Research On Added Resistance And Internal Slamming Effects Of A Moonpool

Moonpool is a structure that is widely installed on drilling ships or drilling platforms,and its existence has a significant impact on hydrodynamic performance of marine structures,such as inducing added resistance of moonpool(only for drilling ships)or deteriorating hydrodynamic performance of the main body,etc.This feature makes the design,development,and manufacturing of drilling structures greatly different from other marine structures.At present,most of the published literature both in the academic and engineering community mainly focuses on the Characteristic of the water body in a moonpool,and there are few studies on the negative effects caused by various strong nonlinear phenomena in a moonpool.In the presented study,the theoretical analysis and numerical simulation are implemented to systematically investigate the added resistance and internal slamming effect of a moonpool installed on an ultra-deep-water drilling vessel.The main work is as follows:(1)The natural frequency of the investigated moonpool is estimated based on the traditional linear potential flow theory.Firstly,the Helmholtz equation is solved based on the method of separation of variables,and then the Helmholtz mode of the bottom-closed moonpool is obtained by the eigenvectors.Then,the theoretical estimation formula based on the domain decomposition method is carried out to solve the natural frequency of the investigated moonpool.The differences between the natural frequency of a moonpool without recess and a moonpool with recess are analyzed and discussed in detail.(2)The "drilling ship-moonpool" coupling system is carried out in a numerical current-making tank,and the inducing mechanism and Parameter sensitivity of the added resistance of the moonpool under flow excitation are investigated.Based on the traditional ship resistance theory,the moonpool added resistance is divided into moonpool added friction resistance,moonpool added viscous-pressure resistance,and moonpool added wave resistance.The momentum equation of the bottom boundary layer of the longitudinal two-dimensional slice of the drilling ship is deduced,and the inducing mechanism of the added viscous-pressure resistance of the moonpool is also explained based on the dimensional analysis.the added viscous-pressure resistance of the moonpool is discussed in detail from the perspective of vortex shedding.Based on the frequency spectrum of the response of the free-surface elevation,the inducing mechanism of the added wave resistance of the moonpool is analyzed,and the Characteristic of the added wave resistance of the moonpool under different Froude numbers are discussed in detail.(3)The "drilling ship-moon pool" coupling system was placed in a numerical wave-making tank to investigate the Characteristic of the moonpool under wave excitation and its slamming effect.The natural frequency of the piston mode is used as the external wave excitation frequency.The non-linear behaviors of the moonpool under pure wave excitation and "wave-current" combined excitation,as well as the non-linear behavior of breaking,run-up,and overturning of the moonpool water body,are studied.According to the typical slamming time-history curve,the most obvious wave-current Parameter combination of moonpool slamming is selected.Moreover,the temporal evolution and spatial distribution of slamming in a moonpool are analyzed and discussed.(4)Based on the conclusions of the added resistance and internal slamming effect of the moonpool in the presented study,the design principle and working performance of the typical added-resistance-reducing and wave-motion-suppressing Device in a moonpool are discussed in detail,and its application in practical engineering is also analyzed.A new type of damping device for a moonpool with recess is proposed,and its feasibility and effectiveness under different external excitations are evaluated based on the numerical simulation.The results show that the moonpool added resistance includes three parts named the moonpool added frictional resistance,the moonpool added viscous-pressure resistance and the moonpool added wave resistance,respectively.The moonpool added viscous-pressure resistance and the moonpool added wave resistance are the key components of the moonpool added resistance.The moonpool added viscous-pressure resistance is induced by the vortex shedding caused by the separation of boundary layer at the leading edge of moonpool,while the moonpool added wave resistance comes from the oscillation of water body inside a moonpool.These two parts of moonpool added resistance are closely related to the speed of a drilling vessel.In practical engineering,the speed of a drilling vessel needs to be reasonably designed to avoid excessive moonpool added resistance.When a moonpool is excited by the wave at zero speed,there will be almost no slamming.However,there will be obvious slamming when the drilling vessel sails at high speed in wave.The locations of highest slamming pressure inside a moonpool appear near the calm water surface of the front wall and the back wall of a moonpool,and the horizontal plate of the recess.In practical engineering,these structures need to be strengthened.