The installation of hot bitumen pipeline (on-lease) requires careful consideration with regards to design and construction methodology. The buried pipelines are routed through congested areas within lease boundary, hence various factors are taken into consideration including design constraints.
Abstract
Buried Pipeline
The hot bitumen pipeline is unique and complexly designed due to high operating temperature (140° to 150°C). The installation technique brings a major role into play in areas where low installation temperature is encountered. As per the design codes, the difference between the operating and installation temperatures is limited by the design stress requirement. In such cases, the pipeline needs to be preheated to a suitable temperature during installation to overcome the stress developed due to high operating temperature. The operating temperature of the buried pipeline will be maintained by insulating the pipe with polyurethane foam (PUR) and high-density polyethylene (HDPE) jacketed sleeve. The application of PUR insulation on pipes and bends needs to be done at shop due to the specialized nature of insulation. All the bends are hot induction bends, since field bending will damage the insulation. A minimum bend radius of 10D is used based on stress design and to allow passing of specialized smart pigging tool. The pre-insulated pipes and bends are joined in the field by thermo-plastic welding technique made of HDPE sleeve material. During installation, when pipe is successfully pre-heated to suitable temperature, the pipeline shall be immediately backfilled with adequate compaction to avoid shrinkage of pipes due to lower ambient conditions.
The health of PUR insulation system is monitored by four copper wires embedded in each quadrant which monitors moisture ingress in the PUR insulation system. The moisture surveillance system is installed at 5000 metre intervals.
The pipeline is cathodically protected by means of sacrificial anode monitored by installing CP test stations.
Above Ground Pipeline
The above ground pipeline is insulated with adequate thickness of mineral wool insulation with electric heat tracing to maintain the required design operating temperature.
Introduction
Pipelines are the safest way of transporting hydrocarbons. There are multitudinous pipelines, but hydrocarbon pipelines fall in the following two categories: Oil and Gas. Internationally a number of codes are followed including ASME B31.4 for oil pipelines and ASME B 31.8 for gas pipelines. In Canada, CSA Z662 is followed for oil and gas pipelines.
The operating and installation temperature of hot bitumen pipeline are:
- Operating Temperature = 140°C to 150°C
- Installation Temperature after preheating = 90°C
- Ambient Temperature (MDMT) = -5°C to -29°C
Pipeline Stress Design
The difference between operating temperature and ambient temperature during installation is more than 150°C. This results in thermal stress exceeding the acceptable limit as per design code. Therefore, the pipeline needs to be preheated to a suitable temperature, backfilled and compacted immediately to reduce the thermal stress and likelihood of uplift buckling during operation.
The pipeline will be modeled using stress analysis software such as CEASAR or Autopipe to ensure that stress, displacement and loads are in compliance with design codes for the following load cases:
- Sustained Case - Stress for design pressure.
- Hydrostatic test Case – Stress for hydrotest pressure.
- Thermal Case - Stress for minimum and maximum design temperature to installation temperature.
- Tensile Case – Stress for maximum design temperature under restrained/unrestrained pipe.
The aboveground and underground sections of pipeline shall be modeled and analyzed together as one system.
Buried Pipeline
The on-lease buried pipeline generally runs in congested areas and has a narrow right of way in comparison to conventional cross country pipelines. Therefore, while routing the pipeline within lease area, factors such as existing underground utilities, potential future development and existing surrounding facilities need to be considered. The routing is also affected due to the thermal stress design.
The hot bends of 10D radius need to be used in on-lease pipeline due to limitation of narrow right of way whereas in cross country pipeline, large bend radius can be used.
The operating temperature of buried pipeline will be maintained by insulating the pipes with adequate polyurethane foam (PUR) and HDPE jacketed sleeve as shown in Figure-1.
PUR foam is bonded to the external steel pipeline surface and the HDPE layer is bonded to the PUR foam layer. The system is designed to expand or contract as an entire unit, with temperature change or mechanical stresses on the pipeline system, ensuring long-term integrity. The polyethylene jacket sleeve provides a very robust protection against mechanical damage during installation and operation of the pipeline.
The PUR insulation and HDPE outer jacket system is designed as per following standard:
- EN 253: Pre-insulated bonded pipe systems for directly buried hot water Networks – Pipe assembly of steel service pipe, polyurethane thermal insulation and outer casing of polyethylene.
The insulation vendor will determine the required material and thickness of insulation system based on pipeline design parameters. The insulation system must be selected for entire duration of design life without any degradation of insulation system.
The recommended properties of PUR foam and the HDPE sleeve from manufacturer are as per Table 1 and Table 2.
Table-1 PUR Foam Properties
(Source – Logstor)
| Property | Description / Value |
|---|---|
|
Material type |
The insulations foaming will be sprayed on to the steel pipe in two layers and with the 4 copper wires for the surveillance systems in between the two layers of foam. PUR foam: Isocyanate : Polymeric MDI, Physical Blowing agent: Cyclopentane |
|
Density |
70 kg/m³ (-10/+30 kg/m³) |
|
Compressive strength at 23°C Short Term Strength at 150°C Long Term Strength at 150°C |
Min. 0.3 MPa at 10% relative deformation 1.2 MPa 0.08 MPa |
|
Closed cells |
Min. 88% |
|
Cell size |
Max 0.5mm |
|
Lambda value, initial Lambda value, aged |
Max. 0.029 W/mK Max. 0.029 W/mK (No change over time) |
|
Lifetime of foam at 146°C |
Min. 30 years |
|
Axial shear strength at 23°C Axial shear strength at 140°C |
Min. 0.12 MPa 0.8 MPa |
|
Water absorption |
Max. 10% |
Table-2 Properties of HDPE Jacketed Sleeve
(Source – Logstor)
| Property | Value |
|---|---|
|
Density |
Min. 944 kg/m³ |
|
Melt flow rate |
0,2 – 0,7 g/10 min. |
|
Thermal stability |
Min. 20 min. at 210°C |
|
Elongation at break |
Min. 350% at 23°C |
|
Carbon black content |
2.5% +/- 0.5% |
|
NCLT (Notched Constant Load Test) |
Min. 300 hours |
The PUR insulation and HDPE Jacketed sleeve shall be applied on straight pipes by spray method as shown in Figure-2.
The application of PUR insulation on bends shall also to be done at shop. All bends installed on pipeline are hot induction bends, since cold field bending is not permitted as it will damage the insulation. The radius of roping/elastic bends shall be determined with conservative approach so as not to damage the insulation during installation.
A bend radius shall be determined based on the stress design in the pipeline. A minimum radius of 10D shall be considered for the intelligent pigging requirements. The insulation on bends will be done by injecting PUR foam in between the pipe and the HDPE jacketed sleeve called injection method in small segments to maintain the bend curve made at insulation vendor shop.
The pre-insulated pipe and bends shall be joined in the field during installation as per following standard:
- EN 489:2009 - Pre-insulated bonded pipe systems for directly buried hot water networks - Joint assembly for steel service pipes, polyurethane thermal insulation and outer casing of polyethylene
The following field joint insulation system has been developed by M/s Logstor, Denmark, as shown in Figure-3. The Field Joint is a tubular style HDPE sleeve where the sleeve circumference overlaps onto the parent coating and is welded.
The moisture surveillance monitoring system is designed as per following standard:
- EN 14419:2009 - District heating pipes – Pre-insulated bonded pipe systems for directly buried hot water networks – Surveillance systems
The moisture surveillance detector system is designed in order to monitor, detect and locate leakage or fault on the underground pre-insulated pipelines. The moisture surveillance detection system is provided by embedding four copper wires in the middle of the PUR insulation at each quadrant (i.e.) 12, 3, 6 and 9 o’clock positions of the pre-insulated pipe. The copper wires are approximately 2.5-mm² diameters.
The detector has a built-in pulse echo meter that makes it a flexible and effective alarm unit. The detectors will be installed on approximately 5000m equal intervals on the pipeline including both ends. The detector surveillance system will be connected to the insulated four copper wires in the pipe system. The current required to measure the impedance in the copper wires is 16 W and 110/230 VAC. It measures impedance and resistance to register moisture that may have penetrated into the insulation system from surrounding soil or a leak in the carrier pipe. The surveillance monitoring system will be based on resistance measurement and impedance measurement both. If moisture or water gets into the area of PUR insulation, the circuit is broken and an alarm is triggered and will get a notification in the control room. The notifications will include the locations of the fault on the underground pipeline.
The surveillance detection system diagram is shown in Figure-4.
Trenching and Lowering
The pipeline trench is required to be excavated to the required depth by conventional excavator. The trench bottom will be appropriately cleaned and sand bedding material will be placed on the trench bottom prior to lowering insulated pipe into the trench.
Preheating and Backfilling of the Pipeline
The most commonly used technique for preheating of the pipeline is by air induction heater. It is specially designed and constructed to heat the hot bitumen pipeline to the required preheating temperature. The heater can preheat the pipeline for a span of 300-1,000 m length depending on pipe size and weather conditions. The heaters will be loaded on the truck and equipped with instrumentation to record the heating parameters of inlet and outlet temperatures, air flow rate and heating time. The construction procedure will elaborate on the acceptable duration of heating and delta temperature based on recorded temperature of inlet and outlet locations.
The following are the steps for preheating as shown in Figure-5:
Step-1
- The truck number 1 will heat the first section until the outlet temperature is reached at acceptable level.
- The Padding material shall be placed on top of the pipe and the pipeline will be locked and backfilled with adequate cover depth as per the stress design with required compaction, leaving enough exposed pipe on the ends for tie-ins.
Step-2
- The truck number 2 will heat the second section until the outlet temperature is reached at acceptable level.
- The Padding material shall be placed on top of the pipe and the pipeline will be locked and backfilled with adequate cover depth as per the stress design with required compaction, leaving enough exposed pipe on the ends for tie-ins.
Step-3
- Do the tie-ins of 1st section and 2nd section while pipe is in pre-heated condition and complete the backfilling with adequate compaction.
- After completing the backfilling of 1st section and 2nd section, the truck 1 and truck 2 will be moved to next section and steps 1, 2 and 3 shall be repeated.
Hydro-testing of Pipeline
The pipeline shall be hydrostatically tested as per CSA Z662 or ASME B31.4/31.8 to ensure the integrity of the pipeline construction.
Above Ground Pipeline
The above ground pipeline including pig launcher and pig receiver shall be insulated with mineral wool and electric heat traced to maintain the operating temperature.
In conventional pipelines, the pig traps are isolated from the main continuous flow by closing the main pipeline pig trap valves and kicker valves. The continuous flow will be through bypass valves as the bypass valves are always in open position.
In hot bitumen pipelines, pig trap valves, kicker valves and bypass valves are always fully open. The pig traps are fully pressurized to keep the hot bitumen product in flowing condition and to avoid dead leg in the traps.
There is also a diesel/diluent line connection to pig traps to flush out solids and debris collected during pigging on the traps.
The pigging procedure of hot bitumen pipeline is unique. During pigging operation, the bypass valve will be slowly closed and flow will be diverted through kicker valve to allow pigging operations. The pig traps are continuously heated by an active electric heat traced system. The pigging tool including smart tools are specially designed for high temperature hot bitumen pipeline.
Cathodic Protection
The Hot Bitumen pipeline is cathodically protected by means of sacrificial anode and CP test leads are installed at approximately at 300 m equal intervals on the pipeline. This is a secondary protection of pipeline when insulation system fails and pipeline is exposed to the soil.
Summary of Challenges and Mitigation
- Narrow ROW due to congested on-lease area with many utilities / roads and creek in the area.
- Design of pipeline stress due to high operating temperature.
- Need of hot induction bends. Field bending is not possible due to the insulation.
- Engineered trench design due to use of hot induction bends only.
- ROW may be shared by other foreign pipelines.
- Due to ever changing nature of active construction site and survey data, it may be challenging to finalize the routing.
- Air induction heater of adequate capacity.
- Creek crossing design.
Acknowledgement
Allen Hansen, Logstor, Denmark
Note: This article has been published on www.piping-world.com with permission from the author Mr. Raju Burande.