Structural skids allow for the mounting of equipment to a piping system, and for fabrication to be conducted in a shop environment. This reduces cost and complexity compared to equipment integration in this field. In this article, you will learn about skid engineering, design, construction, testing, and accessories.
Structural skids are cost effective and can be built quickly. They are designed to contain either a complete process system or a serve a specific function. Structural skids may also systematize a manufacturing process into logical units.
While skid designs vary according to client specifications, the following elements are recognized in all skids:
- Portable design – Skid systems provide easy transport over field fabrication units are self-contained units and built within frames.
- Accessibility – Skids have an accessible layout that is compliant to codes and regulations while allowing access to critical process equipment.
- Small footprint – Tanks, piping, and necessary process equipment are organized to take up the smallest amount of room possible.
- Controlled assembly – Skids are typically built in a controlled facility where access to welding materials, pneumatic air, and overhead lifts is available.
- Process Connections – Process connections can be gathered into one spot on the skid or placed at “tie-points” that have been specified by the client.
- Factory Acceptance Testing (FAT) – The FAT is completed before skids are shipped to the site, ensuring the system operates as designed and on-site startup time is reduced.
Engineering a skid requires determination of main member size, cross member size, intermediate angle iron, and floor plate thickness. Static, dynamic, and quasi-static forces must be considered for beam sizing and construction. Skid rigidity is a major factor of beam size and is measured by allowance deflection in inches. This number is calculated by taking the length of the skid in inches and dividing by a set integer. Typical skids receive rigidity requirements of L/240 whereas demanding applications may see rigidity requirements of up to L/960.
Static skid design evaluates stress and buckling of members under constant load forces. The following static loads are also analyzed:
- Dead loads, including permanent equipment weight.
- Thermal loads, including forces generated by temperature/pressure change.
- Drive torque of compressors/engines.
- Lifting or dragging loads. These loads include a load factor from impact of sudden stops/motion of lifting equipment (load factor of 1.15-2.0 is common.)
- List angle, which creates horizontal loads.
Dynamic loads originate from waves, earthquakes, wind or machinery and evaluated by the resulting vibrations and fatigue. Resonance is a condition where the frequency of dynamic force is +/- 10% of the mechanical natural frequency of the skid foundation. This may occur as a result of dynamic loads. Common dynamic forces include:
- Unbalanced forces generated by rotating/reciprocating weights such as piston assemblies. This is a frequent concern in reciprocating compressors.
- Horizontal cylinder gas forces.
- Vertical forces, created when rotating motion converts to reciprocating motion.
- Pulsation-induced shaking forces, created at elbows/changes in pipe diameter.
- Misalignment of compressor and driver.
- Rolling torque on engines (occurs at higher orders of engine run-speed).
- Torsional vibrations, which may cause horizontal vibrations of the frame.
Quasi-static forces are periodic loads that have a frequency of less than 3 Hz. Their skid design evaluates stress, buckling of members and fatigue. Typical quasi-static loads include:
- Environmental loads like wind, current, wave, live loads, ice, seismic and hydrostatic pressure that occur in any direction.
- Construction loads, including installation, loadout and transportation.
An engineer uses a Piping & Instrument Diagram (P&ID), specifications, 3D model, general arrangement (G/A), and skid calculations as the basis for the structural skid design. The finished product is a highly detailed 2D drawing of how the skid is constructed. The welding method, material type, and lifting requirements are elements to be considered in the skid design.
The following methods are commonly utilized in the development of skids.
- Gas metal arc welding (GMAW) – This process forms an electric arc between a consumable MIG wire electrode and the workpiece metal. Heated metal melts and joins together.
- Flux-cored arc welding (FCAW) – This semi-automatic/automatic arc welding process requires continuously fed voltage to join metals.
- Metalcore arc welding (MCAW) – uses heat generated by DC electric arc to join metal in the joint.
Additionally, weld characteristics are specified on the skid drawing itself, or more generically on a weld map. The type of welds varies greatly. Most welding done on skids are filet welds in a skip or seam configuration. The skip weld alternates between non-weld and welded segments whereas the seam weld is applied continuously along the joint.
Structural skids may require specific materials for construction due to reasons such as heating elements, structural strength and durability. Carbon steel is the most common skid material. Stainless steel appears in the industry at times, with alloys, fiberglass, and plastic being much rarer. Most projects utilize steel materials due to their structural integrity, non-malleable properties, and low cost.
Completed structural skids require lifting studies to be conducted. This ensures the skids can be lifted, transported to site, and withstand environmental loads (e.g induced wind or earthquake).
Transportation methods of completed structural skids from shop to client site is critical to consider and may vary depending on the type of skid. Lifting eyes are a lifting device suitable for vertical lifting from a single point or multiple points and are primarily used for heavy skids. Working load limit depends on the angle of pulling force and can range from 160-24000lbs. Forklift pockets are used for smaller and simpler skids that require less attention and may be installed near the bottom of the skid structure for easy conveyance between the customer and engineer.
Appropriate consideration must be given to the skid structure for the environmental factors of the site location (e.g seismic factors, wind loads, heat factors.) Calculating the center of mass of the skid will aid depicting ensure a safe and secure removal process. Pull tests (a push/pull force on a structure) may be necessary to ensure parts of the skid are fixed and secure, without jeopardizing the structural integrity of the skid.
Skids construction requires the assembly of main beams, cross members and angle iron, floor plate, lifting lugs, drains, and a protective coating.
Main beams are also referred to as perimeter beams. Construction of these beams is critical for correct sizing and build of the skid. The joining of these beams sets the perimeter for cross members to be set in place.
Cross Members and Angle Iron
Cross members and angle iron connect to perimeter beams in a method dictated by the structural calculations. Cross members add strength to the structure, where angle iron is typically to reduce floor plate deflection.
A floor plate, sometimes referred to as a skid plate, secures to skid beams to allow for safe foot traffic. Floor plates generally originate from the same material as the accompanying beams. Checkered floor plates are ideal for skids that face corrosion threats and heavy traffic. Floor plate requirements depend on customer choice of grade, thickness, width, and length.
If the skid is designed for overhead lift, lifting lugs are supplied either in a removable or bolt-on format. Lifting lugs may be cut from a plasma machine at the shop facility or procured externally.
Skids that involve piping require drainage system requirements. The angle between the discharge pipe and attributed pipe must be at least 90 degrees, with the minimum installation depth of the pipeline ranges from .5-.7m.
Paint specifications may be specified in order to prevent corrosion of carbon steel and in rare cases, stainless steel. A multi-coat system is standard and provides an all-weather sealant that protects against corrosion, scratches, impact, abrasion, chemical exposure, and UV rays. Galvanization, the process of dipping a skid into molten zinc, provides a metallurgical bond and a highly protective coat.
Engineering specifications, detailed design, and quality process reviews ensure compliance to the specified requirements. Non-destructive testing for weld verification may also be required to ensure welding is applied appropriately. Skids that are part of a process system undergo a factory acceptance test (FAT). The FAT occurs at the final stage of the project and proves that the equipment integrity and functionality is suitable, and to design and user requirements.
Depending on functional and access requirements, structural accessories may be required. Common accessories include:
- Structural equipment stands
Completed structural skids must also comply with OSHA requirements and local building code.
Punchlist Zero especially thanks Grimes Industrial, Inc of Tomball, TX for their contributions to this article.