Acute Revisiting of Semisubmersible Concept for Ultralow-Motion Floating Production Unit Design
Authors: Demoulins J.F. , Martin Y. , Chefdrue J. 
Published: October 2008

Abstract
To answer the growing needs in deepwater solutions, a new design of ultra low motion semi-submersible production platform has been developed offering at dry tree solutions for deepwater field exploitation.
The ultra low motion response is obtained thanks to combination of innovative columns and floaters spacing and arrangement.
The new design offers motions at least similar to that of deep draft floaters, and also permits a much larger payload capacity together with optimized deck layout.
The paper exposes the research that was performed to obtain these results and how motion responses are sensitive to the whole design of the platform including structural design and topside layout, mooring and risers coupling effects.

Hydrodynamics:
Revisit the semi-submersible concept and use basic principles together with extensive numerical studies and model tests to get a break-through performance in terms of motions compared with conventional semis. A particular attention is devoted to the effects of mooring and risers tensions and apparent stiffness, and their through life variability, in defining the optimum geometry.

Structural:
Select efficient structural arrangements while providing enough redundancy for accidental or other damages in order to obtain a light, slender while strong structure. Incorporate provisions for construction, transportation and installation phases as they are key factors for any offshore development project.

Topside payload and layout:
Efficient design gives reach to topside payload ranges as high as those of big FPSOs, and a "multi centered" arrangement improve safety aspects compared to usually congested topsides of others concepts
Finally, we present the benefits to the oil producer in terms of improvement to the riser behavior, topside safety, construction and installation schedule and costs compared to existing designs.

Introduction
Some years ago, our target when starting the work on a new semi submersible platform design, was to provide to oil and gas producers a new solution suitable for deep and ultra deep waters:allowing to use surface tree systemsable to receive heavy topside loadssuitable for rough environments such as Gulf of Mexicofavoring the topside design and installation specially in terms of safety
The governing factor for this new design is the search for low motions in order to allow the installation of surface tree production risers.
The main constraints concern the heave motion (which governs the sizing of tensioning systems for top tensioned risers) and also the pitch & roll which have both a significant role in riser and drilling interface.

Hydrodynamic Design

The classical semi-sub design basis
The main design features and technical parameters involved in conventional semi submersible designs are well known by offshore engineers.
The primary feature is the so called "cancellation period" for which, in the first order linear hydrodynamics model, the excitation force in heave cancels at a given wave length depending on the geometry of the column and pontoon arrangement. Fig 1