​Construction of Liquid Epoxy Coating Inside Subsea Pipeline Nodes

0 Preface

Subsea pipelines connect offshore oil and gas fields, oil storage facilities or onshore processing terminals into an organic whole, so that each link of offshore production facilities can form an interrelated and coordinated production operating system through pipelines

Liquid epoxy automatic spraying is a high-tech internal anti-corrosion process for subsea pipeline nodes.

1 Introduction to LE coating materials

In the submarine pipeline laying project, the oil and gas mixed transportation pipeline is mainly crude oil, and the medium is moderately acidic (the partial pressure of H2S is 102 Psi, the partial pressure of CO2 is 78 Psi, and the chloride ion content is 14 095 mg/L).

Fig.

2 Construction of LE coating

In order to ensure that parameters such as construction procedures, materials and construction efficiency meet the needs of the project, according to the requirements of the project owner, the procedures of Procedure Qualification Trial (PQT, Procedure Qualification Trial) and Pre-Production Trial (PPT, Pre-Production Trial) must be followed before the laying of subsea pipelines.

2.

The main method of surface treatment is sandblasting, and its technological characteristics are basically the same as those of external anti-corrosion sandblasting of nodes

Figure 2 Sandblasting

Remove contaminants and grease from the steel surface in accordance with SSPC-SP1, and if necessary flush with fresh water to reduce the salt on the steel surface
.
The sandblasting treatment reaches SSPC-SP10 level, the roughness, cleanliness and salinity meet the requirements of the project specification, the roughness of this project is 50~125 μm, and the cleanliness at least reaches ISO 8502-3 level 2, according to ISO8502-6 and ISO8502- 9 Method for salinity detection results ≤ 2 μg/cm2
.
For the treatment of the overlap area, the coating of the end of the pipe body of about 6.
5 mm long is removed by polishing equipment or similar equipment, the purpose is to machine the transition slope, and at the same time remove the damaged coating and the moisture infiltrated between the coating and the substrate, Then, the overlapping area is roughened by sandblasting method with appropriate air pressure, and the overlapping width is 25~50 mm
.

In the process of land storage or transportation, some dust, sand or other pollutants will enter the subsea pipe; during the offshore construction, some equipment (such as grooves, inner mouthpieces, etc.
) will work inside the subsea pipe and come into contact with the inner surface of the subsea pipe or The relative humidity of the air exceeds 85%, which is not conducive to controlling the quality of the surface to be sprayed
.
Therefore, it is necessary to take measures to ensure the cleanliness of the pipe, and to ensure that all equipment in contact with the inner surface of the subsea pipe will not scratch or damage the surface and the FBE coating inside the pipe body.
The main measures include: ① Generally, the subsea pipe is cleaned with fresh water, and the cleaning is completed.
Then seal the mouth of the pipe with a pipe cap to prevent the inner surface of the subsea pipe from being polluted.
During the offshore construction, if it is found that it is necessary to clean up the inside of the pipe, generally, the inside of the subsea pipe can be cleaned with a pigging ball.
The pigging ball bracket is cylindrical and is Metal material, the diameter of the bracket is slightly smaller than the inner diameter of the sea pipe, and the weight of the bracket needs to be properly controlled, otherwise the cleaning effect will be affected.
It is not easy to clean up when entering the pipe, maintain a clean environment in the pipe, protect the coating of the pipe body from damage, and improve the sandblasting efficiency of the cutback (the reserved length of the metal surface of the pipe end) area; ③ All equipment in contact with the inner surface of the sea pipe must The surface or point is protected to prevent damage to the coating of the pipe body or damage to the surface of the treated substrate, and sandblasting needs to be carried out after the groove processing is completed to prevent machining debris from scratching the treated surface; ④ If the relative humidity > 85%, it is necessary to create an environment with controllable relative humidity.
Generally, a shed is built at the end of the sandblasted pipe, and the relative humidity in the shed is less than 85% through a dehumidifier or an industrial air conditioner; The node number is marked inside the piece to ensure the traceability of the spraying quality inside the node.
The marker pen should be of obvious contrast color and free of grease and chloride
.

2.
2 Visual inspection of the inner surface of the root weld

After the end of the pipe end sandblasting, the surface to be sprayed needs to go through welding demagnetization, welding preheating, groove grouping, back cover welding and other links, and finally can enter the spraying process
.
Among them, the groove group pair and the welding link have the greatest influence on the surface quality to be sprayed and the frequency is the highest
.
After the assembly is completed, the groove needs to be preheated before the welding starts.
The grease in the internal plummet may liquefy and drip onto the surface to be sprayed; in addition, some welding defects may occur, such as poor penetration, Reinforces high, non-swipeable weld spatter, sharp surfaces,
etc.
In order to ensure the quality of the surface to be sprayed, a visual inspection of the surface to be sprayed (endoscopy, see Figure 3) is required after the back cover is welded.
It is mechanically connected with the inner centerer and moves synchronously with the inner centerer) to ensure the sprayability of the entire area to be sprayed
.
In general, if the visual weld height is greater than 1.
5 mm, the size of the welding spatter is large, there is a sharp surface or there is oil on the surface, it is necessary to re-weld the incision directly, unless the conditions for grinding and cleaning the surface are available (such as people drilling into the sea) Grinding in the tube or external grinding equipment after the inner plummet)
.

Figure 3 Internal appearance inspection of weld bead

2.
3 Surface shot blasting before spraying

The computer controls the internal anti-corrosion trolley.
First, the centrifugal shot blasting disc is positioned in the weld area, and then the shot blasting cleaning and vacuum recovery procedures are started.
The control parameters in the procedure mainly include shot blasting speed, shot blasting time, and screw operating width.
Parameters have been assessed by PQT and PPT
.
Among them, the shot blasting is first rotated in a clockwise direction, and then rotated in the opposite direction.
The cleaning work is mainly used to remove surface rust, sundries, dust or fine particles.
Larger particles cannot be removed from welding splashes or oil stains.
Finally, rotate the camera 360° on the internal anti-corrosion trolley to confirm whether the dust, welding fumes, etc.
are cleaned up
.

2.
4 Automatic spraying of liquid epoxy

According to the recommendation of the SP-9888 material manufacturer, the temperature of the surface to be sprayed needs to be kept at 15~60 °C (the surface temperature of the substrate should not be too high, otherwise the re-spraying time is too short, and the paint repair operation time is tight), the spraying process parameters (the rotation speed of the spraying disc , paint supply flow, spraying round-trip width, spraying time, etc.
) are finally determined by PQT or PPT test and controlled directly by computer
.
During the construction period, normal pipe laying is the main method, and the pipe laying efficiency is stable.
The welding, welding bead inspection and node coating on the pipe laying operation line are all carried out with high efficiency.
A continuous spraying operation is carried out at each node
.
When the sea condition is poor or some special circumstances have to be laid slowly, the internal anti-corrosion of the node needs to reduce the spraying efficiency correspondingly to match the pipe laying efficiency
.

The width of the spraying area that the high-speed spray disc can reach by moving back and forth is about 200 mm (based on a cutback of 60 mm and not exceeding the roughened overlap area).
Considering that local repairs may be required after spraying, the normal spray thickness is designed to be 500 mm ~1 000 μm, a certain thickness allowance (usually 500 μm) is required, and the maximum thickness of the coating does not exceed 1 500 μm, and the spray thickness is controlled by computer software
.

If local spraying is found to be insufficient, the operator can re-spray the area or spray the entire node by adjusting the position of the spraying trolley
.
However, it should be noted that re-spraying needs to be carried out within the maximum re-spraying interval.
The maximum re-spraying interval is determined by the performance of the material itself.
Generally, material manufacturers will provide recommended values.
For example, when the ambient temperature of the coating is 30 ℃, SP-9888 The maximum re-spraying time interval for liquid epoxy material was 2.
4 h, and when the ambient temperature was 60 °C, the maximum re-spraying time interval was shortened to 25 min
.

2.
5 Final Inspection

Through the 360° rotatable camera on the internal anti-corrosion trolley, the spraying operators and the anti-corrosion QC personnel can visually inspect the surface condition after spraying to verify whether the cutback, weld area and overlap area (at least 25mm) are completely covered, Are there any visible pinhole defects and whether the sprayed area exceeds the roughened lap area
.
If the sprayed area exceeds the roughened lap area very little or if there are scattered spray spots on the unroughened pipe coating, it is acceptable without affecting the overall quality of the anti-corrosion coating in the joint joint
.
Also check the marked node numbers, these video and photo materials are archived and submitted to the owner for archiving
.
Visual inspection results need to be accepted by spray operators and anti-corrosion QC personnel and recorded in the construction daily report
.

3 Construction equipment for LE coating

The LE automatic spraying process requires operations such as sandblasting, post-weld appearance inspection and spraying.
The structure of the post-weld appearance inspection equipment is relatively simple.
The sandblasting system and specially designed automatic spraying equipment are mainly introduced as follows
.

3.
1 Sand blasting equipment and weld bead inspection equipment

Mainly include 2 air compressors (one for use and one for standby), 1 set of air drying equipment, 1 set of air tank, 2 sets of gas preparation systems (adjusting the final blasting gas pressure), 2 sets of blasting recovery systems (sand blasting & Sand recovery, sandblasting head and pipeline); 1 set of portable sandblasting machine, simple structure, low sandblasting air pressure, can only be used for light sandblasting, mainly used to clean dust and remove new floating rust, generally used for Internal sandblasting to remove floating rust after the weld bead incision; 1 set of rotating pipe system, including 2 sets of rotating rollers, 1 hydraulic power station and 1 central control equipment, as shown in Figure 4; due to the higher requirements on construction efficiency for sea pipe laying High, so blasting systems often use rotary pipe equipment to improve blasting efficiency
.
The visual inspection equipment used in this project is relatively simple, only an endoscope, as shown in Figure 5
.
Generally speaking, the pipe end blasting systems are conventional equipment, and the blasting effect is good, but it takes up a lot of space
.

Figure 4 Transfer equipment

3.
2 Automatic spraying trolley

The internal spraying of the node is carried out inside the subsea pipe.
According to the diameter of the subsea pipe, the fully automatic spraying equipment of the corresponding specification is selected.
The spraying trolley integrates multiple functional modules, as shown in Figure 5
.
Including crawling power unit, providing crawling power for the internal anti-corrosion car; battery pack, providing power for crawling, cleaning, spraying and other functional modules; centrifugal shot blasting cleaning mechanism, cleaning the surface to be sprayed The dust, small particle welding spatter and other attached to the surface The sundries on the surface to be sprayed, etc.
; the vacuum recovery unit, to recover the sprayed sand; the spraying unit, responsible for spraying according to the set procedure; the inspection equipment, which is composed of a camera system, mainly used for the positioning of the internal anti-corrosion trolley and the surface before and after spraying check, node number check,
etc.

Figure 5 Root pass inspection equipment

The battery of the internal anti-corrosion trolley needs to be replaced or charged regularly, and sandblasting abrasives and liquid epoxy materials also need to be added in time according to the consumption
.
Taking a 36-inch internal anti-corrosion device as an example, the battery needs to be replaced every 12 hours or every 50 nodes
.
Every time the internal anti-corrosion trolley exits the subsea pipeline, a calibration check is performed, a complete spraying procedure is performed, and the total mass of the sprayed paint and the paint mixing ratio (the epoxy paint is calculated by volume) are recorded to ensure and verify the operation of the equipment.
Stability and precision
.
In addition, after the calibration check, it is necessary to confirm whether the liquid epoxy and curing agent exceed the maximum temperature recommended by the material manufacturer, and use a temperature probe within the validity period of the calibration to measure the actual temperature of each material
.
The maximum temperature of liquid epoxy is 65 ℃, and the maximum temperature of curing agent is 45 ℃.
All test information should be recorded in the exit report of the internal anti-corrosion trolley
.
In addition, it is necessary to check the state of the steel grit abrasive.
Check the state of the steel grit abrasive every time the internal anti-corrosion trolley is charged or the battery is replaced.
An abnormality is found and the abrasive needs to be replaced
.
If the control signal is lost or the equipment fails, the internal anti-corrosion trolley cannot crawl normally, and the emergency locking function needs to be activated and wait for the rescue trolley to rescue
.

3.
3 Control system

The control system mainly includes: 1 host, 1 set of control programs that can display analog animations, several display screens, 1 appearance inspection equipment control system, 1 walkie-talkie (can be combined with the internal pre-blasting workstation of the pipe end and the maintenance of the internal anti-corrosion trolley).
station personnel to communicate, such as conveying operating instructions or confirming some operating information, etc.
), these devices are concentrated in the container, as shown in Figure 6
.
All construction actions are managed and controlled by an experienced team leader
.

Figure 6 Internal anti-corrosion equipment control system

The main functions of the control system include: ①Observing the operation conditions of each work area, including sandblasting in the deck area, maintenance work of the internal anti-corrosion equipment and equipment functional testing work at the 0-1 station of the operation line, etc.
; ②Sending control signals and controlling Appearance inspection action on the back of the weld bead and action of the spraying trolley (crawling, sandblasting, sand cleaning, spraying, and appearance inspection after spraying)
.

4 Conclusions and recommendations

4.
1 Conclusion

This project is the first international pipe laying project with in-node anti-corrosion construction undertaken by our company.
This project has accumulated rich construction management experience for the application of in-node anti-corrosion technology in domestic marine pipeline projects, but also found that the current international The shortcomings of the internal anti-corrosion construction technology in the nodes: ①In international projects, the internal anti-corrosion coating is generally not accelerated to solidify, and the thickness and leakage points are tested.
Only strict process control is used to ensure the coating quality.
The coating quality evaluation method is mainly based on experience.
Abundant spraying operators and anti-corrosion QC personnel rely on the endoscope on the spraying equipment for visual inspection.
Although this coating quality control method has been recognized by Saudi Aramco and some other countries, there is a certain degree of subjectivity; ② Node The internal anti-corrosion equipment is relatively complex.
During the construction process, the stability of the equipment and the stability of the control signal need to be strictly controlled, and the operation of the internal anti-corrosion equipment needs to be tested before and after each spraying, which affects the construction efficiency to a certain extent
.

4.
2 Recommendations

A few domestic companies already have domestic automatic spraying equipment, but there is a lack of extensive verification of spraying quality stability under subsea pipeline construction conditions.
Oxygen coating quality assurance measures or develop new coating quality testing methods, and do sufficient verification
.
It is recommended to strengthen process control from these aspects: ① In the surface treatment of pipe ends, sandblasting equipment occupies a large space on the ship, which generally increases the difficulty of equipment layout.
It is recommended to use or develop sandblasting equipment with a smaller volume; ② After the welding is completed, It is recommended to use the endoscope in combination with laser scanning, which is conducive to obtaining more accurate surface state information and judging the sprayability of the surface; ③For the surface state that cannot be sprayed, it is recommended to use grinding equipment to avoid incisions that seriously affect the coating Pipe efficiency; ④The welding bead is finished after sandblasting at the pipe end.
It is recommended to strengthen the cleaning function on the internal anti-corrosion equipment to better ensure the roughness of the weld bead area; TIG welding or STT (surface tension transfer) automatic welding or CMT (cold metal transfer), etc.
, ensure the smoothness of the internal surface after welding as much as possible
.

References (omitted)

Author | Peng Qinghua, Man Xinbao, Wu Yewei, etc.

(Offshore Oil Engineering Co.
, Ltd.
, Tianjin 300461)