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12 9 THE IMPORTANCE OF OCEAN HYDRODYNAMICS The success of a submarine outfall is contingent upon the designer’s knowledge of the hydrodynamic forces of the ocean that are likely to be encountered by the outfall during the design life of the outfall. Failure to adequately assess these forces and take them into account during the design and construction of the outfall has probably been the leading cause of failure of submarine constructed of HDPE pipe and pipe of other materials as well. The prediction and analysis of the ocean’s forces that can be exerted on an outfall is a topic outside the scope of this manual but a summary of what information is needed and how it is applied is included in the following paragraphs. Current and wave-induced forces are the most important forces to be taken into consideration in the design of a submarine outfall. These forces have resulted in physical destruction of the pipeline by both the movement of the water itself or by the impact of objects such as submerged logs of trees being driven into the pipeline. Failure has also been associated with lateral sliding of the outfall from insufficient ballast anchor weight to withstand the horizontal and lifting forces induced by the current. Undermining of the seabed material from beneath an outfall by current induced differential erosion has resulted in failure by separation of pipe joints. (This is rarely a problem in HDPE outfalls pipe because of its superior flexibility but is a problem with pipe materials that utilize a compressed O-ring type of joint.) Mass movement of seabed material that has been caused by to storm induced waves and currents (and more rarely due to spontaneous liquefaction during earthquakes) has also resulted in outfall failure. There are many areas in the ocean where tidally driven strong currents occur twice a day. Large fiords or bays with relatively narrow and shallow mouths can generate extremely strong currents especially in areas with large tides. Such currents have been measured in excess of 17 knots. Tidally generated movement of water through openings in barrier reefs can also reach high velocities on a daily basis. Frequently such areas have geomorphologically sheltered from the open ocean and are thereby protected from large waves. In such cases the currents are the critical factor in determining the method of stabilizing the outfall. An equation that is commonly used to evaluate the necessary ballast weight necessary to stabilize a submarine outfall against a current perpendicular to the centerline of the outfall is... C ρ WB =μD +CL2(DL)V2 inwhich..... S WB= the buoyant weight (submerged weight of the outfall) of length L CD = the drag coefficient CL = the lift coefficient ρ =themassdensityofseawater(theunitweightofseawaterdividebyaccelerationof gravity) μs = the static friction coefficient between the ballast anchor (or pipe wall) and the seabed D = pipe diameter L = length of pipe section considered (usually a length of 1 meter) V = the velocity of the current moving perpendicular to the pipe It is important to note that WB is the submerged weight of the outfall not the weight in air. This can be calculated by multiplying the weight in air by (the sink factor – 1). The sink factor is explained in the following sections. The friction factor μs varies between about 0.6 and 1.4 for sand and between 0.2 and 0.7 for silt and clay. To obtain the aforementioned coefficients and for complete treatment of current induced forces on ocean outfalls, refer to the publication by Grace and the World Bank publication edited by Gunnerson both listed in the suggested references at the end of this document.PDF Image | SMALL DIAMETER (HDPE) SUBMARINE OUTFALLS
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