Packerless Annular Safety Systems: A Viable Alternative To Packer-Based Systems For Offshore Completions
Authors: Longbottom J.R. , Adams Mike , White P.M. , Taylor R.W. , Guyden Robert 
Published: October 2008

The trend in the North Sea has been toward installing annular safety valve (ASV) systems for gas-lifted platform production wells or for tension leg platform (TLP) completions. To satisfy the requirements for a highly reliable annular safety system for the Norske Shell Draugen project and another TLP completion project in the North Sea area, tests were conducted to investigate the feasibility of a polished bore receptacle (PBR)-based ASV system.

This paper will compare primary design considerations for packerless and packer-based ASV systems and will focus on the testing and qualification of the PBR nipple assembly to fulfill the requirements for a reliable annular safety system.

An overview of completion design options, including packer-based backup systems, is also presented to demonstrate the flexibility of this approach.

Testing has determined that a PBR casing nipple assembly can provide a viable alternative to a mechanical ASV packer and its related seals when used to anchor an ASV system. Packer-based systems have also been tested and are in use in North Sea wells.

Introduction

Demands imposed by industrial, governmental, and regulatory standards concerning personnel and environmental safety on offshore platforms emphasize the need for a fail-safe annular control system.1 These needs can be met by an ASV system, which is designed to effectively contain flow of gas or fluid from the wellbore annulus at a point below the surface, normally at subsurface safety valve installation depth.2,3 When selecting an annular safety system, proper material selection, simplicity of installation and retrieval, the number and nature of the sealing arrangements, dimensional/functional requirements, and economic efficiency are primary considerations for the operator.

A review of these requirements leads to close examination of the PBR concept, which has shown its capability to fulfill most of the design/operational requirements.4 The nipple system has the capability to handle high hang-weight and/or thermally-induced loads; and the PBR nipple, pre-installed in the casing string, will permit a variety of completion designs that can be determined later in the well-development cycle.

Testing verified that 1) only a minimal pressure drop occurred through the system, and 2) there were no compatibility problems between drilling/cementing operations and the PBR system.

Overview-Primary Design Considerations For a Packerless ASV System

Simplicity of System Installation and Retrieval Installation of an ASV system in a PBR assembly is accomplished by simply landing the system and lower tubing string in the PBR. An ASV packer requires hydraulic/mechanical activation to effect setting. For simplifying spaceout of the upper tubing string, the PBR system provides a no-go point for hanging off the lower tubing string, which allows accurate measurement of "stretch" in the upper tubing string. This facilitates spaceout for purposes of landing the surface tubing hanger.