deepwater Horizon met or exceeded regulatory and industry standards, and the company’s proactive review process helped the Deepwater Horizon log seven consecutive years without a single lost time incident or major environmental event prior to this incident.”

He declined to specify the series of “serious accidents and near-hits” that motivated the safety investigations.

The safety reports cite a variety of positive findings about Transocean. “Despite several rig management changes on board the rigs visited, rig leadership was generally praised by the work force,” one said.

Almost 87 percent of workers said they believed that there was enough time to do their work according to rules and procedures.

“Rig management and supervisors were generally seen as approachable, set a good example of the company commitment to safety, and were generally highly visible,” the investigators said.

But their praise often came with a certain ambivalence.

“Generally, the work force thought there was a sufficient number of staff to manage safety,” one report concluded before adding, “There were, however, some questions surrounding the retention of skilled personnel, competency levels for some personnel, and of processes in place for competency development and assurance.”

Although high levels of trust were reported at the rig level, “there was a significant level of mistrust between the rigs and the beach,” the report said, referring to onshore management.

Around 46 percent of workers on the four rigs said that some of the work force was uncomfortable with calling a “time out for safety.” Deepwater Horizon workers polled at about the same rate on this issue.

Transocean’s equipment documents reveal for the first time the severity of the maintenance issues that plagued the Deepwater Horizon, and they indicate that the company was aware of the consequences of the problems.

These new documents refer to at least 36 pieces of equipment in ill repair on the Deepwater Horizon that “may lead to loss of life, serious injury or environmental damage as a result of inadequate use and/or failure of equipment.”

The new equipment documents indicate that an inspection of the Deepwater Horizon rig conducted just days before the April 20 accident found various problems with hydraulic relays that controlled the rig’s watertight doors, two of which had to be opened and closed by hand.

Of the four rigs investigated, the Development Driller II is now being used by BP to drill one of the two relief wells near the Deepwater Horizon. The Marianas was the original rig that was drilling BP’s Macondo well before being damaged in a hurricane. Despite having been built in 2009, the Discoverer Clear Leader had “a few notable safety related incidents on the rig during its relatively short operational history.” It is now being used by BP for oil containment at the Deepwater Horizon site.

It was not clear where the other rigs cited in the safety reports are operating now.