JPH0228514B2 - TAIKYUSEIOYOBIFUSETSUSEINOSUGURETATENSHONREGUPURATSUTOFUOOMUKYAKUKAN - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is a tension leg platform (abbreviated as TLP).
This relates to a leg pipe (called a Tendon) that is a mooring member of a tension-tethered offshore oil and gas production facility. (Prior Art and Problems) In recent years, the pursuit of oil and gas resources has extended to the deep seabed, and the current challenge is to cope with water depths of 400 to 1000 meters. Conventional tower structure systems are considered to have technical and economic limitations in meeting this demand.
Various types of equipment have been proposed to replace towers, but TLP is seen as a promising candidate and is just beginning to be put into practical use. However, the leg pipes, which are the mooring members of TLP's underwater parts, have no similar past experience and are extremely difficult to maintain. A major issue is how to provide long-term corrosion protection, which is estimated to last for years. In general, corrosion prevention methods for marine structures include, for example, "Piping and Equipment", page 34, page 34, published in March 1981, page 6 on the right.
As shown in line 9 and Table 4 on the same page,
The main method used is to apply cathodic protection to the underwater area and a thin film coating with zinc-rich paint as an undercoat to the areas above the tidal zone.
As for the leg tube of TLP, it is used in the underwater area,
If the conventional technology were to be applied as is, cathodic protection would be applied to the entire surface. To ensure a uniform anti-corrosion potential for leg pipes that are connected over a length of 400 m or more, it is better to use a galvanic anode method, which corresponds to a distributed power source, than an external power source method that uses a centralized power source. Are suitable. However, if we calculate the amount of aluminum galvanic anodes required for corrosion protection for 30 years assuming the southern North Sea area, we will need to install 340 kg of anodes for each leg pipe with a diameter of 500 mm and a length of 12 m. .
Next, as to whether it is possible to rely completely on the coating method, the thin film systems conventionally used on the tops of offshore structures cannot be expected to last long under the sea due to concerns about scratches. Even if it is difficult, if it is a thick film type coating with high scratch resistance, it is possible based on various achievements.
It is quite possible that it will last for 30 years. However, it is said that the installation of the leg pipes needs to be done all at once, taking into account breaks in the waves, and there is no time to carry out covering work at the installation site. Most of the leg pipe can be coated in advance at the factory, but the problem is how to protect the uncoated portions from corrosion after screw connection. Even if the threaded joints are coated in advance, there is a high risk that they will be damaged by the tightening jig (tongs) used during connection work. In addition, whether the coating is fixed to the straight pipe body or extended to a threaded joint, the treatment of the ends of the coating, which are generally considered to have low durability, is also a major problem. In addition, as shown in "Ocean Age" published in December 1983, page 59, line 61 on the right to page 60, line 20 on the left, attempts have been made to prevent corrosion of the leg pipes entirely by spraying aluminum. However, it will take many years to determine the suitability of this method. As stated above, even if the conventional technology is applied as is, it will not be possible to obtain long-term corrosion protection specifications suitable for leg pipes. To summarize the above, the conditions that must be met for anti-corrosion measures for leg pipes are (a) the anti-corrosion coating material must have scratch resistance and durability appropriate to the location of the leg pipe, and (b) corrosion protection from the ends of the anti-corrosion coating (c) The anode for cathodic protection must be as small as possible; (d) The anode connection must be durable for a long time; (e) Rust prevention measures must be taken before installation work. (F) Items (B) to (F) above (6) include the ability to omit covering construction work at the installation site. (Means to Solve the Problems) In response to the above request, the present inventors conducted various studies and found that the outer surface of the straight pipe body of the leg pipe is coated with an insulating anti-corrosion coating with excellent scratch resistance, and in contact with the same. The present invention was made based on the knowledge that all of the above-mentioned problems can be solved by providing an aluminum spray coating on the outer surface of each threaded portion and also using cathodic protection. (Structure and operation of the invention) That is, the gist of the present invention is to provide a steel body having a threaded joint at both ends of a straight pipe body and having a plurality of tightening irregularities around the threaded joint. An insulating anti-corrosion coating with excellent scratch resistance is provided on the outer surface of the straight pipe body, and an aluminum spray coating is provided on the outer surface of the threaded joint so that the end of the coating is in contact with the end of the anti-corrosion coating. The tension leg platform leg tube has excellent durability and ease of installation, and features a galvanic anode for electrolytic protection attached to the periphery of the body. The present invention will be explained in detail below. 1 and 2 are conceptual diagrams showing one embodiment of the leg tube of the present invention, in which FIG. 1 is a perspective view and FIG. 2A is a front sectional view of FIG. 1. Note that FIG. 2B is a sectional view showing the a-a cross section of FIG.
FIG. C is a sectional view taken along the line bb in FIG. 2A. Further, FIG. 2D is a partially enlarged view of section c in FIG. 2A. In the figure, part 1 is the straight pipe body of the leg pipe, and part 2 is the threaded joint part. Further, 3 is a spline for tightening the threaded joint portion 2 to an adjacent leg pipe (not shown), and 4 is a rod-shaped projection provided to extend a part of the convex portion of the concave and convex portion. As shown in FIGS. 1 and 2C, a plurality of unevennesses 3 and protrusions 4 are provided at approximately equal intervals in the circumferential direction of the joint portion 2. Reference numeral 5 denotes a galvanic anode for cathodic protection, which forms a ring shape (so-called bracelet shape) after assembly, and is attached to the tubular body 1 in a manner that it is wound around it. In addition, in FIG. 2A, the core metal 7 of the anode 5 is welded to the end of the protrusion 4 via the welding part 6, and when connected in this way, the leg pipe shown in FIG. Since the stress acting on the anode 5 or its support portions 4, 6, and 7 does not act on the anode 5 or its support portions 4, 6, and 7, there is no risk that the weld portion 6 will become a breakage notch with respect to the leg tube. However, this is just an example of how to attach the anode 5, and the point is that any attachment method may be used as long as the attachment part between the anode 5 and the leg tube does not become a break in the leg tube. . In addition, in FIG. 2, the outer surface of the straight tube body portion 1 is coated with an insulating coating 8 having excellent scratch resistance, and both ends thereof are coated with an aluminum spray coating layer provided on the outer surface of the threaded joint portion 2. It is in contact with 9 in a butted manner. However, this also shows an example of contact between the coatings 8 and 9, and both coatings may be partially laminated. The structure of the leg tube in Figures 1 and 2 is as follows:
Although one end of the threaded joint 2 has a male thread and the other end has a female thread, there are cases where a threaded joint 2 is used with male threads on both ends and female threads on both ends. In this case, both ends will naturally have the same type of screw. Next, the coating layer of the leg tube of the present invention will be described. First, the insulating anti-corrosion coating 8 with excellent scratch resistance provided on the straight pipe body of the leg pipe is an electrical insulator, has sufficient anti-corrosion properties, and is resistant to impact and shaking before and after installation. A material with sufficient strength or flexibility is required. Among general-purpose electrical insulating materials, those suitable for 30-year corrosion protection include, for example, epoxy coatings with a thickness of 0.2 mm or more and polyethylene coatings with a thickness of 1 mm or more. These coatings can sufficiently follow the swinging of the leg tube, but because they lack scratch resistance against impacts, etc., it is desirable that the thickness of the polyethylene coating be 2 mm or more.
For epoxy coatings, it is desirable to use the following protective coatings in a laminated form. Examples of the protective coating include a polyurethane coating of 2 mm or more, a glass-reinforced polyester coating, a polymer cement mortar coating, a resin mortar coating, and the like. Among these, a coating consisting of a laminated epoxy thin film and a polyurethane thick film or a polyethylene thick film of 2 mm or more has excellent performance in all aspects and is most suitable. Epoxy thin films can be applied using various methods such as solvent-based paints, two-component paints, powder paints, spray coatings, flow coatings, and electrostatic coatings. Thick polyurethane films can be applied using two-component spray paints, flow coatings,
Applied by casting etc. The polyethylene thick film can also be applied by fluid dipping, spraying, or melt extrusion of a powdered resin, or by heat shrinking a heat-shrinkable sheet or sleeve. The total thickness of the coating is preferably in the range of 2 to 5 mm, considering both performance and economy. In addition, for the purpose of improving the adhesion between the coating and the steel material, a chemical conversion treatment or a primer may be appropriately applied to the steel material before coating. In addition, the aluminum spray coating layer 9 provided on the outer surface of the threaded joint is applied to prevent the parts not covered with resin from rusting until the leg pipe is installed, and to protect the ends of the resin coating during use. . A suitable thickness is approximately 100 to 300 μm, and it can be applied by a flame method or an arc method. If the threaded joint part 2 is coated with aluminum spray coating in this way,
It is stronger than a resin coating, and the coating will not be damaged even when a tightening jig (tongs) is applied to the irregularities 3 and rotated to tighten and connect when installing the leg pipe. Incidentally, the thermal spray coating may be subjected to a sealing treatment, but this is not particularly necessary. Next, the galvanic anode 5 maintains the shielding effect of the thermally sprayed coating by minimizing the elution of the aluminum thermally sprayed coating, and provides electrolytic protection for the exposed steel surface in the event of damage to the coating, and also protects the exposed steel surface from corrosion in the case of damage to the coating. This is to protect the exposed steel surface from cathodic corrosion in the event that the resin coating applied to the part is damaged. Note that it is effective to use an aluminum alloy as the anode material according to a conventional method. Since the leg pipe of the present invention has the above structure, the area ratio of the anticorrosion coating on the straight pipe body is 90%, and the area ratio of the aluminum spray coating on the threaded joint is 10% of the outer area of the leg pipe. %, and further assuming that the damage rate of the anti-corrosion coating is 5%, the only conductive surfaces are the sprayed coating surface and the damaged parts of the anti-corrosion coating, so for the 340 kg of aluminum anode required for 30 years,
The required amount of anode in the configuration of the present invention is 340×(0.1+
0.9×0.05) = 49.3 (Kg), making it possible to significantly downsize the anode. The effects of the present invention will be illustrated in more detail with reference to Examples below. (Example) Five test specimens were manufactured by welding a steel plate dome to both ends of a steel pipe with an outer diameter of 216.3 mm, a wall thickness of 8.2 mm, and a length of 3 m to prevent corrosion from occurring inside the steel pipe. After thoroughly blasting the entire outer surface with steel grit, test specimens were manufactured using the five types of anti-corrosion treatments shown in Table 1. After leaving these five types of test specimens outdoors for six months, the test specimens were observed, and then installed on a coastal barrier wall so that the entire test specimen was completely immersed in seawater, and after two years had elapsed. I pulled it up and observed changes in each part. The results are shown in Table 1. Comparative test specimens C, D, and E developed rust and pitting corrosion after being exposed to the outdoors for 6 months or to the sea for 2 years. If such pitting corrosion occurs in the leg tube of the tension leg platform, it may accelerate fatigue failure. On the other hand, it was confirmed that test specimens A and B, which are examples of the present invention, do not rust when exposed outdoors or underwater, and the amount of aluminum alloy anode wear is extremely small compared to test specimen D. Ta. In addition, the above five types of test specimens correspond to approximately 1/10 of the actual tension leg platform leg tube in terms of corrosion protection target area, and from the above results, the tension leg platform leg tube according to the present invention. It can be seen that this is suitable for long-term corrosion protection.

【table】

[Table] (Effects of the Invention) As is clear from the above examples, according to the present invention, not only is the corrosion protection of each part of the TLP leg pipe maintained before and after installation, but also the galvanic current for cathodic protection is ensured. Since the anode can be made smaller, it is advantageous in both the strength design of the leg tube and the economy, and the industrial effects are extremely significant.

[Brief explanation of drawings]

FIGS. 1 and 2 are conceptual diagrams showing one embodiment of the leg tube of the present invention. 1... Straight tube body part, 2... Threaded joint part, 3...
Tightening unevenness, 4...rod-shaped protrusion, 5...galvanic anode,
6... Welded part, 7... Core metal, 8... Insulating anti-corrosion coating, 9... Aluminum spray coating layer.

Claims (1)

[Claims] 1. In a steel body having threaded joints at both ends of the straight pipe body and having a plurality of tightening irregularities around the threaded joint, an insulating anti-corrosion coating with excellent scratch resistance is applied to the outer surface of the straight pipe body. Further, an aluminum thermal spray coating is provided on the outer surface of the threaded joint so that an end of the coating is in contact with an end of the anti-corrosion coating, and a galvanic anode for electrolytic protection is attached around the pipe body. Tension leg platform leg tube with excellent durability and ease of installation.