Fuel-saving technology

Updated 11 Oct 2019

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There is an unfair depiction of the shipping industry as a profligate user of fuel causing unnecessary emissions and endangering health. In reality shipowners have always been careful about the amount of fuel used and, except for meeting commercial commitments, are usually keen to employ any means to reduce fuel use. In doing so they are indirectly contributing to reducing ships’ environmental impact. Nevertheless, efficiency has become a marketing tool for equipment suppliers and the shipowners themselves with claims for greener operations being made at every opportunity.

Quite obviously the amount of emissions to air is directly related to the quantity of fuel used so any reduction in fuel use contributes to improving the environmental impact of ships. Many of the series of ShipInsight guides contain sections on energy saving measures with Power and Propulsion, Paints and Coatings and Shipping Software featuring the issue quite strongly. While there is no intent to duplicate that material here, a brief resume of the issues is relevant.

In addition to individual items of equipment or ship systems, the design of the ship as a whole is of vital importance. In 2011, following some years of development, the IMO rules on energy efficient design came into effect. Known as the Energy Efficiency Design Index (EEDI), ships are obliged to meet a certain efficiency standard that becomes increasingly strict over a period of time.

Since 1 January 2013, following an initial two-year ‘Phase Zero’, new ship designs must meet the reference level for their ship type. The level is tightened incrementally over time and so the EEDI is expected to stimulate continued innovation and technical development of all the components influencing the fuel efficiency of a ship from its design phase.

The EEDI is a non-prescriptive, performance-based mechanism that leaves the choice of technologies to use in a specific ship design to equipment makers, shipbuilders and shipowners. As long as the required energy efficiency level is attained, ship designers and builders are free to use the most cost-efficient solutions for the ship to comply with the regulations.

The EEDI provides a specific figure for an individual ship design, expressed in grams of carbon dioxide (CO2) per ship’s capacity-mile (the smaller the EEDI the more energy efficient is the ship’s design) and is calculated by a formula based on the technical design parameters for a given ship.

Technology aimed at improving ship efficiency is frequently developed and employed partly to allow newbuildings to meet the EEDI requirements but is mostly driven by operators desiring to reduce fuel costs. This is especially so when a vessel has already satisfied the EEDI requirement, but the owner continues to add features improving efficiency beyond the compliance level.

Some have questioned owners’ commitment to making such improvements but there are good reasons why owners can benefit. Reducing fuel consumption when fuel costs are for the owner’s account is a clear saving but if the ship is time chartered then the owner is not responsible for providing fuel. Even so, the time charterers generally put fuel consumption high on the list of factors affecting their choice of ship to charter so a more efficient ship will have an advantage that can be a premium hire rate or the difference between employment and lay-up in a poor market. Every method of reducing fuel consumption that can be employed has the added environmental benefit of cutting exhaust emissions.

Energy-saving measures

Energy-saving measures can come in many guises, ranging from hull modifications, through to propeller/rudder combinations and appendages and adaptations to engines and machinery. Taking things a little further, the term can include means of exploiting energy from the wind, sun and waves or storing excess power by way of batteries for use later.

Today, energy saving has become linked in the minds of many to the slow steaming strategies adopted by some operators – particularly in the container trades. While it is true that some devices such as turbocharger cutouts and concepts such as variable turbine geometry have come about simultaneously with slow steaming, their use can be extended to vessels for other reasons.

The use of batteries as energy storage systems is of growing interest to many shipowners and the number and types of ships employing them is growing. There is no doubt that ships with highly variable energy loads will benefit most but all ship types have periods when energy production exceeds consumption and a battery could be charged from the ship’s own power sources. Using the battery for port entry and exit would definitely reduce emissions and help improve shipping’s image.

Software too has a role to play in reducing fuel use and so cutting emissions. Two types of application in particular are worthy of particular consideration: trim optimisation and weather routeing. Both have been heavily promoted by proponents of e-navigation although the need for such software has been questioned by some who believe that it undermines the knowledge and expertise of ships’ navigating officers.

Coatings used to protect against hull fouling have different environmental impacts. By preventing hull fouling they allow ships to burn less fuel and therefore play a role in reduction of exhaust emissions. However, even though TBT – which was said to have had an adverse environmental effect causing problems for some marine organisms – has been banned from use, some are saying that the copper-based substitutes are also hazardous.

While criticisms are being directed at the replacement anti-fouling products for still having the potential to hard wildlife, the IMO has recognised the role that anti-foulings can have in preventing species transfer. In July 2011, the IMO issued Resolution MEPC.207(62), Guidelines for the control and management of ships’ biofouling to minimise the transfer of invasive aquatic species. Currently the guidelines are purely advisory, although flag states are encouraged to ensure their use on board ships. It is expected that at some future date, the guidelines will become mandatory.

This expectation has increased recently as the IMO has pushed the question up the agenda by way of the GloFouling project, which the IMO says “will drive actions to implement the IMO guidelines for the control and management of ships’ biofouling, which provide a globally-consistent approach on how biofouling should be controlled and managed to minimise the transfer of invasive aquatic species through ships’ hulls. The project will also spur the development of best practices and standards for improved biofouling management in other ocean industries”.

Energy-saving devices

Measures that are engine-, software- or coatings-related have been complemented by developments in which hull and propeller modifications and appendages have contributed to ship designs that are more efficient than vessels without them.

Some of these measures are relatively inexpensive retrofit options for existing ships and can be incorporated into newbuildings at virtually no extra cost. Others are more expensive but tend to deliver more significant fuel savings and have a short payback period.

Hull appendages
A ship’s hull lines are optimised to both house the intended cargo and enable the ship to move through the water with the least resistance at its intended service speed. Sometimes meeting the two requirements results in something of a compromise and this is especially so if the ship’s design has also required its dimensions to be restricted to allow entry to particular ports.

Often it is possible to reduce the element of compromise that is required by making use of hull appendages that in some way improve the efficiency of the vessel. The bulbous bow is one such appendage and although considered an essential design feature for many years has over the last 15 years or so been rethought.

Today many novel bow forms exist with some of the most well known being the Ulstein X-Bow, Wavepiercing designs developed by VARD and Rolls-Royce Marine’s (now Kongsberg) Universal Shipbuilders Ax-Bow and Ledge Bow, Groot Ship Design’s Cross Bow and Damen’s Axe Bow. Most of these have no bulb but even where the bulb is retained, changes to its shape and size can offer significant savings potential. After adopting slow steaming strategies, several container ship operators had existing bulbous bows on their vessels replaced by a new configuration better suited to the slower speed of their ships.

Smaller appendages are to be found at the aft end of the vessel and are more common on ships with a high block coefficient such as bulk carriers and tankers than on container ships. This is because on wider vessels, the water flow to the propeller can benefit from being modified so as to allow the propeller to impart greater thrust to the vessel.

There is one appendage that has been developed by Japanese builder Naikai Zosen Corp that is placed forward on the hull. The device is known as STEP (Spray TEaring Plate) and consists of a pair of plates around 5m in length, one attached to either side of the bow above the waterline.

At first sight in calm water conditions the device would seem to offer little in the way of energy saving but it is designed to operate by reducing the resistance caused by the bow wave that builds in heavy seas above the waterline in medium and high speed ships including car carriers and container vessels. In tank tests, the device was estimated to decrease the wave resistance by about 18% under heavy wave conditions and to reduce the energy consumption by about 2% under head waves at Beaufort scale 6.

The energy consumption of the first vessel fitted with the device – the PCTC Jupiter Spirit – was compared with ship operation data obtained from a similar car carrier without the STEP, built in the same period. The comparison was carried out by Class NK using sample data referring to design conditions of The STEP, which include full load condition in summer, head wave angle range within +/- 90 degrees, and significant wave height of 3m. As a result, about 3% decrease in energy consumption was proved under the conditions of 2m or more high waves.

Concentrating the flow of water to the propeller on slow, wide-bodied vessels is widely recognised as having a beneficial effect on a ship’s efficiency although the majority of ships of this type have had no modifications aimed at improving their performance. The possible modifications available fall into three categories of appendages fitted upstream of the propeller.

The categories are; fins or spoilers, ducts and pre-swirl stators. The devices can be employed alone or in combination and may even be complemented by more devices downstream of the propeller and optimised rudder/propeller systems. Different builders have developed proprietary versions of one or more of the devices and in Japanese and some South Korean yards they will be offered as part of the package when selling standard designs.

Fins or spoilers are superficially similar to a bilge keel but much smaller and located strategically to concentrate and direct the wake flow to the propeller. As well as improving efficiency, these devices can also contribute to a reduction in propeller cavitation which produces other benefits including more comfort and less noise.

Several shipbuilders have developed designs that feature fins of this type and have given them proprietary names such as Sanoyas Tandem Fin, Onomichi Parallel Fin, Namura Flow Control Fin and Oshima Flipper Fin to name but a few. Although all those mentioned are Japanese, South Korean builders also employ similar devices. Some builders have adopted very similar systems while others have unique patented devices.

Regardless of type, energy savings between 1% and 4% are claimed. For EEDI purposes, the fuel saving effect can be measured either by comparing identical hulls with and without the devices during sea trial or by model testing or CFD.

Ducts that equalise the wake and concentrate it more than fins can deliver significant savings alone and can further enhance the small benefits that fins can confer. As with fins, ducts come in many forms from simple LJ bracket types to more complex versions such as Becker’s Mewis Duct. In some cases, the duct will be formed by structures either side of the skeg but some are fully circular structures placed just forward of the propeller. In the latter case they should not be confused with a nozzle propeller in which the propeller is located inside the nozzle. Ducts are said to be more effective on wide-bodied slow-moving vessels and savings have been claimed in some instances to be in double percentage figures.

Many builders of bulk carriers and tankers have standard designs that feature some form of duct. In recent years, Becker Marine has promoted its Mewis Duct as a retrofit ESD and has managed a considerable number of sales. The device is also suitable for newbuildings and these now make up the bulk of sales.

Since so many have now been retrofitted to vessels, the company claims that statistics of ‘before and after’ consumption figures can now prove an average fuel saving of 6% rising to 8% when combined with an appropriate design of Becker rudder. The basic concept has been developed further into the Becker Twisted Fin, designed for use on vessels with finer lines than the bulkers and tankers the Mewis Duct was developed for.

In the Twisted Fin, the nozzle ring is considerably smaller than the Becker Mewis Duct and has a slimmer profile. The fins on the inside of the nozzle ring extend outwards beyond the nozzle. To prevent the formation of a swirl with cavitation at the ends of the fins, Becker has developed special end caps for the fins similar to the winglets familiar from modern aircraft wings.

The final type of ESD fitted forward of the propeller is a pre-swirl stator. This generates a swirling flow opposite to the rotation of the propeller and creating an additional blade loading through which the delivered thrust per unit of power is raised. A pre-swirl stator can result in fuel savings of around 4%-6%.

The device has the appearance of a small fixed multi-bladed propeller mounted directly forward of the propeller itself. The number and size of blades varies between designers and shipyards and may be described using a proprietary name. The fins inside the Mewis Duct act as a type of pre-swirl stator. It is usually possible for a pre-swirl stator to be fitted as a retrofit on most vessels.

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