Waves

Ocean waves are irregular and random, but they can be described statistically using a few key parameters:

• Significant wave height (Hs) — Classically the mean height of the highest one-third of waves; in spectral work it is computed from the wave spectrum (Hm0), which closely approximates it. This is the standard measure of sea severity used in offshore engineering and corresponds roughly to what an experienced observer would estimate as the wave height.
• Peak spectral period (T_p) — The wave period at which the wave energy spectrum has its maximum. The governing values are site- and season-specific — wind sea and swell carry different periods, and the project metocean basis states both.
• Zero-crossing period (T_z) — The average period between successive upward zero crossings of the sea surface elevation. Related to T_p by factors that depend on the spectral shape.

Both Hs and Tp are critical inputs for heave compensator design — but neither sets the specification alone. Stroke and velocity come from the modelled relative motion of the actual vessel, crane and payload in that sea state, and Tp drives where the dynamic-response and resonance checks land.

Because ocean waves are irregular, engineers describe them using a wave energy spectrum — a function showing how wave energy is distributed across frequencies. Two standard spectral models are widely used in offshore engineering:

• Pierson-Moskowitz (PM) — Describes a fully developed sea in deep water, defined by H_s alone. Suitable for open ocean conditions where wind has blown over a long fetch for an extended period.
• JONSWAP — A modification of the PM spectrum with an additional peak enhancement factor (γ, typically 1.0–7.0). Represents a developing sea with a sharper spectral peak. The default γ = 3.3 is commonly used for North Sea conditions.

The choice of spectrum affects the predicted vessel motions and, consequently, the crane tip heave that the compensator must absorb. JONSWAP spectra with high γ values concentrate energy near the peak period — whether that raises or lowers the response depends on where the peak sits relative to the system’s transfer functions.

Offshore operations are planned around sea state forecasts that specify H_s and T_p (and sometimes directional spreading and swell components). Each marine operation has defined limiting criteria — usually an Hs–Tp envelope, qualified by direction, wind and forecast uncertainty rather than a single Hs number.

The operational limit is typically governed by the most sensitive phase of the operation — often the splash zone crossing. Heave compensation directly increases this limit by reducing dynamic loads, extending the operational weather window and reducing costly waiting-on-weather time.

Where the suspended-load dynamics govern, compensation moves that envelope — the uncompensated and compensated limits are compared case by case in the lift analysis. This can make the difference between a feasible operation and one that requires an impractically calm weather window.

When specifying a heave compensator, the wave environment determines several key requirements:

• Compensator stroke — Sized to accommodate the maximum crane tip heave amplitude, derived from H_s and the vessel’s heave RAO.
• Piston velocity — Driven by the combination of heave amplitude and wave period; shorter periods require faster compensator response.
• Natural period — Establish the system’s natural modes first, compare them with the excitation band, then verify the selected compensator’s stiffness and damping across the modelled cases.
• Damping — Sized to control response near resonance whilst maintaining efficiency at typical operating periods.

Norwegian Dynamics provides engineering support to match compensator specifications to site-specific wave data. Whether using the RIGEL for cost-effective operations or the ANTARES for demanding variable conditions, correct wave characterisation is the foundation of effective heave compensation design.