Delmar system balances short, long lines
Jaime Kammerzell
Gulf of Mexico Editor
The industry-accepted way to moor a drilling rig requires eight or more equal lengths of mooring lines around the vessel. Most of the industry believes that if a short line is needed to avoid a pipeline or other seabed architecture, the same length line must be installed on the opposite side to balance the rig's mooring system, even if there is no pipeline or other obstruction.
Delmar Systems Inc., an offshore anchoring and mooring company headquartered in Houston, is rethinking the balancing methods of a mooring system. Delmar's philosophy is to engineer different configurations of mooring lines, some long catenary lines with drag embedment anchors, some short catenary lines with added weight, and some specially configured short taut-leg lines with suction anchors. When the rig offsets and moves, the short line is designed to pick up about the same load as the long line through a specific band of tension. Essentially, the preset line is engineered to mimic the performance of a standard adjacent line, allowing it to adequately load-share between the two lines.
A hybrid mooring system limits the scope to one side of a rig's mooring pattern, which is made up of standard rig deployed catenary systems and short-scope taut or catenary leg systems. When the rig moves, the catenary line picks up the chain from the seabed, but the weight of the chain pulls the rig back on station. This is called the restoring force of the particular mooring line. A pure or standard taut-leg pre-set will pick up the chain faster than the standard rig catenary line. To make the hybrid mooring system work, the taut-leg system needs to be softer or slow down its tension versus offset.
"With the hybrid system, the long catenary line will pick up load at a certain rate, and the taut-leg will pick up load at a completely different rate," Matt Smith, Delmar vice president of operations, says.
Beyond load-offset performance of the lines, the wave-frequency vessel motions must be considered as well. Standard catenary lines accept a great deal of vessel wave frequency motions without significant fluctuations in tension. Taut-legs, especially in water depths less than 5,000 ft, can exhibit large fluctuations in tensions due to wave frequency vessel motions.
To combat these tension fluctuations, the engineered systems use several tools to dampen the preset line. Buoys the size of a boxcar can be added to the line to create the damping effect. When the line with the buoy moves, it drags a lot of water around it, which creates additional resistance, absorbing the tension fluctuations. Even though the shorter taut-leg is picked up a shorter distance overall, the force of the buoy and additional chain or weight added at the bottom of the line tries to match the behavior of the long catenary line on the adjacent or opposite line on the other side of the rig.
Delmar's hybrid mooring system allowed Diamond Offshore's Ocean Star and Ocean Victory semisubmersibles to moor independently 2,600 ft apart, with overlapping mooring lines. The two rigs were moored using a combination of suction piles and drag embedment anchors. (Image created by Manuel Castro, Delmar Systems Inc.)
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Delmar used its hybrid mooring system to accommodate Diamond Offshore's Ocean Star and Ocean Victory semisubmersibles in the Gulf of Mexico East Breaks blocks 646 and 645, respectively.
Two different operators wanted to drill their wells in 3,800 ft of water, with the rigs only about 2,600 ft apart. The companies wanted independent mobilization and demobilization of their rigs with no impact to the other. The rigs would have mooring lines that cross each other, but the mooring lines would have to maintain clearance with each other even in storm conditions. This added further complication, because one rig's leeward line drops in tension, while the other rig's line, which crosses the leeward line, is a windward line increasing tension. Delmar also had to avoid pipelines and existing wells.
Delmar brought in two pre-sets for each rig before they came on location. The Ocean Victory came in first and left first, but its three pre-set lines had to be left in place until the Ocean Star came off location. Delmar set the pre-sets because lines from both rigs had to cross. The scope of one Ocean Star line was about 3,300 ft and the Ocean Victory had a 3,500 ft, 3,750 ft, and 6,500 ft pre-set line.
In the GoM, because of the number of installed pipeline systems, mooring systems compete with dynamically positioned vessels in congested fields. It is not uncommon to have two operators drill at the same time, or for one operator to work its own field with multiple drilling rigs. Hybrid mooring systems allow the operators more flexibility in rig selection. The hybrid mooring system can be used in both deepwater and shallow water, from 500 ft to 5,500 ft.
