Tensions, derrick and rig equipment weights generally act near the centre of semisubmersible drilling units. Deepwater/ Deephole drilling operations in 1,000 - 3,000 metres water depth and up to 12,000 metres hole depth, also require more tubular storage capacity on deck than work in water depths up to, say, 200-500 metres and hole depths up to for example 7000 metres.
The four inclined centre columns on the FE-P099 semisubmersible represent steel applied directly in the paths of the major global forces resulting from such centrally acting loads. Two additional functions of these comparatively large diameter columns, are as providers of major storage space for powder tanks and as significant hydrostatic stability contributors, which strengthen the total steel application advantage of the FE-P099 over other traditional units with vertical columns and small diameter bracing supports.
The application of steel in the right places and for more than a single purpose leads, in turn, to a lower steel weight, a lower lightship weight, smaller displacement, less required thruster power, a smaller total power plant and a lower price than for other vessels advocating similar deck load capacity and dynamic station keeping capability.