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Coating Selection

Operational requirements and the environment were key influences on the final design of the coating system.

Thermal insulation was necessary to meet the heat-retention requirement needed to maintain the pump ability of dry bitumen even after a 2-day shut­down.

Polyurethane foam's relatively low mechanical strength made an external jacket essential. The jacket had to be ro­bust and water-resistant to withstand the demands of the environment, con­sisting of clay soil, muskeg, and directional drill sections. In addition, the possibility of a fail­ure of the shop-applied coating or the field-applied joint coating drove the re­quirement that the anticorrosion barri­er be suited to immersion service at el­evated temperatures. All factors led to selection of an insulated three-layer coating system.

Historically, there have been similar insulated three-layer coating systems ed around the world to transport fuel oils, sulfur, and sour gas. In Europe, district heating systems using an insu­lated three-layer coating system are quite common.

Insulated coating systems are also available for onshore and subsea pipeline applications. The operating temperatures of most insulated pipelines in North America are less than 110°C.

There were relatively few options for the anticorrosion layer, given that it must be able to withstand immersion conditions up to 130°C.

Three-layer polyolefin coating sys­tems, one of the most robust coating systems available for immersion service, consist of fusion-bonded epoxy (FBE) primer, adhesive, and extruded poly­olefin (Coating System B of CSA Z24S.21-98).

Typically, these systems are limited to operating temperatures of less than 85° C. for polyethylene (PE)-and 110° C. for polypropylene (PP), although some PP systems are rated considerably higher. PE tapes have been used for insulated pipelines operating up to 95° C.

Common high-temperature FBE coatings are frequently designed for use as a primer coat in multilayer coating systems; however, they are rated for high operating temperatures in dry conditions only. A standalone, high-temperature FBE coating, which had just become com­mercially available, was used as the anti-corrosion layer of choice for the MacKay River Pipeline.

A spray-applied polyurethane foam based on a Euro­pean district heat­ing-approved product was se­lected as the insu­lation for the MacKay River Pipeline. This foam has a well-docu­mented service history at high temperatures simi­lar to project re­quirements. The foam system used on the European District Heating systems is mold-injected. Foam in­sulation is normal­ly spray-applied in North America; thus some modifi­cation of the foam was necessary to speed up the cur­ing process.

The foam thick­ness and thermal conductivity were selected to meet the operational re­quirement for an unblended shutdown window of 2 days.

This insulation thickness correspond­ed to an outer jacket temperature of 60° C.

Extruded PE is a relatively low-cost material with excellent properties in­cluding water resistance, toughness, im­pact resistance, and strength.

As a result, extruded PE has become an industry standard for protecting foam insulation.

Outer jackets consist of extruded PE with the option of a PE tape inner wrap between the foam insulation and the extruded PE.

The PE tape inner wrap tends to improve the impact and handling resist­ance of the outer jacket. The PE tape is either joined to the extruded PE outer jacket with an adhesive layer or by way of heat fusion.

A range of PE types is available for extrusion from low-density PE to high-density PE. The latter (HDPE) has the highest tensile strength and hardness of all extruded PE coatings and is the material of choice except for winter con­struction with temperatures dipping lower than -30° C, at which point the impact resistance and elongation at break decrease significantly.

At ambient temperatures of—40° C, HDPE may crack when subjected to moderate impacts or pipe movement. As such, low or medium-density PE is recommended for winter construction.

Since the MacKay River Pipeline was scheduled for late summer or early fall construction, an outer jacket consisting of a PE tape inner wrap heat-fused to extruded HDPE was chosen.