Carbon Emissions and Raft-Based Ship Designs

log raft
file photo: log raft

By Harry Valentine 2017-07-30 00:05:49

While many countries presently seek to reduce carbon emissions from the maritime sector, that sectors has been steadily reducing carbon emissions per ton-mile for more than a century. Efforts at further reducing such emission would require ongoing research into articulated and coupled vessel configurations to improve the sailing capability of such vessels through rough seas.

Reducing Maritime Carbon Emissions

Since the era of the early coal-fired steam ships, the maritime industry has steadily reduced carbon emissions. The final steam powered ships either used multi-stage steam turbine engines or three-stage piston engines that operated at much higher efficiency than the early steamships. Ongoing improvements to propeller design contributed to steadily improved ship propulsive efficiency. Diesel engine thermal efficiency has improved from around 30 to around 50 percent for the latest engines. The steadily increasing ship size and tonnage has steadily reduced the amount of fuel that larger ships consume and steadily decreased carbon emissions on a per-ton-mile or per-container-mile basis.
 
Raft-Based Ship

While the largest bulk carriers and largest container ships are considered to be near their structural design limits, Freedom Ship of Florida used a raft-based construction approach to design a concept super ship of 4,000-foot length. Raft-based construction could form the basis of Suez-max and Cape-max container ships built to lengths of 1,600 to 2,000 foot (480m to 600m) with comparable draft and overall height as present generation large container ships. Raft-based construction may be able to lengthen Seaway-max container ship to 1,000 foot length if Seaway authorities were willing to lengthen the navigation locks to transit such a vessel.

Tug propelled and navigated raft-type vessels of barges coupled four-abreast (140ft beam) by 10 lengthwise (2,000ft) regularly sail along the meandering Lower Mississippi River, between New Orleans and Memphis. Upstream of Memphis, tug propelled and navigated coupled assemblies of 12 to 15 barges sail through the river sections that have 1,200-foot length navigation locks. There may be scope to develop coupled raft-like assemblies of longer, wider and deeper draft barges able to sail through larger waves encountered in coastal ocean service between the islands of Malaysia, Indonesia and the Philippines, also on the Gulf of St Lawrence.

Extended Articulated Tug-Barge

Articulated tug-barges (ATB’s) that feature a transverse-axis hinge between the tug bow and barge stern regularly sail domestic North American oceanic coastal service, including sailing through 20-foot (6m) Atlantic waves. While articulated tug-barges proved problematic sailing through rough seas on the Bay of Bengal, the combination of larger barges and larger tugs may improve rough seas sailing. Several North American Great Lakes ship owners recently removed engines and fuel tanks from ships that they converted into tug-barges, increasing payload capacity and related earnings with minimal increase in fuel consumption and overall operating costs. 

Maritime researchers at University of Michigan modified the tug-barge concept by replacing the tug with a powered ship pushing and navigating a much longer barge. There may be scope to further develop the University of Michigan ship-barge concept using a scale-model that involves a coupling that provides relative roll, yaw and pitching flexibility between leading and trailing sections. 

For oceanic sailing, relative vertical displacement capability of 20 foot (6m) at the coupling would assist in sailing through severe wave conditions. Computer activated bow thrusters on both units would provide directional-navigational control. 

Remotely Controlled Tugs

The crews of present day tug-barges are located on board the tug and navigate the coupled assembly from a raised bridge on the tug. Modern telecommunications technology can allow the crew to navigate a two-section coupled assembly from a bridge located on the barge and especially if the barge were converted from an existing ship to increase payload capacity. Such remote control may be the next step in tug-barge development as it would allow for the operation of longer, higher and wider barges to sail through rougher seas while being pushed and steered by larger and more powerful tugs. 

Super tugs controlled from bridge-equipped barges developed from earlier Panamax ships could initially operate from Nova Scotia carrying containers to Boston and Montreal. Such operation could serve as the basis to develop large ocean capable tug-barges. 

On a smaller scale, a tug-barge conversion of an earlier Seaway-max ship as well as an extended length, raft-based version could be tested between Nova Scotia and Montreal as a possible precursor to extending Seaway navigation locks. There may be scope to develop a two-section barge that a remotely controlled stern tug could propel and navigate on the Montreal – Nova Scotia container service.

Towed Vessels

During the early 20th century as wind-driven sail-powered ships on the Great Lakes neared retirement, some ship owners used them as barges that carried additional cargo and towed by newer steam powered ships. Modern advances in metallurgy, insulated power cable technology and automated navigation can greatly improve directional and navigational control over towed vessels, especially if such vessels included side thrusters and drop-down propellers for rapid speed reduction. The towed vessel could be a coupled barge assembly that will sail part way behind a large ship, then uncoupled to sail to their final destinations.

Towed vessels could operate between Eastern Nova Scotia and Montreal, with smaller towed units sailing to smaller ports. A floating power station powered by any of diesel, LNG, thorium (nuclear), electro-chemical storage battery or thermal battery driving a steam-electric power system may be housed inside a twin-hull, catamaran-style towed vessel. Transferring engine(s) and fuel tanks into a towed unit would increase ship payload capacity while electrically powered azipod (and bow thruster) would provide propulsion and navigation. The propeller backwash would pass between the catamaran twin hulls.

Test/Development Locations

Northeastern United States and Eastern Canada provide a test and development area to sail large coupled vessels. ATBs carrying bulk cargo have successfully sailed on the Atlantic off the coast of Boston, perhaps providing the precedent to sail larger container carrying versions of the technology based at Nova Scotia transshipment terminal. A two-section coupled ship based on the University of Michigan concept could initially sail across the Gulf of St Lawrence on the Montreal – Eastern Nova Scotia service and be further developed to sail across ocean on the Boston – Eastern Nova Scotia service.
  
A two-section assembly would have to be uncoupled at Boston before being docked at quayside. A powered ship towing a computer navigated coupled barge assembly could sail between Montreal and Nova Scotia, with the barges being uncoupled at Montreal for sailing to upstream ports. A towed floating power station coupled behind a ship could sail the Montreal – Nova Scotia service and depending on future oil prices and future battery technology that promises 20,000 recharge cycles, a future grid-scale electrical battery storage technology could store sufficient energy to sail the entire voyage between those ports.

Mississippi vs St Lawrence Rivers

While the St Lawrence River is relatively straight upstream of the Gulf of St Lawrence, the Lower Mississippi River follows a meandering path between New Orleans and Memphis. Extended length barge tows of up to 40 units and 2,000 foot length sail through the abundance of U-curves and extended S-curves. 

Coupled vessels of comparable length should be able to sail along the Lower St Lawrence River with minimal complication. Sections of the Lower St Lawrence River are as narrow as 750 foot (229m), raising concerns in regard to turning around a coupled ship of over 2,000 foot length.

The channel width near Port of Montreal’s main terminal is over 4,000 foot while the channel width to east of the Varennes terminal is over 6,000 foot. Quays at both terminals are parallel to the St Lawrence River and can accept extended length bulk ships and container ships. While navigation locks along the St Lawrence Seaway measure 740 foot length by 78 foot width and 27 foot depth, navigation locks along the Upper Mississippi River measure 1,200 foot length, 106 foot width and transit barges that sail nine foot draft. There is scope to lengthen Seaway navigation locks between Montreal and Lake Ontario.

Further Research

There is scope on a per container basis and on a per ton basis to reduce ship carbon emissions. The combination of switching to a different fuel such as LNG and increasing ship carrying capacity or ship tonnage could help achieve the objective of reduced carbon emissions. Maritime researchers would need to further evaluate future prospects for raft-based single ships, raft-based coupled vessel assemblies as well as extended length two-section coupled ship assemblies. The present availability of redundant Panamax size of ships provides a potentially cost competitive option by which to develop an extended length vessel.

Eastern Canada offers a possible testing ground for extended-length vessel concepts carrying containers and both the Federal Government of Canada and the Provincial Government of Quebec support initiatives aimed at reducing transportation related carbon emissions. However, it is uncertain as to whether transportation officials would be supportive of extended length and super-wide (46.5m) Seaway-max draft coupled vessel assemblies sailing along the St Lawrence River, having previously indicated support for a 44-meter beam vessel built to the same draft, height and length as older Panamax ships, the largest to sail along that river.

Future Mega Ship

The future development of a raft-based or multi-section mega-size ship would depend on the success smaller versions of such vessels in local service. A coupled ship would be developed from a pair or several existing older and smaller designs of ships that may have become obsolete or redundant. An oceanic raft-based ship could involve innovative ways of coupling barges abreast and lengthwise to adapt an already successful river vessel to sailing in rough seas. 

Super-size raft-based container ships may sail the South America – Asia service via Cape Town is they are too large to navigate the Suez Canal. A successful two-section coupled ship developed from modifying older vessel could sail via the Suez Canal, perhaps carrying containers to Port of Newark or the combination of Newark and Nova Scotia transshipment terminal if a section of the coupled super-vessel could be uncoupled. Such uncoupling could occur in the 17-meter water depth located on the storm protected east side of the French Prefecture of St Pierre, with independent sections of vessels sailing the combination of Eastern Nova Scotia and Newark. There may even be potential to develop coupled versions of raft-based super-ships.

Conclusions

Coupled assemblies of vessels that mimic a raft successfully sail along many inland waterways across the U.S. and Western Europe. Research by Freedom Ship of Florida suggests that ocean-capable vessels constructed on the basis of a raft could be built to greatly extended lengths (of up to 4,000ft).

Two-section ATB technology has proven capable of sailing along coastal waters. There may be scope to further enhance the oceanic sailing ability of the concept, to develop much larger two-section oceanic ships capable of carrying bulk cargo and also containers.

Northeastern United States and Eastern Canada provide a potential testing ground to sail extended length vessel technologies that carry containers to a river port (Montreal) with a possible option to sail the technology through ocean water to/from Boston.

There will be a need to widen, deepen and lengthen the tidal navigation lock at Strait of Canso to transit larger vessels that will sail to the St Lawrence River, also to lengthen Seaway navigation locks to transit longer container ships.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.