The S&F double-layer oil tank (full name: steel-reinforced fiberglass double-layer oil tank) has a main steel structure referred to as the "inner shell," while the outer protective and monitoring layer made of FRP is called the "outer shell."

When the S&F double-layer oil tank is buried underground, the inner shell primarily bears the external pressure from soil and rocks. Although the external pressure directly acts on the outer shell, this pressure is synchronously transferred to the inner shell, so the outer shell is not the primary component for maintaining structural strength.