Because of the large market, it is relatively convenient for enterprises to procure raw and auxiliary materials. In pipe-in-pipe R&D and production, determination of polyethylene outer-casing thickness depends mainly on the post-foaming process. Pipe-in-pipe production basically uses one-shot pouring molding. Because the steel pipe is 12 m long, the pipe has a certain density gradient both axially and circumferentially, which is especially pronounced for large diameters. To achieve the minimum core density required by the standard, PU raw materials are overfilled, and the pouring density is much higher than the core density required by the standard. The relatively large expansion internal pressure generated during PU foaming causes the PE outer casing to expand and form residual stress. To keep the stress in the PE outer casing at a safe level, a greater outer-casing wall thickness is used. In contrast, the production process of first spray-foaming and then winding the polyethylene outer jacket does not produce expansion residual stress in the PE pipe, so the stress level of the pipe is very low and the polyethylene protective layer can be relatively thinner.

　　Field-prefabricated insulated pipe: This product emerged in the 1990s as a simplified direct-buried heating pipeline to save heating-pipeline project costs. PU insulated direct-buried pipe not only offers advanced technology and practical performance that traditional trench and overhead laying can hardly match, but also significant social and economic benefits, and is a powerful measure and necessary safeguard for heating energy conservation. Based on the basic properties of thermal pipelines, oilfields adopt appropriate anti-corrosion and insulation measures, optimize the thermal pipeline system, ensure application quality during heating-energy transmission, and eliminate adverse risk factors. Anti-corrosion insulated steel pipes have different processing types and methods; different processing methods determine different performance and functions of the steel pipe, and the scope of use also changes accordingly.

　　Direct-buried insulated pipe, also known as “pipe-in-pipe,” is produced by a “two-step method” and consists of an HDPE outer jacket, rigid PU foam insulation, and a steel pipe. The insulation material is rigid PU foam with a density of 60 kg/m3 to 80 kg/m3, fully filling the gap between the steel pipe and the casing and providing a certain bonding strength so that the steel pipe, outer casing, and insulation layer form a firm composite. The foam of PU insulated direct-buried pipe has good mechanical and thermal insulation properties; it can typically withstand 120°C, and through modification or combination with other insulation materials can withstand 180°C, making it suitable for insulation of various hot- and cold-water pipelines at high and low temperatures.

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