SHIP ТО SHIP TRANSFER GUIDE (PETROLEUM)

(Third Edition 1997)

For Use with Crude Oil and Petroleum Products


APPENDIX 3

Fender Selection Calculation

In order to select a suitable fender arrangement it is necessary to calculate the kinetic energy of a ship and the energy to be absorbed at the point of contact.

 

The following is only a guide and it is recommended that individual fender manufacturers, fender rental or STS agencies are approached for advice when planning an STS operation.

 

Determining the kinetic energy of a ship:

The displacement of the ship should be ascertained at the intended draft prior to the berthing operation; this can be designated W1.

 

Determining the additional weight of the ship:

This is considered to be equal to the weight of water represented by a cylindrical vessel where

(diameter) D = Draft of the vessel in metres

(length) L = Length between perpendiculars of vessel in metres and can be designated as W2 and expressed as

W2 = 0.805 (D)2 (L).

The total weight to be considered for Ship A is WA = Wl + W2.

 

A similar calculation should be carried out for the second ship (Ship B) to determine the value of WB.

 

Approach Velocity:

The approach velocity of the ships can have a dramatic effect on the energy absorption requirements of the fender system. The allowance for velocity should take into consideration the effects of local weather, sea, swell conditions and the physical size and weight of the ships involved. It is common to work within a range of between 0.1 to 0.3 metres per second (0.2 to 0.6 Knots) and it should be noted that an increase of about 0.02 m/sec in velocity can result in approximately 20 percent increase in energy absorption requirements.

 

During berthing operations between two ships it is rare that they make parallel contact with each other and it is more likely that one fender will absorb the initial contact. Under this condition some of the energy is absorbed by the action of the ship pivoting around the fender and it is generally accepted that about half of the energy is absorbed in this manner.

 

Example: for a single point contact the value of E (energy) can be calculated as follows:

E = 0.051(W)(v)2(0.5) tons/metres

were

 

W =

WA x WB

WA + WB

 

and V = approach velocity in metres per second.

 

Energy absorption requirements in the event of a parallel berthing are given by the formula:

E = 0.051(W)(V)2 tons/metres.

It should be noted that in the event of a parallel contact the load will be spread over the remaining fenders in the system.

 

Calculations should be made for both conditions of berthing and unberthing.

 

EXAMBLE:

 

 

Displacement at Draft (tons)

Draft

(metres)

Length B.P.

(metres)

Ship A

65,000

7.5

260

Ship B

312,000

21.0

320

 

Planned approach velocity of a maximum of 0.2 m/sec and allowing for a single point landing at the forward fender:

 

Ship A

Displacement at Draft                        

W1 = 65,000 tons

Additional Weight = 0.805 (7.5)2 (260)         

W2 = 11,773 tons

 

WA = W 1 + W2          WA = 76,773 tons

 

Ship B

Displacement at Draft                        

W1 = 312,000 tons

Additional Weight = 0.805 (21)2 (320)          

W2 = 113,602 tons

 

WB = Wl + W2        WB = 425,602 tons

 

Where

W =

WA x WB   

=

76733 x 425602

= 65,040 tons

WA + WB  

76733 + 425602

 

Energy (E) = 0.025(65040)(0.2)2                E = 65 tons/m

 

On completion of the calculations for berthing and unberthing, reference should be made to the relevant fender manufacturer's performance tables in order to select a fender system that will provide energy absorption capability in excess of that indicated by the calculations.

 

Should the approach velocity be higher than planned, for example 0.3m/sec, then in the case given above, the Energy (E) will be increased to 146 tons/m as worked example below, with a subsequent possible change in fender requirement.

 

Energy (E) = 0.025 (65040) (0.3)2                E = 146 tons/m


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