Introduction

ISO 15926 does not support modal logic. Nevertheless there is, at times, a need for representing modalities. For this the concept of Possible Worlds, as described in 1986 by the philosopher David K. Lewis in his treatise "On the Plurality of Worlds", is being used.

Some essentials [source: 1]:

• The world we live in is all-inclusive, however remote in space and time;
• There are countless other worlds:
• they are just as real as our world
• they differ in content, not in kind
• they are irreducible entities in their own right;
• Possible worlds are unified by the spatiotemporal interrelations of their parts;
• Possible worlds are spatiotemporally isolated from each other;
• Possible worlds are causally isolated from each other;
• Objects in two different possible worlds can be related with a Counterpart relation, as defined in the Counterpart Theory.

Modalities are being replaced with possible worlds in which these modalities are turned into facts. So if something is "required", it exists as a fact in a possible world that is different from the actual world around us.

Applications

In the context of the scope of ISO 15926 the concept of possible worlds is used for these subjects:

• the NonActual possible world of Design & Engineering;
  
• the NonActual possible world of Scheduling;
  
• the Actual possible world that is around us.

Design & Engineering

When we design and engineer a plant we define topology and requirements. The topology is defined with Functional Diagrams, of which the Piping & Instrument Diagram (P&ID) is the most important. The requirements for the physical objects that together form a plant are defined at the level of ClassOfPhysicalObject. Classes are not part of a possible world, they are eternal. Any time a class is defined, it did in fact already exist since the Big Bang and is discovered now.

Often a 3D model of a designed plant is made and we can virtually walk around in that designed plant, located in our possible design world, using VR (Virtual Reality) technologies.

A physical object in a topology for a plant design is a part of the possible world of that designed plant. And like a plant in the real world around us a plant design can also change over time.

Another physical object in a topology for an actual plant is a part of the actual world around us.

In case these two physical objects are deemed to be counterparts a Counterpart relationship can be put on record for as long as that is true.

Please refer to the topic Plant Lifecycle Model for the application of possible worlds.

Scheduling

Even when an Activity is completed "on schedule" it is an Activity in the actual world of which the completion dateTime coincides with the completion dateTime of the counterpart Activity in the scheduling world.

Even in case the completion dates are different, these two Activities are still counterparts.

Note that each Activity in a planning is, in the reality of its possible world, terminated. This means that in that possible world we actually describe a history in which all Activities have been completed. So, in case of an update of the plan in fact a new scheduling world is created, with a new "history". Until now the scheduling world has not been detailed in a topic. This is forthcoming.

ActualIndividual vs NonActualIndividual

In order to be explicit the entity type lci:NonActualIndividual has been added to type the PossibleIndividuals in the design & engineering world and the scheduling world.