The U.S. Coast Guard said the ship's CO2 firefighting system had failed to operate correctly due to leaks, poor maintenance and component failures.

The repairs involved replacing more than 100 miles of electrical cable, according to Carnival. Among other improvements introduced or recommended: better insulation of cables and switchboards, and more fire suppression systems.

After two fires on board cruise ships in the 1990s, the then-chairman of the National Transportation Safety Board said the cruise industry needed to do more in the way of training and fire prevention. The NTSB found that a fire on board the Liberian-registered Ecstasy, operated by Carnival, had been caused by a fire in the laundry room sparked by unauthorized welding.

A fast-expanding industry

The cruise line industry has expanded fast as vacations at sea have come within the budgets of more people and the different operators have competed for customers. By the end of 2010, there were an estimated 215,000 cabins in the industry worldwide. Carnival and its subsidiaries account for nearly half that capacity.

Taking a cruise remains a very safe vacation. According to the Cruise Line Industry Association, known as CLIA, there were 28 marine fatalities involving passengers and crew on cruise ships between 2002 and 2011. During that time, the association says, 223 million people (passengers and crew) sailed on cruise ships. And bookings remain robust despite recent accidents, including the capsizing of the Costa Concordia last year, in which 32 people died.

But there are concerns about how the new generation of mega-ships can be evacuated.

The International Maritime Organization, which regulates shipping worldwide, has introduced new regulations for ships longer than 120 meters built since 2010. Known as "safe return to port," the guiding principle is that a ship is its own best lifeboat. So the International Maritime Organization sets out the amount of damage a ship should be able to sustain from fire or flood while remaining seaworthy, and includes requirements for auxiliary power that could support evacuation for at least three hours.

Patchwork of regulation

Even so, regulating the cruise industry is a haphazard business. Many of the International Maritime Organization's pronouncements are in the form of guidelines and recommendations, and rely on national authorities for their enforcement. Cruise ships operating out of ports such as Galveston, Texas, and Miami carry mainly American passengers but are rarely registered in the United States. For example, many of Carnival's liners are registered in Bermuda and the Bahamas.

Another criticism leveled at the cruise lines is that they use foreign flags to avoid U.S. taxes. Sen. Jay Rockefeller, D-West Virginia, who is chairman of the Senate Commerce and Transportation Committee, said last year that cruise ships "are registered in other countries where they can get cheaper labor and they pay no taxes in this country."

Rockefeller said the cruise industry was in a "world of its own."

"They don't reimburse the Coast Guard, they don't pay taxes that will help with the 20 federal agencies that are watching over them in various ways."

From 2004 to 2011, Carnival paid an effective rate of 1.1% in federal, state and local taxes on $11.3 billion in profits. The CLIA acknowledges there "are some fees and taxation considerations " in incorporating overseas, but says cruise lines pay the taxes in the United States they are required to.

Rockefeller wrote of the Triumph in a letter to the Coast Guard. "This horrible situation involving the Carnival Triumph is just the latest example in a long string of serious and troubling incidents involving cruise ships. Safety must be the No. 1 priority in any transportation industry. It is time that the cruise line industry -- which earns more than $25 billion a year -- pays for the costs they impose on the government since it's the Coast Guard that comes to the rescue every single time something goes wrong on a cruise ship."

Critics charge that "foreign flagging" enables cruise lines to evade tougher U.S. regulations, and allows them to recruit crew members with fewer qualifications from overseas. Cruise line companies defend the practice, saying their operations are global.

The fire aboard the Carnival Splendor was investigated by Panamanian officials, with the help of experts from the National Transportation Safety Board and Coast Guard. A Coast Guard report into the incident is still in draft form.

The fire on the Royal Princess was investigated by maritime authorities in Bermuda, where the ship was registered. Those authorities concluded that securing bolts had not been properly replaced over fuel pipework and the damage was worsened by the failure of the water mist system.

Similarly, the inquiry into the fire on the Triumph will be led by the Bahamian authorities.