Question

1. What are the criteria for selecting the force that I need to apply to my structure when I perform a ship impact analysis?

2. Can we distribute the impact force to more than 1 joint in a Dynamic ship impact?

Answer Q1

The force to set in the analysis depends on the type of element that will receive the impact, If the force is absorbed by pipes lower than 36 inches 1000 kN will be enough to reach the element out to the nonlinear behavior, and with this, the absorption of the energy, remember that the energy es related to the deformation of the element, if we have to evaluate an impact in the leg of jacket we have to increase this force looking for the nonlinear behavior of the element.

(This wiki was created based on the experience of the original creator of this post;  input represents the criteria used by the creator in his projects. It's not a rule or guide to be followed.)

Next, the methods for the estimation of this load: the first is applied for elements lower than 36 inches, and the second for an estimate of the forces in the leg of the jackets.

Energy Calculation

The process to get the static force that we apply to the structure depends on the design code used by the project. We can find the formulation in the next code:

• API RP 2A WSD - Annex B - Section B-17 - Commentary on Accidental Loading
• ISO+19902-2020 - Annex A - Section A.10.2 - Vessel collisions
  
  

First, we have to calculate the energy results of the type of vessel using parameters like velocity and mass provided by this code. Using the recommendations in the API, we have the following formulation:

Where,

E = Is the kinetic energy of the vessel

a = Is the added mass coefficient, 1,4 for a broadside collision, 1,1 for a bow/stern collision

m = Is the vessel mass

V = is the velocity of the vessel at impact

For this case, we are going to work with a vessel mass of 1000 tons and with a velocity of  0.5 mps

 

 

Determining Impact Force Method 1

Once you have the kinetic energy for the vessel we have to propose a maximum dent for the contention elements, in this case, we'll propose a 25% dent for the element, you can work with Table B.17.1 of API  in which we have some recommendations to reach a 12.5% 16.7%  y 25% depending of the diameter and thickness sections.

For this sample, our model has a 273mm x 12.7 mm section

with this data, we will suppose a deformation of 25%, and with this, we can get the force:

As you can see, for the Broadside load we have 854 kN, which is the reason sometimes we set 1000 kN as an impact force. With this value applied to the structure, the software will start the deformation, and with this, the absorption energy process will be registered in the collapse log file.

 

Determining Impact Force Method 2

 

Using this method, you may estimate the applied force for impact on legs or elements that aren't embraced in Table B.17.1 of API as follows:

1. Apply a load in the study zone with a value equal to 1000 kN in the direction that you want and run a simple linear static analysis
2. Get the impact joint displacement (U) from this simple linear static analysis
3. Calculate the stiffness of the structure (the slope of the linear line in the above figure) as  Klinear= P/U
4. Calculate the absorbed energy for a given load F:  E = 0.5 x ( F^2 / Klinear )
   
   

Note: 

• The force estimated above is only based on the model and doesn't include other energies, such as member dent or ship deformation.
• Using this method, the linear force is always overestimated, so ensure you enter enough load increments in the load sequence for your collapse analysis.
  
  

You can set 1000kN or 10000kN whatever value that you want, remember that this load will be applied graduated and the process will stop at the moment in which the structure absorbs 100% of the energy of the ship, doesn't matter if you set as an initial load 10 000 kN the, in this case, the process will stop near of the 900 kN load applied, the total energy that the structure shall absorb will be calculated with the parameters given in the card ENERGY