Chris Landeck, Gery Fryns
Typical STS platform installation.
Nine Indonesia structures in three years
Unocal initiated operations offshore East Kalimantan, Indonesia, in the late 1960s. Production from a number of oil fields ranged from a peak of more than 140,000 b/d of oil in 1977 to a low of 50,000 b/d of oil in 1984.
Between 1989 and 1994, a surge of successful exploration and development activity doubled production rates and transformed the business unit into a major growth area. The development of a series of small oil fields with a jackup drilling rig using a custom-designed minimal structure played an important part in the revitalization of the business unit.
This paper describes the development of the first Stacked Template Structure (STS) and an optimization process that took place during the installation of nine such structures over a three-year period.
Development logicThe STS was conceived after drilling the Seguni prospect in 110 ft of water in July 1995. The discovery well had over 400 ft of net pay. However, the reservoirs in this area are typically highly faulted channel sands. It was not believed that reserves were sufficient to support a typical $15-$20 million, 4-pile drilling/production platform with a 2 to 3-year construction time.
Close proximity (4 km) to existing infrastructure presented an opportunity that the operating unit felt could be approached in a non-conventional manner with minimal facilities. A very small, technically autonomous team was formed to plan the development in an atmosphere where local management was heavily supportive of innovation. The objectives were cycle time reduction, simplicity, and maximizing the use of a limited local infrastructure.
A number of caisson-based, 4 to 6 - well structural concepts were considered. All had undesirable limitations, primarily related to lack of infrastructure and complicated logistics associated with the remote location.
The STS concept began as a sketch on a napkin. It was quickly refined into a workable structure through the combined efforts of a small cross-functional group in the business unit. An outside consultant was used for detailed engineering of critical components. Fabrication of the first STS began less than three months after generation of the initial sketch. First production from the 4-well Seguni-A STS at a peak rate of 13,000 b/d of oil was achieved less than eight months after discovery of the field.
The STS provided a tool that promoted the drilling of shallow water prospects located close to existing infrastructure. A successful exploration program resulted in eight additional STS installations in up to 187 ft of water.
Design, installationUnocal has a long history of operations offshore East Kalimantan, with more than 50 platforms in seven offshore fields. Significant experience has been developed with the use of subsea clamps for various purposes, and the first STS design made use of this experience.
The initial concept for the STS was to construct a trussed structure offshore by clamping tubular braces between freestanding 30-in. well conductors. As the concept progressed from napkin to reality, the truss members took on the configuration of 15-ft high triangular subsea templates with well conductors on 15-ft centers at each corner, spaced between the mud line and the surface to add stiffness to the structure.
The installation sequence developed for the first STS installation is shown on page . The 20-in. conductor of the discovery well had been cut off approximately 60 ft above the mudline. The jackup rig was re-mobilized to the site, positioned, and prepared for operations. The remaining installation spread included a 180-ft construction barge with 4-point mooring system and light duty crane, a small diving support vessel, and a 150-ft material barge used to transport the STS components.
The mooring system was used to position the barge under the rig floor. The templates were picked up individually from the deck of the transport barge and stacked over the discovery well in the correct sequence. The bottom template was built with an integral mud mat, and the upper template was built with a deck large enough to support minimal facilities. After 34-in. holes were drilled through the open clamps on each corner of the structure, the 30-in. by 1-in. wall thickness conductors were run, each consisting of 5-ft by 60-ft joints. A crew of the same welders used in the fabrication yard performed welding and NDT of each joint on the rig floor.
After cementing, the conductors were temporarily left in the freestanding mode. The top template was elevated and positioned 25 ft above the mudline, the middle template being simultaneously positioned via pre-installed slings placed between the upper and middle templates. The lowermost template was left on bottom.
With the templates in position, the clamping process was completed and development drilling commenced. A total of 9.7 rig days were required to complete the illustrated four-step installation of the Seguni STS. Production commenced after the drilling of four wells and the installation of export pipelines and production facilities.
STS optimizationWithin less than three years of the Seguni-A STS installation, eight additional projects were completed (summarized in the table below). Installation times were reduced from nine days to as low as 36 hours by systematically examining every major STS component and installation step. The more significant design changes incorporated during subsequent installations are described in the following paragraphs.
The 30-in. conductors on the first STS were installed through the rig floor in 65-ft joints, which were restricted in length by derrick clearances. On the second and each subsequent STS, quarter-turn connectors were substituted for welded connections to eliminate rig floor welding/NDT work. This saved 4.5 hours of rig time per connection. Subsequent designs have made use of 170-ft joints for the first segment of each leg to minimize connections.
Below the mudline, the grouted 30-in. conductors provide lateral support to the structure. Classical P-Y curve theory was applied for pile design purposes based on generic soil profiles. A conservative design criterion (extra length) was used in order to avoid soil borings for specific STS installations. Changes in soil characteristics due to leg installations were considered negligible in the STS design. In the axial support analysis, it was considered the drilling mud would result in weakening of the soil. The ratio of the peak-mobilized shear stress to the shear strength of the soil was limited to 0.3 and a portion of the axial load is assumed transferred to the single or double inner casing string at the tip of the 30-in. leg.
Three templates were used for the Seguni STS in 110-ft water. For water depths of 110-160 ft, four templates were used. A computer-based iterative process was developed to optimize the elevation of each template as a function of water depth. For water depths greater than 160 ft, additional stiffness is obtained by increasing the conductor spacing from 15 ft to 22 ft. The 22-ft dimension is a limit imposed by the lateral skidding capability of the jackup rig.
Standard template designs allow the two configurations to be mass-produced in support of an aggressive drilling/development program. At 12 and 22 tons, respectively, the standard and wide-base templates can be easily handled without large cranes.
The top deck on the first STS was designed with minimal regard for long term operability. Subsequent installations of the 15-ft and 22-ft versions have incorporated features to enhance the deck areas available for facilities including separators, wireline units, coiled tubing, and snubbing units. Additional strength for the larger decks has been provided by the substitution of beams for the tubulars used in the early STS designs. Deck design improvements have greatly enhanced the efficiency of many aspects of STS operations, with major impacts on wireline and concentric workover activities.
The first STS was designed to support four wells - one in the center and one in each leg. Subsequent installations incorporated proprietary "splitter" wellheads, which effectively doubled the capacity for wellbores on a single STS. The standard STS can now accommodate up to eight wells and the wide deepwater version can support up to 10. It was determined after installation of the first STS that there was no economy in the recovery and tieback of a successful exploration well. Numerous time-intensive activities were associated with this, including inaccurate re-positioning of the rig at the suspended well site and removal of excessive cement debris around the base of the well conductor. For subsequent developments, the STS was always installed in a position to suit development drilling and pipeline configurations.
The first STS was installed by lifting each template individually with slings on a homemade drill pipe adapter. This operation was tedious and diver-intensive. Subsequent STS installations were accelerated by pre-stacking the STS templates on the material barge and transporting the structure to the rig site as a pinned assembly so that it could be lifted and set in a single operation (page ). A J-Slot was built into the middle slot on the top deck to allow the rig to lift and install the stacked templates in one operation. The use of the J-Slot provided rigidity to the assembly, which was useful where leveling difficulties were experienced.
The clamps are an integral part of the STS. Considerable effort has been devoted to the analysis of these critical components for fatigue and strength properties. The floating segments of the three section clamps are rolled from 1-in. plate and the fixed segments are 1.5-in. plate. The clamp height is 5 ft with eight high strength bolts installed per segment. The bolts are installed and torqued by divers using pneumatic impact and manual torque wrenches. Underwater inspection of early installations has proved the adequacy of the clamp designs. Numerous "diver friendly" features were incorporated into the clamp mechanisms through frequent construction yard visits by the diving team in order to improve installation efficiency.
An important factor in the success of the STS program was the ability to fabricate these structures in a remote location using a local work force and mainly locally available materials. Fabrication of the first STS and production of detailed drawings proceeded simultaneously. Although there were frequent changes, the simplicity of the basic design, a very small team and constant informal contact between Unocal personnel, the drilling contractor, the dive team, and the fabricator allowed modifications to be quickly incorporated into the work. The learning curve associated with material traceability, welding procedures, and general quality control was tedious in the beginning; however, the program would not have been successful had a local fabricator not been used.
Development of the first STS provided the Indonesian business unit a tool which was aggressively leveraged for development of a number of small discoveries close to existing infrastructure in a fraction of the previously accepted cycle time.
The 18+ million boe of incremental production attributable to the STS continues to make a significant impact on the business unit. Participation in the development and optimization process has improved the lives of many local Unocal and contractor personnel.
AcknowledgementThe author thanks all of Unocal's employees in Balikpapan for their roles in the development of the technologies described in this article. Thanks also go to Unocal Indonesia and Pertamina for allowing this paper to be published.
Chris Landeck is senior project manager, topsides, on the Unocal Deepwater Development Team in Sugar Land, Texas. He has 22 years of operations and facilities engineering/construction experience with Unocal in the Gulf of Mexico, the North Sea, and Southeast Asia. He spent nine years as chief production engineer for Unocal in Balikpapan, Indonesia, during the development of the Stacked Template Structure. He holds a BS in petroleum engineering from Texas A&M.
Gery C. Fryns started his career in the oil industry in 1983 in Lafayette, Louisiana, and led a number of computer modeling projects in a wide range of areas including well path optimization and basin modeling. He joined Unocal Science and Technology Division in Brea in 1987 as a research engineer. In 1994, he joined the Civil Engineering group, where he became involved in the design and analysis of platforms for Indonesia and the Gulf of Thailand, and later Unocal Indonesia E&C in Balikpapan. He holds an undergraduate degree from the ECAM, Brussels, Belgium, and a MS from California State University, both in civil engineering.
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