Skid Design

 

The skid must support large dynamic forces and loads. There are two common engineering studies related to the compressor skid:

1. Lifting, Loading and Transportation Study This analysis will ensure the skid can be lifted, transported to site, and withstand environmental loads. Skid applications will need to meet industry codes and customer specifications for these loads, and applies to any skid application.

2. Dynamic Skid Study The study is focused on rotating or reciprocating machines. It will assess the vibration and dynamic stress amplitudes in the skid structure and components mounted on the skid to identify if modifications are required.

For many packagers, BETA can also help reduce the cost of the skid (Skid Optimization Study). Contact us for further information on ways to reduce unnecessary costs (labor and materials).

Note that the final skid design is affected by the compressor foundation design (i.e., the dynamic stiffness will vary depending if a concrete, gravel, steel structure or pile foundation is used). 

1. Lifting, Loading and Transportation

Scope

  • Applies to any skid application (supporting process piping and vessels, compressors, pumps, engines, generators, etc.)
  • Evaluate the skid design to ensure the skid member stresses are within the AISC allowable. Other lifting standards may also apply for offshore platform or FPSO applications (such as API RP-2A-W5D).
  • Evaluate skid design based upon lifting loads, transportation and hold-down requirements, environmental loads such as wind, seismic, motion and/or inclination of skid in FPSO or offshore applications.

Features in BETA's Study

  • Skid deflections due to lifting are evaluated to industry or customer specifications.
  • Skid lifting lugs are appropriately sized.
  • The centre of gravity is calculated.
  • Spreader bar and shackle design are recommended.
  • As an option, BETA can perform anchor bolt loading and Finite Element Analysis (FEA) to evaluate localized skid flexibility, gusset and equipment mounting design.
  • Report includes proposed modifications to the skid design and a summary of the applied loading and calculated results.
 

Transportation and in-place analysis
Lifting, Loading and Transportation Study
considers many factors

 

Skid mode shape plot
Example of a skid mode shape plot

 
 

2. Dynamic Skid Vibration and Stress Analysis

Scope

  • Define magnitude and frequency of dynamic loading on the skid. (Dynamic loads will vary depending upon the application.)
  • For reciprocating compressors, the dynamic loads include unbalanced forces and moments, cross-head guide forces, gas rod load forces, pulsation induced forces in piping and vessel, and unbalanced forces and moments in the driver.
  • Calculate skid vibration and stress and compares to industry or customer guidelines.
  • Consider boundary conditions from the foundation (pile, gravel pad, concrete block or steel structure).

Features in BETA's Study

  • All compressor operating forces are applied vectorially to the model as dynamic loads.
  • Vibrations and stresses are calculated for each load step and operating case.
  • Engine unbalanced forces and moments are added to the operating load. Compressor gas forces and cross-head guide forces are calculated by BETA and applied to the FE model as a dynamic load.
  • The appropriate damping ratio is applied based on BETA’s field experience.
  • Design modifications are provided where vibration or stress exceeds guideline.
  • If required, BETA can calculate deflections and stresses on the skid welds and compare to allowable limits.
  • Report includes recommendations for the skid design and a summary of the applied loads and results (calculated mode shapes, MNFs and vibration amplitudes).
 

Customer Benefits

  • Skid design will comply with international standards (ABS, API, AISC) and customer specifications
  • Ensure safe lifting, transportation and operation
  • Optimize skid costs, by reducing unnecessary costs.
  • Ensure sufficient stiffness (at key locations) to avoid vibration problems for components mounted on the skid

BETA Advantages

  • Cost effective and reliable design, based on BETA’s experience in skid and structural dynamics
  • Accurate FE models
  • Fast turnaround
  • Includes full system (machine dynamics and supporting foundation)

Skid lifting analysis