Operational feedback from the Nkossa Drizo gas dehydration unit offshore Congo

For more than 15 years the NKossa barge has been in operation offshore Congo. Design of the facility was to perform deep NGL recovery from the associated gas by a turbo-expander process before using the treated gas for pressure support in the reservoir. A dehydration technology offering below ppm dry gas water content had to be selected upstream the expanders.

Bernard Chambon and Louis Penel, Prosernat
Van Khoi Vu and Thomas Brenas, Total SA

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Article Summary

Glycol dehydration with DRIZO™ regeneration was selected as the most cost effective and best available technology thanks to the high glycol regeneration level provided by the DRIZO solvent stripping, as well as the most adapted process for offshore installation and operation.

From the start-up of the unit until today, a successful partnership was established between Operator Total E&P Congo and Licensor Prosernat to perform long term follow-up of the unit’s performances and tackle all upcoming issues. Several problems were overcome mainly due to a weak engineering design. However very good performances were always maintained while lessons learnt enabled to further sharpen the good reliability of the DRIZO process.

This paper will develop all the above points and in particular give a detailed description of the unit’s evolution over the last 15 years.

The NKP concrete barge was initially designed to process production from the NKossa field, situated about 60 km offshore Congo, at a depth of 170m. This field was discovered in 1983 and put in production in June 1996. In its current development stage it consists of 39 production wells, 8 gas injection wells and 6 water injection wells, distributed between 2 well head platforms (NKF-1 and NKF-2). It is operated by Total E&P Congo, with Chevron and Société Nationale des Pétroles Congolais as partners.

Since start-up it has had to adapt first to different production figures than anticipated, and then to the integration of production from new developments. New fields have over the years been routed to the NKP barge for treatment, and it currently treats oil production from the Nsoko and Tchibeli fields and gas from Moho Bilondo field in addition to that from NKossa.

Liquid production of the NKossa field is mainly based on cycling. In this method, reservoir pressure is maintained by substitution of the condensate reservoir gas (rich gas) in the reservoir by lean (poor gas). Lean or poor gas is obtained from condensate reservoir gas coming from production wells, by natural gas liquid extraction (propane + butane + condensate). To obtain a total conservation of reservoir pressure an extra gas from another gas source is added to the produced lean or poor gas. As of September 2009 production figures were:
Oil : 50 000 bbl/day
Reinjected gas : 12.2 Msm3/day
Propane : 7 000 bbl/day
Butane : 4 000 bbl/day

Treatment performed offshore is very thorough, with extraction of NGL from the associated gas prior to its reinjection and fractionation of the NGL to recover LPG, all performed on NKP. In addition to these processes the barge also includes all the more common processes of gas/oil/water separation, water injection and utilities, as well as living quarters.

The concrete barge, 220 m long and 46 m large, is divided into 7 modules in which the functions shown in Figure 2 are distributed.

The main processes employed on NKP can be seen on the simplified scheme below, where the oil separation and gas treatment stages can be clearly identified in Figure 3.

The main objective of the gas treatment is to recover as much LPG as possible. For this purpose a cryogenic process is installed, based on a turbo-expander to reach a temperature as low as -60°C. In these conditions the upstream gas dehydration is essential to prevent formation of hydrates and thus allow good operation of the plant. The very low temperatures reached mean thorough dehydration is required down to a water dewpoint lower than the operating temperature of -60°C (< 1 ppmV).

Why a DRIZO? General operating principles of the DRIZO Process
Only two technologies can achieve this level of drying: DRIZO glycol dehydration and Molecular Sieve. However the specifics of the offshore environment made installation of a molecular sieve unit problematic: this process requires equipment having a large footprint and heavy weight, both unfavourable characteristics. In addition molecular sieves have a significant energy consumption, requiring larger utilities units.

The DRIZO unit on the other hand could be integrated to the barge in the form of a compact skid with limited footprint and weight, and has much smaller energy consumption. It was therefore the most adapted and cost effective process for the situation, and consequently selected by Total.

The DRIZO process was developed and patented by Dow Chemical in the early 70’s. The technology was bought by the Houston based OPC Engineering company in 1985, which was incorporated by Prosernat in 1998. The technology has been since licensed by Prosernat.

The principle of DRIZO is similar to that of a glycol with stripping gas. However, the stripping agent is not natural gas (i.e mainly light ends such as methane) but a vaporised “solvent” which is in fact a C4 to C9 cut extracted from the wet gas by the glycol.

This has two important favourable impacts. First the solvent cut has an improved stripping efficiency compared to natural gas (“stripping gas”) as it works by reduction of H2O partial pressure (similar to stripping gas) but also by breaking the water-TEG bonds thanks to the polarity of the aromatic molecules of the solvent. Second, the solvent is largely recovered by condensation after use and therefore dramatically reduces flaring and BTEX emissions.

The general principles of a DRIZO unit can be seen in Figure 4 below.
One can see the glycol loop, identical to that of a normal glycol unit with stripping gas, and the specific solvent loop along which the solvent is pumped, vaporised, superheated, used as stripping agent and finally condensed and separated from the produced water. The HP part where the gas dehydration takes place is similar to that of any glycol unit, with its glycol contactor, but thanks to deeper glycol purity it allows to reach much lower dry gas specifications.

NKossa process scheme
The design of the NKP DRIZO can be summarized as follows:
• 9.7 m3/h of lean glycol at 99.99+ %wt
• Dehydration of 13 MSm3/d (460 MMSCFD) of water saturated gas at 35°C down to 1 ppmV
The actual NKossa PFD is shown in Figure 5.
The HP section of the unit consists in 4 shell & tube water/gas coolers, an inlet scrubber fitted with a wire mesh mist eliminator and finally the glycol contactor fitted with 250 m2/m3 structured packing. For regeneration, several important features of the DRIZO unit were included:
• The flash drum overhead system (condenser EC-608 and drum DS-603) was designed to reduce solvent losses. The hot vapour from flash drum operated between 90 and 110°C are cooled down to 35°C in the heat exchanger EC-608 to condense the heaviest hydrocarbons (solvent) and to recover the entrained glycol. The non condensable components and lightest hydrocarbons are separated in the drum DS-603 and sent to flare.
• The liquid solvent is vaporised by heat exchange with the lean glycol from the reboiler in exchanger EC-605.
• There is an integrated water cooled reflux condenser in the still column followed by the water/solvent condenser EC-609, i.e reflux is not external.

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