Would you want your painter to be an interior designer? What if the person mowing your grass was trained in landscaping and soil science? Of course you can hire two separate people in the classic model of project execution however you should always consider the obvious advantages of the design/build model.
Simply stated, pure efficiency is logical and obvious when your contractor is a group of highly trained engineers that can design and construct exactly what is needed.
Zero waste. That means the physical assembly of pieces, parts, pumps, tanks, pipes, instruments into a functioning system that delivers exactly what is needed requires no loss in time, overdesign, poor coordination, finger pointing or delays waiting on request for information. Efficient execution from one source.
Attack the critical path. Your engineer provides you with a design, your construction manager determines how the build the project, solicits the bids from the contractors and manages the construction and the constant flow of documents back and forth between the engineer, the owner, the builder and all other stake holders. Every project manager fully understands the lengthy project schedule that follows the classic model. What if your engineer submitted the design in a logical order based on the longest lead to the shortest lead of components and once approved was immediately able to order the components to begin the fabrication process? Besides the obvious reduction in delays due to communicating the technical documents; the overlap of design and fabrication just makes sense. But you ask, isn’t that too risky putting all my cards on one group? In return, I will ask you “How is it more risky to have one entity responsible for the success?”
Eliminate risk. Will the design work? What if your engineer was someone who spent an equal time building, starting and operating equipment they designed versus someone who never left the office?
Responsible. Why not put the responsibility of a properly functioning system on the desk of a single entity? Eliminate the failures in execution by allowing your engineers to build you exactly what you need. No need to enforce the design firm and the construction firm to resolve the conflicts. There are certainly situations where the classic engineer/procure/construct model is the required method. However, for many projects a more efficient design/build team is the obvious advantage.
You should know your team! Putting your money on the fast horse requires you to know your horse. You need confidence in the team to deliver exactly what you need. They should know your needs as well as, if not better, than you. They have been there. They know without any doubt what works and what doesn’t. Your confidence depends on their confidence. You must know that the team you have selected will deliver exactly what you need.
Quite simply, always consider your options to successful project management.