Inland Workboats Have Much to Gain from Diesel Electric Propulsion

Diesel electric propulsion is not a new concept. Like azimuthing drives, its application to the inland marine market has lagged behind other areas of the marine industry. Diesel electric has gained traction in industries where fuel economy and redundancy of propulsion systems for safety is paramount. In particular, vessels like passenger ferries and offshore supply vessels have enjoyed the benefits of the diesel electric systems. The inland towboat industry shares many of these concerns, and would benefit from the consideration of diesel electric technology as part of a comprehensive engineering design process.

In early 2017, The Shearer Group, Inc. (TSGI), together with the marine division at ABB, embarked upon the study of applying diesel electric technology to the inland marine industry. TSGI originally researched diesel electric propulsion for towboats around 2007, but at that time the technology and price did not make it a feasible option for the market. However, in the last few years, some fundamental industry changes have shifted the equation, making diesel electric a viable option when planning for a new towboat.

Simply put, diesel electric propulsion is a concept that uses an electrical power plant consisting of multiple power generators that develop electrical power to the propulsion plant via the use of electric motors. The electrical propulsion system consists of generators, an electric distribution system, variable frequency drives, electric propulsion motors and a control system.

Mechanical diesel driven systems provide superior efficiency in a very narrow range of operations, typically above 60 percent MCR. As such, it is important to determine the total operational cycle of the engines over a long course of time. Diesel electric improves efficiency of a vessel’s propulsion system by broadening the range of optimal operations and providing more overall efficiency in ranges outside the optimal range of a mechanical drive system.

Under the umbrella of diesel electric propulsion, there are several variants that exist on the market today:

Hybrid systems make use of a mechanical shaft line with a power take off and power take in (PTO/PTI), combined with an electrical generation system. This system combines the benefit of direct drive propulsion through a gear box with the flexibility of a ‘power boost’ from the ship’s electrical system, but also allows for the main engines to run more efficiently by providing electrical power to the ship when the power is not needed for propulsion purposes. When more power is needed, the generators act as motors supplying additional power to the shafts via the PTO/PTI. During lower power operation, the main engines can be clutched out and the vessel can operate on the electrical motor alone, or the main engines can supply propulsion power and also power the PTO/PTI for supply of electrical power for auxiliary loads.

The A/C diesel electric system is the most traditional and common diesel electric system on the water today. It consists of an electric power plant with several generators providing power for all of the loads on the vessel; propulsion, auxiliary, and hotel. Power from the switchboard is provided to the propulsion motors via variable frequency drives in order to allow the motors to operate at virtually any shaft RPM.

The D/C diesel electric system is very similar to the A/C system but provides some improvements in certain areas. The AC current from the generator is rectified to DC at the generator and distributed directly through the switchboards. The DC current is then inverted back as necessary to provide AC power to the propulsion and hotel loads. Direct Current systems are smaller and lighter in general, with fewer parts inside the switchboards. DC based propulsion systems provide the most benefit when combined with some type of energy storage where the DC grid can allow the batteries to provide instant power response to the ship’s electrical system.

When diesel electric systems are combined with energy storage, the benefits of diesel electric systems continue to improve. For typical marine application, the use of Lithium Ion batteries (Li-ion) can help add spinning reserve, peak shaving, and zero emission operations to increase the overall efficiency of the vessels.

To date, most of the focus on diesel electric has bypassed the inland towboat market. However, TSGI has been working with engineers from ABB to develop and fine tune the diesel electric towboat design concept. The impetus for this design effort is the typical towboat’s operational profile. When we asked owners how they operate their vessels, most believe they are between 80 percent to 100 percent load nearly 100 percent of the time. In reality, we found that actual engine data from vessels operating on the rivers was quite different.

Instead of operating at over 80 percent loads the majority of the time (as previously thought), we found that the operational profile of the vessels we investigated spent far less time above 80 percent load than originally thought. For a number of operators on various parts of the inland rivers, the actual towboat spends most of the time below 50 percent total power, with short peaks above 80 percent. This invites opportunity for diesel electric to provide positive impacts on the operational costs associated with running a towboat on the inland rivers.

Vessel operation profiles are very important in determining the suitability of a diesel electric system for a particular vessel. Both sample profiles represent real operational data over the course of 365 days of operation for different vessels.

Diesel electric propulsion provides these savings from a variety of benefits. Propulsion machinery for each 2,400 HP towboat is listed below:

     -Prime Mover: 2 x 1,200HP Tier IV Engines

     -Generator: 2 x 200kW Tier III Generators

     -Power Generation: 4 x 800hp Tier III Generators

For the mechanical towboat, at any engine load from idle to 100 percent you are running at a minimum three engines to maintain full maneuvering plus the house electrical load. The diesel electric however is running an optimized number of engines. From idle to about 25 percent load only one engine is required to be running to provide propulsion to two shafts plus the house electrical load to the vessel. Only at much higher loads (typically > 80 percent) does the diesel electric lose some of its efficiency gains over the mechanical propulsion system.

Diesel electric propulsion systems also provide benefits beyond fuel savings. During the design process, consideration should be given to items like redundancy of propulsion systems, urea consumption and storage, and engine maintenance.

The diesel electric also shines with respect to redundancy and safety. With any engine able to provide power to either propulsion motor, an operator can minimize the impact of a prime mover failure. On a mechanical system, the loss of a prime mover results in the loss of 50 percent of the propulsion and an entire shaft. On the diesel electric towboat, the loss of a generator set only results in the loss of 25 percent of the maximum available power while still being able to provide power to both shafts. Similarly, with the loss of a generator on a mechanical system, the vessel now has no back-up for house and auxiliary loads. The diesel electric system can keep providing power with multiple back-ups for the electrical generation.

Using multiple Tier III generators removes the requirement for Tier IV engines, which use SCR or EGR technology to meet emissions requirements. If a vessel is using urea for SCR, with a typical DEF dosing rate of 5 percent, a new vessel design has to accommodate a urea tank, the SCR, and the additional exhaust piping.

Another benefit involves engine maintenance schedules. By reducing the number of engines running to an optimal number, diesel electric propulsion reduces the number of engine hours spent at partial loads. A vessel that spends 40 percent of its operation time at less than 50 percent propulsion load can keep hours on two engines instead of three when compared to a mechanical driven system. And, because the vessel only has one type of engine on board, the number of spare parts for both the vessel and shore side support can be reduced.

Diesel electric, just like azimuthing drives, may or may not be the best solution for all operational profiles on the river. It does, however, offer distinct advantages in many scenarios and should be considered as part of the overall decision making process when designing a vessel. Benefits extend beyond just operational cost savings by also providing increased operational safety and redundancy for towboats.

Joshua Slade Sebastian, P.E., is Engineering Manager at The Shearer Group, Inc. (TSGI). TSGI would like to thank the Marine Division at ABB for assistance with the electrical design of the concept diesel electric towboat and providing some of the technical information for this article. 

(As published in the March 2018 edition of Marine News)