The ISA-95 equipment model stops at the work unit level since this is typically the lowest level of equipment that is separately monitored and scheduled in the MES and ERP layers. However, this is not granular enough for us to fully model the factory equipment down to the control layer. Often, to model an entire enterprise from the head office down to a single sensor or control input in a factory, the equipment model is extended to include the ISA-88 control and equipment modules:

• Equipment module: An equipment module is part of a larger piece of equipment that can carry out a finite number of specific minor processing activities. It combines all of the necessary physical processing and control equipment to perform these activities. Functionally, the scope of the equipment module is defined by the finite tasks it is designed to carry out. Physically, the equipment module may be made up of control modules and subordinate equipment modules. Equipment modules are typically controlled procedurally through complex logic. They may have multiple states and run through sequences of operations. Equipment modules are connected to other pieces of equipment and systems through control modules.
• Control module: A control module is typically a collection of sensors, actuators, other control modules, and associated processing equipment that, from the point of view of control, operate as a single entity. Direct connections to sensors and actuators in the factory are often made in the control module.

To determine the asset model, we need to follow an iterative process that is based on the following steps:

1. Design the conceptual model of the company asset. To do this, you need to identify the physical asset and set up the customer business from a logical perspective. You can then work out which logical resources you need to manage.
2. Fit the designed conceptual model in the hierarchy of the ISA-95 or the ISA-88 equipment model. This means that you need to associate each logical resource identified in the previous step with the corresponding entity in the hierarchy of the equipment model made available by the ISA-95/ISA-88 standards.

In general, the granularity of the model depends on what a company wants to do with the model. A company needs to look at the work processes and the information flow in light of their own business goals.

The following diagram outlines what an asset model could be in an LNG company, including its hierarchy from a logical perspective and the related entities of the ISA-95/ISA-88 standards: