package doesn't handle moorings with non-buoyant elements properly

[richardsc]

Illustrated with example:

library(mooring)
m <- mooring(anchor(), wire(length=20), float(), wire(length=200), waterDepth = 100)
plot(knockdown(segmentize(m), u=1))

^{Created on 2022-03-07 by the reprex package (v2.0.1)}

[dankelley]

Thanks for the report. We talked about this yesterday. I am not sure exactly what to draw, since I think that in the real world it would be condition and history dependent, e.g.

• if the mooring were placed at a time of low current, I'd imagine the line would end up on the bottom near the anchor
• if there was a current at deployment time, then I'd imagine the line would trail in the downstream direction, at least to some extent.
• the length of the line might matter, e.g. if it's shorter than water depth, it would form a catenary of its own, but if it were longer, then it might form a sort of catenary, with some remainder on the bottom, acting as a sort of anchor.

What I'm saying is that the true solution is not clear to me. However, I agree that what is drawn now is not sensible.

I need to think about the case in which more than one part of the mooring touches the bottom. That's needed for ground lines, for example. Maybe I'll concentrate on that first, because it's a math issue, not just a programming one. I could imagine a setup with buoyant line and small anchors at intervals, and so just altering the math to handle two endpoints in contact with the ground would not be sufficient.

Note to self: the spot in the code where we detect bottom contact is

mooring/R/mooring.R

Line 808 in d07509b

if (z < (-depth)) {

but, as noted, this is primarily a physics problem at this stage, not a programming one.

[dankelley]

This issue has not been addressed, but some things have changed so I am pasting below the results of the (now-altered) reprex that @richardsc sent.

The code changes are

1. waterDepth is now an argument to mooring(), not to anchor(). I think this makes sense because maybe we might want to have two anchors, like with a ground line. (I'm not saying I know how to solve the ground line problem, though, just to be clear!)
2. discretise() is now segmentize() because I found the wording to be hard to guess. I think 'discrete' calls to mind discrete components like ADCPs. I like to evoke segments, like segments of a worm.

The plot change is in where the extra line goes. Here it's going into the sediments. I think I had code at one point to make it go out along the bottom but that doesn't actually make sense because I could imagine a catenary on each side of the float. But that extra line would have drag on the bottom so its position would depend maybe on currents during installation. I do not quite see how to approach this, but I also think the larger problem is the case of a mooring with a top-float that is at the surface for weak currents (or always), as discussed at my blackboard a while back.

library(mooring)
#> Loading required package: S7
m <- mooring(anchor(), wire(length=20), float(), wire(length=200), waterDepth = 100)
plot(knockdown(segmentize(m), u=1))

^{Created on 2024-01-31 with reprex v2.1.0}

[dankelley]

Just for fun, this is what happens with a shorter above-float wire. The extra line just hangs straight down, dejected.

library(mooring)
#> Loading required package: S7
m <- mooring(anchor(), wire(length = 80), float(), wire(length = 70), waterDepth = 100)
plot(knockdown(segmentize(m), u = 1))

^{Created on 2024-01-31 with reprex v2.1.0}