A model test programme studying the hydrodynamic forces on sheltered marine pipelines is presented. The sheltering effect was due to partial burial of the pipeline or because the pipeline was placed in an open trench. The test programme covered wave flow - regular as well as irregular - and wave flow with steady currents superimposed. The reduced force data are used to indicate the decrease in hydrodynamic forces due to the sheltering effects.
The testing technique was the so-called carriage technique, in which the wave flow is obtained by oscillating the model sea bed and pipeline in still or steady flowing water. In this way a KC number range from 5 to 60 and wave-current ratios from 0 to 0.7 were achieved, and the Reynolds numbers were from 80,000 to 300,000.
Pipeline stability has been a subject in focus for at least the last decade. Up until the mid seventies marine pipelines were designed by using a single design wave from which near bed velocities and accelerations were calculated. Hydrodynamic forces were then found from the Morison type force expressions for the in-line and lift forces:
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where U is the time varying near bed velocity, U the acceleration, p is the water density and D the pipe diameter.
CD CM and CL are the force coefficients for drag, inertia and lift.
The values of CD and C were determined experimentally in steady current flows by Jones /1/, and despite the significant differences in flow conditions, those values were also adopted for design of pipelines exposed to waves.
Studies carried out in the late seventies, Grace /2/, Sarpkaya and Rajabi /3/, and in the eighties, Bryndum et al. /4/, Jacobsen et al. /5/ clearly demonstrated that the force levels were significantly higher in wave flows compared to steady currents, a fact that has been substantiated - also for combined wave and current flows - in more recent investigations, Verley et al. /6/, Bryndum et al. /7/.
These findings indicate that the calculated hydrodynamic forces applied up until the mid-seventies in the stability design were much too small in wave dominated areas. If the soil resistance formulation were adequate this would then imply that the pipelines would not be stable when exposed to severe storm conditions. However, reports on large movements or failure of pipelines due to excessive movements appear to be scarce.
The stability has traditionally been expressed by the relation:
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where Ws is the submerged weight of the pipeline and µ is a soil friction factor. This expression implicitly assumes that the pipe is actually sitting on the bottom without any embedment so that soil resistance is obtained by simple Coulomb friction.
