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Content archived on 2022-12-27

A FLOATING BARGE-MOUNTED PLANT, PRODUCING HIGH-PRESSURE N2/CO2 GASES FOR EOR PROJECTS IN THE NORTH SEA

Objective

As the world's reserves of crude oil diminish, the low yield of often less than 30% from a deposit becomes the central aspect of energy policies. The injecting of CO2 or N2 represents one way of stimulating oil recovery. So far the use of such tertiary production measures has been restricted to onshore fields.
Such measures, however, take on greater economic significance for offshore fields. The investment necessary to develop an offshore site exceeds that required for an onshore location by many times. This makes it even more important that the amount of oil extracted from an offshore deposit should be as high as possible. The proposed technical development aims at providing low-cost gas which will then be used to recover the oil offshore by tertiary measures. In awareness of the conditions specific to offshore sites, this means that the production facilities will have to be installed on a mobile carrier, i. e. barges that can be used at several locations irrespective of water depth.
The Norwegian and British oil fields in the North Sea were examined with regard to their yearly production rate and the remaining production capacity. A possible use of the EOR barge can be envisaged for those fields whose regular capacity will be exhausted in the foreseeable future.
First the production capacity of the producing plant was determined. Since the concept of an EOR gas barge aims at a multiple use of the barge in various oil fields, the design value was selected higher than the average of the expected cases of application. The concept of a multiple use precludes from the outset the fixing of a determined pressure. The gas processing plant has therefore been conceived in such a way that an adaptation to various pressures can be made with as low an amount of alteration as possible. The plant has been conceived in the form of a basic outfit for 200 bar injection pressure, and was laid out such that either and additional compressor set can be installed for compression toa higher pressure, or only one additional compressor which can be coupled onto the shaft of an existing set.
Associated gas is used as fuel which is obtained on the oil production platform.
In order to determine the most favourable shape of the barge under the aspect of its sea behaviour, two different pontoons were subjected to an investigation. The size of the deck area is governed by the processing plant which is to be housed. The plant assemblies are partly located on the pontoon's bottom and partly on the deck.
It has been intended to conduct the hydrodynamic evaluation in two stages. At first the barge was tested while freely floating during various sea states without any anchoring (calculation for two types of barge designs). Thereafter an anchoring calculation was conducted using the optimum barge design.

The purpose of the anchoring system is, in the first instance, to anchor the concrete barge together with the gas producing plant in a permanent way, and additionally it is used to anchor the gas supply lines leading from the seabed to the barge. In recent years remarkable progress has been made as far as the production, endurance and safety of flexible underwater pipes for high pressure (up to approx. 450 bar) is concerned.
Due to the relative small dimensions of the barge with a displacement of some 15000 tons (i. e. 10% of an average shuttle tanker size) the possible solution with a tower-anchoring system was abandoned.
A turret mooring system was chosen since this can be applied as a very reliable anchoring device with 6 to 8 chains and accordingly a sufficient redundancy.
Assuming that in future large quantities of CO2/N2 and steam will also be required for the tertiary production of oil offshore, a combined process is proposed which features a particularly high degree of economy. The process is made up of the following three main components:
a. production of an N2/CO2 mix by burning preferably gaseous hydrocarbons, whereby high-pressure steam is also generated, b. production of N2 in an air separation plant,
c. compression of the CO2/N2 mix by using the high-pressure steam produced during combustion.
The combination of combustion and air separation plant results in a substantial decrease in the cost of injection gas when drawing comparisons with each separate process.
The main items of the project:
- CO2/N2 production using a boiler and steam turbine
- CO2/N2 production using an air separation plant with an increased delivery pressure for N2
- application of gas turbines to produce CO2/N2 with gas turbines driving the compressors directly
- basicengineering for the combined production of N2/CO2 and high -pressure steam when using steam and gas turbines
- determination of energy requirements and comparison of the capital investment and operating costs for each alternative
- development of a prestressed concrete barge as a floating carrier for a plant to produce N2/CO2 high-pressure gas
- design of a mooring system with central articulated joint and integrated gas conveyance for N2/CO2 high-pressure gas and high -pressure steam, as well as for supplying the plant with low -pressure fuel
- summary covering capital investment and costs, and final economic analysis.

Call for proposal

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Coordinator

PREUSSAG AG (EX SALZGITTER AG)
EU contribution
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Address
ABTEILUNG FORSCHUNG ENTWICKLUNG POSTFACH 15 06 27
1000 Berlin
Germany

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