High Pressure / High Temperature (HP/HT) design of a flowline system has become a common design challenge as the quest to extract hydrocarbons has pushed the industry to exploit deeper reservoirs. In recent years, industry has classified systems over~10,000 psi and over~300°F as XHPHT (Extra HP/HT). As subsea technology continues to develop, it may not be farfetched to be soon considering UHPHT (Ultra HP/HT) flowline designs.This paper addresses the key features and challenges of the HP/HT design as applied to the production flowlines of a deepwater Miocene GoM (Gulf of Mexico) field. The dual production flowlines are each 10 miles long with SITP (Shut-In Tubing Pressure) of~15000-psi and maximum design temperature of 300°F (classified as XHPHT). HP/HT specific design challenges are associated with large expansion, global buckling, axial creep, and low-cycle fatigue, which are directly influenced by the flowline length. Even though the tie-back distance to the host is relatively long, the definition of "short" and "long" flowline/pipeline loses its significance due to distributed responses (both "Natural" and "Engineered") of system. Additionally, due to field-specific fluid properties, excessive pressure drops across the well perforation and / or the subsea choke can cause Joule-Thomson heating effects, which must be accounted for in design. This paper further discusses optimization of various design components in order to accommodate small variations in the input parameters (soil, operational transients, installation tolerances etc.) that are crucial with respect to global and local responses.This case study uses mitigation techniques at pre-determined locations to address the design concerns such as large expansions, rogue (uncontrolled) buckles, axial creep ("walking"), and low-cycle fatigue. By exercising careful review of the iterative assessments and subsequent optimization of the system's overall design, a safe and reliable solution is achieved. HP/HT design philosophy for deepwater Miocene GOM fields form an integral part of design / implementation processes in the field development program and have significant influence on the overall subsea architecture, procurement and construction strategy, operating philosophy, and integrity management plan. Finally, the paper shows a comparison of numerical prediction and the data acquired from the field observations. The predicted system response is in good agreement with the field observations, thereby, validating the overall assessment/modeling approach and confirming the robustness of the system's response.