The corrosion rate of gas condensate pipelines can be substantially reduced by increasing the pH artificially. The technique is called pH stabilisation and has been used with success in gas condensate pipelines. The reduction is based on the precipitation of protective corrosion products on the steel surface. When corrosion product films are formed, it is the transport of reactants and corrosion products through the film which governs the corrosion rate. Film properties like porosity, thickness and composition therefore become important. All these properties are strongly related to the precipitation process which depends very much on supersaturation and temperature. A large number of flow loop and glass cell experiments have been carried out in order to study these aspects. In addition to the C02 partial pressure, the hydrate preventer, the flow velocity, and the pH, a number of variables related to the steel surface conditions and the operation of a real pipeline were studied in the experiments. The last group of variables included the presence of mill scale and rust on the steel surface prior to exposure, periods without flow (shut down), draining of the pipeline and scratches in the protective film. The paper discusses how these parameters affected the performance of carbon steel in water-glycol(50%) systems with 0.6 MPa C02 partial pressure and with sodium bicarbonate added as pH stabilisator.
Transportation of unprocessed hydrocarbons overlong distances from the seabed to a nearby platform or directly to shore for processing is often a cost efficient alternative for development of new fields. Internal corrosion in such pipelines has traditionally been combated by means of surface active inhibitors. Traditional inhibitors will not give sufficient protection of carbon steels under very aggressive conditions and one alternative in such cases will be selection of corrosion resistant materials, as stainless steels. Another much cheaper method that has been applied for gas/condensate pipelines is the pH- stabilisation technique. The technique which was introduced by Elf Exploration Production 25 years ago has been used in Italy, the Netherlands (Petroland), France (Pecorade) and Norway (Lille-Frigg, Heimdal, Fray, Troll). A survey of systems treated with pH stabilisers is given in Table 1. The experiences so far have been very good. Corrosion rates fm below 0.1 mndyear have been reported for all systems, even when the C02 partial pressure was higher than 1MPa.
Limited information about the pH stabilisation technology is available in the open literature. A large number of flow loop experiments, however, have been carried out at Institute for Energy Technology (IFE) with such systems during the last seven years. Before the pH stabilisation technique was approved for the Lille-Frigg field in 1992, an experimental program was carried out where the performance of MDEA (Methyldiethanolarnine) as pH stabiliser was studied. The experiments showed that when pH was increased to 6.5 by adding MDEA corrosion rates well below 0.1 mm/year could be obtained.
MDEA is an organic amine base and from an environmental point of view it is not the ideal candidate for the pH stabilisation of systems where the pH stabiliser is not filly regenerated. In order to study the pH stabilisation technique in more detail and to find alternative additives, the Kjeller pH- stabilisation Project (KPP) was run at IFE. The main objective for the project was to find environmental friendly additive chemicals which increase and stabilise the pH of the well stream, so that sufficient and permanent corrosion protection of pipelines transporting hydrocarbons is achieved.
