Subsea processing - challenges and opportunities

Håvard Brandt - Det Norske Veritas (USA) Inc.

Michael Else - Minerals Management Service

The subsea/flow assurance track includes this comparative risk analysis and review of subsea processing technologies. Recommendations for technology improvements and qualification and testing required to make this technology ready for future deepwater field applications are based on case examples.

In deep water, transportation of produced fluids is often challenged by a number of factors that can make the exploitation economically marginal, particularly when relying on conventional technology solutions. The need to provide energy to the well stream to reach the treatment facilities is continuously increasing as exploitation moves into deeper waters and operators are evaluating longer tiebacks. Increased energy to the well fluids also has the potential to increase the ultimate recovery and accelerate production. These and other aspects motivate the interest in exploring the opportunities that novel technologies, like subsea multiphase booster pumping and gas compressors, offer.

The potential for slugging and challenges related to managing large amounts of produced water at the surface facility motivates the interest in subsea separation. Subsea separation can be based either on two or three phase separation and pressure boosting to dispatch the liquid phase(s) to the receiving facilities. Two phase separation enables the reduction of the well back pressure by free flowing the gas phase and only boosting the liquid phase. Further, subsea separation could have a positive effect on flow assurance, including the risk related to hydrate formation and internal corrosion protection derived from the presence of the produced water in combination with gas.

The opportunities and possible benefits related to subsea processing technologies are many; however, there are uncertainties related to the performance of these systems.

Significant development and testing work has been undertaken in the effort of qualifying subsea processing technologies, and several systems have also been successfully deployed. While the technology itself is perceived as mature, limited operational experience is available. As a consequence, the anticipated reliability and risks related to applying these systems are subject to uncertainty.

Subsea processing equipment is characterized by use of novel technologies or extended application of existing technologies, increased reliance on remote operations and control systems, and it introduces additional complexity in a deepwater subsea production system.

Further, when moving into deeper waters, the uncertainty related to whether unforeseen events will occur increases as the technology is introduced into an operating environment which is different compared to shallow water operations. These and other factors have motivated the interest in risk assessment and risk comparison of subsea vs. surface-based processing.

Objective

The Minerals Management Service (MMS) requires that new technology is proven to be as safe and reliable as existing technology, with respect to personal and environmental risks. To provide a better understanding of subsea processing technologies and the associated risks and uncertainties, MMS has therefore initiated this comparative risk analysis.

This study has focused on seabed processing, and different subsea processing technologies have been evaluated, including:

•Subsea pressure boosting
Multi-phase booster pumping (twin screw pumps/helico-axial) Gas compression (wet gas/dry gas)

•Subsea separation
Two phase separation (gas - liquid separation) Three phase separation (oil - gas - water separation).

Risk assessment

The study has addressed risk exposure related to subsea processing equipment during the entire life of a field development, including issues related to commissioning, installation, repairs/maintenance and abandonment in addition to the in-service risk exposure.

The first task was to identify the relevant risks related to subsea processing technologies. A qualitative group session, HAZID, was used to brainstorm and evaluate major differences related to field development, applying subsea processing equipment compared to more conventional deepwater field development. The HAZID review included a systematic evaluation of all the relevant risk elements for a quantitative offshore risk assessment.

The risks identified in the HAZID session have been reviewed and evaluated in a comparative risk assessment. One of the challenges related to this project has been to define the basis for this comparative risk evaluation of subsea vs. topside processing.

Subsea processing is an enabling technology, and a direct comparison with surface processing would probably not be realistic. The study has therefore adopted an approach to evaluate the additional implications and risks imposed by subsea processing equipment.

Conclusions

The overall conclusion from the risk comparison is that the HSE risk exposure using subsea processing is not significantly different compared to that of a conventional deepwater field development. In the quantitative risk comparison, a very generic risk picture of a typical deepwater installation was used as a basis, while a typical subsea tieback was developed as the base case when assessing environmental risks.

It was evident that the personnel risk exposure will be reduced by introducing subsea processing equipment. The risk reduction will be very dependent on the specific installation and actual subsea processing technology applied, but in general, the personnel risk is reduced by moving more of the process equipment to the seabed, allowing smaller installation and less offshore operators.

When assessing the environmental risks, measured as release of hydrocarbons, subsea processing resulted in a slightly higher frequency for small leaks compared to a conventional subsea tieback. However, the leak frequency assessment used very conservative assumptions, and the differences were minimal, 4.9 x10-3 compared to 3.2 x 10-3.

The authors conclude with recommendations to advance subsea processing technology and its applications.