The major design consideration for the hydrocarbon caisson is to protect the risers against external impact from supply vessels, intervention vessels etc. It also protects the riser from hydrodynamic loading and corrosion but in contrary makes the inspection of the riser external and caisson internal wall difficult.
A hydrocarbon riser caisson represents a safety critical item of an offshore platform installation. The major threat of loss of riser caisson integrity cannot be over emphasised. Loss of containment could be as a result of corrosion of the riser external and the caisson internal if the caisson annulus is not well managed. The corrosion process will gradually reduce the wall thickness of the riser until failure resulting from pressure build-up of hydrocarbon within the caisson annulus. The likelihood of caisson failure is increased if the caisson itself has suffered from internal or external corrosion.
This paper will describe the processes involved in managing the integrity of riser caissons/j-tubes as highlighted by the failure of a gas lift riser located within a hydrocarbon caisson. A study of the associated riser Caissons is presented highlighting the basic approach taken for the integrity review/re-assessment of all the remaining hydrocarbon riser caisson facilities within the North Sea field complex. The integrity review process applied, involved the detailed review of the general arrangement of the system, the design intent, the current integrity management status and the integrity management requirements for the riser caissons.
A risk based inspection approach could then be used to generate an integrity management plan based on the current integrity management strategy developed. In this way further loss of hydrocarbon containment issues, with their associated risk and cost within the field complex, could be prevented.