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Construction of underwater pipelines with polyethylene pipes (PE)

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The treated wastewaters are normally directed towards the discharge area of the receiving environment at a certain depth and distance from the coast. An outlet of deep water will facilitate an excellent dilution of the wastewaters. The depth of the outlet will vary in an interval of 10-60 m, depending on the self-purification capacity of the receiving environment. The receiving environment can be a river, a lake or the sea.

The pipeline generally starts in the discharge chamber of the coastal area towards where the wastewaters are directed by under gravity or pumping. It is not common to choose the pumping directly in the outlet pipe and it is not recommendable. If it is necessary pump, the best solution will be to pump the wastewaters up to the discharge chamber and direct them under gravity up to the receiving environment.

The main aim of the discharge chamber is to avoid that the air enters the pipe. The air can produce the floating of the pipe due to the thrust force. You must also take into account the variations of the high and low tides when the discharge chamber is designed.

103c• Aspects related to the project

Generally, the specific questions in which an underwater pipeline is based on are the following characteristics:

– Structural
– Hydraulic
– Sanitarian

The execution of a project of an underwater pipeline it must be preceded by a search of basic information through oceanographic tasks of wide duration and extension. Such tasks cover the biological, physical, geological, chemical and meteorological areas, to obtain data about winds, sea currents, waves, tides, bathymetry and nature of the seabed.

The recognition of the nature of the seabed is one of the most important aspects related to the pipes in trench, directly supported in the seabed or a composed of a solution of the two previously mentioned, generally the most usual one.

• Structural Questions

The main acting forces in underwater pipeline are the resulting from the waves and the sea current. The pipes installed along of the edges or rivers crossings suffer the effect of the currents. In the case of an underwater pipeline, besides the effects of the waves and sea currents, it must be also taken into account the resulting forces from the instability of the sea beds.

The testing of the different acting forces is particularly difficult and many times it is a little rigorous.

We must obtain the answer to the following aspects;

– Resistance in the area of breakers
– Resistance in the area of currents
– Resistance to the instability of the sea beds
– Definition of the length of the sections, according to the characteristics of the implantation
– Capacity of utilization of mechanical joints to connect the different sections and to keep the continuous structural of the pipeline in the longitudinal direction
– Capacity of adapting “ballasts” according to the depth and the characteristics of the sea bed

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• Hydraulic Questions

In brief, it is identified the following questions:

– Definition of flows and speeds
– Determination of loss of charge
– Disposition and calculus of the hydraulic organs

• Sanitarian Questions

The biological component in the oceanographic researches is subdivided in different parts. One of them consists in determining the T90 parameter (bacterial decline), which – taking in consideration the effects of the currents, tides and winds – it is calculated the length of an underwater pipeline de residuals.

Also chemical oceanographic researches (quality of the water) allow to characterise the receiving environment before starting the process of dumping or outlet. All the obtained database constitute references to evaluate environmental impacts with the working of the system and its control by monitoring, to know:

– Interaction of the sea currents
– Winds
– Marine fauna
– Recognition of the profiles of temperature and salinity in the point of dumping / outlet.
– Scattering coefficient and of self-depuration
– Capacity of the receiving environment, among others, to definite the length of the pipeline and characteristics of the diffuser.

• Concrete ballasts

It can be distinguished, according different geometrical shapes to type of installation; it is recommended to use eccentric ballasts, to guarantee the highest stability of the pipe.

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As the PE pipes float, even when they are full of water, to sink them it must be incorporated concrete ballasts, generally, constituted by two pieces of reinforced concrete, with dimensions and different geometrical shapes, and which surround the pipe with screws and nuts. In its internal part they normally have two synthetic rubbers of EPDM type to impede damages of the concrete ballasts to the pipes.

The protection of metal elements, as well as the flanges for the connection of sections of pipe, is guaranteed with sacrificial anodes, mainly of zinc or aluminium.

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The ballasts will be placed in equidistant intervals– not more than 10 times of the nominal diameter of the pipe, without exceeding 5 metres – so that it is achieved the coefficients of low length.

“Load” can be defined as the ratio, represented in percentage, between the addition of all the loads of flood and the forces of push of the system, considering the pipe full with air.

During the flood operation the pipe must not be charged more than the 65% of the capacity of floating, decreasing this percentage according to the characteristics of the pipe (thickness) and the sea depth.

The cement used in the concrete must resist the chemical aggressiveness of the water and the steel used must be covered at least of 40 mm. The surface of contact of the ballasts with the pipes will have to be redressed in bands of elastomeric in EPDM or Neoprene of 5 á 10 mm of thickness as minimum.

The inferior and the superior ballast are joined with galvanized steel screws which are protected with sacrificial anodes.

>>> The sea has great capacity of depuration, provided that the dumping are performed with the proper quality and quantity. That’s why, in the coastal cities, the design of the sanitation systems has to take into account the sea as the receiving environment and depurator of the wastewaters. Logically, before being dumping to the sea, the effluent is treated in a treatment plant, optimizing the cost of the binomial pipeline-depurator, always fulfilling the desired environmental aims. In some cases, the dumping to the sea is being substituting by the reutilization of the depurated water. Leave a comment.

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