- Author: Peng Qinghua, Man Xinbao, Wu Yewei, et al.
(Ocean Petrochemical Engineering Co., Ltd., Tianjin 300461)
0. Introduction
Subsea pipelines connect offshore oil and gas fields, storage facilities, or onshore processing terminals into an integrated system, enabling coordinated and interconnected operations throughout the offshore production facility via pipelines. Leaks in offshore oil and gas pipelines are often caused by corrosion, which typically falls into two categories: internal corrosion and external corrosion. In China, internal corrosion protection primarily involves the use of corrosion inhibitors and stainless steel, while epoxy-coated pipelines are mainly used abroad. Extensive experiments have shown that carbon steel is highly susceptible to localized corrosion in chloride- and CO2-rich environments, and that adding an inner coating can significantly reduce the corrosion rate. For example, epoxy-bonded powder (FBE) coated pipelines have been widely used in the Middle East to prevent internal corrosion, extend the service life of pipelines, and reduce the internal resistance to fluid flow. Compared to composite metal pipes, pipelines with FBE inner coatings have a lower manufacturing cost. Pipelines with inner coatings also require patching of internal joints, which is often performed on laying vessels to ensure the continuity of the anti-corrosion and drag-reducing effects of the inner coating.
Liquid epoxy automated spraying is a high-tech corrosion protection process for offshore node interiors. Currently, there are no application precedents in domestic pipeline projects, and only a few international companies possess mature automated spraying equipment and offshore construction capabilities. This article, based on the construction situation of a specific offshore node project abroad, introduces the construction and equipment of liquid epoxy automated spraying, along with conclusions and recommendations.
1、Introduction to LE Coating Materials
In the underwater pipeline laying project, the oil and gas mixed pipeline, which is mainly composed of crude oil, has a medium-acidic medium (H2S pressure is 102 Psi, CO2 pressure is 78 Psi, and chloride content is 14,095 mg/L), with a maximum operating temperature of 71.1 °C and a maximum design temperature of 82.2 °C. According to the project design requirements, the pipe coating is FBE, and the coating inside the node is liquid epoxy. The liquid epoxy (LE) material is SP-9888, which has good compatibility with the FBE coating on the inner surface of the underwater pipe in the project, and is also suitable for automated spraying. SP-9888 is manufactured using a zero-VOC phenolic resin technology. After curing, this material forms a highly cross-linked coating with excellent chemical resistance, solvent resistance, water resistance, 100% solid content, zero VOC, high abrasion resistance, allowing for thicker coating structures, single-layer coating, excellent impact resistance, and good flowability (see Figure 1).
Figure 1: Liquid epoxy coating inside the pipe node.
2、Application of LE Coating
To ensure that construction processes, materials, and efficiency parameters meet project requirements, according to the project owner's specifications, Procedure Qualification Trials (PQT) and Pre-Production Trials (PPT) must be conducted according to the specified procedures before commencing the installation of the coating. These trials are to verify that the coating process, equipment, materials, and personnel's operational skills meet the required standards. The main construction steps for LE include surface preparation, joint preparation, post-welding visual inspection, surface sandblasting cleaning, and coating and inspection of the LE.
2.1 Surface Treatment
The primary surface treatment method is sandblasting, which has similar process characteristics to external anti-corrosion sandblasting at joints. During sandblasting, the sandblasting equipment rotates at a specified speed, driven by the rotating device, while sandblasting personnel use the sandblasting nozzle to apply the abrasive. See Figure 2.
Figure 2: Sandblasting
Clean the steel surface according to SSPC-SP1 standards to remove contaminants and grease. If necessary, rinse with clean water to reduce salt content on the steel surface. Sandblasting should achieve SSPC-SP10 level, with roughness and cleanliness meeting the requirements specified in the project specifications. The roughness requirement for this project is 50-125 μm, and the cleanliness should meet at least ISO 8502-3 Level 2. Salt content detection results according to ISO8502-6 and ISO8502-9 methods should be ≤2 μg/cm.2For joint areas, use grinding equipment or similar equipment to remove the coating on the ends of the approximately 6.5 mm long pipe, with the purpose of creating a transition bevel while also removing damaged coatings and moisture that has penetrated between the coating and the substrate. Subsequently, the joint area is roughened using sandblasting at an appropriate pressure, with a joint width of 25-50 mm.
During storage or transportation on land, the pipes may come into contact with dust, sand, or other contaminants. During marine construction, some equipment (such as beveling tools, internal-to-external tools, etc.) may operate inside the pipes and come into contact with the inner surface, or the relative humidity inside the pipe may exceed 85%, all of which are detrimental to controlling the quality of the surface to be coated. Therefore, it is necessary to take measures to ensure the cleanliness of the pipe and to prevent any equipment in contact with the inner surface of the pipe from scratching or damaging the surface and the FBE coating inside the pipe. The main measures include: ① Generally, the pipe is cleaned with fresh water, and after cleaning, the pipe cap is used to seal the pipe opening to prevent contamination of the inner surface. During marine construction, if it is necessary to further clean the pipe, a cleaning ball can be used to clean the entire interior of the pipe. The cleaning ball support is cylindrical and made of metal. The diameter of the support is slightly smaller than the inner diameter of the pipe, and the weight of the support needs to be appropriately controlled to avoid affecting the cleaning effect. The surface of the cleaning ball should be covered with a towel or similar material; ② Sandblasting should be performed inside the pipe end. A pipe cap should be used to prevent abrasive particles from entering the pipe and making it difficult to clean. This helps maintain a clean environment inside the pipe and protects the coating on the pipe body from damage, and improves the efficiency of sandblasting in the cutback area; ③ All equipment in contact with the inner surface of the pipe should be protected to prevent damage to the coating or the surface of the substrate after treatment. Sandblasting should be performed after the beveling is completed to prevent metal debris from scratching the treated surface; ④ If the relative humidity is >85%, a controllable relative humidity environment should be created. Generally, a temporary workshop is built at the end of the pipe, and a dehumidifier or industrial air conditioner is used to maintain the relative humidity in the workshop <85%; ⑤ Use a marker to mark the nodes inside the sample to ensure the traceability of the coating quality within the nodes. The marker should be a contrasting color, free of oil and chlorides.
2.2 Inspection of the inner surface of welded joints
After pipe end sandblasting, the surface to be coated requires welding, demagnetization, welding preheating, beveling, and bottom welding before entering the coating stage. The beveling and welding processes have the greatest impact and highest frequency on the quality of the coated surface. Before welding, the bevel must be preheated, and the lubricating grease inside the welding vessel may liquefy and drip onto the surface to be coated. Additionally, welding may also produce welding defects such as poor fusion, excessive weld reinforcement, and difficult-to-remove weld spatter, as well as sharp surfaces. To ensure the quality of the coated surface, a visual inspection (using a borescope, see Figure 3) must be performed on the coated surface after bottom welding. This inspection work is carried out using specialized inspection equipment (which is typically located after the welding vessel and mechanically connected to the borescope, and synchronized with the borescope) to ensure the sprayability of the entire coated area. In general, if the visual weld bead height is greater than 1.5 mm, the weld spatter is coarse, there are sharp surfaces, or the surface is oily, direct re-welding is required, unless the surface can be cleaned and re-machined (such as manually cleaning inside the pipe or using external machining equipment after the borescope).
Figure 3: Visual inspection of the weld interior
2.3 Surface Blast Cleaning Before Spraying
Computer-controlled internal corrosion prevention vehicles: First, the centrifugal abrasive disc is positioned in the weld area, and then the abrasive cleaning and vacuum recovery program is started. The program parameters controlled include abrasive speed, abrasive time, and the stroke width of the actuator. These parameters have been evaluated through PQT and PPT. The abrasive cleaning process first rotates in a clockwise direction to clean, followed by a counter-clockwise rotation. This cleaning process is primarily used to remove surface rust, debris, dust, and fine particles. It cannot effectively remove larger welding splatters or oil stains. Finally, the 360° rotating camera on the internal corrosion prevention vehicle is used to confirm whether dust and welding fumes have been removed.
2.4 Liquid Epoxy Automated Spraying
According to the manufacturer's recommendations in SP-9888, the surface temperature after spraying should be maintained between 15~60 °C (the substrate surface temperature should not be too high, otherwise the subsequent spraying time will be too short, and the touch-up painting operation will be rushed). The spraying process parameters (spraying disc rotation speed, coating supply flow, spraying return stroke width, spraying time, etc.) are finally determined through PQT or PPT tests and directly controlled by a computer. During construction, normal pipe laying should be the main focus. This ensures stable pipe laying efficiency, and welding, weld inspection, and node coating on the production line can be carried out efficiently. For internal corrosion protection of nodes, the spraying efficiency should be adjusted to match the pipe laying efficiency, especially when sea conditions are poor or special circumstances require a slower laying speed.
The spraying area width achievable by the high-speed spraying disc, through forward and backward movement, is approximately 200 mm (based on a cutback of 60 mm and not exceeding the roughened joint area). Considering that localized repairs may be necessary after spraying, the normal spraying thickness is designed to be 500~1,000 μm, with a certain thickness allowance (typically 500 μm). The maximum coating thickness is 1,500 μm. The spraying thickness is controlled using computer software.
If localized spraying is insufficient, the operator can re-spray the area by adjusting the spraying vehicle position or re-spray the entire joint. However, it is important to note that re-spraying should be performed within the maximum re-spraying interval time, which is determined by the properties of the material itself. The material manufacturer typically provides recommended values. For example, when the coating environment temperature is 30 °C, the maximum re-spraying time interval for SP-9888 liquid epoxy material is 2.4 hours. When the environment temperature is 60 °C, the maximum re-spraying time interval is reduced to 25 minutes.
2.5 Final Inspection
By utilizing a 360-degree rotating camera on the internal corrosion protection vehicle, spray application personnel and corrosion control QC personnel can visually inspect the sprayed surface, verifying that cutback areas, weld areas, and overlaps (at least 25mm) are completely covered, and that there are no visible pinhole defects. The spray application area should not extend beyond the roughened overlap area. If the spray application area extends slightly beyond the roughened overlap area or if there are isolated spray points on the unroughened pipe coating, this is acceptable provided it does not compromise the overall integrity of the internal corrosion protection coating at the joint. Additionally, the marked joint numbers should be checked and the video and photo documentation should be archived and submitted to the client. The visual inspection results must be accepted and recorded in the daily work report by both the spray application personnel and the corrosion control QC personnel.
3、Equipment for applying LE coating
The LE automated spraying process requires sandblasting, post-welding visual inspection, and spraying operations. The post-welding visual inspection equipment has a relatively simple structure and mainly introduces the sandblasting system and specially designed automated spraying equipment.
3.1 Sandblasting Equipment and Welding Seam Inspection Equipment
This primarily includes 2 air compressors (one for backup), 1 air drying equipment set, 1 air tank, 2 gas preparation systems (for adjusting the final blasting gas pressure), 2 blasting recovery systems (for blasting and sand particle recovery, and for blasting nozzles and pipelines), 1 portable blasting machine, which is simple in structure and has a low blasting gas pressure, suitable only for light blasting, mainly used for cleaning dust and removing newly formed rust, typically used for internal blasting after weld seam cutting for rust removal; 1 rotary system, including 2 rotating rollers, 1 hydraulic power station, and 1 control device (as shown in Figure 4). Due to the high efficiency requirements for pipeline laying, blasting systems frequently use rotary systems to improve blasting efficiency. The appearance inspection equipment used in this project is relatively simple, consisting only of a borescope (as shown in Figure 5). Overall, the pipe end blasting systems are standard equipment with good blasting effects, but they occupy a large space.
Figure 4: Pipe Installation Equipment
3.2 Automated Spraying Cart
The internal application of putty powder is performed within the pipe. A fully automated spraying device with a diameter suitable for the pipe is selected. The spraying vehicle integrates multiple functional modules, as shown in Figure 5. These include:- A locomotion unit to provide mobility for the internal corrosion control vehicle.- A battery pack to supply power for the crawling, cleaning, and spraying functions.- An abrasive blasting cleaning mechanism to remove dust, welding spatter, and other debris attached to the surface to be sprayed.- A vacuum recovery unit to collect the abrasive particles.- A spraying unit to perform spraying according to the programmed procedure.- An inspection system, consisting of a camera, primarily used for locating the internal corrosion control vehicle, surface inspection before and after spraying, and node numbering.
Figure 5: Root weld inspection equipment
The internal corrosion prevention vehicles require periodic battery replacement or charging, and abrasive and liquid epoxy materials also need to be replenished as needed. Taking a 36-inch internal corrosion prevention device as an example, the batteries need to be replaced or recharged every 12 hours or every 50 nodes. After each internal corrosion prevention vehicle exits the port, a calibration check and a complete spraying process should be performed, and the total volume of sprayed material and the material mixing ratio (epoxy material should be calculated by volume) should be recorded to ensure and verify the stability and accuracy of the device operation. Additionally, after the calibration check, it is necessary to confirm that the liquid epoxy and hardener are not exceeding the maximum recommended temperature specified by the manufacturer. Using a temperature probe within the calibration validity period, the actual temperature of each material should be measured. The maximum temperature for liquid epoxy is 65 °C, and the maximum temperature for the hardener is 45 °C. All test information should be recorded in the internal corrosion prevention vehicle exit report. Furthermore, the condition of the steel abrasive should be checked. When charging or replacing the batteries of the internal corrosion prevention vehicle, the condition of the steel abrasive should be checked, primarily to see if the abrasive specifications have significantly decreased, and whether the abrasive has been contaminated by water or oily substances. If any abnormalities are found, the abrasive should be replaced. If the control signal is lost or the device malfunctions, preventing the internal corrosion prevention vehicle from operating normally, the emergency locking function should be activated to wait for the rescue vehicle.
3.3 Control System
The control system primarily comprises: 1 host unit, 1 set of control software capable of displaying simulation animations, multiple display screens, and 1 control system for an external inspection device. It also includes 1 intercom (which can communicate with personnel in the internal pre-shot blasting workstation and the internal corrosion repair station, such as conveying operational instructions or confirming operational information). All these devices are concentrated within a shipping container, as shown in Figure 6. An experienced supervisor manages all construction actions.
Figure 6: Internal anti-corrosion equipment control system
The main functions of the control system include: ① Monitoring the work situation in various areas, including abrasive blasting work in the deck area, maintenance and functional testing of equipment at workstation 0-1, etc.; ② Issuing control signals to control the appearance inspection of welds and the movement of the spray painting vehicle (moving, abrasive blasting, sand removal, spraying, and post-spraying appearance inspection).
4、Conclusions and Recommendations
4.1 Conclusion
This project is the first international pipeline project undertaken by our company that incorporates internal corrosion protection construction. This project has accumulated rich construction management experience in applying internal corrosion protection technology in domestic offshore pipeline projects. However, it also revealed the existing shortcomings of internal corrosion protection construction technology in international projects: ① International projects generally do not accelerate the curing of internal corrosion protection coatings and conduct thickness and leak detection, relying primarily on strict process control to ensure coating quality. Coating quality assessment is mainly performed by experienced spray operators and corrosion QC personnel through visual inspection using endoscopes on the spray equipment. Although this coating quality control method has been recognized by Saudi Aramco and other international clients, it has certain subjectivity; ② Internal corrosion protection equipment is relatively complex, and the stability and control signal stability of the equipment during construction must be strictly controlled. In addition, internal corrosion protection equipment must be tested before and after each spraying, which can affect construction efficiency.
4.2 Recommendations
- only a few domestic companies currently possess automated spray equipment, but they lack extensive validation data for spray quality stability under marine construction conditions. To enable the widespread application of this technology in China, it is necessary to explore scientifically sound and reliable measures for ensuring the quality of liquid epoxy coatings or to develop new coating quality detection methods, and to conduct thorough validation. The following process control measures are recommended:① Surface preparation of pipe ends: The use of abrasive blasting equipment occupies a significant amount of space on the ship, which can increase the difficulty of equipment installation. It is recommended to use or develop smaller abrasive blasting equipment;② After welding, it is recommended to use a combination of endoscopes and laser scanning to obtain more precise surface information and to accurately assess the sprayability of the surface;③ For surfaces that are not suitable for spraying, it is recommended to use grinding equipment to avoid cutting and significantly affecting the laying efficiency;④ After abrasive blasting is completed at the pipe end, it is recommended to strengthen the cleaning function of internal corrosion protection equipment to better ensure the roughness of the weld area;⑤ It is recommended to use high-quality welding processes such as Tungsten Inert Gas (TIG) welding or STT (Surface Tension Transition) automatic welding or CMT (Cold Metal Transition) to ensure the smoothness of the internal surface after welding.
References (omitted)
Article published in "Coatings and Protective Materials Magazine"