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ERA-NET Cofund MarTERA · 2020–2023

NuLIMAS

Numerical modelling of seabed liquefaction around marine structures. Wikki developed an open-source OpenFOAM® model for pore-pressure build-up, liquefaction and re-compaction, validated against wave-flume experiments.

Code on GitHub →Publication →

Four snapshots of a seabed under waves: solid soil (dark brown) liquefies (pale brown), then re-compacts from the base with water on top

In the NuLIMAS project, Wikki developed an open-source numerical model in OpenFOAM® for seabed liquefaction around marine structures such as breakwaters, pipelines and the gravity anchors of floating offshore wind turbines.

Highlights

The model

The model divides the domain into four regions and solves different equations in each. In the water (Ω₁), the waves act as a pressure on the mudline. The soil (Ω₂) is a poro-elastic solid of grains and pore water (Biot), in which the pore pressure builds up wave by wave. Liquefied soil (Ω₃) is a mixture of grains settling in water, described by a drift-flux model. The gravity anchor (Ω₄) is an elastic solid, with the mooring lines as a source of loading.

Every cell switches between the solid and the liquefied description as the soil liquefies and re-compacts. The boundary between the two zones is part of the mesh and moves with the liquefaction front.

Sketch of the model domain: water with waves above, soil below with a liquefied zone, and a gravity anchor on the mudline pulled by mooring-line forces
Model domain: water (Ω₁), soil (Ω₂) on an impermeable base, liquefied zone (Ω₃) and gravity anchor (Ω₄) with mooring-line forces.
Two mesh sketches: (a) a mesh of solid soil cells; (b) the mesh split into liquefied cells above and solid cells below, with the boundary between them marked in red on the liquefied side and blue on the solid side
Dynamic mesh: (a) solid soil before liquefaction; (b) after the onset of liquefaction, the mesh is split along the front into liquefied and solid soil, each with its own boundary.

Validation

The model was calibrated and validated with small-scale experiments at the Institute of Hydro-Engineering of the Polish Academy of Sciences (IBW PAN) in Gdańsk: a layer of loose soil on an impermeable base, 0.5 m water depth and four combinations of wave height and period.

Three plots of pore pressure over time at four depths for three wave cases, numerical solid lines and experimental dashed lines
Period-averaged pore pressure at four depths for three wave cases: model (solid lines) and experiment (dashed lines). Insets: the first 30 seconds. Experimental data: IBW PAN.

The computed pore pressure follows the measurements at all depths, through the build-up, the liquefied phase and the slow decay during compaction. The maximum pore pressure stays within 10 % of the measured values for all four wave cases.

Structures on the seabed

A structure on the seabed changes the effective stress in the soil beneath it, and with it where and how far the soil liquefies. The model resolves the structure together with the seabed. Under waves, the liquefied zone forms around and beneath the structure and grows down towards the impermeable base.

Four snapshots of a seabed with a structure on top: a light-brown liquefied zone forms beneath the structure and grows down towards the base
Liquefied (light brown) and solid (dark brown) soil under a structure on the seabed. Top: initial state. Bottom, left to right: growth of the liquefied zone over time.

Depending on the weight of the structure and the forces acting on it, more or less pore pressure accumulates: a heavy structure restrains the rearrangement of the grains, while a lighter structure or a cyclic mooring force increases the pore pressure beneath it.

Open source and publications

The NuLIMAS toolbox and tutorials are available on GitHub.

Journal articles

Conference papers

Funding

This work was supported by the three-year (2020–2023) research project NuLIMAS (Numerical Modelling of Wave-Induced Liquefaction Around Marine Structures) within the ERA-NET Cofund MarTERA Program under the EU Horizon 2020 Framework (Grant No. 728053), the German Federal Ministry for Economic Affairs and Climate Action (Grant No. 03SX524A–C) and the Scientific and Technological Research Council of Turkey (Grant No. TEYDEB-1509/9190068).

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