Introducing multi-round clustering on capturing the multidirectional extreme current profile for deep-water ROV operation

ROVs (Remotely Operated Vehicles) are playing an important role in installation, construction, and maintenance of subsea systems. During operation, ROVs are usually connected to surface facilities through umbilical lines where the entire umbilical-ROV system is affected by vessel motions, wave, and current induced loads. During the design process, it is commonly assumed that the current profile that leads to the extreme response of an offshore structural system has a typical profile with a uniform direction. However, in deep water, the variation of current direction along water depth becomes more apparent. Hence, the effect of direction variation cannot be neglected when establishing the extreme current profile for deep-water ROV operation. The study focuses on how to efficiently acquire the multidirectional extreme current profiles through a new multi-round clustering method. Firstly, static analysis of a typical deep-water umbilical-ROV system was performed under thousands of multidirectional current profiles extracted from a database of one-year continuously recorded ocean current. Different responses of the system such as ROV offset, umbilical lateral displacement, and maximum tension in the umbilical were investigated and set as a benchmark for further process. Then, a multi-round clustering procedure using a modified maximum dissimilarity algorithm for ROV operation was introduced and applied to extract the extreme multidirectional current profile more efficiently. It is shown that the proposed multi-round clustering methodology can be implemented efficiently and accurately in capturing the extreme vertical current profiles from a current database for ROV operation in different working depths. The study also showed that the effect of bottom current on deep-water ROV operation is more significant than the surface current. Therefore, for ROV operation in deep waters, more precautions shall be made on the bottom current profiles.