One needs to consider the issues of seawater pressure at a depth of 5,000 ft. Sea water has a pressure of .44 psi/ft. while water is .43 psi/ft. 5,000ft. x .44 = 2,220 psi. (.44psi/ft if from wikianswers.com which is historically a reasonably accurate reference.)
For oil to barely leak out of the well at 5,000ft. requires a well head pressure of slightly in excess of 2,200 psi. For oil to gush out of a well at that depth requires a considerably higher pressure than 2,220 psi. The higher the flow rate the greater the pressure required. I have personally seen gauges on gas wells in the area I live in displaying pressures much higher than 2,200 psi. It would not be surprising if the pressure in the oil well is more than 3,000 psi. It could be the original pipe used was not designed to withstand such pressure.
Legenday gusher oil wells drilled on land one hundred years that blew long lengths of drill pipe out of the hole have faded into the memory. Today, oil well companies often must go to great lengths to force oil out of deep formations and bring it to the surface. Perhaps BP did not expect this oil well to have any significant pressure, and to save money they did not specify a sufficiently strong pipe to handle it.
Everything in life, even when drilling oil wells, always comes down to saving a dollar.
There are numerous known pipe applications that show the efficient shape of circular pipe.
Circular pipe is inherently quite strong. For example, plastic pipe is now commonly used everywhere and is buried under tons of soil. Yet a plastic drain pipe has basically zero psi. inside it all the time. Even plastic gas mains are routinely buried as deep as basements under tons of soil, yet only have about 100psi inside them. How can this be possible? Rigid steel or plastic circular pipe, regardless of it's material has a very high crush threshold. It is actually an arch in every direction, which is the strongest shape known in architecture. The same is true for plastic or thin steel culvert pipes made of relatively thin material that pass under roads and do not crush. Force downward from soil, pavement and even tractor trailers is translated to also a sideways force, but no deformation takes place because of packed soil around the pipe supports it.
If an open-ended pipe is dropped down vertically into the ocean the net pressure inside the pipe will be zero. Why? Open to the ocean on the end, the inside pressure is exactly equal to the outside. A sea water column inside the pipe will exert the same pressure inside at the bottom of the pipe. It is the same as a virtual water column of the same diameter anywhere in the ocean. But overall, a compressive force on the pipe material itself is still at work. A submarine retrieval in 1974 brought the destructive nature of this compressive force on steel to light in the engineering world for the first time. The sunken submarine was about three times deeper than the oil well in question.
A Soviet sub in the deep ocean was lost in 1968. The CIA located it (when the Russians couldn't or didn't want to know without a means to bring it up.) The agency commissioned Howard Hughes to build the Hughes Glomar Explorer to bring it up. It was a massive, expensive spare-no-expense ship with a huge rectangular pool in the middle between the two end towers you see in the photo below. A hidden pool in the middle of the ship was large enough to hold a soviet submarine. This ship also had a huge, long cradle with massive steel fingers that could descend to at least 16,000 ft. - the depth of the sunken submarine.
Massive steel fingers were designed and built to go underneath the sub to hold it inside an oversize frame, while it was slowly lifted to the surface by numerous steel cables. Yet several of the massive fingers broke off, even though the entire assembly was designed strong enough to hold the sub. Why? One theory was that seawater pressure on the steel was so great it displaced molecules in the steel, causing embrittlement lowering their tensile strength.
It's also known that a fusion reactor, which generates high numbers of neutrons that constantly bombard the containment vessel during operation, also requires periodic replacement of the containment vessel. Damage is caused by something called neutron embrittlement. (Source- University of Rochester physicist working on their fusion project in 1986.) I'm sure RMC knows more about this than I do.
One cannot simply convert a car engine to run on hydrogen by replacing the fuel system as many amateurs do, and expect the engine to last without replacing aluminum pistons with pistons made of another material like ceramic. Aluminum pistons suffer from hydrogen embrittlement which weakens the metal, creating cracks and leading to failure.
Sea water at 16,000ft. has a pressure just over 7,000 psi. Again, this is the same as a 1" square column of water over 3 miles high.
Yet deep ocean research has proven abundant sea life lives at those great depths. Why? Simply because sea life was born down there. Like open-ended pipe lowered into the ocean, pressure inside these animals is equal to the pressure outside. None of these animals ever survives when brought to the surface by a deep sea reseach sub - they simply fall apart from bursting cells. These animals are often made of gelatenous material, and as such must live their entire lives within a relatively narrow depth range or delicate cells will burst.
Not being a materials expert, I cannot say what 2,200 psi at the well head (combined with the corrosive effects of saltwater) does to steel in pipes and the blow-out preventer stack metals at that depth over a long period of time. Is the speed of steel embrittlement which would ultimately lead to the point of failure, a function of pressure and time? We know in a fusion reactor this is the case. The higher the neutron flux in the reactor, the more molecules in the containment vessel will be displaced by the neutrons over a given period of time. There are numerous known forms displacing molecules which all lead to embrittlement.
I'm sure there are materials experts who have earned their PhD in the field of embrittlement alone that might know the answer.