Studs Loosen and Fallen During the Sea Transportation

• Additional locknut
• Loctite 501 (usual disclaimer) on the threads
• Drill and pin through the bolt above the nut with a split-pin
• Pein the protruding bolt over with a ball-pein hammer
• Mangle the protruding threads with a hammer
• Zap the protruding thread with weld
• etc., etc.

It is a very "hard" assembly thus more sensitive to loosening. As I already mentioned a couple of times bolt assemblies are sensitive to TRANSVERSAL loads and related microslidings. Your tank can move transverse to the yellow frame so that it will get loose very fast since in sea travel transverse loads and vibrations are common.

A solution could be:

make the bolt side more compliant with a bushing so that the bolt clamped length will be longer at least 4..5 d. Tighten the bolts up to 0.6...0.7 yield limit. Use Norlock washers which are today best solution in such cases.

You can as well use as suggested Loctite but in a hard assembly even if you do not loose the nut you risk sliding since you block the thread only and sliding wears painting or other surface treatment and the assembly preload decreases.

You could also ad movement limiters on the frame welding small metal sheet pieces after assembly.

Nylok nuts are a simple solution to this problem. Still use washers, even lock washers to reduce any loosening at sea.

Install rivets instead of bolts. Sure, they have to be ground off at the other end, but that is easy to do with a side grinder. Heck, they held the Titanic together at sea.