KEY BENEFITS OF HYBRID PROPULSION
- Higher average engine load: The hybrid system runs only the engines/generator sets necessary for a particular operating mode.
- Significant fuel savings: Allowing for low-load operation with one small generator set while the vessel is in standby or position-keeping applications may result in fuel savings of as much as 50 percent.
- Reduced maintenance costs: Operating hours on the main engines are reduced.
- Increased uptime: A swing program allows generator sets to be serviced and tested in the shop rather than on board the vessel.
- Increased redundancy: Engines or motors can power azimuth thrusters, and generator sets or alternators can be used for electricity production. In addition, the main engines can be used to power a water pump for firefighting applications.
- Better low-power maneuvering: Running the electric motors at very slow speeds improves fine maneuvering, particularly for vessels with fixed propellers.
- Improved load response time: The high-torque electric motor supports the main engine with RPM increase.
- Reduced emissions: Higher average engine load results in cleaner fuel burn. (NOTE: This may not be true for all emission types if the main engines use an aftertreatment system but the generator sets do not.)
- Smaller DEF footprint: Because the main engines are not running during the majority of operation, not as many DEF storage tanks are required.
CASE STUDY: HARBOR DOCKING AND TOWING, HOUSTON, TEXAS
Harbor Docking and Towing was seeking a propulsion solution for two new tugboats that will operate in Lake Charles, Louisiana. To help the company make an educated buying decision, Cat Marine and dealer Louisiana Cat entered engine load history data from Harbor Docking and Towing’s existing vessels into the Caterpillar Engine Value Analysis tool to provide an accurate lifecycle cost analysis.
The Caterpillar team also entered vessel requirements, operational and resistance profiles, propulsion system data and power generation/conversion information into Cat Select — a new performance and efficiency evaluation tool that compares propulsion system options.
- Minimum of 80 metric tons of thrust
- Minimum of 12 knots of free running speed
- ABS, no ice
- Meet U.S. EPA regulations for January 1, 2018
- 4,000 operating hours per year
- 50% in standby or position-keeping mode
- 35% in transit mode (70% at 6 knots and 30% at 11 knots)
- 15% in towing or ship-assist mode (65% at 5 knots/30-ton thrust, 30% at 2 knots/60-ton thrust and 5% at 0 knots/80-ton thrust)
Assumptions included twin screw Z-drives, 100kW maximum hotel and winch load, and hull resistance based on model tests of 30m and 80 metric ton bollard pull. Maintenance costs were considered, and the financial investment was distributed over 10 years with a 4% interest rate.
CAT SELECT COMPARISON RESULTS
Using this data, Cat Select generated a report showing the differences in utilization, fuel consumption and total cost of ownership between a conventional diesel-mechanic system and a hybrid propulsion system.
With the conventional system, utilization time for the two 3516C engines and C4.4 generator set is 100 percent. Average load for the two main engines only reaches 12 percent. The hybrid system consists of two smaller main engines (3512Cs) plus three larger generator sets (two C18s and one C7.1). Utilization time on the main engines is just 25.5 percent with average load reaching 52.9 percent.