When EEX asked Cal Dive, Inc. (CDI) for a proposal to decommission its Cooper field in Garden Banks 388, CDI's offer came back as a full turn key. CDI took on the first deep water subsea decommissioning project in the Gulf of Mexico as a fully managed turnkey project.
This means EEX pays one price and CDI handles everything else. Everything includes removal of the field's one-of-a-kind freestanding production riser, a 12-point mooring system, floating production unit, and a variety of subsea equipment.
Before removal of equipment could begin, a number of systems would have to be disconnected, cleaned, and transported from the Cooper Field. The EGB floater was anchored in place by 12 mooring lines made up of chain and spiral stranded wire rope connected to anchors.
There was a master, 24-slot template and two satellite templates. The SA template was positioned 50 ft from the master template on the seafloor and handled three wells. The SB template was placed 3 miles from the main well site and handled another three wells.
There were two 12-in. gathering lines running 52 miles to a fixed bottom platform on Eugene Island 315, where production from Cooper was exported for processing. There also was the 1,800-ft self-tensioning freestanding riser, a variety of jumpers, and flowlines.
Lump sum turnkey
CDI took control of the project after agreeing to a lump sum, turnkey-decommissioning deal with EEX. CDI Senior Vice-President of Deepwater Activities Ken Duell said this was the only way the company could ensure that the price it quoted could be met. By choosing its own sub-contractors CDI was able to work with companies it already knew. These were people who had been working with CDI over the last three years in deepwater.
The first step in decommissioning the field was to kill the seven subsea wells. Once this was accomplished, the riser, riser jumpers, flowline jumpers, production trees, gathering and export pipelines were all pigged back to the EGB and then on to Eugene Island Block 315. The entire production system was flushed clean. This was accomplished using an OSCA twin-pumping unit combined with a Cetco separation system, skid mounted on the EGB and EI 315 platform. The Cetco system separated out waste oils and collected the paraffins.
First challenge - pigging
There was major concern about the quantity of paraffins in all the pipelines, especially the 12-in. gathering pipeline, which ran from the EGB to EI315 during the pigging process. To counteract this threat, a special gel pig was developed by OSCA. As suspected, the first thing to be pigged out of the 12-in. gathering line was a 50-ft paraffin slug.
Duell said this is the first time such a large gel pig had been used especially at this water depth. The Gel Pig was blended together onboard the EGB. It consisted of three separate slugs, each 500 ft long, separated by Styrofoam sponge spacers. The decision to go with a Styrofoam spacer was a further contingency.
If the paraffins began to ball up, this design ensured the pig unit would not become trapped in the pipeline. If the unit got stuck, the operator could over-pressurize the line, collapsing the Styro foam spacer. Without the spacers, the gel pigs would blend in with the seawater-pushing medium and be pumped on to E315 for removal. The gel pig performed better than expected, Duell said, considering the paraffin buildup in the pipelines.
Jumper spoolup
Once the pigging and flushing was accomplished, the project moved to disconnecting and reeling up the Coflexip riser jumpers. Using the DP vessel Witch Queen, CDI reeled up 42 3-in. jumpers, one 8-in gas jumper, and a 12-in. jumper. These were then taken to the CDI facility in Morgan City, Louisiana for storage.
After disconnecting the jumpers, CDI recommissioned the derrick on the production vessel to remove the freestanding riser. The EGB rig was used to remove the internal production strings. This consisted of 42 3-in. strings, one 8-in. string, and one 12-in. string. The rotary table was removed and a specially built, four-finger Cameron riser spider was installed. The riser was now ready for removal.
The controlled process of flooding the external and internal bouancy tanks commenced and the riser load transferred to the rig derrick. The rig hydraulically disconnected the riser and brought it up into the moonpool one joint at a time, using the specially designed spider to hold the riser in place while the individual joints were unbolted.
Riser, mooring removal
The riser had a diameter of 84 in. and each joint was 52 ft long. This was a one-of-a-kind riser system, which is re-usable. The removal pro cess was basically a reverse of the installation, Duell said. The joints of riser and associated connecting systems were then shipped ashore to CDI for storage. With the riser removed, it was possible to disassemble all of the riser handling equipment on the EGB in preparation for disconnecting the floater from the 12-point mooring system.
To remove the mooring system, CDI contracted Aker Maritime. The Aker Seastar was brought on site, combined with CDI's ROV vessel Merlin which had a Perry Workclass ROV unit on board. Duell said the remotely operated vehicle (ROV) hooked into the primary chain, allowing the mooring vessel to recover it onboard and disconnected the spiral strand wire 300 KIP subsurface buoy.
This done, the Seastar backed down to the rig, passing off the rig's chain, then laid the mooring wire rope on the seabed with a deepwater ROV recovery buoy to the end. CDI used this process to disconnect all but four of the mooring lines. The remaining four were needed to keep the vessel on station.
Then, three Harvey Gulf tugs were brought out, two to tow the vessel, and one to tail-steer it. Once the tugs were in place, the floater was ballasted up to transit draft. While the tugs held the vessel in place, the Seastar and Merlin moved in to disconnect the final four mooring lines. Free from its moorings, the floater was towed to Newpark yard in Galveston, Texas.
Bottom recovery
Once the production floater was off station, the Aker Dove moved in to recover the anchors and mooring systems. The recovery of the mooring system was accomplished two lines at a time. This was a difficult procedure, Duell said, because all the spiral strand wire still on the sea floor had to be handled carefully for re-use.
The mooring lines are long and very heavy. They consist of 100 meters of 4.25-in. rig chain, and 1,000 ft of spiral wire attached to a subsurface buoy. On the other end of the buoy was another 1,400 ft of spiral wire connected to 1,000 ft of 4.25-in. ground chain. This chain is connected to a 500-ft. length of spiral wire that runs to the anchor.
Duell said the Aker Dove is one of a small number of vessels that could handle the weight and lengths of wire seen on this job. The Aker Dove also features a 300-ft A-frame to accommodate the large mooring buoys and a built-in 10,000 fsw Workclass ROV.
Abandonment
Once the mooring lines, chains, and anchors are moved offsite, CDI will mobilize the Uncle John to plug and abandon all seven wells. The project is scheduled to take approximately 70 days, using a combination of wireline and a specially designed coiled tubing unit. The trees are Cameron horizontal trees.
CDI's preparation and planning of the well abandonments involved meetings and testing (SIT) between FMC and Cameron to develop a tree and control line crossover system. CDI will use an FMC dual-bore riser unit, which contains a lower riser workover package (LWRP).
Because the entire operation will be conducted on DP, the system also uses an emergency disconnect package system (EDP). Duell said this will be another combination of major industry firsts, including a record water depth, abandonment of horizontal trees, and provides the industry with a base case for horizontal trees and the MMS for further abandonments.
Once the wells are plugged, CDI will pull the subsea trees, remove the SB and SA templates, and wellhead stubs. The main template will be left in place on the seafloor. The flowlines and gathering export lines will be filled with inhibited saltwater and sealed. These lines will also be left in place on the seabed. Duell said all the components recovered from Cooper would be stored and preserved for future use at CDI's facility.
Records
In the course of decommissioning the Cooper Field, CDI set a number of records:
- At 2,100-2,500 ft water depth, this was the first deepwater-decommissioning project ever performed.
- This was also the first time wells had been plugged and abandoned at this depth.
- This is the first time flowlines have been pigged, flushed, and sealed at this depth.
- This is the first time a production template has been recovered from such a depth.
- This was the first time a gel pig train had been used at this depth.
Duell said CDI combined this with fast-track methodology and procedures (within 60 days from time of award to offshore activities).
Contractors
The major sub-contractors engaged by CDI on this project were Schlumberger, which provided the wireline and coiled tubing, and surface well control systems; OSCA, which performed the pigging and flushing operations; Cetco provided the filtration and processing for the pipeline waste; FMC is credited with the dual-bore riser and lower riser work over package; Cameron built the subsea tree interfaces and crossover connectors; Cheyenne (Dietsman UK) provided derrick support personal; Aker Maritime handled the mooring recovery; Gulf Harvey provided the tugboats; Fugro, CDI's alliance partner, provided geology support as well as senior drillers for the Uncle John.
