Vortex-induced vibration is a major cause of fatigue failure in submarine oil and gas pipelines and steel catenary risers. Even moderate currents can induce vortex shedding. Pipelines from offshore petroleum fields must frequently pass over areas with uneven seafloor. One of the serious problems for the structural safety of pipelines is uneven areas in the seafloor as they enhance the formation of free spans. Route selection, therefore, plays an important part in design, Matteelli (1982). However, due to many obstacles it is difficult to find a totally obstruction free route. In such cases the pipeline may have free spans when crossing depressions. Hence, if dynamic loads can occur, the free span may oscillate and time varying stresses may give unacceptable fatigue damage. A major source for dynamic stresses in free span pipelines is vortex induced vibrations (VIV) caused by steady current. This effect is in fact dominating on deep water pipelines since wave induced velocities and accelerations will decay with increasing water depth. The challenge for the industry is then to verify that such spans can sustain the influence from the environment throughout the lifetime of the pipeline.
The aim of the present project is to improve the understanding of vortex induced vibrations (VIV) of free span pipelines, and thereby improve methods, existing computer programs and guidelines needed for design verification. This will result in more cost effective and reliable offshore pipelines when laid on a very rugged seafloor.The Ormen Lange field in the Norwegian Sea is one of the examples where the pipeline will have a large number of long spans even for the best possible route (see Figure 1). It was decided to evaluate two different strategies for field development; one based on offshore loading and the other on a pipeline to an onshore gas terminal. A key problem for the last alternative is that the seafloor between these fields and the coast is extremely rugged meaning that a pipeline must have more and longer free spans than what is seen for conventional pipelines. Today’s knowledge and guidelines are inadequate for obtaining a cost effective and reliable pipeline under these conditions, Det Norske Veritas (1998). Significant uncertainties are related to the assessment of fatigue from vortex induced vibrations caused by ocean currents. An extensive research program has therefore been initiated. The aim has been to improve the understanding of VIV for free span pipelines and thereby identify potential unnecessary conservatism in existing guidelines. Some changes have been proposed by Det Norske Veritas (2002), but improved analysis models have not been developed so far.
Two alternative strategies for calculation of VIV are seen today. Practical engineering is still based on empirical models, while use of computational fluid dynamics (CFD) is considered immature mainly because of the needed computing resources. Most empirical models are based on frequency domain dynamic solutions and linear structural models Larsen (2000), but the free span pipeline case has indeed important nonlinearities that should be taken into consideration. Both tension variation and pipe-seafloor interaction will contribute to non-linear behavior, which means that most empirical models will have significant limitations when dealing with the free span case. CFD models may certainly be linked to a non-linear structural model, but the needed computing time will become overwhelming. Then, one of the main focuses of the present research is investigation about time domain model for analysis of vortex induced vibrations for free span pipelines and the other is about multi free span pipelines where neighbor spans may interact dynamically. The interaction will depend on the length and stiffness of the pipe resting on the sea floor between the spans, and sea floor parameters such as stiffness, damping and friction. Each of them has important issues to investigate for improvement of our VIV knowledge.
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