Splash Zone Crossing

What the splash zone is

The splash zone is the region around the waterline where a load is partly in air and partly in water. Its height is not a fixed number of metres: it follows the wave crests and troughs and the motion of the crane tip, so it grows with the sea state and with the vessel’s response. An object crosses it on the way down and, if it is recovered, again on the way up.

Four effects that change the line load

1. Slamming. Water hits the object’s underside and horizontal surfaces as it enters. The force grows with the square of the impact velocity, F_{\text{slam}} = \tfrac{1}{2} \rho\, C_s A_s v_s^2, where C_s is the slamming coefficient (at least 3 for smooth circular cylinders and 5 otherwise, in the simplified method) and A_s the area that hits the water. It is short and sharp, and often what sets the structural check on the payload and rigging.
2. Drag. The water resists the object’s motion through it. The force grows with the square of the relative velocity, F_D = \tfrac{1}{2} \rho\, C_D A_p v_r^2, where A_p is the area seen from below; on a large, flat payload it can exceed the weight in water.
3. Varying buoyancy. As more of the object goes under, buoyancy takes part of its weight off the line, and every passing wave changes the submerged volume and with it the line load, F_\rho = \rho\, \delta V g.
4. Added mass. The water around the object has to accelerate with it, so its inertia grows as it goes under. With the crane tip and the waves moving, that adds a force proportional to the relative acceleration.

How much buoyancy matters depends on the payload. A dense steel structure keeps most of its weight in water; a light-for-volume object, such as a GRP cover or a module with large tanks, loses a large share of it (about half for solid GRP), while its large area catches the waves (Figure 1). Our subsea lifts article works through drag and added mass with numbers.

Slack slings and snap loads

If the buoyancy and the upward wave forces outweigh the object, the slings go slack. When they come taut again a moment later, the line takes a snap load, often far above the static weight. The simplified method in DNV-RP-N103 combines the four effects into one characteristic hydrodynamic force, F_{\text{hyd}}, and avoids slack slings with a criterion on it:

and where slack slings cannot be ruled out, it gives the snap load from the snap velocity, the stiffness of the hoisting system and the object’s mass with its added mass:

What a heave compensator changes

In line between the hook and the slings, a passive heave compensator makes the hoisting system soft. Its gas spring takes up the crane-tip motion as stroke instead of passing it into the slings, which removes the crane’s share of the object’s motion relative to the water; the water’s own motion remains. The drag and the added-mass force fall, and the lower stiffness K cuts the snap load in the formula above. Sized for the case, it keeps the slings in tension.

Three limits come with it. The compensator must not run out of stroke during the crossing, or the peak loads come back. The object’s weight on the line falls as it goes under, which moves the compensator’s equilibrium, so the stroke available is checked for every stage of the crossing. And a soft system has a long natural period: it isolates the load only from waves well shorter than that period and amplifies motion near it, so the lift model checks the tuning against the wave periods of the operation.

An adaptive passive unit such as ANTARES can switch to a stiffer, better-damped setting for the crossing, so the change in weight and the wave loads use less of its stroke, and soften again below it.

Case: a 35 t GRP cover at Hs 1.8 m

In our CONSTELLATION study CS-2026-004, a 35 t glass-reinforced-plastic protection cover is lowered from a construction vessel through the surface at Hs 1.8 m and Tp 7.5 s. The cover is the awkward class of payload: a large, thin shell that is light for its size, so the waves can lift it on entry. The same payload, sea state, rigging and eight wave realisations were run with rigid rigging and with a RIGEL 75 t / 4.5 m.

Across the eight runs the DAF ranged from 3.41 to 5.38 with rigid rigging and from 1.15 to 1.91 with RIGEL, and the stroke used at the governing end from 83 to 98 %. The snap-load ratio is the drop in line tension below the weight in water, as a share of that weight: at 1.00 the slings go slack, and its 0.90 limit is the DNV-RP-N103 slack-sling criterion applied to the simulated tensions. The DAF and stroke screens are our references for this study, not DNV limits.

The compensated runs passed all four response screens in every realisation. The stroke was the limit: four of the eight used more than 90 % of the stroke towards the governing (retract) end, up to 98.4 %, without touching the end stop, so the tuning would be reviewed before a real lift. At Hs 2.0 m no tested tuning cleared the four screens. Read the GRP cover study →

Planning the crossing

1. Screen the sea states. Run the crossing over the wave heights, periods and headings of the operation, with the vessel’s motion at the crane tip. The result is an operating envelope; a weather window needs the forecast to stay inside it for the whole operation.
2. Choose the crossing speed. Lowering faster shortens the time in the zone but raises the impact velocity and the slamming. DNV-RP-N103’s simplified method takes a hook speed of typically 0.5 m/s; our case lowered at 0.16 m/s.
3. Check both ends of the tension. The minimum tension decides whether the slings go slack; the peak load decides whether the crane, wire and rigging hold. One does not replace the other.
4. Confirm it in the time domain. A time-domain model of the waves, the vessel, the crane and rigging, the object’s hydrodynamics and the compensator gives the tensions, motions and stroke for the final check. CONSTELLATION runs it with and without the compensator.