Long Free Spans Exposed to Current and Waves: Model Tests

Abstract A model test programme is described in which the hydro-elastic vibrations of free spanning marine pipelines are studied. A large variety of environmental conditions are covered including steady currents, regular waves, irregular waves and combined steady currents and regular/irregular waves. Two types of models were used simulating a prototype pipeline span. One, a relatively short rigid pipe segment mounted in elastic springs and two, a long flexible pipe. Extensive measurements of displacements and hydrodynamic forces combined with detailed analysis established a number of parameters characterizing the hydroelastic response in each test. More than 3000 tests were performed and analyzed. Strictly 2-dimensional flow conditions were established for the rigid pipe tests while also 3Dimensional phenomena were investigated for the flexible pipe tests. Introduction The development of the technology of submarine pipelines have provided the possibility of conducting projects under extreme conditions with respect to water depth and environmental conditions. In these conditions long sections of spanning pipelines are often unavoidable. Free spans may occur due to natural sea bed irregularities present at pipe installation or they may develop during operation due to erosion, scour, or migrating sand waves. The presence of submarine currents and wave induced flows may cause significant dynamic stresses to the free spanning pipe section. Amplified response due' to resonant fluid-structure interaction, which involve large oscillations of the pipe, may damage the welding. The consequences can be fatigue of the material and reduction of the pipeline life. The sections of free spans may thus represent weak points of the transport system, as they have a low reliability. The determination of the critical length of spans under the various environmental conditions along the pipeline thus becomes an important element in pipeline design with a significant economical impact, especially in deep water where the traditional maintenance and repair technology is inadequate. The codes adopted at present for the design calculation of critical span lengths are based on criteria which could require difficult and very expensive technical interventions. In many cases the accepted length of spans is limited to a few pipe joints in order to avoid the onset of oscillations to the pipeline. In recent years a number of experimental studies have been performed, Refs. /5/, /6/, /7/ and /8/ among others, in which the dynamic response of free spanning pipelines have been investigated when exposed to flow induced hydrodynamic loads. The purpose has been to establish a better understanding of the phenomena involved and on that basis to further develop existing design criteria for free span calculation in order to obtain substantial cost savings in the design, construction and operation of pipelines. An important study in this context is the Submarine Vortex Shedding Project (SVS-Project), Refs. /1/ and /2/, which was initiated in 1984 and now is in its final phase.