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Покрытие соединений
Трубопровод, покрытие на который обычно наносится на заводе или на площадке для хранения до укладки, требует покрытия сварных соединений и участков, поврежденных при транспортировке. После рентгена сварных шов, бригада по покрытию очищает проверенные куски сталив местах соединения труб, и наносит на него защитное покрытие. Покрытием обычно является стягиваемый при нагревании полиэтилен, обернутый вокруг трубы. Материал покрытия нагревают, чтобы он стянулся вокруг соединения, и на соединении сформировалось плотное герметичное покрытие. После нанесения покрытий группа инспекторов проверяет весь трубопровод на наличие царапин и следов абразивного изнашивания на покрытии с помощью устройства для испытаний высоким напряжением – детектора пропусков. Методом испытаний высоким напряжением обнаруживают дефекты в изоляционных покрытиях, нанесенных на электропроводящие подложки. Источник питания в измерительном приборе создает высокое напряжение постоянного тока, подаваемое на соответствующий датчик. Кабель заземления измерительного прибора подключается к непокрытому металлическому участку испытываемой подложки. При перемещении датчика по поверхности с покрытием дефект определяется по искре, возникающей в месте контакта, звуковому сигналу прибора и визуальной индикации на ручке датчика. Датчик может иметь форму щетки или пружины, которая охватывает трубу и движется вдоль всей линии.
Спуск трубопровода в траншею
После сварки трубопровода, завершения гибки и нанесения покрытия, его можно спускать в ранее вырытую траншею. Данная операция выполняется с использованием специальной строительной техники, которая расположена последовательно чтобы относительно равномерно поднимать трубопровод и опускать его в траншею. Следует действовать с осторожностью, чтобы не повредить трубопровод и внешнее покрытие. Трубопровод спускается с брусьев в траншею группой тракторов, оборудованных боковой стрелой. До начала операции по спуску трубопровода со дна траншеи убираются все камни. На участках в гористой местности на дно траншеи укладывают мешки с песком, чтобы защитить трубы и покрытие от повреждения камнями. После укладки трубопровода в траншею деревянные брусья поднимают и переносят вперед, для последующего использования.
Засыпка траншеи
До начала испытания трубопровода траншею засыпают. Иногда для засыпки траншеи используют ранее извлеченный из нее грунт, а иногда выбирают другой материал. Траншея засыпается с помощью машины для засыпки траншеи; в траншее не должно быть посторонних предметов. Засыпка траншеи требует особого внимания в следующих районах:
— на крутых склонах (где грунт обычно каменистый), материал для засыпки,
использованный при создании дна для трубопровода и соприкасающийся с трубой, может быть легко смыт. Действительно, траншея работает как водосток, и спустя годы материал для засыпки съедет вниз по склону. Чтобы избежать этого, принимают специальные меры для укрепления материла на месте (например, стенами из мешков с песком) и для создания регулируемого отвода воды (дренажная труба, отводные устройства).
-при прокладке трубопровода через подземные структуры или во всяком случаи, если невозможно добиться минимального покрытия, принимаются специальные меры, чтобы обеспечить такой же уровень защиты, как и при минимальном покрытии, а это заполнение траншеи бетоном, бетонными плитами и т.д.
Материал засыпки следует надлежащим образом уплотнить вокруг трубы (более плотно, чем в обычных случаях), чтобы придать линии необходимую прочность для защиты от статического и динамического давления нагрузок на траншею.
— Вышеуказанные меры принимаются, если по каким-то причинам нельзя использовать обычный материал для засыпки ввиду проблем, возникших при строительстве: например, обрушение траншеи во время спуска трубопровода или прокладка трубопровода через реки (когда переход создается открытым способом, материал засыпки траншеи следует укрепить для защиты от размывания течением реки).
Испытания
После засыпки трубопровод проходит гидростатические испытания согласно нормам. На самом деле, испытание трубопроводов это больше, чем просто выявление утечки и проверка сопротивления. Испытания трубопроводов проводятся даже при отсутствии соответствующих предписаний. Цели таких испытаний различны:
— удалить все загрязнения изнутри;
— проверить, чтобы не было никаких закупорок или уменьшения диаметра трубопровода;
— убедиться, что трубопровод не поднимается под давлением из-под земли (эффект Бурдона);
— убедиться, что траншея выдерживает дополнительный вес воды, используемой при испытании;
— проверить с максимальной тщательностью наличие утечек;
— проверить сопротивление, убедиться в прочности трубы и сварного шва;
— полностью удалить испытательную воду (даже из небольшого участка под
пересечением реки);
— очистить и осушить (в основном для газопроводов) внутреннюю поверхность трубопровода, прежде чем заказчик пустит через него дорогостоящий продукт;
— закрыть трубопровод с обоих концов, чтобы исключить попадание посторонних материалов и воды.
— избежать коррозии;
— избежать загрязнения окружающей среды большими объемами ржавой
воды, использованной при испытаниях.
Introduction
This section is about the inspection of high pressure cross country pipelines.
Installing a pipeline is much like an assembly line process, with sections of the pipeline being
completed in stages performed by different crews.
The construction process is “linear”, i.e. each operation follows the previous one without any
overlapping possibility. Alike for mass production in a factory, each operation is assigned to a
specialised crew or “spread” with specific equipment; everything is done so that the
construction proceeds smoothly without interference between the spreads. No need to say that
there is a lot of pressure on all the participants, including the inspectors.
For large constructions, there will be many inspectors, specialised each in a specific
discipline; civil work, welding, coating, … But very often a same inspector will have to check
all ongoing operations, requiring from him many various skills.
Cross country pipe laying spreads the operations over long distances; there might be a
hundred kilometres of rough track between the ROW gang and the backfilling. This is to be
considered when planning pipeline inspection.
Various types of pipelines
Pipelines vary mostly by the material they are built from; steel, cast iron, cement,
polyethylene.
Indeed, the material dictates for instance:
– the diameter and pressure,
– the weight and the length of the components to be joined
– how to make curves and take-offs,
– the trench preparation,
– the lowering-in technique,
– the backfilling,
– the anti-corrosion system,
– the testing.
Obviously, the environment will dictate the material:
– high pressure + big diameter will require steel,
– low pressure + big diameter will be concrete of cast iron,
– Small diameters can be any of those and also accept polyethylene (HDPE) and ABS.
This guideline deals with welded steel pipelines for the long distance transportation of:
– liquid hydrocarbons such as crude oil or refined products (diesel, gasoline, jet fuel,…),
– gaseous hydrocarbons such as natural gas (methane) or refined products (ethylene,
propane, etc…)
– water (drinking, raw or sea)
– slurry (a mixture of water and coal or ore powder)
Pipeline Components
A pipeline is not just an assembly of pipe joints welded together. It is a system that constitute
a functional transportation system, like a road or railway network.
Many fittings, accessories and facilities are installed on a line for the operations of this
transportation equipment:
– flanges, bolts, gaskets, casings,
– valves, vents, relief devices,
– thrust blocks
– cleaning facilities: scrapper traps or pig traps,
– metering stations,
– compressor or pumping stations
– cathodic protection, insulating joints,
– line markers,
– monitoring and metering stations
Pipes
Pipelines can measure anywhere from 6 to 60 inches in diameter, although certain component pipe sections can consist of small diameter pipe, as small as 0.5 inches in diameter. However, this small diameter pipe is usually used only in gathering and distribution systems.
Mainline pipes, the principle pipeline in a given system, are usually between 16 and 48 inches in diameter. Lateral pipelines, which deliver product to or from the mainline, are typically between 6 and 16 inches in diameter. Most major interstate pipelines are between 24 and 36 inches in diameter. Pipelines are produced in steel mills, which are sometimes specialized to produce only
pipeline. There are two different production techniques, one for small diameter pipes and one
for large diameter pipes. For large diameter pipes, from 20 to 60 inches in diameter, the pipes
are produced from sheets of metal which are folded or rolled (spiral) into a tube shape, with
the ends welded together to form a pipe section. Small diameter pipe, on the other hand, can
be produced seamlessly. This involves heating a metal bar to very high temperatures, then
punching a hole through the middle of the bar to produce a hollow tube. In either case, the
pipe is 100% tested before being shipped from the steel mill, to ensure that it can meet the
pressure and strength standards for transporting natural gas. Line pipe is also covered with a specialized coating to ensure that it does not corrode once placed in the ground. The purpose of the coating is to protect the pipe from moisture, which causes corrosion and rusting. Coating can also be applied on the inside of the pipes to protect them against corrosion and to reduce the friction loss. There are a number of different coating techniques. In the past, pipelines were coated with a specialized coal tar enamel. Today, pipes are often protected with what is known as a fusion bond epoxy, which gives the pipe a noticeable light blue colour.
In addition, as the coating is never totally perfect and will deteriorate with time, cathodic
protection is often used. This is a technique of running an electric current through the pipe to
ward off corrosion and rusting.
Valves
International pipelines include a great number of valves along their entire length. These valves work like gateways; they are usually open and allow the product to flow freely, or they can be used to stop the flow along a certain section of pipe. There are many reasons why a pipeline may need to restrict flow in certain areas. For example, if a section of pipe requires replacement or maintenance, valves on either end of that section of pipe can be closed to allow engineers and work crews safe access. Some valves are equipped with actuator that can be triggered by a sudden drop of pressure resulting from a major leak. These large valves can be placed every 10 to 30 km along the pipeline, and are subject to regulation by safety codes.
Compressor and Pumping Stations
To ensure that the fluid flowing through any one pipeline remains pressurized, compression
(for gas) or pumping (for liquids) is required periodically along the pipe. This is
accomplished by stations, usually placed at 40 to 100 km intervals along the pipeline.
On gas pipelines, turbine compressors gain their energy by using up a small proportion of the
natural gas that they compress. The turbine itself serves to operate a centrifugal compressor
that compresses and pumps the natural gas through the pipeline. Some compressor stations are
operated by using an electric motor to turn the same type of centrifugal compressor/pump.
This type of compression does not require the use of any of the natural gas from the pipe,
however it does require a reliable source of electricity nearby. Reciprocating natural gas
engines are also used to power some compressor stations. On gas pipelines, in addition to compressing natural gas, compressor stations also usually contain some type of liquid separator, much like the ones used to dehydrate natural gas during its processing. Usually, these separators consist of scrubbers and filters that capture any liquids or other undesirable particles (rust for instance) from the natural gas in the pipeline. Although natural gas in pipelines is considered ‘dry’ gas, it is not uncommon for a certain amount of water and hydrocarbons to condense out of the gas stream while in transit. The liquid separators at compressor stations ensure that the natural gas in the pipeline is as pure as possible, and usually filters the gas prior to compression.
Metering Stations
Metering stations are placed at take offs and at the boundary between states. These stations
allow pipeline companies to monitor and manage the products in their pipes. Essentially, these
metering stations measure the flow of product along the pipeline, and allow pipeline
companies to ‘track’ the product as it flows along the pipeline. These metering stations
employ specialized flow meters to measure the product as it flows through the pipeline,
without impeding its movement. Metering stations may be controlled by the state authorities for fiscal duties purpose.
What to look for?
This section describes briefly each pipeline construction step and for each one gives
indications for its inspection.
Design
As for any other inspection, the inspector uses the approved design and specifications as a
reference for his inspection. Non conformities may only be raised against these requirements.
Before starting any inspection it is important to examine carefully all the requirements of the
design and verify that the contractor uses the last revision of these documents.
Among the many drawings issued for a pipeline construction, a special attention shall be
brought to the typical drawings.
The yards
Pipeline construction starts from the yards. This is where the construction equipment and the
pipes arrive, are stored and sometimes prefabricated and from where they are distributed.
Pre-construction survey
Before construction begins, surveyors check for environmental features along proposed
pipeline segments. Utility lines and agricultural drainages are located and marked to prevent
accidental damage during pipeline construction. This is done not only on the pipeline route
but also on all the site accesses and any other working area.
Surveying and stacking the right-of-way (ROW)
The right-of-way is a narrow strip of land that contains the pipeline(s) and is where all
construction activities occur. Prior to any construction, it is surveyed, cleared of brush and
trees, and levelled to give workers and equipment access to build, inspect and maintain the
pipeline. The route is surveyed and the proposed centreline staked. The outer boundaries of the
construction corridor are staked also (stacks have different colours, typically with a 50m
spacing). The proposed centreline of the pipeline is not in the centre of the construction right-
of-way, but offset to one side. The overburden (excavated material) will be placed on the narrow side of the construction corridor. On the wider side, there is room for two vehicles to pass and a work area for laying and welding the pipe. Depending the type of terrain crossed by the pipeline, the ROW does not have the same width along all the route. The width also largely depends on the diameter of the pipe, as this diameter conditions the size of the construction equipment. Typical widths are from 6m for a 3” pipeline to 20m for a 24” line. At the end of the construction, a narrow band of the ROW may be kept for the maintenance of the pipeline. This is generally not the case in valuable agricultural land.
Right-of-way Preparation
Temporary Fence installation
A fencing crew follows the staked centreline of the pipeline, and installs temporary gates and
fencing where the right-of-way crosses a landowner’s fence. Prior to cutting a fence, it should
be braced at the boundaries of the construction corridor for a minimum width that would
allow construction equipment to pass prior. The bracing allows a fence to be cut and still
maintain integrity of the overall fence. Temporary gates are installed across the width of the
construction right-of-way to allow the ditch to be excavated, as well as provide room for the
pipe and construction equipment to pass.
Timber Clearing
The right-of-way crew clears the right-of-way of all shrubs and trees. Smaller timber are
properly disposed of or cut and stacked on the right-of-way for use by the landowner for
firewood, if appropriate. Special attention is brought to merchantable timber if this has not
been taken care of by the landowner prior the construction begins.
Clearing and grading
After temporary fencing and timber clearing have been accomplished, a crew removes
stumps, shrubs, topsoil and small trees. When the work is done along a hillside, the topsoil
should normally be placed on the uphill side to prevent mixing with other excavated material
during later stages of construction. The right-of-way is then be levelled to allow construction
equipment room to work. In areas along the sides of hills (“side hilling”), two levels may be
necessary. One level would contain the ditch and material removed from it. The second level
would accommodate the pipe fabrication area, as well as construction equipment and passing
lanes. This technique reduces the amount of material that must be displaced during the
temporary construction phase of the work. Access points to the site require a special attention as a lot of heavy equipment will use this passage. Damage to the public road network should be avoided as well, many small country roads or tracks being not used to see so much traffic.
Trenching
After the construction zone is cleared and levelled, trenching machines begin digging the
trench where the pipeline will be buried. In agricultural areas and before trenching, the
topsoil is removed from the work area and stockpiled separately. This is generally part of the
ROW preparation. Materials removed from the ditch would normally be placed adjacent to
the topsoil pile or on the opposite side of the ROW, depending the availability of space.
The depth of the trench can vary, the minimum cover being defined by the regulation (for
hydrocarbon pipelines at least) or the owner’s specifications. The total depth of a trench
depends on:
– the diameter of the pipe,
– the minimum cover required by the regulation (typically 0.8m),
– additional cover required in special locations (road, river, railway crossings) to protect
the pipeline,
– increased depth to accommodate soft backfill before the pipe is laid (in rocky soil),
– to decrease slope on top of hills.
Pipeline layouts will define for each location the minimum design depth of the trench.
The most effective and economical solution is to use a trenching machine. A trenching
machine is capable of cutting through all types of soils except areas that have very large
boulders or rocks. It cuts the trench and side casts the soil. Track-mounted excavators are normally be used to dig the ditch in hilly or mountainous terrain. Extra ditch depth is dug to ease the transition of the pipeline at the bottoms and tops of hills, at water crossings, road crossings and railroad crossings. This requires additional temporary right-of-way width to accommodate the extra material excavated for the ditch. Trenching machines equipped with special blades are also capable of trenching through soft rock. Other equipment called rock saws can cut through rock to the desired depth. Only in rare occasions would blasting be used to trench through rock. If blasting is required, the charges are shaped to limit the amount of outward explosion. To limit the amount of debris spread, heavy mats can be placed over the charges. Special crushers are used to transform the rocks in coarse sand that can be used to prepare the bottom of the trench and the backfill material in contact with the pipe. продолжение
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