TLP TECHNOLOGY SeaStar minimal platform for small deepwater reserves

Stephen Kibbee Atlantia Corporation The hull structure of the SeaStar mini-TLP

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TLP features small size, moderate cost, simple construction and installation

Stephen Kibbee
Atlantia Corporation

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The hull structure of the SeaStar mini-TLP


The SeaStar deepwater mini-platform has been developed and tested to provide production and utility platforms for deepwater applications. These new designs combine proven tension-leg platform (TLP) technology with the cost, schedule, and learning curve advantages of marginal field platform designs that have been used extensively in shallow water. The SeaStar platform has application as an independent production platform on fields having smaller reserves or as a utility, satellite, or early production platform for larger deepwater discoveries.

More than a decade of deepwater exploration drilling has succeeded in finding commercial quantities of oil and gas, and in the process many deepwater discoveries have been identified that are uneconomic to produce with conventional deepwater technology. The SeaStar TLP family has been developed to unlock the economic potential of discovered, but as yet undeveloped, deepwater fields in the Gulf of Mexico and around the world.

Existing deepwater technology is available for fields with large reserves and many wells, but not for smaller fields with fewer completion targets. The maturation of TLP technology and "process insight" from the design and manufacture of hundreds of shallow water marginal field platforms, allow deepwater platform cost to be reduced, so that many smaller deepwater discoveries can now be economically produced.

The SeaStar's relatively small size and low cost are important advantages. Its cost is low enough to be economically supported by fewer proven reserves. The accompanying reduction in delineation activities accelerates schedule and reduces investment. In contrast to large deepwater structures which can be only be built in a few yards, all SeaStar system elements are small enough to be supplied by existing Gulf Coast infrastructure at competitive prices. The small footprint and permanent tension leg mooring allows the SeaStar platform to be installed relatively close to other platforms. A SeaStar could be installed to support future facilities that accommodate changes in field operations over time.

In shallow water, operators can manage risk and cash flow by incrementally developing reserves. The SeaStar deepwater mini-platform provides a vehicle that facilitates incremental development in deepwater. The availability of these small standardized platforms helps to make deepwater development more consistent with the risks and economics familiar to shallow water operators.

Early SeaStar production system development work was partially funded by the US Department of Energy. Improvements and developments in the SeaStar production system since early 1995 have been as a direct result of cooperative efforts and financial support by British Borneo Petroleum.

Production trends

Historically, it is the major oil companies who first enter frontier areas in search of large discoveries. During the maturation of these exploration areas, smaller oil companies enter the region to develop other, usually smaller, discoveries after geology and technology are proven. This transition is now occurring in the deepwater Gulf of Mexico. Deepwater system designs have evolved, and will continue to evolve, in response to the application and need.

TLP technology can preserve many of the operational advantages of a fixed-base platform while reducing investment in water depths to 5,000 ft. and beyond. Existing TLPs have been built to support drilling rigs or completion units which are appropriate for larger discoveries, but this requirement can increase cost and complexity to uneconomic levels for smaller discoveries.

A tension leg mooring can also provide a small, stable platform that can provide affordable real estate for simplifying subsea production operations. This is the basis for the first SeaStar designs.

Traditional floating production systems (FPS) took advantage of a surplus supply and low cost of semisubmersible drilling units to reduce the investment required to support production equipment in deepwater. The same distressed conditions in the offshore drilling industry during the 1980s and early 1990s that provided these low cost rigs resulted in very few semisubmersibles being built during this period. Now, with a limited supply and increased utilization driven by the deep water activity, the value of the MODU fleet has increased dramatically, substantially raising the price of this option for deep water production.

An established approach for developing satellite reserves in the Gulf of Mexico is to complete wells subsea and install a flowline to a host platform. This approach is appropriate when: (a) the host is close enough to the subsea wells that flowline and control systems are economically viable; (b) suitable agreements can be negotiated with the owners of the host platform having available and compatible processing capacity; and (c) the satellite reservoir can be efficiently drained by one or two wells.

As the distance between the host and subsea well increases, flowline and control system cost and complexity increases. This cost and complexity can be reduced by providing strategically located real estate near to the wells.

Seastar TLP designs

The SeaStar platform is a small TLP with a single surface-piercing column. The SeaStar platform is relatively transparent to environmental forces and is designed to efficiently carry a range of payloads. Many SeaStar platform designs have been developed and patented. A basic hull design was developed for British Borneo. Structural benefits of the SeaStar platform are:

  1. The tension-leg mooring system suppresses nearly all vertical motions. This type of mooring configuration makes it possible to have a single, stable column piercing the surface of the water with a small water plane area.

  2. The single surface-piercing column allows the hull and deck to be independently designed and optimized.

  3. The foundation can have either driven piles, drilled and grouted piles, or suction piles. Redundancy can be incorporated by using a template with additional piles.

  4. Tendons are pre-installed which reduce installation risk.

  5. The hull can either be wet-towed or dry-towed to location. After the hull is connected to the pre-installed tendons, the deck section can be lifted into place.

  6. The platform's relatively large base dimensions increase tendon separation and improve their effectiveness.

  7. Key platform components can be standardized.

Global performance

The global performance behavior of the SeaStar is very good for a structure this size. The tension leg mooring provides much better motions than any comparably sized structure using catenary mooring. The motions are comparable with much larger TLPs.

A single leg structure is more compliant over a wider range of wave periods than a conventional TLP. Normal cancellations in the horizontal motion response function are not present. The result is a somewhat larger response in the smallest seastates (four to seven second wave periods, the response is still small), but a comparable or smaller response than a conventional four-column TLP in 10+ second seastates.

The tension response in a single column TLP is typically dominated by surge/pitch coupling forces, and these are controlled in design by the baseline dimension, and by control of the vertical center of gravity.

The SeaStar TLP was model tested recently at the Offshore Technology Research Center Wave Basin at Texas A&M University. These model tests have been funded by British Borneo and Atlantia Corporation. The objectives of the test included validation of the concept, validation and tuning of the numerical models used to analyze the system, and evaluation of ringing/springing responses. Overall, the test results are very positive and indicate that the design is fundamentally sound. The numerical models are able to accurately predict the wave frequency responses of the platform. The structure performed better than expected with regard to the ringing/spring responses. The tests also provided insight on the configuration of the upper column - deck interface design which will be incorporated into the ongoing design work.

Hull

The hull provides sufficient buoyancy to support the deck, facilities, and flexible risers and has sufficient excess buoyancy to develop the design tendon pretension. The hull is designed of stiffened plate and stiffened shell construction.

Compartmentalization in both the column and pontoons is provided to limit the effects of accidental damage. The configuration is designed for ease of fabrication at local Gulf of Mexico fabrication/shipbuilding yards. The design is based on traditional shipbuilding details, and is configured to be assembled from modules which are all less than 200 tons. Plates are sized in standard thicknesses, and are 1 in. or less throughout the design. The node design is somewhat unique for a TLP, and is based on the single column feature of the TLP. The result is an easily constructed, lightweight structure.

The ballast system serves to adjust draft during transportation and installation, and for dewatering in the case of emergency flooded conditions. Since the variable payload for a non-drilling structure is small, the tendons and pretension are designed to accommodate minor day-to-day weight condition changes without ballast changes. The ballast system is therefore only operated during installation and emergency conditions, and is therefore less complex than a ballast system which must remain in continuous active operation for the life of the platform. The ballast pump is designed to be easily recovered to topside for service or replacement at any time.

Deck

The SeaStar deck provides a stable working platform, safely above hurricane wave crest heights, to support the production equipment necessary to process and control production. The deck designs for the SeaStar platform differ from those normally considered for TLPs. This is a departure from existing TLP technology in two respects:

  1. Typically, the deck of a SeaStar is installed after the hull is installed.

  2. The deck and hull can be optimized separately. When the design of the hull and deck are mutually dependent, the marine considerations which affect the design of the hull also impact the dimensions of the deck.

The spider deck arrangement has diagonal deck braces built into the hull section. The deck yields simple fabrication, onshore hookup, transportation, and installation operations. The spider deck configuration facilitates hull re-use because the deck section can be removed by cutting off the stabbed connections, and a given hull can be refitted with a new deck. The system can then be re-deployed at a new location having different water depth, with new facilities.

The deck can be one or more levels with dimensions of 90 ft. by 90 ft. or more, depending on site-specific requirements. A helideck would be included to provide air access. Gulf Coast fabricators are very familiar with construction methods and techniques for this type of deck and it can be fabricated at competitive cost and schedule.

Production system

The ability to provide affordable deck space near the subsea wells has several economic and operational benefits compared to long-reach subsea production systems.

  1. Simpler flow control: Since the flowlines are short, individual flowlines to each well are affordable. Short flowlines also make it affordable to equip each subsea well with a second flowline for a wax removal pigging circuit. The short distance also makes it possible to control the subsea tree with simpler control systems.

  2. Shorter flowlines: Apart from the obvious benefit of reduced flowline cost, short flowlines reduce pressure drop and back-pressure on wells thereby increasing producing rates and recovery.

  3. Simpler hydrate control: In deep water, the seafloor temperature can be sufficiently low to form hydrates that plug the flowline. A straightforward way to suppress the formation of hydrates is to inject methanol at the inlet to the flowline. A simple way to inject metered amounts of chemical subsea is to run a dedicated line from topside to the injection point.

  4. Improved recovery: It is possible to install compression in close proximity to the wellhead to increase recoverable reserves.

  5. Utility: Local electric power can be supplied to surface or subsea power consumers by means of conventional generators.

Tendons

The hull is moored by tubular steel tendons. Tendon systems have been intensively researched for TLP applications and the necessary technology is well established. A basic choice in the configuration of a SeaStar platform is whether to have one or two tendons per corner. The choice between one or two tendons is primarily one of size, desired redundancy, and cost. Both options have been developed and are available.

  • One tendon per corner is a low cost option with half as many tendons and connectors. However, there is no redundancy with one tendon per corner, and tendon removal or replacement involves complete de-installation/installation of the TLP. With one tendon per corner, safety factors are increased to provide additional safety that is normally provided by redundancy.

  • The two tendons per corner option is more expensive, but provides additional safety through redundancy. A tendon can be removed and replaced, without disrupting ongoing operations.

Tendons can be installed either as a single piece or segmented as joints. Both options have been well established by previous practice. The single piece tendons can be applicable when suitable fabrication facilities are located near the installation site, so that the tow distance is relatively short and can be traversed during a predictable weather window.

The single piece tendon is usually designed neutrally buoyant so that it rides slightly below the surface of the water during tow out. The weight of the connectors at the ends of the tendons are supported by buoyancy tanks. The upper buoyancy tank is larger than the lower tank and serves to hold the tendon upright before the hull is installed.

Segmented tendons are applicable when single piece tendons are not practical for reasons of fabrication site or transportation. In this approach, tendon segments are shipped to location on a barge and stalked as it is lowered. Alternatively, the tendon segments could be run from a drilling unit similar to drilling riser. In either case, a temporary or permanent buoy on the top of the tendon is included to hold the tendon upright until the hull is installed.

Foundation

There are some basic configuration choices to be made with regard to the template, piles, and foundation redundancy. The piles can either be driven, drilled and grouted, or suction piles. The main appeal of drilled and grouted piles is that the installation can be done without a derrick barge.

The primary disadvantage of drilled and grouted piles is that in some cases there is uncertainty in the holding power of the pile because of the change made to the soil-pile interface during the jetting and drilling operations. The primary disadvantage associated with the driven pile option in deep water is that underwater hammers, although available, are expensive.

Risers

Since the tension-leg mooring suppresses heave motions and reduces excursions, the performance requirements of production and sales risers are reduced compared to a catenary moored platform. Steel catenary risers (SCR's) are viable for the export system and also may be appropriate for flowlines.

A flexible riser analysis has also been performed under the loading conditions imposed by Gulf of Mexico hurricane and loop current events. Dynamic analysis was performed to determine riser motions under varying wave and current loadings. Analysis was also conducted to ensure that risers and tendons do not interfere.

Installation

In the industry's current worldwide construction fleet, equipment exists which can operate in 3,000 ft. of water and beyond. However, in most cases, this specialized equipment is relatively rare and can command high day rates. A basic premise in the development of SeaStar is that it should be possible to build and install the platform using competitively priced infra-structure. This means avoiding dependence, wherever possible, on sole-source installation equipment.

Conclusions

A decade of deepwater exploration in the Gulf of Mexico has resulted in many discoveries, though few have been developed because of the high costs and long lead times involved. New production methods are lowering the cost of deepwater developments.

In the wake of Conoco's North Sea Hutton Field, tension-leg platforms have permitted economical development of a number of deepwater fields such as Conoco's Gulf of Mexico Jolliet and North Sea Heidrun Fields, Shell's Gulf of Mexico Auger, Mars, Ram-Powell, and Ursa Fields, and Saga's North Sea Snorre Field.

However, high costs have limited the use of conventional TLP's to large deepwater fields. The new SeaStar family of production platforms has been designed to offer an opportunity to economically develop additional deepwater oil and gas fields.

REFERENCES

Kibbee, S.; Chianis, J.; Davies, K. and Sarwono, B.: "A Mini-Platform for Deepwater - The SeaStar TLP", SNAME Conference (23 February 1995)

Kibbee, S.; Chianis, J.; Davies, K. and Sarwono, B.: "The SeaStar Tension-Leg Platform", Offshore Technology Conference (May 1994)

Kibbee, S.; Chianis, J.; Davies, K. and Sarwono, B.: "SeaStar Tension-Leg Plat-form - An Economic Solution to Deepwater Marginal Field Development," Energy Sources Conference, New Orleans (January 1994).

Blandford, J.; Gutierrez, R.; and McRee, R.: "Mini-platform Accelerates Production, Saves Expendable Wells," Oil and Gas Journal (March 4, 1985).

Copyright 1996 Offshore. All Rights Reserved.

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