OFFSHORE TEG DEHYDRATION UNIT PERFORMANCE EXCEEDS DESIGN

Per HolmElf Petroleum Norge Stavanger

Operating conditions on the Frigg gas field in the North Sea have exceeded those for which its natural-gas dehydration unit was designed. The unit is located on the field's central platform (TCP2: Fig. 1).

Nevertheless, the plant has been operated since 1977 by Elf Petroleum Norge with a comfortable safety margin for export gas-water dew point specifications.

Frigg straddles the U.K. and Norwegian border and is the largest gas field so far developed in the North Sea.

The following design criteria have been exceeded:

Gas flow rates: Higher than design criteria as well as literature recommendations
Operating temperatures: Less than both design criteria and values recommended in the literature
Operating pressures: Less than design criteria. Some of the design conditions have been exceeded by 100%. Experience gathered on Frigg has led Elf to several conclusions for future designs.

Care should be taken to avoid over-design of equipment. An important parameter is the feed-gas temperature. Design of a unit with a feed-gas temperature at 15-20 C. greater than maximum operating temperature may result in 100% over-design.

Design criteria with respect to gas velocity (column diameter) and feed gas minimum temperature may be extended beyond values recommended in the literature.

Extending minimum operating temperature down to 0 C. permits TEG dehydration of cold natural gas. This would be applicable for North Sea gases which have been transported to remote locations for treatment and have been cooled to ambient sea temperature during transportation.

Extending gas-sizing criteria from 0.25 mps to 0.30 mps allows reduction in column diameter with consequent cost, space, and weight savings.

SIX CONTRACTORS

The Frigg complex and transportation system delivers gas from Frigg and third-party suppliers (Fig. 1). The gases treated on Frigg contain very little condensate.

The installations on Frigg take care of necessary treatment and compression for transportation as well as fiscal metering of gas and condensate. The gas is compressed before dehydration (Fig. 2).

Transportation to the U.K. takes place in a twin 32-in. pipeline system. Gas and condensate are transported together in two-phase flow to St. Fergus. At St. Fergus, gas and condensate are separated and treated to meet sales specifications.

Through 1992, about 221 billion std cu m (bscm) of gas had been treated on Frigg.

The dehydration facilities installed during development of Frigg consist of six triethylene glycol (TEG) contactors with dedicated regeneration skids (Fig. 3). Each unit is designed according to the following specifications:

Glycol contactor
- 15 million std. cu m/day (MMscmd) gas
- Operating pressure: 140-160 bar
- Operating temperature: 30-50 C.
- Lean gas-water dew point specification: -5 C. at 140 barg (equivalent to 49 mg/std. cu m)
- Vessel rating: 172 bar at 50 C.
- Vessel design: ID, 3.3 m; height (T/T), 8.5 m; 30-in. tray spacing; 8 trays.
Glycol regeneration skid
- Conventional design, rated at 2 million kcal/hr
- Lean TEG specification: 99.5 wt % min. (including stripping gas)
- Lean TEG injection capacity: 4-227 l./hr.

LIMITS STRETCHED

Since start-up of the Frigg installations, several changes in operating conditions have stretched the operational limitations of various process equipment, including the dehydration units.

These changes have been necessitated by changing process plant conditions throughout the field's life.

New fields have been hooked up to the existing facilities for utilization of existing treatment capacity. Treatment of these new fields has resulted in altered operating conditions especially for flow rates, composition, inlet pressure, and operating temperature.

At the same time, the main Frigg field has shown pressure decline and reduced production potential (off plateau production).

An effect of increasing ratio of third-party gases arriving on Frigg at temperatures close to 5 C. is that the water content in the rich feed gas is reduced. With a reduced water content in the feed gas, the amount of water to be removed in the TEG contactor is also reduced (Fig. 4a).

Since start-up of the field, the water dew point measurements have fallen well within specification. Typical measurements at 140 barg show a water dew point around -18 C. compared to -5 C. in the specification (24 mg/std. cu m vs. 49 mg/std. cu m, respectively).

The good performance shown by the units compared to design may be attributable to the over design. An important parameter is the rich-gas temperature which was set at 50 C. for design purposes but has hardly exceeded 38-39 C. since 1977 (Fig. 4b).

At 35 C., the water content in the rich feed gas is about half compared to a feed-gas temperature of 50 C. This reduces the load on the glycol-regeneration unit and thus allows reduced lean-TEG concentration. Normal lean-TEG purity is 96-97 wt % compared to 99.5 wt % design.

Because of reduced water load in the feed-gas stream, the glycol-regeneration units are not fully loaded. With reduced water pick-up requirement and TEG circulation rate, the reboiler duty is about 50% of design.

The over-capacity in the system has allowed reduction in stripping gas rate. Normal stripping gas rate is now set at 10 std. cu m of gas/cu m of TEG.

In the design phase and the early days of production, the flow rate was set at 60 std. cu m of gas/cu m of TEG. A stripping gas rate of 60 std. cu m gas/cu m TEG leads to a lean TEG concentration of 99.85 wt % with consequent over-treatment of the gas.

Typical figures in the literature give a stripping gas rate in the range of 15-70 std. cu m gas/cu m TEG.

CONTACTOR EFFICIENCY

Since start-up of the Frigg field, the maximum operating capacities of the natural-gas dehydration units have been gradually upgraded from 15 to 30 MMscmd. Upgrading the capacity has resulted from the treatment of additional fields, minimization of units in duty, and changing process parameters.

Fig. 4c shows historical data of one of the units (1988-1991).

Testing of the dehydration system has been successfully performed up to 30 MMscmd without noticeable glycol losses. At greater than 30 MMscmd, carry-over of glycol escalates.

Based on these factors, the present limitation for operating quantities treated in one TEG contactor is set at 30 MMscmd at 130 bar and 30 C.

A gas flow rate of 30 MMscmd at 130 bar and 30 C. corresponds to a gas velocity close to 0.3 mps which is 20% greater than the recommended maximum in the literature (0.25 mps).

It is clear that the dehydration units have been oversized and that substantial savings could have been achieved with a more accurate design.

On the other hand, it should be noted that the units have been tested for a gas flow rate through the contactor 20% in excess of recommended limits without operational difficulties. In fact, the water dew point for the lean gas improves with increasing throughput (Fig. 5a).

This observation can be explained by improved gas/TEG contact and shows that over-design of the contactor can adversely affect water dew point.

As an indication of overall contactor efficiency, the following method has been used.1

Eoc = Theoretical number

of trays/Actual number of trays

The results (Fig. 5b) indicate an increase in column efficiency from 15% at 15 MMscmd to 25% at 30 MMscmd.

A clear drawback resulting from an increased throughput is an increased pressure drop across the contactor. The pressure drop increases to 0.6 bar for a gas flow rate of 0.3 mps (30 MMscmd).

The corresponding figure for design conditions (15 MMscmd) is about 0.1 bar (Fig. 5c). A pressure drop of 0.6 bar across the contactor is acceptable and sufficient to keep a liquid seal in the down-comers.

OPERATING BELOW DESIGN

Because of an increased portion of third-party gases, the treatment temperature has been reduced over time. Initially with only Frigg gas to be treated, the operating temperature was about 35-37 C.

A 1987 test determined the minimum operating temperature for dehydration to be around -2 C. At less than this temperature, glycol viscosity and other operational problems were experienced.

In the literature, the minimum recommended temperature is given to be 50 F. (10 C.).

Since 1987, the dehydration unit has been operated at or close to 0 C. on numerous occasions (Fig. 4b).

Because of the reduced water content in the wet gas as well as higher absorption efficiency at lower temperatures, the water content in the lean gas at 0 C. reaches values as low as 5 mg/std. cu m. This would be considerably less than any commercial specifications.

As a consequence of reduced throughput in the transportation system, the operating pressure in the dehydration unit has been reduced accordingly and reached a minimum at 70 bar during first quarter 1990 (Fig. 4d).

As a result of the reduced sales quantities, the number of treatment trains in service has been reduced. This in turn implies that the load per contactor with respect to mass treated remains at a high level.

Because of reduced treatment pressure, the actual velocity in the contactor increases per unit flow of mass. With operating conditions at 70 bar and a gas flow rate of 17 MMcfd at 30 C., actual velocity in the contactor is about 0.3 mps which is 20% greater than recommended limits.