I used to work in a company that was a market leader in offshore cranes and also did a lot of research on UHWMPE cables (interesting because of their neutral buoyancy in water for really deep operations). I am by no means an expert on the subject, but some informations still trickled through to me.
This problem for sure is real. Winching such tether back and forth is difficult.
On the other hand, it seems to me very manageable. Use thicker tether or other materials for the wearing sections. Use existing condition monitoring tools to estimate wear, and replace the tether sections when worn.
Have contingency plans for when the tether eventually breaks.
It all seems like though a worthwhile discussion in terms of engineering, in terms of making AWE happen, its of secondary nature.
And it is perhaps the same for the possible soft kite wear due to the two flight modes in reeling operation.
Soft kites also have flutter and multi axial loads to contend with
For soft kites I think the problem is more interesting in terms of AWE because it’s more domain specific and you wont find many ready made solutions when scaling up.
Flutter is probably a killer for a soft kite (kite producers recommend never leaving a kite fluttering in the wind). This is a quite complex domain to master IMHO, and only a very few people in the world know how to design innovative soft kites (at least if there are many they are not known to me).
On a kind of related subject, pulleys in the kite bridle, with the kite being either rigid, soft or somewhere in between (eg windsurfing sail), is probably difficult to monitor and probably has severe failure modes. My guess is they must be avoided if possible, or perhaps made redundant otherwise. I consider this every time I look at a design incorporating pulleys.
Optimistic comparison with offshore cranes does not account for severe aviation factors of lowest-mass and lowest-aerodrag, that surface cranes are far less sensitive to (in fact most cranes are still wire-rope based, with higher safety factors easily specified).
Our best data is the promotional videos where poor condition of reeling AWES lines is evident after relatively few flight hours. Our best solutions are for short-stroke action of specially heavy sections of line to engage hardware.
For Yoyo, you might have a 500 m line and a 200 m production cycle. Most of the line is only reeled on the winch during launch and land. The part of the tether closest to the kite is where most mass and drag effects occur. The part of the tether being reeled during production cycles may be a lot thicker with few adverse effects, and even another material (steel?). With more R&D the production stroke length might also be reduced a lot
Belt or chain drive sections offer ultimate tensile drive lifecycle performance. If the pumping strokes are short enough, solid mechanical levers become practical.