| Product(s): | Sacs Collapse |
| Version(s): | 16.1 |
| Area: | Collapse Solutions |
Community Question:
What are the criteria for selecting the force that I need to apply to my structure when I perform a ship impact analysis?
Answer
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.
Remember the posts in this wiki are created according to the experience of the creator of this post and this is not a rule or guide to follow, this represents the criteria used by the creator of his projects.
Next, are presented 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 is very simple, this depends on the formulation of 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 next 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 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
whit this data we will suppose a deformation of 25% and with this we can get the force for
As can you see for Broadside load we have 854 Kn, this 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 absorb 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 energy like 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.
In fact, 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