Moonpool Behavior of a Stationary Vessel in Waves and a Method to Increase Operability

Abstract

Potential flow frequency domain calculations have been performed to predict the natural frequencies and water motions for several moonpools. The moonpool free surface has been replaced by a damping lid in WAMIT without changing the kinematic free surface condition. Instead, the free surface modes of motion have been modeled as generalized modes. In this way, different damping values per mode can be applied. Damping values have been calculated based on CFD free decay tests and validated with model tests. Two vessels were chosen for this study with different moonpool dimensions and shapes. The first is a construction vessel with a small rectangular moonpool, whereas the second is a drillship featuring a large moonpool optimized for transit. Free decay CFD calculations have been performed for both vessels, while CFD regular wave calculations have been performed for the construction vessel and model tests were performed for the drillship. Using the aforementioned methodology, a motion reduction device is designed and presented in this paper. The aim of this device is to reduce the motions experienced in the moonpool during vessel operations in a sea state and thus to increase the operability of the vessel with respect to moonpool motion limitations. The increased vessel operability with the new device is shown by a wave scatter analysis. In order to calculate the workability limit curves, suitable limits have been chosen for the free surface elevation and the water vertical velocity in the moonpool.