C8/​1998M Grude Oil Tanker mt NATURA, Ground­ing in Gulf of Fin­land on 13.10.1998

31.03.2000

The grude and product tanker mt NATURA owned and operated by Finnish oil company Neste Oy was on her way from the port of Ventspils in Lithuania to Sköldvik oil harbour in the southern coast of Finland on the early hours of October 13th 1998. She was fully loaden with 87 715 metric tons of grude oil. As she approached the destination under pilotage by a Finnish pilot using the 15 metre fairway leading to port, the master was acting as the officer of the watch.

NATURA was steered using the integrated bridge system. A normal heading change in the fairway of 37 degrees to heading 005 degrees was made. About two minutes after this change of heading at 03:06 hours the clutch connecting the other main engine to the reduction gear opened due to a disturbance input voltage to its logic controller.

The main engine in question went into over-speed and consequently its own automatic control initiated an emergency stop. The stopping of a main engine activated the controller of the controllable pitch propeller (cpp) to steer the pitch to zero. As the vessel had twin engine installation, the other main engine was still running normally. This engine was also still connected to the propeller via the joint reduction gear. The power reserve of the other engine was thus still available to the propeller. The vessel, however, lost its manoeuvrability, because the rudder became inefficient due to the loss of water flow from the propeller. The zeroing of the propeller pitch was not observed onboard during the accident. It was found in technical checks carried out several days after the accident.

With the loss of manoeuvrability NATURA started a port turn out of the fairway area. The master tried to steer the vessel both with manual control of the rudder and the engine telegraph. The propeller did not respond to the control. The vessel turned about 150 degrees and grounded on a shoal. At the end of the turn the master initiated the emergency drop of the anchors. The ship speed was reduced to 3 to 4 knots before grounding thus limiting the structural damage to the double bottom. There was no leakage of oil to the sea. NATURA was refloated 26 hours later by transferring part of the oil cargo to another tanker.

In the accident investigation several defences within the human activity were observed concerning crew, ship owner, shipyard and administration. Had they been in place and functioned, the master and other crew members could have analysed the technical failure correctly. There was two and a half minutes to counteract the turn, which was leading the vessel out of the fairway. The re-control of the manoeuvrability could have been gained by steering the propeller by its back-up control.

The observed defences are the design and testing of the propeller control logic and its alarms, information system presenting the propeller status on the bridge, emergency procedures and task allocations between the master, mate and the pilot, communication during the failure sequence between the engine control room and the bridge as well as the rules and their control by the administration.

The structural defence of the vessel i.e. the double bottom prevented the accident to develop into an environmental catastrophy.