Surface wellhead equipment modified for subsea well control
Jeff Lane, Wood Group Pressure Control
After Gulf of Mexico hurricanes Katrina and Rita, response teams using weld-less surface connection technology created diver-assisted methods for reconnecting to hurricane-damaged conductor pipe. The equipment design provided a testable seal between the temporary “wet” wellhead, the conductor, and the surface casing, along with the ability to attach the wellhead to the surface so that a proper downhole pressure test could be conducted. If necessary, cementing equipment mounted on a work platform could be used for P&A operations. The technology has been used to control hundreds of hurricane-damaged wells and will continue until all damaged wells are secure.
The Gulf of Mexico, one of the nation’s largest sources of oil and gas production, was dealt a one-two punch within a four-week period in August and September of 2005. Wind and waves from two major hurricanes, Katrina and Rita, caused wide-spread destruction. The US Minerals Management Service estimated that 3,050 of the Gulf’s 4,000 platforms were in the direct paths of these two storms. No lives were lost, nor were there any significant oil spills from wells on the outer continental shelf attributed to either storm.
Hurricane Katrina, a Category 5 hurricane when it entered the OCS, destroyed 46 platforms and damaged 20 others. Hurricane Rita, a Category 4 hurricane when it entered the OCS, with its more westward path destroyed 69 platforms and damaged 32. The damage to these platforms resulted in an estimated 1,000 wells out of commission. Most were in water depths of less than 400 ft (122 m), and almost all would have to be plugged and abandoned.
In many cases, the platforms and associated wellheads collapsed to the ocean floor and were damaged beyond repair. Mangled casing strings were entwined with substructure, making reentry into the wells problematic. As is industry practice, most wells had a subsurface safety valve (SSSV) below the mudline to protect from leaks. However, some valves were damaged, and others were not expected to maintain control. These wells had to be tested to see if the SSSV was working, and then safely and quickly abandoned without leaks.
The de-completion system undergoing testing at WGPC.
In 2004, Hurricane Ivan did less extensive damage to fewer wells, but the storm’s impact stimulated discussion at Wood Group Pressure Control (WGPC) about using existing surface technology in a subsea well control application. After Katrina, the discussions turned into action and before Rita had been rated a tropical storm, WGPC was working with several GoM operators and service companies to create an efficient, low-cost, diver-assisted “de-completion” solution for damaged offshore wells.
Modified equipment
Over three months, WGPC had modified weld-less surface wellhead technology for subsea de-completion operations. The equipment design was prototyped, lab tested, and ready for service using special subsea installation procedures. The design incorporated a testable seal between the temporary “wet” wellhead, the conductor and the surface casing; and would employ diver-assist to reconnect to the damaged conductor pipe.
The modified surface equipment included weld-less connector options, conventional wellhead housings (with full-bore access, 7-in. to 13 5/8-in. [17.77 to 34.6 cm] casing), casing guides instead of casing hangers, conventional secondary seal options, multiple tubing suspension options (single, dual, triple), tubing head adapters, continuous control lines, and a control panel.
Work procedures
Because of extensive subsea infrastructure damage, much work was planned and executed prior to the modified surface control equipment being placed over a well. Due to the proximity of many wells, use of ROVs on underwater tasks was not feasible. So, most of the underwater work was done by divers. Work vessels employing surface cranes and usually equipped with dynamic positioning control assisted the divers.
The first task was to remove or stabilize underwater debris as much as possible. Some debris would be cleared aside and positioned to create man-made reefs. Sometimes the debris was removed and “stacked” for later retrieval and transport to land salvage/disposal facilities.
Following removal of the damaged casing and establishment of a “close to round” casing point, the control equipment was stabbed over the remaining casing stub.
Once the area around the well was cleared, the divers could examine the conductor and then cut the casing below the crimp/collapse point with OD casing cutters. This left the remaining stub “close to round,” making stab-over easier. The damaged section of the casing usually was slit vertically to facilitate removal. Divers, assisted by the surface equipment operators, then removed the excess casing leaving a “wedding-cake”-like structure.
At this point, a pressure containment system would be connected to the conductor to allow testing and/or removal of the SSSV, and to provide a conduit for permanently sealing the wellbore in subsequent P&A operations, if necessary. These procedures ensured that pressure would not escape and release hydrocarbons into the Gulf waters. The equipment used for testing and plug cementing was retrievable and could be used on other installations.
The 2007 Atlantic hurricane season officially started June 1 and will last until Nov. 30. These dates represent the period of each year when most tropical cyclones form in the Atlantic basin.
Forecasts of hurricane activity are issued before each hurricane season by research experts at Colorado State University and separately by the National Oceanic and Atmospheric Administration (NOAA). The Colorado State experts define the average number of storms per season (1950 to 2000) as 9.6 tropical storms, 5.9 hurricanes, and 2.3 major hurricanes (i.e., storms exceeding Category 3 strength on the Saffir-Simpson Hurricane Scale). The NOAA defines a normal season as six to 14 named storms, with four to eight of those reaching hurricane strength, and one to three reaching major hurricane status.
A diver, at center, cuts damaged casing using OD casing cutter. At left is a WGPC de-completion system already installed on a nearby well.
On April 3, 2007, Colorado State issued a forecast predicting a very active hurricane season of 17 named storms, nine hurricanes, and five intense hurricanes. The potential for at least one major hurricane impacting the U.S. this year was estimated to be 74%, with the chances for a major storm in the Gulf of Mexico estimated at 49%. The NOAA recently said it expects a busy hurricane season, forecasting 13 to 17 tropical storms, up to 10 of which could become hurricanes.
