The KNT approach

an inflated rubber bag and dimensions vary from 0.5m to 4.5 m in dia and 1m to 12 m in length. The fender bag is protected by wire or chain net with tyres or rubber sleeves. The energy absorption does not decline at inclined compression for these fenders.

6.6.2.4 Gravity Type Fenders

These are generally made of concrete blocks suspended from a heavily constructed wharf work. the Impact energy is absorbed by moving and lifting the heavy concrete block.

6.6.3 Selection Criteria of Fendering Systems

The selection of a optimum fender for a given service depends on the following factors : 1. The type, size, draft and allowable hull pressure of a vessel.

2. Berthing velocity and angle.

3. Distance between the berthing point and the vessels gravity centre measured along the face of the pier.

4. Water level, tidal range, wind velocity, direction of wind, direction and velocity of currents.

5. Behaviour and installation pitches of Dock fender 6. Structure and strength of Berthing facilities 7. Certain human factors involved in berthing.

6.6.4 Berthing Energy of a Vessel

The design of fenders depends very much on the energy to be absorbed by the fenders during berthing. When a ship strikes the fender, it transfers some part of the kinetic energy to the fender and the other part gets dissipated to the motion of ship in water. Some part of the energy absorbed by the fender is transferred back to the ship, after the ship has come to rest, by the fender trying to recoil back to its normal shape. This process of exchange of energies between fender, ship and the loss of energy in water motion continues till the whole of the kinetic energy of ship is dissipated in water motion. The different methods that are used in determining the maximum amount of energy to be absorbed by the fender is given below :

6.6.4.1 Quinn Method

In this method fifty percent of the energy of the ship calculated on the basis of the velocity of the ship normal to berthing structure is assumed as the energy absorbed by the fender.

4

6.6.4.2 Woodruff Method

In this method the following empirical equation is used to calculate the berthing energy.

E = W(0.004 - W x 10^-8 (6.48)

Where W is in tons and E is in ton feet.

6.6.4.3 Vasco Costa Method

Vasco Costa has given the following analytical solution, for a ship moving with translatory velocity u and angular velocity w, having no slip along the berth.

E = (WV²/2g) (1 + 2D/B) (K² + r² Cos²r / K² + r²) (6.49) Where v Distance P= u + aw

The value of k can be taken as 0.2 L to 0.29 L. The following three coefficients are to be considered along with equation.

R. Sundaravadivelu 1 6.6.4.4 IS: 4651 (Part III) - 1974

As per the Indian standard code of practice, the berthing energy is calculated as follows

m e m

D C xC xC

g 2

V x E W

= 2 (6.50)

where

WD = Displacement tonnage (DT) of the vessel, in tonnes, V = Velocity of vessel in m/s, normal to the berth g = Acceleration due to gravity in m/s²

Cm = Mass coefficient

Ce = Eccentricity coefficient and Cs = Softness coefficient.

The approach velocity varies from 0.1 m/s to 0.75 m/s depending on the size of the vessel, site condition and berthing condition. The above equation depends on the angle of approach and l/r ratio where l is the distance from the centre of gravity of the vessel to the point of contact projected along the water line of the berth in metre and r is the radius of gyration of rotational radius on the plane of the vessel from its centre of gravity in metre.

The l/r ratio is in the range of 1 to 1.25. The angle of approach varies from 0 to 20 degrees. The softness coefficient indicates the relation between the rigidity of the vessel and that of the fender. The value of 0.9 is generally used for this factor.

6.6.5 Fender Reaction

The fender reaction depends on the approximate energy to be absorbed and the characteristics of the fender. If the fender reactions are transmitted to the backfill immediately behind the quay wall there will be no problem in absorbing the reaction. If the structure, like open piers and jetties are to be designed for these reaction forces, the forces are critical since they control the design of these structures. If P/E ratio varies from 2 to 7 depending on the type of fender where P is the berthing force in T and E is the energy absorption at 50 % of defletion in T. m, in such cases it is preferable to have fenders with low reaction per absorbed unity of energy (P/E). It is also important to consider the fender performance beyond the rated energy capacity, since the fender reaction increases swiftly.

6.7 SINGLE BUOY MOORING SYSTEM 6.7.1 General

The art of implanting floating structures in the ocean is as old as man’s history. Marker buoys, mooring buoys and navigational buoys have long been familiar sights in the harbours and waterways and along the sea shores. The recent past has seen many large and sophisticated buoy mooring systems deployed in deep waters for a variety of purposes.

A buoy mooring system consists of a buoy or buoys, connected by cables and anchored to the seabed. Being a compliant structure, the system is responsive to external effects and the movements are controlled by the mooring system. Buoy mooring systems are flexible and provide a progressive elastic response to environmental forces absorbing and dissipating energy from the ocean environment. To understand the effect of these constrained or freely drifting buoyant structures often require advanced Engineering knowledge from many disciplines are often required.

A buoy can be considered as a major positively buoyant component in the system. Buoys may be classified, based on their position into three general groups, as (i) surface buoy system where the buoy floats on the surface (ii) subsurface buoy system where the buoy is below the sea surface and (iii) two part buoy system where it is a combination of the above two systems.

Surface buoys can be surface following type with shapes such as spherical, cylindrical, disks etc. or surface decoupled such as spars. Surface following buoys have the advantage of having a large buoyancy to drag ratio whereas the spar buoys have small buoyancy to

R. Sundaravadivelu 1