Offshore Decommissioning Lifting Challenges
The Scale of Decommissioning
Over 600 offshore platforms in the North Sea alone are approaching end of life and must be removed. Globally, the decommissioning market is expected to exceed USD 80 billion over the next decade. Each removal involves reverse-engineering the original installation — cutting, lifting, and transporting structures that have been in service for 20–50 years.
Unlike new-build installation, decommissioning lifts face unique challenges that make them inherently more risky and unpredictable.
Weight Uncertainty
The single biggest challenge in decommissioning lifts is weight uncertainty. Platform topsides accumulate material over decades — piping modifications, added equipment, marine growth on substructures, trapped water in members, and drill cuttings in legs. The as-built weight can be 10–30% higher than original design records indicate.
This uncertainty directly affects crane selection, sling design, and DAF calculations. Conservative weight estimates are essential, but over-estimation means chartering a larger (more expensive) crane vessel than necessary. Weight surveys using strain gauges and jacking tests help but cannot fully resolve the uncertainty for complex structures.
Structural Integrity Concerns
Corroded steel, fatigued welds, and degraded concrete make cutting and rigging decommissioning structures inherently riskier than handling new-build components. Lift points that were designed for a one-time installation lift 30 years ago may not be suitable for the removal lift — pad eyes may be corroded, structural members weakened, or the original lifting analysis unavailable.
In some cases, new lift points must be engineered and welded onto the structure before removal. This requires structural assessment, often using remotely operated vehicles (ROVs) for subsea components, and adds time and cost to the project.
Heave Compensation in Decommissioning
Heave compensation plays a critical role in decommissioning for several reasons:
- Margin management — with uncertain weights, a compensator provides a buffer against unexpected dynamic loads. A PHC reduces the DAF and gives the crane more margin for weight surprises.
- Weather sensitivity — decommissioning campaigns are often scheduled in summer months but still face North Sea weather. A compensator extends the weather window and reduces costly waiting-on-weather days.
- Landing control — placing removed topsides onto cargo barges requires controlled lowering. A compensator prevents hard landings that could damage both the structure and the barge.
- Shock protection — during cutting operations, sudden load transfer events can send shock loads through the crane system. A shock absorber like POLARIS protects the crane from these transient forces.
