Objectives  The moonpool is a type of opening structure through the hull from deck to bottom, but it often brings adverse impacts on the performance of resistance due to its complex flow.

  Methods  The computational fluid dynamics(CFD) method was utilized to perform numerical simulations of resistance and flow field of a ship with moonpool in still water. Meanwhile, the resistance performance of two typical modified moonpools, recess type and corner-cutting type, was verified and the improvement mechanism of resistance increase was studied.

  Results  The total resistance of the ship increases by 51% with the setting of moonpool, and the moonpool resistance is close to the great increment of pressure resistance. The maximum reductions of added moonpool resistance can separately reach 80% and 40% by the recess type and corner-cutting type moonpools. And the flow details show the process of vortex motions in the flow field, the law of resistance increase caused by moonpool and the improvement mechanism of added moonpool resistance by the modified moonpools.

  Conclusions  The resistance variation is closely related to the periodic vortex motions in the flow field, which differs in different speeds. Moreover, the recess type moonpool increases the damping of fluid motions and the corner-cutting type moonpool alleviates the strong effect of the vortex on the back wall of moonpool, therefore the modified types are able to reduce the increased resistance induced by moonpool.