These types of contracts are classified depending on whether or not they include Operation, Maintenance or both (O&M) after delivery of the works.

1. 1. Contracts without operation and maintenance.

This type of contract began to be used in the industrial plant construction sector.

On the other hand, in the case of large construction projects, as each one is different and almost an unrepeatable "prototype", it is configured according to its unique characteristics. Despite these difficulties, turnkey contracts often have the following in common:

• They are generally awarded by restricted or negotiated procedures.
• A single client-contractor relationship is established, although the client may choose an engineer to represent him.
• The contractor is responsible for the concept and design of the project.
• A lump sum price is fixed.
• Although the Contractor assumes most of the risks, there is usually a certain capacity for negotiation of the risks assumed by each party.

As the contractor takes over the design of the project, turnkey methods are considered "Fast Track", as construction can start before the detailed design is completed.

These contracts offer the following advantages over other formulas:

• Closed lump sum price.
• Speed in execution.
• Minimisation of change orders.
• Technology transfer is simplified as there is a single partner.
• They provide legal unity to a complex economic operation, with the global responsibility of the contractor and a single partner with the client.

b. Contracts with operation and maintenance

These tend to be used in large infrastructure projects such as WWTPs, DWTPs, motorways or large bridges or tunnels with tolls, etc. In these contracts, the constructor assumes practically all the risks, both in the design, execution, and commissioning of the work, as well as its subsequent operation, maintenance, and sometimes even the financing of the project.

Build - Operate – Transfer (BOT). It is a turnkey contract in which the contractor is awarded on a lump sum basis. The concessionaire is responsible to the client for the design, selection and commissioning of the project. These projects must carry out four essential operations:

• The search for financing the project
• Construction of the project
• Operation and maintenance
• Transfer of the ownership

There are other such contracting methods, including the following, which are not described as their names are self-explanatory.

• Design – Build – Operate – Transfer (DBOT)
• Design – Build – Finance – Operate (DBFO)
• Design – Build – Operate – Maintenance (DBOM)
• Design – Build – Operate – Maintain – Warrant (DBOMW)

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