The Influence at Vertical First Order Motions Using Appendages in a Monocolumn Platform

Oil discovery in ultra-deep-waters is carrying out the ocean engineering to develop new conceptions of offshore exploration and production systems. A promising alternative is a monocolumn platform with moonpool. Concept design has shown its good stability make feasible the use as offshore solution. In order to solve the problem brings by increase on water deep, the use of steel catenary risers and wellhead dry completion has been shown a good solution because it decreases the maintenance cost. However, this solution needs a very low vertical motion. In general, platform like the monocolumn present resonance on vertical motion (15–20 sec to heave motion, 20–25 sec to roll/pitch motion) close to typical wave energy. This fact is so that designers introduce mechanisms to leave the natural periods out of wave energy. The moonpool as passive absorber motion was a solution found by designers. The water inside moonpool acts out-of-phase with platform motion. This difference on phases can attenuate the platform dynamic response. Other solution is water-line-area (AWL) reduction that reduced the restoring forces and consequently increased the value of vertical motions natural periods. These two alternatives showed very efficient to improve the monocolumn performance. This paper presents the gain, in terms of motions, achieved as the appendages introduction on the platform. These appendages consist of two plates, one in form of a disk and other around it. It can be located on the bottom at platform and inserted inside at the moonpool. Model test conduced on the scaled platform shows that heave and pitch natural period increased approximately 4 seconds when this appendages is allocated on the bottom. Practically any change is noted when it is allocated only inside of the moonpool.