Skid

Key Takeaways

• Jack-up rig cantilever skidding is conceptually different from fixed platform rig skidding in that the entire cantilever structure (the beam that extends beyond the hull over the fixed platform or over open water) moves relative to the hull rather than just the substructure, with the skidding capability in two directions (outreach, how far the cantilever extends beyond the hull; and transverse, how far the rig moves sideways on the cantilever) determining which well slots the jack-up can reach from its single hull position: a modern cantilever jack-up rig with 30-foot outreach and 15-foot transverse skid range can access a rectangular grid of well slots covering approximately 900 square feet from a single jackup position; for platforms with widely spaced wells or non-rectangular well patterns, the jack-up may need to unlatch the cantilever and reposition the hull before all well slots can be accessed; the cantilever skid capacity (the maximum load the cantilever can support at full outreach while skidding, which is less than the maximum load at minimum outreach) determines the maximum hook load the rig can apply at each well position and must be compared to the expected hook loads for each well to confirm that the rig can safely drill all required wells from its hull position.
• Skid beam maintenance and lubrication are critical operational requirements because the friction between the rig substructure and the skid beams is the primary impediment to smooth skidding and the primary source of misalignment if the friction varies across the skid surface: uneven friction (from lubricant depletion on one side of the rig or scale buildup on one rail) causes the rig to advance faster on one side than the other during skidding, creating a rotational misalignment that requires the skidding crew to alternate pushing on different sides to correct the rig's angular position; the standard skid beam lubrication requires a full application of grease before each skid, with the grease applied to the contact surface between the rig's skim plate (the hardened steel base of the substructure that slides on the skid beams) and the skid beam top flange; the skid beams themselves must be inspected for corrosion, weld cracking, and deformation that could cause the skim plate to hang up or drop during the skid, and any defects repaired before the skidding operation begins; the hydraulic system pressure and ram extension rate are monitored throughout the skid to detect unexpected friction increases that could indicate a hung-up skim plate or an obstruction on the skid surface.
• Well slot alignment verification after skidding confirms that the rotary table is correctly centered over the conductor and wellhead of the target well slot before connection work begins and before the slips and BOP are made up: the standard alignment check uses a plumb bob or optical instrument sighted down the conductor to verify that the rotary table center is within the specified tolerance (typically plus or minus 0.25 inch from the conductor center) in both the longitudinal and transverse directions; a rotary table that is not precisely centered over the conductor will cause the BOP stack to land eccentrically when it is lowered, potentially damaging the wellhead connector, misaligning the BOP rams with the pipe bore, or inducing bending stress in the conductor that can propagate fatigue damage to the foundation pile; on fixed platforms with closely spaced well slots (4 to 5 feet spacing), the risk of the rig inadvertently moving too far during skidding and damaging adjacent wellheads or conductors is managed by mechanical stops (steel blocks welded to the skid beams at the target position) and by graduated slow-down of the hydraulic skid cylinders as the rig approaches the target position.
• Disconnection and reconnection of rig services during skidding represents the most time-consuming portion of the skid operation, because all flexible connections between the rig (which moves) and the platform (which is fixed) must be broken before skidding begins and remade after skidding is complete: flexible hose assemblies (mud supply and return lines, choke and kill lines, seawater service lines, pneumatic supply), electrical conduits (power cables from the platform's generation system to the rig's main switchboard, communication cables, signal cables from well monitoring equipment), and mechanical connections (derrick anchorage to platform structure in storm mode, walkway connections between rig and platform) must all be disconnected in a coordinated sequence before skidding and reconnected in reverse sequence after the rig reaches the target well slot; the total disconnection and reconnection time depends on the number and complexity of the service connections, ranging from 2 to 8 hours for simple platform configurations to 12 to 20 hours for complex arrangements with extensive integrated utility connections; the rig and platform design teams on new-build facilities increasingly design the service connections as standardized quick-disconnect assemblies (with minimal make-up time after skidding) rather than custom-welded or flanged connections that require wrenching and pressure testing after each make-up.
• Skidding in adverse weather conditions on offshore platforms presents safety challenges because the rig's weight during the skid is supported on the skid beams with reduced stability compared to its stationary operating configuration, making the rig more susceptible to dynamic forces from wind and wave-induced platform motion: most jackup and fixed platform skidding operations specify maximum allowable wind speed (typically 30 to 40 knots) and maximum platform motion (for semi-compliant platforms) during skidding, with the operation suspended if weather exceeds the operational limits while the rig is in transit between well slots; the rig is most vulnerable to wind-induced overturning moments when it is fully extended on the cantilever (maximum eccentricity relative to the hull support) or when the substructure is positioned between two skid beam support points (maximum unsupported span); skidding operations on platforms in hurricane-prone areas (Gulf of Mexico) or harsh weather environments (North Sea) are typically planned for the most favorable weather window available within the operational schedule, with contingency plans for securing the rig mid-skid if weather deteriorates faster than predicted.