In addition most of these large fields are neighbored by multiple smaller fields, called marginal fields or pockets.
These marginal fields may individually contain far too short reserves to justify the capital expenditure in heavy surface infrastructures but their aggregation may be significant enough to develop a sub-sea exploration plan.
After drilling these marginal fields, and installing all the sub-sea equipment on the seabed, risers are installed to connect them to the floating vessel or platform.
This connection of these risers to the vessels or platforms is called tie-back.
Tie-back will also be used for all the equipment such as the risers and sub-sea connectors.
A tie-back risers can be either a single large-diameter high pressure pipe, or a set of concentric pipes extending the casing strings in the well up to a surface BOP
Physically this tie-back operation is made through the turret and the associated mooring systems.
The first tie-backs were performed for marginal fields located at few kilometers distance from the main field, but today the risers reach several tens kilometers distance and in the near future hundreds kilometers distance may be expected.
The North Sea and the Gulf of Mexico are the regions where the tie-back concept is widely expanded.
Most of the giant fields of these offshore regions are maturing and depleting while the large infrastructures built for these fields are still available for decades of production.
The tie-back system in association with the sub-sea risers technologies fits perfectly in these regions to explore, develop and put in production thousands of marginal fields which can balance the depleting fields and benefits from the available processing capacities on the floating vessels and platforms.