Oh, like this.
First of all, the sections of pipe are joined mechanically, and sealed with O-rings. The O-rings are specified for shallow water pressures (and temperatures), and rather than use adequate deep water parts, the shallow water parts were continued to avoid mandatory Federal oversight and testing.
On top of that, deadlines for completion were already tight, as no schedule variability was provided for unforeseen events, such as severe weather, that might hamper drilling and well conversion efforts. The conversion from an exploratory/research structure into a production well was a hard deadline, and pressure was on internally from the otherwise stagnant middle managers clamoring for achievement. There was no room for failure with a project named Deepwater Horizon.
As engineers' warnings flowed up the chain of command, the wording changed from "grave concern" to "concern" to "noted comment" to eventually "thumbs up!". Inter-hierarchical presentations followed a strict time schedule, so power point mentality and "no bad news up" reigned.
That reminds me of this old classic:
In the beginning was the Plan.
And then came the Assumptions.
And the Assumptions were without form.
And the Plan was without substance.
And darkness was upon the face of the workers.
And they spoke among themselves, saying, "It is a crock of shit, and it stinks."
And the workers went unto their Supervisors and said, "It is a pail of dung, and we can't live with the smell."
And the Supervisors went unto their Managers, saying "It is a container of excrement, and it is very strong, such that none may abide by it."
And the Managers went unto their Directors, saying "It is a vessel of fertilizer and none may abide its strength."
And the Directors spoke among themselves, saying to one another, "It contains that which aids plant growth, and it is very strong."
And the Directors went to the Vice Presidents, saying unto them, "It promotes growth, and it is very powerful."
And the Vice Presidents went to the President, saying unto him, "This new plan will actively promote the growth and vigor of the company with powerful effects."
And the President looked upon the Plan and saw that it was good.
And the Plan became Policy.
And that is how shit happens.