Scandinavian Oil-Gas Magazine

Example of vortex shedding, key loading type for free spans and the main concern together with direct wave loading (illustration: DNV GL)

Offshore pipeline spans, due to uneven seabed or seabed erosion processes, can require huge and costly seabed interventions. The DNV GL recommended practice (RP) on free spanning pipelines has been widely applied in industry to solve the problems related to free spans in pipeline design and operation. The RP has now been updated with project and research experience from the last decade on free span dynamic response, particularly on structural modelling. DNVGL-RP-F105 enables significant costs savings, both in the newbuilding and operational phase.

The scope of the RP is to provide rational design criteria and guidance for assessment of dynamic response of pipeline free spans due to combined wave and current loading. Detailed design criteria are specified for ultimate limit state (ULS) and fatigue limit state (FLS) due to in-line and cross-flow vortex induced vibrations (VIV) and direct wave loading.

The new revision of DNVGL-RP-F105 has improved several key physical models and introduces general guidance to new aspects where no real alternative guidance is given elsewhere. In addition, the document has also systematically been improved based on experience and feed-back provided by users of the previous revision.

“New research through PhDs, published literature and extensive accumulated project experience have advanced the field, and allowed us to significantly refine and improve the recommended practice with more reliable results and greater range of applications,” says Linlin Jiao, Senior pipeline engineer, DNV GL – Oil & Gas. “This update of DNVGL-RP-F105 contains several main changes that strengthens its position as the state-of-the-art document for free spanning pipelines.”

The main changes consist of:

• a new force model for direct wave action, which allows for multi-mode and multi-span analyses,
• highly accurate modal response quantity calculation procedures for short spans,
• general guidance for how to apply and interpret sensor technology on free spans,
• general guidance for how to apply the RP to spools, jumpers, flexible loops and piping systems,
• a new response model for cross-flow VIV in waves with low Keulegan-Carpenter numbers.

“The DNV GL Recommendation Practice F105 has been daily used by pipeline engineers worldwide to solve the main problems for free spans in pipeline design and operation,” says Cathrine Torp, Communication Director, DNV GL – Oil & Gas. “With these key updates, the DNVGL-RP-F105 will increasingly benefit our customers by reducing the enormous cost due to unnecessary span interventions and potential fatigue failure.”

Tags: DNV GL