Newbuild semisubmersible-based production system viable alternative to mini-TLP or Spar

In the last five years, the Gulf of Mexico has witnessed an impressive surge in small deepwater fields developed by either mini-TLPs or mini-Spars.

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The TLPs are all in water depths under 3,500 ft, with the exception of the recently sanctioned Magnolia TLP in 4,700 ft. Most of the Spar platforms are in water depths below 4,000 ft except for the recently installed Horn Mountain platform in 5,400 ft and sanctioned Devil's Tower platform in 5,600 ft. There is little operating history for these platforms in water depths above 4,000 ft.

In Brazil, Petrobras has seven semisubmersible platforms with operating payloads under 10,000 tons that operate in water depths greater than 1,500 ft, with the deepest in 3,250 ft. All platforms have subsea trees, and all are conversions.

Hull and mooring sizing tools, benchmarked with as-built projects, were used to generate hull and deck steel weight data for these mini platforms for topside operating payloads ranging from 3,000 to 6,000 tons and water depths from 4,000 to 8,000 ft. The comparisons may be extended to deeper water.

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There are about 15 small discoveries in the Gulf of Mexico in water depths ranging from 4,000 ft to 8,000 ft that could be developed with small floating hub platforms. A commercial case for using a purpose-built semisubmersible for these developments can be made where direct access to the wells is either not possible because of widely dispersed wells or not essential as in the case of gas developments.

Comparative assessment

A quantitative comparison of steel weight and station-keeping system equipment cost as a function of operating payload and water depth assesses the sensitivities of these capital cost drivers for mini-TLP, truss Spar, and semisubmersible production platforms.

Hull and mooring sizing tools, benchmarked with as-built projects, were used to generate hull and deck steel weight data for these mini platforms for topside operating payloads ranging from 3,000 to 6,000 tons and water depths from 4,000 to 8,000 ft, although the comparisons may be extended to deeper water.

The single or multi-column mini-TLP has the lowest steel weight over the range of depths and payloads. In 4,000 ft of water, the tendons are nearly neutrally buoyant and have small impact on hull size. However, in 8,000 ft water depth, hull weight increases significantly to provide the additional buoyancy to support the tendons and provide required restoring force.

The truss Spar has the highest steel weight over the range, driven largely by the need for permanent ballast to provide hydrostatic stability and pitch stiffness and the deep draft with heave plates to minimize heave motions.

The semisubmersible steel weight falls between the two because of the separation of the columns that provide hydrostatic stability and the large pontoons that provide both buoyancy and added mass to ensure that the resonant heave will not occur in storm seas. The hull weight is relatively insensitive to water depth as the higher riser and mooring loads reduce the need for pontoon seawater ballast needed for stability.

Well systems such as subsea trees, manifold, flowlines, risers, and umbilicals are similar for the three platforms. In addition to hull steel and station keeping system, a major cost differentiator between the three platforms is installation, hook-up, and commissioning costs of hull, topsides, and station keeping systems. The small TLP and Spar must have the deck and topsides installed onsite after the hull is hooked-up to its pre-installed tendons or moorings by a heavy lift crane barge followed by final hook-up and commissioning. Additionally, the mini-TLP tendon, tendon foundation, and hull installation requires a crane vessel. The semisubmersible deck and topsides are installed and commissioned in the shipyard or fab yard prior to hook-up to the moorings.

Case study

The following case study describes the preliminary engineering and execution of a small semisubmersible production facility for deepwater Gulf of Mexico. Steel catenary production export risers are used as a basis, although flexible risers could also be used for this case. The objective is to define the hull configuration, structural arrangement, marine, mooring and riser system costs, and a delivery schedule. Topsides and well systems are omitted because they will be similar to the Spar and TLP. These system costs are then compared with a mini-TLP for the same design basis.

The equipment and material cost of the station-keeping systems of the three platform types as a function of water depth for the 3,000-ton payload hulls.

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Constructability, operational simplicity, and separation of the systems required for operating the hull from those required for producing hydrocarbons is prioritized for this study.

A key to developing a cost-effective design is the ability to optimize the hull for a given payload and water depth. With today's computation tools, designers can obtain almost instantaneous estimates of the weight, stability, and motion performance of a given hull form. The level of sophistication that can be incorporated into sizing the platform has never been higher. Motion estimates based on strip theory calculations and parametric optimizing routines may be used to quickly determine the most efficient hull form for a given payload. Experience has shown that such estimates correlate well with more advanced analysis and model testing.

Structural design places an emphasis on simplicity. This is especially advantageous for smaller vessels. The use of flat panel construction is desirable when considering traditional offshore fabricators over shipyards.

Separating the hull marine systems from topsides minimizes the interfaces between these two distinct disciplines. This has the added advantages of expanding the fabrication options for both the hull and deck as well as reducing the hull/ deck integration time.

The four-column, ring-pontoon configuration design is field proven, structurally efficient, and provides excellent motion characteristics for supporting steel catenary risers (SCRs).

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The ring pontoon provides for a robust and efficient hull structure. The deck is comprised of a strongbox structure providing a flat upper deck and a natural lower deck for drains and utilities. Portions of the box may be made watertight for additional damage stability margin if necessary. The box (along with facility equipment) may be set onto the hull in a single lift. The flat top deck facilitates topside arrangement and integration also permits parallel engineering and fabrication of hull and topsides. Together the ring pontoons and box deck provide efficient global strength.

The pontoon layout is designed to minimize and simplify operations related to the marine systems. All personnel access, piping, and cabling in the columns is routed through vertical access shafts. A pump room is located at the base of each column. Most variable ballast tanks are directly adjacent to the pump rooms so that piping and instrumentation to these compartments is actually in the pump rooms.

Detailed mooring analysis for several configurations was undertaken. An eight-leg mooring system comprised of K4 studless anchor chain and jacketed spiral strand wire proved to be the most cost effective configuration for this case.

The base-case riser configuration considers 6-in. production risers and 14-in. export risers hung off the pontoons. An extensive analytical study was undertaken to determine the required motion performance and offset requirements to support either flexible or steel catenary risers of these diameters. Both extreme (strength) and fatigue analyses were performed using time domain techniques. Resulting motion and offset limitations were then fed into the hull and mooring design to develop the complete system. The results indicate that both flexibles and SCRs can be accommodated on a small semi with damage mooring offsets in the neighborhood of 12% of water depth.

The hull and deck of the small semisubmersible may be fabricated together or separately. If fabricated separately, the pre-commissioned deck may be lifted on to the hull as a single piece. All lifting and final hook-up/commissioning may also be performed quayside. The completed platform may be towed out to site and connected to pre-installed moorings.

The four-column ring pontoon configuration design is field proven, structurally efficient, and provides excellent motion characteristics for supporting SCRs.

The installation cost of a small semisubmersible hull, mooring, and riser system was compared to the costs with a single column mini-TLP designed on the same basis. In the analysis the higher installation cost of the deck and station-keeping system offsets the lower hull and deck cost of the mini-TLP; however, the total costs of the two systems are comparable.

The design basis for a case study cost comparison used the low end of the payload and water depth range, which indicated that the costs for the mini-TLP will increase significantly relative to the semisubmersible as water depth and payload increase. From these figures it is also evident that the mini-Spar will cost more than either of these two options over the range of payloads and water depths because of the higher steel weight and more costly installation of topsides and hull.

Selection drivers

When capital cost differentiators are inconclusive, as is for the case presented, an operator must consider other criteria to select the development option that are related to the managing risk and uncertainty.

A strong commercial driver is the use of the platform as a future hub. This will favor the concept that facilitates ease of expansion of topsides and accommodation of future subsea tiebacks. The semisubmersible hull and deck configuration permits considerably simpler field addition of future topside modules and tiebacks risers, which will be hung off the pontoon. The future topsides must be designed into the system at the outset. Incremental costs for doing so will be comparable for all three platforms, but future module installation is simpler for a semisubmersible.

For small fields, cost overruns and schedule delays over sanctioned costs and schedule can rapidly erode the profitability of a project. The Spar and TLP require a sequential installation of hull, deck, and commissioning. There is no chance for recovery from delays in one of these weather-sensitive activities. For the semisubmersible, the deck to hull integration is conducted in a controlled environment. Hookup to preset moorings and risers is rapid, can be conducted in higher seastates and requires a lower day rate marine spread than either the TLP or Spar, significantly reducing risk of schedule delays and cost overruns.

Another concern for operators is the impact of platform response on production operations. Metocean phenomenon of loop and cold core eddy currents are creating vortex-induced vibration (VIV) problems in several mini-TLPs and Spars. Hull VIV motions on Spars are higher than predicted, even with strakes. TLP tendons are also experiencing severe VIV responses, resulting in retrofit of strakes on tendons and the reinforcement of critical hull and deck structural connections. In short, there is still design uncertainty in predicting global and local response of key TLP and Spar platform components. On the other hand, the semisubmersible hull configuration is a proven workhorse that has thousands of years of cumulative operational experience, in every major offshore venue in the world.

Conclusions

A small production semisubmersible is a commercially viable alternative to a mini-TLP or Spar for small deepwater GoM reservoirs that must or can be produced with wet trees. The commercial case becomes stronger for larger payloads and increasing water depths. An additional advantage is the greater adaptability to serve as a future hub and lower cost, schedule, and operational risk.

Acknowledgements

The authors would like to thank KBR and GVA-C for encouraging and supporting the development of new products and technologies that will enable operators extract more oil and gas from the world's small reservoirs than would otherwise be possible.