A Spar is a deep draft platform for oil and gas production and drilling operations. One of the features of the Spar platform is the presence of a moonpool (or centerwell) inside the cylindrical hull structure. A unique aspect of the moonpool in Spars is the presence of partial closure plates at the bottom. The seawater goes in and out of the moonpool though these bottom closure (or guide) plates. Depending on the design of a Spar (e.g., the number of risers), typical period of moonpool vertical oscillation can be close to the peak of extreme seastate spectra. The Spar heave motion is thus coupled with the moonpool water oscillation and presents itself as a two oscillator problem. The complex behavior of moonpool and Spar heave interaction is largely ignored in current practice, via conservative assumptions (this is also due to lack of a mathematical model). A new model describing this interaction between moonpool water oscillation and the Spar heave has been developed in this paper. The results of the new model highlight the importance of the moonpool hydrodynamics in predicting the heave motions of Spars.

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