An offshore pipeline laid on the seabed is pushed sideways by waves and currents and lifted by the flow passing over it. If those forces exceed the resistance the seabed can provide, the pipe moves. Small, controlled movement can be acceptable, but uncontrolled displacement can overstress the pipe, damage coatings and load subsea structures. On-bottom stability design is the set of checks that keeps that movement within limits.
The forces acting on a seabed pipeline
Water moving past a pipe resting on the seabed generates three main hydrodynamic loads:
- Drag, acting in the direction of flow and proportional to the square of the water particle velocity.
- Lift, acting upwards because flow accelerates over the top of the pipe while the underside is partly shielded by the seabed.
- Inertia, associated with the acceleration of water in oscillating wave flow.
Resisting these loads are the submerged weight of the pipe and the lateral resistance of the soil. Lift reduces the effective weight pressing on the seabed, which in turn reduces friction, so the problem is more sensitive than a simple comparison of weight and drag suggests.
Waves dominate in shallow water, where orbital velocities reach the seabed. Steady currents, from tides or ocean circulation, matter at all depths. In many cases the design condition combines both, and the way they are combined has a large effect on the result.
Metocean data comes first
Stability design depends on reliable environmental data along the route: wave height and period, current speed and direction, water depth and their joint probabilities. Design codes typically ask for the combination of extreme waves and currents that represents a defined return period, and different return periods may apply during installation and operation.
Because water depth changes along a route, a pipeline crossing from deep water towards a landfall will see very different conditions over its length. Stability is therefore assessed in sections, and the nearshore section is often the most demanding.
Seabed soils and pipe-soil interaction
The seabed provides resistance in two ways: friction along the pipe and passive resistance from soil pushed up against its side as it embeds. Sands and clays behave differently. On sand, resistance depends heavily on how far the pipe has embedded and whether the sand is mobile. On soft clay, the pipe may sink into the seabed under its own weight, which can increase lateral resistance considerably.
Geotechnical data from the route survey, including cone penetration tests and samples, feeds into these models. Where data are sparse, conservative assumptions push designers towards heavier pipe or additional stabilisation, so investment in good survey data usually pays back.
Design approaches
Absolute stability
The simplest approach requires that the pipe does not move at all under the design loads. It is a force balance between hydrodynamic loads and soil resistance with safety factors applied. It is easy to apply and audit but can be very conservative in exposed shallow water.
Allowable displacement
More refined methods accept that the pipe may move a limited distance during a design storm, provided that movement does not cause overstress, damage to the coating or problems at fixed points such as tie-ins and crossings. These methods rely on dynamic simulations of pipe-soil interaction and typically follow established industry guidance, such as the recommended practice published by DNV. They can justify lighter designs but need better input data and careful interpretation.
Whichever approach is used, it should be agreed early with the certifying authority and operator, as it shapes every downstream decision.
Ways to make a pipeline stable
When the bare steel pipe is not heavy enough, designers have several tools:
- Concrete weight coating. The traditional solution, adding submerged weight directly to the pipe. Thickness and density can be varied along the route. It increases installation weight and can affect lay vessel selection.
- Increased wall thickness. Sometimes economical for smaller diameters, and it also helps with pressure containment and impact resistance.
- Trenching and burial. Lowering the pipe below the seabed greatly reduces hydrodynamic loads. Natural backfill or rock placement adds further protection, especially from fishing gear.
- Rock placement. Discrete rock berms or continuous cover can stabilise sections, support free spans and protect against trawling.
- Mattresses and anchors. Concrete mattresses and mechanical anchors are used locally, often near structures, crossings and tie-ins.
The right combination depends on cost, installation method, seabed conditions and the other design checks the pipe must pass.
Interaction with other design checks
On-bottom stability rarely stands alone. Adding weight or burying a pipe affects several other parts of the design:
- Thermal insulation. Insulated lines are lighter for a given diameter, and burial changes heat loss, which matters for flow assurance in subsea tiebacks.
- Lateral and upheaval buckling. Hot, high-pressure lines expand and may buckle. Designers sometimes deliberately allow controlled lateral buckling, which must be reconciled with stability requirements.
- Free spans. Uneven seabed leaves sections unsupported, with a risk of vortex-induced vibration and fatigue.
- Installation. Heavier pipe increases lay tension and may limit which vessels and methods are suitable.
An integrated approach, in which these checks are run together rather than sequentially, avoids solving one problem by creating another.
Monitoring through life
Seabeds change. Sandwaves migrate, scour develops around structures and storms can expose previously buried sections. Periodic route surveys using side-scan sonar, multibeam bathymetry and remotely operated vehicles show whether the pipe remains where it was designed to be. Comparing survey results with the original design assumptions helps integrity engineers decide whether remedial rock placement is needed, a process that benefits from the data-driven approach described in our article on digital twins and in-line inspection.
As existing offshore lines are assessed for new roles such as carbon dioxide transport, their stability under the new operating conditions also needs checking. Weight, temperature and pressure may all differ from the original design basis.
Key takeaways
- Invest in metocean and geotechnical data; it drives every stability decision.
- Agree the design approach and acceptance criteria early with all stakeholders.
- Treat stability, thermal, buckling and installation design as one integrated problem.
- Survey the route through life and act on changes before they become failures.