Marine engineering and propulsion company Royston said Monday it had completed the overhaul of diesel generators on board Stena Drilling's semi-submersible offshore drilling rig ‘Stena Spey’.The work saw cylinder heads, pistons, and conrods overhauled as part of a 12,000 running hour service of one Rolls Royce Bergen KVGB12 Diesel generator by Royston engineers onboard the rig while it was located at Orkney…