Abstract: Disclosed is a cost-effective remedy to loss of well control caused by a defective string section upstream of the BHA, and more cost-effective inspection, maintenance, repair and replacement of any tools placed upstream of the BHA. The solution is to deploy one or more tools along the length of the string, either: (i) each tool below a profile nipple, and where each tool is upstream from an adjacent downhole safety valve (“DSV”); or (ii) one or more Flapper-tool-DSV combinations each below a profile nipple, and where the tool is positioned between each Flapper and DSV in the combination. If desired, the valving system described may be deployed along the string without a tool, to allow section-by-section inspection, maintenance, repair and replacement of string sections.