David Paganie, Managing Editor
The potential impact of ice-loading on drilling and production facilities must be addressed if the industry is going to tap the wealth of reserves hidden in arctic regions. A group of seasoned industry professionals has proposed a unique concept to meet this challenge. The new design called MonoCone Arctic (MCAD) platform was introduced at the 2007 International Conference on Offshore Mechanics and Arctic Engineering (OMAE).
Ice loads, depending on ice thickness, can severely damage drilling and production facilities. The challenge is installing and configuring a facility for areas where ice is prominent for year-round operations. The designers of MCAD, Shawn Ghosh, Kevin Huang, and Frank Chou (FCA International), think they have the solution. Their concept, originally designed for BP, can withstand up to 13,000 tons (11,807 metric tons) of ice-load, a maximum wave height of 45 ft (14 m), and a 6-knot current in the warmer season. It is configured with a 32-point permanent mooring system.
Basic design
MCAD is a cone-shaped floating drilling and production platform designed to withstand ice loads and wave loads in up to 410 ft (125 m) water depth. The platform’s hull has two primary components, a conical structure with a 134-ft (41-m) diameter bottom, and the hull’s base, which measures 240 ft (73 m) diameter.
To minimize ice loads, the upper hull is a cone design with a 1:3 (horizontal:vertical) slope beginning at the water line and angling 82 ft (25 m) down. The hull’s 45-ft-tall (14-m) base is circular to provide buoyancy for the unit’s topsides. A square moonpool is cut vertically through the hull and subsequent base structure to accommodate drilling risers.
The platform’s lower base consists of heavy steel ballast that can be lowered to a calculated depth to adjust the unit’s vertical center of gravity for stability. A 32-point steel mooring system reinforces stability.
Feasibility study
Results of the feasibility study for BP indicate the proposed configuration can support 26,700 tons (24,250 metric tons) of payload. The unit’s deck, hull, and mooring equipment have a combined weight of 20,500 tons (18,597 metric tons). Its operational displacement is approximately 108,844 tons (98,770 metric tons), with a 127-ft (39-m) draft, measured up from the hull’s base. The air gap is set at 40 ft (12 m).
Given these characteristics, 35,000 tons (31,751-metric tons) of ballast is needed for the platform to maintain a reasonable metacentric height for regulated stability requirements. According to the platform designers, the unit stabilizes in a manner similar to a submarine - by maintaining a center of gravity lower than the center of buoyancy. This is accomplished by lowering the platform’s ballast 75 ft (23 m). To maintain this draft at an even keel, 2,000 to 6,000 tons (1,815 to 5,445 metric tons) of water are added to the unit’s lower base through built-in flooding compartments to lower its center of gravity.
Stability analysis
The stability analysis was done with the PCSTAB suite of the Marine Engineering Computer Analysis System (MECAS) software package, developed by FCA International Inc. According to MCAD designers, the tool has been used for a number of facility types including jackups, semsibubmersibles, spars, ships, barges, and single-column floaters. It also has been used as the core software for several on-board operational drilling systems for semisubmersibles, they say.
Loads on the platform have been compared for the winter and warmer seasons. In the warmer season, the MCAD is expected to be subjected to larger waves and higher winds. The wind areas were modeled at the projected windages. The hull and drilling rig have been modeled as cylindrical members of varying diameter to obtain equal wind areas from any direction. The PCSTAB calculates the wind forces on any windage as the force due to wind pressure. The unit’s intact stability computed by PCSTAB, based on 100-knot wind conditions, determined that it meets the requirements by regulatory agencies and classification societies, the designers say.
Motion response
The motion response of MCAD has been computed in the frequency domain using the Federal Research Actions (FEDRAS) suite of MECAS. This program uses 3D diffraction and radiation potential with the Higher Order Boundary Element Method (HOBEM).
Motion responses in both regular and irregular seas were calculated. For regular seas, wave periods ranging between 5 and 40 seconds were used. In the irregular seas calculation, 100-year and 10-year storm conditions were used. The 100-year storm has a significant wave height of 44.6 ft (13.6 m), with a peak wave period of 15 second. The 10-year storm has a significant wave height of 27.1 ft (8.3 m), with a peak wave period of 12.4 seconds. Since the waves are expected to be more severe in the warmer season, motion analysis was done for operations in this season only.
The results for surge Response Amplitude Operators (RAOs) show similar characteristics from a typical semisubmersible drilling unit or spar-type facility. The heave RAOs are higher in comparison to a typical semisubmersible, but are much lower than those of a ship-shaped vessel. The pitch and roll RAOs are very low in comparison to other floating drilling and production platform designs. MCAD’s maximum heave amplitude in 100-year storm conditions is 14.9 ft (4.5 m), with a maximum surge and pitch angle of 21.4 ft (6.5 m) and 0.76 degrees.
Mooring system
The platform’s mooring system is designed to withstand ice loads in the range of 26,000 kips. According to the designers, ice loads are substantially higher than loads from wave, wind, and current in the warmer season. If a mooring system can be designed to sustain ice loads, the system will be more than sufficient against expected loads in the warmer season, they say. Loads from wind and current in the ice season are less than 2% of the ice loads.
The ice loads were calculated based on ice pressure of 200 psi at the water line to 10 ft (3 m) below, and 25 psi for ice flows from 10 to 82 ft (3 to 25 m) below the water line.
The mooring configuration consists of 32 seven-in. (18-cm) diameter, 1,200-ft-long (366-m) R5S Vicinay studless chain lines for 360 ft (110 m) water depth, each weighing 425 lb/ft (in air) with a minimum breaking strength of 6,742 kips.
The mooring fairleads are positioned on the side of the bottom base structure, approximately 85 to 90 ft (26-27 m) below the water line to avoid contact by ice and supply vessels. In the ice environment, the 32-line mooring system has a maximum line tension of 3,583 kips. In live operations, the length of the leeward cable can be adjusted to reduce the line tension. By reducing the pretension of the lines on the leeward side, the maximum line tension can be further reduced.
The MCAD designers say a major concern for mooring systems used in ice-prone environments are the heel and trim angles created by the ice forces combined with the mooring line tensions. The maximum calculated displacement is 14.1 ft (4.3 m) with a trim angel of 1.32 degrees, which is very small, they say.
This is due primarily to the location of the fairleads. The horizontal movement produced by the forces from the ice and mooring line tension is compensated largely by the movement induced by the vertical mooring line tension with its arm at 120 ft (37 m) from the center of the rig. The 120-ft (37-m) righting arm allows the vessel to trim by only 1.32 degrees.
The mooring system is anchored by sixteen 10- to 12-ft-diameter (3 to 4-m) piles, penetrating 150-180 ft (46-55 m) below the mudline. The mooring lines are subject to estimated vertical and horizontal loads of 1,035 kips and 3,430 kips, respectively. The piles are equipped with connecting pad eyes at an elevation of approximately 20 ft (6 m) below the mud line. Suction piles also can be used for this mooring system, the designers say.
Riser system
The MCAD concept was developed primarily for drilling. Production can be added by installing top tension production risers, steel catenary risers (SCRs), and flexible flowlines. The top tension risers (TTR) can be run from the moonpool and arranged in a 4 x 4 configuration. If needed, the moon pool can be widened to accommodate more risers. The MCAD has a maximum heave motion of 16 ft (5 m) in a 100-yr storm, which means a tensioner with 20-ft (6-m) stroke can accommodate the riser travel.
Since the MCAD has a significantly higher displacement than a typical floating production unit, according to the designers, the riser tensioner stiffness has a minimal impact on the floater’s heave response. Therefore, MCAD can carry a large number of TTRs without adversely impacting heave response. Since MCAD has nominal pitch and roll motions, the SCRs can either hang from the moon pool through I-tubes or from the hull’s base perimeter.
Result
This MCAD feasibility study determined that the mono cone-shaped platform with adjustable ballast is a viable option for drilling and production operations in arctic conditions.
One significant advantage of deploying this system, the designers note, is that its simple design can be fabricated almost anywhere, and towed to the installation site in one piece. Once on site, mooring lines are connected and the ballast is lowered. Installation does not require a heavy-lift crane, which saves additional time and money. Overall cost for fabrication and installation is estimated at $500-600 million; other competing concepts with similar payload could cost 20-25% more, the designers say.
According to the designers, although numerical analyses of the feasibility study indicate the performance of the platform is superior to that of other competing concepts, the results should be verified by model tests in arctic and wave basins.
