At 1515hrs, i have anchored the tanker at the petroleum anchorage and on the way back to the base, once i arrived at NBCT 6, i saw the tug boat was swung to left and all in sudden she sunk, i called the port control to inform the accident and rushed to help. Managed to save the tugboat captain and two of his crew. The pilot was rescued by the other boat. According to the Pilot, once cleared from NBCT 4 with the tugboat and barge were facing north, due to his justification, the current was slightly flooding with calm sea, the distance to shift to NBCT 2 was not far, he ordered the harbour tug to pull the barge slowly with astern movement. He also ordered the captain to ensure the tugboat to maintain position of facing north with slack towing line. He did not know what had happen when the tugboat started to swing to left with huge slanting. He ordered the harbour tug to stop pulling and released the towing line asap but the the end rope was too tight and end tied. Suddenl...