Corrosion Protection Program For A High-Temperature Subsea Pipeline
by Mike Surkein and Steve LeBlanc, Shannon Richards, John P. LaFontaine (2001)
A subsea pipeline has been installed with a novel corrosion protection scheme. The pipeline and associated components have been protected from corrosion with a combination of a high temperature pipeline fusion bonded epoxy coating, a thermal spray aluminum coating and sacrificial cathodic protection anodes. Due to the high operating temperature of the pipeline, anode bracelet installation on the pipeline was not desirable. A cathodic protection attenuation model was used to design the anodes for the pipeline. The model indicated that the sacrificial anodes could be installed at either end of the two-mile pipeline. To assure adequate corrosion control was achieved by the cathodic protection system, a sophisticated in-situ monitoring system was installed. Also, a diver inspection was conducted to measure cathodic protection performance. This paper presents the corrosion control design approach used for each of the major components including the risers, pipeline and buried expansion boxes. The design basis will be compared to the results of the in-situ and diver gathered monitoring data.
A typical one-well development for a high-temperature well in shallow water includes a three or four-pile platform, wellhead coolers, and utilities. These facilities are required to protect the flowline from the increased corrosion rates and mechanical difficulties that it will experience at full well stream temperatures. For the development being discussed, wellhead temperatures range from 265 F (129 C) to 315 F (157 C). The challenge of this design was to minimize the wellhead facility by eliminating the cooler and its associated utilities, while addressing the corrosion and mechanical difficulties.
The mechanical design concerns for high-temperature, buried flowlines are thermal expansion and upheaval buckling. Thermal expansion will occur when the flowline operating temperature is higher than its as-installed temperature. In buried environments, natural flowline expansion is constrained by the axial and lateral loads of the soil on the flowline. Where there are imperfections in the flowline profile, the line will tend to experience lateral or upheaval movement.
To overcome the thermal expansion challenges; the flowline was designed with expansion doglegs. The doglegs were housed in expansion boxes (Figure 1). The expansion boxes allowed lateral growth of the flowline.
Corrosion concerns at elevated temperatures include high cathodic protection consumption and coatings degradation. Anodic current density increases and its capacity decreases with increasing temperatures. Both of these conditions work against a standard system of bracelet anodes installed directly onto the flowline. The challenge in selecting a suitable coating is to find a system that stands up to high temperatures and resists cathodic disbondment.