2 Answers

+1 vote
answered Jun 26, 2014 by judyp (2,700 points)
Ballast tanks shall not be considered as storage tanks. Storage tanks for, e.g., mud, potable water, dry bulk, etc. shall have internal surface plating.

Furthermore, storage tanks containing hydrocarbons shall not be at the outside perimeter as per industry standards.
0 votes
answered Jun 26, 2014 by antonjoh (2,870 points)
Also want to mention that your question of the ballast storage tank is not a requirement of API-RP-2T or Class Rules for Tension Leg Platforms.

I would add extra weight to the hull if the practice is applied.

It is a common practice in TLP design to consider the storage tank as regular hull compartment with plates and stiffeners and no additional internal surface plating is provided.  The storage is mainly liquid and not difficult to clean or remove and does not require a special surface.  Therefore, the requirement can be removed?

Without all that requirements, the ballast storage tanks will reduce the hull weight and also makes the hull framing design and fabrication easy.