Why did it take so long to develop Rosebank?

Why did it take so long to develop Rosebank?

Rosebank Oil and Gas field, off the coast of the Shetland Islands

Rosebank Oil and Gas field, off the coast of the Shetland Islands

Deepwater Blackrock well finds oil west of Shetland

Deepwater Blackrock well finds oil west of Shetland