Moonpool

How would a moonpool dampen the motion of the vessel/structure it was on?

 

Negatively.

 

Assuming we are talking about the motions of the vessel at rest............

.............and assuming we are talking primarily about heave and pitch.....

The presence of the moonpool will reduce the waterplane area of the vessel. Normally this will tend to increase the natural period of heave of the vessel, which is normally a good thing - and is a special advantage if it can be moved beyond the period of the most commonly occuring sea states in the area of operation.

Depending on the structures/baffles placed inside the moonpool you might get a slight increase in resistance (drag) to vertical motion (which is a form of heave damping, similar to the way a bilge keel provides roll damping) but not enough to make a real difference to the peak amplitudes of heave motion.

If you have more than one moonpool spread out along the length of the vessel, you can have a more marked effect on pitch motion. Not only does the pitch (as well as heave) natural period of motion become altered, but the wave induced pitch excitation moment can also be made lower in waves of the length (hopefully) which will most often be encountered.

Due the greater lever arm, moonpools near the ends of the ship can contribute to pitch damping, but as with heave (but unlike roll) it is difficult to much affect the peak amplitudes of pitch motion by damping.

The more moonpools you have in your monohull, the closer it becomes (in simplistic terms) to a semisubmersible (which, as you know, are optimised for low motions).

This is the philosophy behind the successful design of the three moonpool monohull drillship NAVIS EXPLORER (built around 1999) but now called BELFORD DOLPHIN.

 

IF they are too big the ship might sink...

 

I've also found that, depending on the size of the moonpool, the added mass increases which can significantly dampen roll motions (my last project was an OSV with a moon pool).

CWTeebs

 

Nice one, thanks