COMPARISON BETWEEN ADDED MASS AND ACOUSTIC APPROACHES IN PREDICTING FREE SPANNING OFFSHORE PIPELINES RESPONSE TO EARTHQUAKE

Offshore oil/gas pipelines are usually installed unburied. Free spans in the line may occur initially by an uneven seabed or be created later due to under-scouring. Free spans are usually unfavourable spots in the submarine pipeline. They are vulnerable to vortex induced vibrations, excessive stresses in the pipe wall or may exhibit undesirable responses to strong ground excitations. In this paper, the simplified/conventional added mass method for simulation of water-pipe interactions has been evaluated against a more elaborate coupled acoustic-structural model. With the coupled system the surrounding sea, the pipe body, the sea bed and the free-spanning have all been incorporated in the model. Effects from both irregular (earthquake) and harmonic excitations have been investigated. The validation of numerical models has been achieved by comparison of their dynamic characteristics to some available analytical-experimental relationships. From the results of the current investigation it was noticed that the added mass method generally predicted more conservative free spanning responses in comparison to those from the acoustic model. Under harmonic excitations, the predictions from the two models for the maximum longitudinal strain in the pipeline deviated around 60%. For irregular (seismic) excitations, this deviation came down to about 15%. The disagreement between predictions from the two methods was increasing by the decrease in the span length. The difference was more remarkable for vertical excitations as compared to the horizontal ones.