High pressure/high temperature (HP/HT) production has always posed additional engineering challenges to subsea pipelines. As subsea pipeline use has become more widespread in shallower water such as the North Sea, understanding of these issues and how to address them has improved. HP/HT applications are now increasing in deepwater areas such as West Africa and the Gulf of Mexico, yet the technology and understanding cannot simply be transferred as-is. In deep water, where severe conditions lead to changes in product dynamics, flow assurance and the importance of thermal management systems are critical issues, making pipeline design far more complex. In particular, matters of thermal performance, thermal buckling and installation are among the issues to be considered.
Thermal performance
Given that flow assurance is fundamental, addressing this need must be the central focus in the design and engineering of deepwater HP/HT lines.
Controlling the thermal energy in the product during transportation through the flow line and riser is clearly the crucial factor. This in turn requires consideration of two key elements: (a) steady-state performance (governed by the overall heat transfer coefficient or U-value - a measure of the energy loss per unit of surface area of the system), and (b) transient performance or cooldown requirements (that is, the period of time after shut-in of the flow during which the product temperature must remain above a specified value). In water depths of 1,614 ft (500 m) and more, pipeline design must seek to balance the steady-state and cooldown requirements to meet the specific field conditions, along with issues of weight, installation needs, and other commercial and economic factors.
One of the considerations facing deepwater HP/HT developments is the adiabatic cooling that occurs in the riser. Thermal insulation of the flow line, therefore, needs to be as efficient as possible so as to minimize heat loss in the line to counter the loss in the riser and create as long a cooldown period as possible. Loading the flow line for maximum insulation will be considerably less expensive than loading the riser.
Theoretically, options include coated conventional single pipe or pipe-in-pipe systems, and final selection will depend on a range of factors. Cost will be one of these.
This has to be weighed against thermal performance and structural reliability, however. And given that, as Figure 1 and Table 1 show, single insulated pipe designs can raise concerns over structural performance at high temperature and pressure resistance at depth, pipe-in-pipe systems will still tend to be the first option for deepwater HP/HT lines - albeit the gap shown in Figure 1 between pipe-in-pipe and conventional single pipe systems indicates combinations of U-value and water depth for which there is not an economic off-the-shelf product available.
The design options have only just begun at this stage; however, the next consideration is which pipe-in-pipe design to select. Structurally, choices include sliding (where the carrier pipe is able to "slide" over the flow line), fixed (where the flow line and carrier are fixed axially and laterally at the end of each double or quad joint length) or free-moving (with the flow line concentrically located inside the carrier). Equally, the insulation options are various, from polyurethane foam (the cheapest option - sprayed on or injected into the annular space between flow line and carrier) or granular (fly ash poured into the annular space) or micro-porous materials (bonded spherical particles of fumed silica), to a vacuum (the best insulation but most difficult to create and maintain) or the latest phase-change materials option (storing heat that is released during shutdown as the material crystallizes). Importantly, these choices cannot be made independently, since the use of one will allow or preclude the selection of the other.
Weighty issues
A further consideration is the fact that while pipe-in-pipe may offer the best balance of thermal efficiency and performance reliability for HP/HT lines, it comes with a penalty - increased weight, a factor of considerably greater significance in deep water than shallow.
There are means to address this, however. A solution developed by DeepSea, for example, committed to optimizing design to maximize return, is to apply an "inside-out" design process. This optimizes each layer from the flow line internal bore outwards to minimize thickness, thereby reducing steel volumes and overall outer diameter of the system, and thus weight. The design focuses on establishing the actual required pipe diameters for flow line and carrier, rather than employing API standard sizes, integrating thermal and mechanical design to ensure the project requirements for production rate and steady-state thermal performance are met, while minimizing as-installed cost. In shallow water the benefits may not justify the additional design and procurement complexity, but in deep water, specifically 3,280 ft (1,000 m) or deeper, substantial and valuable cost benefits can be achieved.
Cooldown critical
While the current focus on deepwater flow lines tends to be on steady-state performance, consideration of transient performance is also critical. For HP/HT systems, where maintaining sufficient heat to reach the platform is assisted by high inlet temperatures, extended cooldown periods can provide the greatest operational benefits.
If cooldown criteria drive the thermal management design, however, the use of wet insulation materials developed for good steady-state performance will result in an extremely thick coating, introducing increased cost and seabed stability issues due to low submerged weight.
Where transient behavior is the dominant criterion, the most appropriate design will be one offering good thermal conductivity and high thermal inertia. Density of the material has an important role since the higher the density, the greater the thermal inertia, while the greater density also results in increased submerged weight, in turn alleviating or eliminating seabed stability issues.
Lateral buckling
At higher temperatures, and to some extent higher pressures, pipelines laid on the seabed become susceptible to lateral buckling, resulting in global deflections which can lead to the pipe cross-section yielding. This is caused by compressive axial force building up as the line tries to expand thermally but is restrained due to friction with the seabed.
Key to an effective solution is the pipe's ability to extend axially. Given that movement is required, controlled promotion of this using either buckle initiation points or expansion spools will result in economic relief of the axial force. Various approaches have been tried and tested, and with today's sophisticated tools and advanced numerical modeling, the processes exist to address the issues.
By undertaking route survey data analysis, finite element (FE) modeling of the pipelay, thermal analysis of the pipeline, and FE analysis of the pipeline operation, relevant actions can be established.
Further important factors to be considered in this context include ratcheting (behavior of the line under cyclic thermal loading) and buckle interaction, as well as aspects such as sensitivity to lateral and axial friction coefficients, effect of assumed slip distance in the friction model, influence of end constraints (expansion spools, catenary risers, etc.), and the effect of soil build-up on lateral displacement. Such detailed analysis and modeling of thermal buckling will enable cost-effective mitigation options with a high level of technical assurance to be identified and implemented.
Designing for installation
Finally, an issue to be borne in mind in HP/HT pipeline design is that of installation - considerably more challenging and costly in deep water than in shallow. Given, for example, that pipe-in-pipe technology is widely used in deepwater HP/HT developments; consideration must be given to weights involved with these systems as production goes deeper, incurring problems as they become too heavy for conventional installation methods. Indeed, pipe-in-pipe systems are more installation vessel-dependent than conventional pipe, and this dependency is even more pronounced with increasing water depth and extreme requirements such as HP/HT demands.
A thorough understanding of these systems' structural response during installation is required to allow the acceptable loading regime to be extended. Flow line installation in deep water has been an area of considerable research, looking at improvements to installation lay rates and reliability, and at new methods. While recent developments in pipeline and riser design codes and increased understanding within these allows the installation envelope to be extended, this is only directly applicable to single pipe with simple geometry. More complex geometries such as pipe-in-pipe systems require further investigation.
In particular, issues to consider include the amount and location of plasticity that may be allowed to occur during installation (including the effect of the residual strain on operational performance of the line) and the non-uniform geometries of typical deepwater field joints, which increase the complexity of the challenge.
Recognizing the need for more detailed and rigorous investigation of the pipe-in-pipe field joint area, driven by its geometry, for example, DeepSea has undertaken studies to assess this during installation in both J-lay and S-lay methods. High tension in J-lay, usually applied by the tensioners acting on the carrier pipe, and high bending in the steep S-lay, both result in stress concentrations (at different points) in the field joint region, which need to be evaluated and shown to be acceptable at design stage. The findings of these studies indicate that considerable cost-savings could be realized by using less stringent strain limits for pipeline installation, achieved through a combination of higher-level engineering and analysis than is conventionally used. While moving to strain-based design requires greater sophistication in the analysis, the associated engineering cost is more than recouped in the reduced installation cost and increased certainty of design.
Conclusion
While the challenges posed by HP/HT pipelines are increasingly recognized and understood in shallower waters, the complexity of the issues is considerably greater for deepwater HP/HT pipeline design. Though the factors to consider are many, it is evident that increased investment in advanced engineering to investigate and meet the multiple challenges will ultimately pay dividends in terms of reduced costs and, importantly, reliability and technical assurance.

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