Testing the ROV pressure vessel

July 2016

Well, we’ve built the pressure vessel. But will it hold it’s own against the relentless pressure of the ocean and if so, how far can we push it? Before we put any expensive electonics inside it, we decided to ‘certify’ the pressure vessel to a given depth so we know that it’s not going to die at that depth under normal operation.

To do this, we proposed sealing up the pressure vessel dropping it into the ocean on a rope, letting it touch bottom, pulling it up, re-seating the o-ring and repeating three times. I wanted to get some sense of reliability and repeatability, so I knew it wasn’t a fluke that it worked the first time.

Our initial design goal was 100m, but since working with the LPG cylinder, we were pretty sure it would probably survive to 200m without any trouble, so we’ve adjusted our design goal for stage 1, doubling it to 200m.

By now, it’s June and the middle of winter in New Zealand. The sea around Wellington is notorious for being amongst the roughest in the world – not ideal for testing an ROV from a 5.3m boat. So we had to wait for a break in the weather.

An opportunity came. The weather forecast wasn’t fantastic, but it looked possible. We would need to travel 7 nautical miles (12km) out to sea in order to reach 200m deep. We loaded the boat and set off. Unfortunately as is often the case, the sea conditions were not as described. Facing 3m swells, we had a tough decision to make. We decided to take the one chance we had and do a depth test to 67m, which was as deep as we could get without risk to life and property.

With water occasionally sloshing over our bow, we dropped the ROV over the side. It took three goes to make it sink – each time we had to add more lead weights. Eventually it disappeared into the turbulant waters and settled on the bottom. We didn’t have time to leave it there because the conditions were deteriorating fast, so we hauled it back on board and gunned the engines back to shore.

It wasn’t until we were back on dry land that we actually took a close look at the interior of the ROV – bone dry, not a single leak. We were thrilled. Only 67 meters, but a great validation that our design was headed in the right direction. But we needed to go deeper. So again, we waited.

Two weeks later another opportunity came and this time the forecast was both more favourable and more accurate. With no more than a 1m swell, we pointed the boat out to sea and kept going until the land was distant the the echo-sounded started loosing the plot. We eventually reached 200m deep and stopped.

We sealed up the ROV and again watched it disappear into the briney deep. The current was running strong this time and we reached the end of the rope without reaching the bottom. Frustrating. It took a long long time to pull it back in – 200m of rope is a lot when you’re hauling it by hand. But there were again no leaks showing when we got it back to the boat.

We decided to come up a bit shallower and find somewhere more sheltered from the current before trying again – we knew we had about 200m of rope on the reel. We found the spot we were looking for and again launched the ROV. This time, we struck bottom at 175m. Eager to know if this was our certification depth, we hauled all 200m of rope back in. Again, bone dry. We’d done it – our little sub had successfully withstood the pressures of 175m deep in the open ocean and it had dived three times without even a hint of failure.

Now we know we can get to 175m without any problem, we’re relatively confident in putting all of the electronics, batteries and motors onboard and doing some real sea trials.

We did notice one small crack in the acrylic dome, which we’re going to keep an eye on. We’re pretty sure it’s a stress fracture caused during manufacture and will hopefully not fail under normal operation. We’ll do some closer inspection and make a decision whether or not to rebuild the dome and then re-certify it.

The next step – finish building the motors and assembling the electronics.

Posted in ROV