Exactly one year after the immolated hulk of the drilling rig Deepwater Horizon sank in the Gulf of Mexico, the Coast Guard on Friday blasted the rig’s owner, Transocean, for what it described as poor safety practices that exacerbated the disaster after BP’s Macondo well blew out.

In a 288-page report from the government’s nearly year-long investigation of the tragedy, which took 11 lives and led to America’s worst oil spill, the Coast Guard concluded that Transocean had “serious safety management system failures and a poor safety culture manifested in continued maintenance deficiencies, training and knowledge gaps, and emergency preparedness weaknesses.”

The report says Swiss-based Transocean, operator of the world’s largest fleet of deepwater drilling rigs, has a culture that can be described as “running it until it breaks,” “only if it’s convenient” and “going through the motions.”

Transocean allowed gas alarms and shutdown systems to be bypassed; failed to maintain electricity systems that may have ignited the gas once it leaked from the well; and did not adequately train personnel for how to deal with a gusher, the Coast Guard concluded. The report takes no action but offers dozens of safety-improvement recommendations for consideration by the Coast Guard commandant.

Transocean disputed the findings.

“The Coast Guard inspected the Deepwater Horizon just seven months before the Macondo incident and certified the rig as being fully compliant with all applicable U.S. and international marine safety compliance standards, including those associated with fire and gas detection systems,” the company said. “Further, at the time of the accident the Deepwater Horizon possessed all required valid documents verifying compliance with all international and Coast Guard requirements.”

The long-awaited report is the Coast Guard’s half of the investigation known as the Marine Board of Inquiry, a joint probe by the Coast Guard and the Bureau of Ocean Energy Management, Regulation and Enforcement. The bureau is working on its report and is expected to be finished in late July. That part of the investigation is likely to focus on such issues as BP’s well-design decisions, last-minute changes in procedures for abandoning the well, Halliburton’s cement recipe and the failure of the Cameron-built blowout preventer to seal the well.

The Coast Guard did not examine the causes of the April 20, 2010, blowout but instead focused on its area of expertise, the operation of a ship — the rig, technically known as a mobile offshore drilling unit.

Owned by Transocean as part of a fleet of 139 rigs around the world and leased by BP at a cost of half a million dollars a day, the Deepwater Horizon was largely staffed by Transocean employees. Under a Transocean policy that the Coast Guard skewers, the top person on the rig during drilling operations was the offshore installation manager, but when the rig moved, or experienced an emergency, the chain of command shifted to the captain, or master.

That proved confusing when the well exploded, the report concluded. The rig’s captain, Curt Kuchta, is described in the report as having a “questionable reaction” in the crucial minutes after the loss of well control, when chaos and confusion on the bridge may have delayed attempts to seal the well at the sea floor and detach the rig, cutting off the flow of oil and gas feeding the fire.

Among those on the bridge in that key moment were two BP executives and two Transocean executives who had flown to the Deepwater Horizon that afternoon. A recurring issue during the probe has been whether the presence of the executives distracted the top managers on the rig.

The report said their presence “may have diverted the attention of the offshore installation manager and senior toolpusher from the developing well conditions, limited their interactions with the on-watch drilling crew, and led to their failure to follow the emergency evacuation procedures.”

The crew of the rig had regularly trained for an emergency evacuation, usually on Sunday mornings. The Coast Guard said that made such exercises too predictable. Most of the crew safely evacuated in lozenge-shaped lifeboats, but many jumped to the water some 75 feet below. The final batch of survivors on the rig’s deck struggled to launch a life raft, which tipped and fell, dumping one person in the water — though she survived, as did everyone else who made it off the rig.

“The current lifeboat design and testing requirements do not adequately ensure the safe loading of a stretcher or permit adequate seating to accommodate the physical build of the average offshore worker today,” the report said.

The rig flew under the flag of the Republic of the Marshall Islands, a move with tax benefits for Transocean. The Coast Guard said the Marshall Islands had not adequately regulated the rigs that fly its flag.

Not everything in the investigation is condemnatory. The report recommends a number of mariners for special recognition for heroic actions that helped save 115 people aboard the rig.

Those cited include several Transocean workers; the captain and chief mate of the Damon B. Bankston, the work boat adjacent to the rig during the incident; and three fishermen on the recreational boat Ramblin’ Wreck, who were fishing beneath the rig and sped to safety when they heard the gas come up the well. They stayed on the scene and helped pull from the water a number of rig workers who jumped from the deck.

Staff writer Steven Mufson contributed to this report.