An apparatus for washing solid particles collected from a hydrocarbon-containing fluid produced from an oil and/or gas production facility, the apparatus comprising: a vessel having a lower part defining a volume for collecting a bed of solid particles and an upper part defining an exit for liquid from the vessel, a pipe assembly located in the volume, the pipe assembly having a plurality of outlet nozzles located thereon, a pipe network connected to the pipe assembly and arranged to supply pressurised steam or water or a mixture thereof to the pipe assembly to cause the steam or water or a mixture thereof to exit the outlet nozzles into the volume, a fluidising flushing unit located in the volume, the fluidising flushing unit comprising a fluidisation head, an inlet for supplying fluidising liquid to the fluidisation head and an outlet for conveying a fluidised mixture of solid particles and fluidising liquid from the fluidisation head and out of the vessel, an oil content sensing system connected to the outlet of the fluidising flushing unit, the oil content sensing system being adapted to determine whether an oil content of the fluidised mixture or any constituent thereof is below a preset oil content threshold, the oil content sensing system having a first exit communicating with a recirculation pump for recirculating the fluidised mixture back to the vessel and a second exit communicating with an outlet conduit for outputting the fluidised mixture; and a controller for selectively controlling the fluidised mixture into a recirculation flow through the recirculation pump or an output flow through the outlet conduit in response to the oil content determined by the oil content sensing system. a vessel having a lower part defining a volume for collecting a bed of solid particles and an upper part defining an exit for liquid from the vessel, a pipe assembly located in the volume, the pipe assembly having a plurality of outlet nozzles located thereon, a pipe network connected to the pipe assembly and arranged to supply pressurised steam or water or a mixture thereof to the pipe assembly to cause the steam or water or a mixture thereof to exit the outlet nozzles into the volume, a fluidising flushing unit located in the volume, the fluidising flushing unit comprising a fluidisation head, an inlet for supplying fluidising liquid to the fluidisation head and an outlet for conveying a fluidised mixture of solid particles and fluidising liquid from the fluidisation head and out of the vessel, an oil content sensing system connected to the outlet of the fluidising flushing unit, the oil content sensing system being adapted to determine whether an oil content of the fluidised mixture or any constituent thereof is below a preset oil content threshold, the oil content sensing system having a first exit communicating with a recirculation pump for recirculating the fluidised mixture back to the vessel and a second exit communicating with an outlet conduit for outputting the fluidised mixture; and a controller for selectively controlling the fluidised mixture into a recirculation flow through the recirculation pump or an output flow through the outlet conduit in response to the oil content determined by the oil content sensing system.