This section addresses possible means to vertically re-enter a blowing wellbore and the factors influencing surface and subsea operations. The following general definitions will be used for water
depths relative to well control:
Vertical intervention is one of several techniques proposed and defined to handle subsea blowouts by means of vessel operations at the sea surface. These operations are used to control a blowout from a floating vessel operating over the centerline of the blowout (see Figure below).
See also: Deepwater Drilling Emergency Response Plan
The fundamentals of vertical intervention from a dynamically positioned vessel and the techniques employed to control the blowout from this position are presented in the following sections.
Increased water depth impacts blowouts in several areas including the following benefits (+) and hazards (-):
have the problem severity of tidal surface anomalies as seen in Southeast Asia. Mooring and riser problems have been documented over the years due to these adverse conditions. Surface wind and wave actions can also affect the station-keeping ability of the vessel.
Subsurface currents may increase as water depths become greater. The stronger currents make vertical intervention tool selection more complex and demand stability when intersecting a blowing well. Visibility problems can also be associated with current conditions due to pollution effluent hindering ROV and vertical intervention operations.
Outward radial currents associated with the boil at surface may affect mooring. It has a particularly adverse effect on ship-shape rigs. These currents are easily manageable with semi-submersible vessels if proper techniques are employed.
Ship observations
Ship observations are the initial source for weather information. They record wind, waves, current, and temperature every six (6) hours. The information is radioed and mailed into the agency having jurisdiction over that particular area. Summarized information is put into tables for general publication. This data set is occasionally unreliable due to unreported events and overestimations.
Hindcast studies
Hindcast studies are similar to weather forecasts. They are based on past information rather that short term predictions. The Spectral Ocean Wave Model (SOWM) is a Navy computer program developed to calculate weather conditions at designated grid points covering the northern hemisphere. These calculations are based on current and past weather data and averaged over a period of time.
Measurement
Measurement is the most reliable, but least available, means to forecast weather. Current meters, anemometers, and wave rider buoys are utilized to predict weather patterns in various geographical locations. During sensitive phases of any well control operation, weather is a factor. Vertical intervention, relief well intercept, or crane barge operations all depend on good weather windows. If compromises are made and operations are conducted in less than desirable conditions, injuries or structural damage may result.
Weather forecasts should be plotted and amended on a daily basis during the duration of the well control operation. All vertical intervention, kill plans and/or critical operations need a clear weather window prior to initiating operations.
Chain or a chain/cable combination currently moors a majority of the floating drilling rigs operating in water depths up to 6,500 ft. Ultra deepwater vessels are dynamically positioned (DP). The basic objective for position-keeping of a free-floating vessel is to maintain the drill pipe and riser in a nearvertical position over the hole.
The environmental forces generally associated with this task are wind, ocean currents, and wave action. The most significant environmental force is wind. Wind varies with speed and direction on a continual basis. The data gathered from the monitoring system is input in the DP computer. These calculations utilize the shape of the vessel and the wind drag characteristics of the rig.
Total forces exerted by the wind are calculated and the required power distribution is sent to the thrusters for station-keeping. Well control operations may be suspended if vessel positioning is altered. High wind and surface currents demand stringent station keeping ability.
Acoustic-based DP systems do not function in gas-aerated water. If a DP vessel is utilized where a gas boil is present at surface, thruster efficiency is also reduced due to the aeration of the water. Some vessels have shallow suction headers for main engine salt water cooling systems that will also be affected.
Due to the high activity level, industry has extended delivery schedules that make auxiliary equipment such as ram preventers and subsurface equipment a scarce commodity. Most currently available, rental equipment may not be suitable for a deepwater event.
Equipment compatibility issues need immediate attention. Manufacturers currently do not have standards for equipment compatibility. This could prove to be a major stumbling block in developing sound procedures for handling deepwater events. Items such as wellhead equipment are unique by manufacturer and can have long
delivery lead times.
Proposed kill guidelines should be focused, initially, on techniques unrelated to BOP equipment on the well. This approach avoids the issues of equipment compatibility. Secondary kill guidelines can focus on possible control techniques with existing or new BOP equipment.
As a result, kill techniques should be developed that require little or no visibility at the mudline. Sour fluids should be considered and evaluated on a caseby-case basis. Techniques have been developed that allow safe, working operations in high sour gas concentrations.
Computer modeling will aid with estimations on where possible gas boils may break at surface due to subsurface currents and surface wind conditions. The unknown elements will serve as key points prior to any site selection for a relief well or vertical intervention rig. These systems must be developed independent of the equipment type remaining on the well.
ROV intervention may aid in the following areas:
With technology changing in the deepwater arenas, ROVs have not made many significant advances to aid with floating vessel blowout control. New designs are limited to additional horsepower and larger payload capabilities.
See also: Introduction Drill Emergency Response Deepwater Well
Vertical intervention tools require designs for three fundamental areas:
Guidance design criteria are as follows:
Reentry tools strings can include the following items run individually or combined:
Mechanical packers
Mechanical set packers may play a role in vertical intervention if access to the well can be achieved. This packer type is rigid and has two sets of slips energized by rotation.
The tool has setting limitations due to size and weight criteria of casing strings. If casing has not been damaged due to abrasive flow or mechanical means, the mechanical packer may be used as a plug to stop or reduce formation fluid flow exiting from the BOPs while relief well operations are ongoing, or damaged subsurface equipment is being repaired.
Other limitations include setting procedures for the packer. Rotation is required which may cause damaging actions on the pipe from the seafloor to the rig. Also, the time required for the packer elements to fully expand may allow erosion of rubber elements from blowout fluids.
If the vertical intervention assembly is capable of passing through the BOPs and can be successfully set downhole, the damaged equipment on the seafloor may be retrievable, thus allowing repair and re-use.
Inflatable packers
Some well control operations have utilized inflatable packers for controlling wells on and offshore. Some inflatable packers can seat in large diameter sizes. However, inflatable packers have reduced differential pressure capability the more they are expanded.
Vertical intervention methods may utilize inflatable packers to control several casing sizes. In a situation of a listing BOP, small diameter tools may have the ability to pass through the BOPs and be set in casing. Since this packer style does not have any external slip assemblies as seen on the mechanical type packers, the potential of hanging-up on ram cavities is reduced.
An advantage to an inflatable packer is the setting speed. It can be activated with a dropped ball, which quickly inflates the rubber. This will minimize flow erosion.
Stingers
Stinging blowing wells has been a common practice for many years. Operations in the Kuwait fires utilized this well control method to control over 225 wells. This method can be utilized on wells that do not have substantial volumes of exiting fluids.
For deepwater efforts, seawater hydrostatic may aid in stinging operations. Since many deepwater scenarios may involve large casing sizes, use of a stinger may be restricted due to the upward force created with the stinger in the flow path.
Knuckle joints
Knuckle joints can be utilized as part of the vertical intervention kill string. They assist in enabling passage through the BOP opening or wellhead. Strategically placed throughout the drillstring, these joints can aid with entry and allow the drillstring to move through the passageway without being hung up due to the rigid nature of the assembly.
Vertical intervention kill strings may have knuckle joints as a primary bending point(s) in the bottom hole assembly (BHA). Each BHA may change depending on the blowout conditions at the time of entry.
depths relative to well control:
- Conventional 1,000 ft. - 3,000 ft.
- Deepwater 3,000 ft. - 6,500 ft.
- Ultra deepwater 6,500 ft - 10,000 ft
Vertical intervention is one of several techniques proposed and defined to handle subsea blowouts by means of vessel operations at the sea surface. These operations are used to control a blowout from a floating vessel operating over the centerline of the blowout (see Figure below).
See also: Deepwater Drilling Emergency Response Plan
 |
| Rig over gas boil. |
The fundamentals of vertical intervention from a dynamically positioned vessel and the techniques employed to control the blowout from this position are presented in the following sections.
Factors Influencing Vertical Intervention Methods
Problems existing in deepwater operations may not have the same impact in shallower water depths. Likewise, many substantial problem areas in shallower water blowout control have minimum impact on deepwater events. Problems associated with vertical intervention techniques stem from several areas:- Water depth
- Surface/subsurface currents
- Weather considerations
- Vessel positioning
- Equipment and compatibility
- Blowout effluent(s)
- Surface fire
- Blowout rate
- ROV capability
Water Depth
Water depth plays a role in shallow water well control operations. This is due to hydrocarbons, particularly gas, reaching surface in the form of a boil. In a deepwater well control situation with release of hydrocarbons from the well at the sea floor, it is expected that the surface release would be some distance from the rig due to dispersion and currents.Increased water depth impacts blowouts in several areas including the following benefits (+) and hazards (-):
- + Seawater hydrostatic creates backpressure on the well that affects flow rates.
- + Seawater may act as an H2S scrubber.
- + Disperses wellbore effluent away from well control operations.
- + May aid with relief well operations.
- + Safer working environment for personnel associated with surface operations.
- - Water intrusion into the blowout plume resulting from density defects dilute effluents and increase surface boil radius. Water intrusion into gas plumes is usually substantial.
- - Passive bridging is inhibited.
- - Well control operations are hampered by the distances and extreme forces associated with a deepwater flow.
Well Control Impact of Surface/Subsurface Currents
Surface currents associated with drilling operations vary from area to area. The Gulf of Mexico (GOM) does nothave the problem severity of tidal surface anomalies as seen in Southeast Asia. Mooring and riser problems have been documented over the years due to these adverse conditions. Surface wind and wave actions can also affect the station-keeping ability of the vessel.
Subsurface currents may increase as water depths become greater. The stronger currents make vertical intervention tool selection more complex and demand stability when intersecting a blowing well. Visibility problems can also be associated with current conditions due to pollution effluent hindering ROV and vertical intervention operations.
Outward radial currents associated with the boil at surface may affect mooring. It has a particularly adverse effect on ship-shape rigs. These currents are easily manageable with semi-submersible vessels if proper techniques are employed.
Weather Considerations
Weather forecasts are crucial in the planning stages of floating vessel blowout control. Operations hinge on good weather windows for safe working operations. Industry utilizes three sources to obtain weather information:- ship observations
- hindcast studies
- measurements
Ship observations
Ship observations are the initial source for weather information. They record wind, waves, current, and temperature every six (6) hours. The information is radioed and mailed into the agency having jurisdiction over that particular area. Summarized information is put into tables for general publication. This data set is occasionally unreliable due to unreported events and overestimations.
Hindcast studies
Hindcast studies are similar to weather forecasts. They are based on past information rather that short term predictions. The Spectral Ocean Wave Model (SOWM) is a Navy computer program developed to calculate weather conditions at designated grid points covering the northern hemisphere. These calculations are based on current and past weather data and averaged over a period of time.
Measurement
Measurement is the most reliable, but least available, means to forecast weather. Current meters, anemometers, and wave rider buoys are utilized to predict weather patterns in various geographical locations. During sensitive phases of any well control operation, weather is a factor. Vertical intervention, relief well intercept, or crane barge operations all depend on good weather windows. If compromises are made and operations are conducted in less than desirable conditions, injuries or structural damage may result.
Weather forecasts should be plotted and amended on a daily basis during the duration of the well control operation. All vertical intervention, kill plans and/or critical operations need a clear weather window prior to initiating operations.
Vessel Positioning
Three basic methods exist for positioning a floating vessel:- chain
- chain/cable
- dynamic positioning (DP)
Chain or a chain/cable combination currently moors a majority of the floating drilling rigs operating in water depths up to 6,500 ft. Ultra deepwater vessels are dynamically positioned (DP). The basic objective for position-keeping of a free-floating vessel is to maintain the drill pipe and riser in a nearvertical position over the hole.
The environmental forces generally associated with this task are wind, ocean currents, and wave action. The most significant environmental force is wind. Wind varies with speed and direction on a continual basis. The data gathered from the monitoring system is input in the DP computer. These calculations utilize the shape of the vessel and the wind drag characteristics of the rig.
Total forces exerted by the wind are calculated and the required power distribution is sent to the thrusters for station-keeping. Well control operations may be suspended if vessel positioning is altered. High wind and surface currents demand stringent station keeping ability.
Acoustic-based DP systems do not function in gas-aerated water. If a DP vessel is utilized where a gas boil is present at surface, thruster efficiency is also reduced due to the aeration of the water. Some vessels have shallow suction headers for main engine salt water cooling systems that will also be affected.
Equipment Availability and Equipment Compatibility
The first step after any blowout is a site evaluation. Once the initial site evaluation and rig inspection (if applicable) has been conducted, identification of useable equipment and compatibility needs to be addressed. If the BOPs or riser were lost, an equipment search may be required to locate replacements.Due to the high activity level, industry has extended delivery schedules that make auxiliary equipment such as ram preventers and subsurface equipment a scarce commodity. Most currently available, rental equipment may not be suitable for a deepwater event.
- Few 18.75 in., 10m, or 15m BOPs are readily available in the event of an emergency and integration with existing BOP control systems on the rig would pose additional difficulties.
- Riser availability is even scarcer, with delivery schedules reaching 24 month waiting periods.
Equipment compatibility issues need immediate attention. Manufacturers currently do not have standards for equipment compatibility. This could prove to be a major stumbling block in developing sound procedures for handling deepwater events. Items such as wellhead equipment are unique by manufacturer and can have long
delivery lead times.
Proposed kill guidelines should be focused, initially, on techniques unrelated to BOP equipment on the well. This approach avoids the issues of equipment compatibility. Secondary kill guidelines can focus on possible control techniques with existing or new BOP equipment.
Blowout Effluent
Flow product types affect the situation. If gas is the product, environmental impact will not be such a pressing issue as if it were oil or condensate. Subsea visibility may be a factor for vertical intervention.As a result, kill techniques should be developed that require little or no visibility at the mudline. Sour fluids should be considered and evaluated on a caseby-case basis. Techniques have been developed that allow safe, working operations in high sour gas concentrations.
Surface Fires
Fire at the waterline adversely affects positioning a floating vessel over the blowout for obvious reasons. Fire suppression systems could be used to minimize the heat under the rig or in the moonpool area. However, it may not be such that a fire suppression or water cooling system could be used reliably. Extinguishing the fire is almost impossible.Blowout Rate
The blowout flow rate can create problems with vertical intervention at the point of entry. Stabbing kill assemblies through flow can create an additional set of problems. Knowledge of subsea plume dynamics is required to develop viable reentry procedures. Without foreplanning, reentry will be difficult.Computer modeling will aid with estimations on where possible gas boils may break at surface due to subsurface currents and surface wind conditions. The unknown elements will serve as key points prior to any site selection for a relief well or vertical intervention rig. These systems must be developed independent of the equipment type remaining on the well.
ROV Capability
The remote-operating vehicle (ROV) has limited capabilities. Many are equipped with tool kits, suction dredges, etc. The three families of ROV packages currently consist of the following:- 10-20-40 hp units;
- 75/100 hp units; and
- a third less common 150 hp unit with additional payload and thrust capability.
ROV intervention may aid in the following areas:
- Providing visual support/video capability
- Replacing ring gaskets
- Making or breaking connections/recovering dropped objects
- Detaching/re-establishing guide wire or guide post (if applicable)
- Actuating hydraulic functions with "hot stabs"
- Underwater inspections/explosives placement
- Underwater weld repair/oxy-arc cutting
With technology changing in the deepwater arenas, ROVs have not made many significant advances to aid with floating vessel blowout control. New designs are limited to additional horsepower and larger payload capabilities.
See also: Introduction Drill Emergency Response Deepwater Well
Vertical Intervention Tools
Depending on the situation and BOP condition, several different tool string designs may be required for vertical intervention operations. Several considerations need to be addressed while designing the tool string for easy access into the BOP/s or wellhead assembly.Vertical intervention tools require designs for three fundamental areas:
- Guidance re-entry systems
- Reentry tool strings
- Pipe conveyance mechanisms
Guidance design criteria are as follows:
- Universal designs regardless of equipment types on the well
- Designs for specific equipment types
- Means to handle flow-related, decentralized forces
Reentry tools strings can include the following items run individually or combined:
- Mechanical packers
- Inflatable packers
- Stingers
- Knuckle joints
- Collars or heavi-weight pipe
- Float valves and/or ported subs
Mechanical packers
Mechanical set packers may play a role in vertical intervention if access to the well can be achieved. This packer type is rigid and has two sets of slips energized by rotation.
The tool has setting limitations due to size and weight criteria of casing strings. If casing has not been damaged due to abrasive flow or mechanical means, the mechanical packer may be used as a plug to stop or reduce formation fluid flow exiting from the BOPs while relief well operations are ongoing, or damaged subsurface equipment is being repaired.
Other limitations include setting procedures for the packer. Rotation is required which may cause damaging actions on the pipe from the seafloor to the rig. Also, the time required for the packer elements to fully expand may allow erosion of rubber elements from blowout fluids.
If the vertical intervention assembly is capable of passing through the BOPs and can be successfully set downhole, the damaged equipment on the seafloor may be retrievable, thus allowing repair and re-use.
Inflatable packers
Some well control operations have utilized inflatable packers for controlling wells on and offshore. Some inflatable packers can seat in large diameter sizes. However, inflatable packers have reduced differential pressure capability the more they are expanded.
Vertical intervention methods may utilize inflatable packers to control several casing sizes. In a situation of a listing BOP, small diameter tools may have the ability to pass through the BOPs and be set in casing. Since this packer style does not have any external slip assemblies as seen on the mechanical type packers, the potential of hanging-up on ram cavities is reduced.
An advantage to an inflatable packer is the setting speed. It can be activated with a dropped ball, which quickly inflates the rubber. This will minimize flow erosion.
Stingers
Stinging blowing wells has been a common practice for many years. Operations in the Kuwait fires utilized this well control method to control over 225 wells. This method can be utilized on wells that do not have substantial volumes of exiting fluids.
For deepwater efforts, seawater hydrostatic may aid in stinging operations. Since many deepwater scenarios may involve large casing sizes, use of a stinger may be restricted due to the upward force created with the stinger in the flow path.
Knuckle joints
Knuckle joints can be utilized as part of the vertical intervention kill string. They assist in enabling passage through the BOP opening or wellhead. Strategically placed throughout the drillstring, these joints can aid with entry and allow the drillstring to move through the passageway without being hung up due to the rigid nature of the assembly.
Vertical intervention kill strings may have knuckle joints as a primary bending point(s) in the bottom hole assembly (BHA). Each BHA may change depending on the blowout conditions at the time of entry.