Minimal structures open global production opportunities

Davy and Bessemer monopods during installation (photo courtesy of Charles Hodge Photography)

• Comparison of economic parameters and ratings [69,987 bytes]
• Comaprison of structural behavior ratings [61,403 bytes]
• Comparison of design characteristics ratings [65,402 bytes]
• Comparison of primary characteristics and ratings [80,328 bytes]
• Evolution of the Davy/Bessemer minimum structure design. [37,436 bytes]
• Amoco minimal platform classification system. [39,640 bytes]

Process selection enables marginal development

Smaller oil and gas fields often prove marginally economic for development with large multi-leg space frame platforms or floating systems. Hence, such fields often remain undeveloped until alternate development schemes, technology, and innovative thinking rise to the occasion.

Many technical aspects must be considered in a marginal field development including capital assets (pipelines, structures, subsea tiebacks), downhole processes (vertical, horizontal drilling), contracting and field development programs.

Amoco has a long history of using small, or minimal, platforms to achieve economic development in the Gulf of Mexico. An early example of minimal platforms was the Tampa concept developed in the 1970s by Amoco Production Company (Hancock and Peevey, 1975). Tampa, signifying tender assisted minimal platform arrangement, consisted of at least two bridge linked, four leg, space frame steel platforms.

During drilling operations, one platform supported drilling operations, casing, and drill pipe storage while the other platform provided quarters, dry storage, and mud tanks. After drilling, the quarters platforms were used primarily for production. The Tampa platforms were developed as a lower cost alternative to the eight-leg template platforms which were prevalent in the Gulf of Mexico and North Sea in the 1960s and 1970s. Amoco installed approximately 30 Tampa platforms during the 1970s and 1980s. Significant costs savings were realized through standardization of design and fabrication.

More recently, Amoco focused on minimal in the truest sense: unmanned, remotely operated platforms, with no more wells and topsides equipment than really needed. Since the 1980's, Amoco's Gulf of Mexico Offshore Business Unit has installed several free standing caissons, nine guyed caissons, seven braced caissons, as well as 10 four pile well protector platforms.

Technology transfer

This Gulf of Mexico experience led Amoco to consider the development of marginal fields in other areas of the world using minimal structures. In areas such as the North Sea, where offshore platforms often are manned and fully self contained, the use of small unmanned structures were typically not considered. Operators were concerned with design issues such as boat impact and personnel safety.

In the early 1990s, Amoco UK planned to develop the Southern North Sea natural gas reservoirs Davy (Block 49/30A) and Bessemer (Block 49/23E) but was unable to do so economically using standard field development schemes (Littler, et al., 1996). The water depths for Davy and Bessemer are 43 and 23 meters, respectively. Focus was then placed on the development of a smaller structure which not only could reduce capital costs through reduction of structural steel, but also reduce operational costs through remote operation, well pre-drilling, and wireline and coil tubing workover operations.

Given the fast track nature of the development project, a multi-discipline team was established which included operations, drilling, construction, and resource development personnel. Several development scenarios were considered by the Amoco UK team, including subsea systems and a monopod concept.

Upon consideration of the reservoir and equipment requirements, the monopod option was chosen for further development. Since a monopod had never been installed in the UK sector of the North Sea, several aspects of the design needed careful consideration and close cooperation with the regional authorities. Specifically, typical design requirements for boat impact are too severe for the monopod structure since it is inherently less robust than larger multi-leg platforms. Amoco engineers placed a great emphasis on demonstrating that a monopod, while not able to withstand a direct hit from a typical North Sea work vessel, was actually better for glancing impacts due to the flexibility of the column.

Moreover, the unmanned status of this platform and small footprint of the monopod at the water line actually reduces the probability of impact to the point where boat impact should not be the primary load case. Amoco successfully demonstrated to UK authorities that safety and environment protection are adequate for the monopod.

The initial Davy and Bessemer concept was the MOSS III

platform (CBS Engineering) which is proven in the Gulf of Mexico and other areas. Several critical technical issues were analyzed and subsequently accepted by UK certification authorities for the MOSS III including conductor-through-pile, pile capacity after drilling, water line protection, and external pile/conductors.

Amoco UK decided that, while the MOSS III was technically acceptable for operation in the southern North Sea, the preferred development plan must include capability for installation over pre-drilled wells. Moreover, the Amoco UK production group desired more wells than possible in the MOSS III. Hence, a variation of the STAR platform (Ramboll Hanneman Hojlund, Denmark) was adopted. The STAR-type platform has all wells enclosed in a central column with three grouted skirt piles, however, Amoco opted for four piles to improve the platform's robustness against various damage scenarios.

The STAR concept was acceptable to Amoco's design contractor, however, further in-house structural analysis indicated that a fine tuning of the pile-to-column bracing (Figure 1) results in a more efficient design with improved torsional stiffness. This modified design was denoted AMOSS for Amoco minimum offshore support structure.

Since the production rates from Davy and Bessemer are similar, Amoco UK decided to install similar AMOSS structures at both locations. Due to a shallower water depth at Bessemer, the column height is less than the Davy monopod. The decision to "design once, build twice" led to significant cost savings through design standardization.

This, coupled with the possibility for installation from a jackup, enabled a field development once thought uneconomical. One AMOSS structures was 50% lower in total platform cost than a typical four-leg well head platform. By constructing two platforms at once, the cost savings was estimated to be 70% for the installed platforms compared to the four-pile well head platforms. Additionally, the AMOSS platforms are suitable for operation in approximately 80% of the SNS Amoco locations, hence, an existing AMOSS platform may be moved from field to field as needed, thus further reducing cycle time and project development costs.

The Davy and Bessemer structures are shown during installation in 1995. It may seem odd that, although the platforms were designed for installation by a jackup, it was actually installed by the largest crane vessel in the world (Micoperi 7000). Timing of the Davy/Bessemer installation, together with competition created by jackup installation, made crane barge installation cost effective.

Amoco considers the Davy/Bessemer project to be a breakthrough in marginal field development. A project was made economical through a significant change in operational mind set from past practices through the application of proven Gulf of Mexico technology and simplicity of operation.

The monopod, while quite suitable for the above mentioned project, may not be ideal for other areas of the world. To this end, Amoco Worldwide Engineering and Construction's Offshore and Civil Engineering (OCE) group developed a Guideline for the Selection of Minimal Platforms for use by offshore business units worldwide. Since Amoco has significant operations in the Gulf of Mexico, North Sea, Trinidad, and the Gulf of Suez, OCE pursued the development of a tool which could assist Amoco's business units to determine the appropriate structure for a particular development program.

Worldwide platform study

In order to foster shared learning and encourage good decision making for marginal field development within Amoco, the OCE group commissioned a study to capture as many existing minimum platform types in use worldwide as possible, as well as some future concepts that have been developed but not yet constructed. The study, performed for Amoco by Petro Marine Engineering of Texas (PME), collated information needed for the evaluation of various minimum platform concepts for prospective field developments.

PME was chosen for this project since they have a long history of minimal structure installation for Amoco in the Gulf of Mexico and Trinidad. Amoco obtained data on 72 minimum platforms from design firms that responded to an industry-wide survey. This data, coupled with significant in-house minimum platform studies performed over the last 10 years, facilitated identification and classification of concepts needed to assist Amoco's business units in marginal field development.

An effort was made to classify and group the platforms into a more manageable amount. Hence, 26 different minimum platform types were identified during the minimum platform study. Group I concepts employ the well conductors as principal load carrying members and are classed as having protected or unprotected conductors. Group II concepts do not use the conductors as load carrying members and are classed as caisson structures, monopods, bipods, or three or four legged platforms. They are further differentiated by type, such as caissons being free standing, guyed, braced or jacketed, monopods being spread based, piled legs or braced columns, and platforms as conventional, skirt piled, extended base or suction piled.

The report provides a detailed description of each of the concepts, including discussions of typical usage, advantages, disadvantages, cautions, limits of applicability, characteristic ratings, and industry experience. Drawings and photographs are also provided, when available.

Once the concepts were established, an evaluation protocol was developed which considers five primary characteristics: design characteristics, structural behavior, fabrication and installation, operational characteristics, and economics. Each primary characteristic is further described by several related parameters. These include water depth, storm wave height, deck load, capacity, design and use history, redundancy, robustness, ship impact survivability, rigidity, fatigue resistance, seismic resistance, constructability, installation method, operational flexibility, multiple risers, multiple conductors, reusability, weight and cost. The user rates each parameter on a scale of 1 (poor) to 5 (excellent) and the overall rating is combined by the program.

For each application, there are four governing design parameters: water depth, wave height, deck load and number of conductors. Design limits for each concept, based on the ranges noted in the industry survey and available public information, were set for each of the governing design parameters.

Concept selection tool

An interactive "Concept Selection Tool" was also developed as part of the minimum platform study. The user must supply a limited amount of basic information including water depth, geographical region, oil or gas production requirements, number of conductors and whether or not the conductors must be shielded within a structure. The program has default wave heights for most geographical regions which, like all the default values, may be overridden by the user. Technically feasible concepts are then selected by the program through comparison of the four governing parameters. The concepts that are found to meet the governing criteria are then rated.

The test case assumes that all characteristics are equally weighted. As mentioned, the rating is the sum of the five primary characteristics, resulting in concept 13 receiving the highest ranking for the test case. In the figure, the high rating is due primarily to "structural behavior" (redundancy, robustness, rigidity, ship impact, fatigue, seismic excitation) with operational rating only slightly less.

Three tables show a breakdown of "related parameters" of each primary characteristic. The contribution of each parameter is easily determined so that the user may assess which parameters is driving the rating. Final selection of the most suitable concepts can be made after a review of the feasible concepts and their ratings. The user can then narrow down the list of concepts deserving further consideration and conceptual costs can be determined using the minimum platform cost estimation spreadsheet.

Cost estimation spreadsheet

An interactive spreadsheet that estimates the installed costs of the selected concepts was developed as part of the worldwide minimum platform study. Default values are supplied to estimate costs when particular design requirements are not known, however, the user may override the defaults with project specific data.

From user-supplied or default data defining the configuration, design parameters, and fabrication and installation requirements of the platform, the program calculates the approximate material quantities and duration of activities, and then calculates the costs of fabrication, transportation, installation, engineering, project management, certification, insurance, financing, taxes and import duties, and contingency. Excluded are the costs for drilling, installation of non-structural conductors, pipelines, licenses and permits, land acquisition, leases and escalation.

The Input Form Worksheet includes ten sections, Grand Total Costs, General Information, General Design Data, Topsides Design, Jacket Appurtenance Design, Fabrication, Trans portation, Installation, Hookup and Commission ing, and Miscellaneous Costs. Other worksheets in the program refer to Results, Concept Limits, and a Report Worksheet for the currently selected concept.

The user has the option to override all rate and default assumptions. The more data that is entered by the user, the more accurate the report will be provided that the data is backed up by sufficient experience in platform design, fabrication and installation. Without proper input, the results will not be very meaningful. Accordingly, if experience needed to make suitable override values is not available, it is recommended that the default values be used, and the resultant cost estimates only be used for relative ranking of concepts and rough, scoping cost estimates.

Amoco considers the cost estimation spreadsheet to be a prototype, although with wider industry input its accuracy and applicability should be improved.

Future endeavors

The work performed by Amoco to date needs to be enhanced to include new concepts currently being developed around the world. To this end, an industry effort should be established to collate available minimal structure concepts and data to identify minimal platforms which could make a marginal petroleum development economical. Amoco is willing to provide access to the data described in this article through an industry sponsored effort to expand knowledge and data sharing on minimal structures. Amoco's study coupled with the poster in this issue could provide an excellent starting point for such an industry-wide study.

On the operations and drilling side, it is felt that the industry should move towards developing systems which are truly minimal. Since we often design platforms with space for a larger number of wells than needed, the use of multiple completions per conductor may enable using smaller structures without eliminating the possibility of additional wells.

Consideration should be given to the "string of pearls" concept where several structures are linked together to combine production thus improving the economics through a reduction in directional drilling costs.

Finally, since structures often form only a small percentage of the cost of a field development, effort should be placed on the development of alternative pipeline systems, platform/ pipeline reuse, alternative foundation systems, and lower operating costs (including decommissioning) in order to enable more global production opportunities.

Acknowledgment

The authors acknowledge Ian Ruddy, Adrian Littler, and Richard Lillie of Amoco (UK) Exploration and Production Co. for their leadership in the Davy and Bessemer project.

References

Hancock, J. and Peevey, R. (1975) "New concepts in offshore platforms", OTC 2265, 7th Annual Offshore Technology Conference, Houston 5-8 May, 1975. Littler, A.N., Kearney, C., Schultz, M, Hennington, E., and Parsons, P. (1996) "Marginal gas field developments in the Southern North Sea: Davy and Bessemer", SPE 36873, 1996 SPE European Petroleum Conference, Milan, 22-24, October 1996.

Authors

Pat O'Connor, is Team Leader for Amoco Corporation's Offshore and Civil Engineering group. He has been involved with the offshore industry for 23 years and specializes in marginal field development, platform design, construction, structural assessment, and inspection. He is chairman of the ISO SC7/TC67/WG3 Panel 8 committee on platforms.

Sam DeFranco, is an Engineering Specialist with Amoco Corporation Worldwide Engineering and Construction, Offshore and Civil Engineering group. He specializes in platform structural assessment and underwater inspection. He holds a BSCE and PhD in Civil Engineering from Clarkson University, Potsdam, NY.

Bob Manley,PE, is an Engineering Associate in Amoco's Offshore and Civil Engineering Group. He has 29 years experience in offshore operations and offshore structures design, construction and installation, including six years in LNG/oil port & terminal design and construction. He holds a BS in Ocean Engineering from Florida Atlantic University, and a MS in Civil Engineering from Tulane University.

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