Gas turbine assembly having an effusion cooled shroud segment with an abradable coating

A turbine casing assembly, comprising an annular seal segment assembly for surrounding the turbine adjacent to the turbine blades. An abradable coating (44) is provided on the inboard surface of the seal segments (40) of the assembly and one or more coolant ducts (52) extend from the outboard surface of the seal segment assembly through respective seal segments (40) and the abradable coating (44) , for carrying a coolant towards the blade tips. One or more annular grooves (54) are formed in the inboard surface of the abradable coating, the or each coolant duct opening into one of the one or more annular grooves.