There is currently ongoing interest in the offshore industry to develop a dry tree production semi submersible that has motion characteristics similar as a Spar, but with the functionality of a Tension Leg Platform (TLP) or a Semi in terms of fabrication, installation and commissioning. It has been proposed that the required motions can been achieved by adding a heave plate to an existing deep draft semi with external extendable columns supporting the heave plate. The ESEMI II is such a concept with telescopic structure to lower the Second Tier Pontoon (STP). The riser technology presently used on Spars can be ported to a dry tree semi design assuming the motions are similar. These designs consist of a tensioning system using either buoyancy cans or hydraulic/pneumatic tensioners. The main components of the risers for such a semi-submersible are similar to those of the Spar including Keel Joints and stress joints, where the heave plate on the dry tree semi replaces the soft tank on the Spar. The Top Tensioned Risers (TTR), however, can not be tensioned using simple buoyancy cans and have to be tensioned by non-linear pneumatic tensioners because of exposure in the wave zone. The stroke of the risers in the dry tree semi is estimated to be similar to that of the Spar. The heave plate connectors have to be designed considering not only the hydrodynamic loadings but also horizontal riser loads at the keel as well, which would give enormous moments at the keel and stress joints. The stroke of the riser should also be considered while designing the heave plate connectors. The riser, unlike a Spar, will not be shielded in the hull area, and will be subjected to high currents, and hence the Vortex Induced Vibration (VIV) effects may also be significant. This paper discusses a methodology for riser and VIV analysis, and mitigations, on a dry tree semi-submersible for two water depths.

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