Release time：2020-01-16 18:44 Browse：
First, the application of lengthening screws in the Marine environment
(1) Fields of offshore oil and natural gas exploitation
Fixed offshore platform structure
Offshore oil platform is a large and complex structure, including jacket, deck, module and other parts. Extension screws play a key role in connecting the jacket leg to the subsea foundation. The legs usually need to go tens of meters or more into the sea floor to ensure the stability of the platform, which requires screws long enough to penetrate the multi-layer structure and achieve a solid connection. For example, in the construction of some large offshore oil platforms in the South China Sea, the length of the high-strength extension screws used can reach several meters, and the pile legs are firmly fixed on the seabed, bearing the huge weight of the platform and the Marine environmental forces such as wind waves and ocean currents.
In the superstructure parts of the platform, such as the connections between modules, extension screws are also used to secure various devices and support structures. Like the connection between the crane base and the drilling equipment bracket and the platform deck, the use of lengthening screws can ensure the strength and stability of the connection and prevent the equipment from loosening due to vibration or external force during the mining operation.
The submarine pipeline is connected and fixed
Subsea oil and gas pipelines usually need to be laid in complex Marine environments. Extension screws are used to connect the flanges of the pipes to ensure tightness and connection strength between the pipes. Since submarine pipelines may be affected by seabed settlement, seismic activity, seawater erosion and other factors, it is necessary to have sufficient length of screws to increase the reliability of the connection. Moreover, in the anchorage of the support structure of some pipelines and the seabed, extension screws are also necessary, they will fix the support pile on the seabed, provide stable support for the pipeline, and prevent the pipeline from deformation or displacement due to its own weight and water impact.
(2) Marine renewable energy
Offshore wind farm construction
The basic structure of offshore wind turbine usually adopts the form of single pile foundation, jacket foundation or gravity foundation. In the installation process of these foundations, extension screws are widely used to connect the foundation to the tower barrel. For example, a single pile is a large steel pipe pile driven into the sea floor, and then the tower barrel is secured to the top of the pile by extension screws. Since the height of offshore wind turbines can reach tens or even hundreds of meters, the wind, wave and current forces are huge, and long screws are needed to ensure a firm connection between the tower and the foundation to prevent the tower from tilting or collapsing under extreme weather conditions.
In the internal structure of the wind turbine, such as the connection of the gearbox, the generator and the tower cylinder, the extension screw will also be used. The role of these screws is to accurately fix the position of the equipment, and at the same time withstand the vibration and torque generated during the operation of the equipment to ensure the stable operation of the wind turbine.
Installation of tidal and wave power generation equipment
Tidal power generation equipment is usually installed in straits, bays and other places with a large tidal range, while wave power generation equipment is deployed in the sea area where waves are more frequent and strong. During the installation of these devices, extension screws are used to connect the support structure of the power generation unit to the submarine foundation or floating body. For example, the fixed connection between the support pile of a tidal energy turbine and the seabed, or the connection between the floating body of a wave power generation device and the mooring system, requires the use of extended screws to resist the periodic movement of the sea water and the strong current impact force, ensuring that the equipment can generate electricity normally in the harsh Marine environment.
(3) Port and terminal construction
Dock structural connection
The main structure of the wharf includes dock panel, beam, longitudinal beam and pile foundation. Extension screws play a crucial role in connecting these structural components. In the construction of high pile wharf, the pile foundation needs to be connected with the beam by lengthening screws, so that the vertical support structure and the horizontal bearing structure of the wharf can be firmly combined together. For gravity docks, extension screws can be used to fix ancillary facilities such as dock fender and mooring post, which bear huge impact during the docking and mooring of the ship and require a strong connection, and extension screws can meet this requirement.
Port loading and unloading equipment installation
The installation of loading and unloading equipment such as cranes and conveyor belts in ports cannot be separated from lengthening screws. The base of the crane needs to be connected to the dock foundation by screws long enough to ensure the stability of the crane when lifting heavy objects. The installation of the conveyor belt bracket also requires the use of lengthening screws to fix the bracket on the dock structure to prevent the bracket from loosening due to vibration and impact during the transportation of goods, affecting the loading and unloading efficiency of the port.
Second, special anti-corrosion requirements for extension screws in Marine engineering
(1) Characteristics of Marine corrosive environment
The chemical corrosion of seawater
Sea water contains a lot of salt, mainly sodium chloride, but also magnesium chloride, magnesium sulfate and other salts. These electrolytes make seawater highly conductive, forming a natural electrolyte solution environment. When metal screws are immersed in seawater, electrochemical corrosion occurs. For example, in the anode region, metal iron (if the screw is made of iron-based alloy) will lose electrons to become ferrous ions into the sea water, the reaction is, and then ferrous ions will be further oxidized into rust. This corrosion process is accelerated in the presence of oxygen, as oxygen accepts electrons in the cathode region, and the resulting hydroxide ions combine with ferrous ions to form ferrous hydroxide, which is then oxidized to ferric hydroxide and eventually to form rust.
The dissolved oxygen content in seawater is also an important factor affecting the corrosion rate. In general, the dissolved oxygen content of surface seawater is high, and the dissolved oxygen content gradually decreases as the depth increases. In different sea areas, the dissolved oxygen content is also different, such as in some eutrophication sea areas, due to the photosynthesis and respiration of organisms such as algae, the dissolved oxygen content will change greatly, which also has a significant impact on the corrosion rate of screws.
Marine biological corrosion
Marine biological fouling is a common phenomenon in Marine environment. Many Marine organisms, such as barnacles, mussels, algae, etc., will attach to the screw surface. The attachment of these organisms changes the physical and chemical properties of the screw surface. On the one hand, the biological attachment will form an uneven covering layer, resulting in an increase in the local oxygen concentration difference on the screw surface, resulting in local corrosion, which is often said to be gap corrosion. For example, after the barnacle is attached to the screw surface, a narrow gap will be formed between the bottom and the screw surface, in this gap, due to the limited diffusion of oxygen, an anoxic anode zone and an oxygen-rich cathode zone are formed, accelerating the corrosion process.
On the other hand, some Marine microorganisms secrete acidic substances or metabolites that directly erode the metal matrix of the screws. For example, in the absence of oxygen, some sulfate-reducing bacteria will use the sulfate in seawater as an electron acceptor to oxidize organic matter and produce hydrogen sulfide. Hydrogen sulfide reacts with metal to produce sulfide, sulfide conductivity is worse than the metal itself, will lead to local current density increases, causing local corrosion, and the presence of sulfide will also destroy the metal surface protective film, further aggravate the corrosion.
(2) Anti-corrosion coating requirements
The coating is resistant to seawater corrosion
The lengthening screw coating in Marine engineering needs to have excellent seawater immersion resistance. Commonly used anti-corrosion coatings such as epoxy coating, which has good chemical resistance and adhesion. The epoxy group in the epoxy coating reacts with the curing agent to form a three-dimensional network structure, which can effectively block the penetration of seawater. Immersion tests in seawater have shown that high-quality epoxy coatings can prevent seawater from coming into contact with the screw metal matrix for a long time (up to several years), thus slowing down the corrosion rate.
In addition to epoxy coatings, there are some special coatings, such as polyurea coatings. Polyurea coating has the characteristics of fast curing, high elasticity and excellent weather resistance, which can adapt to the deformation of screws due to temperature changes and stress effects in the Marine environment, and effectively resist the erosion of seawater. For some screws in the tidal range, the resistance of polyurea coating to alternating wet and dry sea water corrosion is particularly important, because in the tidal range, the screws will experience many times a day seawater immersion and exposure to the air process, the coating needs to withstand this frequent environmental changes.
Resistance of coatings to biological adhesion
In order to prevent fouling by Marine organisms, the coating needs to be resistant to biological attachment. A common method is to add an antifouling agent to the coating. For example, when a copper-based antifouling agent is added to the silicone coating, copper ions will be slowly released into the seawater, which has an inhibitory effect on the attachment and growth of Marine organisms. New types of self-cleaning coatings are also being developed, with surfaces that have special microstructure and chemical properties that make it difficult for Marine organisms to attach. For example, the surface of some nanocoatings has super-hydrophobic or super-hydrophilic properties, and when seawater and organisms are near, the special structure of the surface makes it difficult for organisms to attach, thus reducing the risk of biological corrosion.
(3) Material selection and alloying requirements
Corrosion resistant alloy material
In some Marine engineering applications with high corrosion requirements, corrosion resistant alloys are chosen as the material for lengthening screws. For example, 316L stainless steel in stainless steel contains molybdenum, and the addition of molybdenum can improve the corrosion resistance of stainless steel in chloride ion environment. In seawater, the resistance to pitting and crevices corrosion of 316L stainless steel screws is significantly better than that of ordinary stainless steel screws. In addition, nickel-based alloys such as Inconel 625 are also a commonly used material, which contains a large amount of nickel, chromium and molybdenum elements, with excellent seawater corrosion resistance, high temperature strength and fatigue resistance, suitable for high temperature, high pressure and high corrosion parts in the Marine environment, such as screws in deep sea oil mining equipment.
Alloying treatment
For ordinary screw materials, the corrosion resistance can be improved by alloying treatment. For example, adding a small amount of chromium to carbon steel can form a dense chromium oxide protective film on the screw surface. When the screw is exposed to seawater, this protective film can prevent further erosion of the seawater. The degree of alloying and the proportion of elements need to be determined according to the specific Marine environment and application requirements. For example, in shallow Marine environments, a lower chromium content may be sufficient to meet the corrosion protection requirements, while in deep sea or high temperature, high salinity sea areas, a higher chromium content or the addition of other elements to enhance the corrosion protection performance.
(4) cathodic protection measures
Sacrificial anode protection
Sacrificial anode protection is a cathodic protection method commonly used in Marine engineering. For lengthening screws, sacrificial anode materials, such as zinc alloy or aluminum alloy anodes, can be installed around them or on the attached structure. In the seawater environment, the potential of the sacrificial anode is more negative than that of the metal substrate of the screw, so the oxidation reaction will preferentially occur. For example, the reaction of zinc alloy anodes in seawater is that in this way, the sacrificial anodes constantly dissolve, providing electrons to the screws, leaving them in a protected cathode state, thus preventing them from corroding. The size, shape and installation position of the sacrificial anode need to be designed according to the size, number of screws and the corrosion strength of the Marine environment to ensure effective protection.
In practical applications, the consumption of the sacrificial anode needs to be checked regularly, and when the anode is consumed to a certain extent, it needs to be replaced in time, otherwise the screw will lose protection and face the risk of corrosion. For example, in the regular maintenance of some offshore oil platforms, the sacrificial anodes near the structural connection screws of the platform are checked to determine whether they need to be replaced based on their remaining volume and weight.
Impressed current cathodic protection
Impressed current cathodic protection is to apply a direct current through an external power supply to the structure where the screw is located, so that the screw becomes the cathode. In Marine engineering, auxiliary anodes, such as platinum-niobium anodes or mixed metal oxide anodes, are usually installed on protected structures, including those connected with extended screws. The power supply releases electrons to the sea water through the anode, and the electrons are transferred to the screw surface through the sea water, so that the potential of the screw surface is reduced, thereby inhibiting its corrosion reaction.
The advantage of impressed current cathodic protection is that the size and direction of protection current can be flexibly adjusted according to different Marine environments and corrosion conditions. However, it also requires more sophisticated monitoring and control systems to ensure protective effectiveness. For example, it is necessary to monitor the potential and protection current of the screw in real time to prevent the occurrence of over-protection or under-protection phenomenon, over-protection may lead to hydrogen evolution reaction, so that the screw surface hydrogen embrittlement phenomenon, and under-protection can not effectively prevent screw corrosion.
# Lengthening screws # Lengthening bolts # Lengthening outer hex screws # Lengthening inner hex screws # fasteners
