Naval Architecture: Designing Efficiency

Cargo Ship

By Todd Vorenkamp 2015-09-11 12:13:02

Much can be achieved on the hardware side, but it’s the human element that really matters.

The new holy grail of shipping is efficiency, yet no single area of ship design can provide extraordinary gains by itself. Instead, designers and operators try to save fuel and reduce emissions incrementally through different aspects of a vessel’s design and operation in the hope that, taken together, real savings can be achieved.

CFD Software

Hull designs were once tested solely in towing tanks. Today they are being increasingly tested in the virtual world through computational fluid dynamics (CFD) software. Revolutionizing the field, CFD software, once the purview of supercomputers and dedicated servers, is now operating on networked servers in small offices. However, it has not completely replaced the tow tank.

According to Darrel Harvey of Alan C. McClure Associates (ACMA) in Houston, naval architects are “getting more comfortable with the analysis capabilities of CFD software.” But tow tank analysis is frequently used for additional validation and comparison.

He explains that “By optimizing the flows in and around the hull by eliminating areas of high turbulence, orienting appendages more in line with the flow of water they will see and optimizing propeller flow, we estimate we can reduce fuel costs in some cases by as much as five percent.” He says an added benefit of optimizing hull design and flow patterns is the reduction of noise generated from the propeller wakes and rudder interaction, which in turn will reduce repair costs to the aft equipment and hull structure.

Robert Allan Ltd has used CFD to refine dozens of escort tug designs but also for analyzing barge performance and in some instances has reduced barge train resistance by 10 percent or more.

Johan Sperling, Vice President of Crowley Marine Solutions and subsidiary Jensen Maritime, has put CFD into use extensively in the design of the new Commitment Class LNG-powered ConRo ships that are expected to enter service in 2017. Sperling emphasizes that it was a team effort that went into the design of these trailblazing vessels with Crowley, VT Halter, Wärtsilä, MARIN (Maritime Simulation Institute Netherlands) and Jensen all making important contributions.

Efficiency was the name of the game from the start and, as Sperling puts it, “We over-engineered this thing from the start to save money on the back end.” CFD was used extensively, but the vessel was also tank-tested numerous times for both the hull and props to ensure the design was optimized.

Technology Associates of New Orleans recently used CFD technology to refine the hull form of their EnviroMax line of ships. Along with an efficient hull design that gives vessels a speed advantage, the idea behind the EnviroMax concept is to maximize cargo volumes in a multipurpose hull that can be used for different offshore support and cargo roles. With a standard hull type, risk is reduced and production simplified, thereby decreasing unit cost.

Nick Boksa of Boksa Marine Design in Lithia, Florida says that CFD and computer modeling are proving beneficial not only in hull form efficiency but also in reducing production costs. Before computer design, he states, when it came to installing systems, “It used to be that the guy who gets there first gets the spot. The next guy has to work around what has already been installed.” Now, systems integration can be planned long before metal is cut in order to accommodate the engineering needs.

Sperling noted this as well and, especially in a Jones Act build, where labor costs are high, designing the ship to be built efficiently with computer-design technology is a huge cost-saver. He added that saving man-hours at the shipyard level is the most important opportunity U.S. shipbuilding has to be become more competitive with the rest of world.

Aerodynamics

The demand for fuel savings and enhanced maneuverability has led naval architects to take a closer look at aerodynamics. The Nissan car carrier City of St. Petersburg features a forward superstructure that looks as if it was inspired by an Airstream trailer’s graceful curves. The aero-efficiency is said to save 800 tons of fuel annually and 2,500 tons of CO2 emissions.

ACMA’s Harvey says that CFD software can easily look at the aerodynamics of what is above the waterline as well: “The software is very versatile, and we can make changes to our model quickly and see the results in short order.” Aero can give efficiency gains but is not a place where large savings can be had for every type of vessel.

Sperling adds that aerodynamics factored into the design of Crowley’s Commitment Class vessels but, due to the non-aerodynamic nature of containerized cargo, there was only a limited amount to be gained above deck in this area.

Technology Associates Anil Raj says that aerodynamics are an important factor in high-speed craft design like fast patrol boats and fast ferries, whose speed can reach 50 knots. At higher speeds air resistance starts becoming a significant factor. After CFD analyses on the above-waterline shape have been done, wind tunnel technologies come into play.

Propulsion Pods

On smaller vessels, propulsion pods like the Volvo Penta IPS and the Cummins MerCruiser Zeus achieve efficiency gains over traditional shaft drives. Nick Boksa says both designs have been shown to give savings of around 25 percent.

The pod system directs the thrust level to the vessel’s bottom as opposed to at an angle in a shaft-drive setup. Customers have the option of mounting engines with less horsepower than is required with traditional drives or simply running similar horsepower engines at lower power settings to achieve the same speed.

For larger vessels, azimuthing stern drives or Z-drives have made their way from harbor and river craft to large cruise vessels. The Shearer Group of Seabrook, Texas, recently completed a study of the efficiency gains of Z-drive propulsion on inland rivers. The company was contracted by Southern Towing to design a fleet of 3,200-HP towboats with Z-drive propulsion. Now that the vessels have been in service for several years, Southern Towing was able to do a comprehensive study of the performance gains afforded when compared to their legacy fleet. Results showed an 11 percent reduction in voyage time and an eye-opening 28 percent reduction in fuel costs.

Hybrid Power & LNG

Ashore, hybrid power is revolutionizing efficiency with automobiles, but the same technology is not a great fit for the marine environment because, as Boksa says, “A ship is always going uphill.” There is no time when a vessel is coasting or operating in a manner where it can generate and store electricity for propulsion.

Robert Allan adds, “In recent years we have studied in detail every potential alternative for marine propulsion systems on tugs. While hybrid alternatives show promise due to the typical tug duty cycle, the simpler electro-mechanical hybrid systems make more economic sense than those using stored energy.”

Cory Wood, Vice President of the Bristol Harbor Group, says the new trend toward LNG power “is going to keep us busy for the next 20 years.” His firm is currently finalizing the design for the first LNG bunker barges in the U.S. Both Wood and Boksa mentioned that the major obstacle to the expanded use of LNG is the lack of adequate infrastructure for LNG fueling.

ACMA’s Harvey states that LNG fuel storage designs have implications when it comes to integrating systems into a vessel. Sperling says that, when it came to sacrificing cargo space on the Crowley LNG ConRo vessels, “It was a simple equation” that indicated the tradeoff of cargo space for projected LNG efficiency was the right way to go.

According to Anil Raj of TAI, the significantly lower specific fuel cost ($/BTU) of LNG provides an appreciable offset versus Grade 2 diesel and other fuels. When the fuel cost advantage is factored in for select applications, the cargo volume absorbed by the LNG installation can be offset.

Monitoring Systems

How do you measure efficiency? Fuel burn is easy to measure, but there is a better way: Vessel monitoring systems that track multiple parameters and deliver reports to the crew and ashore to the company

Technology Associates’ Advanced Energy Monitoring and Conserver (AEMEC) system measures real-time carbon emissions and fuel consumption and can be installed on both commercial and military vessels. It provides the crew with suggestions so they can adjust vessel and operational parameters to optimize fuel consumption for a desired voyage.

Similarly, NAPA Group’s ClassNK-NAPA GREEN system combines voyage optimization and vessel monitoring to provide operational cost savings. Such systems are crucial for the naval architect as, according to NAPA, “Vessels are traditionally only tested for efficiency under sea-trial conditions but information about their actual operational efficiency isn’t accessible by naval architects, only vessel owners.”

The People Part of Efficiency

The search for efficiency is never-ending. Some vessels have employed auxiliary sail power, solar panels and air lubrication systems in an attempt to gain savings. Savings are savings, but only small amounts are realized through these measures.

According to Robert Allan, one solution is the operator: “The biggest single factor in reducing the energy consumption on a tug is the throttle setting. Training the master to back off on power except when absolutely needed will produce the best returns.”

Erik Seither, Executive Director of the Society of Naval Architects and Marine Engineers, also sees people as the solution, but one not given as much attention as it should. “Hardware is easier because the people side is much more complex. Operators should look more at efficiency in the fleet they have, not the future fleet.” – MarEx

New York City-based Todd Vorenkamp is a frequent contributor to the magazine.

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