AutoPIPE - Foundation Loads calculation for Saddle Supported vessels

Basis Of Calculations:

Reactions per support, bending moment and shear loads are studied using a beam model on simple supported, with one of the supports fixed either to the left or the right of the beam.

The beam is studied using the reduction process which is based on the Falk transmission matrices.
Reference axes are : beam x on the right, y up with positive loads down, moments > 0 from x to y.
Shell, liquid, and bundle own weight are considered as distributed loads while head weight, flange and cover, floating head and nozzle are considered as concentrated loads.

Thermal effects are considered by the friction factor to be an additional moment at the saddle location. AutoPIPE Vessel engine accounts for thermal expansion effects regardless of the design temperature when saddle friction is mentioned. If there is no differential thermal expansion, We recommend setting the friction factor to zero, so AutoPIPE Vessel will not treat it as if differential thermal expansion is present.

A fixed saddle balances all horizontal reactions. The horizontal longitudinal force due to friction is a function of the weight on the sliding saddle multiplied by the coefficient of friction, the additional vertical loads due to the earthquake are not considered.

AutoPIPE Vessel employs a simplified and highly conservative analysis method recommended by TEMA 8th Edition, section 10, to calculate saddle supports reactions for a horizontal vessel.

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Go through the training Video available on YouTube on Interpretation of saddle reactions and forces for horizontal vessel using Link Here.

How Horizontal (longitudinal) reaction (RaHL) is Calculated?

Refer below vessel configuration to proceed further:

Follow the below steps to calculate the horizontal longitudinal friction forces on saddles,

1. Step 1: First change the friction factor at saddle base to zero and perform the calculation
2. Step 2: Open the summary of foundation load and note down the vertical loads (Weight of the vessel),

Here there is no negative vertical load since no horizontal load which can create the moment.

1. Step 3: Now change the friction factor value to 0.3 and perform the calculation.
2. Step 4: Open the summary of foundation load and note down the loads,

1. Step 5: Verify the horizontal loads. Note that in this calculation we need to consider the vertical load on saddle which are calculated with friction factor of zero.
  RaHL2 = Rav2 * Friction factor = 5941*0.3 = 1782 daN
  RaHL3 = Rav3 * Friction factor = 5941*0.3 = 1782 daN
  RaHL4 = Rav4 * Friction factor = 5772*0.3 = 1731 daN
  
  Since system is in statically equilibrium condition sum of all horizontal loads are equal to zero.

RaHL1 = RaHL2 + RaHL3 + RaHL4 = (1782+1782+1731=5295 daN) but in opposite direction of other loads on rest of the saddles.

1. When the longitudinal loads exist, the vertical loads are modified. These modified values are due to the moments.

Why there is negative vertical load at the saddle support?

It is essential to understand that one saddle must remain fixed. This fixed saddle serves to counteract all horizontal longitudinal forces arising from differential thermal expansion. As a result, additional moments are introduced at the saddle location on the equivalent beam.

In example model, the first saddle is fixed while the remaining saddles are sliding. Due to this configuration, the moment at the first sliding saddle is significantly high, resulting in negative vertical loads.

It’s important to clarify that these are not real-time loads as would be calculated using finite element analysis (FEA) tools such as ADINA or ANSYS. Instead, they are theoretical estimates based on initial displacement assumptions.

We trust this explanation provides a clearer understanding of the saddle support loading results. Additionally, it’s worth noting that the TEMA methodology is widely recognized in the industry as a recommended best practice.

To obtain real-time load values, a detailed FEA must be performed. As a practical workaround, you can run the analysis using a zero friction factor. This approach eliminates the negative vertical loads, as seen in the test condition results, which also assume zero friction.