Enhanced flow assurance by active heating within towed production systems

The CDTM involves the onshore construction of the bundle and the towhead structures. The bundle is terminated at each end by these structures, which are designed to accommodate the bundle operational conditions and transmit the launch and tow forces into the bundle.

Once launched from the onshore site, the bundle is transported to its offshore location suspended between two tugs at a controlled depth below the surface. On entry into the field the bundle is lowered to the seabed, maneuvered into location, and the carrier pipe flooded in its final position.

The bundle is designed with excess buoyancy, created by encasing the bundled pipelines and any required control lines or umbilicals inside the carrier pipe. The annulus of the carrier pipe is pressurized with nitrogen to prevent hydrostatic collapse. Subsequently, ballast chains are attached to the carrier pipe at predetermined intervals, providing weight to obtain the desired submerged weight.

The tow speed has a direct lift-straightening effect on the bundle. Controlling the tow speed, in combination with the pretension maintained by the trailing tug, the bundle configuration and deflections are kept under control. The pipeline bundle system has technical and economic advantages over other pipeline systems, such as:

• Use of high-performance insulation materials and transfer of heat between flowlines in the bundle - ensures effective thermal management
• Ability to incorporate active heating systems into the design
• Onshore fabrication, testing, and commissioning to reduce offshore costs
• Carrier pipe gives additional protection to the flowlines and umbilicals against damage and corrosion
• Bundle can be installed in fields where seabeds are congested
• No requirements to anchor work vessels during installation, allowing deepwater applications
• Bundles do not need to be buried or trenched.

Integrated system

A significant advantage of the TPS is that the towhead structures can be used to house all of the equipment required for the subsea facilities, creating a fully integrated system, which can be constructed, tested, and commissioned onshore and installed in one operation.

These structures can be designed to incorporate equipment such as valves, manifolds, controls, data acquisition, and other production systems for field development. All production and injection requirements can be included, and full pigging loops or pig launch facilities can be provided. Various riser bases, well slot configurations, and production options can also be included in the system.

Towed installations can be transported by CDTM or by on-bottom towing to ultra- deep or very shallow water and are highly versatile. The combination of an actively heated bundle system with manifolds, riser bases, and/or subsea-processing systems integrated into a towhead structure could have many applications for deepwater developments:

• Single or multi-well templates or clusters
• Subsea tie-backs of intermediate well clusters
• Well step-out extensions
• Flexible riser base structures
• Pre-installed drilling templates
• Subsea pumping (booster station)
• Subsea separation.

The TPS, however, has additional benefits over other installation techniques, such as:

• Free transportation and installation of the manifolds
• Full testing and commissioning onshore
• The reduced number of subsea tie-ins and spool installations can minimize subsea work and increasing reliability
• Easy accommodation of ESD systems and controls. The manifold structure can be designed to provide permanent in situ protection and can be used to support any additional structures needed to protect the jumpers to satellite wells or future Xmas trees
• Maximum flexibility in the development, both in location and time, minimizing capital expenditure.

Passive insulation

Traditional insulation systems have used a wet insulation material, which is typically polyure-thane, rubber, or glass reinforced plastic. These materials are limited to conductivity or k values of 0.2-0.3 W/mK. To reduce the heat loss from the system requires an increase in the thickness of the insulation. However, buoyancy effects limit the overall thickness. As a result of these properties, the overall heat transfer coefficient or U value is limited to approximately 2 W/m2K.

Dry insulation materials have to be used for U values below 2 W/m2rK. Such materials include polyurethane foam or rockwool, which take their insulative properties from the pockets of gas trapped in their structure, and as a result can achieve U values of approximately 1 W/m2K or better. The presence of water severely degrades the performance of dry insulation; so a pipe-in-pipe system is required to ensure these low U values.

The current trend is to develop systems with lower k values to reduce the thickness of insulation. Recent applications have shown, by the creation of a partial vacuum in the system, U values of 0.5 W/m2K can be achieved.

But, for many deepwater and long distance tieback applications, lowering the U value may still not provide adequate thermal management due to low reservoir temperatures or high wax or hydrate formation temperatures. It is in these scenarios that some form of active heating is critical to facilitate production for the development.

Active insulation

The problem with the passive insulation systems is that, once installed, they do not give the pipeline operator any control over the system. Transient conditions such as startup, shutdown, turndown, or ramp-up are becoming increasingly important for field development, especially in deepwater fields where floating production, storage, and offloading vessel use is predominant. The net effect is that without the use of active heating, a field can still be developed.

The benefit of the active heating systems is that they allow heat to be added to the pipeline to maintain the temperature above the hydrate dissociation temperature without the need to depressurize the pipeline. This allows for the development of ultra-deepwater fields, where it would not have been previously possible. This also reduces the frequency of potentially expensive and time consuming depressurizing and start-up operations.

Active heating is defined as the input of heat into a production system from some external source. Active heating may be required to heat the production fluids during turndown, startup, and/or shutdown scenarios. Two types of active heating can be incorporated into a towed prod-uction system: one based on circulation of a hot fluid within a system, one on electrical heat input.

Hot water systems

The TPS or bundle can incorporate the benefits of a high performance insulation system with circulation of hot fluids. The use of a high performance insulation system is a necessity as it can significantly reduce the heat load for the active heating system.

Two types of hot water circulation, used in shallow water, are direct heating by annulus circulation and indirect heating circulating through dedicated hot water lines. Both systems require provision for a hot water supply tank, a heater, and pumping facilities.

The direct hot water circulation system incorporates the production flowline contained in an insulated sleeve pipe with hot water flowing in the annulus. The hot water can either be injected into any water injection well or returned to the topsides through a separate return line.

The indirect hot water circulation method operates by using the heat supplied by dedicated hot water supply and return flowlines to maintain the temperature in the production flowlines. The production and hot water lines are contained within the insulation layer, which is filled with a low-pressure gas such as nitrogen.

Electrical heating

Another attractive form of active heating is the use of electrical heating methods. These have been successful in the past for treating wax deposition in onshore pipelines; manufacturers are adapting these methods for use on subsea flowlines.

Electrical heating has an advantage over hot water systems in that it can supply a uniform heat input along the entire length of the flowline. The electrical cables are smaller in diameter than the hot water lines, which can give rise to capital expenditure savings through the selection of a smaller carrier pipe. Three methods of electrically heating subsea flowlines that have been used, or have the potential to be used in subsea applications are:

• Direct electrical heating
• Induction heating
• Skin-effect current trace heating.

Selection and design

The key to selecting an appropriate system is specific to the development. The challenge for the industry will be the co-current development of simulation tools to accurately model the increasing complexity of the active heating systems.

If we consider flow assurance to be a risk management, it becomes clear that having accurate information available will impact the final design. Specific design issues, which more optimize the final design, may require the use of 3D software. With the rapid acceleration of computer technology and as the understanding of areas such as multiphase flow increases, it is felt that this technology will become more widespread in the next five years.

Hydrate/wax prediction

An increased knowledge in the prediction of wax and hydrate formation will also be one of the means for enhancing the exploitation of deepwater and ultra-deepwater fields. Existing hydrate prediction techniques using thermodynamic packages can generate a dissociation curve to determine if hydrates will form for a given condition. The system is then designed to operate outside the hydrate formation regions.

It is unlikely that many deepwater and ultra-deepwater opportunities will not be successfully developed without the aid of active heating systems. Incorporating active heating systems in towed production systems can enhance production from these opportunities, however, the challenge for the industry will be to design, install, and operate these systems with the severe conditions associated with deepwater applications.

The relative merits of the types of systems currently available for subsea application and the problems that need to be overcome with increasing water depths have been indicated. For hot water systems, the main issues are the pressure differentials due to water depths and electrical heating systems and the lack of a proven track record for these systems.

Finally, although software currently available is powerful, improvements will be needed to reduce time, costs, and accuracy.

Acknowledgements

Halliburton Subsea Towed Production System Group

References

Kaczmarski and Lorimer, Shell International Exploration & Production Inc.,"Emergence of Flow Assurance as a Technical Discipline Specific to Deepwater," OTC 13123.

Halliburton Subsea "Installation of Flowlines and Equipment for Subsea Developments Using the Controlled Depth Tow Method and Towed Production Systems.

Takaki et al, Chisso Engineering Co., Ltd, "Application of Electric Heat Tracing System to Offshore Pipelines," International Offshore and Polar Engineering Conference, 1993.

Aarseth, Aker Maritime, "Use of Electrical Power in Control of Wax and Hydrates," OTC 8541.

Lervik et al, "Direct Electrical Heating of Subsea Pipelines," International Offshore and Polar Engineering Conference, 1993.

Editor's Note: This is a summary of the paper that was prepared for presentation at the DOT International Conference and Exhibition XIII Oct. 17-19, 2001- Rio de Janeiro, Brazil