Materials selection, cathodic protection and coatings selection on the Great Manmade River Project

Transcription

1 Materials selection, cathodic protection and coatings selection on the Great Manmade River Project Abstract John Boran, Saleh Elkoum, Ken Johnson, Steve Wroe Brown & Root N.A., Leatherhead, UK Telephone: Telefax: Khalifa Enawaa Great Man-made River Authority P.O. Box 9468 Benghazi, G.S.P.L.A.J. The Great Man-made River Project (GMRP) in Libya is the World's largest water supply project. Much of the Project is built in the Sahara Desert. The first two phases will shortly deliver approximately 4.5 million cubic metres of water per day through approximately 2440 kilometres of large diameter main conveyance pipelines from wells in the desert to the coastal regions. The water is intended for municipal, industrial and agricultural use. A contractual requirement of the Project is for minimum maintenance and a 50 year design life for major equipment in the demanding environments in Libya. Several aspects of corrosion control used on the Project are described in some detail. 1. Introduction This paper deals with the material selection and corrosion control aspects of the Project. The external environment is severe, atmospheric exposure comprising a maximum ambient temperature of 50 C and a surface temperature on some equipment of up to 80 C, ultra violet levels are high and abrasion can take place with wind blown sand. The buried environment has a high ambient temperature and high chloride and sulphate levels in some sabkha areas, the water table can reach the pipe burial depth. The internal environment is fresh water, but at temperatures up to 35 C, and average chloride contents of 206 ppm in Phase I and 370 ppm in Phase II. The water also contains varying levels of dissolved carbon dioxide, up to approximately 70 ppm. The principal component of the GMRP is the main conveyance pipeline system, which is made from pre-stressed concrete cylinder pipes (PCCP). The pipes consist of a steel cylinder onto which is welded end rings to form a bell at one end and a spigot at the other end. A concrete liner is cast simultaneously on the outside and inside of the steel cylinder. The external concrete is prestressed with high tensile steel pre-stressing wire wound helically at approximately 75% of the Specified Minimum Ultimate Tensile Strength (SMUTS) of the wire. The pre-stressing wire is covered with cement mortar.

2 2. Corrosion protection philosophy Two important factors in deciding corrosion protection requirements for any particular item are maintainability and design life. A contractual requirement for the GMRP is for minimum maintenance and a 50 year design life for major equipment in the demanding environments in Libya. Briefly, the design life philosophy that was developed for the pipelines and mechanical equipment is as follows:- a) All items of plant where any shutdown would result in a stop in production will have a 50 year design life. Typical examples include buried pipework generally and in particular, the PCCP conveyance system pipework and associated fittings and block valves. b) All items of plant where any shutdown would severely reduce production will have a 25 year design life. Typical examples include wellpumps and motors, pump station pumps and connecting pipework and flow control valves and connecting pipework. The overall corrosion protection philosophy is thus to select corrosion protection measures to ensure that the equipment design life is achieved in the applicable operating environment. Corrosion protection measures that were considered in order to meet the design life included selection of appropriate materials of construction, corrosion allowances, high performance protective coatings and cathodic protection or a combination thereof. 3. Material selection Extended abstract length requirements prevent a full discussion of material selection issues on the GMRP. It is envisaged that a fuller paper will be published in due course. This section is therefore restricted to a brief discussion of some major material selection decisions, whilst the paper concentrates on the PCCP coating, cathodic protection and hydrogen embrittlement (HE) studies on the PCCP pre-stressing wire. 3.1 Hasouna Jeffara System (Phase II) Previous corrosion coupon studies in the wellfield had indicated that grades of stainless steel more resistant to aqueous chloride environments than AISI Type 316 / 316L would be required, especially in view of the sterilisation requirements for the wells. A decision was taken to specify glass reinforced plastic (GRP) for the well casing, 22% chromium duplex stainless steel for the well riser pipes, and Niresist for the wellpumps and motors. The 484 wells, drilled to depths of up to 550 metres represent a novel use of GRP on this scale. A vinyl ester resin was selected for its resistance to alkaline attack as the well casing is cemented on the outside after installation. Long term testing to BS 5480 was performed in elevated temperature alkaline environments to ensure that the 50 year design life requirement was met.

3 The high carbon dioxide levels in the water resulted in the selection of duplex and other high grade stainless steels for valves and instrumentation etc. upstream of the centralised wellfield degassing plants. Copper base alloys were selected for these components downstream of degassing. 3.2 PCCP pre-stressing wire and hydrogen embrittlement (HE) studies A major concern over the durability of the PCCP pipe has been the susceptibility of the high strength steel pre-stressing wire to HE, causing premature fracture of the wire and failure of the pipe due to the loss of the pretension that is supplied by the wires. There have been a number of failures reported of PCCP pipes, in particular in the USA, as a result of hydrogen embrittlement of the pre-stressing wire. HE can be caused by the hydrogen generated in a corrosion pit on the wire if the cement mortar and coating system has failed or by cathodic protection at too negative a potential, causing nascent hydrogen to be generated on the wire surface. The Project undertook a study into the susceptibility of the pre-stressing wire to HE. Three external laboratories undertook the study. The study looked at the basic mechanical and chemical properties of the wires, the susceptibility of the wires to HE in a standard test for HE, the so-called FIP test, and the behaviour of the wires under dynamic and static load when cathodically protected in a cementitious environment. The majority of the wires, which came from a number of wire producers supplying wire to the Project, were susceptible to HE in the standard HE test. The wires also showed a great deal of variability in terms of their time to failure in the test. The following conclusions were drawn. a) The cathodic protection potential at which the most susceptible wires in the FIP test showed failure due to HE under dynamic loading was lower by 100mV than the most negative potential specified for cathodically protected wire on the Project (-1000 mv vs. Cu/CuSO 4, excluding the estimated 50mV IR drop). Therefore, whist the wires were susceptible to HE, normal levels of cathodic protection would not cause this embrittlement. b) The result above for dynamically loaded wires was repeated for statically loaded wires, where at even lower potentials no failures of statically loaded wires took place. c) Recommendations were made with respect to the specification of wire more resistant to HE for Phase III of the Project. It was also recommended that in order to prevent HE as a result of pitting corrosion of the wire, all PCCP should be cathodically protected. In particular all newly installed PCCP should be cathodically protected from the outset, since it is cost effective to install sacrificial anodes during pipe installation. 4. Protective coatings The following is a discussion of the PCCP coating issues.

4 4.1 External surfaces of PCCP The mechanical and primary corrosion protection measure applied to the pipe prestressing wires is a minimum 19mm thick cement mortar coating. In non-aggressive ground this is considered to be sufficient for the 50 year design life. In aggressive ground however, additional corrosion protection is required both to prevent degradation of the cement mortar by sulphates and attack on the pre-stressing wires by chloride. A logic diagram, which uses the results from geotechnical surveys of the pipeline route, was developed to determine whether or not additional corrosion protection was required. Factors considered included stray current interaction, groundwater criteria, soil natural moisture content, soil resistivity, sulphate and chloride concentrations and soil ph. The barrier coating selected for application to the external surfaces of the PCCP in aggressive ground conditions was coal tar epoxy. This was selected on the basis of cost, ability to be applied to green mortar and hence minimal disruption to pipe production, has a track record, albeit limited, and has good dielectric properties. Pipes in marginally aggressive soils were also coal tar epoxy coated. However, it was appreciated that coal tar epoxy has poor ageing properties, rapidly becomes brittle and has poor crack bridging properties. Due to this, cathodic protection is being provided to supplement the barrier coating and this is described in more detail in Section 5 below. 4.2 Internal surfaces of PCCP On Phase II centralised degassing has been adopted to reduce the dissolved carbon dioxide in the wellwater to an acceptable level from a concrete degradation point of view. However, upstream of degassing some form of protective coating is required for the concrete core of the PCCP to prevent attack by dissolved carbon dioxide. Primary characteristics required of the coating are good crack bridging capability (0.5mm minimum), good mechanical and bonding properties, including resistance to pressurisation/depressurisation; resistance to propagation of defects; possess very low permeability to the penetration of water, moisture, oxygen, carbonic acid, hydrogen sulphide and chloride; not degraded by wellwater or disinfection chemicals and does not support microbial growth; low coefficient of friction and resistant to abrasion, erosion and mechanical damage; inert to alkali attack; potable water certified. A desk study was carried out to select suitable candidate materials, which were then subjected to site trials. Following the site trials, coated samples were cut from the trial pipes and subjected to laboratory testing to assess the characteristics above. The four candidate materials included two liquid applied coatings, polyurea and polyurethane, and two ribbed sheet liners, one constructed from polyethylene and one from polyvinyl chloride. The liquid applied coatings were sprayed onto the pipe and the ribbed sheet liners were cast into the concrete core. The laboratory tests were carried out using

5 proprietary test methods. The coating finally selected was 1mm thick polyurea and this has been applied to the internal surface of PCCP installed upstream of degassing. 5. Cathodic protection 5.1 Phase I, Eastern Jamahiriya System On Phase I of the Project, a decision was taken at an early stage to bond the pipe sections to facilitate future cathodic protection. Additionally, extensive surveys of soil resistivity, static and close interval potentials and long line currents were carried out along the whole length of the conveyance system. These were interpreted along with chemical analyses of soil and groundwater and predictions of groundwater rise due to future agricultural use. From these surveys, lengths of line were identified where it was felt that the coal tar epoxy coating required supplementing by cathodic protection. In total, cathodic protection was required for 317km of the total 1546km of PCCP. Sacrificial anode and impressed current cathodic protection were evaluated and testing was carried out to determine the current densities required and current "spread". Due to the possibility of HE of the pre-stressing wires, zinc anodes were selected for most areas with smaller quantities of magnesium anodes where resistivities indicated that zinc would give insufficient output. A computer model was also developed. It was determined that optimum protection would be achieved by using relatively long, slender anodes installed vertically in boreholes filled with gypsum/bentonite slurry backfill. Typically these anode "strings" consist of 6-8 anodes approximately 900mm long and 40mm square. The anodes are cast on a continuous stainless steel wire with a factory connected cable tail. In total, anode strings; 1430 tonnes of zinc and 223 tonnes of magnesium are being supplied. Initially, two sections of pipe were selected as "Optimisation Sections" so that the design and installation methods could be finalised. Based on a literature survey, the protected potential range was set as a polarised (or "off") potential of 710 to 965mV vs. Cu/CuS0 4, studies having shown that the pre-stressing wire was not susceptible to HE at this lower limit. The current density required to achieve protection was more difficult to assess. A literature survey revealed a wide range from 0.15 to ma/m² pipe surface area. These figures relate to different pipe sizes, soil resistivities, new and corroded pipe etc. A complicating factor was that on the GMRA system, protection was to be applied locally so that allowance had to be made for current losses to pipe not in need of protection. Finally, based on field tests, a current density of 1.25mA/m² was selected. The design life of zinc anodes is 25 years and of magnesium 12.5 years (due to the higher rate of self corrosion of this material). 5.2 Phase II, Western Jamahiriya System The Phase II pipeline was under construction when the results of the surveys on Phase I were obtained and analysed. These had shown that a great proportion of the coastal routes would require supplementary cathodic protection. The similarity between conditions on the coastal routes of Phases I and II lead to the conclusion that these

6 would also require extensive protection. Alternative methods of providing protection, which could be incorporated into the construction works, were evaluated. Due to the cost savings over retrofit cathodic protection (approximately 50%) and the fact that time was not available to fully assess the areas requiring cathodic protection, a decision was made to install the "built-in" system over the whole of the 130km coastal section. In accordance with the requirement to avoid overprotection, a sacrificial zinc ribbon anode system with a design life of 25 years was chosen. This was based on the installation of a total of 4 nos. continuous zinc ribbon anodes (two at the invert and two at the crown of the pipe) in the trench parallel to the pipeline. The anodes were surrounded by low resistivity backfill and electrically bonded to the pipe. As the anode was directly connected, it is not possible to measure output currents or polarised potentials. Field tests had indicated the approximate value of the IR drop in the soil to be 50mV, so the criteria for protection were therefore adjusted to the range 760 to 1050mV with respect to a Cu/CuS0 4 electrode placed over the centre line of the pipe. 5.3 Preliminary results and discussion Installation of the Phase I cathodic protection is underway and results from sections already energised indicate that good levels of protection will be achieved. In many areas the available current has been greater than calculated based on spot resistivities and in lower resistivity areas the current output has self regulated or already been trimmed to maximise the anode life. Results from Phase II show that the wire has polarised to protected potentials in the region of 900 to 1000mV vs. Cu/CuSO 4 and, although the installation method here does not allow current adjustment, some self regulation will occur. Full polarisation has taken several months to achieve. At the ends of protected sections, the current output remains high due to drain from the unprotected line. It is noticeable that the polarisation shift may attenuate quickly at the end of protected sections, although some protection is seen up to 10km distant. 6. Conclusions The vast scale of the GMRP has necessitated close attention to the design and corrosion protection measures of many diverse items of plant and equipment. The sheer scale of the Project means that solutions have to be capable of being successfully implemented in practice without unduly affecting cost and schedule. Several innovative solutions to material degradation problems have been used on the project, including the use of GRP for wellcasing, the design of wellscreens for deep wells, the specification, selection and protection of prestressing wire for the PCCP and the design of internal protection of PCCP against high carbon dioxide containing water. These solutions will ensure that the 50 year design life of the project is achieved. Acknowledgements The authors would like to express their appreciation to the Great Man-made River Authority and to Brown & Root N.A. for giving permission to produce this paper.