J.D. Pos, K.S. Russell, J.A. Zwamborn


The design process for the calculation of wave forces and movements for a flexible (plastic) ocean outfall is described. The design procedure is illustrated using a case study of the design of two High Density Polyethylene (HDPE) pipelines of 0,9 m and 1,0 m 0D (4 290 m and 5 45Q m long) constructed at Richards Bay, South Africa, to dispose of dense and buoyant effluent respectively. The pipeline anchor weights are based on the 1 in 1 year wave forces on the pipeline, implying that the pipeline is allowed to move during its design life. Special star anchor weights are used which keep the pipe clear of the bed while maintaining the stability of the pipeline. Friction tests were undertaken with a section of pipeline and two star weights, above water on concrete and sand and below water on sand, to determine realistic friction coefficients for the pipeline design. The results of these tests are summarised in this paper. It was found that the mean friction coefficient for submerged star weights on sand was 0,75. The movements of sections of the 0,9 m OD pipeline were calculated using a finite difference computer programme developed by Prof I Larsen and the results are summarised in the paper. It was found that movements of 1 to 2 m could occur under design wave conditions (50 to 100 year waves) and these were considered acceptable provided that the pipeline was not obstructed by rock outcrops.


wave force; movement calculations; ocean outfall; outfall pipeline; pipeline; flexible pipeline

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