Abstract The traditional method for sealing the top of a casing liner has been to employ a mechanical or expandable pack-off to isolate the parent casing from the open hole surrounding the casing liner. However, in the more complex deepwater wells in the Gulf of Mexico (GOM), more casing strings are being employed in order to isolate a depleted or over-pressured zone. This scenario complicates construction of the wells by creating smaller annular areas that create problems when attempting to effectively achieve pressure isolation at the top of the liner casing. These difficulties occur even when cement and elastomers are used during attempts to achieve isolation during cementation. Recently, a major operator needed to create a seal in the liner top in a deepwater Gulf of Mexico well, and conventional mechanical or expandable pack-offs had not been developed for the reduced annular gaps of .518-in. per side, the dimensions required for this project. To achieve the needed seal, swellable technology was used. This allowed the operator to create a seal with integrity by using a combination of cement and swellable technology when other options were not available. This paper will discuss swellable technology successfully used as an alternative method to effectively isolate the top of a liner. The swellable technology method was custom designed for the particular applications that were needed for this operator, and isolation was performed successfully without surface manipulation in a timely, effective manner. In the case history discussion, the design of the isolation method, the advantages it provided for the operator, and the fact that the application did not affect the Equivalent Circulating Density (ECD) during the cementing of the liner casing will be presented. Introduction Successful installation of a casing liner in deepwater operations can pose significant challenges for the operators and service companies. Magnifying these challenges are the ever changing dynamics of deeper water depths, deeper producing zones, increasingly depleted zones and the need to deploy more casing strings to achieve the desired target safely and economically. The need to deploy more casing strings, either long strings to surface or casing liners hung off downhole, has outpaced the development of liner casing hangers and mechanical pack offs. Under standard conditions, the liner casing string consists of a mechanical liner hanger (MLH) and a mechanical pack off (MPO) (See Fig. 1) that are lowered into the wellbore with tubulars. Before cementation begins, the MLH is manipulated from surface to engage slips which anchors the liner casing into the internal diameter of the parent casing. After the MLH is engaged and fixed, cementing of the liner casing is employed and the cementing wiper plugs are landed and tested with internal pressure. After the cementing wiper plugs are landed and tested, the MPO is manipulated from surface to expand the rubber element of the MPO to affect a seal on the internal diameter of the parent casing. Cement combined with the MPO creates an effective seal to maintain wellbore integrity during the life of the well. There are inherent issues when attempting to manipulate a MPO after cementing has been completed. Partially hardened cement is presumably encapsulating the MPO before the manipulation begins. In many instances the MPO is not able to affect a seal on the internal diameter of the parent casing.