What Are The Common Inspection Methods In Pipeline Application?

As we all know, pipelines play an indispensible part in oil and gas transporting market. Thus, it is crucial to ensure the integrity and good performance of pipelines in real practice. In addition to strict manufacturing process, thorough and professional inspection is also a decisive factor in pipeline performance. Now let’s get to know some key inspection methods.

What is NDT (Nondestructive Testing)

Nondestructive Testing, shorted for NDT, is a widely-used method in pipeline inspection. It is a process of testing, inspecting and evaluating the quality and characteristics of the material without damaging the pipeline itself or affecting the later regular work of the pipe. In other words, after NDT testing, the part can still be put into practical use. NDT utilizes the changes in heat, sound and magnets caused by abnormal inner structure or flaws to inspect the internal and external defects of the material.

The common inspection methods of NDT are visual testing (VT), ultrasonic testing (UT), radiographic testing (RT), hydrostatic testing. Anson International co, ltd uses ultrasonic testing (UT) and hydrostatic testing to inspect its line pipes.

Hydrostatic Testing

What is hydrostatic testing?

Hydrostatic test is an inspection method to reduce the risk of flaws in the pipe that might threaten its ability to withstand the maximum operating pressure. Hydrostatic testing inspects the integrity of pipelines by filling in the pipe with a non-compressible liquid (often water, dyed) to increase the pressure level above the normal pressure to see if there is any defect exists. It can either carried out on pipes prior to being put into service or on existing pipes that are already in service.

Things to notice in hydrostatic testing

In order to carry out hydrostatic testing, the following rules should be strictly stuck to:

• Testing with air is dangerous and not recommended. Therefore, the first step is to release the air in the pipe.
• In the testing process, all the valves and vents should be shut to observe whether there is pressure loss.
• Hydrostatic test should be performed after all the hot work has been done to the pipe, including welding, heat treatment, etc.

Ultrasonic Testing (UT)

What is UT and how does it work?

Ultrasonic testing, also known as UT, enjoyed wide application in the flaw detection and quality control of metallurgy and manufacturing industry. It is able to precisely detect, inspect and diagnose the flaws in the material.
In an ultrasonic test, the ultrasonic transducer connected to a diagnostic machine is put on the pipeline to be inspected. High frequency sound waves can directly travel through the metallic material. If the sound waves hit a defect, normally with a different acoustic obstruction, some of the waves will reflect back to the sending unit and inspectors can see the results in the form of signals on the visual display. By analyzing the reflection, inspectors can locate and test the crack or other hidden defects.

What are the advantages of UT?

Ultrasonic testing bears a number of benefits in the application of pipeline inspection:

• Non-destructiveness is the most important advantage. On one hand, the inspected pipe doesn’t need to be cut or exposed to damaging chemicals, which means the inspection won’t interfere with its future use. On the other hand, it does no harm to operators, leaving no potential risk for them.
• Strong penetrating power makes sure it has a wide detection range.
• High sensitivity allows it to quickly and precisely determine the exact position of the pipe, providing accurate and dependable testing result. 
• Immediate result displayed—The ultrasonic testing equipment makes it possible to get the instant flaw testing result.

In contrast to NDT, there are some destructive inspection methods. They are always carried out on samples of the material instead of on material itself. Destructive inspecting methods are normally used to test the physical properties of the material, such as tensile strength, yield strength, hardness and so on. Suppliers can choose the most suitable inspection methods according to your practical needs.

Anti-Corrosion Coating For Pipelines

What is Pipeline Corrosion and Anti-corrosion Coating

As we all know, pipelines that transport oil, gas, chemicals and petroleum products are of crucial significance to the country economy. Therefore, the performance and maintenance of the pipelines are vital in the process. However, buried pipelines are easy to subject to corrosion attack if it is exposed to a wet environment. Pipeline corrosion refers to the deterioration of the pipes due to its interaction with the surrounding environment. It negatively affects the integrity and longevity of the pipelines. Thus, anti corrosion pipeline protection is the top issue to consider in oil and gas industry. One of the best ways to control pipeline corrosion is anti-corrosion coating. Coating is a method to isolate pipelines from corrosion attack. Corrosion can be greatly minimized by selecting the right coating.

Internal and External Anti-corrosion Pipeline Coating

The internal and external area of the pipe can both be corrosive because of different factors. The external pipeline corrosion can mainly due to the surroundings, such as acid soil, wet conditions and so on. Factors that lead to internal corrosion are the oxygen content, temperature, pressure and flow rate of the liquid carried in the pipelines. Due to the above reasons, the anti-corrosive pipeline coating can be put into two corresponding categories, the internal and external corrosion-resistant coating.

• Internal anti-corrosion coating
  Oil and gas liquid carried by the pipelines contains various chemical solid particles, some of which dissolve in the fluid while some remain. Those chemical particles remaining in the fluid travel in the pipe at a high speed, therefore, the internal coating is supposed to withstand friction caused by the fast moving particles. Besides, the internal coating material is expected to have a low surface roughness, thus, increasing the flow efficiency and capacity.
• External Anti-corrosion Coating
  As is mentioned above, pipelines are installed in diverse environment, thus, external coating is supposed to maintain functioning well no matter in hot deserts, in wet offshore conditions, or in acid chemical surroundings.

Main Types of Anti-corrosion Coating Technology

Considering the various applications and complicated working conditions of pipelines, corrosion resistant coatings are designed and manufactured using different technologies and materials.

Fusion Bonded Epoxy Coating (FBE)

Fusion Bonded Epoxy, also known as thermosetting coating, is an epoxy-based powder coating widely used to protect pipelines from corrosion. Under normal temperatures, resins in the powder coating do not react. However, when the temperatures reaches to 250℉ the power begin to melts into fluid, flow onto the pipeline surface, and quickly stick to the pipe, forming a solid coating. This process contributed by heat is called “fusion bonding”.

During the process, the pipe must be preheated to get rid of moisture and grit blasted to get a clean surface. When the surface cleanness is checked and the preparation work is done, the pipeline and the coating are heated with induction to certain temperature, the FBE coating is formed. Once formed and experienced its cure time, FBE coating will not melt even in higher temperatures--that’s why it is called thermosetting coating.

Dual-layer FBE Coating

Dual-layer coating refers to the use of a second layer of FBE that overcoats the first layer----the anti-corrosion coating. The second layer is applied directly on the first layer to protect the pipeline from abrasion.

Three-layer PE/PP coating (3LPE or 3LPP)

 Three-layer PE/PP is one of the most widely used anti-corrosion coatings in the field. As the names shows, it consists of three layers---the first layer FBE coating bonded to the PE or PP (polyethylene or polypropylene) topcoat using a co-polymer adhesive as the second layer coating. The FBE layer resists oxygen passage while the PP layer prevents water passage, greatly improve the anti-corrosion efficiency. The adhesive layer, which combines the FBE and topcoat together, assists in protecting the coating integrity and prolonging the coating’s service life.

The previous process of the formation of the three-layer PE/PP coating is similar to the FBE coating. After preheating, grit blasting and preparation inspection is done, the FBE layer is formed. Then the adhesive intermediate is applied by extrusion or spray followed by the polyethylene or polypropylene outer layer.

Three-layer PE coating enjoys more popularity in oil and gas pipeline industries, especially in onshore applications. Anson International Co., Ltd, specialized in manufacturing OCTG products and accessories, provides top quality three-layer PE coating for its clients. The standard top layer for 3LPP is high-density polyethylene (HDPE). Compared to lower density products, HDPE guarantees better combinations of properties, higher toughness and greater impermeability to both water and oxygen. The integral three-layer PE coating can ensure strong corrosion resistance, long-term operation at a relatively low cost.

Helical Welded Pipes And Longitudinal Welded Pipes

Welded line pipes enjoy a wide acceptance in oil and gas transporting market for its selection variety and relatively low cost. According to the different welding seams, welded line pipes can be further categorized into two types, the helical welded pipes and longitudinal welded pipes. Both have extensive application in the field, but due to different manufacturing techniques, they have many differences. Now let’s get to know the differences of both types related to the following aspects.

In manufacturing and welding aspects

Comparatively speaking, longitudinal welded pipe has simple manufacturing and welding techniques. As the name shows, longitudinal pipe has a straight welding seam on the pipe surface. Different from the longitudinal pipe, the welding seam of which is vertical to the axial line of the pipe, helical welded pipe is rolled and welded according to certain helical angles. Therefore, helical welded pipe manufacturing, with complicated welding method, is more time-consuming. As for longitudinal welded pipes, to meet the required length of pipelines used in industry projects, short longitudinal pipes always need to be jointed and welded into a long enough pipe. Thus, inevitably, many T-shaped welding seams would appear in this welding process, largely increasing the possibility of welding defects. Besides, T-shaped welding joints incline to cause welding residual stress, which lays potential risk for future fatigue crack. Therefore, though having low cost and simple manufacturing technique, the overall performance of longitudinal welded pipe is less promising than the helical welded pipes.

In strength tolerance

Strength tolerance is the one of decisive factors determining the quality of line pipes. Stress analysis is the basis of measuring strength tolerance. Welded pipes seams withstand two stresses, the radial stress δY and axial stress δX. The compound stress can be derived from the formula δ=δY(l/4sin2α+cos2α)1/2 , α refers to the helical angle. Normally, the angle for helical welded pipe is 50° to 70° while for longitudinal pipe is of course 90°, so it can be concluded that the compound stress sustained by helical welded pipes is 60 to 85% of the latter. Obviously, under same working pressures, the strength exposed to helical pipe is lower, thus, for pipes of same diameter, wall thickness of helical welded pipes could be manufactured thinner than longitudinal welded pipes.

 

From the above, we can know：

• Once pipe pressure beyond its tolerance, the pipe would get burst. For helical welded pipes, normally, the bursting area wouldn’t be the welding seam, because the helical welding methods expose less stress to the pipe.
• Since the radial stress is the largest stress exposed on the pipe, the vertical welding seam of longitudinal pipes withstand the most stress, circumferential welding seam bear the least stress, the helical welding seam in the middle.

Therefore, compared to longitudinal welded pipes, the helical pipe, with lower sustained stress, can guarantee more safety and longer service of life.

In terms of various diameters of pipes

As for longitudinal welding, the width of the steel plate should be strictly stuck to the diameter of the pipe. Once the steel is cut for rolling, it cannot be changed, which means the diameter of the longitudinal pipe is fixed. While for helical welding, it can achieve various diameters of pipes by rolling the steel plate with different angles. Thus, helical welding can be used to manufacture pipes of different diameters. It can use narrower steel to manufacture pipes into large diameter using large helical angle. But for pipes of the same length, the helical welding seam is 30 to 100% longer than the longitudinal welding seam, thus, the welding cost is higher and the welding efficiency is lower. Therefore, longitudinal welding is more used to produce small diameter pipes, while helical welding is always used for pipes of large diameters.

Generally speaking, helical welded pipes own more advantages than longitudinal welded pipes, enjoying more promising market; however, the manufacturing method is more complicated and it is more expensive. Knowing the differences of both pipes will help suppliers and clients choose the most suitable pipes according to your application and budget.

Some Issues About Anti-Corrosion Pipeline And Its Transportation

Corrosion is the enemy of steel, painting is one of the most basic methods to prevent pipe corrosion. Anti-corrosion pipeline will inevitably suffer impact in transit, although corrosion-resistant pipe is generally sturdier, but if we don't pay attention to the buffer protection, it is easy to be damaged. So coating protective is the most important in the course of transporting pipeline. Let’s get to know some issues about pipe coating and considerations for pipeline transportation.

Importance of coating

Pipeline corrosion coating means that the continuous cover layer of insulating material on the metal surface and it prevents direct contact with the electrolyte to the metal. From the principle of electrochemical corrosion analysis, we can understand the electrochemical corrosion must have a few basic conditions: Steel as the anode corrosion, its potential is lowest. The electrolyte solution with low resistance, seep into the iron interface from the outside. In the corrosion process has enough oxygen to maintain at a certain level.
The use of coating to protect the steel is to enhance its corrosion potential and cut off electrolyte so as not to form corrosion cell. As a polymer film, coating can reduce the influence of corrosion factors in order to play the role of anti corrosion. In summary, the protective effect of coating on steel mainly has three kinds, shielding effect, corrosion inhibition and cathodic protection.

Stack anti-corrosion pipeline

In the construction work zone, anti-corrosion pipeline generally placed in the car. The pipe with sandbags gaskets at both ends is not allowed to touch the ground and the minimum distance between the tube and the ground is 0.2 M. If it’s inadvisable to temporary discharge pipe in the construction work zone, temporary stack pipe field should be set up depending on site conditions. Try to set the venue stacking tube in the non-farming area where is convenient construction and the venue should be smooth and compacted.
Pipeline with anti-corrosion is quite important, and there are some questions should be paid attention to when transporting the pipeline.

Anti-corrosion pipeline transportation

• All the anti-corrosion pipelines must pass the factory inspection, when the pipes delivered to the specified address, the transportation management staff and the reception staff check coated pipes and fill out the view record. Serious damage to the coating of the tube should be shipped back to the preservative precast plant.
• Anti-corrosion pipe is transported by special vehicle and the bottom of vehicle chooses thickness not less than 15mm rubber sheet having a width of not less than 100mm in order to reduce the impact of abrasions. Transport of vehicles must be restricted avoiding sudden braking to prevent pipe displacement or damage to the coating.
  
• Loading and unloading pipes using crane or pipelayer. Coated pipes handling should use special harnesses with hanging and dropping gently. In the contact surface pad soft material avoiding bump and should pay particular attention to orifice protection. The angle between corrosion-resistant pipes and the slings should not less than 30°in the hoisting process.
  
• Corrosion-resistant pipes stacking height of not more than three layers stack pipe support to the tube in the center stack prevail, symmetrically arranged, the distance from the ends of the pipe end support for 1.2-1.8m, pipe pile supporting two pipe both sides of the crib wedge should be set to prevent the roller tube.
  
• The strapping material to avoid damage of pipeline coatings as the standard when transport vehicles transporting pipe. Prohibit wirerope directly tied steel pipe and use nylon strapping.  strapping tools parts touching the tube pad a soft material to avoid damage of pipeline coatings.
  

In order to send anti-corrosion pipeline to the hands of customers intactly, to confirm the product has reached the standard before loading. Customers can also mention some packaging requirements to the manufacturers before the dispatch, to protect their legitimate rights and interests.