Bolts for an offshore bolt-socket connection

Abstract

A bolt (1) is disclosed which can be inserted into an offshore bolt-socket connection. The bolt (1) comprises a cylindrical base element (2) made of steel and a coating (4) which is applied to at least one outer surface (6) of the base element (2) using a directed energy deposition (DED) process.

Description

Technical field of the invention The present invention relates to a bolt for an offshore bolt-socket connection according to claim 1. The present invention also relates to a method for manufacturing such a bolt according to claim 7. Background of the invention A bolt-socket connection is a type of structural connection commonly used in the marine energy industry. It is used, for example, to connect beams in floating structures (floating wind turbines), to anchor such structures to the seabed, and so on. Bolt-socket solutions are also common in tidal and wave power plants. In this type of connection, a bolt is typically inserted into a socket, with a sliding contact between the bolt and the socket that leads to wear on the respective surfaces. Marine energy devices, in particular, are designed to operate for 10-20 years without dry-dock maintenance. Therefore, it is crucial that the connections used in such devices can also withstand this lifespan. In this respect, biofouling presents one of the greatest challenges. On the one hand, biofouling is known to reduce the corrosion resistance of many materials. On the other hand, it can also accelerate the wear of moving / sliding parts, such as a bolt in a bushing. Surfaces not subject to wear can be coated with antifouling paint. For example, it is common practice to paint the underwater surfaces of ships, but this type of antifouling paint is too soft to be used on wear / sliding surfaces such as the bolt in a bolt-bushing connection. In any case, the surfaces of the bolt are subject to wear, especially in sliding contact with the bushing, and may be subject to biofouling or other surface damage such as corrosion or the like. Therefore, the objective of the present invention is to provide a bolt for an offshore bolt-socket connection that is resistant to damage caused, for example, by constant contact with water. Summary of the invention This objective is achieved by a bolt that can be inserted into an offshore bolt-socket connection. The bolt comprises a cylindrical steel base element. This cylindrical base element can have a shape corresponding to a complementary bushing into which the bolt can be inserted. To protect the outer surfaces of the base element, a coating is applied to at least one outer surface. Unlike previously used coatings, the coating used here is applied using a directed energy deposition (DED) process. DED is a surface welding process that enables a metallurgical bond between the coating material and the steel base element, thereby providing a DED-bonded surface on the steel base element. The DED process can be laser metal deposition, laser cladding, wire arc welding, thermal spraying, plasma transfer arc (PTA), electron beam melting (EBM), and / or selective laser melting (SLM). DED processes can deposit materials at very high deposition rates (up to 10 kg per hour), thus reducing the time required to manufacture the bolt. Furthermore, DED processes offer dimensional stability, resulting in tight tolerances for post-processing and therefore reduced post-processing, further improving manufacturing in terms of time and cost. In addition, the bolt can be easily adapted to different applications simply by adding a different coating. The DED process for forming the coating can be a DED process using steel wire and / or steel metal powder. In particular, it has been recognized that a high-performance coating can be applied to the surface of the steel base element using a wire- and / or powder-based DED process, where the steel used for the base element can be a cost-effective metal. This allows for the production of a bolt in a cost-effective (due to the cost-effective base element) and time-efficient manner (due to the rapid DED process). Furthermore, the coating can be applied in any desired thickness using DED, resulting in further improved performance and adaptation to different requirements. Another advantage of DED-applied coatings is that the DED process results in a smooth coating surface, particularly free of cracks or pores. Such a smooth surface offers better resistance to sediment deposits and biofouling, as it provides a less vulnerable surface. According to another embodiment, the base element comprises a low-alloy steel. Thus, the base element can be manufactured cost-effectively using inexpensive steel or low-alloy and / or low-carbon steel materials, such as structural steel, for example, S355J2, 42CrMo4, 21CrMoV5-11, 16CrMo4, 25CrMo4, etc. Since the coating is applied to the base element, it can compensate for or supplement the properties that the low-alloy steel lacks. The base element can therefore be manufactured from a comparatively inexpensive material, while the required strength and other properties (e.g., corrosion resistance) of the bolt can be provided by the coating applied to the base element. Furthermore, the coating may comprise a corrosion-resistant material and / or a wear-resistant material and / or a biofouling-resistant material. The coating may comprise at least one of nickel, nickel alloy, copper, copper alloy, bronze, bronze alloy, and stainless steel. Thus, the coating can impart properties to the bolt, such as those required for offshore applications. One of the added properties can be biofouling resistance. Biofouling resistance can be achieved by incorporating a specific proportion of copper into the coating. Copper has been shown to improve biofouling resistance. For example, a wear-, corrosion-, and / or biofouling-resistant coating can be produced by adding a specific copper content, such as up to 5 wt% Cu, to any corrosion-resistant coating used in the offshore industry, such as nickel alloy, duplex stainless steel, or super duplex stainless steel, to enhance the biofouling properties of the existing corrosion-resistant coating. Alternatively or additionally, the coating can be a multilayer coating, with some of the layers comprising different materials. For example, a first layer can be made of a corrosion-resistant material, and a second layer can be made of a copper alloy to ensure biofouling resistance. Thus, different layers with varying properties or characteristics can be applied on top of each other. The layer materials can be selected according to customer and / or application requirements. This offers increased flexibility due to the adaptability of the coating material based on the specific application. At least one outer surface of the base element can be a wear and / or sliding surface. Therefore, surfaces that are subject to wear, for example due to sliding contact, can be coated. This can increase the overall service life of the bolt, as these wear and / or sliding surfaces are better protected by the coating. Because the coating is applied using a DED process, it is more durable than, for example, painted coatings, further extending the service life of the bolt. According to another aspect, a method for manufacturing a bolt that can be used in an offshore bolt-socket connection is provided. The method comprises providing a cylindrical base element made of steel and applying a coating to at least one outer surface of the base element using a directed energy deposition (DED) process. As described above, the DED process can be laser metal deposition, laser cladding, wire arc melting, thermal spraying, or any other type of DED process. In particular, the DED process can be a steel wire and / or steel metal powder DED process. This offers an advantage, for example, when switching between different coating materials, as the steel wire and / or steel metal powder can be easily exchanged, while the same fixture or equipment can be used for the DED process without any modifications. Furthermore, the application of the coating can involve the application of more than one layer using the DED process. This can offer the advantage, among others, of providing layers with different properties, as described above. Furthermore, it has been shown that applying more than one layer, for example 2 to 20 layers, results in a high-performance surface with a satisfactory thickness for more demanding applications. This can also reduce the amount of heat transferred to the base element, which means a reduced risk of cracking in the base element. Optionally, the coating application speed by DED, i.e., the DED speed, such as the laser cladding speed, can range from 0.5 to 1000 meters per minute. According to one exemplary embodiment, the DED speed is higher than 1 meter per minute, for example, higher than 20 meters per minute, e.g., 80–120 meters per minute, which reduces the risk of cracking in the base element. It has been found that a higher DED speed, such as the laser cladding speed, can reduce the risk of cracking. Thus, for example, an improved bolt can be provided by using a higher laser cladding speed. Optionally, if the DED process is laser cladding, the laser power used when applying the coating can be 1 to 15 kW (kilowatts), for example 2 to 6 kW. Optionally, the application speed can be varied during the application of the steel wire and / or steel powder to the base element. This allows for the application of one or more layers with different radial thicknesses. For example, the base element can be rotated about a longitudinal axis during coating application, with the rotational speed being varied as the steel wire and / or steel powder is applied. Varying the speed reduces the amount of heat transferred to the base element and subsequent coating layers during the DED process. In one embodiment, the speed is varied by reducing it at least once during the application of the coating layers.This allows for a relatively high speed to be used when, for example, a first layer is applied directly to the base element, while a relatively low speed can be used when one or more additional layers are applied to the first layer. Consequently, the first layer is thinner than the one or more additional layers. This can result in less heat being transferred to the base element, thus reducing the risk of cracking during the DED process, such as laser cladding. It should be noted that the features and advantages described in relation to the bolt also apply to the manufacturing process and vice versa. Further preferred embodiments are defined in the dependent claims, as well as in the description and the figures. Elements described or shown in combination with other elements may be present alone or in combination with other elements without deviating from the scope of protection. Brief description of the drawings Preferred embodiments of the invention are described below with reference to the drawings, which are only exemplary and are not intended to limit the scope of protection. The scope of protection is defined exclusively by the accompanying claims. The figures show: Fig. 1: a cross-sectional view of a bolt that can be inserted into an offshore bolt-socket connection; and Fig. 2: a flowchart of a process for manufacturing a bolt as shown in Fig. 1. Detailed description of the invention In the following, identical or similarly functioning elements will be designated with the same reference symbols. Fig. 1 shows a cross-sectional view of a bolt 1 that can be inserted into an offshore bolt-socket connection (not shown). The bolt 1 comprises a cylindrical base element 2. This cylindrical base element 2 is made of steel, in particular a cost-effective steel. The bolt 1 further comprises a coating 4 applied to an outer surface 6 of the base element 2. In the example shown, the outer surface 6 is the shell of the cylinder of the bolt 1, which is in sliding contact with an inner surface of the socket when the bolt 1 is inserted into the socket. However, the coating 4 can also be applied to other surfaces of the base element 2 and can, in particular, cover the entire outer surface of the base element 2. To enable a time- and cost-efficient manufacturing method for the bolt 1, the coating 4 is applied using a DED process. This results in a very smooth surface of the coating 4, thereby improving the properties of the bolt 1, for example, with regard to biofouling when used in an offshore application. The smooth surface prevents sediment from accumulating on the surface of the coating 4, thus reducing corrosion of the bolt 1. Furthermore, the coating can comprise four different materials that offer resistance to corrosion, biofouling, wear, etc. For example, the coating can include copper. Thus, the base element (2) can be made of a cost-effective steel, and the coating (4) can be made of a high-performance material, resulting in a bolt (1) that can withstand use in offshore applications. The coating 4 can have more than one layer. In this case, the different layers can consist of different materials to provide different properties such as corrosion protection, strength, wear resistance, etc. It is also possible for more than one layer to consist of the same material, thus giving the different material layers, and therefore the coating 4, a desired or required thickness. Fig. 2 shows a flowchart of a process for manufacturing the bolt 1 from Fig. 1. In a first step S1, the basic element 2 is provided. The basic element 2 is made of steel, specifically a cost-effective steel. In a second step S2, one or more layers of coating 4 are applied or deposited using a DED process. During this DED process, for example laser cladding, steel wire and / or steel metal powder is fused layer by layer to form the coating 4. This step S2 can be carried out until a desired thickness of coating 4 is achieved. By exchanging the steel wire and / or steel powder used in the DED process, four different characteristics or properties can be imparted to the various layers of the coating. This can be done using the same machine or device; only the material used is changed. In summary, the bolt described here, for an offshore bolt-socket connection, and the corresponding manufacturing process provide a bolt that has been improved for demanding applications such as offshore use. The bolt's coating can be easily adapted to various applications and offers a more robust and stable coating compared to previously used coatings. In particular, the coating offers improved biofouling resistance. Reference symbol list 1 Bolt 2 Base element 4 Coating 6 Outer surface S1-S2 Process steps

Claims

Bolt (1) that can be inserted into an offshore bolt-socket connection, wherein the bolt (1) comprises: a cylindrical base element (2) made of steel and a coating (4) applied to at least one outer surface (6) of the base element (2) using a directed energy deposition (DED) process. Bolt according to claim 1, wherein the base element (2) comprises a low-alloy steel. Bolts according to one of the preceding claims, wherein the coating (4) comprises a corrosion-resistant material and / or a wear-resistant material and / or a biofouling-resistant material. Bolts according to claim 3, wherein the coating (4) comprises at least one of nickel, nickel alloy, copper, copper alloy, bronze, bronze alloy and stainless steel. Bolts according to one of the preceding claims, wherein the coating (4) is a multi-layer coating, wherein some of the layers of the multi-layer coating comprise different materials. Bolt according to one of the preceding claims, wherein the at least one outer surface (6) of the base element (2) is a wear and / or sliding surface. Method for producing a bolt (1) that can be inserted into an offshore bolt-socket connection, the method comprising: providing (S1) a cylindrical base element (2) made of steel and applying (S2) a coating (4) to at least one outer surface (6) of the base element (2) using a directed energy deposition (DED) process. Method according to claim 7, wherein the DED process is laser metal deposition, laser cladding, wire arc welding or thermal spraying. Method according to claim 8, wherein the DED process is a steel wire and / or steel metal powder DED process. Method according to any one of claims 7 to 9, wherein the application (S2) of the coating (4) comprises the application of more than one layer using the DED process.

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