BOLLARD

Some pictures are given below for just understanding what a Bollard is.

A bollard is a metal post on a deck of a ship or tug to which mooring lines are fastened. Mooring bollards are used to secure ships and boats to piers or docks. Normally, the material used for construction of Bollard is steel. Cast iron is also used on large bollards for mooring ships. Bollards are also manufactured with cast steel or ductile iron or stainless steel.

Generally all the bollards are vertical to the water line and are mounted on the deck. The bollards should all be as close as practical to the edge of the deck or gunwale but not so close that any rope around them overhangs the ship’s side.

The bollards on the vessel must accommodate sufficient turns of the rope to tie and knot around them. The bigger the diameter of the bollard the more space it takes up on deck and if the bollard is too small it may not withstand the bollard pull and may get damaged.

Polypropylene mooring lines are generally used as ropes. Knots will be put around bollard pipes and the ropes will never be tied onto the pins. The pairs of bollard pipes will be sufficiently apart to accommodate the ropes around them. There will be 3 to 4 turns of ropes round the bollards below the pins.

The pins, projecting through the bollards on each side, are arranged to point fore and aft parallel with the centerline of the vessel.

The line loads are influenced by numerous factors including vessel size, wind conditions, current conditions, passing vessels, elasticity of the mooring lines, vessel deck equipment, tidal levels, bollard spacing, etc.

"Bollard pull" is an industry standard used for rating tug capabilities and is the pulling force imparted by the tug to the towline. It means the power that an escort tug can apply to its working line(s) when operating.

Bollard load ratings are certified using structural analysis and calculations. The bollard itself can be rated based on its yield strength or ultimate strength of the material used in construction of bollard.

Bollard pull may be acting on the bollard in the direction of 0° to 45° in the vertical plane and 0° to 180° in the horizontal plane.

The theoretical point of loading for the line pull shall be the intersection of the bollard vertical axis centerline and the horizontal axis running through the center of the pins. The factor of safety of the bollard against yield shall be 2.5 and the factor of safety against breaking shall be 3.5.

Indian Register of Shipping ( IRS ) specifies certain rules to be followed. With respect to Bollards, the rules to be followed are explained in Part 3, Chapter 15, Section 6. The rules are to be considered at the time of designing a Bollard and are to be complied.

Some of the points are listed below for your reference. They are:

These shipboard fittings associated with the towing and/or mooring, operations at bow, sides and stern are to comply with the requirements given in Section 6.4 and 6.5.

For normal towing operations (e.g. harbour / manoeuvring), 1.25 times the intended maximum towing load (e.g. static bollard pull) to be considered as Design Load.

When a specific SWL (Safe Working Load) is applied for a shipboard fitting by which the design load will be greater than the above minimum values, the strength of the fitting is to be designed using this specific design load.

The shipboard fittings associated with towing and/or mooring, including their welded or bolted connections to the supporting structure, are to be in accordance with a recognized standard, e.g. ISO 3913. Alternatively, the strength of the fittings and its attachment is to be proved adequate for the design load specified in Section 6.4.

Shipboard fittings are to be suitably located on longitudinals, beams and/or girders so as to facilitate efficient distribution of the towing and/or mooring loads.

The net scantlings of the underdeck supporting structure are to be adequate to meet the design loads specified in Section 6.4.1, considering the possible locations for transfer of the load and any horizontal or vertical variation in direction of the applied load. The acting point of the force on the shipboard fittings is to be taken at the attachment point of the towing/mooring line or at a change in its direction.

Allowable stresses given below are not to be exceeded:

Normal stress = σy

Shear stress, τ = 0.6 σy

Where σy = specified minimum upper yield stress [N/mm2];

Normal stress is the sum of the bending stress and axial stress with the corresponding shearing stress acting perpendicular to the normal stress.

Stress concentration factors are not to be taken into account in the above allowable stresses.

The required gross thickness is obtained by adding a corrosion addition of 2.0 [mm] to the net scantling except for oil tankers and bulk carriers covered by the common structural rules, where the corrosion additions specified in those rules would be applicable.

The safeworking load SWL for normal towing operations (e.g. harbour / manoeuvring) is not to exceed 80% of the design load as per Section 6.4.1a) and that for other towing operations (e.g. escort) is not to exceed the design load as per Section 6.4.1b). For mooring operations, the SWL is not to exceed 80% of the design load as per 6.4.1c).

The above requirements on SWL apply for a single post basis (no more than one turn of cable).

The safe working load SWL of each shipboard fitting is to be marked (by weld bead or equivalent).

However, it is most essential to refer the rules before designing the Bollard.

Now, we will see some types of Bollards.

Though, there are bollards with different shapes, some of the types are:

a) Single Bollard,b) Double Bollard, c) Cruciform Bollard.

Fig. Single Bollard

Fig. Double Bollard

Fig. Cruciform Bollard

Bollard pipe material shall be conforming to ASTM Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc- Coated, Welded and Seamless ASTM A 53 Gr B or equivalent. The Schedule no shall be decided on its size and properties. The equivalent BIS specification is IS: 1161 Yst240 (Steel tubes for structural purpose).

The base ( chest ) of the Bollard can be fabricated with steel plates confirming to IS 2062 Gr. B which is equivalent to ASTM-A-36. The fabrication shall be done by means of welding and all structural welds shall be full penetration.

The base of the bollard is provided with stiffeners to give adequate strength and the bollard pipe is welded all round with the top plate and with the stiffeners.

The entire fabricated bollard with base is welded on to the deck plate all the four sides by means of continuous welding to avoid water entering into the inner area and making it to corrode.

Below the deck plate, the structure is strengthened by providing suitable additional stiffeners between the frames and/or longitudinal members to avoid deformation of the deck plate and the structures below it. The strength of the framing below the deck shall also be analyzed and it is seen that all the stresses are well with in limits.

Now, we will try to understand about Bollard Pull, because, based on that only we are going to design the Bollard.

What is Bollard Pull ?

It is a measure of a tugs pulling power. It is usually measured in tons by securing the tug to a scale. The tug then pulls against the scale and its bollard pull in tons is recorded. Bollard pull is determined by a combination of the tug's horse power and the type of propellers that it uses. Bollard pull is the zero speed pulling capability of the tug. It is a measure of the usefulness of the ship in a stranding scenario or in holding a large tanker or aircraft carrier off a lee shore. Ideally, bollard pull is tested when a tug is built and certified by one of the classification societies. Bollard pull tests sometimes are performed after major engine overhauls.

There are basically two approaches for calculating Required Bollard Pull of a Tug for towing a Barge or a Ship. The first method is to determine the frictional resistance, wave-making resistance, wind

resistance, current resistance and towrope resistance for a given tow speed and sum all these up and depending on the units used convert the same to BHP, after taking an efficiency factor.

The second method is to determine the total force (which includes wind resistance, wave resistance, current resistance, all at zero tow speed), that is the force required of the Tug to ensure that it can hold the Tow in a given environmental criteria. As per IMO Guidelines for Safe Ocean Towing, a 5 metre wave, 40 knot wind and 1 Knot current is taken.

The bollard pull of every tug that tows an oil barge on Foreign, Home Trade or Inland Waters voyages should be determined by a recognized test procedure, and checked by retest whenever changes are made to the propulsion system that affect bollard pull.

The towline should

i) comply with the strength criteria given in Table 1 and ii) be supplied to the tug with manufacturer's certificates which attest to the strength rating thereof.

Table 1

VOYAGE DESCRIPTIONBOLLARD PULL

OF TUG(BP) in tonnes

BREAKING STRENGTH OF TOWLINE

in tonnes

EXPOSED COASTAL TOWS≤ 32 te> 32 te

4.5 x BP144 + 0.7 (BP-32)

SHELTERED COASTALTOWS

≤ 35 te> 35 te

4 x BP140 + 0.75 (BP-35)

PROTECTED WATERTOWS

≤ 35 te> 35 te

3 x BP105 + 1.15 (BP-35)

Tug Bollard Pull vs Towline Breaking Strength for tugs not exceeding 100 te bollard pull

The breaking strength of chains, bridles, shackles and other gear used in making up the towline assembly should be consistent with the breaking strength of the towline.

Also the following formula allows a direct rough calculation of BHP (Break Horse Power) but taking no account for external factors such as wind and waves:

BHP = 2D/3*v^2 /120where:D = Displacement of the tow (t)v = towing speed in knots

To pass from BHP to bollard pull in tons use:

Fixed pitch propeller: (freewheeling) BHP / 100 = (t)Fixed pitch propeller and kort-nozzle: BHP * 1.08 / 100 = (t)Controllable pitch propeller: (freewheeling) BHP * 1.125 / 100 = (t)Controllable pitch propeller and kort-nozzle : BHP * 1.26 / 100 = (t)

A quick and rough estimative for engine's power in BHP is 100 to 125 times the bollard pull in tonnes.

Another formula to roughly determine the requested Bollard Pull under consideration of aerodynamic resistance and Seas state:

Bollard pull (tons) = ((D^2/3 * v^3)/7200 + Cmv*B*D1)*K

where:D = Displacement of the tow (t)v = Towing speed in knotsCmw = coefficient for the mean wind speedB = Width of the tow (m) (transverse to movement)D1 = Height of the wind facing area above water level, incl. Deck cargo (m)K = Factor 3 - 8, depending to the circumstances

This formula should only be used during following two situations:• Ordinary towing conditions (Beaufort wind scale BFT. 4) V = 6 knots: Cmw = 0,0025: K = > 3• Keep on station during heavy weather (Beaufort wind scale BFT. 10-11)V = 3 knots: Cmw = 0,015: K = 8.

After calculating the Bollard pull, the Bollard shall be designed based on the bollard pull. Certain basic standard dimensions are taken as guidelines for deciding the shape and size of the bollard assembly. In the diagram given below, you can see the standard dimensions are shown as alphabets and the numeric values are given in the tabular form against the Bollard pull. Taking these values as basis, all the necessary dimensions can be worked out for designing the bollard.

Once the major dimensions are taken as guidelines, then the strength of Bollard pipe, the strength of base frame and the weld strength are analyzed considering the load factors and dimensional factors.

A sample calculation for assessing the strength of a bollard is given below:

Bollard Strength Calculation

Bollard Pull = F = 6T = 6000 Kg.

=6000 X 9.80661 = 58848.66 N = 58.84866 k N = say 59 k N.

Design Load = 1.25 times of bollard pull = 1.25 x 59 = 73.75 k N.

Distance to the point of Load from XX = 21 Cm.

Hence Bending Moment at plane XX = M = 73.75 x 1000 x 21 = 1548750 N cm.

To assess the strength of Bollard pipe, the sectional properties of Bollard pipe to be calculated.

Sectional properties of Bollard pipe:

Outer Dia of Pipe = 273 mm.

Inner Dia of pipe = 261 mm.

Wall thickness of pipe = ( 273 – 261 ) / 2 = 6 mm.

Area of Cross section = A = π ( D2 – d2 ) / 4= 3.1416 x ( 2732 - 2612 ) / 4

= 5035 mm2 = 50 cm2.

Moment of inertia = I xx = π ( D4 – d4 ) / 64= 3.1416 x ( 2734 – 2614 ) / 64

= 44888898 mm4 = 4489 cm4.

Section Modulus = Z = Ixx / y = 4489 / ( 27.3/2) = 329 cm3.

Bending Stress = fb = M / Z = 1548750 / 329 = 4707 N/cm2

= 47.07 N / mm2 = 47.07 MPa

Shear Stress = fs = F / A = 73.75 x 1000 / 50 = 1475 N/cm2

= 14.75 N / mm2 = 14.75 MPa

Normal Stress = Bending stress + axial stress = 47.07 + 0 = 47.07 MPa.

Taking factor of safety as 2.5,

Max Normal stress = 47.07 x 2.5 = 117.675 MPa = say 118 MPa

Yield Strength of Bollard pipe material = 235 MPa

Therefore, yield stress 235 MPa > Max Normal Stress 118 MPa.

Considering Allowable shear stress as 60 % of yield stress,

Allowable shear stress = 0.6 x 235 = 141 Mpa

Therefore, allowable shear stress 141 Mpa > Shear stress 14.75 Mpa.

If the Bollard Pull is acting at 45º inclination,

Then, the force will be resolved into vertical and horizontal components.

Now, vertical component = F sin θ

and horizontal component = F cos θ

Design Load = 1.25 times of Bollard Pull = 59 x 1.25 = 73.75 k N.

The axial Load (vertical component) = F x sin 45 º = 73.75 x 0.707 = 52.15 k N.

Then axial stress due to this load = axial load / c/s area = 52.15 x1000 /50 = 1043 N/cm2

= 10.43 N / mm2 = 10.43 MPa

Horizontal component = F x cos 45 º = 73.75 x 0.707 = 52.15 k N.

Bending moment due to horizontal component = load x distance = 52.15 x 1000 x 21= 1095150 N cm.

Bending Stress = fb = M / Z = 1095150 / 329 = 3329 N/cm2

= 33.29 N/mm2 = 33.29 MPa

Shear Stress = fs = F / A = 52.15 x 1000 / 50 = 1043 N/cm2

= 10.43 N / mm2 = 10.43 MPa

Normal Stress = Bending stress + axial stress = 33.29 + 10.43 = 43.72 MPa.

Taking factor of safety as 2.5, Max Normal stress = 43.72 x 2.5 = 109.30 MPa

Yield Strength of Bollard pipe material = 235 MPa

Therefore, yield stress 235 MPa > Max Normal Stress 109 MPa.

Considering Allowable shear stress as 60 % of yield stress, Allowable shear stress = 0.6 x 235 = 141 Mpa

Therefore, allowable shear stress 141 Mpa > Shear stress 10.43 Mpa.

Sample Calculation for analyzing the strength of Base frame ( Chest ):

When a Bollard Pull of 6000 Kg ( 59 k N ) is acting at a height of 37 cm above the Deck plate,

Moment due to that force at the bottom of the base frame = 59 x 1000 x 37 = 2183000 N cm.

Cross Section of the base frame for Double Bollard :

Length of Base frame = 940 mm.

Width of Base frame = 395 mm.

Thickness of plate = 10 mm.

Therefore, B = 940 mm, b = 920 mm.

D = 395 mm, d = 375 mm

Cross Sectional area , A = ( B x D ) – ( b x d ) = ( 940 x 395 ) – ( 920 x 375 ) = 26300 mm2= 263 cm2.

Distance from neutral axis, y = 395 / 2 = 197.5 mm = 19.75 cm.

Moment of inertia about neutral axis XX, IXX = ( B x D³)/12 – (b x d³)/12

= ( 940 x 395³)/12 – (920 x 375³)/12

= 784704792 mm4 = 78470 cm4

Section Modulus, ZXX = IXX / y = 78470 / 19.75 = 3973 cm3.

Bending Stress = fb = M / Z = 2183000 / 3973= 550 N/cm2 = 5.5 N/mm2 = 5.5 MPa.

Shear Stress = fs = F / A = (59 x 1000) / 263 = 224 N/cm2 = 2.24 N/mm2= 2.24 MPa.

Combined Stress = fc = √ ( fb2 + 3 x fs

2 ) = √ ( 5.52 + 3 x 2.242 ) = 6.7 MPa.

Yield Strength of steel sections of base frame members = 235 MPa.

Factor of Safety = 235 / 6.7 = 35

In the same way the strength of base frame of single bollard can be analyzed.

Calculation for analyzing the strength of welding of Base frame ( Chest ) with the deck plate :

When a Bollard Pull of 6000 Kg ( 59 k N ) is acting at a height of 37 cm above the Deck plate,

Moment due to that force at the weld joint of the base frame = 59 x 1000 x 37 = 2183000 N cm.

Cross Section of the weld joint of base frame for Double Bollard :

Weld Length of longitudinal side of Base frame = 949 mm.

Weld Length of transverse side of Base frame = 404 mm.

Thickness of weld ( throat thickness ) = 4.5 mm.

Therefore, B = 949 mm, b = 940 mm.

D = 404 mm, d = 395 mm

Cross Sectional area of weld , A = ( B x D ) – ( b x d ) = ( 949 x 404 ) – ( 940 x 395 )

= 12096 mm2 = 121 cm2.

Distance from neutral axis, y = 404 / 2 = 202 mm = 20.2 cm.

Moment of inertia about neutral axis XX, IXX = ( B x D³)/12 – (b x d³)/12

= ( 949 x404³)/12 – (940 x 395³)/12 = 387023253 mm4 = 38702 cm4

Section Modulus, ZXX = IXX / y = 38702 / 20.2 = 1916 cm3.

Bending Stress = fb = M / Z = 2183000 / 1916= 1139 N/cm2 = 11.39 N/mm2 = 11.39 MPa.

Shear Stress = fs = F / A = (59 x 1000) / 121 = 488 N/cm2 = 4.88 N/mm2 = 4.88 MPa.

Combined Stress = fc = √ ( fb2 + 3 x fs

2 ) = √ ( 11.392 + 3 x 4.882 ) = 14.2 MPa.

Yield Strength of weld metal ( considering equal to the strength of parent metal ) = 235 MPa.

Factor of Safety = 235 / 14.2 = 16.5

In the same way the strength of welding of base frame of single bollard with the deck plate can be analyzed.

Sample Strength Calculation of Underdeck ( below Double Bollard) :

When a Bollard Pull of 6000 Kg ( 59 k N ) is acting at a height of 37 cm above the Deck plate,

Moment due to that force on the underdeck framing = 59 x 1000 x 37 = 2183000 N cm.

Cross Section of under deck framing below double bollard ( aft side )

Sectional Properties of Under Deck members below Double Bollard ( Aft side ) at frame 1 and 2

  B T y A A x y Inertia

Distance from the

neutral axisMoment of inertia  

Section 1 1016 8 110 8128 894080 43349 24 4849971  

Section 2 8 106 53 848 44944 794011 -33 1699770  

Section 3 angle 65x65x6 59.1 744 43970.4 291000 -27 816712  

Section 4 6 100 56 600 33600 500000 -30 1028612 stiffener

Section 5 65 6 3 390 1170 1170 -83 2667331 stiffener

Section 6 6 100 56 600 33600 500000 -30 1028612 stiffener

Section 7 65 6 3 390 1170 1170 -83 2667331 stiffener

Section 8 6 100 56 600 33600 500000 -30 1028612 stiffener

Section 9 65 6 3 390 1170 1170 -83 2667331 stiffener

total       12690 1087304     18454282  

Y= 86 mm = 8.6 mm    

Area= 12690 mm² = 127 cm²    

moment of Inertia = 18454282 mm4 = 1845 cm4    

section modulus= 215381 mm³ = 215 cm³    

Bending Stress = fb = M / Z = 2183000 / 215 = 10153 N/cm2

= 101.53 N/mm2 = 101.53 MPa.

Shear Stress = fs = F / A = (59 x 1000) / 127 = 464 N/cm2

= 4.64 N/mm2 = 4.64 MPa.

Combined Stress = fc = √ ( fb2 + 3 x fs

2 ) = √ ( 101.532 + 3 x 4.642 ) = 101.85 MPa.

Yield Strength of steel sections of underdeck members = 235 MPa.

Factor of Safety = 235 / 101.85 = 2.3

Similarly, the strength of underdeck members below the double bollard on starboard side shall be calculated. Though the load parameters and bending moment are same, the sectional properties of the underdeck members below the double bollard on starboard side shall be calculated and then using those values, the stresses shall be calculated. In the case of Single Bollard also, the strength of underdeck members below the single bollard shall be calculated similarly.

The Bollard assembly drawing with all the details can be prepared suitably showing the assembly, details, views, sectional views with required dimensions for manufacturing. The list of parts shall also be given in the drawing by clearly naming the parts with correct specification for the material used for manufacturing.