Method and apparatus for subsea wellhead encapsulation

A BOP stack encapsulation system has dual partially hemispherical dome sections which are each supported and guided for linear movement to open and closed positions relative to base plate. The dual containment domes are moved from open positions to closed and hermetically sealed positions about a BOP stack responsive to sensing BOP leakage or a dangerous pressure condition and are controllable from a remote location. When both containment domes are closed they develop concentrically arranged dual hermetically sealed spaces that enclose the BOP stack and capture any leaked fluid and pressure. Captured well fluid constituents can be bled from the internal spaces of the concentric encapsulation domes as desired.