The selective harvesting approach we are progressing with at Impossible Metals has many environmental and economic benefits. Our engineering team prioritizes the reliability of our autonomous underwater vehicle (AUV) components, specifically focusing on reducing the mean time between failures (MTBF).
Because our architecture uses a fleet of vehicles and each vehicle has many arms, we can achieve high production rates. Since the architecture is parallel, even with multiple surface vessels, no single point of failure will stop production. However, since there are many subsystems all operating at the same time, there will be a meaningful occurrence of failures. We have recently progressed our analysis to understand a more accurate expected maintenance cost for our specific architecture.
The following sections discuss the four main components in the system where recurring failures will have a meaningful impact on our economics.
For this analysis, the following operational assumptions are used based on the existing published economic model for a Eureka IV full production system at 6 million dry tons of annual dry production:
Number of AUVs: 128
Payload per AUV: 25 metric tons
Operation days per year: 273.5
Divers per day per AUV: 6.78
Nodule collection time per dive: 1.5 hour
Arms
Each AUV has 58 arms. This arm style is a delta robot with four actuators and a series of linkages to control the end effector. On our vehicles, these end effectors will operate at a 1.5-second cycle time when the AUV is on the seafloor collecting nodules.
The stated mean time between failures from manufacturers typically ranges from 50,000 to 60,000 hours, but studies have found that this can be as low as 1000 hours in harsh real-world environments1.
On each AUV, 58 arms operate for 1.5 hours per dive. Thus, the total combined arm operation time per dive is 86.7 hours. Therefore, one arm will be serviced for every 11.5 dives. The total combined arm operation time per year for all AUVs is approximately 20.5 million hours. Based on a repair cost of $1,500, representing 5% of the total arm cost per repair, annual arm maintenance will cost $30,873,202.
Buoyancy Pumps
The buoyancy pumps operate for 22% of the descent and ascent time, plus they operate for the entirety of collection time, for a total operating time of 1.8 hours per dive. Since there are four pumps per vehicle, the coral combined pump time per dive is 7.3 hours. Using the manufacturer’s recommended 1000 hours, there will be 137.6 dives between pump rebuilds/servicings, corresponding to 1,725 rebuilds annually. The cost to rebuild is $5,000, resulting in $8,624,609 for the annual pump maintenance.
Thrusters
The thrusters are the remaining moving components; thus, maintenance considerations are more costly. With a maintenance program similar to the pumps program of 1000 hours between rebuilds, given total hours of operation per year of 354,864, the total thruster cost for the vehicle is $245,950, and a 25% of the cost to rebuild, the total thruster maintenance is $21,819,000 per year.
The total maintenance costs for the key components are projected to be $61M. The economic model accounts for a total OpEx for maintenance and other expenses pro-rated by the amount of CapEx to be $110M per year, leaving almost $50M for other maintenance and insurance costs.
Batteries
Batteries are treated separately in the model as consumables and considered part of the OpEx. 47% of the pack’s useful life is consumed annually, and given 1,854 dives per year, each pack will see 3,983 dives. Given 128 vehicles and the pack cost of $265k, the total cost for batteries in operation is $34M, and given 47% annual utilization, the yearly replacement cost is $15.8M.
Much like any production operation, we’ll continually look for optimization as we begin operations, including ways to extend the service life of the components, ultimately reducing the operating costs of the nodule collection operations.
Our first production system will deploy tens of small-sized Eureka III systems with a 6.6 metric ton payload, which can be delivered to the transport ship every 4 hours.
