Safe Design of Cross Country Pipelines
J Moris Christopher
JGM, Piping Design Department,

V SivakumarChief Engineer, Piping Design Department,

R BalajiSenior Principal Engineer, Piping Design Department

A long run pipeline carrying liquid or gas over long distance normally takes a route through different terrains such as rocky area, river crossing, highways road crossings, canal crossing, etc. In such routing pipeline integrity is very crucial as its failure can lead to huge safety and environmental issue to nearby society. This article will cover how the safe design of the gas pipeline to ensure this integrity was performed by TechnipFMC in a typical installation from gas gathering station to gas processing unit, considering the conditions such as - Design of pipeline for various terrain conditions including pipe thickness and burial depth; Stress analysis of the pipeline; Design of pipeline to take care of upheaval buckling; Measures for buoyancy protection; Special material requirements for pipeline and components; Surge protection; Cathodic protection; and Pipeline health monitoring.

Transpor tation of hydrocarbons is vital in a vibrant economy to meet various energy requirements. This shall be done in a safe manner with minimum impact to the environment and with least energy. Pipelines over long distances normally take a route through different terrains such as rocky a rea, river crossing, sloped hills, highways, rail crossings, canal crossings , marshy land, etc. In such routings, pipeline integrity is very crucial as its failure can lead to huge safety and environmental issues.The safe design of pipeline star ts from the process design wherein line sizing is done for optimal energy consumption and to negate any impact of erosion induced pipeline failures. Due consideration shall be given for a High Integrity Pressure Protection System to protect the pipeline wherever required. In case of two phase flow, requirement of slug catcher may be explored. This article excludes erection, inspection, testing, operation, maintenance and pipeline routing in sensitive areas prone to wars and conflicts.

Some of the key parameters for safe design of pipeline as experienced by TechnipFMC are indicated below.
  • Minimum cover for buried pipelines
  • Alignment Sheets
  • Materials
  • Thickness
  • Stress analysis
  • Surge Analysis
  • Cathodic Protection System
  • Pipeline Health Monitoring
Minimum cover for buried pipelines
Pipeline passing through agricultural, hor ticultural occupancy, rocky terrains, industrial, commercial and residential area are buried with minimum depth of cover. Pipelines under drainages, ditches and road crossings are provided with extra depth of cover.

Canal/nala crossings, railway crossings and areas under tide influence are provided with additional depth of cover. Also, major river crossings are provided with more depth of cover with consideration of scour depth. Normally a higher thickness pipe is used at these locations.

Alignment Sheets
Alignment sheets graphically show the entire route of the pipeline, location , associated facilities and identifies the land masses associated with its placement and cover the following as minimum,
  • Centerline, outside diameter, grade and coating requirement.
  • Ground profile with intersections points along the alignment reflecting the elevations from Contour maps.
  • Details such as type of soil, hilly, wetlands, etc.
  • Details of crossings such as foreign pipelines, underground cables, river, canal, road and rail crossings
  • Other facilities namely block valve stations, surge facilities, choke valves, etc.
Material design for pipeline deals with selection of appropriate material for the intended appli cation and with due consideration such as sour and other corrosive ser vices. Material normally used are Carbon steel (with /without alloy cladding) , Stainless steel, Nonmetallic plastics, etc.,

There are few special items exclusively used in pipelines. They are briefly described:
  • Barred Tee is a tee which prevents the pig from traveling down a branch connection
  • Choke Valve is for severe ser vice oil and gas applications used for flow rates or pressure control.
  • Isolation Joint is to prevent detrimental electro-chemical interaction and improve the effectiveness of the cathodic protection
  • Sand Trap is a component on downstream of pipelines to remove sand, debris and foreign par ticles that comes from well heads.

Figure 1: Alignment Sheet (Courtesy: Indian Project, HOEC

Thickness of pipeline is calculated based on ASME B31.4 for liquid lines and ASME B31.8 for Gas lines. Unlike process plant piping, emphasis to be laid on population density and environmental factors for thickness calculation by choosing the appropriate location class specified in the Code [1][2].

Stress analysis
Stress analysis involves different conditions and Code [1][2] requirements compared to process plant above ground piping. Allowable stresses as per code are high compared to that of plant piping. Code classifies the line into Restrained (due to soil pressure) and Unrestrained lines. Pipe movements due to pressure elongation and thermal expansion must overcome the soil friction in underground conditions. The portion of buried piping which overcomes soil friction will move and create bending stress and the portion which doesn't move, remains as it is. These transition points are called virtual anchors. Soil forces acting on the buried lines can be categorized into axial friction force and lateral soil force. Above Ground anchors and guides are used to absorb forces cascaded from buried line and to isolate the above ground piping from buried lines. Due to the non-linear nature of the buried pipelines, stress analysis gets complicated.

Normally pipeline will expand towards the end or bend, due to restrained central portion. End movement is directly propor tional to square of the temperature difference between line design and ambient/soil temperatures. To reduce the enormous bending at the ends, block anchors can be installed across the line at equal inter vals. In addition, by making the soil tight around the lateral leg this stress can be reduced. If needed wall thickness of pipes and components can be locally increased.

In pipelines buried under highways or railroad crossings, additional bending stress is created due to uneven soil pressure. Code requires that this bending stress to be combined with pressure hoop stress and the combined stress should be limited to specified minimum yield strength of the material . In addition to this normal Stress analysis, upheaval buckling, buoyancy effect in underwater are some of special stress checks done according to the terrain conditions to ensure safety of the pipeline.

Upheaval buckling
In buried pipeline, the soil resistance offered in lateral direc tions is different between the direc tion above the pipeline and all other directions . This is due to continuous soil mass in other directions. This difference causes pipeline to buckle in the upward direction causing a lateral movement which will displace the pipeline from its intended buried elevation (Ref. Figure.2). When this upheaval is combined with seismic, the result will be catastrophic. Pipelines buried in ocean floors, marshy lands and river crossings will experience less soil resistance in the horizontal lateral directions due to the clumsiness of the soil.

Palmers method[3] is used to evaluate this Upheaval buckling.

Figure 2: Onshore Pipeline Uplift [4]

Buoyancy force
In case of pipelines on seabed or river bed, buoyancy reduces the submerged weight of the pipeline thereby promoting buckling effect (Refer Figure-3). For a given diameter of pipeline this force is a constant. The pipeline weight can be increased by providing a concrete coating with necessary thickness. Although the increase in diameter due to coating increases the buoyancy, the density of concrete is high enough to make the increase in buoyancy negligible.

Case Study
A typical project executed by TechnipFMC had pipeline of size 12", 12km long from Gas gathering station to Gas Processing Plant comprising above ground and buried portion. The pipeline was designed based on ASME B31.8. Analysis was done for operating loads, seismic effects and slug forces. Additionally, a river crossing of 500m width in the buried portion was analyzed for upheaval buckling and buoyancy effect using Palmers method.

The result 1 shows the uplift resistance is less than the required download and the line will be unstable causing upheaval. To solve this a concrete coating with 80mm thickness was considered. The new results are given as result 2. The concrete thickness increases the weight and thereby increasing the uplift resistance (load) required to keep the pipeline without excessive buckling ensuring safe design.

Surge Analysis
Surge pressure occur when fluid flow velocity changes abruptly because of various scenarios like sudden closure of Emergency Shutdown Device,
Figure 3: Subsea Lateral Buckling [5]

pump tripping, slamming shut of a non-return valve etc. causing a pressure wave that moves from one end to another end at the sound speed inside the fluid. The wave therefore potentially subjects the pipe to exceed the pressure limits and leads to pipeline damage, pump casing crack, pipeline leak and by thus contamination and environmental damages. N on-performance of surge study and improper surge protection will result in significant downtime and reduced life expectancy of pipeline. Damage may also occur due to pressure going below the minimum allowable operating pressures leading to cavitation and pipeline collapse. Hence, Surge analysis is an important part of pipeline design.

To prevent failure of pipeline due to surge pressure the following methodologies can be adopted:
  • Surge pressure relief valves and surge tanks
  • Increased pipeline diameter/wall thickness
  • Increase valve opening and closing time
Surge analysis gives force output at various support location which helps to design suitable pipe supports. Cathodic Protection System
In buried pipeline corrosion is one of the major concern due to the environmental and the nature of the soil. External coating is one of the corrosion control measure and is achieved with methods like 3-Layer polypropylene or 3-Layer polyethylene or Fusion bonded Epoxy coating. The corrosion protection by the coating must be supplemented with cathodic protection to achieve complete mitigation of corrosion. Cathodic Protection to pipelines is provided by deep well anode beds, by installing at approximately 100 meter of distance from the pipeline. These anode beds are fed by Transformer Rectifier Unit [6].

During construction of pipeline, temporary cathodic protection is provided using Magnesium anode. In case of Overhead lines crossing, suitable electromagnetic interference study needs to be done. During the crossing of existing/foreign pipelines, one structure may act as cathode and other one as anode leading to corrosion and hence for such cases, the resistance bonding shall be inserted to create alternate path for current flow.

Pipeline Health Monitoring
Pipelines are normally running long length, high value, high risk and often require continuous monitoring. It is of fundamental impor tance to detect, diagnose and monitor damage growth as early as possible to predict the remaining operational life of a system and to minimize the risk of unexpected failures.

Few of the Pipeline Health Monitoring systems are described below.

Non-Destructive Evaluation Method
This is a Non-Destructive Evaluation method routinely utilized in the petrochemical sector worldwide. Guided wave inspection enables the fully -volumetric inspection of several meters of pipeline from a single sensor location, since the entire volume of a pipeline can be monitored. Fiber Optics Sensing Method
The fiber optic sensing leakage detection involves the installation of a fiber optic cable to measure the temperature over the entire pipeline length based on the Brillouin scattering or Raman backscattering effect. Leakages can be detected and localized using distributed fiber optic temperature sensors. Fiber optic distributed sensing systems can be used for distributed measurements of both strain and temperature over extremely long distances using a limited number of very long sensors. Summary
Long distance buried pipelines pose additional design requirements for safe operation during the intended life. Major constraint is the terrain in which the pipeline is routed providing various kinds of challenges to be met.

With proper considerations in material selection, stress analysis, support design, surge protections and careful monitoring the safe design of pipelines can be ensured.

1. ASME B31.4: Pipeline Transportation Systems for Liquid Hydrocarbons and other liquids.
2. ASME B31.8: Gas Transmission and Distribution Piping systems.
3. Design of Submarine Pipelines Against Upheaval Buckling by Andrew Palmer and associates (1990).