Subsea Retrieval

Whilst subsea retrieval is often thought of as simply the reverse of a subsea lift, several factors make it uniquely challenging:

• Suction breakout — Equipment that has been sitting on the seabed develops suction between its mudmats and the soil. Breaking this suction requires a pull force significantly greater than the equipment’s submerged weight, and the release is often sudden and unpredictable.
• Unknown weight — Marine growth, trapped water, sediment accumulation, or missing components can change the effective weight from the original installation value.
• Structural uncertainty — After years of subsea service, the equipment’s condition may be degraded. Lifting points and structural members may have reduced strength due to corrosion or fatigue.
• Snap loads at breakout — When suction releases suddenly, the load on the crane wire drops dramatically. If the vessel is heaving upward at that instant, the load can accelerate upward rapidly, and the subsequent deceleration creates dangerous dynamic forces.

A heave compensator is essential for managing the dynamic forces during retrieval. Its role changes through the operation phases:

• Tensioning for breakout — Before lift-off, the compensator maintains a steady upward pull whilst the winch gradually increases tension. The compensator prevents vessel heave from causing cyclic overloading of the lifting points.
• Breakout absorption — When suction releases, the compensator absorbs the sudden load change, preventing snap loads and controlling the initial ascent velocity.
• Water column transit — During ascent, the compensator provides normal heave compensation. Damping is adjusted as needed.
• Splash zone exit — Passing upward through the splash zone subjects the load to slamming and rapidly increasing weight as buoyancy is lost. The compensator absorbs these dynamic loads.

Retrieval operations benefit greatly from adaptive passive heave compensation because the effective load changes continuously throughout the operation. During breakout, the load includes suction. After release, it drops to the submerged weight. Through the splash zone, it increases to the in-air weight.

A basic passive compensator tuned for one of these conditions will be poorly matched to the others. Norwegian Dynamics ANTARES automatically adjusts its gas spring characteristics to track these load changes, maintaining high compensation efficiency throughout the entire retrieval sequence.

For the most demanding retrieval operations — particularly where the breakout force is highly uncertain — Norwegian Dynamics active heave compensator provides the maximum flexibility, with real-time control that adapts instantly to any load change. For guidance on choosing the right system, see the heave compensator selection guide.