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 stay inside its approved operating-tension band. Low effective tension can allow compression or buckling; whether that can affect well control depends on the riser, BOP, well-barrier arrangement and operating state.

The required tension depends on the riser’s weight, buoyancy, current loading, and water depth. Required top tension is calculated from the riser configuration, mud weight, water depth, vessel offset and operating mode. 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 is specified to keep riser tension inside the approved band over the defined vessel, environmental, offset and operating envelope.

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.

A riser tensioner may use multiple cylinders in parallel. The required redundancy, allowable failed state and load redistribution are system- and certification-specific; no single-failure capability is implied here. The specified stroke also accounts for the project vessel-offset envelope.

Riser tensioning presents several unique engineering challenges:

• High tension, long stroke — Required top tension, stroke and gas volume are calculated for the riser, vessel and operating envelope; deepwater duty can require high force and long stroke.
• Continuous operation — Riser tensioners can operate continuously through approved campaign phases, whereas a lift compensator is specified for its lift sequence. Required duty cycle, availability and maintenance philosophy are project-specific.
• 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 natural frequencies are checked against the relevant wave-excitation range and operating conditions.

For an approved contingency case, when a primary system needs maintenance or is unavailable, a backup riser tensioner can support the defined temporary tensioning duty while the primary is restored. SIRIUS, our backup riser tensioner currently in development, is intended for rapid-deployment contingency duty after project-specific integration and approval.

For applications needing adjustment in service, ANTARES’s automatic gas-spring control can be applied to tensioning duty, holding the configured tension band 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.