Skid Analysis

What is Skid Analysis

A compressor skid is a fabricated steel structure designed to support the installation of the compressor, driver, vessels, piping, and other equipment required for the compressor system.

Skid analysis typically includes lifting analysis, transportation analysis, operating analysis, and dynamic analysis:

Skid lifting analysis: This evaluates the stress and deflection of the skid structure during the lifting process. The assessment of lifting lug strength is often included in this analysis
Skid transportation analysis: This assesses the stress and deflection of the skid structure during transportation.
Skid operating analysis: This evaluates the stress and deflection of the skid structure due to operational loads, such as weight, wind, snow, and seismic forces acting on the skid.
Skid dynamic analysis: This examines the mechanical natural frequencies and vibration levels of the skid structure and its mounted equipment and piping, caused by dynamic loads, including unbalanced forces and moments from the compressor and driver, cylinder gas forces, cross-head dynamic forces, torque variations, and pulsation-induced shaking forces on vessels and piping.

A Skid Structure and the Mounted Equipment and Piping

Why Perform Skid Analysis

Skid analysis is essential for ensuring the strength and stability of the skid structure, allowing the compressor system to operate effectively.

Skid lifting analysis helps mitigate potential risks such as excessive stress and deflection of the skid structure, misalignment of the driver-coupling-compressor train, and even the overturning of the skid. Additionally, it prevents lifting lug failure during the lifting process.
Skid transportation analysis addresses the potential risks associated with excessive stress, deflection, and potential damage to the skid structure due to transportation loads, ensuring the structural integrity of the skid throughout transport.
Skid operating analysis ensures the overall strength and stability of the skid structure, promoting the reliable functionality of the compressor system during operation and preventing unnecessary shutdowns.
Skid dynamic analysis mitigates the risks of mechanical resonance and excessive vibration levels, which can lead to damage to the skid structure and the mounted equipment and piping, ensuring the proper functionality of the compressor system.

Skid Analysis Methodology

Skid Analysis Procedure

Below is a flowchart outlining the procedure for skid analysis of compressor packages.

Information collection involves gathering the necessary input data for the analysis. This includes skid design drawings that provide dimensions, material properties, welding or fastening details, information about the equipment and piping mounted on the skid, including their locations, installation details, weight, Center of Gravity (CG), operating conditions, and other relevant details.
Generation of analysis model is to use analysis software to build the analysis model based on the collected input data. This process includes modeling the skid structure, mounted equipment (such as compressor, driver, vessels, etc.) and piping, and setting appropriate analysis boundary conditions.
Verification of the model involves validating the input data and performing a test run to ensure that the model accurately represents the actual structure, equipment and piping, including their weights and Center of Gravity (CG).
Applying loads and boundary conditions involves defining the applicable forces, moments, and displacement constraints acting on the skid structure based on the specified analysis.

Lifting analysis: The applied loads consist of the self-weights of skid structure, mounted equipment and piping, and any applicable external loads. Restraints are applied at the lifting points.
Transportation analysis: Applied loads include the self-weights of skid structure, mounted equipment and piping, and acceleration loads due to transportation movements. Restraints are applied at the skid supporting locations.
Operating analysis: The applied loads include the self-weights of skid structure, mounted equipment and piping, live loads, and environmental loads such as wind loads, snow loads and seismic loads. Restraints are applied at the skid installation locations.
Dynamic analysis: Dynamic loads include unbalanced forces and moments generated by the compressor and driver, cylinder gas forces, cylinder cross-head dynamic forces, torque variations, and pulsation induced shaking forces on vessels and piping. The boundary conditions for dynamic analysis are consistent with those used for operating analysis.

Developing solutions involves performing the specified analysis to determine whether the skid structure design meets the allowable limits for stress, deflection, and vibration levels. If the design does not meet these requirements, modifications of skid design are necessary. This may include changing skid beam sizes and/or layout, reinforcing areas experiencing excessive stress, deflection, or vibration by adding structural beams, gussets, or enhancing the strength of the lifting lugs. This iterative process continues until a satisfactory solution is achieved, ensuring that calculated stress, deflection, and vibration levels remain within the allowable limits as specified by applicable codes or industry standards.
Generating result files involves creating and compiling analysis results and documentation for all operating conditions being investigated, and presenting these findings for review.

Skid Analysis Guidelines

(1) Maximum allowable stress and deflection

The maximum allowable stress, expressed in terms of Utilization Ratio, and deflection for skid lifting, transportation, and operation, as specified in ANSI /AISC 360-22, are summarized as follows:

Criteria	Lifting	Transportation	Operation
AISC Utilization Ratio	1.0	1.0	1.0
Deflection (mm)	L/360 (Vertical)	L /360 (Vertical) H /400 (Horizontal)	L /360 (Vertical) H /400 (Horizontal)
Note: L represents the span of the respective member, and H represents the height of the respective member.

(2) Maximum allowable vibration levels

The allowable vibration levels for the skid and the equipment (compressor and driver) mounted on it are specified by the compressor vendors and relevant standards. The allowable vibration levels for the Ariel reciprocating compressor skid structure are presented in the following table as an example:

Package Components	JG:A:M:P:N:Q:R:W, KB100	JGJ:H:E:K:T,KBE:K:T	JGC:D:F:B:V:Z:U,KBC:D:F:B:V:Z:U
Skid	<0.10 (<2.5)	<0.15 (<3.8)	<0.20 (<5.1)
Compressor Frame	<0.30 (<7.6)	<0.40 (<10)	<<0.50 (<13)
Compressor Cylinder	<0.60 (<15)	<0.80 (<20)	<1.0 (<25)
Tandem Cylinder	<1.0 (<25)	<1.0 (<25)	<1.0 (<25)
Motor	<0.16 (<4.0)	<0.25 (<6.4)	<0.25 (<6.4)
Note: Values are in inch/sec (mm/s), and are per Ariel Package Standards 2024 for skid and compressor components, and ISO 20816-3 Standard for the motor.

Skid Analysis Tools

A variety of analytical tools are available for compressor skid analysis. At CCPGE, we utilize STAAD.Pro, SAP2000, and ANSYS for this purpose.

STAAD.Pro is a widely-used structural analysis and design software developed by Bentley Systems. Its key features include compliance with numerous international design codes, capabilities for static, dynamic, seismic, and wind analysis, design tools for multiple materials, integration with other Bentley software and CAD tools, and support for BIM workflows.
SAP2000 is a general-purpose structural analysis and design software developed by Computers and Structures, Inc. (CSI). Its key features include support for static, dynamic, and seismic analysis, advanced modeling tools for complex geometries, integrated design capabilities for various materials (such as steel and concrete); built-in support for international design codes, graphical visualization of results, interactive model manipulation, and automated load generation for wind, earthquake, and moving loads.
ANSYS is a well-known finite element analysis (FEA) software that is powerful for addressing mechanical and structural stress, deflection, and vibrations.

Skid Analysis Example

An example of skid analysis with the following specifications is illustrated below. The compressor skid is installed on a concrete foundation, the analysis was performed using STAAD.Pro software in accordance with ANSI/AISC 360-22, ASCE/SEI 7-22, and ISO 20816 standards.

Compressor Model:	Ariel JGJ/4	Power:	250 kW
Motor:	WEG W22Xdb-355M/L-4	Speed:	900 – 1485 RPM
Number of Stages:	2	Flow Rate:	1100 – 3111 Sm3/h
Number of Cylinders:	4	Suction / Discharge Pressure:	0.6-1.5 MPag / 4.6 MPag

The skid analysis model, which includes the skid structure and the mounted compressor and driver, as well as the calculated stress and deflection of the skid structure due to lifting, transportation, and operating loads, are presented in the following figures. The lifting loads consist of the self-weights of the skid structure, mounted equipment and piping, commonly referred to as dead loads. The transportation loads include the dead loads and acceleration loads due to transportation movements. Operating loads include the dead loads, live loads, and environmental loads such as wind loads, snow loads and seismic loads.

Skid Analysis Model on a Concrete Foundation

AISC Utilization Ratios of Skid Structure Due to Lifting Loads

Deflections of Skid Structure Due to Lifting Loads

AISC Utilization Ratios of Skid Structure Due to Transportation Loads

Deflections of Skid Structure Due to Transportation Loads

AISC Utilization Ratios of Skid Structure Due to Operating Loads

Deflections of Skid Structure Due to Operating Loads

A typical vibration mode of the skid structure, and the calculated first eight natural frequencies (MNFs) and interference with compressor operating speeds, representing potential excitation frequencies, are shown below.

Vibration Mode of Skid Structure – Mode Shape 5 (f=20.5 Hz)

Calculated MNFs and Interference with Operating Speeds

Design modifications, including the addition of a beam at location beneath the compressor, have been implemented to ensure that the vibration levels of the skid structure and the mounted equipment remain within the allowable limits specified by Ariel Package Standards 2024 and ISO 20816-3 Standard. One of the following figures illustrates the calculated vibration velocity responses at the compressor cylinder, while another shows the calculated vibration velocities of the skid structure and the mounted equipment in response to vibration excitations during compressor operations, and comparison with allowable limits.

Calculated Vibration Velocity Responses at Compressor Cylinder

Calculated Vibration Velocities and Comparison with Allowable Limits

Dynamic Analysis of Skid Supported by Piles

The methodology for the dynamic analysis of a skid supported by piles is generally similar to that used for a skid on a concrete foundation, with the primary difference being the inclusion of piles in the analysis model.

In this analysis, the piles are modeled as vertical columns integrated into the overall framework. Soil restraints along the length of the underground piles are represented by equivalent soil springs, with dynamic stiffness determined from the soil properties. These properties are typically provided in the geotechnical report for the skid installation site. The table below lists the soil properties used in this example project, including total unit weight, dynamic shear modulus, damping ratio, and Poisson’s ratio.

Incorporating piles into the analysis model ensures that the analysis accurately captures the combined response of the skid structure, mounted equipment, and the piles.

Soil	Total Unit Weight (kN/m3)	Dynamic Shear Modulus, Gmax (Mpa)	Internal Damping Ratio, D (%)	Poisson’s Ratio, v
Sand	21.0	170 to 260	3	0.4
Clay (lacustrine)	18.0	52 to 64	3	0.45
Clay (till)	19.0	58 to 151	3	0.4
Clay Shale	21.0	156 to 234	3	0.3

When evaluating the stress, deflection and vibration level of the skid and the mounted equipment, it is also crucial to assess the pile capacity to withstand the applied loads, thereby ensuring structural stability.

The following figures illustrate the analysis model, a typical vibration mode, and the calculated vibration velocities and comparison with allowable limits for a skid structure supported by piles. The skid structure is subjected to dynamic loads, including unbalanced forces and moments from the compressor and motor, cylinder gas forces, cylinder cross-head dynamic forces, torque variations, and pulsation induced shaking forces on vessels and piping.

Analysis Model of Skid Supported by Piles

Vibration Mode of Skid Structure and Piles – Mode Shape 1 (f=6.5 Hz)

Calculated Vibration Velocities and Comparison with Allowable Limits

Dynamic Analysis of Skid Mounted on Offshore Platform

The methodology for the dynamic analysis of a skid mounted on an offshore platform is similar to that used for a skid on a concrete foundation, with the primary difference being the incorporation of the surrounding platform structure as flexible boundary conditions in the analysis model. The scope of the platform structure included in the analysis model should extend to boundaries beyond which structural responses would not significantly impact the stress, deflection, and vibration of the skid structure being analyzed.

As a best practice, all primary skid beams supporting the compressor, driver, and scrubbers should be correctly aligned and positioned directly over the platform beams, with proper welding between the skid and platform beams. This ensures adequate support for the skid and maintains its structural integrity.

The following figures illustrate the analysis model, a typical vibration mode, and the calculated vibration velocities and comparison with allowable limits for a skid structure mounted on an offshore platform. The skid structure is subjected to dynamic loads, including unbalanced forces and moments from the compressor and motor, cylinder gas forces, cylinder cross-head dynamic forces, torque variations, and pulsation-induced shaking forces on vessels and piping. Modifications to the platform structure beneath the skid, such as the addition of a bracing beam, are implemented to ensure sufficient stiffness in supporting the skid from the platform.

Analysis Model of Skid Structure Mounted on Offshore Platform

Vibration Mode of Skid and Platform Structures – Mode Shape 2 (f=5.2 Hz)

Calculated Vibration Velocities and Comparison with Allowable Limits

Dynamic Analysis of Skid Mounted on FPSO

The methodology for the dynamic analysis of a skid mounted on a Floating Production Storage and Offloading (FPSO) module is generally similar to that used for a skid on a concrete foundation, with the primary difference being the incorporation of the FPSO supporting structure as flexible boundary conditions in the analysis model.

The dynamic loads induced by the ship's motion are schematically illustrated in the following figure. These loads, along with other dynamic forces such as unbalanced forces and moments from the compressor and motor, cylinder gas forces, cylinder cross-head dynamic forces, torque variations, and pulsation-induced shaking forces on vessels and piping, are included in the analysis.

Ship Motions of a FPSO Module with Skid Structure Mounted on It

The following figures illustrate the analysis model, calculated vibration displacements, and calculated vibration velocities and comparison with allowable limits for a skid structure mounted on an FPSO module. Modifications to the FPSO supporting structure are implemented to ensure sufficient supporting stiffness from the FPSO module to the skid structure.

Analysis Model of Skid Structure Mounted on FPSO Module

Vibration Displacements of Skid Structure and FPSO Module
Subject to Dynamic Loads

Calculated Vibration Velocities and Comparison with Allowable Limits

Skid Analysis

Quick Navigation

What is Skid Analysis

Why Perform Skid Analysis

Skid Analysis Methodology

Skid Analysis Procedure

Skid Analysis Guidelines

Skid Analysis Tools

Skid Analysis Example

Dynamic Analysis of Skid Supported by Piles

Dynamic Analysis of Skid Mounted on Offshore Platform

Dynamic Analysis of Skid Mounted on FPSO

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