Are we ready for single flowline subsea tiebacks?

Other pigging and non-pigging technologies for wax mitigation are either less proven, have drawbacks regarding hydrate mitigation, or are otherwise less capable. With further testing and development, or for specific conditions, some of these approaches may be viable.

The authors have taken a functional look at means of delivering waxy product through a single multi-phase flowline. The objectives are to:

• Identify pigging and other technologies that can economically prevent blockage or detrimental reduction in throughput due to wax build-up
• Identify the current state of development of these technologies
• Determine the relative merits of these technologies.

Blockage mitigation methods

The focus of the study was on pigging technologies. The study also examined benchmark systems that could mitigate wax for comparison to the pigging technologies.

The tables present the technologies in ranked order (lower numbers = higher rank). However, note that rank differences between adjacent technologies are not significant and neighboring technologies in the tables are roughly equivalent.

Note that composite liners which are intended to prevent wax adhesion to the pipe wall have also been proposed subsequent to the original study but have no exact parallel in the DeepStar work.

Ranking results

The high level study results confirm that the common approach, dual flowlines, is reasonable and that an insulated line may be appropriate. More advantages and disadvantages of the various technologies are in the DeepStar report.

The ranking method can yield results between a minimum value of zero (meets no objectives) and a maximum value of 5 (meets all objectives). The actual minimum value of any of the technologies examined was 3.2. This is a function of the methodology, which discarded unpromising techniques, and indicates that even the lower ranked technologies have significant wax mitigation capability.

The rankings further suggest that the contraflow pigs be considered for single flowline tiebacks. Pig configuration, operating philosophy, operating procedures, maintenance, and spares should be addressed during system design. For this evaluation the pig is assumed to be operating continually in the flowline, removing wax on the upstream pass and returning downstream with the flow to begin another cleaning pass.

Provision for coiled tubing access would benefit where remediation might be needed and, under some circumstances, might allow coiled tubing to be used as a preventative wax mitigation measure. In such cases, or where backup or contingency capability is desired, provisions for coiled tubing access should be included in designs. Such provisions could include vertical coiled tubing access at the top of the riser, use of a catenary riser, and avoidance, if at all possible, of piping bends in the line.

For longer lines, subsea tees or wyes angled at 45º or less to the axis of the line and with suitable hardware are needed at appropriate intervals.

While the results are reasonable for typical tiebacks an actual project will vary from these examples. Areas of variation to examine when incorporating these technologies into a project are:

• Flow assurance drivers: Variation in production quantity and composition with time
• Operating restrictions: Cool down requirements and availability of pumps/dead oil
• Hydrate mitigation requirements
• Risk tolerance and corporate philosophy
• Regulatory environment: For example the use of large quantities of hydrochloric acid and ammonia by the SGN and Jetstream systems could face environmental hurdles
• Potential variation in predicted versus actual conditions
• Flexibility to handle unpredicted variations, operating upsets, and piggy back tie-ins
• Tieback length: High rated techniques such as insulation and coiled tubing intervention are more suitable for a 5 – 15 mi (8 – 24 km) tieback than for a longer one.

Are we there yet?

Substantial changes are not taking place in the way tiebacks are performed with dual flowline tiebacks still predominating.

This is believed to be due to requirements for additional qualification of the applications in some cases and due to the risk of pioneering new approaches in others. Base case technologies also continue to improve, for example pipe-in-pipe insulation.

Since the original DeepStar study, some new technologies have been identified, for example PigaSys, Aspen Aerogel, Nanogel, Bubbletherm, and composite liners. Subsequent to the original DeepStar study, BP leveraged the study to assess costs for longer flowlines considering both wax and hydrate issues.

The ranking of a technology depends on the specific situation. The dependency of cost on length is shown, but other factors can be equally significant.

Future work

This study focused on wax mitigation technologies. Hydrates were considered only as part of reliability and intervention costs, and could be the object of a similar study.

Further investigation could be done on use of wax mitigation technologies under potential hydrate conditions. For example, release of differential pressure across a hydrate plug could adversely affect a pig or other device in the flowline.

Further identification of design provisions required for facilities in order to use the technologies may be useful. For example, subsea pig launcher modifications are required to use the Jetstream or Power Pig in a single line tieback. Coiled tubing solutions require access points, minimal use of bends, and rig space for the unit. Dual line roundtrip pigging requires that dead oil and pumps be available to circulate pigs.

The use of predictive models has been suggested in conjunction with the progressive pigging system and the wax-in-place system. No documentation was collected on the state of the art of these models. Benchmarking the models against production history and composition data from lines that have plugged would be useful. If the models prove useful to evaluate wax deposition, they could help better understanding the operations of most scenarios in this study.

To evaluate wax remediation by means of coiled tubing, an assessment of the intervention is suggested. This assessment would include determining the effectiveness of the coiled tubing methods at cleaning pipe walls as opposed to releasing blockages. A review of current facilities and field developments should be conducted to determine the availability of fields that could benefit from this technology. Testing of the coiled tubing units in deepwater applications is also recommended.

To further expand the wax-in-place system, flow assurance models and prototype testing may be conducted to evaluate the feasibility of wax-in-place as a viable option. Review of procedures with the regulatory authorities would be carried out to determine methods of decommissioning.

NOTE: This paper identifies and compares alternatives to the conventional dual flowline subsea tieback. The alternatives are for mitigating heavy wax deposition in multiphase deepwater single flowline tiebacks while minimizing lost production. The paper is based on work done for DeepStar (CTR 6302) and subsequent investigations at BP. The prices and costs used are as of the time of writing. Changes since can have an affect on the results.