Waves

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

• Significant wave height (H_s) — The average height of the highest one-third of waves. 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. Typical values range from 5 seconds in sheltered waters to 15+ seconds in open ocean swell.
• 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 H_s and T_p are critical inputs for heave compensator design — H_s determines the required stroke, and T_p influences the dynamic response and resonance avoidance strategy.

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 in a narrow frequency band, which can be more challenging for resonance avoidance.

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 a defined limiting sea state — the maximum H_s at which the operation can proceed safely.

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.

For example, a subsea lift without heave compensation might be limited to H_s = 1.0 m, whilst the same operation with a well-designed passive heave compensator could proceed in H_s = 2.0–2.5 m. 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 — The compensator must be tuned so its natural period avoids the dominant wave period range, preventing resonance amplification.
• 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.