As the search for new hydrocarbon reserves continues, it is inevitable that operations in deeper water will increase. In certain areas, such as Brazil and the Gulf of Mexico, deep water exploration and development is well underway. The speed at which deep water developments proceed worldwide will be dependent on their economic viability. Due to the requirement to use specialized equipment, operations in deep water are inherently more costly than conventional water depths. As is the case in more conventional water depths, standardization of equipment, interfaces and installation methods are seen as the keys to reduction of development costs.
In Brazil, where a large percentage of the deep water developments have taken place, the main areas of standardization have been utilization of a tubing spool, flowline connection methods and ROV interfaces. In the Gulf of Mexico, the Deepstar group has been formulated primarily to look at areas where standardization can take place. To date, virtually all deep water developments have utilized what could be termed as conventional xmas tree technology. By this it is meant that the xmas tree utilizes gate valves in the vertical bore (master and swab) and wing valves in horizontally oriented outlets.
The purpose of this paper is to evaluate the usage of Horizontal Tree Systems in deep water applications. Throughout this document, comparisons will be made to conventional systems. Issues which will be compared are equipment cost, installation times, workover times and risks associated with the Horizontal Tree System. There are numerous variations of conventional systems. For simplicity, it is assumed that a conventional system consists of a dual bore (4" × 2") system both with (Figure 1), and without (Figure 2) a tubing spool between the wellhead and the xmas tree. There is no intention within this document to cover areas where horizontal and conventional systems can be handled in a similar manner. In particular, flowline connection methods, ROV interfaces and gate valve design can be virtually the same regardless of the type of tree. For the equipment cost comparisons, water depth is assumed to be 3,000 ft. The drilling/completion vessel is assumed to be a dynamically positioned drillship which utilizes a guidelineless 16¾" BOP system.
A horizontal tree can be most easily described as a tubing spool with outlets to which gate valves are mounted (Figure 3). The tubing hanger lands in the spool body and directs hydrocarbon flow out through the side outlet. The tubing hanger is equipped with a wireline plug profile above the side outlet. Once in place, this plug serves the same purpose as the swab valve in a conventional tree. There are two major advantages for a horizontal tree as compared to conventional. Firstly, since the tubing hanger is run after the tree is in place, a workover requiring the tubing to be pulled can be achieved without recovering the tree. Secondly, the tree and tubing hanger do not require a purpose built completion riser system. The tree can be run on drillpipe or connected to the BOP stack on marine riser.