Coke Drum Insulation Problem

In the Delayed Coker Unit of our refinery, we have got two coke drums of 30 meters length and 8.5 meters diameter. Circumferential Insulation Support Rings made of standard equal angles are provided at a distance of 3 meters each. LRB Mattress (155 mm thick) with Aluminium Cladding (3 meters X 1 meters) is provided on the drums. After two years of operation, we have observed that the insulation mattress alongwith the cladding has bulged at many locations. What is the preferred type or any better alternate arrangement of insulation technique for such coke drums. Present design seems to be ineffective in long terms of service.

For information, Coke Drums operate in a 24 hours cycle of heating and cooling, during which expansion and contraction of 500 mm is observed along the length.