Innovative Technology for Ultradeepwater Gravity-Based Separators
Authors: Michaelsen Jarle 
Published: October 2008

The Troll subsea separation station is now in operation and is establishing a working track record. The Troll subsea separation station separates water and oil and re-injects the water into a dedicated disposal well. The system is installed on the Troll Field, located in the Norwegian North Sea (360 m water depth). The Troll field would experience liquid handling capacity problems if not for the subsea processing station which relieves this problem.

The Troll subsea separation station is the only full size, subsea separation station actually installed and operating. The adaptation of this technology to ultra deepwater fields is a major challenge, calling for application of new radically improved methods for gravity based separation.

The oil and gas industry is known for its risk awareness, thus the process of adapting subsea separation to ultradeep water fields must be undertaken with care. However, in order to meet the demands of the future deepwater fields, some radical changes will be necessary.

This paper will focus on how subsea separation can be adapted to Ultra deepwater fields, and which principles can be used inthe next generation of ultra deepwater subsea systems, "NuDeep" /1/

Subsea processing concepts for ultradeepwater Fields

It is important to understand that subsea separation is one of several subsea processing concepts applicable to such fields.

Bulk Subsea separation:

Bulk water separation as provided by The Troll subsea separation station (will initially (first 3-4 years) have little significant impact. The water cut ratio is normally less than 20 % and the separation of this water will not have crucial impact on production rates or liquid load on the host.

When water/oil ratios increase to higher percentages, the separation and water removal will cause less pressure drop in the pipeline. In particular, the static liquid head in the riser will decrease and impact on production rates will be significant. Strongly reduced liquid load on the host will enable tie-in of new fields and also reduce discharge to sea of polluted produced water (if treated and dumped and not reinjected). It is important to recognize that for a field with large water production, the gain in driving pressure is very large by applying subsea separation. The calculation example shown below shows how increasing water production slows down natural production as compared to a production with water separation on the seabed.

Figure 1, Comparasion of production with and without subsea separation(Available in full paper)

Complete water separation

If the performance of the subsea separation station can be improved to none hydrate forming amounts of remaining water in oil, the benefits of subsea separation expands significantly. Insulation, heating and injection of hydrate inhibitors can be eliminated or reduced or tie back distances increased. The savings will mainly be in the initial investments, and have a very positive influence on the net resent value of the project. In deep waters the effect of this will increase. Due to the high ambient pressure, the hydrate formation temperature will be higher and the shut down time prior to hydrate formation much shorter (ref. Fig 2), hence the insulation requirements will be higher.