Written by
Nick Blenkey
The Nationwide Transportation Security Board has issued its report on a January 22, 2023 incident wherein the azimuthing stern drive, or Z-drive, tugboat Mark E Kuebler, owned by Bay-Houston Towing Co. and operated by G & H Towing Firm, collided with the 319,287 dwt VLCC Nisalah, whereas the Saudi-owned tanker was transiting inbound within the Corpus Christi Ship Channel.
The tugboat’s hull was breached by the tanker’s propeller, which was additionally broken within the collision. The collision resulted in practically $7 million in damages. There have been no accidents.
5 tugboats had been dispatched for harbor-assist operations for the Nisalah‘s arrival with the Mark E Kuebler assigned the starboard quarter place. Because the tanker was transferring by the channel at practically 10 knots, the mate of the tugboat determined to show the tugboat round and transit backward to get within the tug’s assigned place. Through the flip, the tugboat fell again towards the strict of the tanker. The tugboat mate elevated the pace and energy to attempt to regain place, however was drawn in towards the tanker by hydrodynamic forces and the 2 collided. NTSB discovered the tugboat had inadequate reserve energy to counteract the hydrodynamic forces because of the transit pace of the vessels.
Because the tugboat tried to maneuver into place, its pace elevated to 11.6 knots—simply 1.4 knots lower than its most rated forward pace. Larger pace reduces the quantity of reserve propulsion energy obtainable to the operator.
Following the collision, the tugboat’s working firm instituted a coverage limiting stern-first landings of tugboats on assisted vessels to speeds of seven knots or much less.
The total NTSB report spells out the teachings realized
LESSONS LEARNED
Hydrodynamic forces between vessels in a channel
As a big ship strikes by a channel, a low-pressure suction is especially robust on the vessel’s quarters close to the inlet facet of the propeller, and hydrodynamic forces enhance exponentially with the vessel’s pace. Subsequently, a small vessel working close to a bigger vessel should keep a protected working distance or have ample reserve energy to counteract the hydrodynamic forces to keep away from being pulled into the opposite vessel and risking collision. If a small vessel should function close to a bigger vessel—akin to a tugboat conducting harbor-assist operations—the operator of the smaller vessel ought to pay attention to the hazards attributable to hydrodynamic forces and, if crucial, keep a protected distance till the bigger vessel slows and the hydrodynamic forces are decreased.
Pace throughout harbor-assist maneuvers
House owners and operators of Z-drive tugboats that carry out harbor-assist operations ought to set pace limits for superior maneuvers akin to stern-first approaches. These limits could fluctuate for various courses of tugboats primarily based on design. Tugboat operators ought to talk these limits to ship masters or pilots in control of the vessels that they’re helping earlier than participating in these maneuvers.
- Learn the total NTSB report on the Z-drive tug collision HERE
