While doing a pressure test on the stand pipe, a leak was detected at approximately 39 feet (12 meters) from the rig floor. The Tool pusher assigned a crewman to go up and tighten the hammer union joint. The area was cleared as per Standard Operating Procedures (SOP). Once the adjustment was finished the worker began to come down out of the derrick. At the same time as the worker was coming down, the rest of the crew began normal operations on the rig floor entering the cleared area. At about 23 feet (7 meters) from the rig floor, the worker lost his grip on the 4 pounds (2kg), sledge hammer (which was covered with mud). The hammer fell, striking a cross section brace, changing its direction and then striking a worker on the back section of his hard hat, producing a lesion and cracking his hard hat, producing a lesion.