Insert Tree Completion System
Authors: Brands Klaas W. , Gresham J. Sam , Ball Ian G. , Saunders Neil D. , Cegielski E.J. 
Published: December 2007

This paper outlines the over-all project for development and installation of a low-profile, caisson-installed subsea Christmas tree. After various design studies and laboratory and field tests of key components, a system for installation inside a 30-in. conductor was ordered in July 1978 from Cameron Iron Works Inc. The system is designed to have all critical-pressure-containing components below the mudline and, with the reduced profile (height) above seabed, provides for improved safety of satellite underwater wells from damage by anchors, trawl boards, and even icebergs. In addition to the innovative nature of the tree design, the completion includes improved 3 1/2-in. through flowline (TFL) pumpdown completion equipment with deep set safety valves and a dual detachable packer head for simplified workover capability. The all-hydraulic control system incorporates a new design of sequencing valve for both Christmas tree control and remote flowline connection. A semisubmersible drilling rig was used to initiate the first end flowline connection at the wellhead for subsequent tie-in to the prelaid, surface-towed, all-welded subsea pipeline bundle. The acceptance stack-up tests in the U.S. and the subsequent land tests in Brunei also are described in this paper. paper.

Introduction

The insert tree is the 11th satellite underwater well to be completed in the northwest Borneo area. The earlier completions all have employed the conventional above-seabed type of underwater Christmas tree, and the last five trees stand about 35 ft above the seabed. This has increased concern about the vulnerability of subsea completions. We conducted various studies to improve the safety and simplification of satellite underwater well design. The insert tree completion system evolved from these studies. Although the total system (including the downhole completion design, control system, and flowline installation) is described, the tree, its design, testing, and installation are the primary subjects of this paper. paper.

The Insert Tree Concept

This project is the first attempt to lower substantially the profile of subsea Christmas trees above the seabed. The profile of subsea Christmas trees above the seabed. The insert concept (Fig. 1) entails the installation of the wellhead tubing hanger, master valve block, and flow loops inside the marine conductor of an underwater well. The design was achieved following close study of the purpose of the various valves in conventional surface and purpose of the various valves in conventional surface and subsea Christmas trees. On the basis of favorable long-term experience with TFL as a servicing method, vertical access for wireline servicing by use of swab valves and a wye spool are not strictly necessary, and these items safely can be eliminated if desired. Similarly. for a subsea tree. the working wing valves are at the platform, and the subsea valves are used only to shut in the well safely during an emergency or routine testing. This philosophy yields the valve configuration shown in Fig. 2. As indicated. plugs also can be installed in the tubing hanger or above the Christmas tree valves by pumpdown methods, circulating through the normally pumpdown methods, circulating through the normally open production crossover valve. During the completion stage, these plugs can be set and pulled vertically by wireline from the drilling rig before finalization of the completion.

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