High-density polyethylene (HDPE) pipes have been used in essential service water (ESW) systems of nuclear power plants (NPP) for years. Some NPPs use buried HDPE pipes, while some others use above ground (gallery-installed) HDPE pipes, and this paper will discuss the differences in the design methods of these two construction methods in related technical standards. The design requirements and related load types of buried and above ground HDPE pipes in safety-related Class 3 service water or cooling water piping systems are compared. Pipeline models under different load types are introduced, including model assumption, material simplification, boundary conditions, interactions, etc. The differences in design loads are summarized, i.e., soil and surcharge load, negative internal pressure, flotation due to flood of buried HDPE pipe, and axial forces due to different supporting or fixing methods of above ground HDPE pipe. The design methods of buried and above ground HDPE pipeline are discussed and compared, i.e., design of mechanical load, temperature load, non-repeated anchor movements and seismic load. A case study is presented by using different technical standards of buried and above ground HDPE pipes. This work can provide a reference for the selection of HDPE pipes for safety-related Class 3 service water or cooling water piping systems in different NPPs.