Analysis and countermeasure of the cause of cracking in the winter construction of expansion-type fire-resistant coatings.

01
Introduction
the field of offshore petroleum engineering has many unique features compared with land, such as the compact structure of offshore platforms and the narrow space. On the other hand, offshore oil platform oil and gas production areas, oil and gas treatment areas and storage areas are highly vulnerable to fire danger areas, related to the safety of people and property.
fires on marine platforms, mainly caused by the burning of hydrocarbons, produce hydrocarbon flames that, unlike conventional fiber flames, are extremely difficult to extinguish

. Another type of flame-spraying flame is when pressurized oil or natural gas is leaked from equipment or pipes and ejected, which is more dangerous. As the temperature of the steel increases, so does the yield stress it can withstand, and the associated curve approaches the inverted S-type. Typically, the strength of the steel decreases dramatically at around 500 degrees C (ISO 834-1). According to the design needs of different steel structures, the temperature at which the structural strength of steel failure is reached is called the core temperature. Usually bare steel can reach the core temperature at 5 to 10 min. The fire protection steel structure can greatly extend the time to reach the core temperature, thus winning time for fire extinguishing, evacuation, escape and rescue.
according to petroleum and petrochemical safety standards, steel structures in high-risk areas must be designed for fire protection. Common fire prevention methods are fire protection board and cement-based, expansion epoxy fire-resistant coatings. Fire board installation clean, beautiful decoration, but only applicable to cellulose fire, cement-based fire-resistant materials long fire resistance time, low cost, but the appearance of rough, itself bulky, can not be applied in the humid marine environment;
The principle of passive fire protection coating is to construct an efficient special fire protection coating on the surface of steel structure that requires fire protection, which can greatly reduce the propagation rate of flame on steel surface in a fire scene, and slow down the transfer of heat, delay the time of the steel structure to reach critical yield strength within a certain time range, and win sufficient time for fire rescue and personnel escape.
with the increasing importance of safety and fire prevention, more and more offshore oil platforms have adopted the protection method of coated fire protection in the core area. When selecting fire-resistant coatings, full consideration should be given to the function of marine steel structures, the status of the combustion medium present, the type of fire flame that may occur, the expected time for personnel to escape and start fire-fighting facilities. The most important thing is that the performance of the selected fire protection coating should meet the adaptability requirements of fire safety analysis. According to a large number of reports and practical engineering applications, it is considered that fireproof coatings should have the following properties:
(1) has good integrity and flame impact resistance. When exposed to flame, it can maintain the integrity of the fireproof coating itself and prevent the passing of flame and smoke; The marine environment is harsh and late-stage repair is difficult, so fire protection coatings must be able to provide stable protection for more than 20 years; In the design of the protection time, should be able to block the spread of heat, the temperature change of the unexposed surface can not exceed the specified temperature;
02 The construction of fire protection coatings in extreme cold environment
2.1 construction conditions
is different from conventional fire protection coatings construction, the project in this study is the need to serve in the extreme cold area of an oil and petrochemical project, its construction site in Qingdao Huangdao and Tianjin Tanggu two places. Construction time lasts 2 years, its winter construction temperature is low, summer construction temperature is high, the temperature range is between -10 to 39 degrees C, the construction season span is larger.
the construction of fire-resistant coatings is divided into workshop construction and site construction.
the workshop: mainly for newly built modules, but the parts assembled and welded on the modules need to be reserved. Workshop construction sand blasting and coating are automatically completed by mechanical equipment, construction conditions are better, can control the construction environment conditions, to meet the relevant specifications and standards.
on-site construction of fire-resistant coatings: mainly to repair reserved and damaged parts. Sand blasting and coating are done by hand by the workers, the site construction conditions are worse than the workshop, in the winter or rainy season need to build sheds, control the construction temperature and relative humidity.
2.2 problems
fire protection paint after winter construction, in the spring inspection process, quality inspection personnel in the fire protection paint surface found a large number of cracks, cracks distributed throughout the module, no obvious law, workshop construction and site construction of fire protection coatings can be found cracks, of which the site construction of fire paint cracks the most. Crack depth can reach the primer, in severe cases even in pieces of peeling.
03 Related Causes Investigation and Discussion
Because fire protection coatings are a guarantee of the safety of the project's later operation and an integral part of the integrity of the project. It is understood that the current world in the extreme cold areas of the application of expansion-type fire protection coating experience is very small, in the cold environment construction experience is also insufficient. A full investigation into the incident was therefore carried out. Relevant investigations include fire protection coating performance, fire protection coating construction process, steel structure force analysis and comprehensive performance.
3.1 Fireproof coating product quality
first consideration is whether there is a problem with product quality. Send the actual fireproof coatings used to a third-party laboratory for testing. For fire-resistant coatings attached to steel structures, the first consideration is whether their pressure resistance and stretch strength meet the requirements. Because in the construction, handling, lifting, welding and other processes, will be subject to a certain external force, if the strength of its own fire-resistant paint is too low, it is easy to cause cracking under the influence of these external forces. The test results show that the anti-pressure strength and stretch strength of the inflated fire-resistant coating samples obtained from the field meet the requirements of the product description.
laboratory tests, the conclusion is that the site used in the expansion of fire paint products are qualified quality. But we consider from the scene to get the test block is already peeled off or forcibly peeled off, the test block itself combined well, the performance obtained by the inspection can not be very different. Even if the adhesion of small fire-resistant coatings and other tests meet the requirements, but the lack of large-block samples related to the experiment, so can only prove that on the basis of the laboratory, the quality of fire-resistant coatings in line with product requirements. Fire paint cracking factors need to be looked for in other ways.
3.2 Fireproof coating construction procedures investigation
the construction of fireproof coatings need to go through a number of procedures, to ordinary carbon steel, for example, the first need to go through sand blasting steel structure surface. After sand blasting is completed, primer construction is required and pre-coated for welds, free edges, corners or hard-to-reach areas of spraying before full spray construction. The primer should be firmly bonded to the steel surface, adhesion reaches ISO 2409 level 0, and the thickness of the paint film is 50 to 75 m.
the quality of the last process to meet the requirements before the construction of fire-resistant coatings. The workshop environment should be controlled at air temperature of not less than 10 degrees C, relative humidity is not higher than 85%, and the surface temperature of the substrate should be at least 3 degrees C greater than the dew point temperature. The fire-resistant paint used is divided into A, B two groups, before use, the parts A and B by the quality ratio of 2.5:1 accurate mix. And at least one proportional check should be performed at the beginning of each shift with each shutdown and restart. It is advisable to spray up to 5 mm of film at a time, and the thickness of the last coating must not be less than 3 mm. After the last coating curing, but not completely cured (loss of adhesion), it is best to maintain a slight sticky state, before the construction of the next fire paint. Each coating surface does not allow for inconseconcies, pinholes, bubbles, or partial build-up of overly thick coatings.
through multiple sprays of fire-resistant paint, should also always pay attention to dust, water pollution, etc. , in the construction process to continuously control the thickness of the film, to ensure the quality of construction.
as the last protection of fire-resistant paint from pollution, ultraviolet rays and other damage, and provide color and aesthetics, after the fire-resistant paint cured, contaminated by other surfaces before the construction of a layer of paint.
the above-mentioned complex steps, the final construction completed beautiful fire protection coatings.
each process has a quality inspection and supervision of the certificate, and are under the technical guidance of the manufacturer to complete. Therefore, fire-resistant paint construction procedures to meet the specifications and paint manufacturers requirements, on-site construction in line with the construction procedures. However, due to the thick fire paint thickness, construction is divided into multiple layers of spraying, site construction, workers should avoid the control thickness is not allowed, in order to avoid the thickness does not meet the technical requirements, as far as possible spray more fire paint, resulting in the actual dry film thickness problem.
3.3 Structural Deformation Survey
The structure of the module belongs to the common square frame structure, using finite meta-software for structural stress lifting limit conditions simulation, the average deformation of the simulated single-root structure main beam is 0.08%, the maximum elongation of its corresponding rod parts 0.32%, far less than the maximum damaged elongation rate of 1.07% in the fire protection coating test experiment. According to the simulation results, the extreme stress deformation occurs in the corner position of the steel structure, the deformation and stress of the abdominal plate and the top plate are not obvious, contrary to the fact that the cracks observed at the site occur in the abdominal plate and other positions, therefore, it can be ruled out that the cracking of fireproof coating is caused by structural deformation.
3.4 Coating curing internal stress and cohesion effect
coating film thickness and construction process are in accordance with product requirements after the construction of a large area of cracking, considering that it may be fire-resistant coating materials and their construction process and steel structure construction mismatch caused.
-coated coating material of high-expansion fire-resistant coating is a multi-channel construction, with a thickness of 5 mm per coating. When the curing stress (curing and gel time) produced by the coating during the curing process exceeds the cohesive stress of the coating itself, the coating produces cracking or empty drums. This is influenced by the paint itself and the construction process factors, which require further laboratory certification.
most of the cracking parts of the site are concentrated in the stress concentration areas such as the inflection point of the steel structure, while the abdominal plate and wing of flat H-type steel rarely have large area cracks. This is due to increasing stress in the lacquer film adhesion and curing process (internal stress increases as the lacquer film continues to cure) and environmental stress (changes in ambient temperature) resulting in a higher internal stress than the internalization of the paint film. Therefore, the internal stress of the paint film is greater than the cohesion of the site cracking, which is also the main reason for the fire coating cracking.
3.5 Storage and transportation link analysis
materials inspection and inventory management is an essential key link, fire paint paint in the procurement of the market, according to the feed plan in batches, each batch of products must have the corresponding product quality assurance and product certification. The in-site materials should be equipped with acceptance reports and material sampling re-inspection reports.
this study, the storage environment of the fire-resistant coatings used requires a minimum of 5 degrees C and a maximum of 30 degrees C. According to the site warehouse storage records, fire paint is stored in the warehouse in accordance with the regulations, the relevant reports and information are complete. In the information provided by the parties, excluding the storage link, only the transport and out-of-stock inverted link may expose fireproof coatings to lower temperatures.
the lack of low temperature tolerance of expansion fire protection coatings, the current mature expansion fire protection coatings are required to avoid the erosion of low temperature in the raw material stage. Therefore, it can be considered that fire-resistant coating raw materials in the transport and reverse process by the low temperature environmental impact, resulting in a partial reduction in performance. The part of the coating is mixed and constructed on the surface of the steel structure, causing cracking due to the unearmed force under the condition of temperature change.
the tolerance of high-expansion fire protection coatings in low temperature environment needs to be further demonstrated.
04 Conclusion
with the development of offshore oil and gas more and more in-depth, the higher latitudes of oil and gas fields are gradually beginning to receive attention. In order to protect the safety of oil and gas facilities and property, better and better fire prevention measures need to be developed. At present, the international mature fire protection coating, used in the extreme cold project is very rare, the current actual project construction phase found in the fire protection paint cracking problem, then in the actual extreme cold environment operation will be more difficult to ensure the reliability of fire safety protection.
according to the analysis conclusions of this paper, it can be found that the construction of fire-resistant coatings in the material quality, fire-resistant coating thickness, construction process, construction quality, material storage and transportation, etc. there are areas that need to be improved.
this study on the cracking caused by expansion-type fire-resistant coatings in winter construction, the following suggestions are given:
(1) reasonable design of the thickness of fire-resistant coatings. Combined with the relevant fire safety analysis, comprehensive fire prevention needs, in order to meet the fire demand, to minimize the thickness of expansion-type fire protection coatings,
(2) multi-layer coated fire protection coating system, due to the system itself stress (preheating 50 degrees C after spraying curing residual stress, structural deformation stress and thermal stress caused by changes in ambient temperature) and more likely to cause cracking risk, and with the thickness of the coating increased, the risk of cracking will increase dramatically. Construction procedures can be optimized to ensure that the thickness of the coating within the scope of design requirements, strengthen thickness monitoring,
(3) procurement of fire-resistant coatings in the whole process of transportation and storage, should do a good job of protection measures to avoid exposure to low temperature environment,
(4) expansion of fire-resistant coatings in its low-temperature environment tolerance needs to be further demonstrated by the relevant manufacturers.
for more information, please refer to Coatings and Protection
11th issue
2019, original title: Research on cracking of expansion-type fire-resistant coatings during winter construction
Author / Xiong Rui, Zhang Qing, Wang Hongfu, Song Bo, Wan Mingqi
(Design Institute of Offshore Petroleum Engineering Co., Ltd., Tianjin 300451)
.