Pile Run Protection

During offshore piling operations, large steel piles are lowered through a guide frame and driven into the seabed using a hydraulic hammer. The soil resistance supports the pile’s weight during driving. However, if the pile tip encounters a weak or soft layer, such as loose sand or soft clay, the resistance can drop suddenly, and the pile accelerates downward under gravity.

This free-fall event is known as a pile run. The pile can drop several metres in seconds, and when it reaches a harder layer or the end of the soft zone, it decelerates violently. The resulting shock is transmitted through the hammer, guide frame, crane wire, and into the crane boom and vessel structure.

Without protection, a rigid arrest can drive the hook load far past the static weight; in ND’s modelled clamped-run screening case the factor passed 3, with crane overload, wire damage and harm to the piling equipment on the line. The severity is case-specific: the pile, hammer, soil and rigging boundary conditions set it.

A crane shock absorber is installed in the crane’s lifting system, typically inline between the crane hook and the load, to absorb the energy of a pile run event. It works on the same gas-spring principle as a passive heave compensator, but is specifically designed for the high-energy, short-duration loads characteristic of pile runs.

When a pile run occurs, the sudden increase in wire speed causes the shock absorber to stroke rapidly, compressing its gas charge and absorbing the kinetic energy of the falling pile. While the absorber stays inside its force–stroke envelope, the crane sees at most the absorber’s resistance — a fraction of the rigid-connection peak. Bottoming out forfeits that cap, which is what the stroke check is for.

The key design parameters are:

• Energy capacity — Must take the arrest energy the coupled model assigns to the absorber, including work done during the stopping stroke, without running out of usable stroke.
• Peak force — The maximum resistance force during stroking, which determines the peak load on the crane.
• Usable stroke — Must cover the relative travel through the absorber in the governing arrest case, with end-stop margin — not necessarily the pile’s soil-penetration distance.

For more on the energy balance in shock absorption, see our page on crane shock absorption.

Norwegian Dynamics POLARIS is a lightweight shock absorber specifically designed for crane operations including pile run protection. With capacities from 75 to 4,000 tonnes and strokes from 1.0 to 8.0 metres, POLARIS covers the duty range that offshore piling campaigns typically call for.

Key features of the POLARIS design include:

• Lightweight construction — Minimises the impact on the crane’s usable load chart, preserving lifting capacity for the payload.
• Rapid energy absorption — Tuned gas spring and damping characteristics optimised for the short-duration, high-energy loads of pile run events.
• Simple, robust design — No electronics, sensors, or external power. Works reliably in the harsh offshore environment.

Pile run risk is assessed during the design phase of a piling operation using geotechnical data. Soil borings and cone penetration tests (CPTs) identify potential weak layers where pile runs may occur. The expected pile run distance and velocity are then calculated based on the pile weight, soil resistance profile, and hammer energy.

These calculations determine the shock absorber specification — energy capacity, peak force, and stroke. The shock absorber must be sized to handle the worst credible pile run scenario, not just the expected case, because the consequences of under-sizing are severe.

Pile run protection is also relevant for wind farm installation, where monopiles are driven into the seabed in potentially variable soil conditions. POLARIS is well suited to this application, combining effective protection with the lightweight design that high-capacity crane operations need. See also quick lifting for related techniques that improve efficiency in repetitive installation campaigns.