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Shell Prelude Meltdown: Something went very badly wrong here…

Shell Prelude meltdown: Something went very badly wrong here…

EXPERT COMMENT ABOUT PRELUDE MELTDOWN BY “AN  OLD SHELL E&P ENGINEERING SEA DOG”

With reference to cargo loading operations at Prelude.

It’s been a while since I was involved with these activities but the principles remain the same.  As always there have been many revisions with the objective to improve overall safety for people and property.

Prelude in particular is unique as it exports three differing products with very different properties. 3.6 million tons of LNG, 1.3 million tons condense and 0.4 million tons of LPG.

Condensate

Condensate is a liquid derived from initial gas processing, in other words a light crude oil with a specific gravity, dependant on the field being produced, between 0.74 & 0.82.  The Reid Vapour pressure will be closely controlled in the region of 12.5, any higher the tanker will encounter difficulties whilst discharging the cargo.  Prelude condensate has so far been exported in designated crude oil tankers. Prelude condensate is discharged via the stern and not alongside. Crude oil tankers are provided with inert-gas systems therefore during loading of condensate only the inert vapour contents of the cargo tanks will be displaced therefore should be a very low risk of any hydrocarbon vapours being detected on board Prelude.

LPG

LPG carriers are available in several categories and as Prelude stores these gases as a refrigerated liquid the vessels used to load LPG’s are likely to be in the mid-size (25,000 – 50,000 cubic meters range which are fully refrigerated and used intra regional delivery designations or the VLCC of about 70,000 cubic meters or above used for long haul destinations. Either vessel of this class is provided with vapour recovery and liquefaction equipment. There will also be a fully operation method of inerting equipment provided.

The photograph below is an example. Just aft of the forward mast there are a series of  what may be pressure relief vent stacks, further aft infront of the accommodation is the vapour recovery and re-liquefaction equipment

LPG cargo transfer takes place alongside Prelude.  An extract from Prelude terminal Operations manual.

All three MLAs will be connected onto the LPG carrier. One MLA for butane loading, one MLA for propane loading and a vapour MLA will be connected to the carriers vapour manifold.

Boil-off vapour management for the LPGC cargo tanks should be handled onboard the LPG vessel using her own reliquification plants. However, the primary reason for connecting the vapour arm is to be used as a safety relief if the carriers tank vapour is close to an uncontrolled release through the PRVs. If vapour is required to be sent to Prelude this vapour will be directed to the flare.

It can be seen that there should be no hydrocarbon vapour release from the LPG carrier whilst alongside during loading operations. Any small vapour release could possibly take place during the disconnection of the MLA’s other than that there should be no release to the atmosphere. So as Bill Campbell has rightly observed, why and how did this dangerous occurrence happen, but it did.  So did the safety systems on board Prelude function as design, yes they did, gas was detected fans stopped and inlet dampers closed and all the correct shut down systems functioned as per design. But, and its a big but, Prelude is able to export Butane and Propane separately and both these products in gaseous form are heavier than air ( Butane SG 2.00. Propane SG 1.55). This of course raises the question what volume of gas was released from the LPG tanker?  Methinks quite lot.  If we look at the differing sizes of the Prelude to the LPG tanker then it can be seen that the accommodation intakes are much higher than the vents fro the LPG taker.  Not only was this incident of great concern but should never have happened.  Something went very badly wrong here.

This link goes straight to Prelude Terminal Procedures for LPG cargo handling.

Prelude FLNG Terminal Information Book – LPG – Shell in …https://www.shell.com.au › par › tab_3782 › prel…

I’ve not covered LNG as most of the glitches have been resolved.

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