Simple Subsea Trees for Shallow Water: An Economical Alternative

Abstract Simple, diver assisted subsea completions have been installed and operated successfully in many shallow water oil fields around the world. Although these simple subsea trees have not received the notoriety of their more "glamorous" deep-water relatives, shallow subsea trees are far more numerous. Simple trees have proven themselves to be reliable, cost effective alternatives for shallow to medium water depth completions where "complex" diverless trees are not required. This paper presents a history of the development and evolution of simple subsea trees and describes several typical applications. The relative economics between "simple" and "complex" subsea completions are discussed, along with general guidelines for determining, along when and where the use of simple subsea is appropriate. Background and Introduction "Simple" trees are those which are designed for shallow to medium water depths where installation and maintenance can be accomplished by free swimming divers. Since divers can work effectively and economically at these depths, the subsea tree and associated completion equipment can be very simple and uncomplicated. Complex remote controlled tree components and/or running tools are not normally required. Divers verify equipment orientations, confirm proper tool function, make up connections, operate valves, and intervene manually whenever unplanned events or malfunctions are encountered. Subsea flowline connectors for "simple" trees are often inexpensive jumper hoses which are made up manually by the diver. "Complex" subsea trees, on the other hand, are sophisticated designs for deep water where the use of divers is either not possible or not cost effective. Diverless installation and maintenance capabilities must be provided in the tree and its associated running tools. Diverless trees, flowline connectors, and running tools are inherently "complex" and expensive. "Complex" deep water trees have been the subjects of numerous R&D projects and technical papers over virtually the entire history of subsea completions. These trees represent the current "state-of-the-art" for subsea completions technology, and they are more "glamorous" and technically challenging than simple shallow water trees. Subsea completions are expected to move rapidly into ultra-deep waters where existing subsea completion technology is not adequate. Thus, there is high interest in developing and testing "complex" deep water carpletion equipment to prepare for the anticipated ultra-deep water projects. In fact, many shallow and medium water depth wells have been completed using "complex" diverless subsea trees am remote flowline connectors as "prototype tests" prior to their first use in "diverless" water depths [2]. Many of these cells could have been completed at significantly less cost using "simple" subsea trees an1 diver-assisted flowline connectors if there had been no need to test the prototype diverless equipment. Relatively little has been published on "simple" subsea trees even though they represent a major portion of the world's subsea completions. The large number of simple subsea trees in place, and their many years of successful operation, demonstrate the reliability which can obtained with this economical and relatively unsophisticated equipment. In today's oilfield economy, with depressed oil prices and unstable market conditions, simple subsea trees are being considered more seriously by many operators.