Polyethylene Coating Damage on an Underwater Pipeline in the Southern North Sea

Abstract

ABSTRACT Following a recent inspection, approximately 60 km of a 138 km methanol pipeline was found to be under-protected due to the premature consumption of the original zinc bracelet sacrificial anodes. The cathodic protection system had been designed to last 40 years accommodating uniform coating holidays of up to 3% of the total surface area. However, substantial coating damage (in excess of the design allowance) in localised areas has resulted in accelerated depletion of the system in less than 12 years. A retrofit, sled anode, cathodic protection system was designed to furnish the current required to protect the pipeline for the next 30 years. This system is composed of a 330 kg, Al-Zn-In alloy, stand-off anode mounted upon a low profile sled. The design can provide a 3 amp current for 30 years, which will protectapproximately 200 metres of uncoated pipeline at a current density of 50 mA/m2 During the autumn of 1983, 50 of these sled anodes were installed on the main methanol line. The result was a 200 mV cathodic potential shift within 24 hours. INTRODUCTION Following a recent inspection of all pipelines associated with the Viking Gas Field, all the gas pipelines were found to be in an acceptable condition, but a substantial proportion of the methanol pipelines were found to be in need of remedial action. This paper deals only with the methanol lines, however results of the gas line survey are given for comparison. It addresses the possible causes of observed damage and the remedial work carried out on this polyethylene coated 3 methanol pipeline. The work will be of interest to other operators with lines of similar age and coating type. FIELD DESCRIPTION The Viking Gas Field is located 86 miles due East of the Lincolnshire coast in the southern North Sea (Figure 1). The field consists of two main complexes (A and B) and five unmanned satellite platforms (Figure 2).The satellite platforms, the B Complex and the A Complex are connected by infield feeder pipelines. The main gas export pipeline is a 28" line of 138 km in length between the A Complex and the shore terminal. Current winter production figures for the field are:Dry gas present peak production: 750 MMSCFD Condensate average well production:4 bb1s/MMSCF Connate water average well production:less than 1 bbl/MMSCF To prevent the formation of hydrates, methanol is injected into the gas stream at various places. The methanol is pumped from shore along a 31/2 pipeline piggyback to the 28" gas line, and distributed throughout the field by piggyback pipelines. The produced gas, condensate, water and methanol are separated at the shore terminal. After storage and make-up, the methanol is pumped offshore as required.