This paper presents the parametric 3D modelling of a subsea Inline Tee (ILT) pipe assembly using Autodesk Inventor Professional, applied to an API 5L pipe with an outer diameter of 273.1 mm (10¾ in NPS) and a wall thickness of 12.7 mm (½ in), yielding a D/t ratio of 21.5. The assembly encompasses a main pipeline, tee branch, pup pieces, elbows, and ball valve (BV) sub-assemblies, with all critical dimensions governed by a centralised parameter table comprising seven named variables: L_pipe, L_pup, L_WidthTee, L_elbow, L_BV, L_MainPipe, and L_HeightTee. Two ILT configurations are addressed — Type 1 (single-branch, without left-side BV) and Type 2 (dual-branch, with full BV assemblies on both sides). Parametric update validation confirmed complete model responsiveness with zero feature failures, and the computed D/t ratio of 21.5 satisfies the ASME B31.3 structural limit of 100. The framework provides a reproducible, standards-aligned methodology for ILT component modelling in offshore pipeline design.