Glossary.

Parametric rolling

A non-linear roll instability where a ship in head or following seas develops large roll angles even with small wave excitation, because the restoring moment changes between wave crest and trough.

Parametric rolling occurs when the metacentric height varies cyclically as a wave passes — restoring moment is high in a trough (wide waterplane) and low at a crest (narrow waterplane). When the ship's natural roll frequency is roughly half the encounter frequency, energy pumps into the roll mode and amplitude grows non-linearly. Container ships with fine bow flare and wide flat sterns are particularly vulnerable. Mitigation requires either avoiding the resonance (operational), increasing roll damping (passive U-tubes, anti-roll fins, bilge keels), or both.

CFD (Computational Fluid Dynamics)

Numerical solution of the Navier-Stokes equations on a meshed hull or structure, predicting resistance, propulsion, wakes, pressures, and flow separation.

For naval architecture, CFD typically means Reynolds-Averaged Navier-Stokes (RANS) solutions in commercial codes (STAR-CCM+, OpenFOAM) for resistance and propulsion, and 3D panel methods (WAMIT) for linear seakeeping. Mesh independence, turbulence modelling (k-omega SST, k-epsilon), and free-surface treatment (VOF) are the main accuracy levers. CFD is not a substitute for towing-tank work but a complement: tank tests anchor the calibration; CFD lets you sweep parameter space cheaply.

WAMIT

A 3D panel-method linear-potential-flow code, developed at MIT, that computes wave-body interactions for offshore and ship hydrodynamics.

WAMIT solves the linearised wave-body interaction problem in the frequency domain using a 3D boundary-element (panel) method. It returns added mass, radiation damping, exciting-force RAOs, and motion RAOs for arbitrary geometries. Its strength is mature, well-documented physics for offshore platforms, semi-submersibles, FPSOs, and ships in the linear range. Its limit is the linearity assumption — for parametric rolling, large amplitude motions, and viscous effects, you need a non-linear time-domain follow-on (in-house Ship@Sea, OrcaFlex, etc.).

RAO (Response Amplitude Operator)

A frequency-domain transfer function describing how a floating body responds to unit-amplitude waves — the fundamental output of a linear seakeeping analysis.

For each wave frequency and heading, the RAO gives the steady-state amplitude (and phase) of each motion mode (heave, pitch, roll, etc.) per unit wave amplitude. RAOs are linear: they decompose an irregular sea into wave components, apply the RAO to each component, and superpose the results. They underpin operability calculations (significant motion in a given sea state), wave-induced load assessments, and mooring analyses. Their limit is amplitude — large motions invalidate the linear assumption and require time-domain non-linear analysis.

OPS / Cold ironing

Onshore Power Supply: feeding a berthed vessel from the local electrical grid so its diesel auxiliaries can be shut down, eliminating in-port emissions.

OPS, also called shore power or cold ironing, replaces the auxiliary diesel generators that ships normally run while at berth with a high-voltage shore-side connection. Frequency conversion is often required (60 Hz US ships against 50 Hz European grids). The decarbonisation gain is large where the local grid is clean — and small where it is not. Capital cost is high because of the medium-voltage infrastructure, fast-acting switchgear, and reefer-load shore boxes; payback economics are sensitive to call frequency, port-tariff structure, and air-quality regulation.

CII (Carbon Intensity Indicator)

An IMO-mandated metric, in force from 2023, that grades ships A-E on grams of CO2 per cargo-carrying-tonne-mile — a vessel's annual operational carbon intensity.

CII is calculated from a ship's actual fuel use and transport work over a calendar year. The IMO sets reference lines per ship type and size; vessels are graded A (best) through E (worst). A ship rated D for three consecutive years, or E in any single year, must submit a corrective action plan. The metric is operational, not design-based — it covers what the ship actually does, not what it was built to do — so weather routing, slow steaming, and shore-side downtime all matter. Targets tighten through 2030.

EU ETS Maritime

The extension of the EU Emissions Trading System to shipping (in force from 2024) that requires shipowners to surrender allowances for CO2 emissions from voyages to and from EU ports.

From 1 January 2024, the EU ETS covers all ships above 5,000 GT calling at EU ports. The owner must surrender allowances equal to a phased percentage of CO2 emissions: 40% in 2024, 70% in 2025, 100% from 2026. Methane and nitrous oxide are added from 2026. Coverage is 100% for intra-EU voyages and 50% for voyages to/from non-EU ports. The cost lands somewhere on the freight rate; how it splits between owner, operator, and charterer depends on the BIMCO-style ETS clauses inserted into time charters.

Manoeuvrability prediction

Numerical estimation of how a vessel will respond to rudder, thrusters, wind, current, and waves — typically through a time-domain six-degree-of-freedom hull model.

A manoeuvrability prediction couples a hull model (with its hydrodynamic derivative coefficients), a rudder model, a propulsion model, and external force fields (wind, current, waves) in time domain. The output is a trajectory: how the vessel turns, decelerates, holds station. The accuracy lives in the coefficient set; for high-stakes assignments these are determined by CFD or sea-trial calibration, not generic regression. Probabilistic operability assessments wrap the deterministic simulator inside a metocean joint distribution to answer "how often per year can the vessel do X."

Stability book (Trim & Stability booklet)

The class-approved document a ship's master uses to verify the vessel meets intact and damage stability criteria for any planned loading condition.

A stability book contains hydrostatic data (KMT, BM, GMT curves), cross-curves of stability, freeboard and tonnage calculations, intact stability criteria checks, damage stability cases, a flooded-compartment plan, and worked examples showing the master how to compute GZ and check against the rule. It is approved by the class society and the flag state; copies live on the bridge and ashore. A bad stability book is unusable on day one; a good one survives crew turnover, refits, and conversions for decades.

FEA (Finite Element Analysis)

Numerical structural analysis where a ship hull or offshore structure is meshed into elements and stresses, deflections, and fatigue lives are computed under load.

For naval architecture, FEA covers global hull-girder strength, local panel and frame analysis, fatigue assessment to S-N curves at hot-spot locations, and ultimate-strength assessment. Class societies (DNV, BV, RINA, LR) prescribe load cases, mesh refinement standards, and acceptance criteria. The work splits into linear elastic analysis (most production work) and non-linear ultimate-strength or collapse analysis (special cases, often offshore). Results feed directly into scantling decisions.

Spread mooring

A multi-leg mooring configuration where lines fan out from a floating structure to anchors arranged around it — the most common mooring pattern for FPSOs and semi-subs.

A spread mooring distributes environmental loads across many lines (typically 8 to 16) running in groups to anchor points on the seabed. Compared with a single-point mooring, it has a larger footprint but a smaller motion envelope; compared with dynamic positioning, it has zero fuel cost but no heading control. Catenary, taut-leg, or hybrid configurations differ in line tension, anchor type, and motion behaviour. Fatigue assessment is critical: the number of cycles over a 25-year design life can be in the hundreds of millions per line.

Class society

Independent, non-governmental organisations that develop and apply technical standards (rules) for ship and offshore-structure design, construction, and survey.

The major class societies — DNV, Lloyd's Register (LR), Bureau Veritas (BV), RINA, Germanischer Lloyd (now part of DNV), ABS, ClassNK, China Classification Society — each publish rule sets that a ship must comply with to receive their class notation. Class certificates are required by underwriters and most flag administrations. Designers liaise with class throughout the project: rule interpretation at the start, plan-approval submissions during design, surveys during construction, and periodic surveys in service.