home » Tow and flood of an underwater pipeline with PE pipes

Tow and flood of an underwater pipeline with PE pipes

103a

During the transport operations (tow) of the sections and, especially, in the phase of flood, it is recommended not to subject the pipes to radius of curvature proximal to the critical radius and reduce at the minimum possible in which the determined section of the pipe is required to determined forces (dent). The pipe must be subjected under air pressure during the tow up to the anchorage point and flood.

The internal air pressure in the pipe depends on the depth of flood and the percentage of its load, being independent from the nominal diameter.

Tow

The most common method is to tow the pipe floating on the water surface. The pipe is towed (loaded with ballasts, full of air and floating) up to the point of flood by pulling from one end. According to the length of the section to be anchorage it must be used one, two or more ships of support for the operations of conduction up to their final destination.

110f

Flood

• Previous works

The area of flood will have to be marked, previously it must be inspected the seabed and devices will have to be fixed and tied to the pipe.

• Flood Operation

The method of controlled anchorage due to the progressive flood of the pipe consists in introducing water through the valves placed in the blind flange in the end of the section to be flooded at the same time that the air goes out from the other end.

110c
The introduction of water must be done in a controlled way. It must be taken into account that with the close outlet valve of air, the flood will always continue so that it can be introduced in the water. Only the anchorage will be stopped when it is interrupted the water admission.

The weight of the water makes the pipes start to bend in one end in the flood process. The section of the pipe present an “S” shape – method that is also called anchorage in “S” shape. 110dThe flood can be performed in the earth-sea sense or backwards, although it is preferable to perform the anchorage from land to sea, due to the forces that are produced in the initial phase of the operations, especially in the cases of the heavy sections of great length and placed at great depth.

It is considered anchorages or floods at great depth which have a water depth (H) superior to 15 times the diameter (DN).

During the flood operation in an “S” shape, the pipe suffers very big forces, especially produced by the bending moment and the deformations.

The compressed air used in the anchorage process, reduces the risk of “dent” in the section of the pipe without water. Furthermore, the concrete ballasts, as long as they have been placed correctly, help you to reduce the risk of “dent”.

By applying an axial force at the end of the section of floating pipe, it can smooth the “S” shape of the section and so it can be decreased the acting forces, controlling the operation. The axial force applied in the end of the pipe reduces the radius of curvature of the pipe. Normally the curvature of pipe is more critical on the surface of the water (R1) than in the seabed (R2).

110e
Using a coefficient of security (Cs) equal to 2 it can be accepted radius of curvature of the pipe in the phase of flood according to the chart indicated in the last picture.

For reduced depths – where H < 15 times the DN – the axial force is not used. In the cases of great depth it must be calculated the axial force, besides it must be performed the calculus of the whole anchorage.

All the detailed calculus of the anchorage process must be performed by recognized in such works and using the proper informatics programs.

110g
The anchorage process must be performed with at the most constant possible speed of flood. Like a practical rule and to reduce the operation time of the forces/deformations, the speed of flood must be relatively elevated – 500 / 600 m/hour – it not surpass the 900 / 1000 m/h and it must never be minor to 250 m/hour.

In brief, it can be concluded that the due to the application of axial tensile strength together with the air pressure control within the pipe and the use of speeds of flood relatively elevated, it could be anchoraged the PE underwater pipelines, almost at any depth, without exceeding the data of resistance and the allowed limits of deformation.

The main advantage of such method of flood is the speed of execution. So the long sections could be flooded in much reduced spaces of time and make a planning of the installation operations with total security and according to the meteorological conditions and of the sea.

>>> During the desalinization, it is produced a by-product, that is the extremely salty water (brine). The dumping of the brine to the sea must be performed under certain conditions which guarantee that the impact to the environment will be the less possible. Although sometimes it is dumped directly on the coast, the most habitual thing is that such dumping is performed at certain depth, through a diffuser section that sends the effluent with certain angle upwards (between 45º and 65º), to achieve a good dilution of the “feather” of brine. So in this way, it is achieved a good dilution, which results essential to avoid that the sea species (mainly those which live in the seabed) be affected by the excessive salt of the environment in which they are. In Spain, they are the phanerogams prairies (oceanic posidonia and others) the most sensitive species to these dumping. Thus, the most convenient thing for the dumping of brine is to perform it through an underwater pipeline that ends in a diffuser section. If you want, you can leave a comment.

 
 
 

Leave a Reply

avatar
  Subscribe  
Notify of