New concepts extend dry-tree production into deeper water

May 3, 1999
TLP components (Fig. 1) [85,917 bytes] Response characteristics (Fig. 2) [77,490 bytes] Gulf of Mexico TLP weight summary (Fig. 3) [73,526 bytes] Cost comparison (Fig. 4) [51,456 bytes] Composite riser (Fig. 5) [21,785 bytes] Framing comparison (Fig. 6) [49,361 bytes] Subsea separation and injection (Fig. 7) [47,561 bytes] Single-column floater (Fig. 8) [68,865 bytes] New concepts will allow dry-tree production in deeper water. This is important because dry, or surface, trees can significantly
John W. Chianis, Phillip B. Poll
ABB Lummus Global Oil & Gas-Americas Houston
New concepts will allow dry-tree production in deeper water. This is important because dry, or surface, trees can significantly lower the operating costs of deepwater production.

Compared to subsea wet trees, surface trees allow wells to be readily accessible and easy to maintain.

Tension-leg platforms (TLPs) and spar platform options are the most popular concepts for dry-tree deepwater floating production systems, although other options are being designed.

TLP and spar options are well proven, but these dry-tree systems must be carefully investigated because of their sensitivity to water depth, topsides payload, environment, number of risers, and fabrication and installation methods.

As the industry moves into even deeper water, these factors become critical when performing economic evaluation.

TLP configuration

A conventional TLP hull configuration consists of four vertical cylindrical columns and rectangular pontoons that connect the columns below the water surface. In addition to dry trees, the TLP can support wet trees via steel catenary risers (SCR).

Fig. 1 illustrates a typical TLP configuration.

To date, eight conventional TLPs have been installed worldwide. The first, Conoco U.K. Ltd.'s (now operated by Orxy U.K. Energy Co.) Hutton platform in the North Sea, was installed in 1984 in 486 ft of water.

Shell Deepwater Development Systems Inc.'s Ursa platform was recently installed in 3,950 ft of water in the Gulf of Mexico.

BP Amoco plc's Marlin TLP will be the ninth TLP and will be installed in the Gulf of Mexico mid-1999. Table 1 [56,477 bytes] details the evolution of the TLP concept.

The cornerstone of the TLP concept is the stiffness of the tendon mooring system which effectively reduces the natural periods of the vertical motions to a level that is well below those of the predominant waves (Fig. 2). As a result, dynamic amplification of vertical motion is nearly nonexistent, and the platform has very small heave, roll and pitch motions even under the most severe hurricane conditions.

In the horizontal direction, the TLP is relatively compliant to the environment.

Beyond 4,000 ft

An increase in water depth will soften the mooring stiffness of the TLP thereby moving the vertical natural periods of the system towards the dominant wave energy. This is countered by a corresponding increase in the tendon cross-sectional area.

Although it is technically feasible to design a TLP for 10,000 ft water depth, the weight of the tendon system will grow rapidly, rendering the platform uneconomic. Fig. 3 compares key TLP component weights for a range of payloads and water depths.

Recent work regarding water depth limitations of TLPs has identified a number of substantial possibilities. Fig. 4 compares the TLP to other dry-tree options for a water depth of 4,000 ft in the Gulf of Mexico.

The figure clearly shows that the TLP offers noticeable cost savings over other dry-tree floater concepts for a very wide range of payloads. For water depths exceeding 4,000 ft, the TLP will show similar cost savings, but for more limited topsides payloads.

Beyond project economics, the TLP affords many preferred attributes over all other existing floating concepts. The TLP's reduced motions have significant implications on both structural and process design, and when compared to other dry-tree concepts, the TLP allows for quay-side integration and commissioning of deck and hull.

Substantial reductions in cost and risk are the result, especially for remote areas. Contemporary TLP designs provide for simple construction, well within the shipyard capabilities around the world.

Other TLP hulls

Apart from the conventional four-column TLP, there are other types of TLP hull forms. Typical examples include the three-column TLP and the single column.

Although the hull configurations differ, these TLP concepts are designed based on the same principle of vertical motion suppression to avoid wave dynamic excitation.

The three-column TLP is developed to save construction and installation costs. Hull and tendon costs decrease with fewer columns. The reduced number of tendons also leads to cost savings on installation.

The three-column TLP will have better tendon tension balance and will result in reduced internal pry/squeeze loads. One of the main drawbacks is its inefficient deck arrangement. The wellbay arrangement is also a design concern due to potential interference between the risers and pontoons.

The single-column TLP significantly reduces construction costs because the hull form is a simple cylindrical shape. Tendon attachment points are extended from the center of the structure via pontoons to further suppress roll and pitch natural periods.

While the single-column TLP reduces internal pry/squeeze loads, structural design of the cantilevered pontoons requires special attention.

The three-column and single-column TLP forms are more suitable for small-sized platforms that support limited topsides facility weights. The conventional four-column TLP is preferred for carrying large topsides.

The number of risers and the wellbay arrangement are other design considerations.

The conventional TLP has the advantage of providing a large-sized wellbay and can support a large number of top tension risers (TTRs). The wellbay arrangement becomes difficult for the three-column TLP, and the single-column TLP can only provide a small wellbay within the central column. If necessary, SCRs are attached outside the single TLP column.

Table 2 [26,755 bytes] describes the attributes of these hull forms.

TLP improvements

The TLP continues to provide a competitive solution for a wide range of applications. As such, further development of the TLP concept continues with each new installation reflecting some significant improvement in the design or process.

Composite risers

Water depth limits for TLPs are highly dependent on the required topsides payload for a particular application. Greater water depths are feasible with lightweight composite risers (Fig. 5) that can significantly reduce riser tensions supported by the TLP.

Composite risers can potentially reduce the top tension by as much as 52%, resulting in a cost reduction of as much as 20% of the overall TLP cost.

Composite risers are in an advanced state of development at ABB, including full-scale testing.

Simplifying structure

The 1990s saw the deployment of six steel TLPs for drilling and production. A key feature of the most recent TLPs is a greatly simplified structure within the hull. Much progress has been made in simplifying the structural framing within the TLP hull through an integrated constructability process between designers and fabricators.

The results are shown in Fig. 6, which compares the column framing for the Marlin TLP and a typical spar design. The actual structural framing is highly dependent on vertical load in the column, compartmentation requirements, and other factors.

For the Marlin TLP, however, the framing is simple and includes:

  • Outer and inner shell plating and rings
  • No stiffeners
  • No stanchions
  • No bulkheads near the top of column.
Fabrication of these simple structures can be accomplished quickly, resulting in improved project schedule.

Stepped tendons

Early tendon systems are characterized by constant diameter, constant thickness pipe sections from the seafloor to the TLP hull.

In deeper water, however, tendon design requirements are more pronounced along the tendon length. Near the seafloor, extreme hydrostatic pressure can control the tendon design and require increased thickness to prevent hydrostatic collapse. Note that the tendons are not flooded, but are water tight so as to reduce buoyancy requirements of the TLP hull.

Along the entire length, a certain cross-sectional area must be maintained to obtain the required axial stiffness. For fatigue reasons, the optimum diameter may vary along the length of the tendon.

A new trend in tendon design for deepwater applications is the stepped tendon, where the cross-sectional shape of the tendon changes to satisfy these changing requirements.

Subsea processing

Subsea technology developed by ABB provides an opportunity to reduce the topsides payload requirements of the TLP system, and extend the TLP to even deeper water.

The subsea system called "Subsis," provides separation, water treatment, and reinjection capability at the seafloor. This processing reduces the total fluids being pumped to the platform for processing, not only reducing the topsides weight, but also reducing the deck area requirements.

The first of these systems (Fig. 7) will be installed later this year in the North Sea in about 1,100 ft of water.

Improved design technology

In the 1990s, a revolution occurred in the area of computing and design technology. More powerful computers and improved analytical tools have led to reduced project schedules. Moreover, this increased computational horsepower provides additional capability for another new trend in deepwater design-extending the design codes.

Much of the plated structure is governed by industry design codes such as American Bureau of Shipping (ABS), det Norske Veritas (DnV), and others. These codes provide calculation techniques and design guidelines.

Almost all of these codes allow for a more optimum structure if a more rigorous analysis is provided. For example, fully nonlinear finite element analysis can be used to quantify a structural system's ultimate capacity. These analysis results can then be used to demonstrate required safety factors against ultimate failure.

Spar concepts

The spar is compliant to the environment in both the horizontal and vertical directions. The vertical motion is reduced by keeping the heave natural period well above dominant wave energy periods.

The classical spar platform has the hull form of a deep-draft cylinder. The platform consists of hard tanks that provide the required buoyancy to support its weight and soft tanks that are permanently flooded to increase mass inertia. Near the bottom of the platform there are also solid ballast tanks which lower the center of gravity to satisfy stability criteria, and trim/installation tanks for transport and upending purposes.

Within the spar column, a moonpool is designed that allows the risers to pass through. The risers can be tensioned using long-stroke tensioners, or they can be independently supported using buoyancy cans. The riser buoyancy cans are installed within the spar platform and thus isolated from the external wave excitation forces.

The spar platform is a long slender body with a small water plane area and thus a limited heave restoring force. On the other hand, the platform has a large amount of trapped water and consequently a large mass.

This combination of the small hydrostatic heave-restoring force and the large effective mass keeps the heave natural period of the platform around 28 sec or above, and hence limits the heave motion amplitude.

A typical classical spar has a draft of 650 ft and a column diameter of up to 122 ft. The wave frequency roll and pitch motions are also small since their natural periods are designed to be very long. However, due to its deep draft, the platform static heel and wind/wave generated low-frequency, rotation may be significant, which would have a considerable impact on the platform structural design.

Because of its deep draft, integration and installation of the spar platform and its interaction with the risers are major design concerns.

Apart from the classical spar concept, there are also variations of spar hull forms. A typical example is the truss spar, in which a significant portion of the lower body is replaced by truss members. The benefits are lower structural weight and reduced environmental loads.

Horizontal plates are placed within the truss to increase the added mass and heave damping so that the heave natural period can be kept long and the resulting motions small. Close attention is needed for the structural design of truss members, especially at connections with the upper floating hull.

Other spar platforms include the "step" spar and "gap" spar.

The step spar has a nonuniform hull diameter. The prime intention is to reduce the water plane area near the surface and enlarge the body below the surface to increase inertia. The step spar also increases the vertical damping force.

The gap spar was designed to reduce vortex-induced vibration.

Platform selection

It must be pointed out that both the TLP and spar are suitable for use as deepwater dry-tree platforms. The selection of either platform concept depends on many factors such as:
  • Water depth
  • Required topsides payload
  • Environmental conditions
  • Reservoir characteristics
  • Number of risers
  • Drilling requirements
  • Availability of fabrication and installation facilities
  • Overall field development economics.
Capabilities and deficiencies inherent to the TLP and spar are summarized in Table 3 [20,222 bytes].

In an effort to address the shortcomings of both TLP and spar, ABB has developed the single-column floater (SCF) (Fig. 8). The SCF is a compliant vessel capable of supporting both dry trees and subsea risers. Other significant features of the SCF include:

  • Global performance characteristics similar to a truss spar with motion responses far outside the high-energy regime
  • Steel weight less than that of a classic spar
  • A mooring system not ultra sensitive to increasing water depth, which is similar to other non-TLP floater concepts
  • Capability of supporting both drilling and workover operations
  • A deck that can be installed on the hull at quay-side and precommissioned, and is transported to site vertically in one piece and installed. This is similar to a TLP.
Due to its compact hull form, operations specific to the spar such as hull mating and upending, are eliminated. Because offshore operations are minimized, significant cost and schedule advantages are possible for remote locations without infrastructure.

The Authors

John W. Chianis is vice-president and general manager of ABB Lummus Global Oil & Gas-Americas, Houston. He has participated in a large number of TLP concept developments, feasibility studies, and detailed designs. In addition to TLPs, Chianis has also been very active in the development and design of other deepwater floating systems such as caisson vessels, FPSOs, semisubmersibles, and single-point moorings. He holds BS and MS degrees in ocean engineering.
Philip B. Poll is head of technology development at ABB Lummus Global Oil & Gas-Americas, Houston. The department develops and applies technology to deepwater engineering. Poll has a BS in aerospace engineering from Texas A&M University and an MS in Aeronautics and Astronautics from the Massachusetts Institute of Technology.

Copyright 1999 Oil & Gas Journal. All Rights Reserved.