North East Frigg Development and Experience

Summary The North East Frigg (NEF) gas field is a small satellite of the Frigg field; in terms of recoverable reserves as estimated at the beginning of the project-282.51 cu ft × 10(9) [8 × 10(9) m3] and daily production capacity of 211.9 × 10(6) cu ft [6 × 10(6) m3]NEF is a marginal field. When the NEF subsea multiple well gas field production system (SPS), came on stream Dec. 8, 1983, production system (SPS), came on stream Dec. 8, 1983, it was the first multiwell SPS to deliver commercial gas. The field has now produced for over a year with a remarkably high up-time and is operated in an unmanned production mode as originally designed. This demonstrates the suitability of such a concept for application to remote marginal gas fields. This paper describes and explains the concept and the rationale for the NEF subsea's selection, reviews the main phases of the development (design, manufacturing, phases of the development (design, manufacturing, offshore works, and production operations), and presents the field experience gained. The NEF subsea production facility is located in 328 ft [100 m] of water, 11.18 miles [18 km] from the Frigg field complex. It comprises four major components:a subsea template protecting a six-well cluster and a manifold,an 11.18-mile- [18-km]-long subsea gas pipeline that is 16-in. [40.6-cm] thick, linking the manifold pipeline that is 16-in. [40.6-cm] thick, linking the manifold to the main Frigg field facilities,a field control station (FCS) located 492.1 ft [150 m] from the template, whose main functions are to control and to test the Christmas tree valves, to inject methanol, and to kill the wells if necessary, anda treatment and metering module on the Frigg. The NEF concept was selected as being the best optimization in terms of (1) operational safety, mainly due to separation of critical functions, (2) reliability of gas production by extensive use of conventional and tested production by extensive use of conventional and tested equipment although assembled in a novel configuration, (3) capital and operating costs, and (4) potential adaptation of the concept to future development of marginal fields. The NEF field had produced for more than 20 months when this paper was written, and I stress that the success results from extensive preparation efforts to maintain a high fabrication quality, which is mandatory for SPS's. Emphasis is placed on the need for experienced personnel and for proven procedures to complete safely the personnel and for proven procedures to complete safely the subsea gas wells in the North Sea in conditions approaching the winter season. The project was completed on schedule and within budget. The NEF total capital cost amounted in 1983 to 1,991 million Kroner [250 million dollars]. The NEF's success is also a milestone because it demonstrated that SPS's can be safe, reliable, and economical. This achievement is of prime importance for the future development of our industry in the North Sea, where increased water depths will seriously challenge the conventional surface production concepts. Introduction The NEF project was initiated by Elf Aquitaine Norge (EAN) early in 1979 and was supported by its partners: EAN A/S (operator) 17.40% Esso Norge A/S 58.00% Norsk Hydro Produksjon A/S 13.80% Total Marine Norsk A/S 8.70% Den Norske Stats Oljeselskap A/S 2.10% As a close satellite of the giant Frigg field 70.6 × 10(11) cu ft [2 × 10(11) m3] recoverable reserves, the NEF gas field is placed in very favorable conditions. This opportunity, combined with the demand for new technological concepts for producing underwater gas fields in a difficult environment, has prompted EAN to develop the NEF and to use an unconventional approach. This paper reviews the main components and the different steps of the project development. Particular difficulties encountered are reported, and the major reasons that technical success was achieved at an economical cost are emphasized. Selected technical results, statistics, and cost tabulation are presented to serve as references. The NEF Concept Concept Selection. Four basic schemes were selected for detailed evaluation in terms of cost, safe control of the wells, and reliability of the production:a conventional solution with the wells drilled from an unmanned fixed steel platform,subsea wells with a semisubmersible support vessel permanently anchored above the well cluster,subsea wells with no permanent surface support, production being remotely controlled from Frigg, andsubsea wells controlled from an unmanned local FCS. Scheme 4 was selected mainly because a close permanent support was necessary to obtain a sufficient level permanent support was necessary to obtain a sufficient level of reliability of production control and to provide inhibitor injection and a quick means of well control, including well killing. Scheme 2 was considered unsafe; Schemes 1 and 2 were considered too expensive because of capital costs and operating costs, respectively, and Scheme 3 was thought to be insufficiently reliable. The NEF is actually a combined concept: half is an SPS with a remotely controlled subsea well cluster, and half is an articulated column used as a local FCS. JPT p. 1653