External Corrosion Coating Rehabilitation Of Flowlines

By M. A. Saleem, Saudi Aramco, Udhailiyah, Saudi Arabia | April 2009 Vol. 236 No. 4

The external corrosion protective coating of critical buried flowlines experienced premature coating failure in specific applications at Uthmaniyah Gas Plant in Saudi Arabia.

This article looks at specific issues associated with the external coating’s performance under high operating temperature conditions and continuing efforts to restore the integrity of these pipelines.

Uthmaniyah Gas Plant (UGP) in Southern Area Gas Operations of Saudi Aramco is one of the largest gas processing plants in the world. It was commissioned in 1981 as part of a master gas system to process the associated gas from oil wells and was expanded gradually to process non-associated gas from Khuff formation wells. The gas plant has a capacity to process up to 2.6 Bscf/d of sour gas including 750 MMscf/d of Khuff gas.

The plant has several underground pipelines as an integral part of the operation and their integrity will have direct impact on the plant’s operations.

External Coating Performance

The performance history of the non-associated gas flowlines is discussed below. There are 20 flowlines installed during 1986 as part of the gas expansion program to process the non-associated (Khuff formation) gas from the gas wells. These flowlines are made of carbon steel that run for varying lengths from 5 to 15 km depending on the well location with a cumulative length of more than 300 km. Most of these flowlines are buried, with the fusion bonded epoxy (FBE) coating system as the primary method of external protection with impressed current CP systems installed at the wellheads providing supplemental protection.

External FBE coating failure on hot flowlineFig. 1(source: NACE Corrosion 2008 Paper 08044).

External FBE coating failure on hot flowlineFigure 2: External FBE coating failure on hot flowline Fig. 2 (source: NACE Corrosion 2008 Paper 08044).

The Khuff formation gas was a new source of gas supply and the operating temperature of the system was not able to be predicted during the design phase in 1985 due to lack of adequate information. Hence, an approximate operating temperature of 210ºF was considered as the design basis for the external coating selection criteria. However, after the commissioning, the operating temperature was observed to reach up to 250 ºF. The FBE coating system applied on these flowlines had a temperature resistance of 210 º F and subsequently experienced premature failure to a varying extent, with respect to the temperature excursion and exposure.

Restoring Corrosion Protection

A comprehensive evaluation was conducted to determine external coating condition of these khuff gas flowlines. The objective of this evaluation was to assess the condition of these pipelines, prioritize the pipelines for corrective actions and develop options to restore the integrity from external corrosion. As part of this evaluation, numerous bell holes were excavated to expose these pipelines for a meaningful assessment.