How do you demonstrate the integrity of a buried 18" schedule 40 pipe line that is used to transport crude oil from the middle of a tropical rainforest to the sea, 123km away, where ships can collect and transport the oil worldwide?
The following information is known or can be deduced from the question above:
- Visual inspection and conventional ultrasonic thickness measurement is not possible.
- The commodity (Crude Oil) transported in the line is valuable.
- Spillage of crude oil is environmentally unacceptable.
- The thickness of the line is relatively thin and great care has to be applied.
- The line is impossibly long for conventional inspections (even if it was above ground) and cannot be excavated for partial visual and ultrasonic thickness inspection.
Starting at the source.
Crude Oil is produced from below the ground at the wellheads and is processed through separators and dehydration tanks by means of a network of piping and other process equipment to remove gas and formation water. The processed oil is then pumped from the middle of the forest to the coast where it is kept in storage tanks until it can be pumped into large Oil Tankers offshore. The oil is heated and kept at pour point before it is pumped through the line from the jungle, it inevitably cools down in the line and paraffin wax is formed in the oil. This wax builds up on the sidewalls of the pipe and could effectively block the pipe line if it is not scraped out or chemically controlled.
A wax inhibitor is injected into the line to prevent and/or control the build-up of wax. The line is regularly scraped by means of a cleaning tool that is designed to fit the full diameter of the line. These tools are called "pigs" due to the screeching noise that the first wooden scraping tools made when used to clean lines. Pigs are propelled by the product through the line. Cleaning tools are designed to clean the lines. Some are equipped with steel brushes, discs and/or magnets to scrape the line clean or to remove debris.
Designed for pigging
Lines have to be designed to ensure that they are "piggable". They are equipped with a Launcher at the starting point with a quick closure where the pig is entered into the line by means of valves and piping systems. The Launcher is isolated with bypass lines so as to continue the production when the pig is launched. The line is also equipped with a Receiver at the end point where the pig is retrieved by the same method. The wax that is scraped out by the pig is normally caught in the Receiver together with the pig and is thus removed from the system. The line will normally be designed to have long radius bends and have no large branch connections where the tools can get stuck. Equal Tee's are allowed on the line if they are fitted with bars (guides) to keep the tool from derailing.
Designed for safety
The full length of the pipeline between the launcher and receiver is buried at a depth of approximately two meters to protect the line from human and animal interference. The disadvantage is that it is now prone to soil corrosion.
Design against degradation
For protection, the line has a polyethylene coating along the full length and the field welds are wire brushed and wrapped with the same material after radiography testing is done to ensure that the line does not corrode when it is buried. Impressed Current Cathodic Protection further protects the line against external corrosion. The internal surface of the line is not protected and in this specific case cleaning pigs are launched through the line regularly to scrape out the excess wax. A thin layer of wax remains in the line to form a coating of sorts on the internal surface. The pipeline is therefore unlikely to suffer from internal pitting corrosion or SCC due to contamination by water and oxygen and gases such as Hydrogen Sulphide and Carbon Dioxide. If any of these threats are identified, an inhibitor or scavenger can be injected into the line to protect against the threat
Despite all the measures to protect the line, Technical Integrity has to be demonstrated by performing an inspection.
A special pigging tool is available with Magnetic Flux Leakage capabilities and a memory to record and save data such as wall thickness and crack detection. The tool consists of strong magnets and sensors, an electronic unit that stores all information received, a battery pack to power the electronics and an odometer to measure the distance. Optional is a gyro system that measures directional movement and it can plot the line using X Y Z coordinates on a map after the run was completed successfully. The tool normally consists of three or four units with swivel joints between them to navigate through tight bends (1.5D) without getting stuck.
The tool is launched like normal pigs through the Launcher and is propelled by the product in the line. It induces a magnetic field into the steel and records all the flux leakage from reduced thickness and other features. The Intelligent Pig is retrieved at the Receiver and the data is downloaded and analysed and a detailed report is made that shows the thickness of the complete line and also any defects that were recorded. Accuracy is of the utmost importance as any defect with the potential to leak has to be excavated at the exact location and repaired. The line is plotted with GPS and is marked with permanent beacons at certain distances and all the data gathered during the inspection is reported relevant to the distance markers. Actual defect locations can be found with little effort using a hand held GPS.
This tool is commonly known as the "Intelligent Pig". The 18" Oil Line is inspected every 5 years using In-line inspection or commonly known as "Intelligent Pigging". The technology has improved a lot during the last decade and the method is used with confidence to ensure integrity of pipelines all over the world. Intelligent Pigging is a very expensive inspection campaign but well worth it considering the information that is received. After a successful inspection the line condition and / or defects are known and repairs can be planned and executed or MAOP can be re-calculated if necessary.