TOWMASTER User Manual -...

TOWMASTER User Manual

Version : 1.0.0

Date : 23-November-2014


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License Information

TOWMASTER

TOWMASTER software and source code are property of Technomak Offshore & Marine

Consultancy. The software along with this user manual allows the purchaser to use the

software for commercial and non-commercial purpose.

DISCLAIMER OF WARRANTY

Neither Technomak Offshore & Marine Consultancy (and its group of companies), nor the

author of this program and documentation are liable or responsible to the purchaser or user for

loss or damage caused, or alleged to be caused, directly or indirectly by the software and its

attendant documentation, including (but not limited to) interruption on service, loss of business,

or anticipatory profits.


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Contents _

License Information ................................................................................................. 2

About TOWMASTER ................................................................................................ 4

TOWMASTER Analysis Methodology .................................................................... 5

Range of Validity ...................................................................................................... 5

Unit System .............................................................................................................. 5

TOWMASTER Inputs ............................................................................................... 5

TOWMASTER Resistance Calculation ................................................................. 13

TOWMASTER Results ........................................................................................... 14

Limitations .............................................................................................................. 14

References .............................................................................................................. 15

Appendix 1.1 - Example 1 ..................................................................................... 16


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About TOWMASTER

TOWMASTER is designed for iOS platforms and available on iPhone and iPads. This manual

gives step by step procedure to use the software, which can be used to estimate the bollardpull

requirement for towing tugs. TOWMASTER includes recognized methods (DNV Guidelines for

Marine Operations, Bureau Veritas and tow tested results) for estimation of resistance and

bollardpull. You can download and install TOWMASTER from App Store.


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TOWMASTER Analysis Methodology

Present version of TOWMASTER has two independent methods to predict the barge resistance

and tug bollardpull during tow. The present version uses the following methods for resistance

and bollard pull prediction.

• Bureau Veritas Towage of Sea Vessels or Floating Units

• Resistance of Offshore Barges and Required Horse Power (OTC Paper 3320)

Both the methods are suitable for the conventional material barge tow and give comparable

results. However, it should be noted that none of the resistance prediction methods are exact

and can vary depending on the hull form parameters, tug parameters and environment

parameters.

Range of Validity

Below range of validity may be used as a guideline to verify the accuracy of the results.

However, slight variations in outside the below parameters are also found to give reliable

results.

METHOD Cb V/√L OTC ~0.90 0.0-0.4 BV ~0.90 0.0-0.4

Unit System

Current version of TOWMASTER uses meter-Metric Tons.

TOWMASTER Inputs

Main input sheets for TOWMASTER is similar to the below. The user interface may vary

depending on the platform and equipment (iPad or iPhone).


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Input Sheet Hull Parameters


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Hull Appendages Set Tow Conditions

Definition of input parameters is as below.

Job Name

Your project name or job name. This will be printed in the output file.

Device Name

This field is auto populated with your device name by default. User may change it as required.


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The button will take you to the input sheet

for hull parameters, which are as below.

Hull Parameters

Length WL

Length of waterline in meters.

Beam Mld

Moulded breadth in meters.

Depth Mld

Moulded depth in meters

Draft Fwd Mld

Froward Draft (mld) in meters

Draft Aft Mld

Aft Draft (mld) in meters

Displacement

Floating displacement in metric tones of 1000 kg.

Wet Surface Area

Wetted surface area of the hull. If not known, leave blank. The program uses the following

equation to calculate the wetted surface area, which holds good for barges (Ref 3).

𝑊𝑊𝑊𝑊𝑊𝑊 = 0.92 ∗ 𝐿𝐿𝑊𝑊𝐿𝐿(𝐵𝐵 + 1.81 ∗ 𝑇𝑇)

Where,

LWL is the length of waterline,


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B is the Beam (Mld)

T is the Mean Draft

Cargo Wind Area

This is the windage area of the cargo on the barge in m2.

Area Center abv wl

This is the elevation of center of pressure of the windage area above mean sea level. The wind

speed at 10m above mean sea level will be scaled to the actual elevation as per DNV-RP-C205.

Shape Factor

Shape Factor for the material barge. It ranges from 750 for a shaped bow to 1000 for

rectangular box. Average value of 850 is used as default.

Service Allowance

This is the increase in resistance due to unaccounted factors. A default allowance of 10% is

consided.

Days since drydock

This will determine the increase in resistance due to fouling and marine growth. Default value is

730 days (2 years). Based on the actual number of days, the hull roughness is increased as per

BV guidelines.

Clicking the button takes the user to the

Appendages when applicable appendage area and form factors can be set. The ‘Appendage’

view has various appendages such as ‘Skeg’, ‘Bilge Keel’, ‘Other Appendage #1’ and ‘Other

Hull Appendages


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Appendage #2’. The button next to the appendage has the format of A=xx, f=yy (stands for

Area (m2) = xx and form factor = yy). Clicking the individual button will take you to the

appendage edit page where you can modify the appendage area and form factors. If you do not

know the form factors, leave the default values or refer the note (see below snapshot).

Clicking Done will update the Appendage Areas.

Clicking the button will take you to the tow

condition settings view. By default a ‘STALL’ and ‘TOW’ conditions area provided.


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The default values for ‘STALL’ condition are as per the Noble Denton Guidelies for Marine

Transportation. User may change the values as required. TOW condition is the fair weather

tow condition by default. Other parameters are as below.

Significant Wave Height

This is the significant wave height for stall and tow condition. Default value for ‘STALL’

condition is as per major Marine Warranty Guidelines. However, user may change this as per

the weather data for the tow route and required tow speed for normal tow conditions.

Current Speed

Surface current speed in m/s. It is assumed that the current speed is constant over the draft

and surface current is conservatively used to estimate the drag force on the underwater hull.

Wind Speed

This is the design wind speed for the tow route at 10m above waterline. Normally a 1 hour

mean wind speed may be considered for tow condition.

Start Speed, End Speed and Speed Increment

The required start, end and incremental speed for with bollardpull is to be calculated.

Program Settings

Below screen gives the program settings. Properties of seawater (specific gravity and kinematic

viscosity) and air (specific gravity) are as per standards. Tug efficiency is as per DNV Rules for

Planning and Execution of Marine Operations. Some guideline recommends tug efficiency as a

function of bollardpull of towing tug (such as Noble Denton). In such cases, the user is

requested to modify the tug efficiency manually based on the available tug. By default, both the

bollardpull calculation methods are selected.


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Click to run the bollardpull calculations. Immediately after completion of

calculations, user will be taken to the Report View, where user can review the report and email

as required.

Icons and Symbols

Click the Email Icon on the ‘Report View’ to email the result.


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To display this User Manual, click . User can email the User Manual by

clicking the Email Icon below the user manual.

Click at any stage to go back to the previous screen.

TOWMASTER Resistance Calculation

Total resistance is expressed as sum of frictional, residuary, appendage, wind and sea

resistance.

R Total resistance expressed as

(Rt

f + Rr + Rappendage + Rwind + Rcurr + Rwave) x Service factor

R Frictional Resistance. Frictional resistance is calculated as per ITTC 1957 f

R Residuary Resistance. Calculated as per the specific curves provided as per BV

and OTC Paper 3320 (Ref 3 & 4) r

R Appendage Resistance, based on form factors as per Holtrop & Mennen ’82 (Ref

5, 6) appendage

R Wind resistance expressed as 𝜌𝜌.𝑊𝑊𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 .𝑉𝑉2.𝐶𝐶𝑤𝑤 ., where V is the wind speed at

center of windage area. Note that the wind speed input is the wind speed at 10m

elevation, which is transferred to wind speed at desired height based on DNV-

RP-C205.

wind

R Sea resistance is calculated in accordance with DNV guidelines. sea


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TOWMASTER Results

TOWMASTER results include list of all input data for the vessel, cargo, environmental data and

area and form factor of appendages. Information on the range of validity is also printed. The

calculation results include the individual component of resistance for frictional, residuary,

appendage, wind, sea resistance, total resistance and estimated bollard pull for each of the

analysis method and environment condition selected.

NOTE: In some cases, it may happen that the bollardpull for TOW is more than the STALL condition

and available tug bollardpull is only adequate for Stall condition. The effect of this will be a

reduced tow speed at normal tow, which may be acceptable based on operational requirements.

Limitations

1. TOWMASTER is capable of calculating the bollardpull for conventional material barges

under tow and not intended for higher Froude numbers (V/√L> 0.4).

2. The methodology is limited to conventional barges and shall not be used for other than

material barges under tow (such as drill ships, fast crafts, slender hulls etc.)


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References

1 Det Norske Veritas - Rules for Planning and Execution of Marine Operation

2. Noble Denton - General Guidelines for Marine Transportation (0030/ND)

3. Bureau Veritas - Towage at Sea of Vessels or Floating Units

4. Offshore Technology Conference (OTC) Paper #3320 - Resistance of Offshore Barges

and Required Horse Power

5. J.Holtrop - A Statistical Re-Analysis of Resistance and Propulsion Data, 1984

6. J.Holtrop & G.G.J Mennen – An Approximate Power Prediction Method, International

Shipbuilding Progress,1982

7. Det Norske Veritas – Environmental Conditions and Environmental Loads (DNV-RP-

C205)


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Appendix 1.1 - Example 1


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Objective : To find out bollardpull requirement of a barge with following dimensions. The barge

has a jacket of frontal windage area of 500m2.

LWL = 76.2m

Beam Mld = 27.4m

Depth Mld = 4.88m

Draft Aft = 2.3m

Draft Fwd = 1.7m

Displacement = 3760MT

Wetted Surface Area = Unknown

Shape Factor = Default

Windage area of jacket = 500m2

Center of pressure of wind area = 25m

Skeg area = 20m2


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Input screen for the problem above is shown here.


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Output of Example-1


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