Types of large diameter plastic pipes for Offshore Engineering

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After decades of exploration, there are two types of large-diameter plastic pipes for global marine Engineering: straight extruded solid wall pipe and spiral wound wall pipe.

1. Large diameter straight extruded solid wall pipe

Thermoplastic pipe starts from direct extrusion solid wall pipe. The process is to melt the plastic and extrude it from an annular die and then cool it for shaping. In the range of small and medium diameter, most plastic pipes adopt straight extruded solid wall pipes. However, the problem of extruding large diameter thick wall pipe was not solved in the early stage. The reason is that the molten thick wall extruded from the die head of the large-diameter thick wall pipe will appear "sagging" under gravity before cooling and shaping, that is, the molten resin flows from high to low along the circumference, forming a serious thin wall at the top and thick wall at the bottom. Therefore, in the early stage, the direct extrusion method can only manufacture thin-walled and large-diameter plastic pipes. The application is limited to thin-walled pipes such as "marine water intake and drainage pipeline", "sea drainage pipeline" and repair liner, and cannot be applied to pressure pipes requiring small wall thickness and diameter ratio (SDR). In this century, the world has done a lot of research and exploration on the development and production of large-diameter plastic thick wall pipes. On the one hand, raw material enterprises continue to develop special resin for pipes with better comprehensive mechanical properties and sag resistance. The resin for high density polyethylene pipe has been improved from PE63 to PE80, PE100, PE100-RC, and then to 'anti sag PE100' (LS) 'super anti sag PE100' (XLS). The molecular weight distribution of anti sag resin can be adjusted to increase the viscosity at low shear rate. The other is to improve the pipe extrusion process and equipment, such as innovating the cooling process after pipe extrusion (increasing internal cooling, etc.), and gradually increasing the wall thickness range of large-diameter plastic pipes in recent years.

The mechanical structure of large-diameter straight extruded solid wall pipe is simple. It has its unique advantages for marine engineering. First, it is flexible. Second, it is possible to continuously manufacture 100 meter long extra long pipe (XXL) and transport it to the construction site by floating on the water surface. One of the limitations of large-diameter direct extrusion solid wall pipe is that large wall thickness is necessary to achieve high stiffness. It is only applicable to low-pressure or non pressure pipelines requiring high ring stiffness. Second, it requires a huge and expensive production line, and different accessories are required for different specifications. Therefore, it is concentrated in a few large plastic pipe enterprises.

At present, pipelife group is the leading enterprise in the field of large-diameter direct extruded solid wall pipe for marine engineering. It built a plant at the seaside of stathelle, Norway at the end of last century and completed the famous Montpellier sea discharge project in 2004 at the beginning of this century. According to statistics, pipelife was in Europe, Africa and Asia from 2006 to 2015, 27 countries in South America have completed 48 projects of large-diameter plastic pipe engineering. In recent years, the enterprise actively developing large-diameter plastic solid wall pipe is AGRU group. Its XXL pipe production plant is newly built in the United States. In recent years, several enterprises have made outstanding achievements in the development of large-diameter direct extrusion solid wall pipes, such as UPI (Union pipes industry) in the United Arab Emirates, Firat in Turkey, IPEX pipelines in Australia, etc.

2.  spiral wound wall pipe

Spiral wound structure wall pipe is a process of forming pipe by winding and mutual fusion of melt extruded plastic profiles along the spiral line. The purpose is to avoid the difficulties and limitations of direct extrusion of large-diameter pipes. Large diameter plastic pipes can be manufactured with smaller extruders and supporting equipment, and different structural walls can be formed to meet different strength and stiffness requirements.

After decades of exploration and competition, two kinds of spiral wound wall pipes are widely used. According to Chinese national standards, it is divided into A-type structural wall pipe and B-type structural wall pipe. A-type spiral wound pipe is commonly called 'double flat hollow wall pipe' in China, and B-type spiral wound pipe is commonly called 'Clara pipe' in China. In this paper, they will be called 'double flat hollow wall pipe' and 'B-type structural wall pipe'.

A. double flat hollow wall pipe

The double flat hollow wall pipe was developed by Uponor infra (KWH has been incorporated) in Finland in the 1980s and is called weholite. The basic process is to extrude a rectangular hollow pipe with an extruder, then wind it on a group of cylindrical roller shafts when it remains flexible and semi molten, squeeze molten plastic between adjacent rectangular hollow pipes to make them perfect fusion with each other, and form a structural wall pipe with straight walls in the middle of the inner and outer planes. The wound pipe is continuously pushed out. The inner and outer surfaces of the double flat hollow wall pipe are smooth cylindrical surfaces, and the longitudinal section is a continuous hollow structure supported by evenly spaced vertical walls.

The advantages of double flat hollow wall pipe are:

- Large diameter plastic pipe can be produced with relatively light and economical equipment, and may achieve high ring stiffness. It is suitable for non pressure or low-pressure drainage pipeline.

- The length of a single section of continuously formed pipe can reach more than ten meters (the wall pipe with B-type winding structure is limited by the length of the core die, usually nearly 6 meters per section), which is conducive to reducing the cost of pipe section connection.

The disadvantages of double flat hollow wall pipe are:

- The hollow wall structure formed by the fusion of single-layer rectangular hollow pipe has limited ability to bear the internal pressure of the pipe, so it is not suitable for water transmission pipeline with high pressure.

- Because the end of the pipe is not a solid wall, butt fusion welding is not allowed, and the cylindrical surface is not regular, socket connection is not allowed. The more reliable connection method is fusion welding at the joint with a portable extruder.

Now Uponor infra and the enterprises that have obtained its technology licenses (it is said that there are more than 10 in the world). Nearly 40 production lines) form an exchange and cooperation community, and some international enterprises independently develop similar production processes.

In the 40 years since its development, the double flat hollow wall pipe has developed in the fierce competition and proved its unique advantages. In recent years, there have been many successful cases in Marine Engineering, such as:

- In 2009, the water intake and drainage of CombiGolfe power station in France took DN2,200mm SN4 210m drainage DN2,400 840m

- In 2017, the cooling system DN / ID2,700mm 2,025m of Nghi Son Refinery petrochemical project in Vietnam

- In 2020, the laying of DN 3,000mm 880m HDPE Double culvert with 12 structural wallboard boxes was used for the deepest underground pipe network in the world

B. structural wall pipe

B-type structural wall tube is a polyolefin structural wall tube formed by winding and fusion initiated by krah company in Germany in the 1980s. The basic process is to spiral wind multi-layer hot-melt polyolefin tapes and profiles on the preheated metal core barrel to form a structural wall pipe. Usually, the inner wall is wound by multiple layers of tape to form a solid wall layer to achieve the required strength. Then, according to the requirements for the stiffness of the pipe ring, the hot-melt polyolefin round pipe material is wound outside to form the outer round rib structure wall layer. The socket for connection is made by winding fusion and machining at both ends of the pipe (the socket is embedded with electric heating wire), which can realize the socket connection of electric fusion.

This process can produce large diameter pipes with a wide range of specifications and performance (internal pressure strength and external pressure stiffness) with relatively economical equipment. At present, krah company and the enterprises that have obtained its technology license are said to have formed an exchange and cooperation community with more than 50 companies around the world, and some international enterprises independently develop similar production processes.

The advantages of type B structural wall pipe are:

A set of equipment based on continuous melt extrusion and winding fusion is equipped with core barrels and auxiliary facilities with different diameters, which can manufacture various pipes with different inner diameters, different inner wall thickness (different internal pressure resistance strength) and different outer circular rib structures (different external pressure resistance stiffness). Therefore, the pressure transmission pipeline with internal pressure resistance strength requirements can be manufactured, and the discharge pipeline with no or low internal pressure resistance requirements and different external pressure resistance stiffness requirements can also be manufactured. It can also produce very thick tubular blank for cutting pipe fittings such as flange. It is beneficial for production enterprises to meet a wide range of market requirements with small equipment investment;

All polyolefin plastics with corrosion resistance, impact resistance, wear resistance, easy fusion and light specific gravity are adopted. It is especially suitable for marine engineering: it can float on the sea surface, or it can sink on the seabed and hang underwater by adding concrete weight or pouring mortar into the hollow of the structural wall.

The socket connection of electrofusion is more convenient (especially for large-diameter pipes laid at the bottom of the ditch). Butt fusion welding is also possible for thick inner solid walls.

Flexible structural design: easy to achieve large diameter. At present, the maximum diameter reaches 4000mm, which is the main body of plastic pipes above 2000mm in the world; It is easy to manufacture large wall thickness pipe, and there is actually no restriction on the pipe wall; It is also easy to achieve large ring stiffness and can be wound and welded into multi-layer structural walls.

The disadvantages of type B structural wall pipe are:

It can only be produced in sections, which is limited by the length of the core barrel. Each section is usually 6 meters long, so the workload of connection is heavy.

Krah, the founder of B-type structural wall pipe, has always been constantly exploring, developing and redeveloping on the one hand, and paying attention to market development and cooperation on the other hand. For example, after years of exploration and development, glass fiber reinforced polyethylene material is adopted, and the developed short glass fiber polyethylene material PE-GF has been demonstrated to be rated Mrs = 18Mpa (1.8 times that of PE100), and has entered ASTM and DIN standards.

Krah has done a lot in developing the application of marine engineering. On the one hand, he has carried out technical research and published monographs and exchanges. On the other hand, he has constantly published and introduced the cases of successful application of large-diameter plastic pipes in marine engineering in various countries. Such as:

- production enterprise of dn3000 3900m of Lima sea discharge project in Peru in 2013; Spain power purchase agreement and krah agreement

- in 2014, the sea water in the United Arab Emirates was discharged into 2000 mm 36 km and 3000 mm 22 km PP-B HM production enterprise: UPI-2015 Stockholm port discharge in Sweden: DN 3400, total wall thickness 180 mm 135 m production enterprise Germany hengze Co., Ltd.

- 2018 Argentina water intake project: the world's largest practical diameter plastic pipe DN 3600 10 bar SD R17 wall thickness 220 mm. Production enterprises: Cara America Latin America

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