MOPUs evolving to meet greater depth, flexibility challenges

Risers, mooring dictate production vessel evolution

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Mobile offshore production units (MOPUs) are production units that can be relocated from site to site. The MOPU group includes FPSO (floating, production, storage, and offloading) vessels, production barges, semi submersible production units, jackup pro duction units, spars, and TLPs (tension leg platforms).

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Operating water depth
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In contrast, compliant towers, gravity based structures (GBS) and fixed jacket structures would not qualify as MOPUs. Floating pro duction units (FPUs) are structures that depend predominantly on buoyancy to balance the weight. This includes all of the vessel types mentioned earlier except jackup units, which are bottom-founded through extensible legs.

As part of an ongoing survey of MOPUs worldwide, Aker Engineering, Inc. together with Offshore magazine, has published a pull-out poster on FPSOs in the August 1999 issue of this magazine. In continuation of this effort, the present survey lists semisubmersible, barge-type, and jackup-based MOPUs around the world.

MOPU history

The first offshore production jackup was installed in the Ekofisk Field in Norway utilizing the Gulftide jackup as an early production system. Production began in 1971, 18 months after discovery. The initial field configuration consisted of four subsea wells tied back to the production platform.

Production jackups since have evolved into the sophisticated TPG 500 jackup installed in 1996 on the Harding field in the central North Sea. This giant newbuild, located in 360 ft water depth, supports a process facility of 65,000 b/d of oil and 35 MMcf/d gas. It rests on a concrete gravity base that stores produced oil for periodic shuttle tanker transport, eliminating a costly need for an oil pipeline.

The world's first semisubmersible production system was installed in 1975, in the North Sea Argyll field in 275 ft water depth, by the Hamilton Brothers. The semisubmersible was a leased first generation drilling unit, Transworld 58. The unit was installed with a process facility consisting of a production manifold, two stages of oil/gas/water separation, and an oily water treatment system. Flow to the unit was by a rigid top-tensioned central export riser with individually tensioned production and service risers.

The second semisubmersible production system was installed in the Spanish Mediterranean Sea (Casablanca Field). In February 1977, the Aker H-3 semisubmersible, Bideford Dolphin, conducted a 15-month extended well test, producing to a shuttle tanker. In December 1977, two completed wells were tied back to the semisubmersible Afortunada. The Afortunada, equipped with a 20,000 b/d production facility operated until February 1982, when it was replaced by a fixed platform after producing 12 million bbl of oil.

Brazil's first production semisubmersible began producing in 1977 for the Enchova Field. The vessel, the Sedco 135D, was leased and a process facility installed. Between installation and release of the MOPU in July 1980, this early production system produced over 8.5 million bbl of oil reaching operational efficiencies of 98%. Brazil's entrance into the production semisubmersible fleet is important to note since today, Brazil's production semisubmersible fleet accounts for almost 50% of the operating units worldwide.

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The first application of a production and drilling semisubmersible in the Gulf of Mexico was by Placid Oil in 1988 for their Green Canyon 29 field in 1,500 ft water depth. The unit produced from a 24-well subsea template with production from each well brought to the surface through a freestanding production riser. The unit was converted from the Penrod 72 drilling semisubmersible, built in 1975. The unit was bought by Chiles Drilling in 1991, converted to the Petrobras 24, and is now producing in the Campos Basin in Brazil.

Semisubmersible production platforms have evolved from simply converting existing or slightly modified MODU designs into high capacity purpose built designs employed in the Norwegian North Sea. The Njord, Visund, and the Troll C semisubmersibles, all installed within the last three years can simultaneously drill and produce from wells beneath the vessels. They were selected over TLPs or FPSOs because of the better economic returns provided. Another recent example of large capacity purpose-built semi sub mersibles capable of processing 250,000 b/d of oil is the soon-to-be-installed Asgard B, capable of producing 1,340 MMcf/d and dis placing 90,000 tons.

The paper entitled, "The Semisubmersible Floating Production System: Past, Present and Future Technology", published by SNAME in 1993, is a source of data for the progression of semisubmersible production technology from 1975 to 1993.

Present fleet

The accompanying MOPU pullout poster divides the fleet into semisubmersible and barge or jackup units that are either in operation, being converted or constructed, or stacked. Thirty -five production semisubmersibles, two production barges, and 22 production jackups were identified in the survey.

There are 30 production semisubmersibles in operation, with three in construction and two stacked. Of the existing production semisubmersibles, eight are newbuilds. The remainder are conversions. Seven of the eight newbuild units are installed in the North Sea and have come on line in the latter half of the 1990s.

Of the production semisubmersibles operating, water depth varies from 260 ft to 5,700 ft. Maximum production ranges from 10,000 b/d of oil to 267,000 b/d. The largest displace ment production semisubmersible is Troll B, a concrete semisubmersible, at 189,000 tons.

Two jackups are in construction and five are stacked. Fifteen production jackups are currently in operation, three of which are newbuilds.

Published data indicates existing jackups units worldwide number over 300 units. The number of these vessels operating as production jackups is less than 10% of the total fleet. Production jackup operating water depth varies from 26 ft to 360 ft. The maximum water depth rating for the majority of these production jackups is less than 300 ft. The maximum production for these units range from 3,000 b/d of oil to 150,000 b/d.

Attributes, applications

Jackups and semisubmersibles are proven industry workhorses with a reliable operating track record for drilling, drilling and production, and production only applications. Both have been utilized as central hub facilities, local production or production/ drilling host facilities, and early production or extended well test systems.

The jackup has proven itself an excellent candidate for shallow water early production application. Jackups offer a cost efficient solution for short life fields, compared with jacket or subsea applications.

Mobility enables reduced installation, hook up, and commissioning and decommissioning costs, compared with a jacket structure. For a newbuild, the jacking system cost must be weighed against the installation, hookup, and commissioning costs of the jacket. For an existing jackup as a conversion candidate, the jacking system cost is not as much of an issue as acquisition costs. Production jackups have the added advantage of relocation and re-use at less cost.

Both jackup and semisubmersible platforms provide large flat deck areas offering flexibility for a safe and functional topside arrangement. The large deck area and relatively low sensitivity to topside payload facilitates the building in capacity for future topside expansion, such as gas lift on water injection.

Additional attributes that make the semisubmersible a strong candidate for offshore production development projects are the very characteristics that have proven it as a versatile drilling platform. The platform is relatively insensitive to increasing water depth, with the increased vertical loads of the mooring and riser systems offset by reducing ballast. The hull configuration results in a stable platform with low vertical motions that allows production to continue in severe seas.

The reuse of the Penrod 72 highlights the flexibility of the semisubmersible production platform. The upgrade of the unit to the Petrobras 24 required no major changes to the hull structure or the marine systems. Drilling or workover was not required for the Petrobras 24, so the drilling system was removed and replaced with an expanded process facility. The conversion was completed dockside in Galveston, Texas in 11 months.

For newbuild semisubmersibles the cycle time from start of front end engineering to first oil is historically very short. This is the result of parallel fabrication of hull and topsides, availability of several qualified EPC contractors and hull/topside fabrication yards, as well as ease of installation and hookup to risers and moorings. Semisubmersible platforms have been utilized in the following applications:

  • Drilling, production, workover: One of the first applications, described above, was in the Gulf of Mexico by Placid Oil for their Green Canyon 29 field. This same technique of utilizing a well subsea template with production from each well brought to the surface via a free standing hybrid riser was later used by Enserch on their GB 388 development. Both platforms were converted from existing drilling semisubmersibles. Subsea architecture can range from individual wells, cluster wells, or an integrated template. In water depths to 1,000 meters, production risers can be flexible risers in a pliant wave configuration or a free-standing riser. In water depths beyond 1,000 meters, a free-standing riser is technically feasible and cost effective. Many free-standing riser designs are being developed for ultra-deepwater. When the well count is low (10 or less), consideration can be given to a workover/production semisubmersible with wells predrilled by a mobile drilling unit and competed from the production platform.
  • Central hub facility: For shallow reservoirs with large aerial extent, or for an area-wide development consisting of several widely separated (10-15 km) drill centers with 4-6 wells, the semisubmersible is an attractive candidate for a centrally located host platform. The cluster manifolds can be easily tied back via flexible risers or, if in ultra-deepwater, by a steel catenary of lazy wave risers. If wax or hydrates are a problem, these may be round trip pigged, or accessible for coil tubing intervention.
    The platform is equipped primarily for production only and to support the production risers. This has been the most widely used application for semi submersible floating pro duction systems, part ic ular ly in Brazil. In the North Sea, semi submersible central hub facilities have been selected on the Troll B, the Troll C and the Asgard B. The latter is going to be the world's largest steel semi submersible production platform.
  • Local production host facility: Similar to a central hub facility but on a smaller scale, this is a production-only application to produce a small shallow re servoir of large aerial extent, that is to be produced by a small number (5-7) widely scattered wells on the seabed.
  • Early production or extended well test systems: The relatively insensitivity to water depth, ease of topside modification, transport and riser/mooring installation makes the semisubmersible platform a natural candidate for an early production system. It is particularly applicable where drilling or frequent well intervention is required, function that are not easily conducted from FPSOs.

Conversion design considerations

Major considerations in selecting a unit for conversion to a production unit, be it a jackup or semisubmersible, are the naval architectural aspects of the conversion, modifications to the existing systems, and the repair and life extension. The initial stage in the vessel section is the vessel screening process, the review of the existing drilling fleet and the identification of suitable candidate vessels. For the semisubmersible, the naval architectural aspects include the vessel arrangement and stability. An increase in deck loading may result in the addition of buoyancy enhancements (column and/ or pontoon sponsons).

System modifications will include but are not limited to the upgrade of fire and gas detection and suppression system, the utility systems, the ballast system, the lifesaving system, and potentially, the mooring system.

Unlike drilling semisubmersibles, the MOPU is intended to remain on station for an extended period of time. The engineering of the conversion must address this critical issue. A corrosion protection plan must be implemented during the conversion. The fatigue life of critical structural joints must be considered and the lives of critical joints not accessible to inspection and repair restored to an as new condition. The accessibility of equipment and piping must be considered and systems deemed expensive and difficult to repair at site overhauled or replaced during the conversion.

Future market

The future for production jackups appears promising. Designed originally for shallow water, jackups are now capable of operating in harsh environments in water depths up to 360 ft. Their flexibility and mobility offers operators a shallow water, early production solution as well as a production solution for shallow water fields with a short field life.

An area for future growth is the use of purpose-build jackups for long life shallow water gas fields as an alternative to jackets or floating MOPUs, especially in areas where mobilizing of derrick barges may be an expensive option. Companies are developing low cost units that are easy to fabricate with inexpensive jacking and locking systems.

Published data for 1999 shows production semisubmersibles making up about 30% of the worldwide floating production fleet. The same industry data states current orders exist for four additional production semisubmersibles, two new constructed vessels slated for the North Sea and two converted vessels slated for Brazil.

Most semisubmersible floating production platforms, in the past, were converted from drilling platforms. Utilization of an existing semisubmersible allowed operators to take advantage of a reduced fabrication schedule and cost. The high utilization and day rates for the semisubmersible drilling platforms have lately precluded this option by driving up the acquisition price. This, combined with the limited availability of second generation candidate vessels suitable for conversion, has led the industry to develop purpose-built hull designs that are simple and cost efficient to fabricate and have short cycle times by enabling parallel fabrication of hull and topsides.

Extending drilling/production semisubmersibles to water depths to 10,000 ft, in competition with alternative drilling/production platforms such as TLPs and spars that offer dry completions, is a major challenge. The main drawback is the high cost of the low pressure 18 3/4-in. drilling riser and associated buoyancy. Conventional drilling practice also results in larger durations to drill wells because of running and tripping of risers.

Advances such as riserless drilling and slimhole drilling will reduce these costs significantly. A potential breakthrough is the freestanding drilling riser that can be pre-installed by another vessel. The semisubmersible need only carry a short length of drilling riser. The freestanding riser is never pulled and greatly increases efficiency of drilling and heavy workover operations. It will also permit conducting simultaneous light workover while drilling. For a drilling/ production semisubmersible, freestanding hybrid risers are qualified to 5,000 ft and no foreseeable barriers exist to extend this to 10,000 ft. A similar advance that will achieve the same result is the artificial seabed, where trees and blowout preventers are located about 250 ft below the waterline.

A second challenge with deepwater pro duction semisubmersible are the production risers. Industry is currently engaged in proving the application of simple steel catenary risers hung off semisubmersibles. Petrobras is utilizing a 10-in. steel catenary riser (SCR) on the P-18 in 3,000 ft water depth.

In today's field development selection pro cess, there is no singular answer to determine which platform type (FPSO, TLP, spar, or semisubmersible) is the most attractive for the particular field parameters. The solution must be considered in the context of the total field - the field layout, drainage strategy, subsea facilities, drilling and workover requirements, the offloading systems, and the corresponding life cycle economics.

In some cases, economic issues may not be the sole justification for platform selection. Where the economic differences are inconclusive, issues such as captial expenditures, fabrication, schedule, risk, flexibility, safety, and operability should be considered. The semisubmersible scores highly on many of these issues.

Semisubmersibles, like all other development solutions, will be faced with design challenges to remain competitive. The versatility of the semisubmersible has been demonstrated by its track record from different geographical locations. It is believed this platform type will be competitive as extended well testing/early production units, central hub facilities for large aerial extent reservoirs, and combined drilling and production platforms.

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