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. 

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.

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.

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.

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.

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.

Flow Assurance is an ever evolving discipline that aims to address a number of threats related to design and operation of any system that conveys hydrocarbon fluid, with consideration also given to ancillary systems, which may include injection or artificial lift. 

​Solids deposits such as wax, hydrates or asphaltenes, and problems associated with multiphase flow behavior, such as slugging, are highlighted as being some of the main challenges facing developments and operations in the industry today, see Figure 1.

Furthermore, formation water chemistry and pressure maintenance of the reservoir can lead to a variety of severe scaling issues, while souring can be a major concern and dramatically impact system design and cost.

​I​n order to address the challenges associated with transporting live production fluid, particularly from the wellbore to the host, an integrated approach is required to consider all aspects of the system, so that the production flow path can be maintained over the life of field. 

In general, the core flow assurance strategies are well understood, and vary as a function of developing technology e.g. electrically heat traced pipe-in-pipe. Often, ​a primary strategy will be supplemented by secondary strategies to facilitate safe-out of a system from the various modes of operation. To be the most effective, strategies need to be applied early in the design phase while also giving consideration to material selection and monitoring at key locations, since late additions can have significant impact on cost and schedule, or may be omitted completely.

​There is certainly an impetus on being risk tolerant rather that completely risk adverse, where layers of conservatism can create cumbersome solutions that penalize any given project.​ But that's more about identifying the threats and strategies, and understanding the various drivers and interfaces, versus assumed risk from not applying an integrated approach.

In understanding the system as a whole, and applying learnings and experience from operations and previous projects, representative simulation models can be created and used to produce the necessary deliverables while minimizing man-hours and costly re-runs. It's a result that everybody should be looking to attain.