Partial removal best option for large US Pacific deepwater platforms

State-of-the-art technologies for removing large offshore platforms located in deepwater were identified and reviewed by the US Minerals Management Service (MMS - Dept of Interior) in November 2000. The platforms emphasized at the time were located in the Pacific Outer Continental Shelf Region (POCSR). Compar-isons were made to similar Gulf of Mexico platforms.

Deepwater platforms were defined as those with jacket weights exceeding 10,000 short tons and located in water depths exceeding 400 ft. The overall goal of the MMS was to determine and examine the issues relevant to decommissioning deepwater platforms and to quantify them in the context of economics, risk, and available technology. Following were the specific goals:

• Develop decommissioning plans for selected POCSR (Pacific) deepwater platforms using the best technology currently available.
• Prepare decommissioning cost estimates for selected platforms and cases, including an evaluation of cost sensitivity (risk) issues, and the cost of alternative technologies (explosive vs. non-explosive severing methods).
• Assess environmental and human safety risks for current and evolving decommissioning technology.
• Provide a review of the state-of-the-art in decommissioning technology for lifting, transportation, disposal, and explosive and non-explosive severing techniques.
• Prepare specific recommendations for industry and federal/state support for future developments.

The work consisted of selecting platforms to be evaluated, selecting and evaluating removal technology, developing decommissioning plans and cost estimates, analyzing safety and environmental issues, and reviewing and describing other removal technologies, their current development status, and prospects.

Project challenges

Offshore platform decommissioning is a challenge under any circumstances in terms of planning and executing the work in an environ- mentally sensitive, safe, and economical way. Regarding large deepwater platforms in the POCSR, this is particularly true. Among the issues are:

• Lack of local infrastructure: There currently are no derrick barges of significant capacity or other types of major marine construction equipment based on the US west coast, and none are likely to be in the foreseeable future. Additionally, there are no onshore facilities in California (or Mexico) capable of accepting jackets or topsides of these sizes, even in small pieces. The nearest such facility is in Portland, Oregon, eight to ten days sailing time away.
• Challenging marine environment: The offshore California marine environment is very challenging to the type of large construction equipment and operations required for offshore platform decommissioning. Research has shown that conditions vary widely, with areas such as Point Arguello being subjected to rough seas, long period swells, and dense fog much of the year.
• Limited availability of equipment: There are only four derrick barges in existence today, worldwide, with lifting capacities in excess of 5,000 short tons. These are generally committed to projects years in advance.
• Environmental regulation constraints: California has a large number of regulations and a wide variety of federal, state, and local agencies enforce them. This has a direct impact on the application of decommissioning technologies and the resulting economics.
• Depth challenges: The industry has limited experience in applying decommissioning technologies at depths beyond 300 ft. New systems and procedures will likely be required for both explosive and non-explosive severing techniques. Safety will drive the use of divers and remotely operated equipment.
• Lack of artificial reef legislation: Unlike the Gulf of Mexico oil and gas producing states, California currently has no enabling legislation for a rigs-to-reef program for offshore facilities. Such legislation is currently being discussed, but the timeline for its enactment is uncertain.

Topsides containing oil and gas processing equipment are assumed to be taken to shore and scrapped in all cases. Three methods for jacket disposal were selected to cover the range of costs and what are expected to be the most likely choices in the actual execution of POCSR platform decommissioning. These methods are equally applicable to Gulf of Mexico removal. The selected methods are complete removal, partial removal, and remote reefing.

Hidalgo, Gail, Harmony

The three Pacific platforms selected to cover the range of issues are Hidalgo, Gail, and Harmony. These platforms encompass the wide range of decommissioning options available and provide a review of issues related to decommissioning deepwater offshore structures. The water depths for these platforms range from 430 ft to 1,198 ft.

Standard cost components that should be accounted when developing offshore platform decommissioning estimates are as follows: project management and engineering, heavy-lift vessel (HLV) mobilization, cargo barge mobilization, well plugging and abandonment (P&A), platform removal preparation, pipeline abandonment, conductor removal, platform removal, site clearance and verification, and onshore disposal.

The MMS provides detailed descriptions of each of the above phases of the decommissioning process. However, the removal alternatives considered - complete removal, partial removal, and remote reefing - vary only in the method the jacket is removed. Site clearance and onshore disposal methods also differ in that they are dependent on the removal alternative selected.

Cost results summary

The removal method selected is a driving factor of each estimate. The complete removal scenario requires more heavy lift vessel work, more cuts to be made to jacket legs and conductors, more cargo barges, and more travel time to tow all platform components to shore.

Additional costs include site clearance/verification activities at each location the jacket is set down and cut and scrapping fees. The larger each platform is, the higher the complete removal cost. Partial removal is the least expensive option. Cutting operations, heavy-lift vessel and cargo barge usage, and site clearance/verification time are minimized.

While the remote reefing option is less expensive than the complete removal scenario, heavy-lift vessel usage, and travel time make this removal method more expensive than partial removal. Additionally, the size of the platform directly correlates to its decommissioning cost.

Deeper water depths often denote larger, more complex structures, which require larger heavy lift vessels and more conductor cutting. All of these factors increase the cost of platform removal.

Decommissioning methods

A key objective of the MMS was to provide a quantitative analysis comparing the three decommissioning methodologies for each of the platform removal scenarios. In order to perform this analysis effectively, categories were developed and a ranking system was employed to facilitate this task.

• Issues that influence decommissioning method selection were categorized and ranked in order of importance from 5 to 1 (5 being the most important). The evaluation categories used were safety (5), technical feasibility (4), environmental impacts (4), permitting (3), disposal options (3), cost (2), and scheduling (1).
• Using the categories above, each decommissioning method was compared to the other two and ranked 1, 3, or 5 (5 being the best case method). The ranking for each method was then multiplied with the weighted value for each task. The resulting numbers were then added to determine the total score. The decommissioning method with the highest values is considered to be the best option.
• Complete removal, partial removal, and remote reefing were found to be equally comparable in all decommissioning phases, except jacket removal. The following discussion highlights the issues where the differences exist between the three decommissioning methods categorically.

Safety is considered the most important factor in selecting the decommissioning method. Safety issues evaluated are directly related to the complexity of the work, duration to complete the work, and equipment required.

Partial removal was considered as the safest of the three methods. This method considers cutting the jacket at 85 ft below the waterline, a depth commonly accessed by commercial divers. Since explosives are not used, the local marine life is not affected. In fact, a major portion of the habitat is maintained.

Remote reefing was rated in the middle of this category, in that divers are used longer and at greater depths and the local marine environment is affected. Divers will assist attaching bouyancy bags to the jacket. This extra work requires the use of divers at depths greater than the 85 ft (partial removal) for longer periods of time. Additionally, the explosives used to sever the piles will greatly affect the local marine habitat.

Complete removal is deemed the least safe method due to the large amount of work required to remove the jacket. The heavy-lift vessel handles, cuts, lifts, and loads the jacket in many sections. This will require extensive personnel. If the jacket were cut up in-situ, the safety issue would be even more of a factor.

• Issues considered for technical feasibility are the planning, engineering and execution required to complete the jacket removal. Partial removal was considered the most feasible method. Cutting the jacket at the specified location only requires the use of divers for a short period of time at the specified depth.

Remote reefing is more challenging in that ballast calculations are required to determine the additional buoyancy needed to re-float the jacket. Deballasting the jacket at the reef site will also need to be planned and engineered.

Complete removal was ranked as the most technically challenging method. A detailed engineering study is necessary to determine how the jacket will be cut (size, weight, and location). Locations to cut the jacket are then established, based on the jacket sections configurations. Rigging and handling the jacket sections will at times be very challenging to the heavy-lift vessel. Removal in situ would be more formidable.

• Three issues were considered for environmental impact: air pollution, use of explosives, and impact on the attached flora and fauna and other marine life. Reefing jackets in place (in situ) has the least impact on the environment. The marine equipment utilized is on the job for site less time than for the complete removal scenario. In addition, the work is performed in one place, unlike in the remote reefing scenario. This avoids the emission of pollution in other areas outside the general platform location (as found in the complete removal and remote reefing scenarios). The less time the equipment is on site, the less impact it will have on the environment.
• Issues considered in evaluting permitting requirements were the number of agencies involved and the process by which the method will be permitted. In the Gulf of Mexico, the MMS Regional office is the lead agency by which the platform decommissioning method is approved. More agencies (federal, state, county, and others) and special interest groups are involved in the OCSR (Pacific).

In the POCSR, the complete removal methods is the permitting mechanism identified to be most straightforward in accordance with existing regulation. Partial removal and remote reefing requires that a reef program (similar to the Texas or Louisiana artificial reef programs) be established and the guidelines approved.

• The amount of material being removed and taken to shore was the issue in evaluating this category. Partial removal and remote reefing send only the topsides to shore. Complete removal sends the topsides and all of the jacket and conductors to shore for recycling. Partial removal was given a high score of 5, while remote reefing scored next. Complete removal will require the most effort to dismantle and recycle the platform. For this reason, Complete removal scored the lowest in this category.
• Complete removal, partial removal, and remote reefing costs for Hidalgo, Gail, and Harmony have been compared. Partial removal is the least expensive of the three methods and thus scored the highest. Since remote reefing is very close to partial removal, a score of 4 was assigned. As expected, complete removal (the most work-intensive method and thus the most expensive) scored a 1.
• Assuming that three to five platforms will be decommissioned concurrently, the schedule will influence the method selected. Partial removal can be completed in the least amount of time (20 days per platform) and thus scored a 5. Remote reefing was assigned a score of 4 in that the time to remove the platform is very close to partial removal (23 days). Complete removal scored a 1, in that it takes as least three times longer than the other two methods.

Acknowledgement

The US Dept of Interior's Mineral Management Service supported the development of this study. Sharon Buffington was technical leader, assisted by Jon Smith and Glen Shackell of the Pacific Outer Continental Shelf Regional Office. TEC-Demix provided the explosives data and report.

Editor's Note: This is an updated and summarized version of OTC 12972, which was presented at the 2001 Offshore Technology Conference in Houston.