Seems easier to just use that same mass on land with a crane.
They don’t need to do much underwater, only install. But I agree capital costs might be higher because of this.
> all the same concrete requirements because they need ballast
Depends on the geology of the sea bed. Where I live, the sea bottom is mostly solid rock, it’s possible to attach stuff to the bottom without putting much ballast.
> use that same mass on land with a crane.
The idea’s nice I just worry about disasters. I read in local news about falling construction cranes every couple of years, because stormy weather for a few months each year. It’s possible to make cranes strong enough even for bad weather (like bridges), but such weather proof cranes will become more expensive than regular ones.
> Depends on the geology of the sea bed. Where I live, the sea bottom is mostly solid rock, it’s possible to attach stuff to the bottom without putting much ballast.
It's likely to be more expensive to attempt to attach to the sea floor than it is to just sink large weights. In fact, I'm not aware of any large structures that are anchored to the sea floor in a way that resists an upward force rather than a downwards one. It might be a wholly unsolved problem.
> The idea’s nice I just worry about disasters.
Falling construction cranes are dangerous because they tend to happen in inhabited areas, adjacent to other buildings. The energy storage cranes, on the other hand, would likely be in the middle of nowhere where land is cheap, so even if they do fall there's nothing to damage beyond the energy storage installation.
Marine vessels can be very large and helical screws are widely used, here’s an example https://helixmooring.com/square-shaft-anchors-2/
> even if they do fall there's nothing to damage beyond the energy storage installation.
Very expensive installation. Maybe they don’t need the reliability of bridges but they need to be able to withstand any weather.
Global marine community has developed special signs to mark various locations on water, http://www.sailingissues.com/navcourse9.html
Captains normally follow these rules because the ships they’re controlling are often very expensive.