Guidelines for economic conversion design and methods
After quick conversion, the Gulftide began productin operations in 1995 on the CI-11 field off the Ivory Coast.
The practice of converting jackup drilling rigs to mobile offshore production units (MOPUs) is becoming more common in the oil and gas industry. This option is an economical choice for early production or long-term testing applications. In addition, this strategy helps to minimize field installation costs and can provide a low-cost alternative to platforms for marginal fields.
Installing process equipment on drilling rigs offers a unique set of challenges, involving the following:
- Locating and installing equipment
- Imposing new loads on the rig
- Storing hydrocarbon liquids
- Maintaining gas-free hull ventilation
- Gas venting and flaring
- Installing pipeline risers.
Paragon Engineering has had experience with these issues on two projects: Dresser-Rand's Porta VI and VII Compressor's MOPUs on Lake Maracaibo, Venezuela, and United Meridian Corporation's (UMC) Ivory Coast CI-11 development.
Locating, installing equipment
For safety reasons, all process equipment must be located on the MOPU's deck. If possible, no rig-related utility equipment in the rig's hull should be used when the MOPU is handling oil and gas production.
The removal of drilling equipment from the rig's deck frees considerable deck space. Large production modules can be installed in this area as well as in pipe laydown areas. Smaller equipment packages can be installed along the edges of the deck. Utility equipment packages such as instrument air compressor packages can be installed on top of the quarters. The modules and equipment should be prepackaged to the fullest extent possible to minimize interconnect work at the shipyard.
Rig geometry often severely limits access by dockside cranes; this factor must be taken into account early in design and construction planning efforts. Careful planning and execution can allow large equipment modules to be skidded onto the rig from a transport barge rather than requiring that the modules be lifted onto the deck with an expensive crane barge.
Imposing new loads
Drilling rigs are initially designed for substantial drilling loads that usually are greater than the new production equipment loads. Even with lighter loads, however, conversion to a MOPU requires that the rig be certified by a marine certification agency, such as the American Bureau of Shipping. To reduce conversion costs and simplify the certification process, designers can minimize the structural modifications that will be made to the hull due to the equipment loads.
The simplest way to minimize hull modifications is to take advantage of the load-carrying capabilities of the MOPU's drilling rig skid rails. The modules and other heavy equipment packages should be designed to be supported by these skid rails. In addition, the drill derrick substructure that spans the drill slot should be evaluated for use as a support platform for major equipment. Smaller equipment packages can usually be supported on the rig deck without the need for any internal hull modifications.
Storing hydrocarbon liquids
Careful design allows the conversion of existing hull ballast tanks - which are located in the hull at the end opposite that of the quarters - to tanks that can store oil that will be pumped to a pipeline. This conversion should allow the tanks to operate as ballast tanks while the MOPU is being transported or elevated into position. The design should also allow the tanks to be isolated for oil handling and storage.
A blanket gas system, ideally using an inert gas such as nitrogen and a natural gas backup, should be installed on the tanks to prevent oxygen from being drawn into the tanks when oil is being pumped from the tanks. A primary gas vent system should be installed to prevent tank overpressure when oil is filling the tanks. Secondary vacuum safety valves and emergency vent valves provide safety backups for primary blanket and vent systems.
Gas-free hull ventilation
Maintaining a gas-free hull and quarters while handling hydrocarbons above the deck is of prime importance. Gas detectors must be installed above and below deck to allow for shutting off gas flow, electric power, and burner flames. Gas detectors that are installed on air intake fans for the quarters and hull should shut down the fans upon detection of gas.
Rig hull ventilation equipment must be upgraded to provide adequate ventilation and a slight positive pressure. This positive pressure will ensure that gas will not enter the hull when hatches are opened. To achieve this goal, designers can make the hull air intake fans slightly larger than the exhaust fans. The larger fans will move more air into the hull than the exhaust fans will remove from the hull. Nevertheless, the positive pressure cannot be too great; if it is too high, hatches and bulkhead doors will be difficult - even dangerous - to open and close. At any rate, the hull and intake fans must be relocated to areas that are not likely to contain gas under any conditions.
Gas venting and flaring
Gas vents and flares must be located a safe distance from personnel and equipment and must be arranged in accordance with API RP 521. Typically, a stack tip height that provides an acceptable heat radiation rate for personnel and equipment also provides safe dispersion of gas to the atmosphere if ignition does not occur.
Locating the tip a safe distance away by using a typical offshore-type cantilevered flare boom would require major reinforcement to the hull at the point where the boom would be attached. In addition, this boom may necessitate installation in the field with a crane barge. To avoid this work, the designers can evaluate the length of the rig leg that extends above the deck when the MOPU is elevated to its design position as a support for the vent or flare.
In the Porta MOPU project, sufficient leg extended above the deck to support an 18-in. vent stack that terminated 10 ft above the top of the leg and 150 ft above the deck. In the UMC MOPU project, 30 ft of rig leg extended above the deck. The triangular shape of this rig leg was able to support a 125-ft tall vertical microwave-type flare tower that placed the flare tip 155 ft above the deck. This microwave-type flare tower was erected on site by the tower builders using only the builders' tools and the rig crane.
To prevent flaming liquid carryover from the vertical flare tip onto the MOPU deck, an emergency liquid spillover line was installed on the flare scrubber of the UMC MOPU. This emergency spillover line will route liquid overboard if the scrubber's high level switch fails to initiate a shutdown of incoming flow. The spillover line was terminated 100 ft below sea level, requiring a seawater head of 100 ft to be displaced before any oil is discharged and dispersed into the sea. The remote risk of an oil discharge in this manner was considered acceptable when compared to the increased protection provided to the personnel on the MOPU.
The Gulftide jackup drilling unit is shown being converted for use on UMC's West Africa (Ivory Coast) CI-11 project. The rig was converted in Pascagoula, Mississippi.
Installing pipeline risers
The UMC MOPU was designed to be a fixed production platform that handles oil and gas production from two remote wellhead caissons and one bridge-connected wellhead caisson. Field flow lines from the remote caissons to the MOPU were required, along with gas and oil lines leaving the MOPU. The original plan called for attaching the risers for these lines to the outside legs in the field.
This effort would have been costly and time-consuming, requiring work in 230 ft of water. Therefore, a design was developed to pre-install the risers inside the rig legs at the shipyard, resulting in considerable savings in time and cost. Other risers that were pre-installed in the legs included pipe for the fire water pumps, jockey pump, vent scrubber spillover, and water discharge.
Paragon's experience in the recent UMC and Dresser-Rand MOPU projects has led to the following conclusions:
- When designing and placing large equipment packages and modules, designers should take rig geometry and crane access at the shipyard into consideration.
- Structural modifications to the rig hull can be greatly minimized if designers take advantage of the rig skid rails' capacity to support large equipment modules.
- With careful design work and the use of safety devices, designers can use existing hull storage tanks to store liquid hydrocarbons.
- Below-deck space that must be accessed during production operations must be adequately ventilated with a slight positive pressure to prevent gas from entering the hull space.
- Without the need for modifications to the rig hull, a vent stack or flare tower can be supported by the rig leg section that extends above the deck.
- The flare scrubber can be fitted with an emergency liquid spillover line to minimize the possibility of flaming liquid carryover from the flare tip onto the MOPU deck.
- If the geometry of the rig legs permits, pipeline risers can be pre-installed in the shipyard; this strategy saves field installation cost and time.
Note:This is a summary version of a paper presented by John Barnes and Richard Duhon at the Society of Petroleum Engineers (SPE) Annual Technical Conference & Exhibition in Dallas in October, 1995.
John Barnes has more than 20 years of project management and design engineering experience. At Paragon, he has been involved in the design, fabrication, installation and hookup of offshore structures, production facilities, pipelines and marine terminals.
Richard Duhon has more than 20 years of project management and design engineering experience. At Paragon, he has worked on onshore and offshore compressor stations and has overseen engineering design and procurement on several projects.
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