The “Flexible” underwater pipelines are subaquatic pipes, generally of big diameter and length, which work under pressure and are used for the final conduction of wastewaters previously treated, for the conduction of salt water (brine) and also like a means of seawater collection (outlet).

In the Iberian Peninsula, through the last decades it has been built a great diversity of pipelines of great diameter and length, in which it has been used Polyethylene High Density pipes (HDPE).


Normally, it has been used the technique construction of long sections on the shore by butt welding, transporting them on the surface (floating) and the controlled flood in an “S” shape.

The concept of flexible underwater pipeline is based on the high capacity of deformation, added to its capacity of relaxation of tensions, own characteristics of the materials of the range of the polyolefin (PE and PP).

So, the PE pipes guarantee a good behaviour of the exceptional actions produced by;

– waves

– sea currents

– inestability of the seabed

The great flexibility and the low weight of the PE pipes; the possibility of executing long sections; the possibility of connecting long sections with mechanical joints, which guarantee the structural continuation of the pipeline in longitudinal direction; the facility collection and transportation of long floating sections up to the anchorage point; the speed of the works of controlled anchorage in an “S” shape, a great depth, and it is not necessary a previous preparation of the seabed; and the considerable capacity of adaptation of the pipes to the changes of the seabed, they make it possible the installation of PE pipes in points and in conditions in which the conventional materials couldn’t be technical and economically designed.

Collection of pipes in work

The supply of PE pipes is normally performed in bars from 12 to 15 m long, due to the capacity of the overland transport. The pipes must not be tied on the ends to be handled, at least it must be fastened the 50% of its length in the central area. Don’t handle pipes by using wires or metal chains.


Respect to the storage, the ground of collection must to be cleaned and it must be avoided the contact with stones or sharp elements. It is recommended a height (h) of collection related to the Relation of standard dimension, SDR (diameter/thickness).

– SDR 33: h < 2.1 m

– SDR 26: h < 2.8 m

– SDR < 26: h < 3.0 m

Place of the work (open space)

The workplace in which we are going to build long sections of a pipeline should be placed near the water, protected enough from the conditions of the sea and the closer possible of the area of the final installation. They must have a space for collection of pipes and, if it is possible, underwater pipelines of the concrete ballasts.

27qNormally ramps are used for the conduction of the sections to the sea (welded pipes and with ballasts supported in metal rails to facilitate the operation of placing the section in the sea). The use of ramps with curvatures of small radius or in an “S” shape is not recommendable because the irreversible damages that can produced the pipes.

Finally, the long sections need a protected area for parking, in the water, before having the own conditions de of the sea to start the flood.


Advantage of the flexible PE pipes

The use of PE pipes in the construction of underwater pipelines has important advantages before other materials:

‒ Quick supply in bars from 13 to 15 m.

‒ It allows the execution of long sections on the ground.

‒ It is a material with great resistance to corrosion from the sea water.

‒ Due to the little radius of curvature which admits the factors which are introduced in the tow manoeuvres and anchorage are minor than in other materials.

‒ The possibility of fittings long sections on the ground, it reduces the number of joints in the seabed.

‒ The low roughness of the material which give sit a very good hydraulic behaviour.

‒ The reduced adherence in its internal walls makes that the useful life fulfils the expectative.

‒ It doesn’t condition to a large extent the constructive process.


– Plastics Pipes for Water Supply and Sewage Disposal
Lars-Eric Janson

– Polyethylene Pipes
Einar Grann-Meyer

– Manual Técnico para instalaciones submarinas de tuberías de Polietileno
Pipelife Norge AS

– Emisarios submarinos para saneamiento y conducciones submarinas para desaladoras
Eloy Pita. Avila 2005

– Manual Técnico tuberías de polietileno (PE)

I want to thank to my partner and friend Antonio Mendonça (a.mendonca@sapo.pt) for his master classes about underwater pipelines as well as for the provided information.

>>> The underwater pipelines play an essential role within the water supply in the coast areas. In dry areas, the drinking water supple is more and more guaranteed, with the support of the desalination plant, which takes the water, frequently through the underwater pipelines. Besides, the sanitation system normally ends in the sea, through an underwater pipeline. Leave a comment.