Skid Calculations

How to Calculate Skid Footprint, Weld Inches, and FAT Duration for Process Skids

Work through the five core calculations for shop-fabricated skids, from footprint and spool count to weld diameter-inches and FAT hours, with real inputs and worked examples.

A skid estimate lives or dies on five calculations done before steel is cut. Start with footprint, because it sets shop bay demand and shipping class. The Skid Footprint calculator takes plan area in square feet: length times width of the structural frame, plus a clearance band. A 24 ft by 8 ft base frame is 192 sq ft, but add a 2 ft maintenance aisle on two sides and the effective envelope becomes 28 ft by 10 ft, or 280 sq ft. That 46 percent jump is what actually blocks a bay and drives the trailer permit class, so always carry the clearance, not the bare steel.

Pipe spool count converts the P&ID into fabrication work. The Pipe Spool Count method multiplies spools per weld station cycle by available cycles for gross capacity, then applies uptime and first-pass yield. With 4 spools per cycle over 480 cycles you get 1,920 gross spools; at 90 percent uptime and 97 percent yield, net deliverable spools drop to about 1,676. That is a 13 percent haircut. The rule of thumb: a mid-size process skid carries 40 to 120 shop spools depending on line count, and you plan labor against the net figure, never the gross cut list.

Weld inches are the true labor unit for piping, expressed as diameter-inches (DI). The Weld Inch Estimate takes total welds, average joint diameter, and a schedule factor. Count field-fit and shop welds separately: a 6 inch schedule 40 butt weld is 6 DI, and a skid with 80 butt welds averaging 4 inch line size is 320 DI. At a shop productivity of 8 to 14 DI per welder-hour for carbon steel GTAW root plus SMAW fill, 320 DI is roughly 23 to 40 welder-hours. Stainless GTAW all-pass runs slower, 4 to 7 DI per hour, so material choice can double the hour count.

Instrument loop labor is driven by loop count, not pipe size. The Instrument Loop Labor calculator multiplies loops by a per-loop rate and a completion factor, then adds fixed calibration and mobilization cost. Count discrete control loops, where one loop may span a transmitter, controller, and final element. For 100 loops at 45 dollars per loop and an 80 percent completion factor, captured labor is 3,600 dollars; adding 250 dollars fixed gives 3,850 dollars total, or 38.50 dollars per loop. Simple analog loops run 1.5 to 3 technician-hours each; SIS and analyzer loops run three to five times that.

FAT duration sizes the witnessed test window that anchors the ship date. The FAT Duration calculator divides test points by an execution rate, then applies a retest allowance. Pull the point count from the approved test procedure, including functional, alarm, and interlock checks. At 120 test points and a demonstrated 12 points per hour, base time is 10 hours; a 10 percent retest allowance gives 11 hours of active testing. Alarm and interlock points run slower than straight functional checks, so blend your rate honestly. Setup, utility hookup, and teardown are separate: add 2 to 4 hours so the scheduled day reflects the full window.

Chain these together and units must reconcile. Spool count feeds weld inches (welds per spool times DI per weld), weld inches feed fab hours, loop count feeds both loop-check labor and FAT points. A worked mid-size skid: 60 spools, 3 welds per spool at 4 inch average equals 720 DI, or about 60 welder-hours at 12 DI per hour. Ninety loops at 2 hours each is 180 I&C hours. FAT of 110 points at 12 per hour plus 15 percent is roughly 10.5 hours. Keep everything in the same clock and diameter basis or the rollup silently drifts 10 to 20 percent.

Two conversion errors sink these calculations. First, mixing nominal pipe size with actual diameter: weld labor is always keyed to nominal DI, so a 2 inch line is 2 DI even though the pipe wall varies by schedule. Second, blending simple and complex spools into one per-cycle figure inflates capacity. Segregate a 1 inch instrument-air spool from a 12 inch header spool, because the header may carry 6 welds and 3 hours while the small line carries 2 welds and 20 minutes. Estimating them at an average buries the labor on the large-bore lines where most hours actually sit.

For a defensible number, run each calculation against a recent as-built, not the design intent. Take uptime, yield, execution rate, and loop hours from a comparable completed skid, then adjust only for the deltas: more stainless, more SIS loops, a larger footprint. If your net spool count comes in above 90 percent of gross, your uptime or yield input is optimistic and worth re-checking against the weld log. If FAT base hours land within 5 percent of adjusted, your allowance is too thin for anything but a repeat build. Calibrate the inputs and the five formulas will hold to within 10 percent.

Published 2026-07-01.