On August 21st 2021, after lowering the substructure, crew connected mast raising cylinders to the mast and attempt to lower the mast, at 17:30hrs, and during mast lowering (approximately at 80 degree), suddenly mast descended uncontrolled due to failure of both mast raising cylinders. No injuries were incurred. Findings Based on metallurgical laboratory test results, the failure of Driller side mast raising 4 stage telescopic hydraulic cylinder assembly attributed to the fracture of circumferential weld seam between rod eye and stage 4 cylinder, which lead to the loss of cylinder integrity: Further, Weldment fracture of the rod eye and 4th Stage due to the presence / entrapment of slag (confirmed with the help of EDS elemental analysis) forming a discontinuity / potential stress raiser at the weld root side due to poor workmanship. This slag-entrapment or stress raiser at weld root area triggered fatigue cracking. These damages of the welded parts in the cylinder assembly caused compression loading on the gland end of off-driller side cylinder fist stage cylinder which subsequently bulged and fractured.