Polyethylene (PE) liners are frequently applied in corrosive water services. As part of the a progressive integrity management and the drive for lowest life cycle cost, the use of PE liners in Oman has been extended to wet hydrocarbon services for new flowlines and pipelines as well as for retrofits. After some teething problems, installation is straightforward and operating experiences are good. Liners prove to be an effective means for a fit and forget system to prevent and eliminate leaks in aging systems.
Petroleum Development Oman LLC (PDO) operates an extensive network of pipelines and flowlines with a total length of some 11000 kin. Services vary from wet and dry oil, wet and dry gas, to water transport. Pipelines are usually buried, whereas the majority of flowlines are installed above ground. Originally, by default, the material applied was C-steel with limited corrosion protection by corrosion inhibitor or internal coatings. Corrosion remained the prime lifetime limiting parameter. For example, in some areas the lifetime of C-steel flowlines was only 3 years. Many of the lines are aging and when integrity can not be ensured and derating is not feasible, they need replacement. Nowadays a suite of alternative materials is available that on basis of life cycle cost can compete with C-steel. As such, Polyethylene (PE) applied as a liner is now extensively used. PE liners are also applied as retrofits to extend the life of existing flowlines and pipelines. Since 1988, extensive operating experience has been gained in Oman with PE lining of C-steel water injection flowlines. The first PE lined wet crude pipeline was installed in Yibal, in 1992 and the first above ground flowlines were installed, late 1997. In October 1999 a 34" wet crude PE lined transmission line was successfully commissioned.
LINER DESIGN
The author's company has developed a specification for PE liner applications in Oman, which comprises the materials design and procurement requirements. It also provides guidelines for the installation, commissioning and operation of PE lined flowlines and pipelines.
The specification determines the wall thickness of the liner by the following requirements:
a) Minimum thickness required for successful application: Starting with the ID of the C-steel pipe, a minimum wall thickness of the liner is established, such that the PE pipes are robust, can be joined by butt fusion welding and pulled/pushed into the C-steel pipe.
b) Thickness required for liner stability (buckling resistance): When high annulus pressures are not anticipated, the liner stability will be determined mainly by the PE modulus, affected by liquid absorption and temperature (swelling + thermal expansion including high temperatures in non flowing conditions). Calculation of the wall thickness in this case will include stability calculations, where swelling is taken into account.
c) Thickness required for collapse resistance: The design aims to prevent collapse as a result of expansion of the annular gas upon depressurization of the bore. The calculation of the critical liner collapse differential pressure includes the effect of swell induced stresses.
Requirement
a) typically pertains to water services,
b) typically to stabilized crude oil and
c) mainly to live crude (multi- phase lines) or gas.
When the liner wall thickness is sufficient to withstand an annulus pressure equal to the pipeline bore pressure (MAOP) the design is intrinsically safe against collapse. Experience has shown that this requirement is over-conservative however and a lower collapse pressure is acceptable, when the annulus pressure during normal operation is manag
