Buoyant leg structures: An economic solution for marginal deepwater GoM oil fields

There are large numbers of offshore oil and gas field discoveries with recover-able reserves ranging from about 25 MMboe to 100 MMboe.

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C.C. Capanoglu
R.W. Copple
D.W. Kalinowski
International Design, Engineering, and Analysis Services Inc.

There are large numbers of offshore oil and gas field discoveries with recover-able reserves ranging from about 25 MMboe to 100 MMboe. Many of these fields are in the deepwater Gulf of Mexico and are considered marginally econom-ical as stand-alone developments. These fields, which may collectively contain 4-6 Bboe, may not be produced unless oil prices rise signi-ficantly or more cost-effective field development solutions are brought forward.

One potential solution is the buoyant leg structure (BLS). The BLS is heave-restrained and provides lateral support to the wells from the seafloor to the deck. It, in effect, extends the less expensive, fixed-platform operational methods to deepwater. This capability could produce a better than 50% internal rate of return (IRR) on a field with recoverable reserves of 55 MMbbl over a 10-year field life, based on $18/bbl revenue. Even if a pessimistic approach is taken to assume that the rate of return for the field will be limited to 26 MMbbl during an eight-year field life, the IRR is still 21%. (The accompanying tables and graphs present the capex, opex, and economic data.)

The BLS is a positively buoyant structure inherently stable in both free-floating and tethered mode. It consists of a circular water-piercing column/hull that supports the deck structure and a restraining leg that tethers the column/hull to the seafloor. Several seafloor connection options are possible, including the use of a suction pile, a template with drilled and grouted piles, or a hybrid gravity-base structure.

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The proposed multi-cell BLS with a 4,000-ton deck payload.
Click here to enlarge image

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The foundation type will depend both on management philosophy and the intended function of the BLS unit. A wide range of BLS configurations were developed to accommodate a small support facility with a 1,000-ton deck payload to a large drilling and production platform with a 16,000-ton deck payload.

A BLS resembles a Spar but behaves more like a TLP. Both the BLS and the TLP are heave restrained. While the TLP is completely pitch and roll restrained, a BLS is only partially restrained. The bending stiffness of the restraining leg and the BLS hull configuration provide the BLS with adequate rotational stiffness so that the pitch and roll motions are negligible in an operating environment, while the maximum single amplitude pitch and roll in a hurricane environment is under 4°. This represents only about one-third the pitch and roll of a Spar. The BLS has smaller silhouette and displacement near the water surface than a comparable TLP, and is subjected to wave and current forces that are substantially smaller than those on a TLP.

A Spar has 6° of freedom, and its keel has to extend far below the water surface to minimize the dynamic heave motions and achieve acceptable operating motions. Conse-quently, a large hull displace- ment is required, yielding a high displacement-to-deck payload ratio.

A TLP has 3° of freedom, and the restriction of pitch and roll results in large tendon tension variations. Thus, high initial tendon pre-tensions are required to prevent the tendons from buckling under compression. Conse-quently, a substantial percent- age (20-25% for the largest to 40-45% for the smallest) of the TLP hull displacement is dedicated to pretension. Though necessary, this increases the hull displacement-to-deck payload ratio.

A BLS does not need a deep keel to reduce the heave motions because its restraining leg restricts this motion. It does not require large restraining leg pre-tensions, as it is partially compliant in pitch and roll. The magnitude of BLS pre-tension is determined by the desirable limit on quasi-static offset, where the horizontal component of pre-tension equals static offset forces. Typically, less than 15% of the BLS displacement is dedicated to pre-tension. In this way, the BLS retains some advantages of both the Spar and the TLP while avoiding the less desirable characteristics.

The BLS hull consists of either a single ring-and stringer-stiffened cylindrical shell or a column consisting of multiple cylindrical shells. In either case, it does not have complex nodes or joint details. This means it can be constructed quickly and cheaply with relative ease at any offshore fabrication site. The restraining leg consists of a transition cone and small diameter cylinders stiffened with rings only.

Construction simplicity and the ability to construct the components in parallel reduces the unit cost and shortens the construction time. The buoyant hull and the restraining leg(s) can be joined quayside, wet-towed to the installation site, and upended by flooding the selected compartments. If the BLS is transported dry from an overseas yard to the GoM installation site, the transition cone and the components of the restraining leg will be assembled and lowered to the seafloor separately. The buoyant leg is upended by free-flooding the lowest three compartments. Since the BLS is stable in free-floating mode, the deck structure can be either floated-over or lift-installed. Then, the restraining leg is secured to the foundation system and several compartments deballasted to achieve the desirable pretension.

The in-service stresses on the restraining leg are small for the operating environment and moderate for the extreme environment. Thus, design fatigue lives are readily achieved, and member utilization ratios for the extreme environment remain reasonable. An added restraining leg redundancy can be achieved by using two concentric cylindrical shells, multiple tubulars, or a combination of a tubulars and a wire rope. The choice depends on the size of the BLS and the management philosophy.

Design status

A BLS unit is yet to be constructed and installed. Numerous engineering studies and several preliminary designs have been completed, however, resulting in further improvements to the design. Improvements include the introduction of a multi-cell hull to facilitate construction and localize vortex induced vibrations (VIVs).

The BLS system is made up of standard, proven components. A two-phase joint industry project on BLS was completed successfully in March 2001. Several preliminary designs were developed, and model tests at the Offshore Model Basin in Escondido, California, validated the analyses results of two designs. Construction specifications were prepared and, together with design drawings, transmitted to several construction yards. Cost quotes confirmed the accuracy of estimated constructed costs.

Sample case

The BLS developed for a marginal oil field in 4,000-ft water depth in the Gulf of Mexico would support a deck payload of 4,000 tons. This facility would have a processing capacity of 35,000 b/d.

The hypothetical field has four production wells, which are predrilled with a semisubmersible drilling rig. A subsea template foundation option was chosen over a suction pile option because predrilling allows easy installation of the foundation template and the piles.

The cost of these two options remains similar. The buoyant leg consists of a central, 25-ft diameter cylinder surrounded by 17.5-ft diameter auxiliary cylinders. Variable spacing between these cylinders is intended to minimize VIVs and keep them localized. The restraining leg consists of an 80-ft by 10-ft section attached to a 3,500-ft by 5.5-ft diameter section. The total deck, buoyant leg, and restraining leg steel weigh 7,400 tons.

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The buoyant hull and the restraining leg(s) can be joined at the quayside, wet-towed to the installation site, and upended by flooding the selected compartments.
Click here to enlarge image

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With 600-ton appurtenances, 7,200-ton ballast, and a pre-tension of 2,300 tons, the total displacement is 21,500 tons.

The total installed cost of the basic BLS unit is estimated at $45 million. Adding the installed and commissioned cost of topsides equipment of $41.5 million and pipeline cost of $18.8 million, the total cost equals $105.3 million. When the cost of the development wells is added, the total capex reaches $151 million.

For the purposes of this economic study, three separate reservoir conditions were assumed. The base case (case A) was assumed to have all four wells initially producing 8,000 b/d with a well decay factor of 0.155. This yields recoverable reserves of 55 MMbbl in 10 years.

The other two cases are based on more pessimistic well production. Both case B and case C are based on the wells initially producing 6,000 and 5,000 b/d, respectively, and the fields having economic lives of eight years each. The wells in case B are assumed to have a decay factor of 0.18, yielding recoverable reserves of 34 MMbbl. The wells in case C are assumed to have a decay factor of 0.2, yielding recoverable reserves of 26 MMbbl.

The economic study is based on an $18/bbl price with both the opex and revenue subjected to 3% annual escalation. In addition to the IRR on investment, it was considered desirable to determine the net present value (NPV). The IRR for case A is 54%, and the NPV is $420 million without a discount rate. Even with a pessimistic case B, the IRR is 33%, and the NPV $192 million. For a very pessimistic case C, the IRR is 21%, and the NPV is $105 million.

In addition to BLS's superior in-service performance characteristics, this economic study clearly shows that a BLS can be utilized to develop truly marginal fields and achieve a very good return on investment.

The BLS also offers operational advantages. The main well system advantages include drilling and surface completion of wells, protecting the wells from the environment, and laterally supporting them by running them through the restraining leg and easy and inexpensive entry for workover. Simple cylindrical components of the BLS facilitate its construction as well as its inspection and maintenance, thereby contributing to a reduction in both capex and opex.

Unlike most TLPs, stability characteristics of the BLS permit the unit to be in a free-floating mode. Since the BLS is inherently stable in a free-floating mode, it can be disconnected and relocated from one marginal field to another in an upright position. This characteristic makes it feasible for a BLS-based drilling unit to drill several wells in one location and several at another.

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