Studies On The Dynamic Difference Of Truncated Mooring Line In Hybrid Model Test

With the development of deep water oil and gas exploration, more and more deepwater platforms will be used for production. Model test is still one of the most important steps for deepwater platform design. However, full water depth model test still can not be performed under conventional scale in current wave basin. Hybrid model test is the most advanced method to solve the problem. And the key problem in hybrid model test is designing an equivalent truncated mooring system. How to design the truncated mooring system which can simulate the full mooring system including static and dynamic characteristics is the ultimate purpose of the truncation design Currently, designing of truncation system is mainly based on static equivalence.There are obvious dynamic differences between the truncated mooring system and full depth mooring system if the truncation is only based on static equivalence. The dynamic difference will lead to error of prediction of the mooring line tension. In this paper, the author choose a semi submersible, which will be set up in South China Sea, for truncation design based on static equivalence. Then the truncated single mooring line and truncated mooring system are used for dynamic difference studies. The semi submersible can work for 1500m water depth with mooring system and up to 3000m with dynamic positioning.The following aspects are included in this research:1: Introduction to the development of the model test, technique of the deepwater platform model test and hybrid model test.2: Brief introduction of mooring line calculation, including catenary equation for static analysis and numerical model and calculation methods for dynamic analysis. 3: Based on the static equivalence, the truncated mooring systems of semi submersible for 1500m full water depth are designed respectively for 1200m, 900m, and 600m water depth. The static characteristic curves are provided for all truncated mooring systems.4: Time domain coupled analyses are performed for all water depth numerical models, including truncated and full water depth model. In the analysis, same hydrodynamic parameters are adopted for all numerical models, including truncated model and full depth model. In fact, this process is equivalent to the numerical extrapolation in the hybrid model test. Research on the motion and tension prediction difference between truncated mooring system and full water depth mooring system, under same hydrodynamic parameters; we try to prove that truncated mooring system based on static equivalence has some prediction capability, but dynamic characteristics difference of mooring line still can not fully meet the requirement. 10-years-wave in South China Sea is selected for the analysis. Two types of sea states will be included: 1: 180°waves and wind, without current; 2: 180°waves, wind and current.5: 600m truncated mooring system is selected for dynamic characteristics difference research, including single truncated mooring line and truncated mooring system. In the single mooring line dynamic studies, the top end of the mooring line will be given regular oscillation with different amplitudes and oscillation periods, and irregular oscillation, including horizontal oscillation and vertical oscillation. The same top end oscillations are performed for the truncated mooring system; with the difference that top end is changed to floating body--semi submersible instead of top end of mooring line. Comparative items for dynamic difference research includes mooring line damping, dynamic tension force per unit amplitude, tension spectrum and tension RAO. Definitions of the items are explained in the comparative process. Especially, mooring line damping is explained in details, including the definition and calculation methods. Through changing the pretension, amplitude, frequency, stiffness, normal drag coefficient, and current speed, the author obtains the results of individual parameter effect on the mooring induced horizontal damping and vertical damping respectively. In this paper, comprehensive dynamic characteristics are calculated for truncated mooring line and mooring system. The dynamic differences between truncated system and full system are given for further research. The understanding and research of the dynamic difference between truncated mooring system and full mooring system have great benefit for future truncation design.