Modularization for hydrocarbon pipelines in Africa
One of EPCM’s major focus areas is modularization for hydrocarbon pipelines in Africa. As a company, EPCM services the whole African continent. From the beautiful beaches of Cape Town in the South to the rocky outcrops and rift valley as far north as Djibouti.
Being very experienced in delivering projects in Africa, EPCM also understands how to make our lives easier in these areas. One of the major ways this can be done is to understand modularization. Although it sometimes cost a little bit more from a capex perspective to skid mount certain equipment, it saves all of this and more during the construction phase.
EPCM as a company started as an oil and gas business and has recently moved into other chemical and industrial process plants. EPCM’s core is built on years of cross country oil and gas pipelines and tank farms. It is thus very familiar with the concept of remote locations, construction environments that keep on changing and stranded infrastructure that needs to be supported.
When we look at modularization, EPCM developed 3 levels to define and understand the process better.
The first level of modularization is where the complete unit can be supplied in modular format – the unit is completely pre-fabricated in the workshop and delivered as a single unit to site. From a modularization perspective, this is complete modularization. This however is not always possible as location and logistics play a major role.
The second level of modularization is where skids are made up of a certain facility. The skids are then shipped to site and the skids are assembled together to create the final unit. Although this is not complete modularization, it also works very well and takes logistics and location into consideration.
Level 3 of modularization is where no skids are manufactured. The equipment and materials are sent to site individually and then assembled on site. The level of modularization does however have certain OEM (original equipment manufacturers) components skid mounted and supplied to them from the OEM’s in this format.
In this article EPCM will discuss the major items that can be supplied in modular format for hydrocarbon pipelines.
Table of Contents
1 Pump Stations
One of the major components of any hydrocarbon pipeline is the pump stations. These stations are responsible for moving the product from point A to point B. Although it sounds like a simple task, there’s a lot more to it. Depending on the length and diameter of the pipeline, there might be a number of pump stations that work together to move the liquid fuel through the pipeline. In most cases a pump station would consist of at least two large pumps, one would always be running and the other would be on standby. Where two pumps cannot meet the demand of the pipeline, it can be increased to 3 pumps, with two running and one on standby. In order for the pumps to have the correct inlet pressure called NPSH (Net Positive Suction Head), the pump station would either have small booster pumps feeding them or the static head of the tanks would be enough to provide the larger pumps with enough pressure. Another very important component of a pump station is to understand what drives the pumps. When the pump station has electricity available in the vicinity, it would be the cheapest option to connect to the existing power grid. This will then require large electric motors to be fitted to the pumps and connected to the power grid. Typical electric power needed for large pipeline pumps would be in the range of 1MW of power per pump. Even with electricity available, the pump station would still need backup power, in this case one or two backup generators would be included as part of the pump station design. These generators would be designed to start automatically if the pump stations have a power trip. In very remote areas where no power is available, pump stations can be designed to run directly from diesel motors. Thus, the pumps are not connected to an electric motor powered by the generator but connected directly to a diesel fired engine. This would reduce energy losses but would then be very limited to future power connections as the utility companies expand to cover the remote areas.
Although pump stations have a few more components, this article focuses more on the modularization of the stations and thus will only discuss the major components.
1.1 Level 1
When we look at the pump station design, you have a few levels of modularization. Level one would be a complete modular pump station with all its components. You would have a plinth, electrical connection point and pipeline connection point available on site. The pump station would arrive as a complete skid or even a containerized solution on site and would then get connected. This would be the perfect world of modularization and in many cases, it can happen exactly in this fashion. Typical examples of pump stations that work this way are smaller pump stations that can fit everything into a container (or worst case an abnormal load). Larger pump stations can also be used – these are usually situated directly next to the ocean where a large vessel could transport the station and drop it into position without having to drive on any roads. Unfortunately, in Africa you do not find a lot of these perfect locations. In the cases where level one modularization does not work, you can move on to level two.
Level 1: Complete modular pump station
1.2 Level 2
In level two cases you split your pump station into multiple skids. So, you would for instance have the pump already connected to the electric motor skid mounted and ready to slot into its position on site. One of your pumps and its motor would be skid mounted and fits on the back of the truck delivering the pump. Your inlet piping as well as booster pumps would be another skid already welded and assembled in the workshop. Your outlet piping, and metering would form another skid that can be assembled with the rest of the pump station. All of the skids would be delivered and fitted like Lego blocks together to assemble the pump station. This is a very practical solution for Africa and used regularly on the cross country multi-product pipelines. In certain cases where pump stations are close to existing towns where they have skilled resources, a level 3 modularization could also work.
1.3 Level 3
Level 3 modularization is the least amount of modularization. This is mostly done where skills are existing around the facility where the pump station would be installed. Most of the items would be bought as separate loose standing items that get installed on site. A skilled workforce would have to assemble and build this pump station. The area where modularization would still occur would be, for instance, the generator sets – the contractor on site would get a complete generator with the engine and generator already assembled. He would probably not try to fit actuators to valves and the pumps would already be fitted to the electric motors and laser aligned from the OEM (Original Equipment Manufacturer). He would thus do most of the assembly and even welding on site as part of his construction and not really have skids that he would assemble together.
Level 3: Modularization.
All three levels of modularization can work for pump stations in Africa, but the strategy would have to be clearly decided during the design and planning phase of the project. Using BIM (bill information modeling) principles during the design of these stations would help the team identify the level of modularization needed.
2 Compressor Stations
Compressor stations are very similar to pump stations, but they are used to move gas from one point in a pipeline to the next. The big difference between gas and liquid is that liquids cannot compress, and gas can. In the gas hydraulics, the compression factor thus needs to be taken into consideration. From an infrastructure point of view, compression stations are very similar to pump stations and they have the same general layout. Gas that enters the compressors needs to be clean in order to prevent damage to its structural integrity. Furthermore, when gas is compressed and the pressure increases, the temperature of the gas rises. This means that usually after gas has passed through a compressor, it has to be cooled down to a temperature that falls within the design limits of the system. Compressors can be electrical, diesel or gas driven but gas is usually preferred owing to its cheaper burning costs.
As discussed in the pump station section, gas compression stations could also be executed using either of the 3 levels of modularization. Level one would be the best for small stations that can fit on the back of a truck whereas level 2 and 3 would be applied to larger stations depending on the location and the skilled resources in the area.
Level 2 and 3 combined: Compressors to be skid based, etc. piping to be installed onsite
2.1 Customer Metering Stations (CMS)
The use of customer metering stations are most commonly used in distribution networks. You would have the large transmission pipelines with their own metering systems used for leak detection and billing purposes, and then you would have a distribution network with hundreds of small users all using gas, for instance. The gas would thus be taken from the transmission pipeline, pressure dropped using a pressure reducing station (PRS), and then fed into a distribution network running at a lower pressure. At each of the customer sites of the distribution network, you would have CMS stations that measure the amount of gas the customer uses, and the gas company bills them accordingly. These CMS stations are small stations that could be one or two legs depending on availability requirements from the client. CMS stations can easily be skid mounted or even containerized and shipped to site. From a gas company perspective, it makes a lot of sense to standardize these units as they often have to maintain meters and exchange spares.
CMS stations could thus be modularized using level 1 or level 2. In some cases, contractors still prefer to build them on site, but this is becoming less as technology and logistics improve. The one thing to consider during these installations is that the meters do sometimes have a long lead and might have to be slotted in at the end of the fabrication or even on site.
Level 1: Skid base metering installation
Level 2: Spool piece metering installation
3 Pressure Reduction Stations (PRS)
As with CMS stations, the PRS station is used in transmission and distribution networks. It can be used for any product, but most of the time it’s used for gas networks. When a gas pipeline needs to drop the pressure, they install a pressure reducing station. As with a CMS station, it can have a single leg or double leg. Most of the customers prefer to have double leg stations to improve availability. These stations are usually designed to have a control valve in each leg with a shutoff valve downstream from the control valve. The shutoff valve is set at a pressure slightly higher than the bottom end of the control valve – if it reads at a higher pressure, the shutoff valves closes, and the gas is directed to the alternative leg. This way, the problem in the original leg can be sorted out before it’s used again.
Depending on the gas pipeline, PRS stations can be large installations. In many cases the gas needs to be heated before the pressure can be dropped. This is due to the Joule Thomson effect that reduces the temperature of the gas when the pressure of the gas is reduced.
From a modularization point of view, the small PRS stations can most certainly be skid mounted or even containerized and sent to site. Level one would thus work very well for the small gas pressure reducing stations.
Level 1: Skid mounted PRS installation
As the size of the PRS station increases, it might be necessary to move towards level 2 and level 3 of the modularization process. In two leg stations, each leg could be spool pieced and sent to site, on site the two legs get assembled and connected to the rest of the installation. It might be that the heating skid would have to be installed separately and then connected to the whole system.
As most of the skids discussed in the document, a number of factors will influence the level of modularization. But standardizing on the design and trying to skid mount as much as possible makes a lot of sense for pressure reducing stations.
4 Cathodic Protection
Cathodic protection is a very specialized area of cross country pipeline design. Many factors like soil resistivity, stray currents, proximity to existing power lines, pipe area and material as well as pipeline coating play an important role in designing a cathodic protection system for a pipeline. When all factors are considered, there are two major types of cathodic protection systems that can be installed. The first one is called ICCP (Impressed Current Cathodic Protection). The ICCP system uses electricity from the grid, solar or generators to drive a current through a TRU (Transformer Rectifier Unit). The TRU converts the AC power to DC and steps the voltage down to the required voltage. The current then flows from the TRU to silicon iron anodes that are installed into deep wells that will be drilled close to the TRU location. Typical depth of a deep well is about 60-100m. The electrodes then move from the silicon iron anodes through the soil towards the pipeline. If the pipeline has any damage to the coating, the electrodes will collect on the pipeline and prevent it from rusting.
The second cathodic protection system used in pipeline design and construction is the sacrificial system. In this particular case, magnesium anodes would be installed and connected to the pipeline every few hundred meters. They have no electricity connected to them and depend on the natural flow of electrodes from the magnesium anode to the steel pipe. If the pipeline coating gets damaged, the electrode will flow towards the steel and thus protect the pipe from rusting.
Both systems have positives and negatives depending on the location and specific pipeline design.
From a modularization point of view, both systems can be supplied in modular form to pipelines.
ICCP stations could be supplied as level 1 modularization. But the complete system would probably be a level 2 modularization as the deep wells and anodes would have to be separately delivered and installed. The whole power supply, TRU and monitoring device can be containerized and supplied as a complete container or skid. The deep wells would have to be drilled separately and then the anode tails with silicon iron anodes could be a module that fits into the deep wells.
When we consider the sacrificial system, it would probably have to be installed as a level 3 system with each sacrificial anode system installed separately. You will however have a certain amount of modularization with the test points that can be assembled in the workshop as well as the anode tails that can be fitted before installation.
Complete installation of the CP system. Combination of level 1 – level 3
5 Main Line Valve Stations
When designing a hydrocarbon pipeline, the placement of valves stations are very important. In general, the design codes give good guidance although they don’t all agree on the distance between valve stations. In the American code, the maximum distance between valves stations for a gas pipeline is about 32km. In the Australian code, they leave it open to the engineer but in some cases they go as far as 100 km from each other. In ASME 31.4 that governs multi-product fuel pipelines, they also only give guidance, with authority given to the engineer to make the final decision. They do however explain why you have valves stations and where you should look at installing mainline valves. In summary, the most important reason mainline valves are installed for cross country pipelines is to keep the amount of product that can be lost during rupture to a minimum. Safety is a major driver when considering valve station placement and in ASME 31.8 it gives you classes based on the population around the pipeline. If the area is densely populated, you need to have more valve stations in that area compared to when the area is not populated. Environmental impact is another major driver of main line valve placement. For multi-product pipelines, ASME 31.4 requires main line valve stations to be placed on either side of a river crossing or wetland to ensure minimal spill if a rupture occurs.
The normal design of a main line valve station usually has a large full-bore ball valve installed in the line. This allows the pipelines to still be pigged, but shut off can easily and quickly be obtained by closing the ball valve. Depending on the location and control philosophy, the mainline valve can be actuated or not. If the valve is actuated, it can be done electrically, hydraulically or pneumatically. Again, this depends on the product, location and utilities available at the valve station. Another component of mainline valve stations is the bypass line and valve. When commissioning pipelines or sections of pipelines, the bypass valves are used to equalize the system. If the pressure difference is too high between the two sections of the pipeline, the torque needed to open the main line valve will be very high and operators would battle to open the valve. In gas pipelines when they are commissioning sections and the one section has a high pressure and the other section is still low, the Joule Thomson effect causes a major temperature to drop in the gas. They thus use the bypass lines to regulate the pressure and flow, to prevent damage to the mainline valve.
Mainline valve stations can be buried under the ground with extended spindles, installed into a valve pit or above ground with 5D elbows taking it above ground as needed. The selection of the correct layout for a mainline valve station is up to the pipeline engineer and he should take a number of factors into consideration when designing the station.
Modularization of mainline valve stations would always be a good idea. When looking at the remoteness of these installations as well as their size, it’s a perfect fit for modular fabrication. Depending on the size of the bypass line, mainline valve stations could either be a level 1 or level 2 modularization.
When the level 1 modularization principle is used, the complete valve station including the bypass line, actuator and instrumentation would be installed in the workshop and the complete assembly would be transported to site and installed.
Level 2 would allow the main line valve station to be installed using a few spools that get welded together on site. Most of the assembly would be done in the workshop, with some final welds done in the field.
In certain cases the contractors might choose to go with level 3 modularization, but we would not recommend going this route unless they have a specific reason this works better for the exact location.
6 Control rooms/Data Centers
The control room of a pipeline forms the heart of the operations. From this room the pipeline owner can inspect the operations of the pipeline as well as control various items on the pipeline. The control rooms works with the fiber network to control all pumps and compressors as well as valves and storage facilities. It has a number of alarms and warnings to ensure the system is functioning the way its suppose to. A lot of development is going into improving the user interface and recent development includes mobile applications to control pipelines and plants. With the movement from voice to data, data centers are becoming more and more popular to lump with control rooms. These data centers thus store and process all the pipeline data. With clever tools like Watson, the data can now be processed into meaningful patterns and used to improve the pipeline operation and maintenance.
Depending on the size and location of the control room or data center, these items get modularized on a regular basis. Both data centers and control rooms are made up of hundreds of small electronic components talking to each other. It would thus be very difficult to assembly all these components on site.
Although control rooms are mostly in existing buildings, the bulk of the components get assembled in workshops and modules are sent to site to fit into existing buildings.
This market is mostly dominated by large international companies that offer a complete solution to the problem and offer all their own equipment as well.
Control room and data center equipment would mostly be installed using level 1 or level 2 modularization and we would also not recommend using level 3 modularization for these systems.
7 Pig launchers and receivers
In order to clean, commission or inspect pipelines they need to have pig launchers and receivers installed. Pig launchers and receivers are usually installed at pump stations or compressor stations. The norm is to have a pig launcher and/or receiver for every 100km of pipeline. Although, these days it seems like the latest intelligent pigging technology can collect up to 200 km of pipeline inspection information in one run. Intelligent pigging is governed by the design codes as well as the law of the country the pipeline is installed in. In most cases, pipelines are required to be pigged every 5 years to inspect wall thickness and ensure they are still safe to operate. If pipeline companies find defects during the pig runs, they have to repair them to ensure the integrity of their pipeline is still intact.
The physical pigging station is not a very complex item but forms an important part of the pipeline. Pigging facilities are usually above ground installations and, in many cases, especially at intermediate pump/compression stations, the launcher and receiver are both fenced in the same facility. The way a pig launcher works is it uses the product in the pipeline to propel the pig, pig stations thus have a barrel like a rifle that has a door on the one side and is connected to the pipeline on the other side with a double block and bleed valve between the barrel and the pipeline. It then has a kicker pipeline that directs some of the product behind the pig in order to propel it forward. Once the door is closed with the pig inside the barrel, the valve at the kicker line opens and the product propels the pig into the pipeline.
Pig launchers and receivers are perfectly suited to be modular components in pipeline construction. In many cases, pipeline companies choose to have the launchers and receivers mobile and they move them from one scraper station to the next. Quick opening doors are also a very nice extra to have and if the budget allows it, we would recommend adding it to the design.
In most cases, you would see that pig launchers and receivers are supplied as level 1 modules that the pipeline contractor can install as a whole.
You can however also do a pig launcher as a level 2 modularization. The best split between the two skids would probably be to do the launcher barrel, door and valve as one skid and have the kicker line and its valves as the second skid and assemble the skids on site.
If the project wants to investigate a level 3 modularization, it would probably be the best option to completely fabricate the launcher on site as it does not have many components that will fall into a level 3 modularization.
Level 1: Skid mounted pig launchers and receivers
Level 2: On site assembly of launcher/receiver
8 Fiber optic systems
Communication has always been a challenge for cross country pipelines. In the early years of pipeline design and construction, they would only dream of having all the ends of the pipeline talking to each other. As technology improved, pipelines started pushing towards different ways of communicating: from the standard telephone communication to radio to fiber optic. These days most of the pipelines either have fiber optic installed or are busy looking into retrofitting fiber optic lines onto the pipelines. The fiber optic cable is installed in a duct on top of the pipe. The cable usually has about 32 fibers that can be used for communication. In general, a pipeline communication only uses a few of these fibers, the remainder then gets sold to the telecoms companies and the fiber line becomes a second income for the pipeline company. The fiber optic cable makes use of light signals that gets sent via the fiber to give signals as needed between the ends of the pipeline. By using fiber optics, the pipeline operator can give signals to open and close valves, shut down pumps, and provide time of arrival for products, to name a few examples. The latest technology with fiber optics is called intrusion detection. The fiber uses acoustic sensing to detect interference on the pipeline and can pinpoint where the intruder is busy digging or driving over the pipeline. This is especially helpful in areas where 3rd party intrusion is high, like West Africa and Mexico.
The fiber optic system will run for the full length of the pipeline. The duct that houses the fiber is installed during the construction and the fiber is blown in afterwards. This way reduces the damage caused to the fiber during the pipeline installation. Inspection pits are installed every few hundred meters. When the intrusion detection systems are installed, a black box gets added to the fiber installation every few kilometers. This handles the acoustic sensing of the fiber system.
Although fiber is essential to ensure communication on pipelines now and into the future, it’s not a very modular installation and most of the fiber optic systems would probably be installed using level 2 or 3 of the modularization process.
9 Filter/Dehydration Stations
Filter skids are used in a wide application in the hydrocarbon pipeline industry. In certain industries like the aviation industry, it’s extremely important that the cleanliness of the product is maintained. In general, most pipeline systems get designed to have a filtering system before they enter a facility and as they move to the next portion of the facility. Filter systems are also very common to protect specialist equipment. In gas pipelines for instance, the gas is filtered to less than 2 microns before it can power the gas turbines, as any large particles can damage the turbines. Filter skids are also often seen before compressor stations to ensure compressors are not damaged. During pigging operations, most of the dust and debris in the pipelines are removed by means of a pig, which then needs to be taken out of the system at some stage. Filter skids are perfect for this and as part of the pigging exercise, filter skids are usually connected to the kicker lines at the pig receiving facilities. When gas is found at wells drilled in the ground, its usually wet and has particles trapped in it as part of the stream. Filtering and dehydration skids work perfectly to clean up the gas before moving it into the gas gathering network. The most common filter skids use cyclone filters to remove the larger particles and then make use of candle type filters to remove the smaller particles. When dehydration is needed, desiccant dryers are often employed to remove the water from the system.
Filter skids are perfect for modular fabrication and installation. Although filter skids can also get relatively big, most of the times a typical filter skid can fit onto a truck and be transported to site. Level 1 modularization would thus work very well when it comes to filter and dehydration skids. In some cases, contractors might choose to go with level 2 modularization due to size or location, but you would seldom see a level 3 modularization approach taken on remote filter skids.
10 Heating Stations
Heating skids are most commonly used in gas applications as well as heavy fuel oil and crude oil applications. When the pressure of a natural gas pipeline or system is dropped, the Joule Thomson effect drops the temperature of the system as well. The rule of thumb for these calculations is 0,5 degC for every 1 Bar pressure dropped. Heating skids are thus very often used before pressure reduction stations are installed. The most common heating skids use water bath heaters that get heated by burning the gas or alternatively using electricity. The water bath heater heats the gas to the required temperature before the drop in pressure reduces the temperature again. In some of the latest PRS (pressure reduction station) designs, the energy from the pressure drop is converted into electricity and the electricity is then used to power the heating system. Another application where heating skids are frequently used is HFO (heavy fuel oil) installations. As with crude oil, the product does not flow easily and if it’s not heated, the viscosity blocks the pumps and filters. Heating skids are used in this particular application to keep the product at the correct flowing temperature. With certain pipelines that transport waxy crude, the whole pipeline needs to have heating pads or stations to ensure the product can flow as required. Heating skids play an important role in the oil and gas industry and should be engineered correctly to ensure integrity of the system.
Modular heating skids are very common and can either be skid mounted or containerized. Heating skids would thus be great for level 1 modularization but could also be modularized up to level 2 and 3 as needed.
11 Cryogenic/Cooling Stations
During the compression of natural gas, the reverse Joule Thomson process occurs, and the natural gas is heated as the pressure increases. Usually aftercoolers are installed downstream from gas compressors. These aftercoolers are usually part and parcel of the compressor skid that you purchase from the OEM compressor supplier and they specify the outlet temperature from their skid. It’s thus already modularized as part of the compressor package you purchase. Aftercoolers are also used at the exhaust gas of engines or burners. Burners often have waste heat that is then recovered by boilers or condensers. Condensers or boilers are also basically aftercoolers; they take the heat produced by something and convert it into a different phase by cooling it down.
An additional cooling technology, known as cryogenics, takes cooling to the next level. Cryogenics occurs when products are taken below 0 degC. At the temperature of -161 degC, natural gas changes from a gas to a liquid at atmospheric conditions. The latest trend in gas pipelines is to install small liquefaction plants on the pipeline that liquefies the gas. Once the gas is liquefied, it’s then moved to different locations where pipelines cannot reach. This process of liquefying natural gas is called LNG (liquid natural gas). LNG consists of the liquefaction process where gas is turned into LNG and also the regas phase where the LNG is turned back into natural gas. Both the liquefaction skid and the regas skid work perfectly in the modular form.
When cooling and cryogenic skids are evaluated, both work well with level 1 and level 2 modularization. Level 3 could work for certain applications, but would over complicate the site construction process and potentially delay the project.
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