Research On Some Key Issues In The Design And Manufacture Of Octagonal Pile Legs Of Offshore Wind Power Installation Vessel

The construction of offshore wind power is an important national strategy to solve the energy crisis,adjust the energy structure and achieve the ‘double-carbon’ goal.China has an ocean environment of long coastline,complex seabed geological conditions,blustering wind,heavy waves,rapid flow and frequent typhoons.The annual time of effective offshore wind power construction conditions is very short and the installation of large wind power equipment is extremely difficult.Manufacturing offshore wind power installation vessel with good wind and wave resistance,high installation efficiency and high safety factor is an important way to solve the ‘strangle’ problem of offshore wind construction.Pile legs are the key components of the self-elevating and self-propelled offshore wind power installation vessel,which ensures the hull of the wind power installation vessel lifted off the sea level to obtain a stable installation environment.As a new type of shell pile leg,the leg with octagonal section has good manufacturing and assembly process,higher space utilization rate and higher overall strength than circular shell legs,which can effectively avoid the problem of stress concentration.This thesis takes the octagonal pile leg of a self-propelled self-propelled wind power installation vessel as the research object,Focusing on the goals of low stress,light weight and high reliability of the structure.Several key issues of its design and manufacture are studied as follows:1.Aiming at the welding defect of cold crack deformation of octagonal pile legs,a precise preheating method by selective electromagnetic induction technology before welding is proposed.Based on the electromagnetic-thermal coupling finite element simulation method,the influence of three different induced current loading methods on the temperature field is studied.By optimizing the linear current loading amplitude,a uniform temperature field which meets the requirements of the specification is obtained,which has a theoretical guiding value for the improvement of the welding manufacturing process of the octagonal leg.2.In view of how to reasonably evaluate the structural safety in the design process of octagonal pile leg,a simplified equal-stiffness model of the main hull and octagonal legs is established,and the wind,wave and current are loaded on it.The static analysis of the beam element is carried out to obtain the displacement and bending moment values of the legs at each wave angle.The plate element of a single pile leg is established.The static stress analysis and yield strength check of the outer plate of the pile legs are carried out.The results show that: the octagonal pile legs meet the overall stiffness and strength requirements;the overall yield strength reserve redundancy of the pile legs is relatively high;The natural vibration period of the pile leg is much smaller than the wave period so that the risk of resonance is low.3.To avoid the risk of dangerous working condition of pile leg due to dynamic load,the dynamic response analysis is carried out based on the rigid-solid model,and the displacement and stress amplification factors are calculated.The results show that the displacement and stress of the legs under the action of dynamic loads are larger than the static results,which means that dynamic response analysis is the necessary process of evaluating the pile leg structure.Orthogonal tests are carried out on four load factors with the dynamic amplification factor as the test index.The results show that the operating water depth and wave height are the main sensitive factors of the dynamic amplification factor,and the wave direction angle and the hydro-static air gap are the secondary sensitive factors of the dynamic amplification factor.4.In view of the high redundancy of pile leg yield strength,the thickness of the enclosure plate,guide plate,platform partition and enclosure plate rib is optimized,and the overall quality of pile leg is reduced by 19.3% compared with the original design scheme.The maximum stress and maximum displacement are within the allowable range and the initially goal of lightweight and reducing stress redundancy has been achieved.In summary,this thesis studies several key design and manufacture issues of a new type of shell legs for wind power installation platforms-octagonal legs,and obtains some meaningful conclusions,which provide some references for the independent design,manufacture and structural optimization of octagonal pile legs.