JPS6351853B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a steel pipe whose outer surface is coated with plastic for corrosion protection, and particularly to the anticorrosion coating of steel pipe piles used in marine structures.

Steel pipe piles have long been used as structural members for offshore structures such as bridges, bridges, and platforms because they have many excellent properties. However, the problem with steel materials is that they are prone to corrosion, and the problem is even more serious than that of civil engineering and construction materials on land, especially since the ocean is a much harsher environment for corrosion than on land.

According to the knowledge so far, the ocean can be divided into five areas based on the state of corrosion in the marine environment: the oceanic area, the splash area, the tidal area, the underwater area, and the ocean floor area. The ocean area is exposed to sunlight, wind and rain, and the air contains more salt than the air on land, so corrosion is more severe than in the air on land, but less so than in the splash and tidal areas. The splash area is directly above the ocean and is exposed to seawater spray, repeating wetting and drying, and increasing temperatures due to sunlight, all of which accelerate corrosion, resulting in corrosion of 0.5 to 1.0 mm per year at the most severe locations. The next tidal zone has relatively little corrosion. This area undergoes repeated drying and wetting due to the ebb and flow of the tide, and it is said that this is because the highly corroded areas just below the horizontal and the ebb and flow areas create macrocells (corrosion batteries), and the ebb and flow areas act as cathodes for the area just below the sea level. . The closer to the sea surface, the more dissolved oxygen there is in the ocean, and corrosion is the second most severe after the splash area. However, the closer you get to the ocean floor, the less corrosion occurs. The ocean floor generally has the least amount of corrosion due to the lack of dissolved oxygen.

There are broadly two methods that have been used to prevent corrosion of steel materials for marine structures in such corrosive environments. One method is to increase the thickness of the steel material by preparing in advance a corrosion allowance commensurate with the amount of corrosion. However, this method is economically unreasonable as the larger the structure is, the greater the increase in overall weight becomes. Another method is to apply anti-corrosion methods. Typical corrosion prevention methods include coating with zinc-rich paint, tar epoxy resin paint, chlorinated rubber paint, etc., metal linings such as Monel, stainless steel, and sacrificial steel, concrete linings, and organic polymer linings such as rubber, polyethylene, and polyester. . Furthermore, cathodic protection is used in the ocean and undersea areas. Note that seawater-resistant steel with chromium added thereto may also be used. Appropriate methods for these corrosion prevention methods are used alone or in combination depending on each area, such as the offshore area or the splash area.

However, few of the conventional methods have been satisfactory in terms of performance or economy, and a better corrosion prevention method has been desired. For example, painted systems have a short coating life and must be repainted every 3 to 5 years, and Monel or stainless steel linings are not common because they are extremely expensive even though they have sufficient performance.
Concrete and organic polymer linings also had to be constructed on-site, resulting in high work costs.

The present invention was made with the object of solving these problems and producing a plastic-coated steel pipe that is economical and has excellent long-term corrosion protection performance.

It is well known that steel pipes coated with plastic such as polyethylene or polypropylene are used in pipelines for transporting gas and oil. Mill scale on the surface of a steel pipe is removed by shot blasting, then heated to over 140°C and coated with polyethylene or polypropylene to a thickness of 2.5 to 3.0 mm using a modified polyolefin adhesive. Due to its excellent properties such as chemical stability and mechanical strength, it can prevent corrosion of steel pipes installed underground or on the seabed.
It is expected to have a useful life of several years.

Naturally, even if plastic-coated steel pipes developed for pipelines are used as steel pipe piles for offshore structures, the long-term corrosion protection effect will be sufficient, and the conditions that are managed at factories with high productivity will be sufficient. Because it is manufactured in

However, the coating thickness of plastic-coated steel pipes is determined by taking into consideration moisture permeability, oxygen permeability, etc., and the film thickness required for corrosion protection, as well as handling during transportation and storage of the steel pipes, and when burying. It is determined based on the total thickness of the coating required to protect the coating from being damaged by external blows caused by backfilling with earth and sand, and for buried steel pipes, it is determined according to the outer diameter classification of the steel pipe. , for example, in JIS G3469
1.2mm to 2.5mm, and DIN30670 specifies 1.8mm to 3.5mm. However, in the case of plastic-coated steel pipe piles used for marine structures, even though the damage caused during handling during transportation and storage is the same as that of buried steel pipes, even when they are used as marine structures, the tidal areas and splash areas are covered with driftwood. There is always a risk that the coating will be damaged by damage from contact with boats or boats, and the degree of corrosion varies greatly depending on the area, as mentioned above, between the splashed area and the underwater area. That is,
For plastic-coated steel pipe piles, the corrosive environment and risk of damage to the coating are significantly different between those used in the ocean or under the sea, and those used near the sea surface or in areas exposed to splashes. It is not a good idea to decide on the thickness of the coating, but it is desirable to make the plastic coating thicker on the parts near the sea surface or in splash areas than the plastic coating on steel pipe piles used in underwater areas or on the seabed.

Steel pipe piles for marine structures can be over 40 meters in length by connecting steel pipes of a predetermined length, but the length of the ebb and flow part is about 3 to 7 m, and the standard length of the steel pipe is Since the length of the pile is 12m or 18m, it is desirable to have thick covering in some parts of one steel pipe pile. The present invention provides a method for manufacturing a partially thickened plastic coated steel pipe pile.

That is, the present invention relates to the production of a thermoplastic-coated steel pipe in which the outer surface of a steel pipe moving in the longitudinal direction as shown in FIGS. In the process, the upper lip 8 of the flat die 3 is moved up and down by the hydraulic cylinder 5 to arbitrarily adjust the interval between the slits 9 of the flat die 3, and the upper lip 8 is moved up and down by the cylinder 5. Interlockingly, the screw rotation speed of the extruder can be increased/decreased to adjust the amount of polyethylene extruded, and the thickness of the plastic sheet extruded from the middle of a single steel pipe can be increased/decreased to continuously provide coating with different thicknesses. This is a method for manufacturing plastic-coated steel pipes.

Next, the present invention will be explained in detail using specific examples.

A spiral steel pipe with an outer diameter of 508 mm, a wall thickness of 9 mm, and a length of 9 m is shot blasted to remove mill scale and rust from the outer surface, and then heated to 140°C to 210°C. Next, an adhesive containing maleic anhydride or a copolymer of acrylic acid and ethylene is coated with polyethylene. As shown in Fig. 1, the steel pipe 1 is conveyed while being rotated by the skie rolls 2, and the polyethylene, which has been heated to a semi-molten state by an extruder (not shown), is extruded into a strip from the slit of the flat die 3. The material is then wrapped spirally around the outside of the steel pipe, partially overlapping each other. The overlapping sections of polyethylene are fused together to form a continuous coating. After this, it goes through a cooling inspection process and becomes a product.

The thickness of the polyethylene coating is determined by two factors: the slit spacing of the flat die 3 and the drawdown rate, which can be expressed as the ratio of the polyethylene extrusion speed to the speed at which the polyethylene sheet 4 is wound around the steel pipe. The thickness of the sheet is determined, and it is further determined by the number of times the sheet is wrapped around the steel pipe.

Polyethylene is extruded under great pressure, and when it goes into the atmosphere and the pressure is released, it expands a little.
If no tension is applied when it is extruded from the flat die 3, it will be thicker than the slit spacing. When the circumferential speed of the steel pipe is made higher than the speed at which the polyethylene is extruded, tension is applied to the polyethylene sheet, which is stretched and becomes smaller in width and thickness. Naturally, the larger the spacing between the slits, the thicker the polyethylene sheet becomes.

Therefore, the first method for increasing the thickness of the polyethylene sheet is to increase the extrusion speed while keeping the circumferential speed of the steel pipe constant, thereby reducing the tension applied to the polyethylene sheet. These conditions can be changed arbitrarily during the process of coating a steel pipe with polyethylene, so the thickness of the polyethylene coating can be gradually increased, and the thick film coating can be applied for a predetermined length. After that, it is possible to apply the coating by gradually thinning it again and returning it to the original thin film coating. However, in this method, the slit spacing of the flat die is kept constant, and the thickness is changed only by the amount of tension applied to the extruded polyethylene sheet. Therefore, considering the coating workability and coating quality, the thickness There is a limit to the scope of change, and it is difficult to make too large a change. At most, a 50% increase is an appropriate range.

The second method is to greatly widen the slit spacing of the flat die and increase the amount of polyethylene extruded. This is accomplished by using a die constructed such that the upper lip 8 of the flat die 3 can be moved up and down by a hydraulic cylinder 5, as shown in FIG. The slit spacing can be set arbitrarily using the lower limit 7 and upper limit 6, and if the screw rotation speed of the extruder can be increased or decreased in conjunction with the vertical movement of the lip by the cylinder, the amount of polyethylene extruded can be increased. It is changed according to the slit interval. Therefore, while covering a single steel pipe with polyethylene,
It is possible to widen the slit interval from any point to increase the film thickness, coat a predetermined length with a thick film, and then narrow the slit interval again to return to the original thin film coating. In the case of this method, the range in which the film thickness can be changed is wide, and it is easy to change the film thickness up to about 3 times, so it is possible to sufficiently cover the practical film thickness range.

Considering the ease of coating work, equipment cost, etc. between these two methods, the best method is to change the second slit interval.

Example 1 A spiral steel pipe with an outer diameter of 508 mm, a wall thickness of 9 mm, and a length of 9 m was shot blasted to remove mill scale on the outer surface, heated to 140°C, and then mixed with a modified polyethylene adhesive and low-density polyethylene to a combined temperature of 2.5 m.
The slits are coated with a film thickness of mm, and the slit spacing is increased from 1.5m from the tip to cover a length of 7m with a film thickness of 5.5mm.From there, the slit spacing is narrowed again and the slits are coated with a film thickness of 2.5mm. did. After that, it undergoes a cooling and inspection process to become a product.

Diameter for 2.5mm and 5.5mm thick parts
When an impact test was performed using a 200 mm punch, the results were 63 Kg-m with a film thickness of 2.5 mm, and 173 Kg-m with a film thickness of 5.5 mm, indicating a significant improvement in impact resistance. Moreover, the amount of polyethylene used is smaller than when the film thickness is 5.5 mm over the entire length, making it more economical.

As mentioned above, there are three methods in the present invention, but all of them indicate a method of increasing the thickness of the plastic sheet from the middle of one steel pipe, and these are the methods for manufacturing this type of plastic-coated steel pipe. This is a particularly practical method.

[Brief explanation of drawings]

Fig. 1 and Fig. 2 are both explanatory diagrams related to the method of the present invention. FIG. 3 is a diagram showing the internal structure of a flat die. 1... steel pipe, 2... ski roll, 3... flat die, 4... polyethylene sheet, 5...
Hydraulic cylinder, 6...Upper limit, 7...Lower limit, 8...Upper lip, 9...Slit, 10...Lower lip.

Claims (1)

[Claims] 1. In the process of manufacturing a thermoplastic plastic-coated steel pipe, in which the outer surface of a steel pipe moving in the longitudinal direction is coated by spirally wrapping a semi-molten plastic extruded in a band shape from a flat die 3, the upper lip 8 of the flat die 3 is hydraulically wrapped. The flat die 3 is moved up and down by the cylinder 5.
The amount of polyethylene extruded can be adjusted by arbitrarily adjusting the interval between the slits 9, and by increasing or decreasing the screw rotation speed of the extruder in conjunction with the vertical movement of the upper lip 8 by the cylinder 5. A method for manufacturing a plastic-coated steel pipe, which is characterized by increasing or decreasing the thickness of a plastic sheet extruded from the middle of the pipe to continuously coat the pipe with different thicknesses.