TECHNOLOGY Low cost mudslide resistant platform exploits delta gas

Mudmats, shown onshore next to the platform, supported the platform during installation prior to driving the piles (Fig. 2). Photo by Michael Klein.

Project team

Beginning in 1993, EDC assembled a team which estimated that a typical mudslide-resistant platform and production facility to exploit the SP 47 gas reserves would cost about $22 million, a prohibitive expense.

But through the unique alliance among specialists and working closely with the U.S. Minerals Management Service (MMS), the EDC team was able to cut this cost by more than half.

The project team included the following members:

• Paragon Engineering Services Inc., which provided project management and design engineering

• Robert Bea, a professor at the University of California at Berkeley who did the reliability evaluation

• Jim Hooper of Fugro-McClelland Geosciences who examined the geotechnical conditions

• Joe Suhayda, a professor at Louisiana State University who evaluated the oceanographic conditions.

The project team conducted a preliminary hazard survey of the field, selected an optimal platform location, and undertook a reliability analysis to assure that the design assumptions resulted in appropriate risk levels.

The resulting innovative design involved the following:

• A stretched four-leg deck for simultaneous drilling and production

• Individual well conductors

• Removable mudmats to minimize soil loading.

Location

The platform in SP 47 is about 5 miles due south of the Mississippi River Delta's South Pass. The gas reserves are in both SP 34 and 47.

The delta area is marked by seafloor instabilities that have caused dramatic mudslides. In fact, during Hurricane Camille in 1969, several platforms failed as a result of soil slides or flows.

The following factors have contributed to the instability of the delta area's seafloor:

• Rapid sedimentation, which results in widespread sedimentary loading of the upper delta front slopes

• The presence of fine-grained delta deposits, which, due to their rapid deposition, are generally unconsolidated

• Rapid biological degradation of organic material, creating large volumes of bubble-phase methane gas, which, in turn, creates excess pore pressures within the sediment

• Hurricanes and winter storms, which cause cyclic, wave-induced loading of the seafloor, contributing to the weakening of the sediment.

Cost trade-off

In most locations where bottom conditions are stable, the forces imposed on the structure are uniform over a wide area, and platform locations are chosen to minimize drilling costs. However, mudslide risks and the forces imposed on a structure by a mudslide are not uniform even over a small area.

As a result, the optimum surface location from a drilling standpoint must be adjusted to account for the cost effects on the platform structure. This optimization requires a greater-than-normal interaction between drilling engineers, platform designers, and geotechnical consultants because each potential location incurs differing costs due to drilling, depth of soil movement, and potential for soil movement.

The selected location for the EDC platform lies amid two gas fields in a relatively stable location between existing deep mud flows. Although the site was believed to be stable during 100-year storm conditions, geotechnical evaluations determined that the site could be overrun by as much as 25 ft of soil from the adjacent mud flows, caused by such events as severe storms. In addition, the soil under the platform would liquefy and flow to a depth of 15 ft.

The total anticipated soil movement of 40 ft would be coupled with little or no lateral soil resistance to a depth of 250 ft.

Platform configuration

On most mudslide-resistant platforms, individual well conductors are shielded from the direct force of flowing mud by large-diameter caissons that are driven through the jacket legs. By choosing a site where the soil is expected to liquefy only to a thickness of 40 ft (and, thus, not imparting large horizontal forces on well conductors), the EDC team was able use a more conventional platform configuration that included individual well conductors, and, thus, smaller-diameter battered legs.

Because of the unstable sea floor at the site, MMS would not permit drilling wells with a bottom-founded rig. As a result, the platform was designed to support a full-size, self-contained drilling rig.

At this water depth (224 ft), some platforms are configured as eight-leg jackets and decks. By using a four-leg design that includes a deck with extended cantilevers to accommodate simultaneous drilling and production, the design team reduced the lateral loads due to both the hydrodynamic forces and the soil overrun forces imposed from adjacent mudflows.

Because the soils are extremely soft, it was necessary to design large-plan-area mudmats to support the jacket from the bottom during various phases of loading until the piles were installed. Two 125-ton mudmats were installed with the jacket. Unfortunately, these mudmats could cause tremendous lateral and vertical forces on the structure under the design conditions of soil liquefaction and soil overrun. Therefore, the mats were designed to be removed as soon as the piles were driven and welded to the jacket.

Fig. 2 shows the mudmats on the beach at the construction site.

Because of the unstable sea floor at the site, Minerals Management Service would not permit drilling wells with a bottom-founded rig; therefore, the platform was designed to support a full-size, self-contained drilling rig (Fig. 3). Photo by Michael Klein.

Wave-soil interactions

A common practice in the Gulf of Mexico is to define as acceptable the risk of failure associated with platforms designed according to API guidelines to withstand 100-year-occurrence storm conditions. However, the risks associated with soil movements and their effects on platforms are not the same as those associated with oceanographic forces.

For example, a platform designed for a 100-year storm in a stable soil area will statistically have a relatively high survival probability in a 1,000-year storm and even in a 10,000-year storm. However, in a potential mudslide area, a step function increase in lateral load occurs once the soil is mobilized and begins to flow.

Thus, a mudslide-area platform designed to withstand a 100-year storm may have a relatively low survival probability in a 1,000-year or 10,000-year storm, if the increase in wave energy causes the mud to start to flow.

To obtain MMS approval for the platform's design assumptions, it was necessary to show that the platform's reliability was similar to that of other platforms in the Gulf of Mexico. The reliability analysis was required to account for not only the uncertainty in forecasting oceanographic forces and their occurrence intervals but also the greater uncertainty associated with predicting wave-soil interactions and the resultant soil forces imposed on the structure.

Timetable

A soil boring was taken at the proposed platform site in August 1993. Preliminary structural design began in December 1993, based on the initial results of the soil analysis. Pile and conductor sizes were then estimated.

In February 1994, EDC presented its SP 34/47 development plans to the MMS. The SP 47 gas reserves could not be independently produced because they were uneconomical on a stand-alone basis. In addition, the SP 47 lease was due to expire soon, and the MMS required a development plan for approval and lease extension.

The MMS, which was interested in working with EDC to find an economical means of recovering the SP 47 gas reserves, encouraged EDC to have a third party review the design philosophy and prepare a relative reliability report for placing a conventional platform that is designed for the anticipated soil movements at the site.

EDC then commissioned Robert Bea to coordinate the design review and designate required parameters for Paragon's analysis of various storm and soil loadings to assess the relative risk of the design concept.

Bea reviewed mudslide-resistant platform designs and their performance to date and evaluated the response of the SP 47 platform design to the various loading conditions to determine the concept's reliability. He also assembled data from the team's structural, geotechnical, and oceanographic consultants for 10, 100, 1,000, and 10,000-year storms to compare the reliability of the selected platform configuration with that of platforms located in stable areas of the Gulf of Mexico.

The report, submitted in August 1994, showed that the SP 47 design had only slightly greater risks than a platform in an area with stable soils. In addition, the platform will entail minimal safety risks, because it will be unmanned after drilling, and because it poses minimal environmental risks because it will produce only natural gas.

After examining the report, the MMS approved EDC's plan and gave the "go-ahead" to proceed with detailed platform design. The design, fabrication, and installation work for the platform was then performed in accordance with the requirements of the MMS's Certified Verification Agent program.

The structure was installed in August 1995, and the drilling rig was installed in November. Fig. 3 shows the platform after installation of the drilling rig.

Production will begin after completion of the second well, which is scheduled for late in the first quarter or in the second quarter of 1996.

Drilling will continue into the summer. The platform is designed to produce 60 MMscfd.

The Authors

Castor M. Surla is chief construction engineer for Energy Development Corp. (EDC) in Houston. He is responsible for EDC's worldwide engineering project and construction management of platforms and facilities. Surla has BS and MS degrees in civil engineering from Tulane University. He is a member of the American Society of Civil Engineers and the Society of Petroleum Engineers. He is a registered professional engineer in Louisiana and Texas.

Lee D. Danner is a project manager and structural engineer for Paragon Engineering Services Inc., Houston. He has 21 years of experience as a design engineer and construction manager. Danner has a BS in civil engineering from the University of Houston

Michael C. Kreinsen is a senior structural engineer for Paragon Engineering Services Inc., Houston. He has 18 years' experience in the design of offshore platforms and marine structures. Kreinsen has a BS and an MS in civil engineering from Cornell University.

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