The Derrickman and Assistant Driller (AD) were working outside the sludge tank in an effort to clear a blocked main flow line to the tank. The flow line was heavily plugged with barite and could not be cleared using only the high pressure washer from the top, therefore the original plan had to be modified. The AD had the Pumpman operate the high pressure washer while the Derrickman entered the tank to use a rod to clear the blockage. Once the blockage was cleared the Derrickman attempted to exit the tank and slipped on the mud at the bottom of the tank and fell against the ladder impacting his chest causing severe bruising. As a precaution he was sent ashore for medical examination, he was certified fit and returned to the rig on normal duties.