Field Experience on the Inspection and Performance of Subsea Pipeline Cathodic Protection Systems

Abstract

Abstract DPC has one of the most complex offshore pipeline systems in the world. The field is mature with many of the pipeline CP systems at or approaching the end of their life. A regular inspection program has to be set up to monitor these systems. With good historical inspection data, future inspections are now based on the condition of each individual CP system, rather than carried out at fixed time intervals. This has resulted in significant cost savings. Premature depletion of the pipeline anodes has been seen which is due to a number of factors, including the interaction of pipeline and platform CP systems. With careful planning this same interaction can be used to reduce anode retrofitting costs on pipelines. This has only a marginal cost benefit to DPC, but will have a significant cost benefit in other areas of the world where diving conditions are more severe. Introduction DPC operates one of the most complex offshore areas in the world, in terms of number of platforms and number of pipelines (Figure 1). There are 2 main fields, Fateh and South West Fateh and 2 smaller fields, Falah and Rashid. The fields are characterized by a large number of jackets in relatively shallow water linked by short in-field pipelines (typically 10,000 ft.) carrying waterflood water and gas lift gas to the platforms and produced fluids from the platforms. The fields are linked by longer transfer pipelines (typically 40,000-50,000 ft). In total there are 76 jackets. Water depths vary from 100 ft to 191 ft with an average of 144 ft. The oldest jackets and pipelines are 22 years old. There are 112 pipelines in service, with a total length of 276.7 miles. Line sizes range from 4" to 42", with the majority of the lines in the 12" to 18" range. Cathodic Protection Inspection History Early Life of the Field Originally all the pipelines were electrically isolated from the platforms, using insulation flanges. The platforms and pipelines are protected by sacrificial anodes. As these systems were then relatively new, the levels of cathodic protection (CP) were good. Annual dip check readings were started on the platforms, taking readings on each leg and on each riser. Due to the risks of entanglement of the reference electrode lowered in the water with the jacket structural members, readings were only taken down to -25ft. Over a period of time shorting occurred over the insulation flanges, such that pipeline anodes were protecting platforms and vice versa. Whilst it is not cost effective to have pipeline anodes protecting platforms, in a few instances this was certainly the case. The first platform, "C", was retrofitted in 1983 (ref. 1). The CP system on "C" platform was designed for a 10 year life; after 15 years the potentials were -940mV (ref Ag/AgCl), with most of the anodes totally consumed. The platform was being protected by the pipeline anodes.