The present study deals with the dynamics of a semi-submersible platform coupled with a free-hanging vertical rigid riser during the re-entry operation in ultra-deep water. The reentry operation occurs after the drilling phases are accomplished which only use the bare drillstring. The re-entry operation entails positioning the subsurface Blow-Out-Preventer (BOP) above the wellhead and installing it to the wellhead. During this operation, the BOP is suspended by the drilling riser. The placement and installation is time consuming and requires a great amount of accuracy. One challenge is the effects of platform motions on the installation procedure. The present work was carried out with the aim to understand the influence of platform motions during the re-entry operation in ultra-deep waters. Another benefit of this study is the knowledge of the effect of platform motions on the drilling riser and BOP which leads to the possibility of drilling more than one well without the need to raise the BOP to the surface. This could greatly reduce the amount of time needed to drill subsurface wells which in turn, reduces overall operational costs. A numerical simulation in time domain has been carried out using a non-linear model for the platform dynamics. The dynamics of a free hanging drilling riser and a dynamic positioning system (DPS) of the semi-submersible platform are included in the numerical model. Simulation results in time domain of the platform displacements with DPS and riser displacement are shown. A discussion of riser displacement and DPS control is also included.

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