Multi-body Dynamic Study Of Lowering And Installing Ultra-large Subsea Equipment Based On Heave Compensation

The heave stability of subsea structures is an important aspect affecting the bearing capacity of the subsea structure lowering system.Improving the heave compensation performance of the subsea structure lowering system is also an important approach to improve its lowering capability.However,currently there's less research focus on the heave stability or heave compensation performance of subsea structure lowering system.In this paper,a heave compensation based subsea structure lowering system——HCLS is proposed,with the background of lowering ultra-large subsea structure.Regarding the disturbance on the lowering system as the point of penetration of the problem,by studying the disturbance and resonance characteristics,multi-body dynamics research on the heave stability and heave compensation characteristics of HCLS are conducted,the main contents are as follows:1.Quasi-static structural design and strength verification of HCLS are completed.The quasi-static design involves in strength design and selection of main components of the system based on API and other rules of Classification Society,which establishes the model foundation for subsequent dynamic analysis.In the strength analysis,strength of each component is checked,and the strength safety factor reaches the standard of designing hydrodynamic equipment by static method.In addition,the dangerous point of the system is determined.2.Hydrostatic characteristics of HCLS are analyzed by using fully coupled method.Firstly,the hydrostatic strength of HCLS is checked,and the results of calibration are compared with the results of quasi-static structural design calibration,which verifies the rationality of the structural design preliminarily.Then,heave motion characteristics of HCLS in the static dimension and the mechanical properties of bearing components are analyzed,which provides a basis for the system dynamic characteristics and hydrodynamic response study.3.Vibration dynamic characteristics of HCLS are analyzed by two methods: non-coupled method and fully coupled method.The non-coupled dynamic characteristics analysis divides HCLS into two parts: the catenary system and the main lowering system.The non-coupled dynamic characteristics analysis mainly focuses on the disturbance source that affects the stability of the main lowering system and the vibration dynamic characteristics of the lowering system under disturbance.In the catenary system analysis,firstly,the mechanical model of the catenary is established based on the catenary theory and the lumped-mass method.By writing MATLAB program,disturbance source vibration characteristics and dynamic conduction characteristics of the catenary are studied.Moreover,the software Ocra Flex is utilized to verify the theoretical calculation results.Then,the dynamic characteristics of HCLS under disturbance are analyzed by modal characteristic analysis of the main lowering system.The full coupled dynamic analysis is based on the three-dimensional potential flow theory to analyze the hydrodynamic frequency domain characteristics of HCLS by using the hydrodynamic numerical software AQWA.The consistency of non-coupled analysis results and fully coupled analysis results is determined.The hydrodynamic characteristic analysis lays the foundation for the hydrodynamic response analysis.4.The hydrodynamic response of HCLS is mainly conducted in the frequency domain and time domain under irregular waves.In frequency domain,statistical results of RAOs and motion response of the subsea structure are analyzed.In time domain,displacement response of the system and mechanical response of the bearing component are analyzed.And the effect of resonance on the stability of the system is determined.Among them,the study of heave compensation performance of HCLS is emphasized.Finally,according to the comprehensive dynamic characteristics of HCLS,a HCLS structure design optimization scheme is given based on dynamic heave stability.