Two recent reports over the past few months, one a grounding the other collision and both with vessels under pilot’s advice, serve as useful lessons regarding hydrodynamic effects and vessel safety even when an expert is aboard.
In the case of Arklow Raider, she grounded as she passed the bar at the mouth of the River Boyne, Eire.
The Marine Casualty Investigation Board report says: “As the vessel approached the river bar, its speed was reportedly reduced. The data from the port’s VTMS gave a speed over the ground of approximately 5.4
knots between the Green and Bull light marks. The speed was 5.1 knots as the vessel passed Aleria light. The speed then dropped to 4.9
knots. At 19:30 hrs. the course was 053°T at 4.3 knots.
“The predicted time of high water was 19:54 hrs., the grounding occurred 20 minutes before the predicted high water. At 19:34 hrs. the vessel would not respond to rudder commands. The Master used both engine and bow thruster in an attempt to resume the correct course. At 19:35 hrs. the vessel touched bottom, veered to port and ran aground”.
Vessels moving from fresh water to salt water will undergo a change of draft. The draft will decrease as the salinity increases. The vessel will also undergo a change of trim, that is, the forward and after drafts will alter, normally increasing by the stern in a laden condition. The vessel will also be affected by squat as it passes through shallow areas where the under keel clearance is
reduced. For this vessel when travelling at 5 knots the effect of squat can be as much as 0.30 metres.
- The weather, the wind and waves would be close to beam on as the vessel emerged from the relative shelter of the training walls and cause the vessel to roll and possibly pitch.
- The under keel clearance of the vessel, that is the difference between the depth of water available in the channel and the draft drawn by the vessel in its laden condition and the combined effect of squat on the clearance.
- The Master’s knowledge of the port was good and he knew the Pilot well.
- •The Pilot controlled the vessel during the major element of the river passage with the Master taking over as the vessel approached the bar. Thus, the Master was not free to observe what was happening and take evasive action sooner (Refer MCIB report No.58 published on 20/02/2004).
- There were no reports of alarms on the bridge that would indicate either engine or steering gear failures.
The MCIB’s recommends “All Masters, Pilots and Harbour Masters should be aware of the effect that rolling has on increasing the draft of a vessel and take this into account when determining a safe under keel clearance for a vessel. It may be safer to postpone departure especially when speed must be reduced for squat reasons asthe effects of weather are greater at slow speed.
On the otherside of the pond, the US NATSB reports that “the collision of a chemical tanker with a containership in the Houston Ship Channel in October 2011 was likely caused by the inappropriate response of the pilot of the Elka Apollon to changes in bank effect forces as the vessel transited the Bayport flare, causing the vessel to sheer across the channel and into the MSC Nederland, the National Transportation Safety Board said today. Contributing to the accident was the combination of the narrow waterway and traffic density at the time, which, when combined with the channel’s bank effects, increased the challenges in a waterway with a limited margin for error”.
Based on its investigation, the NTSB recommended the Coast Guard develop and implement a policy to ensure adequate separation between vessels in two Coast Guard-designated precautionary areas of the Houston Ship Channel and in similarly configured locations. The two specific areas include the site of the current collision as well as another accident less than three months earlier. The NTSB also recommended that these precautionary areas be identified on Houston Ship Channel navigation charts so they are more readily identifiable to mariners.
“Our investigation highlighted safety issues on the Houston Ship Channel, one of the nation’s busiest and most challenging waterways, where the ships are large and the margin for error is small,” said Chairman Deborah A. P. Hersman.
The Greek-flagged chemical tanker Elka Apollon was traveling outbound in the Houston Ship Channel as the Panamanian-flagged containership MSC Nederland was inbound. The pilots on the two deep-draft oceangoing vessels agreed by radio to pass one another just south of the intersection of the Houston and Bayport Ship Channels. The pilot of the MSC Nederland had slowed to let the Elka Apollon pass before turning into the Bayport channel. As the ships were about to pass, the Elka Apollon pilot was unable to correct the ship’s path to prevent the vessel from crossing the channel and striking the MSC Nederland. There were no fatalities.
NTSB’s investigation of this accident, occurring in one of the busiest cargo tonnage ports in the United States and internationally, highlighted the significant challenges that pilots in the Houston Ship Channel face. The channel hosts many large vessels as well as towboats with barges transiting a relatively narrow waterway. Bank effect forces develop and interact with vessels as they proceed along the Houston channel. In addition, insufficient separation between vessels when they turn, pass and overtake one another near intersections can create unsafe situations.
“More active and precise monitoring of vessel traffic and guidance by the Coast Guard for mariners on this unique and challenging waterway could enhance safety,” said Chairman Hersman.