The power pack is lowered into the water for testing.Structure and characteristics
SHPS consists of three main modules: HPU, tools, and the surface support apparatus. Normally, the power unit is mounted and maintained on the deck of a support vessel. A crane hoists the HPU over the gunwale of the support vessel when it is going to be launched for work. The power unit can be lowered into the sea and placed as close to the working spot as possible, reducing the length of tubing between the HPU and the tools, which minimizes pressure loss. This also makes it convenient for a diver to manipulate the power unit.
All hydraulic components in the HPU are water hydraulic components, developed independently at HUST. A filtration unit protects the water hydraulic system from sand, mud, microbes, and any other contamination in the ocean. Multiple layers of stainless steel meshwork are used as a filter medium, to allow reversible flushing.
Experimenting with SHPS
Although divers are now able to work 900 ft below the surface, they need the support of a whole saturation diving system. To verify the power unit under pressure at a 900-ft depth, an unmanned experiment was carried out in a pressure hull of 560 psi, the same pressure as a 1200-ft depth. The HPU was adjusted to work at 1450 psi. Results show that the power unit can operate in this type of environment with all components bearing full load.
Practical experiments of SHPS were held both in the laboratory and on the bottom of South China Sea. All components were tested and verified separately in the laboratory. The experiment in the South China Sea was carried out 90 ft below the surface. Two divers manipulated the power unit and tools to accomplish two kinds of underwater jobs (cutting wire rope and grinding a metallic structure).
Lessons learned
The accumulator is effective at absorbing vibration and noise in the power pack, which prolongs the lifespan of hydraulic components, especially the relief valve.
The aperture of the filter mesh-work should be carefully selected to prevent premature filter failure. The filter aperture of 10 µm, tested in the laboratory, is not suitable for open circuit. Somewhere in the range of 30 µm would be a good compromise.
Pressure unloading is important, not only at startup, but when the quick-acting couplings need to be disconnected to change tools.
As far as future changes, the following should be considered:
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a self-renewing filter system should be developed to ensure a longer mean time between failure for the filter
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more styles of tools need to be developed, to extend the purpose of the power pack, and
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all hydraulic components need more experimentation, to ensure higher reliability.
Yu Zu-Yao, He Xiao-Feng, Nie Song-Lin, and Li Zhuang-Yun are with the School of Mechanical Science & Engineering at Huazhong University of Science & Technology in Wuhan, China. Email Zu-Yao at yuzuyao@ 163.com. This discussion is adapted from a paper given at the ICFP 2005 conference held in Hangzhou, China (http://sklofp.zju.edu.cn/icfp).