A Wireline-Retrievable Wellhead Plug System For Use With Horizontal Trees
Authors: Vick James D. , Miller James W. 
Published: October 2008

One of the recent enhancements to offshore completion technology has been the development of a new horizontal tree design in which a wireline retrievable wellhead plug that sewes as a primarybarrier and permits vertical access to the well is used. The basic features of this removable plug system and what advantages it offers the operator will be presented in this paper. Also discussed will be the installation and retrieval techniques that are required to ensure system integrity.

This wireline-retrievable well head plug system can be used with any horizontal tree for vertical access to the well and is particularly applicable to subsea completions in which wireline is the method of choicefor installation and retrieval.

The paper will focus on:Economic advantages of a wireline-retrievablewellhead plug system over methods that employ rod or diver manipulation.Proven seal technology with added redundancy to provide a primary barrier.Assurance of proper installation and retrievai through wireline tool design.Safety features of a wireline-retrievable wellhead plug system.

Actual field expedience that demonstrates the economic and operational advantages that the new wellhead piug system often, both initially and in daily operation, will be provided.

INTRODUCTlON

With a conventional subsea tree system, the Production string is run into the well bore before the tree is landed. With a horizontal subsea tree system, the tubing hangeris an integral part of the subsea tree and is run through a blowout preventer (BOP) attached to the subsea structure. Access to theproduction string of a typical horizontal subsea tree is through the riser, BOP, and tree. This paperaddresses a horizontal subsea tree system thatrovides an alternative method of access to the production string, production string of a typical horizontal subsea tree isthrough the riser, BOP, and tree. This paper addresses a horizontal subsea tree system that provides an alternative method of access to the production string.

Although conventional and horizontal subsea trees are applicable for use in subsea wells, the horizontal tree design offers several advantages over the conventional tree design. In order to understand theadvantages, a brief description of the configuration of the horizontal tree follows.

HORIZONTAL SUBSEA TREES

The horizontal tree can be described simply as a "through bore system without any valves in the vertical run" (See Figure 1). All valves that are provided with the tree are outboard of the main spoolbody. This system provides access to the wellbore through the tree without removal of the tree assembly. After installing the tree on the wellhead, the tubing hanger is installed and aligned in the spool body. The production outlets in the tubing hanger align with the utlets in the main spool body and redirect the flow horizontally. The flow control valves (production, wing, crossover, etc) are outside the main body. inaddition, the tubing hanger has a plug profile above he production outlet that accepts a plug. The plug serves the same purpose as a swab valve in a conventional tree. After installing the plug, a highpressure cap that sea is the main bore of the spool body is installed. The high pressure cap maybe soiid as shown in Figure 1 (single wellhead plug) or may have a through bore that is plugged by a second wireline-retrievable wellhead plug (dual plug application). The advantage of the dual piug completion is that the high pressure cap does nothave to be removed for wireline service to the well. Operational benefits that are derived from use of Horiz