Avoidance! Yes that's the key word as you've surmized.
If somebody hangs an anchor on you, your line is going to get pulled no matter what you've put on top, at least if its any kind of a reasonably sized ship. I've seen sidescan images of anchor drags 2000 feet long.
I haven't been working in the US since 1990, but at that time, SOP was to jet sled & bury all pipelines to 3 ft cover in water depths less than 200 ft. Since much of the GOM bottom slope between Port Lavaca and the La Delta is 1 ft/mile, the 200 ft depth line can get pretty far from the beach, so most of the heavy traffic shrimp trawl areas wern't a factor for the buried pipe, only exposed bits at tie-ins were vulnerable. We put trawler cages over valve handwheels at tie-ins.
Shore approach cover depths varied with sediment type but normal practice was 10 ft depth in surf zones and beach approaches. The intracostal canal crossings were buried to 10 ft cover with large lighted NO ANCHORING advisories on both sides.
Over 200 ft depths, no burial was required by the regulations.
Platform placement in designated anchoring areas was not encouraged, but somehow two of them managed to get installed off Galveston to which I had to build a connecting flowline and the gas carrier pipeline to the beach. There was about 10 miles of it in the designated east anchorage area. It is a busy anchorage area, as Houston is probably the highest volume port in the US and there are always ships there. As I recall, the majority of ships transversing the area have anchors with flukes of 10 ft lengths, so COE required 15 ft depth of cover on our permit. We had stiff clay for most of it and they were able to get a good trench profile (they said) to the 15ft depth, but I didn't verify that myself. 15 ft is hard to get if the bottom is sugar sand though, so it also depends on the bottom material as to if its possible to do it. Jetting rock wouldn't work, so you'd probably have to dump rock on top to get some protection, as they do in the North Sea. The profile you would need would depend on current velocity and wave drag velocity at the bottom. Probably the best references for those kind of structures is the COE Shore Protection Manuals. They're available for donwload.
Anyway, with the over 4000 platforms and several thousand miles of pipelines in the GOM, and the high traffic cargo, tankers and fishing fleet, the above practice seems to work well, as there are relatively few incidents. Get out there at night and in a lot of places you can't tell you've left the city. At least there are a lot less incidents than I would expect, but the MMS does report a few platforms out of service from time to time due to anchor drags on connecting pipelines. Most of those apparently from rig service boats that don't know NOT to anchor near platforms .. da!
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
