Developing a Procedure for Welding Thermoplastic Seawater Piping Procedures were qualified and a 450-m-long seawater intake pipeline was fabricated with high-quality welds BY Y. V. HARINATH AND T. V. KRISHNA MOHAN NOVEMBER 2016 / WELDING JOURNAL 51 Many industries around the world, including power generating plants, are located on the seashore because they require seawater for cooling equipment and/or use the sea as the ultimate heat sink. Desalination plants need large amounts of seawater for producing potable water (Ref. 1). Aqua farms also need large amounts of fresh seawater continuously for culturing various marine organisms (Ref. 2). These industries draw seawater using an appropriately designed intake system. For one such facility at the Bhabha Atomic Research Centre in Kalpakkam, India, a seawater intake pipeline was constructed that consisted of a 355-mm (14-in.) outer diameter (OD) high-density polyethylene (HDPE), 600-m- (1998-ft-) long pipeline laid into the Bay of Bengal. The pipeline includes 150 m (500 ft) onshore and 450 m (1500 ft) offshore parts connected to onshore pumps. The entire pipeline was fabricated by welding 12-m- (40-ft-) long standard pipes. The offshore part, after fabrication, was towed into the sea for deployment at a required position and the onshore section was buried in the soil at 1 m (3.3 ft) depth. The cross-sectional schematic layout of the intake pipeline is shown in Fig. 1. The pipe weld joints in this type of system must be strong enough to withstand the erection and operational loads. This can be obtained only when a proven standard welding procedure is followed. Although there are many standards available for carrying out HDPE thermoplastic fusion welding, these contain only certain guidelines. Detailed welding parameters for various steps have not been mentioned. These parameters have to be standardized and the procedure has to be qualified specifically for any project. This is because these parameters depend on the pipe size, wall thickness, and thermal conductivity. Hence there was a need to study the effect of various weld parameters and prepare a unique welding procedure that could be adopted for this project. By following this procedure, supplemented with a stringent inspection plan, sound weld joints could be obtained. Pipe Material Metallic pipes corrode under seawater conditions, and are rigid, heavy, and expensive. Therefore, use of plastic pipes has increased for applications like distribution of potable water, discharge of waste water, transport of gas, and intake and discharge of seawater. High-density polyethylene (HDPE-IS 4984) (Ref. 3) is considered to be the most appropriate material for seawater applications because it is resistant to corrosion and UV radiation (Ref. 4). In addition, it is light in weight, highly flexible, and has good strength to withstand erection loads. As per certain design criteria (Ref. 5), PE 100 grade of HDPE material and PN 10 pressure rating with wall thick- Fig. 1 — Cross-sectional schematic layout of the HDPE pipeline of a seawater intake system.
Welding Journal | November 2016
To see the actual publication please follow the link above