During a recent luncheon hosted by the Marine Technology Society, John Greeves, the technical director at Versabar, addressed the company’s deck-raising methods and how they can help prolong the working life of offshore platforms.
Over the life of an offshore production facility, a decrease in reservoir pressure may result in compaction and consolidation of the reservoir rock. Production from the reservoir decreases fluid pressure in the pore spaces and increases the stress on the rock formation. The increased stress can result in compaction of the reservoir. The compaction, in turn, may lead to seabed subsidence. A consequence of subsidence is the reduction of the air gap between the average sea level and the deck of a fixed offshore platform.
“The reservoir pressure acts as a spring for the platform,” Greeves said. “Decreases in the pressure can result in significant displacements over time. Maintaining an adequate air gap is very important to prevent waves from hitting the platform.”
A decreased air gap puts the platform and personnel at greater risk in extreme storm conditions. If the deck becomes inundated by waves, the topsides equipment may be washed overboard or destroyed. The platform’s structural reliability is also put at risk.
Platforms built from 1946-1948 in the US Gulf of Mexico (GOM) typically had deck heights 20 ft to 40 ft above the mean sea level, according to the Interagency Ocean Observation Committee. Following Category 3 (Betsy, 1965) and Category 4 (Hilde, 1964) hurricanes and the resultant destruction of 21 platforms and the disappearance of Zapata’s Maverick oil rig, the American Petroleum Institute’s Committee on Standardization of Offshore Structures was created in 1966 to develop better design standards through industry cooperative efforts. Today, the standard deck height is about 65 ft.
The causes of decreased air gaps include the following:
- Old codes were used in the original platform design
- Seabed subsidence has occurred
- Design waves have become larger—as in the GOM after the 2004 (Ivan) and 2005 (Katrina and Rita) hurricanes
Greeves said that the options for remediation of a reduced air gap include wave load reduction by removing the unused conductors and marine growth; strengthening of local members of the platform with grouting, clamps, or the addition of members; reinforcement of the foundation by retrofitting skirt piles; and elevation of the platform’s topsides.
Greeve described the company’s first deck-raising project in 2006. Following hurricanes Katrina and Rita, two of Devon Energy’s platforms were damaged and had reduced air gaps due to seabed subsidence. Each topside was raised 15 ft using 32 synchronously controlled hydraulic cylinders, each with a 26-ton capacity.
The technology includes the use of split sleeves to fully encapsulate the deck legs to provide lateral stability for the topsides during jacking. The sleeves become permanent leg extensions to support the topsides at the new elevation. A leg pin connection keeps the platform storm-safe during the cutting and welding process. Custom-engineered hydraulic power units, designed with redundancy (two engines, two fuel tanks) are used to operate the rams.
The deck-raising system was designed with synchronous and manual control modes. The primary mode was synchronous and used a computer system to send signals to the flow-control valves of the rams. Computer monitors observed the raising procedure, including leg displacements, cylinder pressures, and hydraulic oil temperatures. In manual mode, operators adjust the flow control valves for each leg.
The company’s deck-raising system was used to simultaneously lift a bridge-linked platform complex off the coast of Jakarta, Indonesia, in September 2013. The LIMA flow station, operated by Pertamina Hulu Energi Offshore NorthWest Java (PHE ONWJ), comprised six structures: staff quarters, production, compression platforms, two flare bridge supports, and a bridge support on the wellhead platform (combined weight of 3,300 tons). Subsidence had reduced the facility’s air gap over the years and PHE ONWJ was concerned that platform safety could be compromised before the planned end of the station’s life in 2026. The complex needed to be raised 13 ft to restore adequate air gap.
After evaluating options, such as conventional lifting, construction of new platforms, use of a mobile offshore production unit, and the use of a float-over barge or hydraulic jacking system for deck-raising, the operator concluded that hydraulic jacking offered the best solution when also considering technical feasibility, shutdown period, project schedule, and risks.
The LIMA project was the first to use Versabar’s synchronized hydraulic jacking system with a programmable logic controller combined with encapsulated leg sleeves.
Because the existing air gap did not provide enough clearance for installation of rams long enough to perform the entire lift, the deck-raising process needed to be broken down into multiple stages.
Six 400-hp hydraulic power units (HPUs) weighing 20 tons each were used with 108 rams. Because space was limited on the platforms, the HPUs were placed on the bridges between the platforms.
In Stage 1, the first set of rams raised the platforms 38.5 in. In Stage 2, after the legs pins were inserted, a set of dual-rod rams raised the platforms to 106 in. After pinning off, the dual-rod rams lifted the platforms to 159.5 in.
The offshore schedule beginning with the rigging up for Stage 1 through recommissioning of the platforms required about 4 months. Greeves said the cost of 2the deck raising was about USD 200 million, compared with an estimated cost of USD 1.5 billion to rebuild the platforms.
Pam Boschee is the Senior Editor for Oil and Gas Facilities.