To cap greenhouse gas emissions, the International Maritime Organization (IMO) has introduced the Energy Efficiency Design Index (EEDI). This

14 Highlights 64 / 2017 – fw – the unknown factor

fw – the unknown factor

Legislation on the Energy Efficiency Design Index (EEDI) requires the determination of a “weather factor” – fw – reflecting the percentage of its calm-water speed that a ship can maintain in Beaufort 6 and corresponding waves. The higher the fw value, the smaller the speed loss and the better the ship will perform in a seaway. But how do we determine this unknown fw factor, and how do we strike a balance between accuracy and costs? As a member of the International Towing Tank Conference (ITTC) SSPA is actively involved in developing guidelines on fw and as an independent maritime consultant and established model testing basin we have the knowledge and tools required to realistically determine the “unknown weather factor”.

Added waveresistance

Power setting = 75% MCR

Calm water resistance

WIN

D +

WAV

ES Propeller curves

CALM

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ER

fw =Vw /Vref

Added wind resistance

Wind tunnel /SSPA Database

Total resistance

Vw

speed in wind and wavesVref

speed in calm water

Power prediction(ITTC 1978 method)

Speed power curve in calm water

Speed power curve in wind + waves

Calm water resistance

Power prediction(modified ITTC 1978 method)

Two things are required to obtain a more realistic and above all ship-specific value of fw : determination of the calm water speed and determination of the ship speed in Beaufort 6. At SSPA, we have the knowledge, tools and facilities to help with both.

where fw is the so-called “weather factor” that takes the influence of wind and waves into account. It is defined as:

index is a measurement of the amount of carbon dioxide that a ship emits in relation to its cargo capacity and speed:

Ship speed

Eng

ine

Pow

er

75% MCR

CALM W

ATER

WIND +

WAVE

S

fw=Vw/Vref

VrefVw

place at the concept stage of a design. However, if manoeuvring and seakeeping tests are schedu-led to prove a final design, why not test for fw as well?

Power prediction and fwOnce the total resistance in wind and waves is known, the corresponding engine power to overcome this resistance can be calculated using the principles of the “ITTC 1978 Performance Prediction Method”. As illustrated in the speed power plot, such predictions are made for several ship speeds and a speed-power curve in wind and waves is plotted. Finally, the fw value is determined from this.

Here VW denotes the speed of the vessel in Beaufort 6 (3 m significant wave height and 12.6 m/s wind speed) and Vref is the speed of the ship in calm water.

The crude way – fw from IMO’s standard curvesA very simple, if somewhat rough, method of obtaining an initial estimate for fw is described in IMO Circular MEPC.1/Circ.796. The method is based on regression analysis of full-scale measurements, i.e. on the actual speed reduction of existing ships, and only requires ship type and cargo capacity as input. Three kinds of standard fw curves are provided for bulk carriers, tankers and containerships. The disadvantage of this simplistic method is obvious: it will not give a ship-specific fw value, i.e. it cannot distinguish between a good and a bad design.

Two things are required to obtain a more realistic and above all ship-specific value of fw: determination of the calm water speed and determination of the ship speed in Beaufort 6. At SSPA, we have the knowledge, tools and facilities to help with both.

Calm water model tests already mandatory under existing EEDI rulesAs shown in the flow diagram, these speeds are determined separately. Finding calm water resistance and calm water speed Vref is relatively straight forward and part of the model tests that have been mandatory under the EEDI rules since 2013. As shown on the right-hand side of the flow diagram, this includes a combination of model towing tests, model self-propulsion tests and model propeller open water tests.

Speed in wind and wavesFinding the total resistance in wind and waves and the corresponding ship speed – Vw – is a bit more complex and also requires knowledge of wind resistance and added wave resistance.

As illustrated on the left-hand side of the flow diagram, the aerodynamic resistance of the ship is best determined through wind tunnel testing. As an alternative, we can normally find values for similar ships in our wind-force database.

A ship sailing in a seaway experiences a hydro-dynamic force component known as “added resistance in waves”. This force can be determined using CFD calculations or free sailing model tests in SSPA’s Maritime Dynamics Laboratory. The choice of method depends on how far the design of the ship has progressed and on many other factors that need to be considered carefully. A full seakeeping test would clearly be out of

Photo of MDL, the Maritime Dynamics Laboratory for seakeeping and manoeuvring tests.

What is fw?A “weather factor” reflecting the percentage of its calm water speed that a ship can main-tain in Beaufort 6 and corresponding waves.

What is EEDI?The Energy Efficiency Design Index (EEDI) is a measurement of the amount of carbon dioxide that a ship emits in relation to its cargo capacity and speed. The achieved EEDI value of most new-build ships has to be below a certain limit set by the Interna-tional Maritime Organization (IMO). If a ship fails to comply with the required EEDI, it is not allowed to be taken into service.

Is it mandatory to determine fw?Yes, but currently a poor fw value will not influence the attained EEDI. It is expected that this situation will soon change.

Why test for fw at SSPA?To obtain realistic and ship-specific fw values. The generic IMO curves are too pessimistic. We have the knowledge and the tools to provide cost-effective fw predictions. SSPA’s fw database makes it possible to compare a new design to other ships and their energy efficiency.

Can fw tests be carried out in a towing tank equipped with a wave maker?No, fw determination requires a performance prediction in short crested waves. Tests in head waves alone are not sufficient.

Can you calculate fw?Yes, this is a viable and sensible option at the early design stage.

16 Highlights 64 / 2017 – fw – the unknown factor

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For shipowners: A realistically determined fw value makes it possible to compare design options and ships from different yards. A ship with a high fw value is fuel-efficient under realistic conditions; bear in mind that wind speeds between Bft 4 and Bft 6 prevail for 65 per cent of the year in the North Atlantic.

For shipyards: SSPA’s measurements show that the “fw standard curves” from IMO Circular MEPC.1/Circ.796 are too pessimistic, i.e. they overpredict the speed loss in a seaway. It is possible to obtain a more favourable and ship-specific fw value.

Frederik GerhardtSenior Technical Expert. Frederik received a Dipl.-Ing. in Aeronautical Engineering from RWTH Aachen in Germany in 2005 and a PhD in

Mechanical Engineering from the University of Auckland in 2011. He has been employed at SSPA since 2011 and has mostly worked on research and consultancy projects related to seakeeping and manoeuvring issues. Frederik has been a member of the International Towing Tank Conference (ITTC) Seakeeping Committee since 2014.

Contact informationE-mail: frederik.gerhardt@sspa.se

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f w

Capacity [DWT]

SSPA fw databaseFull-scale measurementIMO standard curve

Typical LNG carrier

SuezmaxAframaxVLCCProduct carrier

LNG bunker vessels

SSPA database for fw of tankers As part of a strategic research project, SSPA has by way of seakeeping model tests determined fw values for a number of tankers ranging from 2,500 to 150,000 deadweight tonnes. The results are illustrated in the fw values versus “Capacity” plot above. Also shown are full-scale values from IMO Circular MEPC.1/Circ.796 and the “Standard Curve” from the same IMO publica tion. As can be seen, agreement between full-scale data and SSPA’s model test results is good. The generic IMO standard curve on the other hand is extremely conservative because fw is underpre-dicted, i.e. speed loss is significantly overpredicted.

Want to know more?Further reading at our webpage:http://www.sspa.se/ship-design-and- hydrodynamics/fw-the-unknown-factor