Ballast:

For ships, water taken onboard into specific tanks to permit proper angle of repose of the vessel in the water, and to assure structural stability.

2.

For mobile offshore drilling rigs, weight added to make the rig more seaworthy, increase its draft, or sink it to the seafloor.

Seawater is usually used for ballast, but sometimes concrete or iron is used additionally to lower the rig’s center of gravity permanently.