Well Intervention Wellhead access, equipment support vital to well intervention

L. William Abel David A. Barnett Wild Well Control Wellhead capping assembly. Pre-determined firewater ring connection site. Wellhead spacing for capping stack.

Re-boarding kill pumps, emergency tie-ins, cranes, firewater pump fuel important to control blowing wells

L. William Abel
David A. Barnett

Wild Well Control

It is possible for the design of offshore drilling and production platforms to enhance and facilitate well control intervention operations. The recent emphasis on emergency preparedness has led many operators to seek the input of well control specialists during the conceptual and initial phases of new platform design. These efforts are aimed at providing low-cost design enhancements and additions to the facility, which will reduce the complexity of major well control intervention.

The criteria used for platform design typically involve functionality, safety, and economics.

The extremely high cost of design, procurement, and construction of an offshore facility tends to lead the design team toward a smaller, more compact design. These efforts have yielded commendable results which have reduced size, increased efficiency and safety and allowed otherwise unrealizable development projects to proceed.

On the other hand, they have resulted in facilities that will significantly increase the time and effort required to control a blowout. In some instances, the platform design creates a situation where capping is virtually impossible due to lack of access to the wellhead.

The historical view has been that the conflicting goals of cost effectiveness and well control intervention are mutually exclusive. Recent involvement by well control experts in platform design has contradicted this assumption. The keys to cost effectiveness in such a project are to:

  • Identify a well control company that combines practical experience with engineering capabilities.
  • Involve the well control company while the design process is at the conceptual stage.

The well control/engineering company can provide hands-on experience and engineering skills to identity potential problems. If done during the initial stages of the design, these potential problems may be resolved without significant cost.

The role of the well control/engineering company should include:

  • Provide the design team with an overview of commonly applied well control techniques and equipment.
  • Review facility design and well control practices.Recommend design specifications which will enhance the ability to apply well control techniques safely and efficiently.
  • Develop a specific action plan or well control intervention based on platform design.

The role of the design engineering team with respect to well control intervention should be to:

  • Develop the overall design philosophy.
  • Provide the well control company with adequate information for review.
  • Consider the impact of facility design on the ability to implement well control procedures.
  • Evaluate the feasibility of recommendations and implement if practical.

The efficiency of any major well control intervention is dictated by the time required to apply the necessary techniques which will bring the well under control. Generally, the requirement for an efficient intervention are:

  • Capabilities to re-board the facility under blowout conditions.
  • Adequate firewater protection during initial mobilization and once the intervention is underway.
  • Access to the facility.
  • Isolation of personnel and equipment from hazards.
  • Lifting capabilities, preferably stationed on the platform itself.
  • Overhead and lateral access to the wellheads.

Since each facility is unique, the way these requirements are satisfied vary from one platform to the next. Brief discussions of these issues are contained in the following sections.

Re-boarding

Early on in the intervention, the well control team must re-board the platform to assess the situation and develop a plan of action. In some cases, being able to climb the platform leg from the splash zone is necessary. Another platform attribute which is beneficial in this regard is the placement of stairways such that they are not obstructed by the well flow.

A minimum of two protected stairways should be included to provide access under varying wind conditions. In some situations the re-boarding will take place from a crane barge or multi-service vessel (MSV). A safe area (or possibly two) should be designated for this purpose which will allow the placement of personnel and light equipment.

This safe area should be isolated from well flow/heat and have unobstructed overhead access for the placement of a personnel basket and equipment. These can serve dual purposes, such as muster points for evacuation and safe briefing areas.

Firewater protection

Adequate firewater protection for personnel and equipment is essential. In the initial stages of a blowout, sufficient firewater may prevent ignition of the well flow, which will have a huge impact on the complexity of intervention, This is especially true for platforms with large enclosed topside facilities.

A number of rules and recommended practices have been use to establish fuel supply requirements for platform fire pumps which typically range from 8 to 24 hours. From a practical point of view, the firewater system should be capable of affording protection while the initial response and mobilization are underway.

In remote regions and areas which do not have an abundance of marine vessels with firefighting capabilities this length of time may be significant. An estimate of the time required to mobilize auxiliary firefighting equipment should be used to determine the necessary fuel supply for the onboard pumps.

In certain instances, the firewater protection time can easily be extended by tapping into crane pedestal fuel tanks for gravity feed or using natural gas from an available source on the facility.

The design of the firewater ring should include the ability to install temporary fire monitors and hose lines at pre-determined connection sites. The firewater ring and wellbay deluge system should include external tie-in points which will allow them to be charged from the splash zone. The external tie-in points should be positioned at boat landings on opposing comers of the platform.

Internal access

Access to the wellbay and wellheads is critically important during the intervention. It may be impossible to gain entry into an enclosure while a fire or significant well flow is present. Ventilation of the wellbay area provides a means to dissipate heat and prevent the accumulation of combustible vapor/air mixtures. This can often be accomplished by simple features such as replacing deck plate with grating or relocating equipment to provide ventilation from another side.

For facilities operating in harsh environments, natural ventilation may not be feasible. Proper planning can provide methods of removing certain walls and decks without the need to employ cutting equipment.

Fluid accumulation in certain areas can sometimes hinder the intervention. Layouts and drainage equipment should insure that critical areas are not subject to fluid accumulation due to the well flow or water application.

Lifting capabilities

Major interventions will involve the removal of equipment and various structural components. Since lifting capabilities play such an important role, it can be very beneficial to have input from the well control company with regard to proper crane selection. Issues such as power (hydraulic, electric, combination, etc.), crane placement, shut-dawn/override systems and lifting capabilities should all be reviewed from a well control perspective.

The cranes should be positioned so that they are usable during a blowout. Certain situations may require the override of electromechanical shut-down devices. This should be reviewed to determine what can be done to move the crane if, for instance, it is obstructing access or otherwise hindering intervention. This will require re-starting the crane in ESD level 0 conditions.

A review of the anticipated drilling, casing and completion plans will be necessary to determine what size and pressure rating BOP equipment may be required. This, in turn, can be used to determine the capabilities of the onboard crane in case they are used for well capping.

Wellhead access

To perform a capping operation, the well control team must be able to remove and replace damaged equipment. This requires an unobstructed area above the wellhead. In addition to the removable components mentioned earlier, overhead access can be enhanced by insuring that piping runs do not obstruct the area above the wellheads. This additional constraint does not usually add greatly to the piping design and can have a significant impact on the ability to quickly regain control of a blowout well.

Well spacing

A capping plan should be developed for each phase of drilling, completion, and production operations. The capping plans will be used to determine the dimensions of the largest anticipated capping stack. Wellhead spacing should be reviewed based on these BOP dimensions, Ideally, the wellhead centerline spacing should allow the placement of the largest anticipated BOP at any orientation.

This will enhance the ability to place straight unobstructed diverter lines. After placement of the capping stack there should be adequate space to open the BOP bonnets for ram replacement or repair.

Wellhead spacing can have a significant impact on platform design. Every effort should be made to identify a means of reducing wellhead spacing without compromising the ability to safely cap and control a well. One possibility is to use spacer spools to raise the capping stack above other wellheads. Certain platform designs will allow this while others may not. Capping procedures can also be aided by placing the wellbay along the platform perimeter as opposed to the center.

Wellhead equipment

All wellhead equipment will need to be constructed of suitable material for the anticipated service temperature, pressure and environment. Some aspects of the type of wellhead equipment used will impact well control intervention.

Secondary involvement of adjacent wells on the same platform is always a concern during platform blowouts. The risk of secondary involvement is due primarily to the potential for damage to the adjacent wellheads either by impact from falling debris or heat damage.

Several wellhead manufacturers offer fire resistant equipment. Fire resistant wellhead assemblies typically incorporate the following features:

  • Solid-block christmas tree with fire resistant, integral double master valve.
  • Fire resistant seal assemblies, either Laurent-type metal-to-metal seals or graphite foil seals.
  • Fire resistant clamps of forged heavy wall construction with recessed stud and nuts.

Fire resistant gate valves are also commercially available. The main feature of these valves are:

  • Eutectic Ring which melts at a specified temperature and allows the pressure within the valve to move the stem to the backseat position. When the stem backseats against a special shoulder, a metal-to-metal seal is formed to prevent leakage. The valve backseats in the open, closed, or any intermediate position.
  • Metal-to-metal stem backseat confines pressure within the valve body, even if outer parts of the bonnet assembly are damaged by fire.
  • Fire-resistant stem packing made from high temperature fluoro-elastomer to ensure it does not melt before the eutectic ring.
  • Shielded bonnet studs and nuts.

From a capping and intervention standpoint, the features of the fire resistant wellhead equipment are less than ideal. The shielded studs and nuts may present a problem with direct intervention and the clamps used on the metal-to-metal connection are not as suitable for capping purposes as a flanged connection.

These conflicts will need to be studied to determine the best compromise. A comprehensive survey should be made of all wellhead equipment to determine the exact components which are suitable for a given platform.

Kill lines, pumps

Onboard kill pumps are often justified for facilities which involve high pressure, high temperature production. These pumps should be positioned in a protected area which will allow them to operate without creating an ignition hazard. The volume and pressure capabilities of the kill pumps should be based on well pressure and configuration.

Pump lines which connect the kill pump to the wellheads should include remotely operated valves which allow pumping of kill fluid into the production tubing and/or into the tubing/casing annulus. Valves should also be included which will isolate the kill pump from the auxiliary tie-in pump line.

Kill fluid of adequate density should be stored on board the platform. The volume of on board kill fluid will have to be determined, but as a rule-of-thumb should be equal to 1-1/2 times the largest well volume (tubing and annulus). This rule may not apply for a dynamic kill operation.

If a brine solution is used for the kill fluid, no agitation will be required. Other weighted fluids may require the addition of an agitation system to ensure proper density is maintained.

Emergency tie-ins

When a facility suffers a major blowout it should be assumed that all power will be lost. Plans should be developed to re-energize critical components such as the firewater ring, kill lines, onboard cranes, BOP control systems, and rig skidding mechanisms.

This can be accomplished by providing auxiliary tie-in points at the boat landing level.

These tie-in points should have standard connections and be positioned at opposing corners of the platform to allow access regardless of wind and current direction.

Conclusion

The input of well control specialists during the conceptual design phase has proven to be a cost effective method of preventing complications during well control operations due to the platform design. The key to cost effectiveness is to involve the well control company during the conceptual design stage.

This allows simple modifications to be incorporated while avoiding the expense of re-design. The benefit of this pre-planning is to enhance and accelerate capping and control operations which will result in significant savings of time, money, and resources should a well control emergency occur.

Copyright 1995 Offshore. All Rights Reserved.

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