A very interesting article on the difficulties of such a task:

http://www.redorbit.com/news/science/948325/troubles_run_deep_on_gulf_oil_platform/index.html

"Thunder Horse's oil reservoir is nearly 5 miles below the water's surface. At that depth, oil will gush from the drill pipes at a temperature of 275 degrees Fahrenheit, under a metal-crunching 17,400 pounds per square inch of pressure."

The article describes how they were trying to find leaks 6000 feet underwater....

It's risky because of that amount of pressure. But this was also a careless mistake that could have been prevented if they ckecked the wields months before. The fact that it's located in the Gulf of Mexico poses the threat of natural forces.

Read the whole article. It explains why the pipes failed...

Hint: nothing to do with earlier checking....

I have already. Here is the referrence:

Sleuthing at 6,000 feet

Perhaps a valve was left open. Perhaps a coupling on a pumping station wasn't properly tightened. "We figured we would find out in our spare time," McDaniel said.

Two weeks passed, then a month. One pressure test held for eight hours, and then failed. The team injected ink into the piping network and sent a remotely operated, unmanned submarine 6,000 feet down into the water to photograph what was going on.

Outfitted with cameras and high-precision robotic arms, the sub was capable of spotting any problems, and fixing many potential mishaps.