dolphin/piling design 1

[Leslie]

I am looking for information about dolphin/piling design, particualy regarding the pull out design. This may be more of a geotechnical question, so I'll also post there. Thank you.

 

[OSMEL]

Leslie that's a pretty broad question, but I will try to respond to all what the question implies.

The first important thing is to have the theorical capacity of piles in compression and tension. They are of course different, because in compression you have point-bearing capacity plus friction capacity, and in tension, you will only have friction capacity. For calculation of pile capacities, you can check any soil mechanics book. There are different formulas and they will give you different results. So you can take a pair of them for calculation. Moreover, some authors don't take the same value for friction in compression and in tension.

By the way, tension is important because in a dolphin and other maritime structures many loads are basically horizontal: quake, berthing, and mooring, and this usually produces tension in some piles and compression in others.

You also need a profile of your soil, and according to the proposed alternative designs obtain the capacities of various pile types and pile diameters or sizes in several foundation quotes, if there are more that one alternative for the sub-structure. In theorical calculation, a safety factor of 2,5 to 3 is advisable, although it depends on conditions of the soil (If it is or not homogeneous). Safety factors can be reduced if you have site information from pile capacity tests, or pile dynamic analyzers.

Dolphins are designed for the next load cases (also for compare with soil capacity):
1. Self dead weight
2. Live weight
3. Earthquake (if it applies)
4. Berthing
5. Mooring
6. Wave forces

If you are in a seismic zone, earthquake load is generally greater than berthing or mooring forces. So, if it is your case, most of the times, you only have to compare the values for berthing and mooring forces, to see if it is greater.

Berthing and mooring forces depends on a "Design Vessel", that is a definition of a vessel with a dead weight tonnage and its dimensions Length, width or beam, and depth or draught, lateral area affected in and up the water (in most cases this is a real vessel selected by the designer).

If you have fenders in your dolphin, for berthing, you must take into account the "Berthing Energy", that is the total energy that must be absorbed by the fendering system, that depends on the movement of the ship and the water surrounding it during the impact at berthing.

One of the formulas for berthing energy is:

E= (W1+W2)*V^2*K/(2*g)

Where:
E= Effective Berthing Energy (Ton-m, kgs system)
W1= Displacement Tonnage that is equal to weight of ship plus cargo (Ton)
W2= ro.water*L*H^2*pi/4
K= 1/(1+(L1/R)^2)
Ro.water= water density= 1,025 Ton/m3
Pi= 3,1416…
L= Length of ship (m)
H= Full draught (m)
L1= distance of line parallel to wharf measured from the contact point to the center of gravity of the ship (m)
K= excentricity factor. Because must ships berth at approximately 1/4 of length, K value becomes 0,5.
V= Design berthing speed, usually in the order of 0,1 to 0,15 m/s. It depends of the natural site conditions.
g= Acceleration of gravity= 9,81 m/s2

The berthing energy is dissipated in the fendering system and have associated a reaction force, that will be in the simplest way the berthing force you must design your dolphin (The real thing is that the berthing energy is also dissipated in your structure also, but you don't need to complicate that much).

You can talk with a fender representative to have a manual of the fenders system, which will give you an idea of the rated reaction force for a selected fender.

For mooring you must take into account the wind and sea current forces (separately -the mayor of it, or in some combination depending on site conditions). The general formulations for these forces are:

W= c1*ro.wind*A1*Vw^2*sen(fi1)
C= c2*ro.water*A2*Vc^2*sen(fi2)

Where
W= Total water force to the ship (Ton)
C= Total current force to the ship (Ton)
C1= Coefficient that depends on the configuration
C2= coefficient that depends on the configuration and clearance below the ship
Ro.wind and ro.water are respectively the wind and water density (Ton/m3)
A1= Total area below the line of water (m2)
A2= Total area up the line of water (m2)
Vw, Vc are the design speeds for wind and current (m/s)
Fi1 and fi2 are in that order, the angles of the principal directions of wind and current with the axis of the ship

This value is, of course, for all the ship, so you must assign with some criteria the forces for each mooring (Easy as total forces/ number of moorings, or other more complicated and that depends of the mooring configuration. If the moorings are uniform the first approach will be enough).

Because, usually moorings take different angles (vertical, and horizontal), it is good to take several cases in a range from 30-90 degrees horizontal (in the pier axis) and 0-30 degrees in vertical. A simpler way is to take the mooring force at 90 degrees horizontal and 30 degrees vertical.

Moreover, it is advisedly to check the structure for the maximum pull out value of your selected mooring (You will not like that the vessel pulls out your dolphin...). If the mooring is already selected for some reason, you just have to check for wind and current values, and design for that pull out value.

For most cases waves forces are negligible because piers are usually located in sheltered areas. If it is not your case, and waves are bigger, say in a marine platform, you must take into account that effect, and if you need some information about this I can give it to you.

Load combinations must be (the number is for load case)- no load multipliers are mentioned:
I. 1
II. 1+2
III. 1+2+3
IV. 1+2-3
V. 1+2+4
VI. 1+4
VII. 1+2+5
VIII. 1+5

For the V Combination it is not necessary to take all the live load, because the piers aren't charged with much weight during berthing operations, you can take say 25% of it.

Also, it could be possible that if you have a pier in which the vessel berth in two or more sides of it, you must check for a combination of berthing and mooring, depending of the operations, and this requires some type of criteria.

You must compare the maximum axial forces (compression and tension) in each combination with the design capacity of the pile in tension and compression. For dynamic loads (this includes wind, currents and quake) you can reduce your safety factor to 2-2,5.

Many dolphins require weight in order to minimize tension on the piles. In the strictest way you must check for the moments and the reaction of the soil for them, but that is another long history. Usually it is only necessary to check the axial force.

Your structural members (piles, beams- usually don't used-, and slab) are designed as usual for bending moments and axial forces following anyone of the codes or methods. A last reminder is that for the structural analysis you can suppose that the pile is fixed at a point below the sea bed level, just a pair of meters (1-2). There are some formulas for that, but my time is up, so this will be for another occasion.

 

[Donald]

I like this reply to the question of mooring forces, but could somebody please clarify the values for the coefficients for wind and current forces, as I find very different results form British Standards and other sources.

Donald

 

Can anybody provide me with some info on mooring design. I am planning to replace big buoy mooring systems by dead weigh, chain and nylon rope without the buoy as a structural part of the mooring system. However a buoy has to be attached to the system so the mooring line can be retrieved.
I'd like to know how to calculate the size and shape of the dead weight if it is made of concrete.

Thankx