The incident occurred in September after gas leaked into the utility leg of the rig, where the men were inspecting a temporary repair to an oily water drainage pipe.
The Scotsman newspaper reported that an internal inquiry by Shell found that the temporary repair was inadequate. In addition it was found that a series of valves designed to keep extracted gas separate from drainage water failed, causing a build-up of the gas condensate in the utility leg.
Jake Molloy, the general secretary of the Offshore Industry Liaison Committee, the offshore workers' union, told the Scotsman: "Shell has admitted that the patch on the pipe was not up to the job, and identified a failure in the system which allowed it to be used. The valves designed to isolate gas extracted in the oil process from oily water failed to do their job.
"The report found that this allowed gas to migrate into the oily water, and when this combination leaked out of the damaged drain-pipe and met with the air, it turned into the gas condensate, and it is this gas which took the men's lives.
"The men who died had no perception that there was a risk that the gas and oily water could mix, and the lesson is that the workforce must be made aware of this possibility by employers."
"Shell have been very thorough in their report and put their hands up and said that they have identified a number of shortcomings in management practice, such as risk assessment, hazard identification, supervisory systems and prioritisation of safety critical maintenance."