Monday, November 16, 2009

Design of Cross – Country Pipe Line - 1

Introduction:

In the modern age of Industrial word, the Oil refineries, petroleum products, and petrochemicals form the major part of the industrial set-up all over the world. It is often economical and practical to carry the liquid and Gaseous products through pipe-lines rather than by Tankers over ling distance. When a pipe-line has to carry such products like crude oil, refined oil, chemicals like naphtha, ethylene, propylene etc. over long distance ranging from 10 km to even 1000 km,. Passing through land, rivers, sea, mountains, marshy areas, private and public land and land, rivers, sea, mountains, marshy areas, private and public land and crossing other services like roads, railways, transmission lines, underground Pipes/Cables etc, such a pipeline is called “Cross-Country Pipe-Line”. As the name suggests it transfers the liquid/Gas products from one place to another at far distance.

Engineering and Installation of Cross-Country Pipe-Lines form a special branch of piping
design and engineering., as it involves many aspects and parameter which are normally
faced with, in inplant piping system within the boundaries of refinery or a chemical /
petrochemical plant Special Techniques have to be adopted for design, laying
welding/jointing, corrosion protection, testing, commissioning etc. The most common line
familiar to all, is water-line from reservoirs to different consumption points, like, water-main from vaiterna/ Tansa dam to city of Mumbai Unlike water line, the hazardous chemical conveying Pipelines, involves many more stringent precautions in their design and installation. This is mainly due to fire and explosion hazards associated with the oils and chemicals.

This write-up highlights the main features of the eng ineering and construction of Cross-
Country Pipe-Lines. The objective of this note is to make the reader familiar with the broad perspective of the Cross-Country Pipe-Line Work and the pipe-lines which are already installed in India. These also include the submarine lines and en-land pipe-lines.

Advantages of Cross-Country, Pipe-Lines over transport by Roads / Railways / Waterways

The most common modes of transport known to all include Trucks running over Roads,
Railway goods train and Ships/Launches/Boats/Barges on waterways. The transport by Airway be cargo Air-crafts is also another way of bulk-transport. These modes of Transport have following limitations.

a) Availability of sufficient roads, rail-tracks and port-harbour facilities to take
up the traffic load.

b) Condition of the tracks.

c) Hurdles and conditions of the vehicles

d) Maintenance and conditions of the vehicles

e) Procedures and control involved in the Transport operation (permits/ licenses/octroi/toll/RTO etc.)

f) Manpower to run and maintain the transport system

g) Availability of fuel and power required to run the system.

h) Effect of Nature on the system like rains, storms, earthquakes, thundering, mist etc.

i) Pollution generated by the transporting vehicles.

k) Safety, insurance and security of the transported goods and materials.

l) Time taken for transportation and delays

m) Overall efficiency of the system.

While transportation by roads, railways, water and air-ways is widely used all over the world, it has its own limitations due to the features used all over the world, it has its own limitations due to the features (a) to (m) mentioned above. These limitations especially restrict or forbid their use when large quantities of Oil, petroleum, water, chemicals are to be continuously supplied from the source to the consumption point at users’ end. Hence the most reliable and efficient system can be provided only be Cross-Country pipe-lines.

The advantages are as given below :

a) Continuous un-interrupted transport is ensured.

b) No dependence on availability of roads, railways, bridges etc.

c) Least manpower requirement to operate the transport system except for inspection and maintenance of minimum required level.

d) No hindrances on way due to any reasons which are listed in problems (a) to (m) for surface transport, air/water ways.

e) Possibility of crossing any odd areas like seas, oceans, rivers, mountains and underground space.

f) Safety & purity of the product is ensured. The product reaches exactly in the same condition from source to the supply point, with minimal loss of quality or quantity.

g) Once laid down, the system works automatically especially with the help of modern instrumentation, safety devices, interlocks, communication system and remote control devices.

h) Minimum or no tampering on the way.

h) Cost of Transport per Unit of the product conveyed is far less than the transport by
Trucks/railways/ water/ Airways.

j) Fastest mode of transport even between two countries or continents.

k) Comparatively much less hazardous than surface transport & minimum dependence on human factors.

There are of course certain disadvantages but they are offset by the advantages, to a large extent, so as to make them ignorable as far as safety & techno-economic aspects are concerned. They are listed as given below :

a) Right of way Acquisition to run the pipeline, especially thru’ private & agricultural land
& habituated areas.

b) High fire & explosion Hazards potential.

c) Problem of corrosion & leakages & repair work involved.

d) Daily on-route inspection, testing & quick arrangements for attending to repairs and rectification work.

e) Possibility of laying other services in future (like other pipe-lines due to ignorance of
its existence, among other agencies) causing damage.

f) Special Techniques & Agencies are required to design, engineer, install & operate the pipe- line system.

g) Expensive cathodic protection required for the protection of u/g lines running in close
proximity of overhead High Tension electrical Transmission lines which induce the currents in the metallic pipelines, causing the corrosion by stray-currents.

The modern techniques are well developed to offset the effects of the above disadvantages. Even if a line has to shut-off for a day or two, the storage facilities at the users end take care of such stoppages even for 15 days to 1 month.

Preliminary work for A Cross-Country Pipe-Line Project :

The following necessary work on planning & collection of information/data is required to be for preproject activities, once it is decided to install a Cross-Country Pipe-Line.

Data on the Product to be carried :

- Name, Qty/day, properties of the product

- Source of supply & location details

- Names & location of consumers.

- Qty/day to be supplied to each consumer

- Storage facilities at suppliers’ end & consumer end

- Pumping facilities at Suppliers’ end

- Unloading facilities at receivers’ end

- Safety requirements for the product

- Risk& Hazards associated with product.

- Interruptions in supply at suppliers’ end & at receiving end.

Route Survey & Analysis

There may be many alternatives for routing the pipe line from supplier to the consumer. It is necessary to study the techno-economic comparison of the alternative routes. This survey includes the following activities :

a) Spot-level survey at every 50 to 100 metres & at least over 10 m on either side of the probable route.

b) Soil Conditions in the form of bore-logs, trial pits, chemical tests on subsoil & ground
water etc.

c) Alignment Map With lengths, bearings, angles etc. to know the exact route & the total
length of the pipe-line.

d) Details on the route and their locations dimensions etc sea, roads (crossing and along
side the route) rivers, Nallas, pipe-lines, bridges, rail-tracks, transmission lines, underground services including cables/pipes etc, Hills and mountains, buildings, plantation, forests, agricultural land etc.

e) Cadestral Survey –The route may be passing thru’ so many lands belonging to private
owners, farmers, govt. authorities, defence wings etc. En-route information and data has to be collected for such land pieces. Such data will include :

- Type of land and the owner’s name

- Length of the route thru’ the land.

- Problems in acquiring Right of Way (R.O.W.)

- Authority which will permit/grant Row

- Survey maps for the land available from the local Land Authorities (such as collector,
Tahasildar, Gram-Panchayat etc.)

- Land records regarding the title and ownership of the land

- Approx compensation required for acquiring the R.O.W.

- Status of Habitation on the land.

- Similar information of the adjacent plots on 50 to 100 m on either side of the route.

- Plans for future installations by others on the proposed route and/ or in the vicinity such as roads/ rail-tracks/ buildings/pipe-lines etc.

f) Availability of construction Materials, Labour & facilities

Since the pipe-line has to pass thru’ different areas and over a long distance, it is essential
to know the availability of construction Labour and Materials on the way. Such as excavation labour, transport facilities, access roads, construction material like stones, aggregates, sand, cement, steel, structurals, etc., workshop facilities. This information will be useful in working out project schedule and cost estimates and assessing the problems in construction.

g) Soil Resistivity Survey – required for design of cathodic protection system.

Names and addresses of the statutory and public bodies required to be contacted for
acquiring ROW, construction permission, blasting licences, excavating the public facilities
(Roads, rivers, rail-tracks etc.) and cathodic protection work, power supply/water supply etc.

Such authorities include the following but not limited to the listed ones.

  • Local land authorities – distr. Collector, Municipal corporation, Tahsildars, D.I.L.R. etc. Owners of the respective Land.
  • P.W.D. authorities – Local Office
  • Irrigation Dept.
  • Electricity supply Agencies/bodies/Boards.
  • Water-supply and Public Health Dept.
  • Controller of Explosives and use of Hazardous chemicals.
  • Industrial Development corporations
  • Railway Authorities
  • Marine and Port Authorities
  • Salt-commissioner and controller
  • Competent Authorities for Land and Row acquisition.
  • State and Central Govt. for necessary permissions, licences, clearances etc.
  • Import/export rules/ regulations authorities
  • Controller of Quarrying and Mining
  • Navy/Army/Air force (Defence Authorities)
  • Plants for future installations.
  • Forest authorities

Project Schedule

Base on various data collected as in 3.1 and the cost Estimates, over all project schedule
has to be prepared based on past experience, and specific problems unique to the project
under consideration. This schedule should cover only broad activities to serve as a guide
line for preparation of detail activity schedule.

This should generally include :

a) Preliminary Survey / Data Collection

b) Finalising the route

c) Cost Estimates / budget sanctions

d) Acquisition of R.O.W. and land

e) Basic Engineering package

f) Detail Engineering work

d) Construction work (Civil/Mech./Piping/Electr, Marine crossing, river crossing etc / cathodic protection)

h) Testing/Flushing/Pigging.

j) Commissioning and Hand over

This will establish the overall completion time for the entire project work.

Finalising the most optimum route

This involves the comparison of alternative router surveyed as in 3.1. The analysis should
include various parameters which are tabulated in the following format :

Under parameter columns, following minimum items should be included :

1. Estimated Cost

- Row Acquisition

- Land Acquisition

- Statutory Permission

- Basic Engineering

- Detail Engineering

- Material Procurement (pipes/Valves/Equipments)

- Construction Cost

- Civil

- Piping

- Mechanical

- Electrical

- Cathodic Protection

- On line buildings

- Marine / River Crossings

- Testing Commissioning

- Cathodic Protection

2. Overall completion – Time

3. Total length

4. Cost per km.

5. Other features

- Rock area (L)

- Marine Zone (L)

- No. of Road Crossings

- No. of Railway Crossings

- No. of Nalla Crossings

- Underground portion lengths

- Above ground portion lengths

- No. of Isolation Valves

- Pipe-line dia.

- No. of rectifier stations.

- No. of Diode Stations

6. Cost of operation / year

7. Cost of Maintenance / year

8. Hazard Classifications

9. Risk-Factor

10. Disaster Management Category

11. Stoppages / shut down due to ext. factors

12. Threat to Security and Safety etc.

Value analysis should be done for each alternative routes considering appropriate weightages assigned to these parameters and costs of the same. Thus final and most
optimum route can be selected.

Salient Steps in Detail Engineering

After deciding the final route, cost estimates, broad project schedule and engineering. The
detail engineering will involve following main steps.

Detail Design of each system

§ Civil works including trenching, sand filling, back filling, buildings, concreting, river-weights, valve-chambers, Test points, markers and construction infrastructure like site office, construction water, power, site godown/open yards etc.

§ Construction Equipment required for transport, laying, welding, erection testing etc.

§ Piping : Stringing/ Welding/ Laying/ Testing pipe support system

§ Catodic protection system design, diode stations, sacrificial anodes, UPSinstallations,
on-line test-points, insulation flanges

§ Specific designs for submarine portions and river-crossings

§ Designs of all crossings, pipe-bridges, supports

§ Preparing Detail Design and Fabrication Drawings for all Systems

§ Quantity calculation for materials and work items.

Implementation Planning and Organising

§ Selection and appointing Agencies/Contractors/Suppliers for various activities and
materials

§ Division of work among the staff on the project.

§ Progress monitoring and reporting system

§ Mobilising planning (manpower deployment planning) (Resource-planning)

§ Implementation work packages

§ Payment to subcontractor system

§ Inventory-control-planning

§ Safety/Security Guidelines

Organising Revisions/Change/alternatives/improvements in system design/drawing
during the project-process.

Preparation of As-Built construction drawings and final costing.

Data-Bank for the executed project, useful for future project.

Salient Features of Construction

Trenching : See. fig. 1

Generally Cross-Country Pipe-Lines are laid underground in an excavated Trench while
crossing the land-areas. Minimum depth of the Trench should be Trench while crossing the land-areas. Minimum depth of the Trench should be (1 M + Pipe dia + 150 mm). 1 M – is the depth of overburden i.e. back-filled soil, and 150mm is the thickness of sand cushion to be laid before lowering the pipe in the trench. Width of the trench is general minimum 1 M or as required by higher dia. Pipes. Thus width should be (Dia. Of Pipe + 0.4 M on either side) or 1 metre whichever is higher.

Minimum Trench Dimensions

Pipe-Preparation in Yard

- Inspection/Testing/Stacking of Pipes/Numbering

- Edge-Preparation for welding

- Wrapping/Coating (generally reinf. bitiminous) and its testing

- Testing/Stacking bends/elbows/Tees

- Pipe-Sleeves for road crossing

- Valve-testing/stacking/numbering

- Other accessories like blinds/spectacle blinds, gaskets, bolts, nuts, washers etc.

- Selecting/ Stacking welding machine/electrodes etc.

Stringing at Site and Welding

After trenching is ready over substantial length pipe-lines made ready in the yard as in 5.2
are transported to the site and lined up over sleepers placed across the trench for welding
and lined up over sleepers placed across the trench for welding the joints. The joints are welded continuously in 2 or 3 shifts. They are subject to inspection by D.P. check and Radiography. Wrapping/ coating is completed over portions about the weld.

Lowering

Once a fairly long length say 100 m to 150 m is welded/Tested, then if is lowered into trench over sand-bed already laid-Necessary small/big cranes, lifting tackles are used for lowering the line. Back filling with soft earth free from stones is done after lowering.

Hydrotesting

A long length after lowering a back filling is hydrotested for the test-pressure which is
generally 1.5 times the operation pressure or as stipulated for specific service.

Overall Total Welding

After each 100 to 150 m length is lowered, tested, then they have to be welded to form a continuous pipeline.

Testing of entire line is then taken up by filling the whole or section of line with water &
pressuring. Any leaks found are repaired and tested.

Pigging


For flushing and cleaning the entire length of all muck, dirt, welding rod bits etc, a pig is
passed thru the line, from one end, and it is pushed by water pressure. The pig travels
through the pipe, scrapping the muck and pushing it forward. At intermediate points flanged joints are left to pass-out the muck. If a pig gets stuckup, its location is detected by passing an ‘ISOTOPE’ and detecting its location by external instruments which tracks the isotope as it is travelling through the pipe. The pipe line is cut, pig removed, pipe cleaned and rewelded. The pig is passed through from that point onwards to flush the remaining portion in the forward direction.

Commissioning

It is done as per the procedure laid down for the specific product to be carried through the pipeline.

Cathodic Protection

This provides the protection to the underground pipe from the corrosion by electrolytic
process in subsoil water, whenever in e-m-f is induced’ in it (when pipe material is a good
conductor e.g. carbon. Steel)

Basic Principal and Phenomena

H.T. overhead Transmission lines conduct A.C. current under very high voltages of the order of 11 kv to 33000 kv and more. Due to fluctuations in voltage, magnetic field around the conductors also changes continuously. Any conductor in the magnetic field, thus cuts the magnetic flux and e-m-f is induced in it. (Ref. Fig). 2 below)

Induced E.M.F Phenomena

u/g pipeline of metal (viz. carbon steel) is a conductor of electricity. If it lies within the
magnetic field of electricity. If is lies within the magnetic field of the O.H. lines, then it
developes a potential higher than the ‘Ground’ potential. As we know, earth i.e. ground is at ‘Zero’ potential. When the u/g pipe is subjected to an induced e.m.f. if is supposed to be higher potential than the surrounding ground. The subsoil water always contains many
dissolved salts of sodium, potassium and other elements. This makes the subsoil also a
conductor of electricity. Thus the current flows from the pipe at flow of current depends on the resistivity of the subsoil. This phenomenon sets up an ‘Electrolytic’ process between the

pipe which acts as ‘ANODE’ and the ground which acts as ‘CATHODE’. Once this process
starts, pipe starts losing the positively charges ions say Fe++ or Fe++ into the subsoil
around the pipe. This is the corrosion process by which the pipe. This is the corrosion
process by which the pipe gives up its material & develops a hole or a reduction in
thickness.

Prevention of Effects of Induced EMF

To prevent this phenomenon, it is necessary to prevent the current from ‘PIPE’ to
“GROUND’. This is not possible, but it is possible to reverse the flow i.e. from ‘GROUND’ to ‘PIPE’. In other words pipe must act as ‘CATHODE’ and ground should act as ‘ANODE’.
This type of system is called ‘CATHODIC PROTHECTION’. In this case (ref. Fig. 3), the
current flows from ‘ground’ to ‘pipe’ and the pipe is said to be at –ve potential since the
ground has zero potential. The fig.3 is self explanatory. Applying a voltage by means of a battery set or D.C. current (rectified from A.C. supply or from D.G.) to the ‘anodes’ inserted in the ground surrounding the pipe and at regular regular intervals along the pipe This system, is maintained to see that the pipe always acts as ‘cathode’ then there are no chances of corrosion of pipe.

Current Flow from Ground to Pipe

3 – [Current Flows From Ground to pipe]

Diode Station in the Vicinity of Rail-Tracks with Electic Traction

The system described in 5.10.2, is often disturbed due to presence of other sources of
electric conductors in addition to the O. H. transmission lines, (e.g. Electric Traction). In this case, as shown in fig.4 stray currents flow from rails (which act as the path of return current) to the surrounding ground. This phenomenon, causes ground at-ve potential and quite often at lower potential than the pipe. This causes the flow of current from pipe to ground and the corrosion can take place. By supplying the current to a diode introduced in the circuit joining the rail to the pipe in such magnitude that the surrounding ground will conduct the current from ground to pipe,

Diode - Prevents the flow of current from pipe to ground

but not in the reverse way as diode acts like a ‘NON-RETURN’ valve. The calculations of
diode capacity, voltage to be applied etc. have to be made based on the survey conducted on fluctuations in the Traction field voltage. This arrangement prevents the pipe working as anode, but maintains it at cathodic level and thus prevents the corrosion.

Sacrificial Anode

In addition to the impressed D.C. current as in 5.10.2, sacrificial anodes are introduced into the ground, which maintain the pipe at lower potential than the ground, in case the D.C. supply fails for some reason. These anoces are made of Metals which are ‘NOBLET’ than the pipe materials. By this, it is meant that the metal which electrons surrounding or adjacent metals in contact is called a ‘NOBLE’ metal e.g. Mangessium (REF. FIG. 5)

Fig-5 Presence of Mg-Anode keep ground water at +ve potential water pipe

This makes the GR. WATER at + ve potential with respect to the pipe. Therefore the current flows from magnesium to the pipe. In this case magnesium Anodes slowly lose its own metal to the surrounding and in course of time dis This is why it is called ‘SACRIFICIAL’ anode periodically they have to be replaced by new anodes, say every 3 to 4 years.

Normal Subsoil Corrosion

Ever of there is no presence of Electric O.H. lines, or ‘Traction’ lines, any conducting metal
buried in the ground gets corroded by the similar phenomena. The subsoil water which itself is a ‘solution’ of so many salts, contains +ve and –ve ions. The presence of metal conductor such as ‘pipe causes movements of there ions and often the current from pipe to ground. This causes corrosion of the pipes which get electrostatically charged due to friction between pipe and the fluid flowing through it. Hence the sacrificial Anodes are required to be provided (Ref. fig. 6)

Fig-6 Normal Corrosion

On-Line installations

a) Test Points: Once the system of applying impressed current into ground to keep the
pipe at –ve potential, it is necessary to check the potential difference between the pipe and the ground at regular intervals, say at every 500 m. ‘Test points’ are installed close to the pipe. (se fig. 7) T.P. Box indicating voltage between ground and the pipe. Ground should be at least 1.5 v above pipe potential Volt-meter is carried by the inspector and the voltage between Terminals inside the T.P. Box is checked and recorded.

Fig-7 Test-Points at 500m

 

b) Insulating Flanges : If any portion of the pipe is above ground, then the same has to
be ‘Electrically Isolated’ from the under ground portion. This is required so as to prevent the flow of any other currents from sources outside and also the path of least resistance which the current may find through above ground pipe resting on steel or metal supports. (see fig. 8 on next page)

 Fig-8 Schematic Detials of Insulating Flange

All installations and systems described in section 5.10.1 to 5.10.6 from the Total System of Cathodic Protection.

Additional Information on Cathodic Protection (C.P.)

1.Necessity of cathodic protection is established on following criteria

a) Type of soil with its constituents like PH value, contents of chlorides and sulphates.

b) Soil resistivity which determines corrosion level.

c) Importance of line(s) to be protected.

d) Study of stray currents i.e. induction from EHV/HV lines, rail lines.

e) Dis-similar metals structures in the vicinity

f) Life of object to be protected e.g. 30/50 years

2.Two methods of C.P.

a) Sacrificial

Sacrificial is adopted for less important object & remotely located objects where electric
supply is not available early. Zinc, Aluminium & Magnesium are used as anode material.

b) Impressed current

Impressed current system is used for important objects and is dependent on electric power supply. Hi-silicon coast iron graphite are normally used as anode material for impressed current system.

3. Impressed current system comprises the following major equipment and accessories

a) Transformer Rectifier Unit

b) Anodes with tail cables

c) Anode junction boxes

d) Reference cell/electrode

e) Backfill material

f) Cables.

4. Criteria of selecting anodes and electrical equipment for buried pipelines.

Generally pipe lines which are buried, are buried, are coated and wrapped. This brings down the current level (and potential level) which is required to be provided. Generally 10 mA/sq. meter current criteria is used. For Pipe lines which are in sea water higher current upto 110 mA/sq. meter are used.

5. Following Data is necessary for engineering of C.P.

a) Dia meter of Pipe-line(s)

b) Length of buried pipeline

c) Material of construction

d) Type of coating.

6. Monitoring/maintenance of C.B. System

a) Potentials are measured on frequent basis (or daily)

b) Maintenance of electrical equip on periodic basis.

7. When construction period is long, during such period temporary cathodic protection has to be provided, until the permanent C.P. system is ready and commissioned.

Schematic Layout of A Typical Cross-Country Pipe-line 

7. Major Cross – Country Pipe Line in India

Major Cross – Country Pipe Line in India

NOTE: No. of crude oil lines (Submarine and Land) are in use for last several years, running between Gulf – Countries.

Continued to Design of Cross – Country Pipe Line - 2

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4 comments :

  1. Abdullah ElmahdiApril 6, 2012 at 3:08 AM

    for cross country pipelines how the break up for the design is done? I mean say for a 1000 km pipeline, do we design CP system as a whole or break up the length in smaller parts and do the design.

    ReplyDelete
  2. Basically the CP system design & requirements are calculated by CP engineers and it depends upon pipe thickness, driving voltage, coating on pipe and soil resistivity. The Numbers of CP Rectifiers over the length of Pipeline depends on above requirements.

    ReplyDelete
  3.  Salam Abdullah!!!

    Basically the CP system design & requirements are calculated by CP
    engineers and it depends upon pipe thickness, driving voltage, coating
    on pipe and soil resistivity. The Numbers of CP Rectifiers over the
    length of Pipeline depends on above requirements.

    ReplyDelete

 

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