Mr Wotherspoon said that he had previously put a temporary repair on the leak, but may have taken a different course of action if he had known the pipe had contained potentially hazardous material.
He had been working on the rig as a mechanical technician at the time, when he was asked to go to the utility shaft to check out a leak.
The platform's operations supervisor and another colleague were there and Mr Wotherspoon said he could see an absorbent cloth collecting the water from the leak, which had been caused by pipe corrosion.
He decided to make a temporary repair patch using clips and rubber.
'Failures in duty'
He said he was told by the operations supervisor that the pipe contained "oily water".
"I was not told there was a potential of hydrocarbons coming through the line," he said.
He was asked by Procurator Fiscal Ernest Barbour: "If you had known that this was a hydrocarbon service, would you have regarded that as being an appropriate kind of patch to put on?"
"No, definitely not," he said.
Earlier, Shell lawyer Stuart Gale QC made an opening statement to the FAI expressing sympathy to the families of the men.
Mr Gale said: "That expression of sympathy is made now, as it was at the trial, in the knowledge that certain failures in duty on the part of Shell led to the events which caused the deaths.
"I wish also to emphasise at the outset of this inquiry that Shell will not depart in this inquiry from the rationale which led to the decision to plead guilty last year.
Tragic events
"That involved an acceptance of certain deficiencies at a corporate level."
He said that the inquiry's task must be to indicate the lessons learned from the tragic events and to set out the steps taken by Shell to minimise possible repetition.
Mr Moncrieff, 45, from Invergowrie, Tayside, and Mr McCue, 22, from Kennoway, Fife, were working on the platform 116 miles north-east of Lerwick, when the incident occurred.
Shell admitted breaching three health and safety regulations and received what was thought to be the biggest fine given to a company following a North Sea accident.
The inquiry also saw video footage and photos from the platform's utility shaft.
The hearing could last up to three months.
