Offshore Platform Design Calculator

This calculator sizes a gravity-based offshore platform foundation the way a preliminary geotechnical check would: it sums dead, live, and vertical environmental loads into a total vertical load, then divides by the allowable seabed bearing capacity — multiplied by your chosen safety factor — to get the minimum required base area, which it then expresses as the diameter of an equivalent circular footing. From that footing it derives two stability checks that matter for any structure resisting lateral wave and current forces, but the two are not equally sensitive to the footing's size. The overturning check compares the restoring moment (total weight acting through the base radius) against the overturning moment (horizontal environmental load times its lever arm above the seabed), so it grows directly with footing radius — a larger required base area, whether from a higher safety factor or a lower bearing capacity, mechanically inflates the overturning FoS even if nothing about the soil's actual sliding resistance changed. The sliding check is independent of footing geometry entirely: it compares frictional resistance at the soil-foundation interface (total vertical load times your friction coefficient) against the horizontal load, with no term for base area, diameter, or radius anywhere in it.

So a comfortable overturning FoS driven by an inflated footing size should not be read as validation of your bearing capacity or safety factor assumptions, but a comfortable sliding FoS is not affected by that inflation at all — it only improves if the vertical load, friction coefficient, or horizontal load actually change. Both ratios should exceed roughly 1.5 for a stable design, per API RP 2A-WSD and DNV-OS-J101 guidance. This is a preliminary sizing tool for gravity-based structures specifically; jacket platforms with pile foundations require a different, pile-capacity-based analysis entirely.