Plant and Work-Two Complex Entities

It is beneficial to view an engineering project as containing two complex entities the plant to be created and operated in order to satisfy the stakeholder requirements, and the body of work required to create and operate it. They are often called the system and the project , or also the Works and the Work we shall stay with our previous nomenclature of plant and work. These two entities are obviously tightly coupled, but they are of very different natures. The relationship is somewhat analogous to that of a glove and a hand the size and shape of the glove is determined completely by the hand, but they are otherwise very different. 

The description of the work as a set of elements is captured in the Work Breakdown Structure (WBS), where the elements are Work Packages (WPs), defined in terms of their results or outputs, the activities required to produce these outputs, and the resources or inputs required to support the activities and complete them in a certain time period. However, it is often overlooked that the word structure has two quite different meanings here. The one directly linked to 

Describing the plant as a system is also generally a step-wise process, but due to its dual nature mentioned above, it is best viewed as two parallel and closely coupled processes. One is the development of the description of the plant in terms of its components. Depending on the industry and the type of project, the resulting artefact has various names, such as Bill of Materials, Schedule of Works, or Works Definition Document (WDD), but in any case it should fulfil two essential requirements  

The other process is the development of the set of requirements on these components. It is contained in documents with various names, such as Technical Requirements, Requirements Definition Document (RDD), or Technical Specification. This document (or set of documents) is developed in parallel with the PBS, and as with the PBS, there is a requirement for completeness at each level. However, this completeness is partly explicit and partly implicit, with the fraction of requirements that are explicit increasing with each step in the development until, at some level, all the requirements at the top level, i.e. the stakeholder requirements, are explicitly satisfied by the requirements on the components at this level. 

In Sec. C2.4 we look at some issues involved in developing the best PBS. 

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