In 2014 CTS Thailand was awarded a contract by MTT to convert 3 74m and 1 78m external floating roof tanks to covered external floating roof tanks, installing aluminium domes on top of these tanks. Apart from that MTT ordered changing the tank seals for the a seal combination consisting of a CTS30 liquid mounted shoe plate seal with wax scraper and a CTS20IT rim mounted independently acting secondary seal.

The driving reasons for these tank conversions were the following:

Reducing tank emissions
Eliminating rain water ingress on top of the floating roof, allowing removal of the drain lines, also preventing rain water to pick up product remains inside the tank and requiring treatment before disposal
Reducing tank maintenance costs
Eliminating wax built up inside the tank shell

Dome design:

To minimize the reactions of the dome supports on the top of the tank MTT selected to use a dome with an internal tension ring, positioned on sliding supports. This ensures minimal forces and moments are transferred into the tank shell, resulting in no or minimal modifications to the existing tank shell when positioning the dome on top of the tank.

To ensure the tank internal is properly vented as does not built any explosive concentrations the domes are vented as per API 650 requirements, installing a 20” centre vent and fitting peripheral venting. Several international studies (amongst others from BP and Vopak) have shown that minimal product concentrations are expected below the aluminium dome roof, amongst others being the result of the reduced heat influx and lower emissions for a covered floating roof tank.

Suspended rolling ladder:

To keep access to the floating roof MTT has requested CTS to suspend the rolling ladder from the dome, still allowing access. This is something made possible by reviewing the bearing capacity required locally for loading the dome with the added weight of the rolling ladder.

Dome construction:

The domes are constructed inside out, while suspending the dome structure using tripods that can be moved around easily, allowing the workers to add consecutive dome rings while having their feet on the ground. During construction the dome is elevated above the foam dam, and the seal combination present.

Dome lifting:

After constructing the dome on top of the steel floating roof it is hoisted to the top of the tank, using the CTS’ designed and manufactured pneumatic driven synchronised set of grip hoists, allowing safe positioning of the dome on top of the tank. All grip hoists are load tested and certified for a safe working load (SWL) of 3 tonnes each.

Ladder suspension:

After the dome has been put in position the ladder is suspended from the dome, allowing for safe access, where of course one has to realise access is given to a confined space, so corresponding safety measures and vapour measurements are to be conducted to facilitate safe access to the tank interior.

Seal installation:

After all this has been done and as a final step to completion of the tank conversion the seals are installed.

Now from this point onward these MTT tanks do represent the best in class, for tanks reaching minimum emissions and a maximum level of safety, allowing many decades of trouble free operations against minimum maintenance costs!

Mitigation

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