PART II: This article is the second in a two-part series on hydraulic workover techniques and procedures, specifically on achieving a safer working environment and reducing costs through the use of a modified HWO stackup tower.
A Gulf of Mexico operator contracted workover services for a caisson platform well in the Eugene Island field. The purpose of the workover was to remove the existing production tubing and all downhole equipment. The well had been shut in the previous year because of paraffin buildup in the wellbore.
To further complicate the operation, the well had a concentric string of 1 3/4-in. coiled tubing (CT) installed in the wellbore as a means of gas-lifting the well. Well economics required that the operation be performed with one size of unit. The decision was made to use a 200K hydraulic workover (HWO) unit.
Before the operation could safely begin, a detailed engineering study was performed to determine the caisson's ability to support the cantilever effects of the 200K HWO unit, as well as the transverse wind and wave/current loading that could occur during workover operations.
In this particular operation, the HWO unit was to be stacked directly onto the wellhead, requiring a vertical standing orientation during operations. To support the stack effectively, a means of guying-off the unit was necessary. Since the small footprint of the platform prevented guying the unit using normal procedures, a different method was needed to ensure that the HWO unit remained erect during operations. Also, because the cassion was a single-legged structure, there was concern about the structural integrity and stability of the caisson platform once the unit was fully rigged-up and operating.
There was also concern about the possible cantilever effects of the HWO unit connecting to the caisson, the bending moments from vertical weight loads, and the transverse wind and water current loads. The caisson well was a single drive pipe that extended more than 96 ft above the mudline and more than 40 ft above the waterline.
For the HWO unit to perform the operation, it would have to be rigged up and extended at least another 55 ft above the caisson wellhead. The entire structure and HWO unit would extend more than 130 ft above the mudline, acting as a vertical cantilever beam and directing additional loads and bending moments into the structure. Guying-off the HWO unit also presented problems, since the unit extended above the upper section of the caisson well by another 50 ft or more. After the platform was inspected, personnel suggested that the wind-load tower design be installed instead of outrigger beams.
A guy line wind tower is a stacked scaffolding-type structure that is assembled over a HWO stackup, imparting lateral stability to the HWO stackup without the use of extended guy line cables. Instability can occur because of winds on the stackup and the platform as well as the water current forces on the caisson platform.
The guy line wind tower converts lateral forces imparted by wind loads on the HWO stackup into vertical reactions inside the caisson platform deck. Multiple sets of internal guy line attachments from the HWO stackup are possible because each stacking unit has a guyline attachment eye at each inside corner of the unit.
An Ansys finite element model of a proposed 200K HWO stackup with the guy line wind tower was designed, including the helideck, the welldeck platform, and the caisson structure. Two wind-load studies were performed to determine the effects of wind speeds of 80-100 mph. Later, additional studies were performed to determine the effects on the structure if the gin pole mast and the workbasket were removed from the structure to prepare for adverse weather. The customer was also concerned about the effect of the combined loadings of the structure on a 7 1/16-in. 10M flange on the wellhead stack.
Once this study was completed with satisfactory results, a tower was designed and fabricated that could be attached to the caisson, and could support the unit and any loading effects experienced during operation. The tower was designed for easy transporting and rig up. It was built in sections that could easily be stacked and flanged together on location as the tower is erected above the caisson wellhead.
The HWO unit was mounted within the tower, and guy line attached from the unit to the tower, providing vertical support for the HWO unit. The tower was constructed and fabricated to meet all API and industry requirements. An HWO design engineer and supervisor visited the well site to examine the existing structure and determine how the tower should be attached to the caisson.
Because of the small area around the wellhead, the project called for the use of a self-elevating workover platform vessel (jackup boat) as the primary work area. The deck of the vessel served as the pipe rack, placement for the fluids package, and storage for backup equipment and supplies.
The production tree was removed and the HWO unit rigged up with the recommended blowout preventor (BOP) requirements. The first stage of the operation was carried out as a through-tubing operation during the 1 3/4-in. CT fishing process. The HWO access window was the key component used to eliminate the need for an additional unit. The access window allowed additional stationary slips, accommodating the 1 3/4-in. CT without having to rely on the larger stationary slips to support the CT load.
During the recovery of the CT, the large bore preventors were dressed with the proper ram guides and blocks to accommodate the CT, ensuring that the well could be secured safely if pressure was encountered. Since the well had produced paraffin during past production operations, the chances of trapped pressure did exist, and the CT was tapped every 40 ft to relieve all pressure before each section was cut and layed down. Once the CT was removed from the wellbore, the small stationary slips were removed, BOPs dressed to accommodate the existing production tubing, and the workover operation continued.
Because of the high carbon dioxide partial pressure and the lack of internal coating on the existing production string, a number of holes in the tubing string made it impossible to effectively bullhead fluid into the well and properly kill the well. Once the HWO unit and BOPs were rigged up and tested, tubing pulling operations began. The landing joint was made up into the hanger and attempts were made to pull the seals free at the bottom. New tubing could withstand a maximum pull of 260,000 lb, but because of the carbon dioxide problems, it was decided to chemically cut the tubing above the seals on the bottom to recover the tubing. To use the existing packer and seal bore, the old seals were fished with overshot and hydraulic jars.
After jarring and finally recovering the old seals, the existing packer and seal bore were available for use. The well was completed with 2 7/8-in., 7.9 lb/ft tubing, with a TK-7 coating inside for corrosion protection. All the necessary completion equipment was installed in the tubing string as needed, and the well completed as engineered by the customer. The CK-3 sand was re-perforated upon completion to help establish better continuity with the reservoir, and the well brought online.
The HWO unit and tower helped eliminate formation damage normally caused by kill operations, with enhanced operational safety for personnel. Other benefits included the following:
- The HWO unit size allowed quick rig-up on the small caisson platform.
- High pressure capability provided security in potentially dangerous trapped gas situations.
- Tower design and hookup in this situation reduced applied forces and bending moments on the wellhead flanges.
- Cost savings were achieved through eliminating (1) mobilization and demobilization costs, (2) slow rig-up and testing, and (3) high day rates of a jackup rig.