Accelerated testing and results of underwater electric thrusters for mini observation class ROVs

Thrusters are an integral part of the propulsion system of electric underwater vehicles, and they are loaded in very harsh conditions. The current commercial market segment for electric thrusters possesses a steep increment in cost with increasing reliability and operational lifetime. As a result, a major void exists in the category of commercial thrusters with high thrust output, high power to thrust ratios and affordability for mini observation class remotely operated underwater vehicles (ROV). This is due to the rapid escalation of various challenges in thruster design related to overall size, tight tolerances in manufacturing, electrically accelerated corrosion, pressure compensated encapsulation, seal design, thermal effects and so on. The challenge during an optimal thruster design process is further propagated by the limitations in performing accelerated testing in the laboratory with conditions simulating hostile loading environments. Also, given that such thrusters are designed for a very long lifetime, it is challenging to test them for their full life. In addition, their loading patterns are also highly dynamic in nature, further making the simulation of environmental conditions even more challenging. Given such challenges in product design and testing, a minimalistic design methodology and test results are presented for a thruster developed at Planys Technologies targeted specifically for mini observation class ROVs.There are many models available for accelerated testing of commercial products. However, conventional accelerated testing methods most often use statistical methods to analyse the test data. This leads to the necessity of testing a large number of samples to get a reliable life estimation data. Also, the overall test design is further complicated by the fact that the number of variables affecting the life of the product are very high. In view of the said complications, and also since underwater marine robotic products are expensive to manufacture, the proposed test method uses a minimum number of samples for life estimation. This is achieved by utilizing a test strategy centered around the anticipated failure modes of a thruster. This methodology not only saves the overall cost, but also the overall time required to release the product to the market. In addition, the test setup and the results are also presented for the particular model that was tested using this methodology.In this article, the test design philosophy with key considerations is explained followed by a deeper emphasis on the test methodology which has been prepared with the aid of the experience gained by operating indigenously designed ROVs in tough underwater conditions across the globe. It is to be noted that the proposed solution may not be as accurate as the conventional statistical test methods in all cases, nevertheless, is very much useful in giving an insight into the product lifetime range to an acceptable degree. The proposed test methodology can also be extended to other product life testing in marine robotics and beyond.