Riser Tensioning

A drilling riser is a large-diameter steel pipe extending from the vessel to the seabed, sometimes over 3,000 metres long. The riser must be kept in positive tension at all times to prevent buckling, which could lead to loss of well control — one of the most serious hazards in offshore drilling.

The required tension depends on the riser’s weight, buoyancy, current loading, and water depth. Typical top tensions range from 100 to over 1,000 tonnes. This tension must be maintained within tight limits despite the vessel’s continuous heave motion, which can cause the vessel to move several metres up and down every few seconds.

If tension drops too low, the riser buckles. If it rises too high, the riser, connectors, or wellhead may be overstressed. The tensioning system must keep the riser tension within this narrow operating band throughout the full range of sea conditions.

Riser tensioners are typically hydro-pneumatic systems consisting of multiple hydraulic cylinders connected to large nitrogen-charged accumulators. The cylinders apply an upward force to the riser through a wire and sheave arrangement or through direct-acting rams.

As the vessel heaves up, the cylinders extend and the gas expands slightly, maintaining a nearly constant upward force. As the vessel drops, the cylinders retract and the gas is compressed. The large gas volume ensures that the pressure change — and therefore the tension change — over the full stroke is small.

Modern riser tensioner systems incorporate redundancy: multiple cylinders work in parallel so that failure of any single unit does not compromise riser integrity. The system must also handle the slow drift of the vessel (surge/sway) without running out of stroke.

Riser tensioning presents several unique engineering challenges:

• High tension, long stroke — The combination of high tension (hundreds of tonnes) and long stroke (up to 15 metres on deepwater vessels) demands very large gas volumes and robust hydraulic components.
• Continuous operation — Unlike a heave compensator used for a single lift, riser tensioners operate continuously for weeks or months during a drilling campaign. Reliability and maintainability are paramount.
• Variable requirements — As water depth changes between wells, or as riser joints are added or removed, the required tension and stroke change. The system must be reconfigurable.
• Resonance — The coupled riser-tensioner system has natural frequencies that must be kept well away from the dominant wave periods.

Norwegian Dynamics is a cost-effective backup riser tensioner designed to provide emergency tensioning capability when primary systems require maintenance or in the event of a primary tensioner failure. SIRIUS offers a compact, rapidly deployable solution that maintains riser integrity during contingency operations.

For applications requiring adaptive performance, the ANTARES system’s automatic gas spring adjustment can be applied to tensioning duty, maintaining optimal tension accuracy as conditions and requirements change. The ANTARES piston rod locking feature also provides a secure hold function when tensioning is not required.

Selecting the right riser tensioning solution depends on the vessel, water depth, riser configuration, and operational requirements. Norwegian Dynamics provides engineering support for tensioner specification and can advise on the most appropriate solution — see our compensator selection guide for an overview.