Jack-Up Vessel
Vérins de levage hydrauliques
Leg Jacking · Spudcan · Storm Lock

RACK &
PINION DRIVE

The leg passes through a jacking unit containing multiple motor-driven pinions engaging a toothed rack on the leg chord. Hydraulic lift cylinders in this system are used as: (a) pinion engagement actuators — small cylinders pressing each pinion into engagement with the rack; (b) anti-rotation locking cylinders — securing the leg against rotation during jacking; and (c) fixation (storm lock) cylinders — clamping the leg once the hull is at operating elevation. The jacking force is transmitted through the rack-and-pinion teeth, not through the lift cylinders directly.

GRIPPER &
CYLINDRE DE LEVAGE

The sequential gripper system uses large hydraulic cylinders as the primary jacking actuators. Two pairs of grippers (upper and lower) alternately clamp the leg chord while a large central cylinder extends or retracts to advance the leg or the hull. When the cylinder is fully extended, the lower gripper engages, the upper gripper releases, the cylinder retracts, the upper gripper re-engages, and the sequence repeats. This system uses the largest individual lift cylinders in any offshore application — bore 300–500 mm, stroke 1 500–3 000 mm, generating 2 000–8 000 kN per cylinder. Typical installation: 4–8 lift cylinders per leg, 3–4 legs per vessel.

1

Initial leg touchdown. Hull floated to position, all legs lowered by the leg jacking lift cylinders until each spudcan contacts the seabed. The lift cylinders are operated in coordinated sequence to maintain hull trim within ±0.3°.

2

Hull lift-off. All leg jacking lift cylinders apply upward force simultaneously to lift the hull clear of the water — the full hull weight now transfers through the legs to the spudcans and seabed. Penetration monitoring begins.

3

Preload ballasting. Seawater ballast is pumped into dedicated preload tanks, increasing the hull weight transferred through the leg jacking system to the spudcans — typically to 1.3–1.5× operating vertical reaction. The additional load drives the spudcan deeper until the seabed bearing capacity equals the applied preload force.

4

Preload proof hold. The full preload force is maintained for a minimum period (typically 1–2 hours per classification society requirement) to confirm penetration has stabilised. If penetration continues, the soil bearing capacity has not been reached and additional preload must be applied.

5

De-ballast and air gap. Preload ballast is discharged, hull is raised to the operating air gap above the highest wave crest, and storm lock is engaged. The lift cylinder jacking system can now be set to maintenance mode.

HYDRAULIQUE
VERROUILLAGE

After reaching operating elevation, the leg jacking lift cylinders are isolated by closing their supply valves and activating high-pressure lock valves at each cylinder port. The trapped oil column holds the hull position. However, hydraulic lock alone is not classified as an approved storm lock system by any major classification society — all jacking lift cylinders experience some internal leakage over time, and a month-long offshore campaign would allow the hull to drift downward. Hydraulic lock is the first barrier; mechanical lock is the mandatory primary structural hold.

MÉCANIQUE
STORM LOCK

The mechanical storm lock engages a positive mechanical connection between the leg and the hull structure — typically a pin or wedge driven hydraulically into a matching recess in the leg chord or rack. Once engaged, the storm lock transfers all leg-to-hull forces through the mechanical connection, relieving the leg jacking lift cylinders of their structural function completely. The lift cylinders in storm lock mode carry only the weight of internal components — the full environmental loading (wave, wind, current) passes through the storm lock pins. Classification rules require two independent mechanical storm locks per leg, each capable of carrying 100% of the design load without the other.

SURVEILLANCE
& ALARM

Modern jack-up vessels continuously monitor leg jacking lift cylinder pressure, storm lock engagement status, hull inclination, and leg penetration depth throughout the on-location period. Any anomaly — unexpected pressure change in the jacking cylinder circuit, partial storm lock disengagement, hull trim deviation, or continued leg penetration — triggers an alarm in the marine control room. Continuous monitoring supplements the mechanical storm lock to provide real-time situational awareness of foundation and structural integrity for the duration of the campaign.

Q01

A leg jacking lift cylinder is showing hydraulic drift — the hull position is changing slowly during a campaign. What action is required?

If the hull is drifting while the mechanical storm lock is engaged, the storm lock itself is carrying the structural load correctly and the hydraulic drift in the jacking circuit is not a structural safety issue — the lift cylinder internal leakage is not causing the hull to move, because the hull is held by the mechanical lock. The correct action is to note the leakage rate in the maintenance log, monitor for any increase, and schedule seal replacement at the earliest opportunity when the cylinder can be accessed safely without compromising the hull support. If the storm lock cannot be confirmed as fully engaged, or if the vessel is in a port approach or transit mode where the mechanical lock is not engaged, hydraulic drift in a leg jacking lift cylinder is a jacking system failure that requires immediate investigation and correction before any jacking operation continues. The distinction between “drifting hull with storm lock engaged” and “drifting hull without storm lock confirmed” is the difference between a maintenance issue and an emergency.

Q02

Is it possible to repair a damaged seal on a jacking lift cylinder while the vessel is on location without taking the leg out of service?

Only if the remaining leg jacking cylinders on that leg can collectively maintain the required support load while the defective unit is isolated for repair, and only if the mechanical storm lock can bear the full leg load during the repair period. The engineering assessment must confirm: (a) the remaining cylinders on that leg and the storm lock together meet the load condition requirements of the vessel’s jacking analysis; (b) the defective lift cylinder can be isolated without disturbing the storm lock engagement; and (c) the repair can be completed within the risk window before environmental conditions deteriorate. In practice, in-situ seal repair of a jacking cylinder on location is uncommon and carries significant risk — most operators prefer to accept the controlled leakage and continue operations to the end of the campaign, then perform the repair at the yard during the next scheduled dry-dock period. The decision must involve the vessel’s marine superintendent, the classification society surveyor, and the jacking system OEM engineer before any work proceeds.

Q 03

What is the typical lead time for replacement jack-up leg jacking lift cylinders with class approval?

Class-approved offshore jack-up leg jacking lift cylinders typically require 16–28 weeks from order placement to delivery, depending on bore size and the classification society involved. The timeline is dominated by the documentation process — design submission to the classification society (3–5 weeks for review), EN 10204 3.2 material ordering and testing (6–10 weeks), manufacturing (8–12 weeks for large-bore units), and the witnessed pressure test scheduling (1–3 weeks depending on surveyor availability). Vessels approaching their SPS should place orders for replacement lift cylinders at least 8–10 months before the required delivery date to allow adequate contingency in the documentation and manufacturing schedule. Last-minute expediting is possible but is possible but typically doubles the cost and is not always achievable within the vessel’s schedule constraints.

Q04

Does the corrosion protection specification for an offshore jack-up lift cylinder differ from a standard industrial or mobile machinery cylinder?

Yes, significantly. A jack-up vessel leg jacking lift cylinder operating in the offshore marine environment faces continuous salt spray, wave wash, UV exposure, and — for the portions of the leg and cylinder submerged during transit — full seawater immersion. The outer coating specification must be rated to at minimum 1 000 hours salt spray (ISO 9227 NSS) — compared to 200–480 hours for industrial or mobile equipment. The standard specification for offshore structural cylinders is: blast cleaning to ISO 8501-1 Sa 2.5, zinc silicate shop primer (DFT 20–40 μm), high-build epoxy intermediate coat (DFT 100–150 μm), and a polyurethane or isocyanate topcoat (DFT 75–100 μm). Total DFT minimum 200–290 μm. Rod surface protection is hard chrome at minimum 50–70 μm (twice the standard industrial specification) or HVOF tungsten carbide where exposure to splash zone wave action is anticipated. Sacrificial zinc anodes are fitted to the cylinder mounting structure on legs that are regularly submerged during transit. Every offshore cylinder must have the coating specification documented and the application method verified by the coating inspector during manufacture — offshore coating failure accelerates at a rate that makes in-service touch-up difficult and structural corrosion progression a realistic risk within the class survey interval if the initial specification was inadequate.