Truncation Design And Numerical Reconstruction And Extrapolation For Hybrid Model Testing Of Deepwater Platform

Because deep-water platforms have high cost and work in harsh environment,it is necessary to carry out wave basin tests to ensure their safety.However,owing to the limited scale of wave basin,it is difficulty to carry out model tests for large-scale mooring and riser systems with normal scaling ratio.Therefore,hybrid model testing technique which combines experiments with numerical simulations is proposed to overcome the limitation of wave basin.The classical hybrid model testing technique includes three stages: truncation design,model tests,numerical reconstruction and extrapolation.The feasibility of hybrid model tests has been verified by many researches,while problems still exist,for instance: strategies are needed to decrease the discrepancy of vertical force and pre-angle after truncation;curse of dimensionality occurs in truncation design of asymmetrical system owing to the large number of objective functions,thus traditional multi-objective optimization algorithms are invalid;inequivalence of static and dynamic characteristics occurs when the platform has large drift;the model used in numerical reconstruction and extrapolation has limitations because it is based on the potential flow theory and linear vibration equations.This thesis elaborates and analyzes the existing problems of hybrid model testing in truncation design,numerical reconstruction and numerical extrapolation.Truncation design method specifically for deepwater asymmetric mooring and riser system is studied,and empirical formula for truncated design is proposed through supervised learning method.The feasibility is verified by applying to various deepwater floating system.Then,effective method of numerical reconstruction and extrapolation is studied considering the nonlinear effects induced by motion coupling and bracings and the nonlinear environmental load induced by viscosity and slamming.Specifically,the criteria of equivalence for truncation design of asymmetrical system are firstly described.To fulfill the equivalence criteria,a four-level screening method is proposed,and meanwhile lump-mass method which is used to perform static and dynamic numerical simulation is deduced.The four-level screening method classifies the objective functions into levels and optimizes them in sequence,thereby curse of dimensionality encountered in truncation design of asymmetrical system is overcome.The first three levels are based on the static equivalence,and the length,axial stiffness and wet weight of truncated lines are acquired;and the fourth layer is based on dynamic equivalence,and the diameter and weight in air are acquire.To prove its accuracy and effectiveness,the four-level screening method is then applied to truncation design for varies types of mooring and riser systems,including the asymmetrical mooring systems,the asymmetric design for symmetrical mooring systems,and the asymmetrical riser systems.Compared with traditional multi-objective optimization algorithms,the four-level screening method has the advantages as below: faster decreasing speed of the objective function,shorter iteration time and capability of parallel computation.Empirical functions for truncation design are generated based on supervised learning method.Based on the static equivalence,empirical functions for truncation of single lines and mooring systems are generated using supervised learning.The truncation design can be complete in seconds using the functions,and the functions have high applicable rate.The process of supervised learning includes: determining the problem and object;generating a training set and a test set;generating input features,determining the structure of the empirical function and the training algorithm;training the model;and testing the accuracy of the function.The test results of the empirical functions show that for the truncation of single mooring line and mooring system,the empirical function can generate proper properties of truncated system,and can be applied to most of the full-depth model.Pre-offset and asymmetric design method and bundle truncation design method are proposed.Based on four-level screening method,a new pre-offset and asymmetric design method is proposed to overcome the mechanical inequivalence when the platform has large drift under harsh sea conditions.The method enlarges the offset limit in truncated design,and the equivalence between the truncated and full-depth system is achieved with a relative error less than 5%.In addition,for the systems with a large number of risers,bundle truncation design method is proposed.The effectiveness of the method is verified by applying the method to a deep-water semi-submersible platform system.The results show that it can simplify the truncation system while ensuring the mechanical equivalence before and after truncation.Mathematical model considering nonlinear effects such as nonlinear coupled motion and influence of under-water structure on buoyancy is established and thoroughly deduced for numerical simulations.Program of time domain analysis using the model is coded for the use of numerical reconstruction and extrapolation.Focused on a semi-submersible platform with bracings,model tests and numerical reconstruction are performed using the program to verify its accuracy.In the model tests,instability of pitch motion and deviation of resonant frequency are observed.The results show that the newly proposed model can better predict those nonlinear effects than traditional model.In addition,using mathematical model,numerical simulations are performed to investigate the parameters inducing the nonlinear effects,and the scope of the model is given.A numerical reconstruction and extrapolation method based on inverse identification of environment load is proposed.The method extracts environment load by subtracting the force of mooring system and the hydrostatic restoring force from the external force,and the external force can be obtained from the inertial force.Using the method,we can properly reconstruct and extrapolate the phenomena caused by viscosity such as vortex-induced motion.The method is used to perform numerical reconstruction and extrapolation for a three-column semi-submersible platform system.The feasibility of the method is verified by comparing the results with the experimental results of truncated and full-depth systems.In addition,numerical reconstruction and extrapolation are performed using traditional method.The results show that the newly proposed method can better reconstruct and extrapolate the vortex motion of the platform.In summary,the methods proposed in this paper can effectively solve the existing problems of hybrid model tests,enlarge the scope of application of hybrid model testing technique,improve the precision of hybrid model tests,provide reliable support for prediction of hydrodynamic performances of deepWwater platforms.The methods have been successfully applied to nearly twenty domestic and international deepwater platforms.