Newest Through-Flowline Retrievable Safety Valves for Subsea Completions
Authors: Yonker John H. 
Published: December 2007

Summary
Two new retrievable-ball-type safety valves are now available in the through-flowline (TFL) (pumpdown) configuration. These valves offer greater depths in a more simplified configuration to operators planning subsea completions. They were developed at the request of major producers for use in the North Sea and Southeast Asia. Design of these new valves began in 1977 and early 1978 by engineers from our R and D Dept. One valve is a balance-line valve with a fail-close leak protection feature. The other valve is a single-line, deep-set valve that allows installation at greater depths than previously achieved, They have tested successfully in test wells and test loops by the manufacturer and in two land test wells by the operators. The safety system packages for these valves currently are installed in two subsea wells, one in the North Sea and the other in the South China Sea. This paper discusses how the capabilities of subsea production are expanded by the development of a truly failsafe, production are expanded by the development of a truly failsafe, balance-line, retrievable-ball valve and a deep-set, single-line ball valve that can be placed at or near the packer, below paraffin levels, or below the kickoff point. paraffin levels, or below the kickoff point.

Introduction
As completion technology has kept pace with drilling technology in the push for oil into deeper waters and hostile environments, so has TFL (pumpdown) completion and service technology. This is true also for TFL retrievable-ball safety valves as two new valves have been developed that help extend subsea completion technology. In 1977 a design team consisting of our R and D engineers and engineers from a major North Sea producer met and established design criteria for a new subsea safety system. 1 This system would consist of a tubing-retrievable ball valve and a separate backup insert TFL ball valve, both being of the balanced type. The exact details of the design criteria for the tubing-retrievible valve and the overall safety system are presented in Ref. 2. We were approached in early 1978 by a major producer from the Southeast Asia area regarding new TFL completion equipment. This equipment was to be for a new below-the-mudline subsea completions and was part of an ongoing development program in which five TFL subsea completions had been made with an above-the-mudline wellhead configuration. Discussions centered around the updating and compatibility of the TFL completion and service equipment with the new system, but also in particular on the safety system. The safety system would, as in the past, be a single-line system, However, for this completion it would consist of both a tubing-retrievable valve and an insert valve. Five previous completions had consisted of only a landing nipple and TFL previous completions had consisted of only a landing nipple and TFL insert valve that spoke well of the reliability of and confidence in the insert valve's operation and retrievability. The previous single-line insert valves were designed for approximately 500-ft [152-m] depth and generally were landed in depths of 200 to 300 ft [61 to 91 m]. In our discussions, it was decided that the requirements for the valve for this completion, however, would be a minimum depth of 1,000 ft [305 m], if possible, while still maintaining the same maximum bore and other features of the previous valves plus control system compatibility.

Equipment Design
Deep-Set Valve. A standard-configuration, retrievable ball valve is shown in Fig. 1. The valve shown in this figure is a pump down ball valve but also could represent a wireline-retrievable valve because of the basic similarities. In this valve the piston is at the top, the spring is below the piston, and the external packing is placed on the OD somewhere in the area of the spring-i.e., above, alongside, or just below the spring. In this configuration the outer housing in the area of the spring chamber must be resistant to external pressure either from the well or from the control pressure, depending on external packing location. This determines the size of the spring that can packing location. This determines the size of the spring that can be used by virtue of not only the thickness of the packing but the added wall thickness for pressure resistance. The spring size is critically important because the amount of spring force coupled with the piston area is the amount of' pressure put up against the control-line fluid column for valve closure. Another limitation of this valve design is the inner mandrel On some nonequalizing type designs the inner mandrel may be exposed to differential pressures if the secondary seat leaks. In addition, the mandrel also must withstand column loading when the ball is fully rotated open and the high control line pressures are applied to the piston. When designing the valve the inner mandrel must be made thick enough to withstand these loads. This again will limit spring size. When a valve of this type is made up in a string with its associated equipment similar to that shown in Fig. 2, additional potential problems can be encountered from the scaled joints potential problems can be encountered from the scaled joints between the upper and lower external packing. The upper packing and sealed joints are required because the control fluid entry port is above the external packing on the valve (Fig. 1).
JPT
P. 1922