This study focuses on the design of a Reinforced Thermoplastic Pipeline (RTP), made of High-density polyethylene (HDPE) inner pipe, reinforced by glass fiber wrapping (GFRP), and inserted into an outer HDPE pipe, under internal pressure. GFRP wrappings with different stacking thickness and fiber orientations are investigated in the numerical design. Finite element analyses are validated based on available analytical solution of orthotropic cylinders under internal pressure and are used to predict the internal pressure capacity of the RTPs. Overall, from the parametric study, it is understood that (i) the pressure is mainly carried by the reinforcement layers and marginally by the inner HDPE, and (ii) GFRP with 45° orientation works more efficiently than those with 0/90°. At higher internal pressures, either the thickness of glass fiber should increase, or the fiber orientation should be changed from 0/90° to 45° to maintain the stress on glass fiber layers within allowable utilization ratios (UR). Assuming a consequence factor of 1.3 (UR of 77%) needs to be maintained, in a RTP with a DN80 HDPE inner pipe and at 45° orientation, GFRP with thickness of 1.1 mm is adequate to carry up to 8 MPa internal pressure. If a 90° orientation is used, the RTP can take up to 6 MPa with the same UR. The parametric study proposes required minimum thickness for RTPs of different fiber orientations and at internal pressures. This study prepares a design for future experimental studies as well as investigation into performance of RTPs under combined loadings.