Hydraulic rig for marginal fields: Design and use of the G-125 Bruno Pini, Saipem SpA THE LAST FEW YEARS have produced major changes and innovations in technology to answer today s challenge: how to provide cost-effective solutions in numerous operating environments. Challenges are not only those related to extreme scenarios or frontier technologies. There are market niches that normally do not attract the attention from the media, but, nevertheless, are very im-portant for the energy industry at large. Onshore marginal fields exploitation and high-risk exploration ventures are 2 niches that have drawn the attention of many oil companies and led to the decision to design and develop a fit-for-purpose drilling rig. new project was set up with a partial fundingfromtheeuropeancommunity within the framework of the Thermie program. The project consisted of the following stages: Conventional oilfield equipment from an idle drilling rig was adapted to the water well rig to keep investment to a minimum while proving the applicability of the original concepts. This first rig was called G-100, and was intended as the prototype from which to derive the engineering required to design the actual rig; Field testing, consisting of drilling 3 wells (2 verticals and 1 deviated), was undertaken to assess the reliability of the basic technological aspects and to obtain indications of equipment efficiency and limits, as well as to draw specifications for the subsequent purpose-design-ed G-125 rig. Saitre Drilling, an ad hoc drilling contractor consortium combining Saipem and Trevi, Soilmec s parent company, now operates the G-125 rig. After a period of testing, the G-125 in July 1996 began its current campaign, drilling a series of onshore wells in different parts of Italy. This paper presents the criteria adopted to design a new rig, called G-125, specifically to exploit marginal fields. The paper also illustrates actual results achieved during 22 months of operations. The rig is actually operating in Italy, in the specific field of application that it was designed and built for. DESIGN PREMISES In Italy, there are a remarkable number of small prospects that have never been exploited because of their limited expected reserves. Typically, reservoirs hold a few hundred million cubic meters of gas. Vertical depth usually ranges from 1,200 m-3,000 m, and expected production rates generally range from 20,000-70,000 cu m/day. To access these pros-pects, a drilling unit was needed that would not only reduce dimensions and volumes related to the drilling activity, but provide increased mobility and a smaller crew complement. Also, the goal was to minimize both capital expenditures and environmental impact. Having assessed these requirements, a Hydraulic operations: The G-125 was designed as a hydraulic rig because it was seen as easier and smoother for operations, such as pipe handling, drillstring rotation, mast and substructure erection. A hydraulic cylinder operates the telescopic mast, resulting in substantial reduction in height and weight. This also enables rig transport with standard road loads. When on location, 4 hydraulic pistons lower and raise the substructure. (Maximum height is 6.1 m.) The rig s tongs and slips are automatic and are also hydraulic. Set up a dedicated project team of an oil company (Agip), a drilling contractor (Saipem) and a rig manufacturer (Soilmec); Select a suitable hydrau-lically driven drilling rig. The choice was a compact drilling rig developed by Soilmec, which specializes in water well rigs; Modify the rig for oilfield operations. TECHNICAL FEATURES The rig is hydraulically driven, with all components in a self-contained configuration that is never disassembled. Hydraulic power was preferred because of its ability to operate rig functions more smoothly and easily. These functions include push n pull substructure and mast self-erecting, pipe handling, drillstring rotation, etc. A hydraulic cylinder operates the telescopic mast, resulting in substantial reduction in weight and height, and making transportation possible on standard road loads. When on site, 4 hydraulic pistons lower and raise the substructure to proper height (up to 6.1 m), directly from the trailer. Another couple of pistons erects the mast; both operations require some 45 minutes. Rig up/down operations are consequently much easier and faster, compared to a conventional rig; all hydraulic control lines being already in place and connected. The mast is self-standing and does not require any windbracing. Another innovative aspect is the vertical pipe-racking system (any pipe can be September/October 1998 D R I L L I N G CONTRACTOR 47
stored and transported in containers from base to well site) along with the jib crane fitted to the rig. A total of 12 containers, each containing eighteen 3½-in. range 3 drill pipe can be accommodated around the rig, enough to drill in excess of 3,000 m deep. The rig is equipped with automatic hydraulic slips and power tongs. The mast features a cable and sheaves system that doubles the stroke and speed of the hydraulic piston. The rising length is 16 m, allowing to handle range 3 casing and drill pipes. The resulting maximum rising speed is 1 m/s. 2 persons are normally required to handle tubulars. 4 hydraulic motors operate the top drive, which is capable of torques to 3,600 Kgm at 60 rpm. During pipe connections, extending and tilting braces allow the top drive to connect the single joint from the mouse hole, then rise and connect it to the drill string. The top drive is also equipped with a special device for casing lifting and make-up, enabling the rig to separately and independently prepare casing. The casing can be run in and filled with mud at the same time. The entire system (substructure and mast) is rated for 136-ton static load capacity. A special feature provided by the hydraulic system allows for a pushdown action equivalent to a force of 20 tons. The rig also operates very quietly. The maximum combined noise level measured at rig site during operations is 82 db. Studies are under way to even lower this level. 4 totally sound-proofed diesel-electric generators provide a continuous total power of 1,600 Kw. Furthermore, the hydraulically operated rig does not require standard brakes, which are typically the most difficult source of noise to eliminate. The drilling rig is mounted on a 3-axis semi-trailer, weighing 42 t. The rig is equipped with : 11-in. x 5,000-psi annular BOP; 48 D R I L L I N G CONTRACTOR September/October 1998
11-in. x 5,000-psi single blind/shear ram BOP; 11-in. x 5,000-psi dual ram BOP; 13 5 /8-in. x 3,000-psi diverter spool. Three 600-hp pumps provide optimum performances in all drilling operations. The 3rd pump is normally kept as back up (thus minimizing down time), as well as for cementing operations. For these operations, a bulk cement surge tank and 2 batch mixer tanks (18 cu m and 7 cu m, respectively) are available. The rig can independently run cement jobs. The mud system is fully integrated into the rig equipment. The pit system consists of 3 mud tanks (theoretical total mud volume is 122 cu m), plus an additional 50-cu m tank, that can be used as a cement or water tank. The mud cleaning system includes two dual Brandt ATL 1000 shale shakers, a degasser and an on-line M/V CD 350 centrifuge for fine cuttings. AUTOMATIC DRILLING Thanks to the hydraulic design, automatic drilling can be easily achieved: Constant WOB or, alternatively, a pre-set rate of penetration or a constant torque can be selected from the control panel. The driller ensures that set parameters are properly working. Alarms can be set, along with automatic stop of drilling operations in case of anomalies. In case of stuck pipe, pre-determined values of overpull and/or torque prevent accidental damage of the drill string. A complete set of sensor control all essential rig functions. The on-line drilling parameter data-acquisition system displays real-time information on the drilling console in the driller s cabin, also managing the alarms. Drilling data are also displayed and stored on PCs in the toolpusher s office. Hard copies of the drilling parameters may be produced from playback or real-time data. The same set of data may feed the Company s drilling database in real time. The minimum rig crew consists of 10 people for drilling operations: 4 daily (1 toolpusher, 1 mechanic, 1 electrician, 1 roustabout), and 2 shifts of 3 (1 driller, 2 assistant drillers). Presently the rig is operated under a contract that includes extra services (cementing and casing make up). MAIN ADVANTAGES The design features numerous advantages. First, as stated above, mobilization is much faster. Rig up and rig down is reduced by some 35-40%. Assuming a 100-km move, the G-125 needs less than 6 days to complete rig down, moving and rig up on the new location. About 28 standard truckloads are required to complete moving. Rig footprint is also smaller. Thanks to the rig s lower height, rig layout can be reduced to 30 m x 30 m. The vertical pipe racking system allows for pre-packed pipe transportation from the base to the well-site, reducing to a minimum the land space required and the inspection and preparation times at the rig site (casing and tubing running). The hydraulic top drive works satisfactorily and gives good performances during fast drilling. The tripping times have been cut to a minimum by using its back reaming capabilities at the best. FIELD EXPERIENCE With construction completed by May 1996, the new rig G-125 underwent field trials on a series of 7 AGIP wells in northern Italy near Milan. The first well was Gisolo 1, a vertical exploration well in a hilly area. Total depth was planned at 3,000 m with 4 1 /8-in. hole size. A 3 ½-in. string was used. For geological reasons, the 6 1 /8-in. phase was deepened from 2,400 m to 2,510 m. It was decided to stop drilling after logging. Drilling performance was satisfactory in all phases 12 ¼ in., 8 ½ in. and 6 1 /8 in. Time for casing running and wellhead operations were comparable to conventional rig performances. The San Gervasio 2 Dir well was a deviated well with a maximum drift of 53. Total depth of 2,400 m was reached in 8 days (190 hours), including 3 strings of casing. This compares very well with the 13 days (309 hours) planned based on the best area performances. This corresponds to nearly 40% reduction in drilling time. In this well a compact (or unitized) wellhead was used, along with 2-in. coiled-tubing completion for dual concentric production. Performance in the 4th (Cortemaggiore 57) and 5th (Barzaniga 1) wells is not comparable with the other wells, because different hole diameters and other experimental drilling tools were used. Over the other 5 wells, ROP increased from 15m/hour to 35 m/hour, and meters drilled per day increased from 75 to 300. In addition, flat time was reduced from 450 hours in the 1st well to less than 120 hours in the last (including wellhead/bop, casing and cementing operations, tripping, down time, etc). Reduction in drilling costs ranged from 12% to 24%, compared to estimates based on traditional rigs. The G 125 rig is currently operating in Sicily, where 3 wells have been drilled so far (Monteferrante 1 and Acquavena, Fiume Salso 1 Dir. The fourth, Comunelli 1, was spudded on 31 March 1998. REMARKS The G-125 drilling rig is based on innovative concepts in order to make marginal fields development economically viable. It has those characteristics that allow for easy and fast mob/demob; thus a well where drilling time is short if compared to moving time is the ideal application. 50 D R I L L I N G CONTRACTOR September/October 1998
The rig layout permitted reduction of the area dedicated to the rigsite. This smaller area and the soundproofing made the rig more suitable for applications in environmentally sensitive and populated areas. Rig automation and reduced number of personnel combined to produce both a safer environment and a better performance. The economic analysis demonstrates the good results obtained; the learning curve and the use of complementary technology (unitized wellheads and CT completion) can further improve the benefits. The G-125 rig has been built to the highest safety standard in order to improve the efficiency in drilling operations. However, due to the innovative system, rig personnel had to adapt to the new system, especially concerning tubular handling. Some accidents occurred early on. As the crew has gained experience and safety procedures were improved, accidents have been rarer. In the last year, the operation has suffered just a single accident, and that one a minor incident. FUTURE IMPROVEMENTS A continuous optimization process is under way, both in terms of operational performance, personnel training and rigsite. Thanks to the good results and experience gained with this first version of the near-prototype hydraulic drilling, Saitre has undertaken the construction of a more powerful version of the rig, still based on the same concepts and principles. The new rig, named G-200, will be ready for operations by the third quarter of 1998. Offshore applications of the G series rigs are also under evaluation. These could range from workover rigs on fixed structures to installation on semis or multi-purpose vessels. CONCLUSION The G-125 has been designed to meet and exceed demanding technological requirements that characterize onshore drilling activity today. The focal point of the design is reduced weight and size and enhanced automation to reduce crew size. Interdisciplinary cooperation between operator and contractor has made possible the development of an innovative drilling rig that performs economically and efficiently for drilling shallow and medium-depth wells. ABOUT THIS ARTICLE Bruno Pini joined AGIP in 1981 as drilling supervisor and has worked both on land and offshores. In 1994 he transferred to Saipem as coordinator of the drilling engineering section. He is responsible for Saipem s drillingengineering department and Project Manager for the company s cheap/- slim hole drilling, which includes the G-125 rig. This paper was adapted from a presentation made at the 1998 IADC European Conference, held 14-15 May in Paris. n September/October 1998 D R I L L I N G CONTRACTOR 51
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