High Pressure and High Temperature (-HPHT) conditions, with pressures greater than 69 MPa and temperatures greater than 150℃ C, pose significant challenges for well tubular designs.In particular,an elevated design standard is required for the leakage resistance of casing and tubing connections in HPHT wells.The traditional deterministic design approaches may be insullicient which evaluate the sealability of connections by considering the effect of the worst geometry, minimum yield strength and worst combination of loads.To take into account the uncertainties related to the variability in tubular connection geometry,material properties,and operational loads, a Reliability-based Design and Assessment (RBDA) methodology should be This paper presents an RBIDA methodology that characterizes connection sealability using a quantitative estimate of reliability.In the RBDA approach, the connection sealability is considered as a serviceability performance limit state,with the sealing assessment criteria established based on a review of existing testing and analysis data.The RBDA is performed by Monte Carlo simulation using a surrogate function implicitly defined by the connection geometry, material properties, and load conditions, based on parametric Finite Element Analyses (FEA) involving the variation of several design parameters.A case study of a premium connection is also presented to demonstrate the use of the proposed methodology.