Subsea tiebacks, which link new discoveries to existing infrastructure, are now becoming viable and are set to become a major factor in the development of new oil and gas reserves in the 21st century. With larger oil and gas discoveries becoming less common, attention has turned to previously untapped, less economically viable discoveries. If a deposit is isolated, it is usually not economically viable. However, rising crude oil prices and increasing demand for gas means that previously uneconomical deposits are now starting to look attractive. Indeed, exploiting new discoveries using existing production facilities is an important way of obtaining maximum value from existing infrastructure.

Of course, this still does not solve the technical difficulties of laying subsea pipelines and the installation of subsea production platforms and processing units. How much easier and less expensive it would be if existing infrastructure could be used to recover the oil and gas and transport it to market. This is where subsea tiebacks are now becoming an appealing option.

Subsea tiebacks can require significantly lower initial investments compared with developments using floating production, storage and offloading (FPSO) or fixed installations. The economics of a field are, however, governed by a number of factors specific to that field such as: Distance from existing installation; Water depth; Recoverable volumes, reservoir size and complexity; Tariffs for processing the produced fluids on an existing installation; The potentially lower recovery rates from subsea tiebacks versus standalone development, due to limitations in the receiving facility’s processing systems; The potentially higher recovery rates from platform wells, due to easier access to well intervention and workovers.

The Scandinavian oil giant Statoil has instigated a number of recent subsea tieback developments. One of these has recently come online, the Skinfaks / Rimfaks project in the North Sea. Field recovery in the project will be increased using several wells via tieback to the Gullfaks C platform.

The technology for subsea tiebacks has been widely available since about 1990-1995, but more established and field-proven technologies are sometimes preferred because they reduce the risks in small or marginal developments. Modern advances in flow assurance and multiphase transport now allow the use of tiebacks over much longer distances, while the introduction of subsea processing will strengthen the business case for subsea tiebacks in future developments. In addition, new tools have become available for boosting pressure and removing sand and water from the wellstream, and for assisting reservoir pressure on subsea tiebacks, and this has boosted the case for the development of marginal projects. One of these tools is Advantica’s grouted tee hot tapping, developed as a lower-cost alternative to other pipeline modification methods for subsea installations. Such technologies are essential to the economics of subsea tiebacks.

Subsea tiebacks provide a number of important benefits in the development of oil and gas fields. Because much of the infrastructure is already in place, projects can be fast tracked and brought into production much more quickly. There can be flexible and phased developments in certain fields, which are beneficial for small / marginal developments. In addition, as production capacity becomes available on existing installations and infrastructure, subsea tiebacks are very important in maintaining production levels.