Alternative approaches to subsea intervention, workover

Subsea completion technology enhanced

Dr T. J. Leeson
The Expro Group

Dual-bore access to tubing hanger from monobore riser. [17,115 bytes]
The cost of hardware and rig time required to deploy subsea christmas trees and completions has traditionally been a significant element of subsea well costs. However, a number of recent innovations have reduced costs by simplifying operational programs and equipment configurations, while adding to operational flexibility.

The development of a dual-bore subsea test tree has permitted conventional open hole completions to be run while retaining an emergency quick disconnect facility and protecting reservoir sections from damage from kill fluids. Bore selector tools permit both dual-bore completions and christmas trees to be deployed on simple monobore risers while retaining access to the production annulus.

Finally, large bore intervention trees (up to 7-3/8-in. through-bore) have provided disconnect/relatch facilities and well control functions such as coiled tubing cutting and sealing while installing completions or intervening on horizontal-style christmas trees.

All these tools reduce operational time and riser complexity while increasing emergency response options during intervention operations. Savings of US$1 million per well have been indicated by operators.

In addition, the opportunity to lease workover riser systems has become a cost-effective method for securing equipment currently in short supply. This approach could be expanded to other subsea equipment.

Subsea completions are now considered a viable option for field development plans, particularly for deepwater wells and for marginal fields unable to economically support fixed infrastructure. As operators become more comfortable with the technology required to install and maintain this type of development, and gain the reliability and expertise associated with the necessary equipment and operations increases, the manufacturing and service sectors must improve the cost-effectiveness of these solutions.

A holistic approach to the costs associated with subsea developments, including equipment purchase and rental, rig operating time and associated services, production delays and deferment, and well productivity and ongoing support costs, can save money.

Completion installation

The conventional method of installing a subsea completion is to run the production string and tubing hanger through the subsea blowout preventer (BOP) on a tubing hanger running tool (THRT) and landing string. A tubing hanger orientation joint (THOJ) is included to ensure the correct orientation of the hanger prior to landing and setting.

When access to the annulus bore of the hanger is required, the landing string is comprised of a dual-bore riser. This access is usually required in order to install a wireline suspension plug. However, this method has a number of limitations.

There is no provision for disconnection of the string, nor are there any well control devices in the production conduit to permit disconnection. This becomes a serious limitation at any location where bad weather may disrupt operations and where an emergency disconnection function is required, such as in the case of a dynamically positioned rig.

In order to address these limitations and to increase the operational flexibility available, a derivative of conventional subsea test tree technology has been developed to provide dual-bore well control and an emergency disconnection facility. The tool is inserted inside the THOJ above the THRT and provides access to both bores of the tubing hanger.

The ball-valve technology developed for subsea test trees is used to provide well control barriers in both bores. In addition, the tool carries hydraulic penetrations for both downhole and hanger setting functions, and retains the emergency disconnection and subsequent relatch capabilities of the most advanced test trees.

This includes a primary hydraulically activated unlatch via the workover control umbilical, a secondary hydraulic function using annulus pressure and a tertiary disconnection mechanism by ensuring the tool is sufficiently short to allow shear rams in the top section of the subsea BOP to act on a shear sub. Systems with electrical penetration for downhole gauges and electrical submersible pumps have also been supplied and operated successfully.

Such systems have been engineered and supplied to interface with equipment from a number of tree manufacturers and have gained a track record of successful deployment of both 5-in. by 2-in. and 6-3/8 in. by 2-in. completions.

Now that well control barriers have been introduced to the landing string, intervention via wireline or coiled tubing can be undertaken during the completion program with much more confidence. The ball-valves are designed, tested, and certified to cut slickline, braided line, and coiled tubing up to 2-in. in diameter with 0.190-in. wall thickness, should this be required, and to subsequently seal. Sealing can be by metal-to-metal or elastomeric as appropriate.

In addition, and perhaps most importantly, the well can be cleaned up during the hanger installation trip. This not only minimizes the number of trips, and thus rig time, but also minimizes the sand-face exposure to kill fluid and thus the risk of impairment to well productivity. Savings of US$1 million per well have been estimated by operators at current rig rates.

Once the well is cleaned up, it can be suspended with a deep-set plug, kill fluid above this, and finally a plug in the hanger. This is particularly useful when tree installation may be delayed by availability or batch installation program for a number of wells. Operators have estimated productivity improvements of up to 30% using this methodology, depending on reservoir type and conditions.

Dual-bore riser alternative

Access to a dual-bore riser can be difficult, potentially involving long delays and significant capital investment. In addition, trip times are significantly longer with 2 or 3 joints run an hour compared with up to 10 an hour for conventional monobore landing strings, increasing operational costs. Therefore, it was decided to develop an elegant method for dual-bore access from a monobore riser.

The design involved installing an hydraulically activated hinged gate immediately above the dual-bore subsea completion tree (DBSCT) which can be positioned to direct tool strings to either the annulus or production bores. The body of this bore selector tool carries penetrations for all the functions of the DBSCT, the hanger setting system, and any additional downhole communications required, but no well control devices are included in order to simplify the design.

A riser circulation path can be provided by introducing coiled tubing or by porting the annulus bore of the DBSCT to provide communication with the choke/kill line of the BOP stack. This requires the insertion of an additional valve in the annulus bore to maintain well control integrity.

This system has now been successfully deployed on a number of wells in a 6-3/8-in. by 2-in. configuration, providing a number of advantages, including the elimination of the cost of dual-bore riser and associated equipment, and a reduction in rig-up and trip times. Savings have been estimated at US$1.2 million and 12 hours per trip on this project.

Bore selection systems

The next obvious step for bore selector systems is to eliminate the need for dual-bore risers during tree installation.

An adaptation of the above design has now been produced for open water deployment, and having successfully completed function and stack-up testing, was deployed for the first time in June 1998. The tool is inserted in the landing string immediately above the emergency disconnect package (EDP) and below a stress joint designed to dissipate the bending moments created by the external forces expected on the riser string.

Again, the principal advantages are the elimination of the cost of dual-bore riser and reduced rig-up and trip times. In the first deployment, a dual-bore stress joint was utilized due to availability, with the annulus bore redundant. However, in the future, it is envisaged that a simplified monobore stress joint can be deployed, reducing the complexity and costs still further.

Horizontal tree installation

Horizontal or spool trees are increasingly being seen as an alternative option to dual-bore trees, particularly for low pressure applications or where interventions and/or workovers are expected during the well life. The systems are designed for completion installation and intervention/workover to be carried out through the BOP, thus eliminating the need for an open water lower riser package (LRP).

Disconnection capability, however, is still required. A range of effective installation and intervention programs can only be undertaken in this manner if emergency disconnection and re-latch capability is provided while maintaining sufficient through-bore within the landing string.

Intervention, and the provision of disconnection capability, requires well control devices to be installed in the landing string, and these have been developed from subsea test tree technology to interface with the tree manufacturers' equipment. A successful track record has now been established around the world for access with a through-bore up to 7-3/8 in. This permits large diameter suspension and crown plugs to be run and pulled and thus allows large bore, high productivity completions to be installed.

As annulus communication can be established through the rig BOP systems, a monobore intervention string can be used for completion installation and well intervention. A derivative of monobore subsea test trees is employed to provide the well control barriers, and this carries hydraulic penetrations for all the THRT or tree connector functions plus a number of penetrations for downhole equipment such as gauges, flowmeters, safety valves, chemical injection and packer setting lines.

During a recent operation this system's expediency was demonstrated beyond doubt when a coiled tubing string parted at surface during a well clean-up operation. Failure of the surface shear systems resulted in a high pressure gas release at the drill floor, and only the capability of the intervention tree to cut the coiled tubing, and subsequently seal off the well, averted a major incident.

Although the unit cost of horizontal trees is not significantly lower than that for conventional trees, the total project costs can be significantly reduced when the simplified installation equipment and procedure are considered. Workovers are also likely to be less expensive, with rig time to retrieve and replace completions or electrical submersible pumps expected to be significantly shorter.

Costs can be reduced still further through the use of rental and lease agreements for the intervention equipment, since it can be configured to interface with equipment on a number of separate developments. Savings up to US$3 million and 4 days rig time have been estimated for a single well installation when all the above options are utilized.

Alternative approach

An alternative method for reducing the cost and lead-time associated with provision of dual-bore systems has been successfully undertaken recently in the Asia Pacific region. A dual-bore flowhead, riser and lower riser package (LRP) is often purchased from the tree manufacturer and held in readiness for any future intervention requirements.

Because delaying intervention when required can be very expensive in terms of deferred production and cash flow, buying dedicated equipment can be justified.

However, equipment utilization is usually extremely low and there is therefore the opportunity to spread the costs by pooling equipment. This is a commercial arrangement that has been used successfully elsewhere in the world, and is becoming more applicable in the Asia Pacific as the number of operators with subsea completions increases.

A variation of this novel commercial approach was undertaken recently.

The service contractor was given an incentive to pursue additional utilization of the equipment during periods of inactivity.

The complete package was mobilized for a second operator, and employed in the installation and kick-off of a new field development. Cost reductions in excess of 50% were estimated by the client and the lead-time for equipment preparation and supply was reduced to less than four months.

There are clearly a number of interface issues which require attention before this type of approach can be used. These revolve mainly around scheduling of operations and identifying any adaptations required in order to manage the mechanical interfaces between the various packages.

The recent experience with this equipment should give comfort that all the hurdles can be crossed so long as sufficient time is set aside for preliminary discussions and equipment specification, and that competent engineers are employed to ensure compatibility in every respect.

Costs can be reduced

Subsea completion and tree installation is a costly element of subsea field developments, but costs can be minimized in a number of ways. It is clearly possible to significantly reduce overall project costs compared with conventional, traditional tooling and commercial arrangements. Potential exists to reduce rig time, lead-time, equipment purchase costs, and weather delays, and to maximize well productivity.

Innovative approaches to engineering design have delivered both capital cost reductions and increased operational flexibility. The hazards associated with intervening on live wells can be managed effectively while delivering improved well performance through a wider choice of installation sequences and intervention options.

Rental, and/or pooling, of capital items can deliver cash flow and cost saving benefits without limiting field development options.

An understanding of all the issues affecting field development economics is essential to driving innovative engineering. It is essential that this approach is maintained throughout the service sector for subsea technology to continue to deliver cost-effective solutions to the field operators.