Team Towing

Much has been written recently about the projected growth of worldwide waterborne transportation. In the United States alone, some experts are projecting that waterborne commerce is expected at the least to double over the next two decades.

Crowley Harbor class tugs demonstrate Team Towing while performing a Power Indirect move to starboard to generate a turn to port during live testing in the Port of Long Beach with the tanker S/R Long Beach.

Many U.S. ports are planning to improve port access and expansion in response to these projections. Some have begun significant dredging projects to deepen and widen channels to accommodate larger, deep draft vessels more safely. As some studies support new port features, others are being conducted to examine the ability to meet these future demands using existing equipment and support services.

One such evaluation was conducted in the Port of Long Beach, Calif. The purpose of this article is to share what has been learned at this port. With channels dredged to accommodate a safe draft of 62 feet (19 m), the local pilots recommended that the maximum vessel size be increased to an LOA of 1,200 feet (365 m), 200 foot beam (61 m) and draft of 62 feet (19 m) due to other channel restrictions.

The operational challenge was how to control a deep draft ship safely in the unlikely event that it experienced a loss of power or steering during its inbound passage. Of particular interest to the escorting mission in Long Beach is the fact that after passing the Long Beach breakwater, the ship must make an immediate 47¼ turn to port into the Long Beach Channel before maneuvering through the 1,200-foot wide, two-mile channel.

Considering the highest horsepower tractors in Long Beach are Crowley’s Harbor class with Voith Schneider cycloidal drive propulsion (VSP) rated at 4,800-hp, the pilots, tug operators and industry representatives worked together to evaluate a unique approach to increasing the effective use of these tugs.

California escort regulations for Los Angeles/Long Beach require that the escort tugs must be able to:

  • Turn a ship, traveling at an initial speed of 5 knots, 47° to port within a 4,000 foot (1,220 m) head reach limit.
  • After completing the turn, stop the ship from a minimum of 4 knots within a 4,000-foot (1,220 m) head reach limit when tractor type tugs are employed because the speed will decrease while turning. When conventional tugs that steer by pushing are utilized, then a minimum speed of 5 knots is used since the speed will not decrease during the turn.
  • (For simulations) tethered tractors are to use a standard 15 second/15 second failure recognition/response time delay and a 30-second ramp up to full power.
  • The Harbor Safety Committee rules restrict the number of escort tugs to two in the LA/LB complex.

    Preliminary Evaluation

    Simulators were used to evaluate how to safely control a large vessel that experienced a hard right rudder failure while approaching the Long Beach breakwater. Seattle-based Glosten and Associates used their Shipman simulator to explore the performance variations of using one, two and even three 4,000-hp tractor tugs applying maximum towline forces at various positions on a 211,000 dwt vessel with a draft of 61 feet.

    The simulations described below were run at 5 and 6 knots and used the tugs in two different configurations. First, to oppose the ship’s rudder and return the ship to its original course. Then make the required 47¼ course change to port. The second exercise deployed the tugs in a way that would accelerate the ship’s turn to execute a successful round turn in case the rudder failed outside the breakwater or before entering the dredged channel. For simplicity’s sake, we will discuss only the 6 knot tests, where the tugs were used to oppose the initial turn to starboard caused by a hard right rudder and then complete the 47¼ turn to port, as they are representative of the relative performance of the tugs used in these positions. The initial set of simulations used a generic VSP tractor rated at approximately 50 tons (100,000 lbs.) of static bollard pull.

    Case A

    In Case A, a single tractor made fast at the centerline of the transom was used. The single tractor simulations were run to establish a baseline to compare the relative performance of the other positions and combinations. Using the single tractor, an off-track error to starboard was noted of approximately 700 feet (210 m) with an advance of 8,100 feet (2,470 m) to complete the required 47¼ turn to port.

    Case B

    In Case B, one tractor was made up through the stem and the second at the centerline of the transom. Using the two tractors fore and aft, we noticed a significant improvement was cited with an off track error to starboard of approximately 250 feet (75 m), but the advance to complete the turn required 7,700 feet (2350 m). Because, in theory, the pivot point of the ship has moved forward with headway, the tug at the stem has a much smaller lever arm to the pivot point than the stern tug and is therefore less effective.

    Case C

    In Case C, two tractors were made up at the outer corners of the ship’s transom. Using the two tractors aft, the noted off-track error continued to improve to approximately 200 feet (60 m) and the advance was significantly improved at 3,700 feet (1,130 m).

    Case D

    Finally, in Case D three tractors were made up at the stem, starboard quarter (because the port quarter is usually the weather quarter which precludes a tug pushing in this position) and centerline of the transom. It was recognized that the tug at the quarter could only help oppose a starboard rudder failure or turn the ship to port. Using three tractor tugs in this configuration, an off-track error to starboard of approximately 200 feet (60 m) (equal to Case C) was noted but the advance increased to 4,300 feet (1,310 m). The increase in the advance in this case vs. Case C is caused by all three tugs applying steering forces only to the ship.

  • By Professional Mariner Staff