A System Level Approach to Subsea Hydraulic Control Line Reliability Issues
Authors: Ellis Scot Thomas , Siappas George , Colyer Angus , Mitchell Robert Franklin 
Published: October 2008

In general downhole control lines are not subject to routine failures, but a study of downhole completion failures identified the downhole control line to be a principle cause of the failures. The cost of recovering downhole completions to repair downhole control line failures warranted the investigation of the reliability of the downhole control equipment of control line clamps, control lines and control line fittings.

Early work indicated that the downhole control line components were not engineered as a system but rather as individual components driven by the hardware interfaces of more substantial components at the physical limits of the system. Hence the mechanical attributes of the collection of components had never been closely studied before.

In order to remedy this, a system level approach was developed to understand and resolve the problem areas involved in downhole control line reliability. This system level approach defined the following workflow;Identify actual downhole control line failures and reasons for the failures,Evaluate the operational impact between the control system, control fluids, completion string and the downhole control line equipment,Seek industry participation in the analysis and testing of the downhole control line equipment,Generate dedicated software to identify the downhole equipment installation and operational loads,Test downhole control line equipment to establish the working envelope of the components,Cross compare the analytical and test data results to identify equipment short falls.

The outcome of implementing the above workflow and associated tasks has lead the project team to a greater understanding of what processes work best for control line and clamp manufacturing. Some questions were raised during the study regarding connections and connection make up indicating limitations in the components and interfaces. To answer these questions the vendors of the components as well as of the interfaces the components relate to were involved, in order to drive component design improvements but also improve the downhole control line system overall.

Finally the highest impact finding was a distinct lack of recommended practices specifically for control line systems. A next step is the determination of recommended practice and qualification testing for improved reliability and reduced work-over cost for control system repairs.

Introduction

In a recent study of downhole completion failures utilizing an industry recognized completion reliability database, downhole control lines were identified to be a critical component in the failure analysis. Current well scenarios and conditions are requiring greater use of hydraulic downhole control systems connecting to intelligent completions and deep set SCSSV's. Well configurations with geometry changes from multilaterals and high deviations are putting greater stress on the lines. The reliability of the control line is paramount to the success of operation for any of the downhole components to which it is connected, i.e. failure of an SCSSV to operate because of a ‘leaking’ control line could cause catastrophic well failure (loss of well - MM$ or worst loss of life).

Specific analysis based on cost of intervention for just the control lines revealed that control line failures alone could amount to approximately $150K USD/well-yr (average 4 lines per well).