"Design of the project must be set in stone, no changes."

That is not the way this project is approach at all. The Airport will be a "design build". The plan is for a single contract with the project owner to provide their own design and construction services. One entity, one contract, one unified flow of work from initial concept through completion. The costs to fully engineers and design a project like this are enormous. Theoretically the cost would be fixed or not to exceed in the contract. To count on that is always naive. Something will likely be discovered along the way that the people of the BVI will want to pay for rather than suffer the shortfall. Potential issues with the environment come to mind.

You can look up the benefits of "design build" and why many use that approach today.