Orhan Degermenci
Author: Orhan Degermenci
Dr. Orhan Degermenci (BSc., M.E. and PhD in Petroleum Engineering from the Technical Univeristy of Clausthal in Germany) with his full-time of 28+ years job experience is involved as both lead and specialist engineer in engineering design (FEED & DD & EPC & PMC and O&M Projects) of numerous oil+gas+water+refinery product piping & pipelines.
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This article gives an overview on design engineering aspects of both offshore and onshore pipelines used for oil and gas transportation. It takes into account the international codes and standards such as ANSI/ASME B31.4 and ANSI/ASME B31.8.

Introduction

Design engineering of pipelines covers mainly the following issues: Risk assessment dealing with the safety aspects, environmental impacts, and economic issues. Pipeline routing taking into account the route selection and surveys, location classes, proximity to the foreign objects or obstacles, and pipe laying technologies. Pipe strength calculations considering different factors such as; the design factors, steel qualities, wall thicknesses, pipe stresses, and safety against external collapse (offshore). Crossing which applies methods of open-cut and directional drilling at roads, railways, rivers and waterways. Burial and protection of pipelines, which is considering minimum pipe cover, and pipe markers. Pipeline stability realized by means of concrete weight coatings, special anchor points, trenching and burials. Corrosion protection and corrosion monitoring of pipelines, which considers external corrosion (avoided by anticorrosion pipe coatings, cathodic protection CP, and insulating joints), internal corrosion (prevented by linings, and inhibitors), CP measurements, intelligent pigging, control of process parameters, corrosion probes, corrosion coupons, and debris analysis. Pipeline pigging which deals with the piggability measures, pig traps, and utilization of intelligent pigs. Pipeline sectionalizing and emergency shutdown using block valves and actuators, emergency shutdown valves, leak detection, and blow down systems. Overpressure protection of pipelines, which takes into account the Maximum Allowable Operating Pressure, surge pressure, and thermal effects. Telecommunication and SCADA system; used for supervisory control and data acquisition of pipeline during pipeline operation. Material procurement of line pipes, fittings and special components, anticorrosion coatings, and concrete coatings. Construction defining construction procedures and methods, hydrostatic pressure testing, records, and health, safety, environmental protection measures. Pre- Commissioning carried out by means of cleaning and dewatering of pipelines.

Pipeline Engineering

General

A pipeline (Fig. 1) is defined as a system of pipes for transportation of fluids in the liquid or gaseous phase (or a combination of both phases), between wellhead facilities, production plants, pressure boosting stations, processing or treatment plants or storage facilities (not included in the pipeline scope). Pipeline extents from pig trap to pig trap including pig traps and associated pipe work and valves.

Pipeline Scope Boundaries

International Codes and Standards

The first design and engineering issue of pipelines considers the definition of applicable basic codes and standards. Many countries have their own codes and standards explaining how to design and engineer the oil and gas pipelines. However in international oil and gas transportation projects the pipelines are being designed, engineered, constructed and operated usually in accordance with those standards which are issued together by the American National Standardization Institute (ANSI) and the American Society of Mechanical Engineers (ASME). According to these standards, the fluids transported trough the pipelines are categorized generally into four (4) groups, depending on their hazard potentials. Namely; categories A, B, C and D. Crude oil belongs to the Category B fluids as being flammable, toxic and unstable, whereas the natural gas is part of the Category D fluids as being also flammable, toxic and unstable. Pipelines carrying fluids (liquids) of categories A and B are designed, engineered, constructed and operated in accordance with code ANSI/ASME B31.4, whereas the pipelines carrying fluids (gases) of categories C and D are being designed, engineered, constructed and operated in accordance with standard ANSI/ASME B31.8.

Material Selection

Another pipeline design engineering activity covers the pipe material selection, whereby its main parameter is governed by the corrosiveness of the transport fluids. Under sweet corrosive conditions (that means the expected corrosion rate is less than 0.5 mm/year) line pipes of carbon steel material are used. In the international pipeline industry the basic standard for the steel quality selection is the API SPEC 5L. According to this standard the line pipe steel materials are governed commonly by the range of the steel grades X42 (low), X52 (medium) and X65 (high). The higher grades of carbon steel line pipe materials such as X70 and X80 are creating weldability problems and therefore are not being used commonly for oil and gas pipelines.

Risk Assessment

One of the main concerns of the pipeline engineering is related to risk exposure on the pipelines. Thereby, it has to be analyzed on quantitative basis during pipeline design and engineering phase. Risk is defined as a product of expected failure frequency and associated consequence depending on transport fluid (in terms of its flammability, stability, toxicity and polluting effect) and pipeline location (in terms of its ignition sources, population density, proximity to occupied buildings and climatic conditions). There are usually three types of pipeline risks considered during pipeline design, namely; the risk of safety to the people (Quantitative Risk Assessment), the risk of damage to the environment (Environmental Impact Assessment), and the risk of income loss (Economical Risk Assessment). The pipeline failure sources are originating from corrosion, mechanical impact or external third party interferences, fatigue, hydrodynamic sources, geotechnical forces, material defects and thermal expansion forces. The risk analyses is targeted to minimize potential risks as much as practicable and preventing leaks. The risk reducing factors are; applying lower design factor, using higher line pipe wall thickness or stronger steel, rerouting, additional protection to pipe and installation of line block valves for minimization of any released fluid volumes, controlled methods of operation, frequenter maintenance and inspection of pipelines. The Quantitative Risk Assessment is targeted to optimize both the selected design factor and the proximity distances to foreign objects within the route corridor of pipeline. The Environmental Impact Assessment aims at identification of possible impacts of pipeline on the environment, determination of significance of these impacts, to eliminate or minimize pipeline impacts, to choose the adequate design, construction method, and reinstatement as well as pipeline operation philosophy. The Economic Risk Assessment considers the deferment of income, cost of repair and other costs such as liabilities to the public and clean-up costs for case of pipeline emergency or failure situations.

Pipeline Routing

The pipeline routing is another main design issue, which is carried out in such a manner that safety and environmental risks alongside the pipeline are minimized, pipeline access during maintenance and inspection phase is guaranteed, and pipeline construction costs are as low as possible. The basis for detailed route selection is given by the survey data collected within possible defined route corridor. These survey data covers the information on population and building densities, location of inhabited buildings, topographical data showing location of rivers. Furthermore, it comprises information on roads and railways in case of land pipelines, and seabed conditions as well as rocks or corals and data on fishing and shipping activities in case of subsea pipelines. Soil investigations for foundation design (burial and supports), location of subsidence areas such as mining activities, soil resistivity measurements for cathodic protection design, and environmental data are also part of pipeline survey data. Depending on the selected route the location classes along the pipeline route are determined in order to define the pipeline design factors (Table 1 and 2). According to the above mentioned standards the following routing parameters are considered for route selection covering pipeline installation techniques (Table 3 and 4):

 Design factors for steel pipe construction

Design factors for onshore steel pipelines

Pipeline routing parameters depending on line pipe nominal diameter

Pipeline to Building proximity distances

Design and Maximum Operating Pressure

Most important pipeline design activity is to calculate the pipeline design and maximum operating pressure based on the pipeline strength as specified by the pipeline design engineer. As a first step, the line pipe steel quality is chosen as described before. The main criteria to calculate the line pipe minimum wall thickness, which guarantees good construction conditions (without collapse) is defined by the term of D/t ≤ 96 for onshore pipelines and D/t ≤ 60 for offshore pipelines. For the simple case of straight line pipe, which is staying under internal pressure of the transport fluid, the Specified Minimum Yield Strength (SMYS) of a pipe is calculated under the given operation conditions as follows:

SMYS = (Pi · D) / (2 · t)   ..........................(1)

Where, Pi , D , t are internal pressure, external pipe diameter and calculated pipe wall thickness, respectively. The pipeline design pressure (Pd) is derived from this formula as follows:

Pd = ((2 · SMYS · t) / D) · E · F · T  .........................(2)

Where E, F, T are longitudinal joint factor, pipeline design factor and temperature derating factor, respectively. The pressure calculation factors E (1.0 - 0.60 acc. to pipe welding method), F (0.80 - 0.40 according to location classes) and T (1.0 - 0.867 according to pipeline temperature) are shown in Table 5 and 6. The maximum (steady state) operating pressure of the pipeline shall never exceed the pipeline design pressure.

Longitudinal joint factors for steel pipe

Temperature derating factors for steel pipe

Crossings

Another aspect of pipeline design engineering is technical planning of pipeline crossings at different obstacles, which might be located along the line route such as railroads, highways, waterways and rivers. The basic design requirements of pipeline crossings with railroads and highways are described in standard API RP 1102. In case of long pipeline crossings with rivers and waterways the directional drilling is suitable particularly. Main achievements of this method are large burial depths and being insensitive to current and water traffic. The minimum cover of pipeline in case of crossings with public roads and railways is 1.5 m for normal grounds and 1.2 m for rocky grounds. The minimum vertical separation of 0.3 m should be kept between pipeline and other buried existing structures such as foreign pipelines, cables and foundations.

Burials

The onshore pipelines should be buried to protect them from mechanical damage, fires and tampering. The recommended burial covers are defined for Class 1 locations to be 80 cm for undisturbed or normal grounds and 60 cm for rocky grounds. For Class 2 locations burial depths of 1m for undisturbed or normal grounds and 80 cm for rocky grounds are considered to be sufficient. In case of Class 3 and 4 locations a burial depth of 1.2 m for undisturbed or normal ground and 1m for rocky ground is applied. For agricultural areas a minimum burial cover of 1m is sufficient, whereas the ploughing depth and drain systems are considered. For grazing land of common fencing activities 0.8 m is sufficient. Locations of buried pipelines are clearly identified by markers. A warning tape has to be installed in the excavation above the pipeline in areas of highly interference risk by mechanical excavators to further lower the risk. Under normal circumstances there is no requirement for trench and bury conditions of offshore pipelines. Only the shore approaches will be buried to avoid pipe erosion. In cases of suitable soil properties on seabed and environmental conditions (no current) offshore pipelines are self-bury removing need for physical protection burial.

Stabilization

In case of submerged pipelines (in areas of offshore, swamps, flood, high water table, rivers and waterways), they are stabilized on bottom against combined action of hydrostatic and hydrodynamic forces. Such on-bottom stability measures include increasing the pipe wall thickness, application of concrete weight coating (Fig. 2), spaced anchor points, trenching and burial.

Concrete weight coating of pipelines

Corrosion Protection

Since pipelines are exposed to corrosive soil conditions (Fig. 3), all metallic buried pipelines are protected against external corrosion by a suitable anti-corrosion coating using polyethylene, polypropylene, fusion bonded epoxy, and asphalt bitumen of 25 mm thickness at least. External anticorrosion coatings will be supplemented by cathodic corrosion protection. To ensure that an adequate cathodic protection can be demonstrated at all times, whereas pipeline is electronically isolated (by means of mono-block isolation joints) from the plants to which they are connected. There are existing two types of CP systems such as sacrificial anodes and impressed current system. CP system (Fig. 4) consists of DC voltage source, which is a transformer or rectifier being fed by an AC power supply (engine generator set). The maximum voltage of a DC power source is 50 volts. For buried or submerged pipelines, the occurrence of coating damage is normally monitored by CP measurements. For carbon steel pipelines the control of internal corrosion is implemented normally by applying a tight control of the process parameters such as water dew point in gas transmission systems and injection of corrosion inhibitors. The further provisions are the use of corrosion probes and corrosion coupons as well as analysis of debris recovered following pigging operation. In case of corrosive conditions with significant corrosion damage risk a complete inspection of the pipeline is required by using of intelligent pigs in order to prove the continued pipeline integrity by evaluation of the pipeline criticality.

Pipeline - Typical causes of corrosion

General principle of cathodic protection

Pigging

A pipeline design engineer has to take into account design provisions for pipeline pigging operations during construction and operation phases: Pipelines must always be piggable, whereas pigs are used for pre-commissioning, commissioning and decommissioning, cleaning as well as corrosion control of pipelines by removal of wax, debris and stagnant liquids, batch inhibition, control of liquids hold-up in gas lines, inspection with intelligent pigs and pipeline repairs. For use of pigs in pipelines, pigging facilities such as pig traps (launchers and receivers; Fig. 5) are needed. In order to guarantee intelligent pigging of pipelines, the internal diameter variations of the mainline pipes are limited, when pipes of different internal diameter are connected together. Therefore, the angle at transitions shall not exceed 14º, the mainline valves must be full bore having same internal diameter as the pipeline. All mainline bends must have sufficient radius to allow pig passage such as; 7 - 10 D bends for DN ≤ 250, 5 D bends 250 < DN < 400 and 3 D bends for DN ≥ 400.

Schematic of typical horizontal pig trap system

Sectionalizing and Emergency Shutdown

Sectionalizing and emergency shutdown are important design requirements for the pipelines. In order to limit the release of the pipeline content (transport fluid) between adjacent valves in case of its leaking or rupturing, remotely operated pipeline block valves (called also sectionalizing block valves) are installed at the pipeline in certain distances. For offshore pipelines there is no spacing or distance requirement for sectionalizing block valves. For oil pipelines mainline block valves are to be installed on the upstream side of big river crossings and crossing of public water supply reservoirs, whereas on their downstream sides block or check valves are installed. For gas pipelines the spacing between valves shall not exceed 30 km (20 miles) in areas of predominantly Location Class 1, 25 km (15 miles) for Location Class 2, 15 km (10 miles) for Location Class 3, and 7 km (5 miles) for Location Class 4. For the case of incidents (explosion, fire, etc.) within plant boundaries, the pipeline asset needs to be isolated from the plant in order to prevent incident escalation. To meet this requirement, automatically actuated emergency shutdown valves (ESD) are installed at each end of the pipeline, and on the incoming and outgoing sections of pumping and/or compressor stations. ESD valves are forming part of the pipeline and are located close to the plant fences as being nonhazardous area. Additionally, leak detection systems are also required for pipelines.

Pressure Protection

During pipeline design and engineering works particular attention is paid towards overpressure protection of the pipelines. In order to safeguard the pipeline against overpressure during operational phase, which may have different causes, there is one governing level of pressure, which is defined accurately by the pipeline design engineers, namely; MAOP (Maximum Allowable Operating Pressure) of pipelines. The MAOP shall never be exceeded at any point along the pipeline during normal continuous pipeline operations. This pressure limitation value is related to the design or maximum operating pressure, pipeline test pressure and SMYS (Table 7). As additional design requirement, pipelines are protected against overpressure, whereby the overpressurization may originate from facilities such as pumping / compressor stations located upstream of pipelines. As overpressure protection installation pressure control valves, pressure relief valves, pressure safety valves are commonly used. In case of pipeline overpressurization originating from the surge pressure created by a change of momentum of the moving fluid stream (valve closure, pumping stop; particularly critical for liquid pipelines), the pipeline is designed in such manner that this surge pressure cannot exceed the MAOP value at any point along the pipeline. This effect is analyzed during pipeline hydraulic calculations taking into account the unsteady transient conditions. As installation of protection against surge overpressure the use of pressure relief valves is common procedure. A pipeline section can be blocked-in, while containing a medium with a low compressibility such as liquid fluids. In this case the effect of possible thermal expansion of the blocked-in fluid on the internal pressure of the concerned pipeline section (due to solar heating) is investigated as well.

All pipeline branch connections are provided with valves to permit isolation of the branches from the main pipeline, whereas the number of flanged connections in pipeline systems is minimized.

pipeline test requirements and the maximum  allowable operating pressure (MAOP) of pipelines

Telecommunication and SCADA

Design engineering of telecommunication and SCADA systems for pipelines is an important issue. These systems are installed to provide assistance during operational and maintenance activities covering both inspection as well as end to end communication for pigging operations, and emergency situations. For pipeline monitoring from a central location and remote control operations installation of the telecommunication is imminent. The Supervisory Control And Data Acquisition (SCADA) system is an important installation of modern designed oil and gas pipelines nowadays.

Material Procurement

After completion of pipeline design the next stage of pipeline engineering covers the procurement of materials needed to construct the pipeline. During procurement phase it is important to take into account the needed quantity of line pipe materials particularly. The spare materials are needed to cover for route deviations, testing and possible pipe damages during construction activities. Spare pipes are also needed for emergency cases during operation. Therefore, a material contingency stock will be set up for pipeline operational phase. The following spare quantities are usual as guidance only for each pipe size (onshore); 60 m pipes for 1 km route length, 250 m pipes for 10 km route length, 750 m pipes for 100 km route length, and 0.5% of route length pipes for longer than 200 km route length. For offshore pipelines, a length of twice the water depth and one riser is needed additionally. For contingency stock of operational phase; line pipe materials of 60 m for each pipe size and 250 m for offshore pipelines are needed as being guidance in the pipeline practice. As stated before, line pipe materials are specified according to API SPC 5L in case of using carbon steel pipe materials. The main criteria covers the manufacturing requirements of the line pipe depending on pipe diameter as follows; seamless pipes for 50 ≤ DN ≤ 150, HFI (High Frequency Induction) welded pipes for 150 < DN ≤ 500, and Longitudinal Submerged Arc Welded (LSAW) pipes for DN > 500.

Construction

The pipeline construction works are consisting mainly of the following activities: ROW will be prepared by grading, and thereafter be trenched for pipe laying. Line pipes will be stringed on site and cold bended, where necessary. Pipelines or risers (offshore) will be installed. The obstacles (roads, railways, rivers, etc.) are to be crossed. In case of offshore pipelines shore approaches will be constructed. Furthermore, construction works are comprising of pipe welding and nondestructive testing (ultrasonic or radiography), field joint coating, lowering-in, back-filling and site reinstatement (onshore), concrete coating of river crossings, installation of markers, tapes and barriers, CP installation, filling the pipeline, cleaning and hydrostatic pressure testing, precommissioning, and preparation of as-built drawings. After several pipes are welded to a string, it is lowered into the trench created before pipe welding, then the trench will be back-filled with fine grain materials. Thereafter the construction site will be reinstated before leaving to the next construction spread. In case of offshore pipeline installation several methods are applicable depending on the pipeline characteristics such as diameter, length and water depth, and availability of suitable equipment; the S-curve method (conventional pipe laying in shallow waters), reeling and Jlaying (in deep waters only).

Testing

After construction and burial works the pipeline is tested to prove the strength and leak tightness by applying of hydrostatic pressure test with water (Fig. 6). Prior to testing pipeline is gauged either by a gauge plate pig or an instrumented caliber pig to ensure that no dents or buckles are left in the line. The strength test pressure (Table 7) will be set in such manner that at least a hoop stress level of 90% of the SMYS is applied and maintained for duration of 4 hours minimum. The crossing pipeline sections are tested separately prior to incorporation in the pipeline. Following the strength test, a leak tightness test is carried out by test pressure set at 80% of the strength test pressure with 24 hours minimum.

Schematic diagram of typical pipeline test section

Documentation

During construction works a comprehensive set of as-built documents are produced and retained for the pipeline life. As-built construction records consist of as-built drawings in form of updated design drawings. These are covering changes to the original design, pipe books giving pipe construction information records such as on line size, grade, wall thickness, ID number, coating details, location of pipe joints, welding and coating specifications, weld radiographs, as-laid position survey results, burial depths, pigging records, and hydrostatic pressure testing certificates.

Cleaning

Every new constructed pipeline is thoroughly cleaned to remove any remaining construction debris and loose scales performed by pushing several cleaning pigs through the pipeline with water or air. After cleaning the pipeline is dewatered and dried (when transporting dehydrated fluids); Fig. 7. The appropriate drying techniques include use of methanol or glycol swabbing, air or nitrogen drying and vacuum drying, or combination of these techniques (Fig. 8).

Pipeline Dewatering

Pipeline Glycol Swabbing

Conclusion

Design engineering of crude oil and natural gas pipelines needs specific skills and capabilities. Pipeline engineers designing pipelines have to be familiar with applicable international codes and standards.

References

  • ANSI/ASME B31.4 – Code for Pipeline Transportation Systems for Liquid Hydrocarbons and other Liquids.
  • ANSI/ASME B31.8 – Code for Gas Transmission and Distribution Piping Systems.
  • SHELL Design and Engineering Practice for Pipeline Engineering (DEP 31.40.00.10-Gen.), Nov. 1993.
  • TOTALFINAELF Design Specification of Onshore Pipelines (SP-STR-516), May 1996.
  • BP Group Recommended Practice and Specification for Engineering of Onshore Transmission Pipelines (RP 43- 1), June 1992.

Note: This article has been published on www.piping-world.com with permission from the author Dr. Orhan Degermenci. Please note that this article refers to old version of ASME standards. Pipeline Engineers are cautioned to use the latest version of standards.