Tools provide dual-bore access in monobore risers

Subsea operations are an expensive necessity in remote and deepwater locations due, in most part, to the complex functionality required of the tooling and the costs as sociated with the high specification vessels required on location. This is particularly true of completion and tree installation programs where traditional dual-bore systems are specified.

Aug 1st, 1998

Reducing subsea hardware needs

Tom Leeson
The Expro Group
Subsea operations are an expensive necessity in remote and deepwater locations due, in most part, to the complex functionality required of the tooling and the costs as sociated with the high specification vessels required on location. This is particularly true of completion and tree installation programs where traditional dual-bore systems are specified.

The Expro Group's subsea development engineering department in Aberdeen has been responsible for two recent innovations in this area, with both completion and open water bore selector systems suitable for providing dual-bore access from monobore risers. The goup has been commissioned to support a major field development in the North Sea region.

The tools have been designed to dispense with the need for a dual-bore riser, an expensive element of the installation tooling package, in order to reduce the capital cost of the required equipment and to reduce the rig-up and running time during the operations sequence. These latter factors become a significant issue due to the increase in day rates for high specification rigs for deepwater operations and the increased time to trip the string to the seabed.

Completion installation

The completion system is designed to be run as part of the tubing landing string, providing wireline access to both the annulus and production bores of the tubing hanger. The tool can be controlled from the surface in parallel to the tubing hanger running tool (THRT) control system and directs tool strings to one or other bore as required via a hinged gate actuated by a sliding sleeve.

The tool is designed to interface either directly with the THRT or with a dual-bore subsea completion tree (DBSCT), another Expro development introduced in 1995 and now employed to provide dual-bore well control and an emergency disconnect (and subsequent relatch) facility in the landing string during completion and clean-up operations.

This modular concept has allowed the design to be kept simple with well control and tubing hanging functions provided in com plementary tools. The potential for a string circulation path has been retained through use of a side-port in the annulus bore of the DBSCT accessed via the BOP choke/kill lines.

The tool, in a 6-3/8 in. by 2 in. configuration, has now been used to successfully install eight subsea completions with no attributable downtime. These have been installed in approximately 500 meters of water in a harsh environment, utilizing a hydraulically actuated tubing hanger requiring suspension plugs in both annulus and production bores.

Tree installation

Similar dual-bore riser requirements have traditionally existed for conventional tree installation operations, with monobore systems requiring a round-trip to surface to re-align the riser for annulus bore access - an operation that can be expensive from a high-specification rig in deep water.

However, application of the bore selector concept will deliver capital and potential operational cost savings by interfacing with existing lower riser package (LRP) design. The tool is positioned in the string immediately above the emergency disconnection package (EDP), and below the riser stress joint. Selection functions are controlled hydraulically via the workover control system (WOCS). A position indicator, viewable with an eyeball ROV, has been incorporated to provide feedback.

The tool utilizes a similar mechanism for the completion tool, ensuring simplicity and allowing lessons learned during engineering and fabrication to be incorporated into both designs.

Having recently completed stack-up and integration testing without problems, the system will be deployed for the first time in a program of tree installations due to commence in the near future.

Summary

Clearly, there remains room for improving the cost-effective design of subsea equipment. However, the success outlined above shows that it is possible to incorporate novel concepts while maintaining effective management of the interfaces between several complex tool packages.

Each item can no longer be viewed in isolation. Rather, a holistic approach incorporating the overall impact on both capital and operational expenditure is required. As subsea developments become the norm in the deepwater provinces, the potential benefits of innovative tooling will increase, requiring all engineers to question the validity of employing well-proven but expensive technology.

With the increasing pressures caused by high-cost developments and low crude prices, engineers will be forced to investigate alternative solutions in order to maintain expected returns on development in vestment.

Author

Tom Leeson is General Marketing Manager for Subsea for the subsea division of the 'Ex pro Group, based in Aberdeen. Previously, he was with Shell in Oman and The Netherlands, and after that was Expro Group's principal engineer for floating production, with special ization in extended well testing and early production facilities. He holds a Bsc and PhD in Chemical Engineering from the University of Birmingham (UK).

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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