An offshore universal riser system (OURS) and injection system (OURS-IS) inserted into a riser. The OURS/OUR-IS provides a means for pressurizing the marine riser to its maximum pressure capability and easily allows variation of the fluid density in the riser. The OURS-IS includes a riser pup joint with provision for injecting a fluid into the riser with isolation valves. The OURS includes a riserpup joint with an inner riser adapter, a pressure test nipple, a safety device, outlets with valves for diverting the mud flow, nipples with seal bores for accepting RCDs. The easy delivery of fluids to the OURS-IS is described. A method is detailed to manipulate the density in the riser to provide a wide range of operating pressures and densities enabling the concepts of Managed Pressure Drillin g, Dual Density Drilling or Dual Gradient Drilling, and Underbalanced Drilling.