200101 Why Did This Collision Occur?

Why Did This Collision Occur?
Report No. 200101

I was serving as Chief Officer on an medium sized oil tanker which was engaged in lightening operations. We were steaming at about 5 knots on a parallel course to a VLCC (the "mother vessel") with about 40m between us. There was a safety tug employed who was steaming about two miles ahead of us and transmitting SECURITÉ messages about the operation on VHF Ch. 16.

A cargo vessel was heading on a north westerly course about 4nm off on our port side and was crossing our course. The safety tug called the cargo vessel on Ch. 16 and asked the OOW to pass astern of us. The OOW on the cargo vessel agreed and confirmed that he would alter course to starboard. About 8 minutes later, when we were only 30m from the "mother vessel" and maintaining the same speed, it was observed that the cargo vessel had not altered course or speed.

Our Mooring Master checked the ARPA and ascertained that the cargo vessel was going to cross our bow at a distance of 0.4nm. The Mooring Master decided to maintain our course and speed and let the cargo vessel continue crossing the bow. About 4 minutes later the cargo vessel was right ahead of the "mother vessel". She then started altering course to starboard, probably with the wheel hard to starboard. In less than 30 seconds she hit the bow of the "mother vessel" at a speed of about 18 knots. Our Mooring Master reacted immediately and manoeuvred our vessel out of danger.

As a result of the collision, the "mother vessel" sustained two small holes in her fore peak tank. The cargo vessel was severely damaged along the port side including the bridge wing. Can anyone give a reason for the cargo ship's strange manoeuvre?