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In the development of the 3`d edition of the Steel Tank Institute SP001 Standard (July 2005 edition), an inspection schedule was devised that is based on risk management concepts. However, the SP001 inspection schedule is prescriptive and does not rely on assigning values to various likelihoods and consequences. Rather, a table of types of tank installations and tank sizes is given varying periods between inspections. Single-wall tanks sitting directly on soil without secondary containment or spill control were deemed to pose the greatest risk for an incident and therefore require more frequent inspections. The basis for this assessment was empirical and determined by experts in the field based on good engineering practice. SP001 TerminoloQy Key to understanding the inspection schedule in SP001 is an understanding of the terminology used in SP001. Here are some key concepts: CONTINUOUS RELEASE DETECTION METHOD (CRDM) — a means of detecting a release of liquid through inherent design. It is passive because it does not require sensors or power to operate. Liquid releases are visually detected by facility operators. The system shall be designed in accordance with good engineering practice. Several acceptable and commonly used CRDM systems are as follows: • Release prevention barrier (RPB) described in definition of release prevention barrier. • Secondary containment AST including double-wall ASTs, double -bottom ASTs, or other ASTs described in definition of secondary containment. • Elevated AST with release prevention barrier FIELD -ERECTED AST — a welded metal AST erected on-site where it will be used. For the purpose of this standard, ASTs meeting either of the following descriptions are to be inspected as field -erected ASTs: a. An AST where the nameplate (or other identifying means such as accurate drawings) indicates that it is a field -erected AST. These are limited to a maximum shell height of 50 feet (15.24 meters) and a maximum diameter of 30 feet (9.14 meters). b. An AST without a nameplate (or other identifying means such as accurate drawings) that is more than 50,000 U.S. gallons (189,271 liters) and a maximum shell height of 50 feet (15.24 meters) and a maximum diameter of 30 feet (9.14 meters). FORMAL EXTERNAL INSPECTION (FEI) — a documented external inspection conducted by a certified inspector to assess the condition of the AST and determine its suitability for continued service without entry into the AST interior. FORMAL INTERNAL INSPECTION (FII) - a documented internal inspection conducted by a certified inspector to assess the internal and external condition of the AST and determine its suitability for continued service. This includes the inspection requirements of a formal external inspection. A formal internal inspection satisfies the requirements of a formal external inspection and shall be considered equivalent to or better than a formal external inspection for the purposes of scheduling.