Experiences Using DNV-RP-F105 in Assessment of Free Spanning Pipelines

Free spans often become a challenge in pipeline design and operation due to pipeline installation on uneven seabed or seabed scouring effects. The costs related to seabed correction and span intervention are in many projects considerable. Therefore it is relevant to investigate whether such intervention work is necessary or not. Despite the complexity inherent in free span response, the spans are often designed applying presumably conservative concepts and very simple analytical tools. The DNV guideline no 14 (GL14) for free spanning pipelines was issued in 1998 and has later been updated and issued as an Recommended Practice (DNV-RP-F105) to account for recent technical research and development and accumulated experience applying GL14 in pipeline projects. This code allows vortex induced vibrations (VIV) as long as the pipeline integrity is ensured, by for example checking that the fatigue life is sufficient. By giving design methodology and acceptance criteria for fatigue, the DNV-RP-F105 approaches the real physics of free spans in a better way than older codes and makes it possible to select cost-effective methods both in the design phase and later when re-assessing spans in the operational phase. This paper will briefly discuss some experiences obtained by using the DNV-RP-F105 in free span design/re-assessment. Some examples of pipeline failures due to free span and vortex induced vibrations will also be presented.