Monday, November 20, 2017
Maritime Reporter and Engenering News White Papers
Published: Jul 2017
Authors
  • Gregory W. Beers
    • Gregory W. Beers
    • Company: P.E. / President
Categories
  • Naval Architecture Software

Designing for Efficiency

Designing efficient vessels is the heart of what naval architects do. Efficiency in this case is defined as the ratio of useful travelled distance of goods or cargo; divided by the total energy put into the transportation propulsion means. In other words, a more efficient vessel design will require less fuel (cost) to move a given cargo a certain distance. This paper will explain why efficiency is important and will explore some aspects of small coastal vessel design that can improve vessel efficiency. We will also investigate several of the techniques that are used to analyze a vessel’s efficiency utilizing Articulated Tug-Barge (ATB) units as case studies.

Background

Bristol Harbor Group, Inc. (BHGI) is a full service naval architecture and marine engineering firm located on the harbor in Bristol, Rhode Island. We have been in business for more than twenty years, and have produced more than 100 unique designs to which hundreds of vessels have been built. We specialize in commercial vessel design and consulting and have experience with tugs, barges, passenger vessels, dredges and yachts.

Our team of dedicated professionals hail from some of the best naval architecture schools in the country. The executive team is comprised of Greg Beers, P.E. and Cory Wood, who met while students at the University of Michigan. Greg and Cory incorporated BHGI (formerly FG Marine Design, Inc.) with two other college friends in 1995. The greater technical team includes University of Michigan, Webb Institute, Virginia Polytechnic Institute, University of Rhode Island, and Rensselaer Polytechnic Institute graduates. Our engineers have much real-world experience, enabling us to make sure that our designs are innovative, yet practical.

BHGI provides naval architecture, marine engineering and project consulting services to clients worldwide. Quality is an important part of engineering, and at BHGI, our engineers follow strict document checking procedures. All drawings and calculations that leave BHGI are checked by a senior or principal naval architect and marine engineer prior to release.

We utilize state of the art computer modeling and design practices to develop innovative and functional designs that meet our customers’ diverse requirements. Our engineers also have many years of hands on experience both aboard vessels and in shipyards. This practical experience allows our engineers to better understand both operational and technical issues. BHGI’s marine engineering practice focuses on the design of new vessels, but our engineers also work on repowerings and mechanical and electrical upgrades.

At BHGI, we endeavor to over deliver to our clients. This does not mean that we over think issues, but rather that we deliver drawings with more detail than the client might be expecting, and calculation packages with more depth than the client has seen in the past. We do this because we have discovered that this is our preferred work method, i.e., this is what “floats our boat”.

Through BHGI’s Indefinite Delivery/Indefinite Quantity (IDIQ) contract with the U.S. Army Corps of Engineers (USACE), Marine Design Center, Philadelphia district, BHGI has been tasked with jobs ranging from conceptual vessel designs; to biodiesel and LNG studies; to detail design and analysis of vessels.

Recent experience includes the design of a USACE wicket lifting vessel for operation at Olmsted Lock & Dam on the Ohio River. Currently under construction, the vessel is expected to be delivered in the 3rd quarter of 2017. An interesting aspect of this USACE IDIQ contract is that other federal agencies use the contract vehicle to reach BHGI. A recent example of this is that BHGI will continue to provide design and engineering support to the USACE for the NASA barge PEGASUS. Through BHGI’s first contract with the USACE, BHGI was tasked to redesign the PEGASUS’s hull, lengthening it from 260 ft. to 310 ft. to accommodate NASA’s new Space Launch System.

Other recent experience at BHGI includes the design of several 80,000 BBL oil barges that are part of ATB units for Vane Brothers Company. The barges are being built by Conrad Shipyard and the lead vessel is expected to be delivered in 3rd quarter 2017 with two follow on units at six month intervals. Another recent project is for the New York Power Authority (NYPA). BHGI was contracted to design and perform construction support services for two new tug vessels. The tugs will be used for the deployment, retrieval and maintenance of the Lake Erie ice boom. The construction for the first of the two, the JONCAIRE II, has been completed and NYPA will soon be soliciting bids for the construction of the B-2 BREAKER tug.

History

Marine transportation is exceptionally efficient to start with. When comparing inland barge transportation to trucks and rail, the differences are stark. One train operator, CSX, runs a wonderful advertisement that notes that trains are four time more fuel efficient than trucks1. However, they fail to mention that barge transportation is even more efficient. The advert speaks in terms of ton miles, noting that one ton of freight is moved 471 miles on one gallon of fuel by CSX trains. However, a report for the U.S. Maritime Administration and the National Waterways Foundation2 notes that inland towboats and barges move one ton of freight 576 miles on one gallon of fuel. Both modes primarily use medium speed diesel engines as prime movers, so the comparison is quite valid. Although this paper does not focus on inland barges and towboats, a white paper by our sister company, The Shearer Group, Inc., slated for later this year will focus on similar efforts in that market, so stay tuned!

Historically, vessels have been either fast or efficient. The two attributes often oppose one another. In this paper, we will focus on efficiency, not speed. There are many things vessel owners do operationally to increase efficiency, such as slow steaming, coating the hull with slick bottom paint, and keeping the hull clean. Additionally, there are many aspects of a vessel’s mechanical design that can improve efficiency such as the use of diesel electric and / or battery technologies. When hunting for efficiency, these other avenues are ripe with opportunity, especially as the U.S. fleet wrestles with the implementation of Tier IV complaint engines. However, this paper will focus on the some of the things that naval architects can do to simply create more efficient hull designs rather than operational or mechanical improvements.

Techniques

First Principles

Naval architects have been working to reduce the resistance of vessels (a.k.a. increase the efficiency) for centuries. A simple truth about marine transportation that is well understood is the efficiency of scale. Marine vessels can generally move more cargo at the same speed with less power per ton of cargo as they grow in size. Further, the lighter the part of the ship that is NOT cargo (i.e. the lightship weight of the vessel), the more tons of cargo the ship can move at a given speed for a given horsepower. Therefore, lighter vessel structure and minimizing the use of ballast both contribute to the efficiency of a vessel.

These first two concepts are simple and easy to understand. A large light vessel is more efficient than a small heavy ship. However, there are other aspects of hull design that are harder to understand. For instance, a longer, finer vessel will produce less wave making (and eddy) resistance, but will likely have more wetted surface which adds to the vessel’s frictional resistance. Similarly, appendages add resistance, and propellers can be designed to maximize the thrust needed to overcome said resistance. There are rules of thumb and empirical regressions to aid in the design of vessels and their appendages, but we will look at two specific methods of testing and analysis that can be used to quantify the resistance of a vessel. We will first investigate the use of model testing in towing tanks, and then the use of Computational Fluid Dynamics (CFD) which can be used to test hull models virtually (i.e. on a computer).

Tow Tank Testing

Tow tank testing is accomplished by using a ship model basin to perform hydrodynamic tests to refine the design of a vessel to improve its performance. The world’s first facility to perform this type of testing was a shipbuilding company called William Denny and Brothers in Dumbarton, Scotland in 1883. Modern towing tanks vary in size with one of the longest being the 2,968 ft long high-speed basin at The David Taylor Model Basin at the Carderock Davison of the Naval Surface Warfare Center. Generally, a carriage runs on two rails on either side of the basin and is equipped with computers that are able to control the speed, propeller thrust, torque, etc. to run resistance and propulsion tests to determine how much power the vessel will need to achieve the desired speed.

Case study:

Bristol Harbor Group, Inc. has tested many vessel designs in several different hydrodynamic laboratories, including the Marine Hydrodynamic Laboratory at the University of Michigan and the tank at BC Research, Inc. Ocean Engineering Center. For this paper, we will investigate a 311’ x 68’ x 24.5’ 60,000 BBLS barge that is part of an ATB unit. The requirements for the barge were that it needed to be efficient when being pushed by the tug in the notch, but also stable (dynamic towing stability) when being towed by the tug astern on a hawser. Therefore, a model of the barge (Figure A) was built with two different bow designs that could be swapped out.

One bow design was a more conventional spoon bow (Figure B), and the other was a ship shape bow (Figure C). This model and these two bows were tested at the University of Michigan Marine Hydrodynamics Laboratory. The barge was towed on a model hawser to evaluate its the dynamic towing stability with each bow, and to evaluate the resistance of the hull in each configuration. As with most engineering problems, the results were conflicting. The spoon bow barge had higher resistance, but tracked better (i.e. had better dynamic towing stability) than the ship shape bow.

In this case, the client opted for the 5% to 8% increase in resistance in favor of the dynamic towing stability gains that the spoon bow offered. However, had the increase in resistance been higher, say 20%, perhaps the ship shape bow would have been chosen. This case study is an excellent example of the use of towing tanks for comparative design.

Computational Fluid Dynamics

CFD is an alternative to tow tank testing that allows naval architects to obtain engineering data they need to verify or alter a design early in the design process. This form of hydrodynamic analysis looks at the interaction between the hull, its propulsor, its appendages and how they all interact with environmental conditions. Performing physical hydrodynamic testing at a model scale as described above can produce uncertain results regarding vessel performance. Further, by using CFD technology such as STAR-CCM+, naval architects are able to virtually test hulls and improve the design of the vessel without building the expensive physical models that are required for tow tank testing.

Case Study

ATB’s are unique vessels, especially from a hydrodynamic perspective. The interaction between the tug bow and barge stern notch is inherently inefficient. Turbulence in the notch creates increased resistance and reduced propeller efficiency, the latter of which is further exacerbated by the fact that tugs are relatively shallow vessels.

For this case study, BHGI started with an existing design for a 399’ x 74’ x 30’ 80,000 BBL double hull oil tank barge and a 120’ x 40’ x 18’- 6″ twin screw ocean service tug. CFD and Finite Element Analysis (FEA) were used extensively during the design effort for these vessels making them a good starting point for hydrodynamic experimentation. CFD analysis had been previously performed to determine the calm water resistance of the combined unit and to optimize the tug forebody and stern rake geometry of the barge. The vessel specifics follow:

ATB Tug:

Dimensions: 120’x 40’x 18.5’

Classification: ABS *A1, *AMS, ABCU, Oceans Towing Vessel Unlimited Service certified under SOLAS/IMO/MARPOL

ATB Double Hull Oil Barge:

Dimensions: 399’x 74’x 30’

Capacity: 80,000 BBL

Classification: ABS *A1, Unlimited Oceans,

Permissively Manned

The goal of this additional experimentation was to determine if the resistance of the ATB unit could be reduced further by incorporating changes to the barge’s stern and the bow design that were not possible for the existing design. BHGI was able to reduce the resistance of the combined unit by upwards of 20% by further refining the barge stern design as shown below in Figure H (as compared to Figure G).

BHGI then looked at alternate bow designs for a vessel of this size and service. Interestingly, a fuller “elliptical” bow (Figure I) provided an additional reduction in resistance for the combined unit. This reduction was greater than 10% at certain speeds.

CFD Results

Finally, BHGI looked at combining the improved bow and stern designs to quantify the total potential reduction in resistance. Although the improvements were not additive, incorporating both alterations to the baseline design showed a greater reduction in resistance at a given speed than only modifying the bow or stern.

Real World Observations

As anyone who works with computers knows, there is always the potential for “garbage in — garbage out” when it comes to computer based analysis. To ensure that BHGI is not falling prey to this truism, we make good use of real world observations. One good source for this is AIS software such as MarineTraffic3 . Using this tool, we track actual ATB units to see what kind of real world transit speeds they are attaining. In one example, we tracked a unit for 10 months, looking at her speed when loaded and when empty, and we note that her loaded transit speed lines up very well with that calculated for the unit operating at 80% of the tug’s main engines’ maximum continuous rating (see Figure K).

Conclusion

As noted above, BHGI is a firm believer and user of the latest technology to create more efficient vessel designs. In fact, BHGI has been involved in custom propeller and appendage design using similar tools (model testing and CFD analysis). However, we also understand the importance of grounding our analytic efforts in the real word. This is evidenced by the first case study where a less efficient hull design was selected because it had better operational characteristics (better dynamic towing stability); and by our efforts to monitor real world observations and check our analytic work against this data. Said simply, we “trust but verify” the tools in our tool chest.

BHGI / TSGI Efficient Together

BHGI has designed and provided construction oversight for both conventional and z-drive ocean-going tugboats ranging from 660 bhp to 5,000 bhp. Our sister company, The Shearer Group, Inc. (TSGI) has designed a myriad of conventional and z-drive towboats for the inland waterways. Most notably, TSGI recently designed two different revolutionary z-drive towboats for Southern Towing Company, eight of which were built (six 3,200 HP and two 2,400 HP vessels). TSGI is a leader and innovator in the inland towboat industry. BHGI and TSGI are intimately familiar with the current U.S. Coast Guard loadline, and class requirements. Further we were involved in critiquing the recently released Subchapter M rule. This has positioned BHGI and TSGI well for designing new Subchapter M compliant tugs and towboats or bringing existing vessels into compliance.

BHGI & TSGI have completed hundreds of unique barge designs ranging from 90’ deck and crane barges to 400’ double hull oil barges, and 400’ loadline deck cargo barges. The companies have also designed passenger vessels range from high speed catamaran ferries, to 600 passenger only ferries, to a double-ended ferry for the Texas Department of Transportation. Other passenger vessel designs include dinner vessels and paddlewheelers. BHGI has also provided owner’s representation and construction oversight services for passenger and cargo / passenger vessels for both public and private operators. In regards to aluminum vessels, BHGI has designed a variety of small custom patrol boats and water taxis.

BHGI and TSGI both have extensive knowledge of liquefied natural gas (LNG) and have been involved in LNG projects since 2009. BHGI was contracted by Conrad Orange Shipyard to design a 232’, 2,200m³ LNG bunker barge. It is the first bunker barge in North America and is expected to be delivered in 2017. As for TSGI, they too are engaged in LNG projects, having developed the design of a LNG powered towboat utilizing a proven towboat design. The team was awarded AiP by ABS for this design as well. TSGI is currently working with Pittsburg Region Clean Cities (PRCC) and Clean Fuels Clean Rivers (CFCR) on the conversion of an inland towboat to dual fuel (diesel/LNG) to reduce diesel emissions of marine vessels.

As described above, both companies are committed to their core value which is “To Create”, and to using the latest technologies to design more efficient vessels. Later this year, TSGI will issue a white paper continuing this theme but focused on efficient design of inland towing vessels. Many of the concepts discussed in this paper relate to the brown water industry and we look forward to sharing some of our thoughts and observations on efficient vessel design for the inland market next.

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