Methods and apparatus for installing deepwater completions and performing well intervention from a vessel that can perform other duties while not running completions or performing interventions. The system for installing deepwater completions and performing well intervention may comprise a surface pipe handling and deployment package including a horizontally operated rig that may also be operated in a slanted mode. Deepwater completions may be deployed from the vessel via a buoyant horizontal riser (BHR), which may be supported by a submerged buoyant tensioning system (BTS). In this manner, the cost of performing completion or intervention operations may be significantly reduced compared to such operations run from a drilling rig.