(Article originally published in Nov/Dec 2014 edition.)
***From Nov-Dec 2014 Edition of The Maritime Executive magazine***
Seacat Services recently unveiled the Seacat Intrepid, the first DNV GL-certified 26-meter crew transfer vessel to profit from a unique, three-year workboat tank-testing program. The vessel is hailed as a turning point in design evolution as it is the result of detailed research examining the specific weather conditions it will operate in.
Such highly specialized designs are becoming increasingly common, and they are taking advantage of new propulsion technologies as well as new design initiatives. The focus on operational specifics is leading to greater use of electric drives in various configurations. AHTSs, for example, have much higher power demands for anchor-handling than for transit and are increasingly designed with a diesel engine driving propellers through a reduction gear including a power take in (PTI) for efficient boost power, says Oddvar Deinboll, Head of Machinery & Systems at DNV GL.
The latest EDDY tug built by Holland Shipyards exemplifies the trend. It features a Schottel PTI electric motor, which provides 460kW at 1100 rpm for maneuvering and transit speeds up to 10 knots when the diesel engine is switched off. The investment costs are attractive because it enables the use of a smaller diesel engine and fuel savings can be achieved across the operating profile.
Thrusters & Waterjets
The use of thrusters is on the increase. “Thrusters offer good maneuverability and thrust in any direction, so you eliminate the need for aft tunnel thrusters,” says Deinboll. There is an array of options in both push or pull modes and, to reduce fuel costs, thrusters with two propellers, either rotating in the same or opposite directions, are also being used.
Balanced against the gains of matching the propulsion system to a vessel’s operating profile is the developing trend toward using workboats, including tugs, as multipurpose vessels. “This can entail having to maneuver very precisely in adverse weather conditions and standing still over ground,” says Frank Lutz, DNV GL’s Head of Propulsion & Engines. “Dynamic positioning (DP) enables them to do this and is especially relevant for workboats used in drilling operations.” DNV GL is currently working on drafting new rules for DP in response to the need for more vessels to work in close proximity in remote offshore locations, and DP technology is increasing in sophistication to support this demand.
During development of its energy-efficient DP system, experts at GE Power Conversion used model-based engineering and simulation to ensure accurate test results. Studies making use of weather patterns for a full year showed that a 12 percent reduction in fuel usage could be expected for representative scenarios. At the same time, wear – in terms of stick-slip cycles on the thrusters and propulsion system – could be reduced by a factor of 10 or more.
DP systems normally maintain vessel position at a single defined point. This leads to constant corrections by the thrusters as the vessel makes small motions about this point. GE’s energy-efficient mode allows the operator to define an “aim circle” within which the vessel is permitted to move. A novel predictive control algorithm anticipates future motion and only updates the thrust demands if the vessel is expected to move close to or outside the permitted area. Thus far fewer small corrections are needed.
Thrusters have a rising competitor: waterjets. As older offshore oil and gas fields approach the end of their useful lives, the industry is moving to deeper waters to find replacements. Significant ultra-deepwater fields have been found off the coasts of Brazil and West Africa and in the Gulf of Mexico. These fields are typically several hundreds of miles away from the closest port.
“The support and supply vessels serving drillships, FPSOs and other floating facilities must be capable of traveling these long distances within a reasonable time,” says Joe R. Bekker, President of Thrustmaster of Texas. “A traditional OSV cruising at 13 knots takes 15 to 24 hours each way bringing and returning fluids, supplies and equipment to the rig. In many cases that is too long. This has created the need for fast supply vessels (FSVs).”
FSVs have cruising speeds of 25 to 40 knots as well as DP. To accommodate both, Thrustmaster offers waterjets as the main propulsion. The waterjets are equipped with reverse buckets of double-cheek design. In combination with the steering nozzle, this allows a large envelope of jet direction angles, facilitating the kind of vector control needed for DP. To complete the DP capability of FSVs, Thrustmaster also offers aluminum tunnel thrusters and retractable azimuth thrusters for the bow.
Reducing Weight and Size
Given the wide range of available propulsion options, engineers are turning to increasingly lightweight materials and compact designs. These include Wärtsilä’s compact steerable thrusters, the latest generation of Voith inline thrusters, and Man Diesel & Turbo’s 12-cylinder version of the MAN 175D engine, suited to OSVs and tugs.
The Caterpillar twin-fin system has been designed to reduce fuel consumption and improve performance and reliability. Primarily intended for diesel-electric applications, the company claims it is the most compact system of its kind for offshore vessels, offering increased cargo-loading capacity in comparison with podded propulsion systems.
Other propulsion components are following suit. “In terms of materials, there is a renewed increase in the demand for components made of carbon fiber composites in the propulsion chain in order to increase a vessel’s efficiency by weight reduction,” says Axel Albrecht, Senior Engineer at DNV GL Hamburg. “Ten years ago shafts made of carbon fiber laminate were especially popular in high-speed ferries. This trend is now being applied to other ship types, such as swath vessels and crewboats.”
Another example is American Vulkan’s new combination clutch and coupling for hybrid systems. Among its environmental benefits are that it doesn’t require external cooling, doesn’t leak, and is very lightweight and compact.
There is also increasing interest in the environmental benefits of dual-fuel and gas engines. “Dual-fuel engine systems would be well suited for vessels that frequently operate in coastal waters and emission control areas (ECAs), such as OSVs and tugs,” says DNV GL’s Lutz. Sanmar shipyard in Turkey delivered the world’s first LNG-fueled tugs, Borgøy and Bokn, for Norway’s largest tug owner, BB, this year under DNV GL class. The two tugs feature Rolls-Royce Bergen C26:33L6PG engines fueled purely by LNG.
Damen, Rolls-Royce subsidiary MTU and Svitzer have teamed up to build the first-ever reverse stern drive compressed natural gas tug. Due for launching in 2016, the tug will feature the new 16-cylinder pure gas engine being developed by MTU. It will be complemented with a multipoint gas injection system and dynamic engine control.
“We are developing our new gas series in order to meet the extreme load profile of the tugboat,” says Dr. Ulrich Dohle, CEO of Rolls-Royce Power Systems. “The acceleration will be comparable to the level of our diesel engines. Due to the clean combustion concept, compliance with IMO Tier 3 emission legislation will be ensured without the need for additional exhaust gas after-treatment. The 2,000 kW MTU gas engine is characterized by high power density and low fuel consumption.”
Proximity to the customer has become increasingly important to service providers looking to support new technologies. Some, such as MSHS Group, are building infrastructure. “Whether a scheduled drydock or emergency repairs, the challenge is to ensure that you are there when the customer needs you,” says Vice President Rodrigo Quilula. “This is not only changing the location of service hubs in relation to ports, but it is also developing the need for onboard emergency and specific services such as on-site machining. Our Turbo Shop in the Bahamas supports customers who maximize docking time to include overhauls. Additionally, supply chains are continuing to evolve, ensuring the parts are dockside when the ship arrives.”
Chris-Marine and IOP Marine have introduced a series of tools to monitor cylinder condition so that overhaul decisions can be based on the actual condition of the equipment. For 4-stroke medium-speed engines, the tool is a lightweight measuring instrument called the DEC-TESTER that can be used to quickly determine a cylinder’s ability to seal when the piston is in top position. Testing, including setup, takes a matter of minutes, and if trend plotting is applied it is possible to plan the time of overhaul well in advance.
ScanTech Marine, established in February of this year, has a service division for troubleshooting propulsion equipment. Additionally, Managing Director Kristian Hansen says consulting on green technologies is a focal point for the company, assisting shipbuilders in meeting high standards in the installation of new technologies ranging from automation to waste heat recovery and exhaust gas cleaning systems.
The company also has its own technologies including ScanTech Voyager, a voyage data recorder and control unit that interacts with the propulsion controls and onboard green technologies to ensure SOx and NOx abatement technologies are running in ECAs. Voyager allows shipowners and maritime authorities remote overview and control of the vessel’s operational performance.
“We are the practical engineers with the knowhow to ensure the reliability of green technology installations. Such systems should not have any negative impacts on engine performance, either in terms of fuel consumption or reliability. Today, this is sometimes still a challenge.” – MarEx
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The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.