Sandeep Khurana
Petro-Marine Engineering of Texas Inc.
Houston
New computer technology and past experience now enable platforms and jackets for 500-1,300 ft of water to be designed in a few months. Previously for a deepwater jacket of a fixed platform, the finalized design might take up to 1 year, and fabrication and installation another couple of years.
The Gulf of Mexico Viosca Knoll Block No. 817A (VK 817A) platform is one recent example of a fast-track project in 671 ft of water. The accelerated schedule provided 15 months from the start of the project to the offshore installation, hookup, and start-up. Only 3 months were needed to complete the structural design of the platform.
Leviathan Gas Pipeline Inc., a subsidiary of Deeptech International Inc., is operator of the project. Petro-Marine Engineering of Texas Inc, provides the engineering, procurement, and construction management.
The jacket is being fabricated at Aker Gulf Marine's Ingleside Yard, near Corpus Christi, Tex. Fabrication is ahead of schedule, and the platform is to be installed in July.
Despite the expedited schedule, no technical quality was sacrificed.
FAST-TRACK PROJECTS
Recent low oil and gas prices have put enormous pressure on the offshore oil and gas industry to cut costs and to respond fast without sacrificing quality.
Fast-track design of a deepwater platform requires special technical and managerial expertise. The regular challenges in the deepwater platforms are amplified by the fast-track nature of the project.
On the technical side, one has to be prepared with sensitivity studies to respond quickly to the various options presented. On the managerial side, one has to identify the critical path and expedite it.
BLOCK B17A
The platform on Viosca Knoll Block No. 817a will be placed in 671 ft of water (Fig. 1)(55731 bytes). Although the reservoir is not large (six gas and four oil wells), this project is economical because the platform acts both as a producing platform and also a gathering and a pipeline junction platform.
Production lines, both oil and gas, from nearby blocks and future deepwater prospects will tie into this platform. This additional scope required a large facility to accommodate initial and future production, and compression needs.
Jacket design, fabrication, and installation were the critical path in the schedule for previously installed deepwater fixed platforms, such as Kilauea (1989 in 620 ft of water) and Texaco Tick (1990 in 720 ft of water).
In the accelerated schedule for the VK 817A, however, the jacket design, fabrication, contract bidding and awarding, and construction schedules were compressed. Therefore, the critical path became the deck structural design and fabrication, facilities design and equipment procurement, and the mechanical and electrical/instrumentation hook-up.
DESIGN CHALLENGES
One main challenge in a fast-track project is the handling of design criteria changes caused by delays in the availability of critical data.
Deepwater platforms of this kind attract nearby prospects. This added production requires more deck area and increases deck load as well as riser wave loads on the jacket. However, it is possible to commence the design with criteria based on the client's projections before running a final optimization when more definitive data become available. This has to be achieved quickly to avoid changes in long-lead materials.
In line with the philosophy of cutting costs, personal computers (PCs) can replace mainframe IBM and VAX computers for designing the platform. For the structural engineering design of a deepwater platform, PCs now routinely can handle the enormous amount of data required for a detailed model and a rigorous dynamic and fatigue analysis.
The key to facing challenges is to start various processes in parallel. That was done for VK 817A.
To take the jacket away from the critical path, the first step is to involve the fabricator and installer at an early stage. For VK 817a, this was achieved by maximizing the use of an existing design.
Once the jacket configuration was established, a similar existing platform design was selected and, with limited analysis, adjusted to the required water depth. Bid packages included very detailed definition of all the jacket features, appendages, etc. and a set of specifications.
Very competitive bids were received based on a perton basis with an option to establish a lump-sum price with the approved-for-construction (AFC) definition.
While detailed platform design was being pursued, the fabricator and installer gave their input into ease of fabrication and installation.
An electronic data file played a pivotal role. Once the fabricator was selected, the electronic data files on the structural model were sent by modem to the fabricator. Using the model, the fabricator performed the roll up and constructability analyses. Also, the fabricator began ordering the main steel for the platform.
Focus tends to be on the structural design of deepwater jackets, but one must not forget that the purpose of the jacket is to support the deck and thereby top-side facilities. As mentioned earlier, an important aspect of deepwater gathering platforms is to, attract nearby prospects.
VK 817A has the other dimension of serving as a junction/compression platform for a major pipeline system. This required very close liaison with the pipeline operator to establish metering and pigging specifications. Because in this fast track the facilities were on the critical path, the various production/pipeline scenarios were established at the start.
The equipment packages were sized at an early stage, so that one could proceed with the structural design quickly. The number of risers also had a significant impact on the jacket design. Working together, the structural design and facility teams responded quickly and efficiently to design changes as they surfaced.
To size the topside, another design input required early was the identification of the drilling rig and consumables. The operator provided a good definition of their drilling requirements and a number of drilling contractors provided rig layouts and loads to meet these requirements.
Using an envelope of space and weight that would allow for a number of rigs, drilling deck arrangements were finalized very early in the design.
The top deck provided sufficient space for future large turbine pipeline compressors.
Once the fabricator, installer, and facility group started to finalize their input, the structural design team had to respond quickly to make the final jacket Optimization. This was done by automating and organizing the analysis procedures. The structural designers performed various sensitivity studies and were prepared with what-if scenarios.
The successful design of the VK 817A platform may be attributed to a combination of experience and computer technology.
JACKET CONFIGURATION
The main aim while developing the jacket configuration is to have minimum cost based on design, fabrication, and installation. For the VK 817A platform, various options of jacket main' leg piles along with the skirt piles were available.
For example, main leg piles and skirt piles combinations included:
- The 8+4 concept (8 main leg piles with 4 skirt piles), such as the Zapata platform in 1984 in 658 ft of water
- The 4+8 concept (4 main leg piles and 8 skirt piles) such as GB191A in 1994 in 721 ft of water
- The 0+8 concept (4 main legs with no main piles and 8 skirt piles) such as the Kilauea and the Texaco Tick, mentioned earlier.
The main governing factors for the VK 817A platform design were the large facility (5,500 tons of design payload) that had to be supported and the installation cost.
Main piles cost more because of their additional weight, larger jacket leg diameters needed, and increased pile installation time. Consequently, an all-skirt pile configuration, the 0+8 concept, was selected. The final jacket weighed (including appurtenances) 7,600 tons and the piles weighed 1,540 tons.
FABRICATION AND INSTALLATION
To fast-track the jacket, items for fabrication were prioritized. When fabricating a launched central drilling structure, the first item constructed is the launch box. To avoid delays, the launch box was kept as simple as possible, with many of the risers outside the box.
The vent and flood lines were rerouted away from the launch box. This helped in detailing other areas of the jacket while fabricating the launch box.
The mud mat is one of the last items fabricated. Because a detailed seafloor survey was unavailable, the design included pancake-type, adjustable mud mats. These can be adjusted, without incurring major costs, after obtaining the survey.
Involvement of the fabricator and the installer at an earl stage not only helped the fast tracking but also the costs. Once the preliminary bid packages were sent out, the fabricator started providing estimates on various items.
One such area was grouted piles-vs.-the Hydra-Lok system for the piles. The installer did not have any preferences but for the fabricator, the grout lines required intensive labor, while the Hydra-Lok system required rolling extremely thick (5.25 in.) Hydra-Lok sleeves.
The cost estimate prepared with the fabricators showed the Hydra-Lok system to be more cost effective, and it was finally chosen.
Jacket installation will follow procedures for similar sized jackets in the Gulf of Mexico. To minimize changes after fabrication started, various installation related aspects were considered early in the design phase.
A tow analysis was performed for various available installation barges. The jacket was designed for the motion envelope of these barges. Similarly, a launch analysis was performed for these barges.
The skirt pile guides were designed for both underwater hammers or a surface hammer follower system. To encourage competitive bids from prospective installation contractors, both the option of vertical and the battered piles were kept in the bidding phase. The installation contractor finally selected the battered pile option.
STRUCTURAL DESIGN
Fig. 2 (109861 bytes) shows the systematic design approach for fast-tracking the jacket design. The first step, as discussed earlier, was to select the jacket configuration and immediately involve the fabricator and the installer.
The next step was to perform a dynamic in-place analysis. As platforms are set in deeper water, the dynamic characteristics of the structure become more critical. To avoid large dynamic amplification due to wave action, the structural period must be kept low.
The leg diameter, spacing, and batter, along with the major bracing pattern, are important parameters affecting the structural period. The initial dynamic in-place analysis helped fix the jacket geometry.
In the VK 817A project, another challenge was to design the structure using the latest API-RP2A (20th edition) criteria. There is about a 5-10% increase in wave loads with the 20th edition as compared to the 19th edition. This leads to a heavier jacket. The 20th edition, however, encourages the designer to include wave directionality.
Wave directionality was used to offset this increase in wave loads. This led VK 817A to be an unsymmetrical structure to the extent that the piling at the four corners had different penetrations.
The main legs of the jacket varied in diameter from 60 in. at the top to 84 in. at the bottom. All four main legs had the same diameters, but the thickness of each leg was different depending on wave direction orientation. Similarly, depending on the wave direction, all piles had 72-in. diameters with pile penetration ranging from 350 to 280 ft.
To fast-track the design, various designs and analyses were performed in parallel. After all the analyses were performed, the results were collected in a large data base file or a spreadsheet and, simultaneously, stress and hydrostatic checks were performed for each resizing. This was different from previous deepwater jacket designs.
For example, in earlier jacket designs, fatigue was analyzed after other analyses and resizing were completed. However, because of organization and automation, all designs and analyses, including fatigue, could be performed simultaneously.
In one case, if stress analysis dictated a change in brace diameter, one could perform an interactive fatigue design and immediately determine the impact on the joint.
This strategy helped to move the time-consuming fatigue analysis away from the critical path, while maintaining the fast-track design.
At the end of each analysis cycle and resizing, the various scenarios and trade-off alternatives were studied. This included, for example, a comparison of changing the member wall thickness-vs.-providing for hydrostatic ring stiffeners. Additionally, at the end of each cycle, the jacket flotation was checked. The entire cycle (Fig. 2)(109861 bytes) was repeated until the jacket was fully optimized.
The fast-track design of the VK 817A platform was successful. It should be noted that the jacket design and fabrication were compressed in schedule, and a major emphasis was placed upon the procurement, fabrication, and hookup of the equipment and the deck fabrication.
It is projected that about 2 months will be cut off from what was at first thought to be the minimum schedule. Besides being completed ahead of schedule, the management approach and the technical expertise helped in cutting costs. By working with the fabricator and installer during the early stages of the project, the designers helped cut expenses and prevent delays.
Existing data bases and past experiences proved most helpful, particularly in expediting the bid packages. In the jacket design, the reliable computer set-up with personal computers helped for quick response and again cut costs for the client.
Above all, the key to success lay in using a highly experienced design team.
