Direct Electrical Heating (DEH) is applied to subsea oil and gas flowlines for hydrate control and avoiding flow problems during production and after shut-ins. In a DEH system the flowline to be heated is thermally insulated and the steel wall of the flowline is made an active conductor for electric current. Single-phase alternating current (AC) from a topsides power supply is introduced into the wall of the flowline via apiggyback electrical cable resulting in resistive or joule heating which maintains the pipe contents above the wax melting point and hydrate formation temperature. The flow of AC can lead to electromagnetic interference in other metallic components and structures that are proximal to the flowline. Such components include collocated umbilicals and the armor wire used in the protective sheathing of such umbilicals. A laboratory investigation was conducted to study the effect of AC interference on the corrosion of two types of umbilical armor wire – zinc-galvanized and bare carbon-steel. AC corrosion testing in mildly-sour anoxic synthetic seawater conditions representative of the anticipated deep-sea service environment revealed significant differences in the corrosion behaviour of the two types of wire as a function of the average AC current density.
Deposits can play an important role in the corrosion rate and morphology of carbon steel in a production environment as well as affect the efficacy of an inhibitor. A test method has been developed to investigate the corrosion characteristics of carbon steel in a stratified flow regime where deposits of solids accumulate in the bottom of a pipeline.Previous testing showed higher under deposit corrosion rates of API X-65 carbon steel when the deposit consists of iron sulfide (FeS) versus sand in a 100%H 2 S environment. There is a needto investigate the effect of CO2 on these findings as the lower pH should affect the corrosion rate morphology and galvanic action particularly with respect to the FeS deposits.This paper presents the effect of mixed gases (50%H 2 S + 50%CO2 ) on corrosion behavior of API X-65 carbon steel coupons in the FeS and sand deposits. The average corrosion rate thickness loss of the coupons and pitting at the solid/liquid interface will be presented.Keywords: Under deposit corrosion Iron sulfide (FeS) sand mill scale acid gases
Alternating Current (AC) interference affects pipelines collocated near high voltage transmission lines. While overhead high voltage AC transmission line collocations are common underground high voltage AC transmission line collocations while less common can cause similar interference effects on pipelines. This paper describes a feasibility evaluation of a proposedunderground transmission line route in the United Kingdom affecting a nearby pipeline. The proposed transmission line was to consist of either three 220 kV circuits or six 132 kV circuits; both cases were evaluated. The proposed transmission line phase cables were to be installed with concentric sheath conductors direct buried in a close trefoil configuration and bonded to underground splice boxes at various locations. In addition to the proposed transmission line route the project evaluated a set of base cases: a typical pipeline paralleling at varying offsets for various lengths under a variety of operating conditions and a typical pipeline crossing scenario. The effect of soil resistivity on the interference levels experienced by the pipeline was considered. The feasibility study assessed the expected steady state fault state and AC corrosion effects that the proposed transmission line could have on any nearby pipelines. Results for all cases and scenarios will be discussed.Key words: AC interference case study underground AC power transmission pipeline integrity