PERFORMANCE OF PIPELINE INSULATION SYSTEMS IN ARCTIC CONDITIONS
12^th International BHRA Conference - Paris, France - November 4-6 1997

Author
Mike Alexander
GARNEAU INC.

Carmine D'Agostino
NOVA CHEMICALS
6711 Mississauga Rd., Suite 200
Mississauga, Ontario L5N 2W3 Canada

ABSTRACT

In the conditions of Arctic winter it is not unusual for the ambient temperatures to drop as low as -40°C. Insulated pipes overcoated with extruded polyethylene have a tendency to fail drastically due to mechanical impact, rupturing the outer polyethylene layer and damaging the polyurethane foam.

A systematic investigation into main reasons for this phenomenon was conducted, resulting in specific recommendations to permanently solve this problem.

INTRODUCTION

Pipelines insulated with polyurethane foam have been used in the oil and gas industry for several decades. Main area of application include insulating sour gas pipe for gas transmission from the producing well to the gas plant. Depending on the gas field, the level of hydrogen sulfide can reach in some cases as high as 30%. For such high hydrogen sulfide level pipe insulation is necessary to prevent hydrate formation.

The other main area of application involves transportation of heavy oil. Canada is blessed with abundance of the largest deposit in the world of heavy oil, asphalts and tar sands. Heavy oil must be heated for the purpose of shipping via pipeline in order to reduce viscosity and hence pumping cost. One of the most recent projects involving 150 km of 12 inch pipe required oil temperature of 90°C in order to reduce viscosity and lower the pumping resistance.

HISTORICAL PERSPECTIVE

In the past it was customary to insulate the bare pipe with spray applied polyurethane foam. This kind of system is known to have failed due to steel pipe corrosion, especially if the moisture or water found a direct access to the steel pipe. In the presence of water at elevated temperature the polyurethane foam has a tendency to decompose into its basic building blocks, polyol and isocyanate. Isocyanate will react with water forming a weak organic acid, which will attack steel. Typical pH of decomposed polyurethane foam has been measured to be around 4.5-5.5. In such a situation the pipeline owner is faced sometimes with tough decisions to make, whether to continue to operate pipe partially attacked by corrosion, to repair it or replace.

Ideally, the insulated pipeline system should fulfill basic design requirements in addition to having excellent insulation values:

The insulated pipe system is quite advanced and quite expensive. A well designed system consists of many different layers, shown in Figure 1.

Anti-corrosion coating

During last few years numerous pipeline companies started looking at different options to address the problem of pipe corrosion under the polyurethane foam. The most obvious solution was to install the ant-corrosion barrier coating under the foam. Several anti-corrosion barrier coatings with their respective properties are discussed as follows:

Fusion Bond Epoxy (FBE)

FBE is applied sometimes as the anti-corrosion barrier coating under the foam. In the experience of the authors, its application is rather limited due to the fact that at specified thickness of 0.12-0.18mm (5-7mils) it does not provide totally holiday-free surface. At higher thickness (0.35-0.4 mm) this coating becomes uneconomical under the foam. In the opinion of the authors this coating is used very rarely now under the foam.

Polyethylene tape

High shear polyethylene tape is arguably the most frequently used anti-corrosion coating under the foam. Numerous recent pipelines, such as ECHO line in Canada, use high shear polyethylene tape under the foam. This system is suitable to operate at temperature up to 93°C without major shearing problems, as shown in various tests, due to the fact that the tape is bonded to the pipe with a vulcanized adhesive. Many recent insulated pipeline projects utilize the tape as anti-corrosion barrier coating.

Three Layer Polyolefin

This is probably the most expensive anti-corrosion barrier coating. It consists of FBE primer, polymeric adhesive and extruded polyolefin, all applied according to accepted international standards, such as CSA Z245.21, or French NF A 410, with the only difference that the polyolefin layer might be thinner, if specified. This coating has excellent properties as a stand alone coating and under the foam and comes very close to the definition of an "ideal" system for insulated pipelines operating at temperatures not exceeding 85°C for polyethylene and 110°C for polypropylene.

Outer jacket

Polyurethane foam is an excellent heat insulation material, one of the best in the middle-range of temperatures with the average thermal conductivity values ranging around 0.022 W/mK. However, polyurethane foam possesses very little mechanical strength and has to be overcoated by an external jacket. Several choice for such a jacket include polyethylene pipeline tape, extruded polyethylene and spray applied elastomeric polyurethane coatings.

Over the years it became accepted in the industry that the polyethylene jacket is the best method for protecting the pipe, due to its relatively low cost and excellent properties of toughness and strength. High density polyethylene has been used predominantly for this purpose. However, at low temperatures, during the Arctic winter, when the ambient air temperature might reach -40°C, the high density polyethylene outer coat develops decreased elongation and reaches the lower brittleness point, resulting frequently in severe damage. Sometimes the pipe outer coating will develop a tendency to spontaneous cracking and sometimes it will crack under slightest movement or impact.

Some examples of such polyethylene insulated pipe damage are shown in attached photos, fig. 2, 3 and 4. The outer jacket and foam insulation are damaged, in the experience of the author, under following circumstances:

EXPERIMENTAL PROCEDURE

High Density Polyethylene

Following failures of insulated pipe at low temperatures in the field, an extensive laboratory testing program was initiated. The samples were prepared by insulating a 12 inch pipe with 50mm polyurethane foam, followed by extrusion of external high density (0.945 g/ml) polyethylene jacket. Average thicknesses of polyethylene were

The testing was conducted to check the mechanical properties of the system at various temperatures. The test results are shown in Table 1 and 2.

From the test results it became apparent that

 Linear Low Density Polyethylene (LLDPE)

In the next series of tests a comparison test was conducted in the lab with lower density polyethylene. Linear low density polyethylene (LLDPE) was found especially immune to low temperature damage. A complete list of tests conducted on lab extruded sheets of LLDPE is shown in Table 3. Some very important differences which become instantly obvious include:

• impact properties at 0°C and -40°C remain essentially not changed.
• tensile strength at yield at -40°C increases by a factor of 50-75%, compared to the same property at 0°C
• elongation at break decreases at -40C by 30-40% compared to 0C, but it is still in excess of 1000%
• tear resistance at -40°C is about 25-40% lower than at 0°C

Based on the test results shown in Table 3, a decision was made to try polyethylene shown as LLDPE 1 on the line as an outer coat for the insulated pipe system and to conduct similar series of testing, as with HDPE.

The test results fully confirmed that this type of polyethylene performed much better at low temperature, as shown in Table 4.

Polyethylene tape/ extruded LLDPE jacket.

Another modification of the outer jacket was tried in the plant, consisting of wrapping the "green" polyurethane foam with polyethylene pipeline tape, followed by extrusion of LLDPE. This system proved to be much superior, resulted in higher impact resistance and better handling properties. The authors believe that this system has tendency to balance the stresses in the outer jacket. Quite obviously, the stresses in the tape are directed circumferentially around the pipe. The stresses in the extruded jacket are directed longitudinally, therefore both stress directions counteract each other, resulting in much stronger matrix, as shown in Figure 5.

Black outer jacket versus white outer jacket

Another series of experiments was conducted by applying a white extruded outer jacket and a black extruded outer jacket. The surface temperature of the outer jacket was measured on a cold winter day, as shown in Table 5. The black outer jacket was found to be much warmer than the white jacket, in some cases on a warm sunny day the temperature differences was as high as 15°C due to absorpotion of infra red and visible radiation.

INSULATED PIPE BENDING PROPERTIES

During the bending operation the insulated pipe is subjected to major stresses. It should be explained at this stage that the compressive strength of the foam used to be only 200 kPa (30 psi), whereas the forces exerted onto the pipe during the bending process are measured in thousands of kilograms. Frequently the insulation system is crushed by the bending force, resulting in serious insulation damage. Some of the examples of foam compression are shown in Figure 6 and 7. To prevent foam damage it is recommended to increase the foam compressive strength to the value of 275-400 kPa. In the experience of the industry, this higher compressive strength results in much improved bendability and significant less damage to the insulated pipe systems.

CONCLUSIONS

A well designed anti-corrosion coating is a must for successful operation of the insulated system

Linear Low Density Polyethylene polyethylene used as outer jacket is superior to high density polyethylene, especially at low temperatures.

Compressive strength of polyurethane foam of at least 300 kPa is recommended for insulated pipe in order to minimize insulation damage during bending.

Black outer jacket should be used in the winter, as it has a tendency to get warmer due to higher absorpotion of infra red rays.

ACKNOWLEDGMENT

The authors wish to express gratitude to Garneau Inc and NOVA Chemicals for permission to present this paper.

Table 1 - Insulated pipe system using HDPE as an outer jacket, mechanical properties.

Table 2 - Impact properties of insulated pipe system, using HDPE as an outer jacket.

Table 3 - Test Summary Low Temperature Sheet Properties

 
Table 4 - Insulated pipe system-using LLDPE as an outer jacket, physical properties.