해양플랜트의 스콜에 대한 폭풍안전계류 평가에 관한 연구

Abstract

The design methodology of offshore mooring system has been settled as industrial standards and guidelines by abundant references throughout long times. This offshore mooring system has various design requirements in the installation engineering. As the representative engineering item in offshore mooring installation, storm safe mooring is to identify the minimum number of mooring lines required to meet the storm safe mooring condition during installation. During mooring line hook-up operations, mooring lines will be successively transferred and connected to the offshore structure. Once the minimum required lines considered to be storm safe are hooked-up to the offshore structure, the supporting tugs for station keeping will depart the installation site, and the offshore structure is to be completely supported by the mooring lines. The oil field of West Africa is governed by squall which are wind episodes characterized by a sudden increase in wind speed and direction over a relatively short duration. Although a large number of studies have been made on the offshore mooring system design including storm safe mooring, little is known about the design procedures settled for this squall condition so far. In this study, it was performed with storm safe mooring analysis for squall in quasi-static and dynamic. After the results were compared, the several alternative analysis methods were proposed for the time saving in installation engineering and the results were also compared with the conventional analysis results. Based on the results presented in this study, the following summarization and conclusion are made: 1. FPSO with 12 mooring lines and the squall environment were defined respectively. The evaluation process and limitation were established, then the quasi-static and dynamic analysis were performed. 2. The quasi-static and dynamic analysis results have the differences by 11~58% from the criteria of maximum tension and 10~23% from the criteria of maximum offset. 3. The maximum tension was selected for the significant index of the storm safe mooring analysis, it was pointed out the relation between the maximum tension and shorten analysis time window throughout the investigation of time series results in squall environment. The three kinds of dynamic analysis and one additional quasi-static analysis were proposed as alternative evaluation procedure in squall environment. 4. First, alternative dynamic analysis I (Dynamic*I); constant maximum wind speed of squall for 20 minutes calculation was proposed. The results have 0.1~1.8% difference with the 3 hours conventional dynamic analysis methodology. 5. Secondly, alternative dynamic analysis II (Dynamic*II); time series wind speed of squall but fixed wind direction for below 20 minutes calculation was proposed. The results have 1.0~2.8% difference with the 3 hours conventional dynamic analysis methodology. 6. Thirdly, alternative dynamic analysis III (Dynamic*III); time series wind speed and direction of squall for below 20 minutes calculation was proposed. The results have 0.7~3.8% difference with the 3 hours conventional dynamic analysis methodology. 7. Additionally, alternative quasi-static analysis (Quasi-static*); constant wind speed by peak value of 1 minutes averaged wind velocity in scaled squall time series for quasi-static calculation was proposed. The results have 0.3~4.6% difference with the 3 hours conventional dynamic analysis methodology. This study needs following verification with various additional squall time series data. There remains a second question: how different in various parameter, e.g. floater type, size, weight and mooring lines configuration, etc. Finally, it is remained also more calculation cases by the variation of irregular wave seed. Despite of the above limitation, it seems reasonable to conclude that this study would be effective as alternative procedure to provide time saving engineering.