Research On Truncation Technology Of Deepwater Mooring System In Model Test

With the ever increasing demand for oil and gas, the resources in shallow water can hardly satisfy our requirements. We turn our attention to deepwater, and many types of new platforms have been designed and rapidly developed. Model testing is an important method to verify the design of a platform and its mooring system, but for model tests of deepwater floaters, we often face a common problem, which is neither the depth nor the width of the basin is adequate to accommodate the complete mooring system in a reasonable model scale, and the mooring lines have to be truncated with acceptable equivalent characteristics. Therefore, truncation designs of deepwater mooring systems are systematically researched in this thesis, and the results are verified by model tests. Associated with the deepwater mooring design and tests, the following topics are studied in detail.Firstly, since the static characteristics analysis of mooring lines is the basic method for initial analysis of mooring systems and basis for truncation design, and for the convenient use in the following chapters, the static characteristics are derived according to the catenary theory, and a corresponding computer program is compiled. The reliability is verified through analyzing case examples. This program can be used for static characteristics analysis of different kinds of mooring lines.Secondly, numerical simulations of a deepwater internal turret moored FPSO are investigated for the preparation of model tests and subsequent verifications of testing methodology. The wave loads, RAO, QTF, etc. of FPSO under regular waves are derived through calculations using potential flow theory in frequency domain. Considering the coupling effects among mooring lines, risers and FPSO, the global response analysis of the complete system are performed in the time domain. The dynamic analysis and quasci static analysis are both considered and the resultant mooring line loads are compared. It is found that the calculated results of static characteristics of the whole system and FPSO global response under environmental loads all agree well with the available past MARIN test results. The predicted mooring forces compare well with MARIN results when using dynamic analysis, but the loads are lower when using quasci static analysis, which is well expected. This exercise verifies the reliability of the numerical simulation. Thirdly, to solve the problem of limited basin depth in model tests of deepwater mooring systems, truncation designs are performed in three aspects, catenary mooring system, SCR, and taut mooring system. The truncation methods, procedures and calculating formulas are proposed respectively. For a catenary mooring system, the catenary characteristics must be ensured even after truncation, and accordingly, the line length, anchor positions, and also other parameters can be determined. For a SCR, the riser is made up of only one type of material, and the truncation method is similar to catenary mooring system. For a taut mooring system, to adapt to different conditions, the truncation methods are proposed respectively when the truncation factor is large and small. On the base of foregoing numerical analysis on the DEEPSTAR internal turret FPSO mooring system and according to the proposed truncation schemes, equivalent truncation designs are carried out on the same FPSO system under many cases with different working depths. After comparison, both the static and dynamic results of truncated cases agree well with those of the untruncated mooring system, which prove the truncation designs are robust.Fourthly, to solve the difficulties of the limited basin width in model tests of mooring systems, three truncation methods are proposed according to different truncation factors. When the truncation factor is large, only the bottom lines are truncated in method 1 and only the middle lines are truncated in method 2. When the truncation factor is small, both the bottom and middle lines are truncated in method 3. The DEEPSTAR FPSO mooring system is also selected as research example and the mooring lines are truncated to adapt to different basin widths. After comparison, it is found the static results in method 1 have the best agreement with those of untruncated system, but for the results of coupled analysis in time domain, the FPSO movements and mooring forces of the three truncated methods all agree well with the untruncated results.Finally, model tests on the internal turret FPSO mooring system are carried out in the 50m x 30m x 10 m basin of Harbin Engineering University, which include free decay tests of FPSO and RAO tests under regular waves, static characteristics tests of FPSO mooring system in calm water, tests of FPSO mooring system under irregular waves. For the mooring system in 914m water, mooring systems are truncated to prototype water depths of 736m and 460m respectively and then model tests of these truncated designs were conducted. The test results of 736m and 460m depths agree fairly well with test results of 914m depth and also numerical results, which verify the veracity of foregoing truncation designs and numerical simulations. The 736m and 460m truncation model tests are numerical reconstructed, and then extrapolated to 914m full depth, and the extrapolated results coincident well with the full depth results. The exercise in a way verifies the commonly used hybrid model test method.In conclusion, systemic investigations and analysis on deepwater mooring system are carried out in this thesis, which includes numerical simulations, equivalent truncation designs and model tests in deepwater basin. Different truncation schemes are proposed to satisfy different model test requirements, and significant conclusions on optimum truncation design, mooring system dynamic response, verification of hybrid testing method, and advanced numerical analysis vs. model tests are obtained. The work presented in this thesis is hoped to provide a good reference for future research in this field.