My kakis in the industry says that most of the safety procedures and protocols like the mandatory Risk Assessments were just lip service and wayang meetings to generate more documents that doesn't really make the work any safer. The members of these meeting are generally the lowest ranking blur fucks that can't be useful anywhere else. So they are intellectually inadequate for real safety discussions.
My friend say for incidents like these, a proper gas test, ventilation, sufficient air displacement, continuous monitoring and calculations on dilution and rates are performed. It can be very technical and not many engineers know how to do it.
But he as an experienced gas tester knows not to disturb sludge as that will release potential harmful gases. If necessary he says the workers must put on breathing apparatus.
His suspicion is inadequate gas monitoring and workers not briefed not to disturb the sludge.
I can only surmise that there is plenty of incompetent people working in Singapore.