Resistance due to open moonpools on offshore ships
MetadataShow full item record
- Institutt for marin teknikk 
A moonpool is a vertical well in a vessels hull that is exposed to the sea. Many types of offshorevessels are equipped with such a moonpool, to facilitate maritime operations. The water columninside of the moonpool is known to oscillate violently, and in model test, these oscillationshave been connected to a resistance component. It is also well known that, during transit, anopen moonpool will increase the resistance of the hull. Even so, most vessels are not equippedwith any measures intended to reduce the added resistance from the moonpool. This thesis has an objective aimed at building an understanding of the added resistance dueto moonpool on offshore ships. The focus as been on forward velocities in calm waters, with anoverall goal to derive general expressions describing the resistance. After a thorough literature study, surprisingly few directly related experiments were found. Itwas established that the resistance consists primarily of the base resistance as the moonpoolis a modification on the hull. Secondly it has been shown that the amplitude of the verticalpiston-mode oscillation, is directly connected to a second resistance component. Expressionsthat estimates the amplitude development, and its connection to the resistance were found. Numerical simulations were conducted using PVC3D, a software developed by MARINTEK. Thissolver is based on a hybrid method that combines the viscous flow with boundary conditionsfrom potential flow theory. The simulations were used to confirm the prediction models for the oscillation amplitudes. Theoscillations are only observed within a smaller range of velocities. The expected range can beexpressed by the normalised reduced velocity U => [0.35 -0.60] in calm waters. In conclusion, it is expected that the total resistance of an offshore vessel increases in the area of5-10 %, due to the existence of a moonpool. The piston-mode oscillation might contribute withan additional increase in the magnitude of 20-30 %. However, this is related to the maximumexpected amplitude, which can be expected to be present in velocities well below the servicespeeds of offshore vessels. The resistance component related to the amplitude is related to ahigh level of uncertainties and should be a topic for further investigations.