I did the oil rigs dives 2 weeks ago. I am nowhere near calling myself an experienced diver but I do have a fair bit of experience diving weekly in California and off boats here where, as
@vladodessit mentioned, you need to be pretty much self sufficient and have your s**t together. The reason I mention this is because even with all that being said, these were extremely challenging dives for me.
Divers more familiar with the rigs can chime in but this was my personal experience as a first time oil rig diver.
- The boats get a very short window (usually 20min) from the oil rig manager to offload divers. This means that there is a fair bit of urgency just getting in to the water. The boat crew's commands and directions reflect this and can be quite stressful as it may feel like they're yelling at you to get in the water. This didn't bother me as my buddy and I were ready to go but I certainly saw some divers get pretty flustered by it.
- The boats can't get super close to the rigs due to security reasons so there's always a surface swim to the rigs, sometimes a 100ft or so in pretty significant swell. You also have to swim as fast as possible since there can be boats coming and going from the rigs and, from what I saw, they were not very observant of divers in the water. Same for the swim back to the boat. For me, this was absolutely exhausting and the hardest part of the dive.
- This is open ocean diving with very little protection from swells. Even what looks like a small swell in the forecast has a much more pronounced effect here than closer to land.
- Depending on ocean conditions, the top 20ft can be quite rough. Even deeper, there was a fair bit of surge horizontally and vertically - we were getting tossed around as much as 10ft in every direction.
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The next statement is just relevant to me but it was significant enough that I felt I should mention it. The blue water below you is absolutely hypnotic. I was anxious enough as it is about this dive and not having anything below me was nerve wracking. It's not hard for me to imagine this taking a large mental toll to the extent that you can lose situational awareness. My solution was to always swim over a crossbar for pretty much the entire dive (lame, I know).
I was talking to the boat crew on the ride back to port and they told me a few stories of almost accidents that were pretty terrifying - losing awareness and ending up around 120-150ft seemed to be the most common as that's where one of the crossbars is. We actually even had a rescue on the trip I was on - a diver, new to California diving and in rental gear, freaked out on the surface under the rig and actually climbed up on it in panic (huge no-no, people on the rig were yelling at him to get off, also had his reg out, mask pulled down to his neck, not responding to any instructions); the safety diver had to swim out and get him.
Again, the above is only my personal experience and assessment of the dives (based on 1 trip there). Not adding any specifics about this tragedy, just trying to provide some more context for those reading. Always sad to read these ...