This paper develops a systematic method to assess the suitability of buoyancy control systems to different mission profiles, risk acceptance thresholds and environmental sensitivities. We analyse various existing variable buoyancy engines (VBE) for different platforms and mission profiles. Our results identify a capability gap for low-energy seafloor survey platforms such as the Lagrangian imaging float that conduct most of their buoyancy control manoeuvres at depth and rely on end of missions recovery for data extraction. Based on these requirements, we introduce a novel VBE concept that uses the energy mechanically stored in a pre-tensioned spring to passively surface in the event of power loss and reduce power consumption when changing buoyancy at depth. The equations to size the pre-tensioning spring are developed and simulations are performed to demonstrate the reduced power consumption at depth. Unlike safety systems that use drop weights, the pre-tensioned VBE minimises environmental impact by achieving net positive buoyancy without leaving anything behind in the environment.