KHISTYAEVA.A. JSC «Momeftegazproekt», Russia

Development of gas production at Arctic shelf of Russia is linked above all with Stockman Gas Condensate Field (SGCF) discovered in 1988.

When speaking about supply of Russian LN gas we assume the USA as main market for such fuel. Difference between supply and demand at this market increases and due to this it is especially important to use the best endeavours for acceleration of the Stockman Field development. It is not a secret that nowadays some other projects relating LN gas are under performance, i.e. in Trinidad, Catar. Nigeria. Besides, there is a possibility to construct a new gas pipeline from Alaska. As result of these projects performance the supply can in future exceed the demand. So, for Russian LN gas as a whole and for Stockman gas in particular speed of the project performance is of great importance as it permits to leave behind the competitors. Secondly, cost of the development shall be competitive in comparison with the similar indexes of the other LN gas projects. That is why during preparation of the field development project it is important to select the most economically effective and rational design of the Field development.

The SGCF is located on shelf of the Barents Sea 290 km to West from Novaya Zemlya and 655 km to Northeast from Murmansk. Field sea depth is 280-380 m. This field is a unique object for development not only in terms of Russian but also of International practice of offshore gas production. This is due to volumes of stocks and possible production per year and also to work conditions relating the field development. Field Facilities Construction is rather difficult due to work conditions: sea depth, distance to shore, severe climatic conditions, complicated sea bed topography, probability of gas hydrates and permafrost presence in bottom layer deposits, probability of heavy ice formations within the area, and absence of sufficient monitoring data and data relating to hydrometeorological conditions of the sea area, and finally absence of experience of the work under such conditions.

Within more than ten years different design and research organizations were involved in problems relating SGCF development.

During basing of the optimal system of the SGCF development a large group of options was examined, these options varied in the design gas production, capacity of the process equipment installed on the offshore platforms, number of the production wells, limit wellhead pressure, hydrocarbons treatment and transportation method, ratio between wells injected by underwater and above water methods etc.

In 2003 a project of the SGCF development considering the existing experience was prepared.

The project examines the following option as the main one:
- design gas production 67.5 billion m3/year;
- design gas condensate production 350 ths.ton/year;
- allocation of four production objects:

I object - deposit Ю0 of reservoir;

II object - deposit KDi of reservoir;

III object - deposit Ю2 of reservoir;

IV object - deposit Юз of reservoir.

- priority objects of development - deposits Юо and KDi of reservoir;
- construction of three multipurpose ice-resistance platforms;
- total design number of wells in case of the offshore facilities complete construction -156 pes;
- gathering and complete treatment of well product on platform;
- construction of three underwater pipelines for gas transportation in single phase state;
- construction of underwater pipelines for gas condensate transportation to shore facilities;
- limit wellhead pressure 2.0 MPa;
- gas production capacity of single platform 22.5 billion m3/year;
- duration of gas continuous production - 25 years;
- gas recovery ratio for 50 years of the field operation - 82.9 %.

The field development is performed in three stages to achieve the design gas production 67.5 billion in /year. Separation into stages assumes step-by-step commissioning of facilities. Each of the stages increases the annual production by 22.5 billion m3/year. Such approach permits collections of the process and design solutions of the field development based on study of the performed stages and using new technologies.

The platforms are connected by gas lines to provide high reliability and continuous product transportation to shore by offshore lines. Intrafield pipelines shall be out of steel pipes with diameter 762 mm (30 in.).

Pipeline from SGCF rims on bed of the Barents Sea with output to Korabelnaya Gulf near settlement Teriberka on Kolsky Peninsula. The design length of the pipeline is 564.5 km. The route shape is characterized with significant drops of sea bed marks. Maximum sea depth along the route is 335 m.

Based on experience of pipelines construction in North Sea by foreign companies rate of pipeline laying is assumed as 2.4 km/day. Duration of the offshore gas line construction shall be 8 months.

For selection of the optimal option of the platform construction Russian and foreign participants of the project studied different designs of the platform including gravitation, piling and float with anchor systems for holding in place of operation of TLP, SPAR. BUOY, TPG types and some others. The earlier performed work examined also stationary gravity platforms.

The feature of gas production platforms being designed for SGCF is the necessity to withstand external actions of different nature during operation. In accordance with natural conditions in the SGCF area, external load has two main components: wave load and ice load.

Correct determination of ice loads effecting on base characteristics of the offshore ice-resistant oil and gas production facilities is one of the most important problems.

Difficulty of the ice loads determination is caused by a great number and wide range variability of a random factors, such as physico-mechanical characteristics of ice, movement velocity, geometric parameters of ice formations, etc. and in number of cases - by dynamic processes which take place at the facilities interaction with ice.

Basing on the design researches and testing of different platforms models in the ice and wave testing baths of the Krylov Cental Research Institute, the conclusions were done that the platforms with tensioned bindings of TLP class are the most optimal and safe for conditions of SGCF. However, certain difficulties have arisen during the final selection of the platform type.

The reason is that at the present time requirements for operation stability of ice-resistant platforms with tensioned legs are not specified either in international practice of Classification Society Regulations and Nouns of Vocational Organizations, or in practice of Russian organizations and societies (e.g.. Russian maritime Register of Shipping). At the same time, the experience of design and construction of common (not ice-resistant) TLPs, generalized in recommendations of, for example, [API, Exxon], gives ground for definition of stability conditions for ice-resistant TLPs with taking into account anchor bindings strength. Stability of the mentioned facilities as a floating object in general should be considered in the following aspects: 1) stability of the anchored facility in miming condition under the impact of irregular sea state - gale of the assigned heaviness, 2) stability of the anchored facility in miming condition under the impact of hummock field of the assigned parameters, 3) strength of anchor bindings and devices with taking into account nonuniform tightening because of effect and/or forces of waves drift, wind and stream, 4) strength of anchor bindings and devices in conditions of cyclic loads of a sea waving and ice effects.

In accordance with Regulations of known classification societies including Register for Mobile Offshore Drilling Unit/Fixed Offshore Platform, consideration of the above listed parameters should be performed for three cases: a) miming condition during operation, b) during heavy gale, c) emergency condition caused by failure of one anchor binding, d) emergency caused by compartment flooding. Concerning requirements to the anchor bindings strength, the

Regulations requirements are limited by specification of margin coefficients.

In course of analysis, the conclusions were done that the following additions should be taken into account in contrast to the existing Regulations of different societies concerning the ice-resistant afloat facilities:

a) Including of ice impact moment in to the design heeling moment;
b) Design of the platform set-down and recovery moment in case of compartment emergency flooding, taking into consideration strain in the anchoring binding, and besides this, working out requirements on the platform emergency stability in case of one anchoring binding or group of bindings abruption under effect of ice;
c) Checking of the platform stability under the impact of sea state or ice, taking into account dynamic behaviour of facility with anchoring bindings, including the case of bindings bundle abruption.
d) Specifications of such requirements on the platform behaviour in running and emergency condition under the impact of ice, that will exclude damage of risers.

TLP with low and TLP with large displacement meet the stated requirements in a best way. However, assessments and comparisons of the two platform types, which have been performed in the previous sections, indicate that the both options have certain disadvantages, which not allow to recommend reliably one of the options for the further development.

So, for example, nature of waves impact on the TLP with large displacement is such that the vertical wave force is comparatively low, and the overturning moment has a high value. TLP with low displacement suffers considerably higher vertical load, but the lower overturning moment. Horizontal wave load for the both types of platforms is approximately equal. Ice load on the TLP with large displacement is lower, than on the TLP with low displacement. The lesser dimensions of the TLP explain that with large displacement in the area of waterline.

So, basing on the test results and considering the performed studies a suggestion concerning TLP of new design was prepared, and the following basic requirements to the support part of platform, which combines advantages of the TLP with low and large displacement, were specified:

- a structure, base of which can be performed as one or two blocks in shop is the preferred one. Block width is - 26*38m, length is ~ 150 m. Block completion should be carried out at quayside in horizontal position. If the structure consists of two blocks they should be mated afloat;
- support part body should be preferably fabricated out of flat sections. In this connection, its is practical to replace cone and cylindrical surfaces with many-sided structures;
- to reduce use of clad steel, the support part should have minimum overall dimensions in the area of interaction with ice (if possible).

The next stage of design is the selection of equipment for the field development and analysis of possible schemes of the equipment layout on the platform.

This option of the field development is assumed as main option.

But at present time considering the progress in offshore oil and gas production the idea of SGCF development based on product transportation without large platforms use looks more rational. May be this option is more economically effective and considering presence of the icebergs and large ice formations in the SGCF area it looks more safety.

At present time the following option of the field development is suggested. Upon maintenance of the same number of wells and production rates equipment for wells underwater injection and subsequent transportation of product to shore without lifting to surface is installed on seabed. For example, this may be equipment FMC Energy Systems. This includes underwater separator for water and condensate separation and equipment for its further well injection (for example, equipment of Framo Engineering AS or FMC Energy Systems companies).

The option based on the product multiphase transportation is another scheme of the field development, which allows avoiding use of expensive platforms for gas production. Implementation of this conception allow to reduce the cost value of the SGCF gas for 30 percent approximately, thus making the project more competitive and attractive for investors. This option have been rejected earlier because of long distance of transportation and great bends along the pipeline run, which cause hydrates formation in the pipeline. However at present time taking into account example of Hydro

Company at the Onnen Lange field development, we can consider this scheme as the most rational.

Considering that temperature near seabed is below zero, the monoethylene glycol (or methanol) shall be injected in the system to prevent hydrates formation. To reduce costs, at the end of transportation monoethylene glycol shall be separated, regenerated and reused in the process.

Besides that, special method of hydrates formation control can be used in this option of development, thus increasing safety and respectively, cost effectiveness of the multiphase transport. By the way, this method is used for a half of year by BP Company at the Horn Mountain field. Gulf of Mexico.

Besides that, Ondeo Nalco Energy Services Company has accomplished successfully testing of new generation of the low-dosage hydrate inhibitors - LDMIs, which were called "anti-aglomerators " (AAs). The last RXD achievement in this area - FreeFlow AA inhibitor. This inhibitor can be used at high pressures and below-zero temperatures, i.e. it fully meets requirements of the SGCF conditions and allows virtually completely exclude hydrates formation at transportation. So the option based on the multiphase transportation of product should be considered more thoroughly.

Main argument for use of the platforms was that at some moment upon decreasing of the initial reservoir pressure gas compressors installed above surface should be necessary. But at present time there is conceptual suggestion from foreign companies (Hydro, Onnen Lange Field) being designers of the underwater compressors operating under conditions similar to SGCF. So, use of the platform can be omitted at the second stage of the field development.

Considering that major of the equipment suggested under the second option is unique, and that we have no experience in its use the final option of the SGCF development will be prepared after thorough study of all factors at further stages of work aimed on providing of the maximum possible economic effect and safety.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004