Appendix D

OPTIMOOR Users Guide Appendices

These are from the old DOS guide and have not yet been updated.

Most of the information in these appendices is still applicable. But they do notcover changes and enhancements in the newer Windows versions of OPTIMOOR. Also, the screen images do not represent the Windows version.

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Appendix D

WIND COEFFICIENTS AND CALCULATION METHODSUSED IN OPTIMOOR

D-1 Introduction

This Appendix documents the methods and coefficients used within OPTIMOOR to calculate windforces. Reference should also be made to the following OCIMF documents:

Prediction of Wind and Current Loads on VLCCs 1

Prediction of Wind Loads on Large Liquified Gas Carriers 2

Guidelines and Recommendations for the Safe Mooring of Large Ships at Piers and Sea Islands 3

Mooring Equipment Guidelines 4

D-2 Wind Force and Moment Calculation Method

D-2.1 Wind Force Equations

The equation for Longitudinal Wind Force is:

The equation for Lateral Wind Force is:The equation which is used within OPTIMOOR for Yaw Moment is:

The more traditional equation for Yam Moment due to Wind is:

Thus the Yaw Moment Arm is :

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where: AL = Lateral (side) (broadside) wind areaAT = Transverse (end) (head-on) wind areaCXw = Longitudinal wind force coefficientCYw = Lateral wind force coefficientCXYw = Wind yaw moment coefficientFXw = Longitudinal (end) wind forceFYw = Lateral (side) wind forceLpp = Length between perpendicularsMXYw = Yaw moment due to windVw = Velocity of windXw = Moment arm at which Lateral Force is applied in producing Moment����a = Density of air

Some of these concepts are illustrated in Figure D-1.

The value 7600 is a conversion factor of (Tonne-Metres)/(Kilogram-Knot-Second).

D-2.2 Wind Velocity Elevation

The coefficients used in the wind force equations are based on wind velocity at an elevation of 10 meters(33 ft) above the surrounding free surface. The following formula can be used to convert a wind velocitydetermined at some other elevation to that 10 meter elevation:

where: h = Elevation of known wind velocityVW10 = Wind velocity at 10 meter elevationVWh = Wind velocity at elevation h

D-2.3 Wind Velocity Duration

Prediction of Wind Loads on Large Liquefied Gas Carriers 2 states "A 30 second average windvelocity is recommended for use in the wind force and moment equations for mooring analyses.

If the wind velocity for some other duration is known, it can be adjusted to the desired duration usingFigure D-2.5 6 For example, the ratio between a 30 second average and the one hour average is 1.32, andthe ratio between a 5 minute average and the one hour average is 1.09. Thus to convert from a 5 minuteaverage to a 30 second average, multiply by 1.32/1.09.

D-3 Wind Force Coefficients

Five sets of pre-programmed alternative wind force coefficient data from several sources are includedin OPTIMOOR. These data are stored in the file WINDCOEF.DAT. Other wind force coefficients maybe added by the user, as described later.

To access the Drag Data Screen, first bring up the Vessel Data Screen by the command Alt-V and thengive the command Alt-D. That Drag Data Screen is shown in Figure D-3.

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D-3.1 OCIMF Tanker Coefficients

The OCIMF Tanker Wind Force Coefficients used within OPTIMOOR are those given in Predictionof Wind and Current Loads on VLCCs.1 That document states these coefficients are for vessels in the150,000 to 500,000 class. However, the later OCIMF document Mooring Equipment Guidelines 7 states:

"The wind and current coefficients of Reference 5 [here-in Reference 1] were defined originally for VLCC sizeships above 150,000 tonnes deadweight. More recent model test data on the modern tanker forms which have allaccommodation superstructures aft and segregated ballast configurations confirms that these same coefficientsare, in most cases, sufficiently accurate when applied to smaller ships, and that they may therefore be used for arange of ships down to approximately 16,000 tonnes."

OCIMF published tanker wind force coefficient data for two different bow shapes, the "Vee-shapedBow" and the "U-shaped Bow". These bow shapes are illustrated in Figure 2 of Prediction of Wind andCurrent Loads on VLCCs.2

The "Vee-shaped Bow", in which the bow shape at the water line is relatively sharp or V shaped, isreferred to as "conventional" in the OCIMF document. It is probably appropriate for most tankers. The "U-shaped Bow, referred to as "cylindrical" in the OCIMF document, is appropriate for tankers with a veryrounded bow shape extending down to the water line.

This difference in bow shape appears only to effect the ballasted tanker longitudinal wind forcecoefficient. At oblique angles of between about 40 and 90 degrees, a suction effect takes place whichsignificantly increases the longitudinal force. The OCIMF wind force data shows no other differencesbetween these two bow shapes.

The OCIMF tests were conducted on models representing four different tankers in the 155,000 to500,000 dwt range, in loaded and ballasted condition. Other than state of loading, fully loaded vs. ballasted,and the difference ascribable to bow shape, the OCIMF document makes no distinctions as to hull orsuperstructure shape. The OCIMF document says that the data for the various tankers were adjusted todefine representative mean curves and "The total variation about the mean curves ... was within ±10 percentof the maximum value ..."

The OCIMF document then says the data were further adjusted to reflect "uncertainties inherent in modeltest results". The ballasted lateral force and yaw moment coefficient data accordingly were increased by10% and the longitudinal force coefficient force data were increased by 20%.

Thus the OCIMF wind force coefficients used in OPTIMOOR are probably conservative for mosttankers. Caution should be used in comparing the OCIMF data with those from other sources because ofpossible differences in how the coefficients were defined, determined, and adjusted. If the user ofOPTIMOOR has better quality wind coefficient data, then that data should be used instead.

D-3.2 RINA Berlekom Tanker Wind Force Coefficients

These wind force coefficients are taken from the Royal Institution of Naval Architects paper "LargeTankers - Wind Coefficients and Speed Loss Due to Wind and Sea", by Berlekom, et.al.8 The data reportedin that paper were based on tests conducted at the Aeronautical Research Institute of Sweden and financedby the Swedish Board for Technical Development.

Most of the work was carried out on a model of a 282,000 dwt ore/oil carrier. Alternate superstructuresand scale factors were tested to represent tankers over the range from 100,000 to 500,000 dwt. Severalalternative aft house were tested on the 282,000 dwt model. The data programmed into OPTIMOOR is that

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for the line representing the approximate mean though the various data, as plotted in the original paper byBerlekom.

D-3.3 OCIMF Gas Carrier Wind Force Coefficients

The OCIMF Gas Carrier Wind Force Coefficients used within OPTIMOOR are taken from Predictionof Wind Loads on Large Liquefied Gas Carriers2. Those data are based on tests conducted on modelsof gas carriers in the 75,000 to 125,000 m3 class.

OCIMF published wind force coefficient data for two different gas carrier tank configurations, PrismaticTanks and Spherical Tanks. These are illustrated in Figure B-1 of the report Prediction of Wind Loadson Large Liquified Gas Carriers.2

The Prismatic Tank gas carrier models which were tested appeared to have higher freeboard thanconventional tankers but otherwise did not appear to have much additional above-deck area. The SphericalTank gas carrier model appeared to have essentially the same freeboard as the Prismatic Tank vessels, butin addition had pronounced spherical tanks projecting above the main deck. Thus they appeared to havean appreciably extra transverse above-deck area.

D-4 Windage Areas

The values AT for end windage area and AL for side windage area are used in the OPTIMOOR windforce calculations. The terms transverse and lateral are used in some other references, but those terms cansometimes cause confusion.

The windage areas use within OPTIMOOR are the sum of the approximate hull area and the above-deckarea. This above-deck area includes all hull components and superstructure above the level of the hull maindeck. Figure D-4 illustrates this concept.

The user should enter the actual end and side above-deck areas if these are known. The user can callon OPTIMOOR to calculate approximate areas if they are not know.

For gas carriers the user is strongly advised to measure and enter the actual above-deck areas. Fortankers which have unusually large or small superstructures, the user is strongly advised to measure andenter the actual above-deck areas. When OPTIMOOR is to be used aboard a particular vessel, the user isurged to enter the actual above-deck windage areas for that vessel.

D-4.1 Hull Windage Areas

OPTIMOOR calculates approximate areas for the side and end areas of the hull along, using. Thosecalculated hull areas only extend up to the main deck (as defined by molded depth) and do not include theareas of forecastle, poop, or other above deck portions of the hull. These hull areas depend on the vessel draft and trim at any particular time. The end hull area iscalculated as the product of the beam and freeboard (depth minus draft plus 0.4 times the absolute value oftrim). The side hull area is calculated based on the defined length between perpendiculars and the presentvessel freeboard. The effect of trim on the approximate center of side hull area is also used in calculations.

The program adds the end hull and above-deck areas to achieve the total end windage area. In likemanner, the program adds the side hull and above-deck areas to achieve the total side windage area. Thesetotal windage areas are used in subsequent calculations of wind force.

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D-4.2 Above-Deck Windage Areas

The End Windage Area and the Side Windage Area are displayed in the second row of the VesselScreen. These values are entered on the Wind Drag Screen (accessed by Alt-D from the Vessel DataScreen).

The End Windage Area should be based on measurements of all areas which appear above the main decklevel in the end view of the vessel. This includes any portion of the forecastle or poop which projectsbeyond the outline of the superstructure in the end view of the vessel.

The Side Windage Area should be based on measurements of all areas which appear above the maindeck level in the side view of the vessel. This includes the forecastle, poop, and any other above-main-deckportions of the hull, as well as all superstructure and other significant items.

The program will suggest typical side and end and side above-deck areas if the ? option is exercised onthe Drag Screen. These calculated areas are based on data for several typical vessels and an empirical ratioof hull area to above-deck area which varies with vessel size.

D-4.3 Calculation Errors Introduced by Tanker Area Errors

In most cases, only small error will result from using the computer calculated approximate values forabove-deck areas for tankers. Wind forces and moments are of more concern on ballasted tankers than onloaded tankers. The ratio of side above-deck area to total windage area is usually very small for ballastedtankers. The following table, using data from Prediction of Wind and Current Loads on VLCCs1, shows theratios of "above deck area" to "total area" for ballasted and loaded tankers.

Ballasted 280,000 dwt Tanker Loaded 280,000 dwt Tanker

Side Area End Area Side Area End Area

Hull Area 6800 1200 2500 400

Above-Deck Area

600 700 600 700

Total Area 7400 1900 3100 1100

Above-Deck to Total Area Ratio

0.08 : 1 0.36 : 1 0.19 : 1 0.63 : 1

The end above-deck areas constitute significant portions of the total end windage areas. For the loadedtanker, this end above-deck area is about 63% of the total end area.

The side above-deck areas contribution very little to the total side windage areas. For the loaded tanker,the side above-deck area is less than 20% of the total side windage area. In the case of the ballasted tanker,the side above-deck area is less than 10% of the total side windage area.

A 10% error in estimating the side above-deck area would cause less than a 1% error in the broadsidewind forces and also in the moments on a ballasted tanker. (0.10 x 0.08 = 0.008) In the case of the head-onforces on a loaded tanker, a 10% error in estimating the end above-deck area would cause only a 6% errorin the results. (0.10 x 0.63 = 0.063)

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For wind loads, the ballasted case is usually much more of concern than the loaded case. Also, the sidewind forces and associated wind moments are usually much more of concern than the end wind forces.Thus for most tanker cases, only small errors will probably result from accepting the computer calculatedwindage areas. Never-the-less, the actual above-deck windage areas should be input if known.

D-4.4 Calculation Errors Introduced by Gas Carrier Area Errors

The wind forces and moments on most gas carriers are much more affected by errors in above-deckwindage areas. This is because the hull area constitutes a smaller proportion of the total windage area fortypical gas carriers. Also, wind forces and moments are generally of greater concern on such gas carriers.

The following table, using data from Prediction of Wind Loads on Large Liquefied Gas Carriers2,shows the ratios of "above hull area" to "total area" for ballasted and loaded Spherical Tank gas carriers.

Ballasted 125,000 m3 Gas Carrier

Loaded 125,000 m3 Gas Carrier

Side Area End Area Side Area End Area

Hull Area 3000 700 3100 700

Above-Deck Area

4100 700 3500 600

Total Area 7100 1400 6600 1300

Above-Deck to Total Area Ratio

0.57 : 1 0.50 : 1 0.53 : 1 0.53 : 1

For this particular gas carrier, the above-deck areas constitute significant portions of the total windageareas. For all cases, the above-deck area constitutes between 50% and 57% of the total area.

Thus the user should input actual above-deck end and side areas for gas carriers when ever these can beobtained. Significant errors might result from using the default values provided by OPTIMOOR for manygas carrier designs.

D-5 WINDCOEF.DAT File

The wind force coefficient data used within OPTIMOOR are stored in the file WINDCOEF.DAT. Thatdata can be viewed and edited with a text or word processor program. D-5.1 Description of WINDCOEF.DAT File

The WINDCOEF.DAT file is divided into data sets by lines of ***. The left portion of the first line ofeach data set gives the title of the data set, as it will be displayed on the OPTIMOOR Wind Date Screen.The right portion of the first line, reading "End Area Parameter Side Area Parameter" are simply labels fordata entered on the second line.

The second line contains these "Area Parameters". The "End Area Parameter" is the ratio of the end areaabove the vessel freeboard to the beam or breadth of the vessel. The "Side Area Parameter" is the ratio ofthe side area above the vessel freeboard to the length between perpendiculars (Lpp) raised to the 0.8 power.

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These parameters should be determined from elevation plans for the vessel which was model tested indetermining the wind coefficients.

The third line is blank. The fourth line consists of labels for entries on the fifth line.

The first value in the fifth line is the number of draft conditions, normally two (2). If data for only onedraft condition is used, then OPTIMOOR will make no adjustment in coefficients for the draft condition.Data for as many as three draft conditions can be entered.

The second, third, and fourth values in the fifth line are the ratios of twice the total lateral area to thesquare of length over-all, 2AL/L2

OA for each of the entered draft conditions. These are used within theprogram to scale the data for different tanker sizes.

The last value in the fifth line is the datum for the ratio x/L. The OCIMF data is based on MIDSHIP.The NMI data is based on the longitudinal center of side windage area.9

The sixth line is blank. The seventh line contains labels for the coefficient columns which follow.

Nineteen lines of coefficient data must be entered, corresponding to the coefficients at 10 degreeintervals, beginning at 0� (bow on) and ending at 180� (stern-on). Cx is the longitudinal wind forcecoefficient. Cy is the lateral wind force coefficient. x/L is the ratio of the lateral wind coefficient momentarm to the length between perpendiculars. Further comments can be entered after all the lines of coefficient data and before the next line of ***.

D-5.2 Cautions in Editing Data

Caution must be observed in creating and altering the WINDCOEF.DAT file. Professional advice shouldbe sought if you are not completely familiar with the basis of the data that you are entering or with editingASCII files on your computer.

The WINDCOEF.DAT program has the DOS "Read Only" attribute. This attribute must be changedbefore the file can be edited and should be restored after it has be edited. This change can be done througha DOS utility program such as PC-TOOLS or Norton DeskTop. It can be done directly within DOS by thecommand ATTRIB -r WINDCOEF.DAT to disable the "Read Only" attribute and then ATTRIB +rWINDCOEF.DAT to re-enable it.

Make a back-up copy of the WINDCOEF.DAT file before making any alterations. This can be by thecommand COPY WINDCOEF.DAT WINDCOEF.BAC. A DOS utility program can be used forcopying.

The WINDCOEF.DAT file can be edited using a word processor program such as WordPerfect.However, the resulting file must then be saved as an ASCII file without the formatting functions used bysuch word processors. The file may preferably be edited using a plain text editor, which retains the originalASCII file format.

Serious errors can result from mistakes in data entry. A single error, such as a sign error - omission ofa minus sign, or a decimal error - omission or misplacement of a decimal point can alter the resultingmooring force calculations over a range of 20� with possible serious consequences in the results producedby OPTIMOOR.

Ap-D_Txt.gid : January 16, 2000__________________________________________________

REFERENCES:

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1. Oil Companies International Marine Forum, Prediction of Wind and Current Loads on VLCCs,2nd Editon, Witherby & Co., Ltd. 1994.

2. Oil Companies International Marine Forum, Society of International Gas Tanker & TerminalOperators, Prediction of Wind Loads on Large Liquefied Gas Carriers, Witherby & Co., Ltd.,London, 19??

3. Oil Companies International Marine Forum, Guidelines and Recommendations for the SafeMooring of Large Ships at Piers and Sea Islands, Witherby and Co., Ltd., London, 1978.

4. Oil Companies International Marine Forum, Mooring Equipment Guidelines, 2nd Edition,Witherby and Co., Ltd., London.

5. Vellozzi, Joseph, and Edward Cohen, "Gust Response Factors", ASCE Structural Division, ST6,pp. 1295-1313, June 1968.

6. Flory, John F., et.al., Exxon Research and Engineering Co., Guidelines for Deepwater Port SinglePoint Mooring Design, U.S. Dept. of Transportation, U.S. Coast Guard Report No. CG-D-49-77,Washington, DC, 1977.

7. Oil Companies International Marine Forum, Mooring Equipment Guidelines, Witherby & Co.,Ltd. London, 1992.

8. Berlekom, W.B, van, P. Tragardh, And A. Dellhag, "Large Tankers - Wind Coefficients and SpeedLoss Due to Wind and Sea", Trans. Royal Institution Naval Architects, Vol 116 pp 41-58, 1974.

9. Gould, R.W.F., "The Estimation of Wind Loads on Ship Superstructures", Royal Institution ofNaval Architects Monograph No. 8, 1982.

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Edit this ASCII file to add or change wind drag data for OPTIMOOR.You are advised to make this file read-only after saving.Data can be entered in free format, but line numbering MUST correspond.End Area Parameter = Typical end area above freeboard / BreadthSide Area Parameter = Tyical side area above freeboard / Lpp^0.85th line of entry allows linear interpolation for draft based on values of 2A/L²Datum for x/L is either LCA (eg NMI data) or MIDSHIP (eg OCIMF data).If LCA entered then OPTIMOOR will ask for longitudinal centroid of windage area.19 coefficients from 0 to 180° are required for each draft.Any comments after table of coeffs will be ignored.Vessel type entry starts after next line starting with ****************************************************************OCIMF Tanker (Vee-shaped Bow) End Area Parameter Side Area Parameter 15 11.2

No of draft conditions 2A/L²(1) 2A/L²(2) 2A/L²(3) Datum for x/L 2 0.058 0.13 MIDSHIP

Cx(1) Cy(1) x/L(1) Cx(2) Cy(2) x/L(2) Cx(3) Cy(3) x/L(3) 0.96 0.00 -0.075 0.87 0.00 0.182 0.91 0.09 -0.078 0.75 0.13 0.167 0.83 0.19 -0.089 0.61 0.32 0.113 0.73 0.30 -0.110 0.46 0.52 0.086 0.62 0.43 -0.116 0.33 0.67 0.073 0.48 0.55 -0.120 0.21 0.79 0.062 0.34 0.64 -0.125 0.13 0.89 0.046 0.20 0.68 -0.137 0.07 0.95 0.024 0.07 0.71 -0.146 0.05 0.98 -0.010-0.04 0.725 -0.154 0.00 0.99 -0.055-0.14 0.72 -0.175 -0.07 0.96 -0.101-0.21 0.71 -0.205 -0.18 0.89 -0.143-0.29 0.68 -0.241 -0.28 0.79 -0.188-0.39 0.63 -0.268 -0.37 0.68 -0.237-0.51 0.55 -0.294 -0.46 0.57 -0.292-0.65 0.43 -0.335 -0.55 0.45 -0.340-0.74 0.28 -0.379 -0.61 0.31 -0.357-0.78 0.13 -0.385 -0.64 0.16 -0.337-0.75 0.00 -0.390 -0.61 0.00 -0.333Any comments here before next line of stars

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Table D-1 Portion of WINDCOEF.DAT File

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Figure D-1 Nomenclature and Conventions Used For Wind Forces and Moments

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Figure D-2 Adjustment of Wind Speed for Duration (Vellozi - 1968, Flory - 1977)

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Figure D-3 OPTIMOOR Wind Drag Screen

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Figure D-4 Illustration of Hull and Above-Deck Windage Area