Process optimization during the construction of cold splash coating systems

Introduction:
natural gas (LNG) as an affordable, green, high safety performance of energy is more and more widely used in various fields. At the same time, because LNG is prone to leakage, volatile, flammable and explosive characteristics, once leakage, will produce low temperature, combustion explosion and other hazards, may cause catastrophic accidents. In the past LNG steel structure project, due to the environment and the volatile characteristics of LNG, the steel structure surface coating only pays attention to the hazards caused by fire, and only uses the method of applying passive fire protection coating to reduce the speed of flame propagation . The project liquefact module described in this paper is located in the extreme cold region of the Arctic, and not only should the coating system consider the flame damage to the steel structure, but also the failure of the steel structure caused by the ultra-low temperature leakage. This paper mainly aims at the cracking problem encountered by CSP system during the construction of the first liquefaify module, and verifies the same module after process optimization, which is instructive for the subsequent CSP coating thickness design and quality control.
01, CSP cracking problem description and cause analysis
1.1 CSP cracking problem description
In an LNG project, offshore oil engineering co., Ltd. Qingdao site (sea oil project) is responsible for the construction of three columns of the same core modules. After the installation of a module 114-V-013 device (291.4t) in the first column, a large number of cracks were found in the CSP paint system, as shown in Figure 1 of the module schematic and in Table 1 of the coating system. The equipment-structure contact position is located in the small mezzanine between the 3rd and 4th and 5th floors (EL-128.785), and during the inspection period from December 2015 to February 2016, the cracks continued to expand from the initial 104 (12.4 m) to 851 (69 m).
Table 1 Project Coating System
Figure 1 A module 3D diagram
crack cumulative length mainly occurs in the 3 layers, namely EL 119.000m, the length of the total 33.95 percent, the length of 23.58 m. The most severe cracking conditions are located in EL-131.700 m, part of the area CSP and substrate completely separated, coating and substrate clearance of 1.5 cm, as shown in Figure 2, Figure 3.
2 CSP cracking picture 3 CSP cracking picture
1.2 CSP cracking cause analysis
1.2.1 Construction environmental conditions and material storage conditions investigation and analysisConstruction dates are mainly concentrated in June-September, material storage environment to meet the minimum requirements of 5 degrees C, construction environment to meet the requirements of the procedure air temperature minimum 10 degrees C, relative humidity ≤85%, steel plate temperature higher than the dew point of 3 degrees C.
1.2.2 Coating curing and adhesion performance testing
through CSP field coating material sampling for appearance and physiculation performance testing and equipment parameters recording information, material curing is good, the spraying process equipment parameters and surface treatment records to query, all meet the construction requirements. The appearance and thickness of the coating meet the requirements of the procedure, the overall shedding of the coating does not appear layered shedding phenomenon, after construction through chisel experiments to confirm that the procedure adhesion requirements are met. Chisel experiments can qualitatively measure adhesion between different coatings by:
indicates a rectangle about 250mm x 250mm in size. Using a round grinding wheel, cut down the substrate along the indicated boundary line. Care should be taken to ensure that the corner position is cut through to ensure that the area is completely unbound. Using a sharp chisel no wider than 50mm, cut in the center of the patch in. In general, the adhesion of the system is considered acceptable if the 250mm x 250mm area becomes 4 or more blocks when removed. See Figure 4, Figure 5.
Figure 4 Chisel experimental location selection Figure 5 Chisel coating adhesion is good
1.2.3 steel plate surface roughness check
Steel suitable surface roughness is conducive to the improvement of paint film protection, roughness caused by large paint film thickness distribution is uneven, especially at the peak of the paint film thickness is easy to cause early corrosion; After on-site review, the roughness range is 62 to 70 m, to meet the project roughness 50 to 75 m requirements.
1.2.4 Coating thickness test
by measuring the thickness data of each deck sheet, it was found that all test points exceeded the required thickness, with a maximum thickness of 60 mm, 46.3% of the required thickness, and an average thickness of more than 17.56%.
1.2.5 crack distribution analysis shows that the
cracks are mainly concentrated in the area close to the equipment. The closer the equipment, the more severe the crack, according to the EL-131.700m cracking situation, the profile here has been deformed, and CSP can not completely change with the shape of the structure, resulting in a serious cracking and stripping state; At the same time, due to the conduction of force and the gradual release of stress within CSP, the phenomenon of crack expansion occurs.
1.2.6 Lifting Process Impact Analysis
Using SACS (Marine Analysis Computer System) software to simulate the deformation during the transport of cold splash paint area structure lifting, by analyzing the deformation value of cold splash paint area and CSP material performance comparison, it is concluded that CSP cracking is less relevant to lifting and transportation structure.
02, CSP construction process optimization and comparison of the
program optimization process: early in the project, CSP did not consider each area risk factor, the system is designed in accordance with the most stringent conditions, the construction process owner requirements "on the high not low", all construction can not be lower than the required film thickness, did not set the maximum film thickness limit, resulting in all construction thickness is higher than the procedure requirements. As the CSP thickness decreases, the extruse rate increases gradually, increasing the extrusivity by 83% to 12 mm. See table 2. System optimization from the thickness of thinning CSP materials to optimize the coating system, taking into account safety factors and material performance, CSP thickness from 26 mm thick to 15 mm, optimized CSP coating system see Table 3, and the maximum film thickness is set to no more than 1.1 times the required.
Table 2 CSP thickness and stretch growth rate
Table 3 Optimized CSP coating system
effect comparison: the optimized CSP coating system is all applied to the same structure on the third column of the same module, the equipment after installation did not appear CSP cracking phenomenon.
Conclusion
(1) CSP thickness design should take full account of the service environment and safety factors, under the premise of meeting the requirements to minimize the overall thickness of the CSP system, the construction process should be strictly in accordance with procedures and instructions for construction, the thickness of a single coating and overall thickness should be strictly controlled, to prevent the coating construction too thick so that the pressure in the coating increased, resulting in an increase in the risk of cracking;
(2) For the structural framework of large CSP system, the lifting scheme should be optimized to prevent uneven stress during lifting caused by CSP cracking,
(3) the construction process should ensure that material storage, construction environment, and steel plate surface treatment meet the specifications, in strict accordance with the procedures for construction, to ensure that the construction process meets the requirements.