System expected to remain on station in
100-year storm in South China Sea
Masahiro Nakamura, Kozo Yokokura
Japan Drilling Company
Japan National Oil Company
Deepwater petroleum development is increasing throughout the world, bringing with it a host of engineering and economic considerations. Harsh conditions and deepwater cause proper evaluation of a project to be problematic, meaning that the risks of deepwater development are high, although the rewards correspond.
Economically, deepwater development faces substantial capital costs and lead time prior to first production. As well, uncertainties about ultimate production rates and total reserves increase the risk and proportionately reduce the expected value of a project.
Reducing uncertainties therefore has definite advantages when economically evaluating a project. The use of extended well testing or early production and testing systems (EWT/EPTS) can allow better evaluation or reservoir production characteristics and the most appropriate exploitation concept may be developed and optimized, with possible corresponding decreases in total development cost and initial capital cost outlay. In addition, depending on the length and scale of the EWT/EPTS, early cash flow can, especially in the case of marginal field development, improve a project's finances considerably.
The engineering and design of facilities also has risks. Data about a reservoir is necessary for the optimization of production facilities, but the most useful way to characterize a reservoir is through actual production. A technical solution to this chicken and egg problem is to carry out EWT/EPTS to characterize a reservoir. EWT/EPTS have seen successful applications since the 1970s, with specialized systems such as the Seillean and the Tentech series being constructed in the 1980s.
Depth and environment
From the deepwater standpoint, a critical factor is the mooring system limit on existing floating systems. There are no systems capable of carrying out EWT/EPTS in very deep water, with the current record for a chain and wire mooring system being 1,416 meters for the Ocean America.
For long-term production floating, storage, and offloading (FPSO), the record was set by the Barracuda Field FPSO at a depth of 840 meters. In addition, stationkeeping and offloading is problematic in severe weather, resulting in the need to drive off location.
Thus, two future requirements for deepwater EWT/EPTS systems are defined by depth and environmental conditions. To address these challenges, Japan National Oil Corporation and Japan Drilling Company undertook a series of studies to investigate market conditions, to prove the technical feasibility, and to develop the conceptual design for a system that would allow the economic development of deepwater fields in Asia and Oceania.
The resulting DEPTS design (drilling, early production, testing, and storage) is an independent system capable of taking a field rapidly from the drilling phase through to early production, and includes offloading facilities and onboard storage for production when offloading is not possible. This fifth generation vessel should have clear advantages in the exploration of oil prospects in deepwater areas of Asia and Oceania.
The DEPTS semisubmersible will be able to operate in the harsh environmental conditions of the target areas, and in waters up to 2,000 meters depth. The inclusion of 100,000 bbl of crude oil storage and large amounts of deck space for production and drilling equipment will allow the system to be used in any or all of the drilling, early production, and testing phases of oil field development.
Drilling and production systems on the same vessel allow EWT/EPTS to take place while further drilling is carried out. Oil storage allows continuous production even when shuttle tankers are disconnected or unavailable due to weather conditions.
With Asia and Oceania as the intended area of operations, the severity of the design was for conditions in the South China Sea. With an average field development project assumed to take about five years from initial drilling to full field production, the DEPTS vessel is expected to be able to be on station with no required evacuation of the vessel, and to withstand weather forces for the project duration. Systems were design to withstand a 100-year storm in maximum survival mode, a 50-year storm in survival mode, and a 1-year storm in operations (drilling and production) mode.
For a DEPTS type of project, or any production system that relies on offloading, the duration of storms becomes critical when estimating maximum production rates, and storage capacity. The production and storage capacity was decided from a 5,000 b/d average production for Asia and Oceania. The production capacity of 20,000 b/d and a 5 day required storage capacity resulted in a 100,000 bbl requirement for DEPTS.
As this study assumed relatively harsh environmental conditions for drilling and EPTS, high stability, minimum motion characteristics, and operating depth requirements resulted in the choice of a semisubmersible type vessel. A buoyant motion analysis software simulator was used to obtain the multiple frequency wave RAO function for each possible column design.
Four, five, and six column hull designs were evaluated for stability, mobility, dynamic motion characteristics, total balance, layout, riser characteristics, structural weight, and mooring efficiency. The five column vessel showed the best overall stability, but at a cost of being over 50% more massive than the four or six column vessel. The characteristics of the four column design were virtually as good, resulting in the adoption of the four column design.
This four column and two crossbeam, perfect box structure resulted in a relatively low hydrodynamic load with corresponding load capacity reductions in mooring line diameter and number, as well as the reduction of total steel weight. Another benefit was to allow mooring winches to be placed inside the columns, lower than the center of gravity of the unit.
The mooring system evaluation was done with the assumption that loss of one mooring line should not hamper operations. Modified quasi-static catenary and lumped mass analysis methods were used to design mooring systems. A thruster-assisted, conventional mooring system was selected. This system was shown capable of holding position at the maximum operating depth of 2,000 meters in the worst case storm.
No analysis was carried out on estimated mooring operation time or fuel requirements, but the combination of the two different types of mooring systems should provide for an optimization of both. Depending on site conditions, either system could be used independently. The system, as designed, uses 12 mooring lines, consisting of wire (122 mm by 3,000 meters), and chain (100 mm, K4 by 1,400 m). Assisting these are four azimuthing thrusters, each with a 3.6 MW power rating.
Weather factors and expected production rates required a 100,000 bbl crude oil storage volume as estimated above. Three possibilities were considered for placement of the storage tanks:
- Lower hulls in only the center
- Over the entire length of the entire hull
- Inside the corner columns.
The three cases were examined for the effects on stability, ease of offloading, and required structure size. For the DEPTS, the most efficient placement of the storage tanks was found to be inside the columns, with placement such as to minimize potential damage from collision.
The deck was designed to accommodate the latest generation of drilling equipment including a 3,000 hp drawworks, three 1,600 hp mud pumps, a fully automatic pipe handling system, an automated solids control system, and 15,000 psi blowout preventers (BOP). Production and testing equipment was designed to handle 20,000 b/d of production. Deck layout was set up for optimized, fully automatic drilling, testing, and production operations to be able to function simultaneously.
Structurally, the simple box layout allows straightforward module replacement and/or exchange. Equipment such as riser, drillpipe, casing, and production equipment and gas injection systems was placed on the upper deck. Solids control and BOP systems were placed in the middle deck, and mud pumps placed near the stern. Utilities and generators were placed at the bow. For operational safety reasons, the helideck, flare boom and burner arms were set on the four corners of the vessel. The lower deck was devoted to environmental protection equipment such as waste handling and oil/water separation.
The offloading system consists of three subsystems: flowlines from the semisubmersible to the ocean floor to the transfer point, the transfer point itself, and the shuttle tankers. Three possibilities were considered:
- SALM type single point mooring with a subsurface flowline
- DPS shuttle tanker and a floating hose
- DPS shuttle tanker and submerged turret loading.
On the basis of offloading equipment requirements, cost, and operational characteristics in deep water, the DPS shuttle tanker and a floating hose option was the most appropriate.
The DEPTS design project was begun in 1993, and a variety of studies on systems have been carried out by the companies concerned and other international subcontractors since that time. At all times, proven, commercial technology was used in order to obtain a fully realizable system. This resulted in a basic estimated cost of the DEPTS unit of US$ 345 million.
The latest available advances in system technologies may be adopted in the next level of design. The use of these technologies is expected to enhance the economic feasibility of constructing the system, but by how much is difficult to predict at this point.
As well, the final overall system optimization, from hull to crown block has not yet been carried out, and cannot be carried out at this stage of the conceptualization / design process. This too would have the effect of lowering the construction costs, and enhancing the economic feasibility of the system.
Economic case studies were performed for DEPTS operations in different areas of the world for comparison purposes, although the intended range of operations is primarily Asia and Oceania. For illustration purposes, only two cases are presented, with currency in US dollars. Operations were assumed to allow the drilling, completion, and production of three wells at 15,000 b/d by the start of the second year.
Well count: 3 wells Production rate: 5,000 b/d/well EWT/EPTS period: Two years DEPTS day rate: $128,000/day Drilling and Completion: 90 days Drilling OPEX: $30,000 Logistics: $30,000/d Shuttle tanker cost: $1/bbl Subsea completion cost: $1.5 million/well Flexible riser cost: $1.5 million/well Oil Price: $18/bbl Total Revenue: $197 million Total Expenses: $172 million
Deepwater MODU rate: $80,000/d FSO day rate: $58,000/d Shuttle tanker offloading: $0.50/bbl Total Revenue: $197 million Total Expenses: $178 million
The DEPTS is the more economically advantageous of the two cases, showing a clear dollar advantage at the end of the second year. Longer term projects will increase the comparative advantage of the DEPTS.
The DEPTS concept, based on a semisubmersible drilling rig design with the functions of EWT/EPTS, and with 100,000 bbl oil storage and 2,000 meters water depth capability, is feasible from an engineering standpoint. The determined optimum variable deck load of 8,000 tonnes and 14,000 tonnes of oil permits the installation of the latest drilling and production equipment.
Design comparisons showed that a four column twin lower hull with a thruster assisted mooring system was the most suitable for usage in 2,000 meters water depth. The DEPTS mooring system especially has been shown to be capable of operating in extremely harsh environments of the target areas.
The economic case comparison showed the advantage of using a DEPTS type system over a combination of a specialized drilling vessel with an FSO. This advantage depends on many factors, but shows that a market does exist for a DEPTS type unit. As well, although this system is equipped with facilities for different stages of exploration and development, it can also be used for a single phase of an exploration project, or in the phased development of a project from drilling through to the EWT/EPTS stages.
This multifunctionality will widen the market niche of the system, lowering any risks due to fluctuating demand. The resulting expectation is that this system's construction can be considered to be competitive at this time.
Acknowledgment: The authors would like to thank the personnel of the Japan Drilling Company and the Japan National Oil Corporation for their support and assistance with this DEPTS project. Thanks also to GVA Consultants, Natco Japan Co., I.D.E.A.S. Inc. and the drilling equipment makers for their assistance in providing engineering and data.
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