Offshore Decommissioning Lifting: Weight, Cutting and Release

Why a removal lift is harder than an installation lift

The scale of decommissioning

OSPAR’s 2023 inventory of the North-East Atlantic counts 1,948 oil and gas installations, 1,144 of them subsea structures. Of these, 1,385 were operational, 277 closed down or being dismantled, 171 decommissioned, and 10 left in place under derogations from OSPAR’s rule of full removal.

In the UK, the North Sea Transition Authority’s August 2026 update puts the cost of the remaining programme at £43.4 billion (2025 prices), about £21 billion of it by 2032. It expects 173 assets to cease production from 2026 to 2031, and counted 10 topsides and 8 substructures removed in 2025. Its benchmarks show the cost of large jacket and topsides removals, over 3,000 t, rising with global demand for heavy lift vessels. On the Norwegian shelf, Offshore Norge’s February 2026 outlook covers 31 assets with decommissioning planned from 2025 to 2034, 21 of them with a planned end of production in that period, and about 190,500 t of topsides and 107,500 t of jackets to remove.

Topsides come off in one of three ways, or a mix: piece small, cut up offshore and shipped in containers; reverse installation, module by module with a heavy lift vessel; or a single lift of the whole topsides. The largest single-lift vessel takes topsides of up to 60,000 t. Each method has its own lifts, and each lift has its own weight, cut sequence and load transfer.

The weight: what the rules ask for

A lift is designed on a weight multiplied by a contingency factor that grows with the uncertainty. The published factors depend on how the weight is known:

DNV-ST-N001 replaced these documents in 2016 (current edition December 2023, lifting in its Section 16), and DNV-RP-N102 replaced the removal practice. The project works to the factors of the edition it is contracted to; the older documents show how the factor follows the evidence.

For removal, DNV’s guidance lists what service adds: equipment and structural modifications, new piping and cables, concrete fillings, flooded members and tank residues, with marine growth taken from surveys and treated as a permanent load. It recommends weighing where possible, and warns that conservative factors alone are often not enough: the lift is also checked with extreme centre-of-gravity positions, the lowest possible weight and the highest feasible buoyancy. Water trapped in a subsea structure can raise the hook load considerably as it leaves the water, and perforating the structure first is one answer. A non-skirted structure lifted off a clay seabed needs extra force at least equal to its submerged weight to break free; see subsea retrieval.

Published close-out reports show topside weights that held up well, and far less marine growth reaching the yard than was estimated. A 23,500 t topsides removed in one lift in 2017 came out within 0.25 % of the weight recorded at the lift, and about 600 t (under 3 %) below the estimate in its decommissioning programme. Marine growth was estimated at 6 to 10 % of the jacket weight on three large North Sea jackets, but yards receive far less, because it falls off in cutting and lifting and dries out on the way. For the lift, plan on the estimate, not on what reaches the quay.

Cutting free: the release

DNV’s removal guidance asks for the cuts to be verified complete before the lift, sometimes by two independent methods, and for a structure that may not be stable once fully cut to be kept partly cut or secured. Its lifting standard, written for cutting seafastening before a lift-off, asks for the cut to leave no vertical restraint and for the crane operator to have live load readings to stop the lift if something is still attached. The removal guidance also asks for crane load monitoring accurate to better than ±5 %, with a contingency procedure for an overload reading. Among the cases a removal analysis considers is a random reaction force from an incomplete cut.

The dynamics depend on how the last support lets go (Figure 1). Where the final cut is made with the crane attached, the crane is preloaded and the supports carry the rest of the weight. If that support then lets go at once (a hanging or cantilevered member, a connection in tension, or a cut face the load can slide or drop off), the remainder moves to the hook suddenly, and the rigging overshoots the new static load, by as much again when undamped.

In a simple model with 5 % damping, a crane preloaded to 90 % of the true weight, for example to an estimate that was 10 % light, briefly carries about 109 %; preloaded to 70 %, about 126 % (Figure 2). A load that sits on its cut faces does not drop: it stays there until the crane lifts it off, and the transfer becomes the lift-off described below. A shock absorber in the line strokes near its setting instead, as long as the setting sits above the heaviest credible weight and the unit has stroke left.

So where the last cut is made under load, the preload is brought as close to the true weight as the evidence allows, but not above the lowest credible weight, or the load jumps up when the cut goes through; the peak at release is part of the crane and rigging check. Many removals are cut ahead instead, with the load held by its own weight, friction at the cut faces and temporary restraints, so the release is the lift-off. A single lift in 2017 worked that way: the legs had been cut months before. The vessel took up nearly all of the 23,500 t topsides over about 1.5 hours of deballasting, then separated with a 15-second fast lift that gave 1.5 m of clearance above the cut legs. It is the same idea as a quick lift: take the load before the release, and clear fast after it.

Lift-off from a structure, landing on a barge

A crane vessel lifting from a fixed platform, or from a barge, lifts off a structure that moves differently from its crane tip. DNV-RP-N103 treats that lift-off as a snap load: the snap velocity is the hoisting speed plus the relative vertical velocity between the object and the crane tip, and the snap force grows with the square root of the hoisting system’s stiffness. After lift-off, the load can meet the barge deck again as the barge heaves, or the platform as the crane tip drops. At set-down on a barge, the deck can rise into the load, so the contact speed is the lowering speed plus the deck’s upward speed. Our transfer lifts article works through both.

Where a compensator or a shock absorber helps

None of them changes the static weight, proves a lift point or completes a cut. They are sized once the weight basis and the load path are established, from a model of the lift. CONSTELLATION models the release, the lift-off and the landing with and without the unit, and reports the peak loads, the stroke used and the clearance.