Abstract:

The East China Sea experiences extreme waves and frequent typhoons. Currently, the installation design of jacket platforms in the East China Sea is mainly based on the sea state restrictions and standby coefficient obtained from installation experience in the South China Sea. Consequently, the actual installation period and investment of jacket platforms in the East China Sea deviate significantly from design expectation. Therefore, studying the operation windows of jacket platform construction in the East China Sea plays an important guiding role in ensuring their safe and economical installation. At present, a large number of studies have focused on the operation windows for offshore wind turbines in shallow waters, and some studies have analyzed the operation windows for offshore operation in the deep waters of the South China Sea. However, research on offshore operation in the East China Sea remains limited. The area of the East China Sea investigated in this study is classified as mid- to deep-water, with depth ranging from 80 to 110 m, and its waves are larger than those of the South China Sea. The jacket platforms operating in this area have significantly greater sizes and weights than offshore wind turbines, making the installation of offshore oil and gas platforms much more difficult than offshore wind turbines in this area. The stability of the floating crane vessels and the property of the cranes are very important for the offshore installation. Generally, both daily reports during the operation period of the installation and the simulation analyses according to the design parameters of floating crane vessels such as Huatianlong, Blue Whale, are essential. Code checking based on Noble Denton and API standards was performed on the operation performance of floating crane vessels and cranes. By comprehensively analyzing the actual installation conditions of the platforms, the stability of floating crane vessels, the capability of cranes, and the environmental conditions were identified to meet the requirements for jacket platform installation by the crane vessels. The results showed that when the jacket was installed, the wind speed was required to be no more than 10 m/s, and the significant wave height must not exceed 2.0 meters. Otherwise, when the stability of the floating crane vessels deteriorated, the installation conditions could not be satisfied. When the topsides were required to nest the Christmas tree, stringent demands were imposed on the floating crane vessels regarding sea conditions, limiting the wind speed to no more than 10 m/s and the significant wave height to no more than 1.5 meters. When the topsides were installed conventionally, wind speed for the floating crane vessels was required to be more than 14 m/s, and the significant wave height was not more than 2.0 meters. During the installation phase of the jacket platform, the feasibility of offshore installation and the period of the installation mainly depended on the stability of the floating crane vessel and the strength of the platform. The jacket platform mainly consisted of a jacket and topsides. The installation design reports of 3 jackets and 4 topsides implemented in the East China Sea were investigated. All jackets were designed according to the significant wave height not more than 1.5 meters and the wind speed less than 10 m/s. In addition, only one of the four topsides installation was designed with a significant wave height not exceeding 2.0 meters, while the other topsides were designed with a significant wave height not exceeding 1.5 meters, with the wind speed less than 10 m/s. It could be concluded that the sea conditions adopted in the installation design of a jacket and topsides were calmer than the sea conditions allowable for the stability of the floating crane in the actual installation process. Therefore, the sea conditions adopted in the installation design of a jacket and topsides should be used as the governing sea conditions during the installation process. A high-precision model was used to simulate the sea state conditions of the target sea areas in the East China Sea, and the simulation results were calibrated based on both the prediction and observation data of the nearby sea area. Consequently, the high-precision wind wave data of the East China Sea from 2015 to 2023 were obtained, enabling the establishment of an innovative sea state database with long time span and large data capacity. On this basis, research methods such as comprehensive analysis method and reverse inference method were used to analyze the wind and wave data. Wind speed and wave height were identified as two key factors to be considered in evaluating the operation windows. It should be noted that the requirements for weather conditions varied significantly at different stages of offshore installation of the jacket platform. Specifically, the launch of the jacket imposed relatively higher requirements for sea conditions, while the requirements for pile driving and grouting operation of the jacket were relatively lower. To further evaluate the duration of the window period in terms of time and safety, a comprehensive analysis method for offshore operation windows considering the effects of wind, waves, and sustainable operation time conditions was innovatively proposed. In this analysis, any period of continuous offshore installation shorter than the required duration was excluded from being counted as an operation window. According to the large amount of data involved, it was assumed that the wind speed did not exceed 12 m/s, and the significant wave height was not more than 1.5 meters to simplify and accelerate the statistical analysis. Then, the duration of installation periods meeting the wind and wave requirements were statistically analyzed for intervals of at least 24, 48, and 72 hours, respectively. A code was developed via MATLAB to read the wind and wave data day by day, and subsequently, the annual and monthly operation windows were evaluated. Finally, reverse inference analyses were performed. The results showed that the average number of operation windows exceeded 15 from March to August, and the number of operation windows in October was at least 8.44 under the conditions of 24-hour continuous operation. Under the conditions of 48-hour continuous operation, the average number of operation windows exceeded 10 from March to September, and there were at least 4.75 in December. Under the conditions of 72-hour continuous operation, the average number of operation windows was more than 10 from April to August, and at least 2.38 in December. In conclusion, the optimal window for offshore installation of the jacket platform in the East China Sea is from April to June. If the significant wave height of the installation operation considered in design is increased from 1.5 meters to 2.0 meters, the weather standby coefficient will be reduced by 20% to 40%. Therefore, to improve the offshore installation efficiency of jacket platforms and reduce the waiting probability, it is recommended to adopt the significant wave height of 2.0 meters for jacket installation design. When the topsides are required to nest the Christmas tree, the installation design should adopt the significant wave height of 1.5 meters, and the installation design of the remaining topsides should adopt significant wave height of 2.0 meters. These research findings fill the gap in the operation window data for the East China Sea and effectively ensure the safety of the offshore installation of jacket platforms.

Key words: East China Sea, platform installation, design sea state, vessel performance, operation window