Analysis of Corrosion Failure Causes of Short Joint of Booster Pump Outlet Pipeline on an Offshore Platform

introduction

The outlet pipeline of the booster pump of an offshore production platform is short, the size is 10"×6", the material is carbon steel, and the transportation medium is crude oil with a water content of about 40% to 50%

In this regard, based on the platform's process flow and previous pipeline failure research experience, this article starts from the corrosion mechanism, combined with indoor experimental results, analyzes the root cause of the corrosion failure of the booster pump outlet pipeline short section, and proposes corresponding protective measures

1 Macroscopic morphology analysis

Figure 1 The actual working position of the failed pipeline short section

Observed from the outer surface, the short section of the pipeline is welded to a section of 152 (6″)×7mm straight pipe by a section of 254 (10″)×10mm pipe through a necking and reducing diameter, and the other section of the straight pipe section is welded with a flange

Figure 2 Appearance corrosion diagram of short section of outlet pipeline

Cut the pipe section longitudinally, it can be seen that there is a groove at the welding seam that accounts for almost 45% of the circumference of the entire pipe inner wall.

Figure 3 The topography of the inner wall of the outlet pipeline after the short section is cut

2 Physical and chemical tests and results

2.

On the pipe section on the side of the intact side of the inner wall of the short section of the outlet pipeline, the base material of the weld, the base material of the neck reducing pipe and the straight pipe section are respectively taken for chemical composition analysis

2.

Metallographic analysis of samples of necked reducing pipe, weld and straight pipe section is carried out.

Figure 4 Metallographic structure of sample

2.

Scanning electron microscope (SEM) was used to analyze the microscopic morphology and component energy spectrum of the corrosion products on the surface of the weld grooves, pits and flat parts of the steel pipe

XRD analysis was performed on the surface of the non-corroded pits and the pits on the inner wall of the pipe.

2.
4 Electrochemical analysis of pipe short joint weld

Three materials of weld metal, base metal of straight pipe section and base material of necked reducer were selected, and the self-corrosion potential in artificial seawater solution was measured according to actual working conditions.
The results showed that weld metal and necked reducer There is no significant difference in the self-corrosion potential of the base material in artificial seawater under the conditions of 35℃ and 85℃ (the difference of 8～18mV is a normal measurement error); the self-corrosion potential of the base material of the straight pipe section at the two temperatures is higher than that of welding The base material of seam metal and necking reducing pipe is corrected, and the potential difference is 48～88mV
.

2.
5 Galvanic corrosion test of weld and base metal

According to the measurement results of the self-corrosion potential of three kinds of metals in the artificial sea water solution, select the weld metal-the base metal galvanic pair of the straight pipe section and the weld metal-the neck reducer base metal galvanic pair for the galvanic corrosion Test
.
According to the curve of the galvanic current with time and the actual area of ​​the anode in the galvanic couple, the average galvanic current and galvanic current density in 360 h are obtained respectively, as shown in the following table
.

Galvanic current and galvanic current density value of a pair of materials

By calculation, the average galvanic corrosion rate of the weld metal (anode in the pair) in the weld material-straight pipe base metal pair is 0.
015 9g/(m2·h)
.

3 Analysis and discussion

There is no abnormality in the metallographic structure of the welded part of the pipeline short section.
The inner wall of the pipe is mostly smooth and flat.
There are few corrosion products on the inner wall surface of the entire pipe section.
The corrosion products in the grooves and pits are loose.
The results of component energy spectrum analysis and XRD analysis show that the corrosion products contain A large amount of Fe, Cl, Na, Mg, Ca, S and C, O and other elements, it is inferred that the corrosion products are mainly composed of various iron oxides and FeCO3 with loose structure.
Because of the seawater-containing crude oil transported in the pipeline, these elements are The original conveying medium in the pipeline is consistent
.
However, it has been placed for dozens of days after the sample has been replaced before the sample is sent for testing, and the surface is wet, and the surface of the grooves and pits must be covered with floating rust
.

In addition, the medium transported in the short section of the pipeline is crude oil + produced water + gas.
The pump inlet operating pressure is about 0.
73 MPa, the outlet pressure is about 1.
2 MPa, and the temperature is about 90-95°C
.
The corrosion part of the inner wall of this section of the pipeline is in the direction of the flow of the medium.
Under the action of the pressure pump, local surface stress is uneven in the flow direction, and the surface corrosion morphology presents grooves, horseshoes, etc.
This series of characteristics shows that the corrosion at this place is a kind of relatively high corrosion.
Typical erosion corrosion
.

Combined with the evaluation results of galvanic corrosion, the possibility of galvanic corrosion caused by the coupling of the reduced diameter pipe and the weld metal under the test medium conditions is small.
There is a certain potential difference between the straight pipe section and the weld metal, and electricity will be generated after coupling.
The condition of occasional corrosion, but the corrosion rate is very low
.
Therefore, the main cause of corrosion of the inner wall of the short section of the booster pump outlet pipeline is the result of the combined effect of erosion corrosion and galvanic corrosion
.

4 Conclusions and recommendations

(1) Under the premise of ensuring the welding quality, the weld should be fully passivated to ensure the formation of a protective oxide film;

(2) Try to avoid the structure of small anode and large cathode
.
It is recommended that the welding material should be as close as possible to the electrode potential of the pipeline;

(3) It is recommended to increase the Cr element content in the short section of the outlet pipeline and use low-alloy steel or stainless steel that is more resistant to erosion and corrosion
.

References (omitted)

Authors | Jin Xi, Zhou Xiaojie, Sun Jixing, etc.

(CNOOC Changzhou Coating Chemical Research Institute Co.
, Ltd.
)