During routine Bop testing, the high pressure mud pump(pump#1) was being used to flush the lines and fill Bops / riser with water in preparation for performing the first pressure test with the Cup Type Tester.

The operation of the hand-throttle (on the driller's SCR control panel)yielded erratic response of the pump's stroke counter (at driller's instrument panel) with zero indications of pressure response on any of the pressure gauges at the driller's console, the HP standpipe manifold and/or the remote control Swaco choke panel.

A 'bang / shockwave' was heard and felt, as if the mud pump's safety relief valve had tripped.

Inspection of the scene of the incident revealed that 4" discharge valve in the HP mud delivery line to the drill floor was in fact closed it was furthermore observed , that the mud pump's (manual reset type) relief valve had failed to trip . The immediate pressure build - up resulted in failure of (splitting) the middle cylinder / fluid end module