A mudmat for a subsea pipeline accessory has a body with an accessory support structure arranged to lie underneath and support an accessory. At least one foundation element attached to the body is movable from a stowed position at least partially under the accessory support structure into a deployed position extending laterally from the accessory support structure after overboarding the accessory from a vessel and before landing the accessory on the seabed. Two or more foundation elements may be deployed simultaneously in laterally-opposed directions transverse to the longitudinal axis of a pipe that supports the accessory.