LOPA is a semi-quantitative tool for analyzing and assessing risk. This method includes simplified methods to characterize the consequences and estimate the frequencies. Various layers of protection are added to a process, for example, to lower the frequency of the undesired consequences. The protection layers may include inherently safer concepts; the basic process control system; safety instrumented functions; passive devices, such as dikes or blast walls, active devices, such as relief valves; and human intervention.
Reference: Chemical Process Safety – Fundamentals with Applications, 2nd Ed.