Pipeline free span design has evolved from basic avoidance criteria in the DNV ’76 rules [1], to fatigue and ultimate limit state considerations in Guideline no. 14 [2]. Modern multimode, multi-span free span design is predominantly performed according to DNV-RP-F105 [3].
In 2006, the latest revision of DNV-RP-F105 [3] was written as a direct result of extensive research, performed due to significant free span challenges in the Ormen Lange pipeline project. DNV-RP-F105 was at the time, and still is, the only pipeline design code giving contemporary design guidance for vortex induced vibrations (VIV) and direct wave loading design for pipelines in free spans.
The last revision of DNV-RP-F105 included a few, but highly important advances, particularly the consideration for multi-mode and multi-span pipeline dynamic response behavior. In the 10 years that have followed, no breakthroughs of similar magnitude have been achieved for pipeline free spans, but a large number of incremental improvements to existing calculation methods, and some novel advances in less critical aspects of VIV understanding have been made. As a result, DNV-RP-F105 has recently been revised to account for these advances, which include improved frequency-domain analyses of wave-induced fatigue, a new response model for cross-flow VIV in low Keulegan-Carpenter (KC) regimes in pure waves, new analytical methods for dynamic response calculations of short spans in harsh conditions, and extensive guidance on how to apply the recommended practice for assessment of fatigue and extreme environmental load effects on curved structural members such as spools, jumpers and manifold flexloops.
This paper gives an overview of most of the important changes and updates to the new revision of DNV-RP-F105. Case studies are used to demonstrate the importance and effects of the changes made, and to some extent how the revision of DNV-RP-F105 can enhance its applicability and ease of use.
