DNV Releases new Recommended Practices on Riser Fatigue

DNV has released its new Recommended Practices (RP) on riser fatigue and riser interference, for industry hearing. New challenges occur when moving into deeper water. The industry’s reaction is that DNV is prepared for that move – one generation ahead of the market.

As water depth increases, current forces, external pressure and longer riser system lengths require thicker, heavier components to limit the stress. In addition, larger spacing between the risers is critical to avoid riser collision, a design change which results in larger structures to support the component loads. An optimal riser design is a key to the coming projects success.

To bring extended knowledge into engineering companies and making the results available to the end user in a transparent format, DNV believe that the new Recommended Practices, developed as part of the NDP (Norwegian Deepwater Program) sponsored work, perfectly matches the intended objectives. They are in the timeframe of deepwater business requirement.

An Enhanched Risk Based Fatigue Criterion to establish case specific fatigue safety factors for risers is included within the RP, which provides adequate target safety levels and also potentially cost savings to the industry.

The RP is also the first of its kind to include a simplified design procedure for the complex but important problem of Vortex Induced Vibrations (VIV) of risers.

Related to the design practice of today, interference between each single riser is not allowed. When production is moving to water depths of 3.000 meter or more, this requirement will increase the costs to unacceptable heights. The alternative is to accept interference and design for it. In the Recommended Practice, riser collisions are allowed provided that the riser array is subjected to state-of-the art analyses and sufficient fatigue and ultimate capacity are documented. The RP discusses the main items regarding riser interference comprising hydrodynamic interaction, local response of risers in contact and acceptance criteria for design.

The objective of the new RP is to recommend a methodology for engineering analysis and to provide rational design criteria and guidance for assessment of riser interference.

The new RPs are recognized to be a supplement to relevant national rules and regulations globally.