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Deck & Support Systems
Deck Cranes
Deck cranes are pedestal-mounted slewing cranes that lift between a drilling unit and its supply vessels and move loads across the main and pipe decks. The Category covers lattice boom, knuckle boom and telescopic designs, in diesel-hydraulic and electro-hydraulic drive, for jack-ups, semisubmersibles, drillships and fixed platforms. It also includes cranes with active heave compensation for subsea lifts.
How an offshore pedestal crane works
The crane slews on a large slewing bearing bolted to a pedestal adapter. The superstructure carries the boom, the main and auxiliary (whip) hoists, the luffing system and the power pack. Hydraulic motors drive the hoists and slew through planetary gearboxes, with spring-applied brakes that hold when pressure is lost.
Offboard lifts set the design. A load lifted off a heaving supply vessel deck can snatch, and a lowered load can be hit by the deck rising again. Design standards turn the hook speed and the vessel deck velocity in a given significant wave height (Hs) into a dynamic factor. The load chart therefore carries an onboard curve and lower offboard curves, each tied to an Hs value.
Boom types and what they suit
| Type | Strengths | Limits |
|---|---|---|
| Lattice boom, wire luffing | Long outreach, high SWL for its weight | Needs a boom rest, more rope to inspect |
| Knuckle boom | Folds compactly, precise hook control, pipe and riser handling | Many pins and cylinders to inspect, shorter reach |
| Telescopic boom | Compact stowage | Wear pads between sections, capacity falls fast with radius |
Active heave compensation (AHC) and constant tension modes suit subsea lifts and landing loads on the seabed.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Onboard SWL at radius | Lifts within the rig's own deck | t at m (lb at ft) |
| Offboard SWL and Hs | Sets the weather window for supply work | t at m for an Hs in m (ft) |
| Hoist speed | A faster hook clears the vessel deck sooner | m/min (ft/min) |
| Rope length | Reach to a vessel deck or the seabed | m (ft) |
| Prime mover | Power supply and hazardous-area fit | Diesel or electric, kW (hp), Zone 1 or 2 |
| Pedestal interface | Must match the slewing bearing or adapter | Bolt circle in mm (in), bolt count |
Standards and certification
- API Spec 2C covers design and manufacture, and many cranes carry its monogram. The edition matters, because editions derive offboard ratings differently.
- EN 13852-1 covers general-purpose offshore cranes. It requires AOPS, which lets the hoist pay out if the load snags on the vessel so the structure is not overloaded, and MOPS, which lets the operator release an overload deliberately.
- DNV-ST-0378 and the matching ABS and Lloyd's Register rules apply on classed units.
- API RP 2D covers operation, inspection and wire rope removal in service.
Failure modes and inspection
The slewing bearing ends most crane lives. Inspectors trend the tilt clearance (rocking) measurement against earlier readings and the maker's limit, test grease samples for metal and check bolt pretension. A rising trend matters even inside the limit.
- Booms. Check chords and lacings for dents, then magnetic particle inspection at boom foot lugs, the heel pins and the A-frame.
- Knuckle and telescopic booms. Check pin and bushing wear, cylinder rod pitting and wear pads.
- Rope and sheaves. Assess each rope against ISO 4309 discard criteria and gauge sheave grooves. Rotation-resistant rope can fail inside before the outside shows it.
- Hoists. Run oil analysis on gearboxes and a holding test on the brakes.
- Safety systems. Function test the rated capacity limiter, hoist and boom limits, AOPS and MOPS where fitted, and emergency load lowering.
Documentation
- Certificate of conformity or class certificate, with the standard and edition
- Load chart and summary of design calculations
- Last proof load test certificate and annual inspection reports
- Rope certificates and slewing bearing measurement history
- Running hours, hydraulic oil analysis and repair records
Integration on your rig
The pedestal and deck structure need a check for the crane's overturning moment and offboard reactions. Plot lifting arcs against the supply vessel position, laydown areas and blind spots. Check power supply, hazardous zone and the boom rest location, and add anti-collision with any neighbouring crane.
Refurbishment and recertification
A major overhaul usually covers the slewing bearing, hoist and slew gearboxes, brakes, hydraulic hoses and seals, new ropes and often a control system upgrade, followed by a proof load test. Unused surplus cranes from cancelled newbuilds can save build time, but check preservation records and the age of hoses, seals and electronics. Liebherr, NOV, MacGregor, Seatrax, Huisman and Palfinger Marine are well-known makers. For hooks and slings, see crane attachments and rigging.
FAQ
Frequently asked questions
What does offboard rating mean on an offshore crane load chart?
The offboard rating is the load a crane may lift to or from a moving supply vessel at a stated significant wave height. It is lower than the onboard rating because vessel motion adds dynamic load. Higher sea states give lower offboard ratings, so read the curve for the weather you expect to work in.
How do you check the slew bearing on a used offshore crane?
Measure the bearing's tilt clearance and compare it with earlier readings and the maker's limit. A steady trend matters more than one reading. Inspectors also test grease samples for metal particles, check bolt pretension and watch for uneven or noisy rotation under load.
What are AOPS and MOPS on an EN 13852-1 crane?
AOPS and MOPS are the automatic and manual overload protection systems that EN 13852-1 requires for offboard lifts. AOPS lets the hoist pay out if the load snags on the supply vessel, so the crane is not overloaded. MOPS lets the operator release a load deliberately in an emergency. Both need a function test on a used crane.
Do I need a load test after buying a used deck crane?
Yes. Once the crane is on its new pedestal, class rules, API RP 2D practice and regulations such as LOLER expect a proof load test and thorough examination before service. The test also proves the new pedestal connection, which no previous certificate covers.
Is a knuckle boom or lattice boom crane better for a drilling rig?
A lattice boom gives more outreach and lift for its weight and suits general supply boat work. A knuckle boom folds into less deck space and controls the hook more precisely, which suits pipe and riser handling and subsea lifts. Many drillships carry both.
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