Thermal Management as a Flow Assurance Strategy

Thermal management is a cornerstone strategy when it comes to mitigating flow assurance threats such as wax and hydrate formation. Through its application, the objective is to manage thermal losses in the system such that arrival temperatures at the host are maintained about targets during normal operation, and sufficient cooldown times are available in the event of a shutdown.

It's a tried and tested solution that has served the industry well for decades. Thermal management solutions are however field specific, and depend on a number of factors, particularly as we look to increase tie-back distance and move to harsher, deeper water environments. The solution needs to encompass not only normal operation, but transient events like shutdown (cooldown) and restart. For any given project, both the thermal and hydraulic impact of the system should be vetted with the following considerations:

• Fluid type (e.g. API gravity / GOR)
• Tie-back distance
• Available wellhead flowing temperatures

• Flow assurance threats (e.g. wax appearance temperature / pour point)
• Rheology (including emulsion forming tendencies)
• Turndown limits
• Host configuration (e.g. water depth / available riser slots)
• Target arrival temperature/pressure

Traditionally, in deepwater (>3,000 ft), we tend to apply production loops with sufficient insulation to mitigate the risk of deposits during normal operation, where dead oil is used to displace the live fluid during a shutdown. In such an instance, we may apply passive insulation to the flow system. There are times however, where creating a production loop is not feasible based on host infrastructure, or desirable, particularly as we look to extend tie-back distances to economically develop new fields and opt for a more hub-and-spoke arrangement. 

Passive Insulation

A passive system relies on the overall heat transfer coefficient that can be achieved for a the flowline, which is approximately 0.50 Btu/hr.ft².°F based on OD for a wet insulation system such as Glass Syntactic Polyurethane (GSPU) or 5 Layer Polypropylene (5LPP). Burial of the flowline can also be used to provide thermal resistance to heat transfer. U-values better than 0.20 Btu/hr.ft².°F can be achieved for a pipe-in-pipe system, where superior grade insulation material is maintained in a dry environment with or without a vacuum in the annulus, see Figure 1.

Figure 1 - Wet Insulated and Pipe-in-Pipe Flowline Configurations

In deepwater applications, there can be significant heat loss in the riser, as gas breaks out of solution, then expands and cools on its way to the surface, so the it cannot be an after thought. The following
following figure shows the influence of GOR and insulation system in a water depth of 10,000 ft for a 10 mile, 8" NB flowline producing at 30,000 STB/d. As is evident, the largest portion of heat loss in the system can be the riser.

Figure 2 - Heat Loss in Wet Insulated and Pipe-in-Pipe Flowlines in 10,000 ft of Water

Even for the best conditions, tie-back distances for passively insulated system are limited to less than 50 km. To provide greater operating flexibility to the field, or increase the tie-back length, active heating can be applied.

Active Heating

Active solutions involve the addition of heat to the system through circulation of hot liquid or an electric heat source. Applying heat can be extremely beneficial through all modes of operation, whether it be a turndown event, temperature maintenance above the hydrate dissociation temperature during a shutdown, or warming the flowline out of hydrate formation region prior to a restart. Some active heating success stories include:

• Britannia Production Bundle (hot water circulation)
• King pipe-in-pipe (hot water circulation)
• Papa-Terra (Integrated Production Bundle)
• Lianzi (Direct Electrical Heating)
• Manuel (Electrically heat traced pipe-in-pipe)

Figure 3 - Active Heating and Passive Insulation Types (courtesy of Subsea 7)

A lot of thought is required before applying active heating technologies to a field, and include considerations such as heat source, power requirements, tie-back distance, mechanical design, installation feasibility, technology maturity and reliability. If the riser, for whatever reason, cannot be actively heated, it becomes the weak link in the system given that the static portion of insulated pipe on the sea floor will always cool faster than an actively heated flowline. 

From a flow assurance philosophy standpoint, additional thought needs to go in to the appetite for heating hydrate plugs to melt them. Even if heat is supplied to a flowline in a uniform manner, the variation in liquid hold-up along the system length means that any gas phase will warm-up faster than and oil or water phase.

Hot Topics

Recent advances in technology is allowing the industry to move to a low power electrically heat traced systems for deepwater, either in a pipe-in-pipe configuration, or potentially buried with heating wires. The latter, developed by Salamander Solutions, certainly offers less constraint when it comes to mechanical design, installation loads and tie-back distance, where lengths upwards of 50 km can be achieved depending on power requirements. Active heating, used in conjunction with multiphase boosting, has the ability to unlock ultra-long tie-back distances for oil systems, well beyond anything that has been seen before.

On the opposite side of the spectrum, Cold Flow removes the temperature maintenance element, and allows seeding of hydrate and wax particles through a novel recirculation loop located near the wellhead. The formed particles become inert, which allows ultra-long cold multiphase wellstream transport in uninsulated flowlines without the risk of deposition or blockages. It's a concept that has been around for the last number of decades, and continues to be developed by Sintef.

Figure 4 - Cold Flow Technology (courtesy of Sintef)