This paper discusses the evolution of the horizontal subsea tree and its utilisation in increasingly demanding applications.

Initially, the Horizontal Tree was seen as a solution for wells requiring high well maintenance, such as those using electrical downhole pumps. Today, the horizontal tree is being applied to natural drive wells because of ttractive whole life economics.

This raises concern in a number of areas.

Tubing hanger seals are permanently exposed to well fluids, and must provide high integrity sealing. This required the development of a new type of radically energised, all metal sealing system, specifically forhorizontal tree applications, suitable for oil or gas service at working pressures to 15,000 psi.

The crown plugs are also exposed to the same environment and so should preferably provide reliable metal-to-metal sealing, equivalent to a gate valve.

In addition, horizontal tree pressure integrity depends on the master valve stem, gate and bonnet seals. The need for high seal integrity also dictates that the first master valves should be integral to the main body of the tree, to maximise dropped object protection.

Tree connector capacity, too, can be an issue, as horizontal trees must withstand higher than normal BOP induced loads. These loads may exceed the capacity ofcurrent low cost tree connectors.

This paper describes the development of a new family of horizontal subsea tree technologies, designed to address these concerns, and shows how these problems have been addressed.

The commercial aspects of the project are fully explored and it will be shown that the use of a standardised wellhead product line approach to horizontal tree design, coupled with modular flow control components, candeliver a new range of horizontal subsea trees, designed to address these safety and environmentally critical areas, at lower cost than conventional vertical access trees.


In the 1960?s land trees were supplied into the Middle East which were probably the first example of the horizontal tree concept, These were 6" spools with side mounted valves. Production was via the annulus and a concentric kill tubing string was suspended from a flange above the side outlet.

In 1988, we led an EC funded joint industry development programme to investigate the subsea application of ESP technology. The programme concluded that horizontal trees were the correct approach for subsea ESP wells, and carried out the initial design of the horizontal subsea tree.

Recently, an in-house programme was initiated to research operator?s concerns regarding current horizontal tree designs, develop technology to address these areas of concern, and manufacture and test a prototype "second generation" horizontal tree system. The programme identified a number of key issues, which are described below.


The most critical area of the horizontal tree with regard to pressure integrity lies in the section of the production bore which is separated from the reservoirs only by the subsurface safety valve. This section is bounded by three critical seal systems.

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