Three Dimensional Time Domain Wave Load Calculation Method For Trimaran And Its Application

Trimaran has attracted much attention in recent years and becomes a high performance ship to be developed in international shipbuilding industry by its advantages such as high speed,spacious deck platform,good stability and seakeeping.While the unique trimaran’s structure brings some advantages,it also increases the difficulty on the trimaran’s design.The high proportion of structure weight to the whole ship weight makes the structure design of trimaran affect the ship performance advantage directly.In order to guarantee the good performance of trimaran,in structure design,the methods of designed wave load and direct structure strength calculation are often used to assist the detailed design on trimaran structure.And these methods are always based on the wave load forecasting.So the research on trimaran’s wave load forecasting is very important.The two-dimensional strip method and three-dimensional wave load method based on frequency domain Green function without speed,which are widely used in engineering,are difficult to fully consider the influence of speed effect on trimaran’s motion and wave load,due to its assumption of high frequency and low speed in these methods.The existing rule for trimaran has certain limitations on the load design and structure assessment of high-speed trimaran whose side hull is at stern.Therefore,a kind of wave load forecasting method for high-speed trimaran and the corresponding structure assessment method are urgently needed to meet the needs of wave load design and structural optimization of high-speed trimaran.Based on these needs,a series of research on the wave load prediction of trimaran is carried out in this paper.The application of wave load prediction method in trimaran structure design is also presented.Finally,the wave load forecasting and structure strength evaluation system for trimaran is established,which will provide technical support for subsequent trimaran structure design and ship type development.The main contents are as follows:1.In order to observe the characteristics of trimaran’s wave load and provide an effective verification basis for the subsequent wave load forecasting method,according to the model test technology,a series of self-navigation tests using the segmented trimaran model are designed and implemented.Based on the experimental data in regular waves,the variation of trimaran’s motion,sectional wave load and its high frequency and low frequency components with different environmental parameters,such as wave height,sailing speed and wave length,are analyzed.And on the basis of test data in irregular waves,the influence of wave heading on the trimaran’s motion and wave load is introduced.The slamming pressure at the trimaran’s bow and cross-bridge structure is also measured synchronously.And the time-domain characteristics of these slamming loads are described carefully.By summarizing on the average peak value at each pressure monitoring point,the slamming pressure rule along the ship length and depth is found.2.Based on the time-domain Rankine source method,a method for predicting the trimaran’s motion and wave load is given.In solving the time-domain differential equation of trimaran’s motion,according to the structure characteristics of trimaran,the transient slamming load happened in the main hull bow,the side hull bow and the wet deck of cross-bridge structure is added to form a three-dimensional time-domain nonlinear wave load prediction method for trimaran.This nonlinear wave load prediction method considers the influence of the fluctuation on instantaneous hull wet surface at the same time.In this method,the slamming load influence,caused by the interference between the main hull and side hull,is also included.In order to reflect the trimaran motion and wave load in real waves,the numerical simulation towards irregular waves is carried out.And the prediction method of motion and wave load under irregular waves is established.Then,the calculation method of sectional load is found out.In order to obtain the hydrodynamic mesh accurately and quickly,an autonomous mesh generation method for trimaran hydrodynamic grid,by combining the B-spline method and the infinite interpolation algorithm,is introduced.Finally,the validity of the time-domain wave load method is verified by comparing the value of numerical simulation with model test result.3.The trimaran’s motion and wave load calculation model,based on time-domain Rankine-Green matching method,is established by combining the Rankine source and the time-domain Green function.The flow field around trimaran is divided into two parts.In the inner sub-domian,the Rankine source is used to solve the velocity potential.And in the outer sub-domian,the time-domain Green function is applied for solving boundary integral equations.So the motion and wave load of trimaran in waves can be simulated quickly and effectively.In solving the trimaran’s motion equation,the influence of transient slamming load and the fluctuation on instantaneous hull wet surface is also considered.Finally,the time-domain nonlinear prediction method of trimaran’s motion and wave load is formed.In order to improve the efficiency of time-domain Green function calculation,a fast algorithm for solving the wave term of Green function is presented,which combines the precise time-integration method and the table interpolation method.Compared with the model test result,the frequency-domain theoretical prediction value and the time-domain Rankine source theory prediction value,the validity of the trimaran’s wave load prediction method based on the Rankine-Green method is verified.The characteristics and application scope of these load forecasting methods are also analyzed.4.The application of time-domain wave load prediction method on designed wave load and structure strength assessment is given.Based on the wave load prediction results,a calculation method of designed wave load is introduced.Compared with the design load obtained by the trimaran specification,the characteristics and application scope of these calculation methods are given.Based on the design wave method,the direct structure strength calculation towards trimaran is realized.The concrete implementation approaches of the hull longitudinal strength assessment,yield strength assessment,buckling strength assessment and fatigue strength assessment are found.The influence on the structure strength assessment,caused by different load forecasting methods,is analyzed by comparing the strength assessment results between the frequency-domain load forecasting method and time-domain load forecasting method.Finally,a complete wave load prediction and structure strength assessment system for trimaran is formed by combining the time domain load forecasting method with the structure strength assessment method.