well ,there is a problem with this sort of project and that is if some terrorist attack it ,it would be very hard to fix it .
this is a gas pipeline so environmental effect of accidents are far less than an oil pipeline , but I still worry about espionage and terrorist attacks right now its very easy for private companies and treasure hunters to order small robots that can work at the bed of seas ,and as we all knew some of the terror groups have far more funds at their hand compared to some governments. and those robots can be used for attacking these pipelines.
another matter is that such pipeline is extremely expensive and I guess would be tens of time more expensive than estimated 10 milliard dollar , that 10 milliard dollar is more in line of the price of putting the pipeline on the land.