VESSEL REVIEW | National Energy Sea Test No 1 – Unmanned cable detection mothership for use at Chinese offshore wind farms
Chinese energy company the China Longyuan Power Group recently placed a new unmanned boat into service.
National Energy Sea Test No 1 (国能海测1号; Guoneng Haisi No 1) was developed as an unmanned exploration vessel to support wind farm operations in Chinese offshore waters.
Specifically, the vessel will primarily be used by Longyuan Power to deploy remotely operated vehicles (ROVs) to assess the status of subsea cables and to detect corrosion and damage of offshore booster stations and wind turbine jackets, thus making it a mothership for smaller unmanned craft.
The vessel has a length of 18 metres (59 feet), a beam of 4.1 metres (13 feet), a draught of only 0.76 metre (2.5 feet), and a range of 460 kilometres (290 miles). Operation of the vessel can be done either remotely or autonomously, though there is ample space for an embarked crew for manual navigation if needed.
The vessel will also take on roles such as precise identification of subsea cable fault points, thus significantly reducing cable fault location time as well as windfarm operation and maintenance costs. This then minimises instances of power outages.
Unmanned navigation permitting longer-endurance monitoring
The deployed ROV can also undertake comprehensive acoustic, optical and electromagnetic scanning of subsea cables, high-precision 3D modelling of seabed topography and landforms, and 360-degree scanning of pile foundation equipment in support of offshore wind farm maintenance activities.
The decision was made to develop an unmanned boat in anticipation of monitoring and detection voyages that are to be conducted year-round. Using an unmanned vessel would allow monitoring to be conducted even under extreme conditions such as strong winds and large waves much farther from shore, as no human operators would be put at risk.
Monitoring can also be done for sustained periods, such as when working with cables that are more than 300 metres (1,000 feet) long.
Ren Shaoyi, Engineer at the High Voltage System Institute of Longyuan Power Engineering Technology, said that an unmanned vessel would also ensure greater accuracy in subsea cable fault detection. Detection was traditionally conducted with a large crewed vessel with nine personnel and one subsea detector, but this arrangement typically delivered a detection accuracy far below operational requirements.
Improved data accuracy compared to traditional crewed monitoring
The vessel is fitted with a dynamic positioning (DP) system that is accurate within three metres (10 feet) of a given point even under sea state three conditions with wave heights of up to 2.5 metres (8.2 feet). The DP system enables the vessel to remain in a fixed position while the ROV executes the actual monitoring and detection, manoeuvring accordingly throughout the process.
The ROV itself also boasts positioning capability, being able to rely on multiple navigation technologies such as global satellite navigation, hydroacoustic positioning, and inertial navigation.
The other onboard electronics include a radar, lidar, and cameras that provide a 360-degree real-time view of the surrounding environment.
The vessel’s operating profile consists of four stages: mission planning and preparation, which also includes setting waypoints and the status of the onboard equipment; navigation and collision avoidance; deployment of the ROV; and data processing and analysis, when the vessel transmits data collected by the ROV to an onshore control station in real time for the purpose of generating detailed detection reports.
Trials showed that the use of the unmanned boat can reduce cable fault detection time from the typical 22 days to only two or three days. The project team said detection accuracy improved by more than ten times, and the overall cost has decreased to about one-tenth compared to traditional detection methods employed overseas.
