As far as I know thy were only beached once a year, just before refit. I don't think any of them ever beached operationally. Certainly not in the Falklands. Before every beaching the beach was very carefully surveyed for obstacles. I dont't know about the stress on the hull, but the crew thoroughly enjoyed it - it gave us the chance to get some unusual photos.
As far as I was concerned, because we only practiced going up the beach, we only went in at around 1 knot..... I would have prefered at least 8 knots...then that would have been fun. It was normally done on a beach in Warbarra bay near Lulworth Cove in Dorset, and we would have two tugs standing by.
During my 12 months on Sir Lancelot, 1964 / 1965, we beached 3 times, Little Aden, Das Island in the Arabian Gulf, ( on exercise with the RN) and Tenby . This latter beaching was on a Sunday morning. Putting a tank on to the beach caused a few raised eyebrows among the dog walkers.
The landing at Das Island was to put ashore a squadron of tanks. We were told that the RN LCT which went in to picked up the tanks after the exercise 'got stuck on the beach'.
Although it wasnt operational, Sir Percivale beached in Loch Ryan in 1987 with a full load of vehicles. To get the trim, we had to reduce bunkers to 10%. We had to hit the exact spot as they had pre-laid the trackway to impress visiting Warsaw Pact Generals. No tugs either.
When the Sir Galahad (Original) was taken over by the RFA in 1970 we used to beach on Pulau Tioman at weekends and this happened on quite a few occasions. Eventually the powers that be put a finish to such regular practices. A great pity since the local kids used to come out and play football with the ship's company..the older locals kept well out of the way. Gentleman Jim Foster had just got his first command and was enjoying playing with his new toy.