A New, Small Semi Concept for Marginal Fields
Authors: Molin Bernard 
Published: October 2008

Abstract
A new type of semi-submersible platform is proposed, which is claimed to be well suited for the development of offshore marginal fields, where light and economic supports are required. It consists in shrinking and perforating the pontoons which become flooded and act as passive dampers, Duoyancy is then insured by the columns only.

Introduction
The performances of such floaters are demonstrated by theoretical and numerical analyses, and comparisons with model test results. Semi-submersible platforms are floating structures characterized by relatively deeply submerged elements - pontoons - and vertical columns piercing the free surface and supporting the deck. Most often they are being used as drilling units - in which case minimization of their vertical motion is a major design objective. Here we are concerned with their utilization as production supports - in which case heave minimization becomes a less stringent requirement if flexible risers are in use.
The design of semi-submersible platforms obeys a number of rules or constraints, the most obvious ones being of hydrostatic nature: that the displaced volume should balance the weight in all circumstances, and the metacentric heights be larger than some minimum values. For voluminous and deeply submerged pontoons this means that the columns should be sufficiently far apart. The other design criteria are more subtle, and of a dynamic nature: they result from the existence of a so-called cancellation frequency, at which the vertical dynamic loads on the columns and l)on toons cancel out (notwithstanding phase cancellation effects which playa role at somewhat larger wave frequencies).
For usual designs the heave natural frequency is just slightly below the cancellation frequency, As the heave motion is 180 degrees out of phase with the free surface elevation in between these two frequencies, semis are designed in such a way that their cancellation frequency be smaller than the wave frequencies they are likely to encounter in their lifetime. For North Sea conditions this means cancellation periods around 20 seconds. The result is that these semis have voluminous pontoons, that typically account for 70 to 80 %of their total displacements (the higher the cancellation period, the larger the pontoons' displacement, as compared to the columns').
For drilling units this large available volume permits deballasting and sailing off location quickly to another drilling site. For permanently moored structures used as production units, it may be wondered whether the cost of such voluminous pontoons is not prohibitive when the topsides weights are limited. It may also be wondered whether their motion response is not too good, since reduced vertical motion means higher deck elevation and increased structural cost. Cutting off the pontoons would significantly decrease the structural cost (with the columns alone still providing enough buoyancy to support the deck and topsides!), but then the motion behavior would be frankly too bad, with the heave, roll and pitch natural frequencies in the wave frequency range.