Dry tree wells are more robust than subsea systems in the face of mechanical problems and human fallibility. This paper will review historic and recent developments for dry tree wells in deep water.
In the 1980s and early 1990s, dry tree structures were extensively studied as alternatives to subsea wells in water depths up to 5500 ft (Marshall 1993a). The dry tree structures being designed and tanktested included compliant towers, tension leg platforms, moored spars, and tethered spars. Semisubmersibles were initially rejected because they required unreasonably large tensioner strokes. Patents covering the successful concepts and their enabling technology will expire soon, enabling more unfettered use. As in the shallow Gulf of Mexico, many wells with frequent intervention were anticipated in the early days of deep water. Two things changed this paradigm: The discovery that turbidite wells could produce 30,000 bopd, dramatically reducing the required number of wells. Also the development of better flow assurance technology and more reliable subsea systems designed for ROV intervention, for which high per well production rates could pay the upkeep expense. Nevertheless, economics continue to favor dry tree drilling from an onboard rig or with tender assist, e.g. Perdido in 8,000-ft water depth, or Kikeh in the South China Sea. The authors' early interest in dry tree structures has continued, and over the last four years led to a series of student design projects at the National University of Singapore (NUS). Light weight dry tree structures were studied for water depths of 400-ft to 5000-ft, for offshore Borneo. They do not carry the full drilling rig, but are supplemented by a tender or over-the-side semi, e.g. the Sedco 135 which drilled dry tree wells through guyed caissons in the South China Sea in 1970. Recent events have revived an interest in dry-tree drilling.
