The Potential for a Mega-Sized Tug-Barge on the Upper Great Lakes

The inland ship transportation industry moves significant volumes and tonnage of bulk cargo on the Upper Great Lakes. While the tugs push and navigate barges of 740-feet length across the Great Lakes, there may be prospects of sailing even larger tug-barges in the region while making minimal changes to the locks between Lakes Superior and Huron.

Introduction

The Great Lakes shipping industry has at various times modified vessels to carry great volumes and tonnage of payload. During the early years of the 20th century, one shipping company removed boilers and coal storage from a vessel that they converted to sail propulsion, to increase payload capacity. Other companies would use a steam-powered ship to tow a former sail-driven ship in order to increase payload capacity. During an even later era, ships were converted to barges after removing engines, fuel tanks, and crew quarters, and large tugs would push and navigate the former ships across the Great Lakes.

Ships sailing between Lakes Superior and Huron need to transit a navigation lock of 1200-feet interior length. Articulated vessels built to lengths of over 1200-feet would need to uncouple to transit in a sequence through the same navigation lock. Any initiative at converting a ship of 1,000-feet length to a barge would require much research into articulated vessel dynamics and vessel structural integrity. Wave conditions would likely require the development of a coupling mechanism capable of relative vertical movement between the stern of the barge and the bow of the tug.

Precedent and Research

The precedent of tug-barge operation on the Upper Great Lakes involves barges built to 740-feet length by 78-feet width (the maximum for the MacArthur Lock). Structural research would determine the necessary modifications and reinforcements required for a vessel built to 105-feet width by over 1,000-feet in length (the maximum for the Poe Lock) to operate as a barge. The coupling mechanism between the barge stern and tug bow would be subject to repeated vertical stress loads caused by waves inducing vertical forces along the length of both hulls. Future structural research would determine as to whether or not to include relative vertical movement capability into the tug/barge coupling.  

Navigation Lock

An over-length articulated vessel would split in the navigation lock after the tug has pushed the barge into the lock. Cables would attach to restrain the barge as the tug would uncouple from and then reverse from the lock. Both tug and barge would need to make the transit simultaneously transit involving both the MacArthur and the Poe locks. If the tug is under 50-feet width by under 240-feet length, it might be possible to negotiate with Canadian authorities for the tug to make the transit between Lakes Superior and Huron via Canada, in both eastbound and westbound service.

After the cable restrained barge has changed elevation, the tug would tow the barge and sail for a short distance before mooring it at dockside. It would uncouple from the barge bow and couple to the stern before continuing its voyage.

Locks Tug

There is the option for a tug stationed at Sault Ste. Marie to pull a mega-size barge from the locks. Its presence would depend on the number of mega-size barges that enter service on the Upper Great Lakes in the years ahead, as well as the headway between ships that need to transit through the locks. A generous headway between ships would allow a mega-barge and a mega-size tug to transit via parallel locks simultaneously and without causing undue delay to other ships. A close headway between ships would require a tug assigned to the locks.

Cost Aspects

The conversion of a full-size vessel that sails the Upper Great Lakes from ship to barge, would increase payload carrying capacity and increase earnings for the ship owners and especially when sailing between Chicago and ports located around Lake Erie. However, sailing between Duluth and ports around Lakes Erie and Michigan would result in the tug-barge incurring higher transit fees from dual lock transit for both tug and barge. The delay would include transit duration for both tug and barge along with duration to uncouple from the barge and then re-attach to the barge.

The economics of operating a mega-size barge on the Upper Great Lakes would depend on the increased earnings that result from increased payload capacity along with savings in fuel cost per ton-mile for the season, being able to exceed the higher cost of making the transit between Lakes Superior and Huron. While iron ore accounts for a large proportion of the bulk freight that sails across the Upper Great Lakes, a mega-size barge would be especially suitable for carrying the lower density bulk freight that requires additional onboard volume, such as agricultural produce, fuel oil and compressed gas.

Tug-Barge Coupling

Structural analysis would determine stress levels at the coupling between a barge of over 1,000-feet length and a tug of 230-feet to 350-feet length. It is possible that coupling mechanism might need to provide for relative vertical freedom of movement, to reduce bending loads on the barge superstructure as waves induce pitching motions. The coupling mechanism might combine the proven transverse hinge with triple sets of vertical rails that enclose two sets of double-flange railway-type wheels between them, to transmit forward and reverse thrust loads while allowing relative vertical movement between barge stern and tug bow.

Temporarily locking the vertical mechanism upon departure from port would allow for acceleration while preventing the bow of the tug from lifting upward. Once underway, the vertical mechanism would be unlocked and be operational as the tug-barge sails through waves, with the stern of the barge and bow of the tug rising and falling at different rates. Research and development undertaken to meet such an objective would likely have application in future ocean service, involving mega-size tugs and mega-size barges assigned to coastal service and short-distance trans-ocean service.

Future Developments

Construction is underway to rebuild the north side navigation lock between Lakes Superior and Huron, on the American side of the border. The project is scheduled for completion in 2029, with the rebuild lock being open to ship traffic either by the summer of 2029 or early 2030. If the Great Lakes shipping industry has any long-term interest in sailing extended length vessels on the Upper Great Lakes, there would be need to undertake research into the economics of operating such vessels and their structural integrity. Without such interest, the navigation lock will remain at 1200-feet interior length.

Conclusions

It may be technically possible to develop a tug-barge where the barge is over 1,000-feet in length. Such a barge would carry increased payload involving lower-density freight such as agricultural produce, liquid fuel or compressed gas. However, the Great Lakes shipping sector would need to decide as to whether such a vessel would find market application carrying bulk freight across the Upper Great Lakes. If there is long-term future interest in operating such vessels, there would likely be scope to negotiate with the Department of Transportation in regard to the interior length of the navigation lock undergoing reconstruction.