An Assessment of Techniques for Removing Offshore Structures (1996)

Previous chapters have described practices and policies governing offshore platform removals and dispositions, the prospective and current availability of explosive and nonexplosive techniques for cutting well conductors and platform components, the effects of both explosive and nonexplosive cutting techniques on living marine resources and the marine environment, and the potential interactions between platform removal and disposition policies with the interests of other ocean users. For each chapter, the committee relied on experts with experience to sort and evaluate the facts, probabilities, and uncertainties. It is impossible to reduce the key considerations and conclusions of each chapter to a common denominator that directly addresses the central policy consideration at issue. Thus, in this chapter, the committee tries to capture and summarize the collective wisdom that evolved during the deliberations.

This chapter has two objectives. The first is to present a broad comparative summary of the principal options for platform disposal (complete removal, partial removal, and toppling in place) and the cutting techniques (explosive, mechanical, abrasive, and diver cutting) considered by the committee. The second is to identify and discuss more fully the findings on which the committee based the conclusions and recommendations, which are outlined in chapter 6. The broad summary is given in table 5-1, and the findings that are especially relevant to the committee's charge are enumerated.

The criteria used to compare options for platform disposal and cutting techniques fall into three broad categories: (1) economic and operational, including considerations of relative costs, reliability, and risks to human health and safety; (2) environmental, encompassing effects on sea turtles, marine mammals, and fish; and (3) effects on other ocean users, recreational and commercial fishing, shipping and defense.

Cutting options are compared for the complete removal of platforms, the most common disposal method, although exceptions are noted for the two other disposal options. Cost comparisons are given as ratios showing differences among cutting options relative to the cost of explosive removals (more detailed cost estimates from previous chapters averaged for different water depths). For a shorthand comparison of environmental effects, it was assumed that the option under consideration would be subject to relevant federal or state statutes.

1. There are many different types of platforms in the Gulf of Mexico. The platforms are owned by both major and independent operators who use them for a variety of functions; they were installed at different times in the past 50 years; and they are located at varying depths and on sea bottom of various soil types and compositions.

2. Current regulations require that platforms be removed within one year of the abandonment of a lease. All structural elements of the platform and all well conductors must be removed to a depth of at least 15 feet below the mudline and disposed of onshore unless a special exception is granted. The principal basis for an exception is using the platform in a rigs-to-reefs progam administered by the state of Louisiana or Texas. Under these progams, some platforms are deposited at designated sites where they provide habitat for reef fish that are valued by recreational and commercial fishermen.

3. Simple platforms in shallow water can be removed relatively routinely. As the platform size, complexity, and water depth increase, the removal process becomes more complex and costly, creating greater risks to the lives and safety of workers and incurring additional costs to the operators. Procedures for removing large platforms in deep water inherently involve elements of uncertainty that can increase health and safety risks if they are not properly planned for and analyzed prior to and during the removal process. Financial risks to operators are created primarily by the high costs of using specialized equipment, such as large derrick barges. These costs range from $50,000 to $100,000 per day, so interruptions of or delays in schedules can mean a rapid escalation of removal costs.

4. Current estimates are that between 100 and 150 platforms (including caissons) will be removed annually for at least the next 5 to 10 years; a slightly lower number of new

platforms will be installed each year over the same period. More of the removals are expected to be from shallower to more accessible waters (i.e., less than 200 feet deep), and more of the installations are likely to be in deeper and less accessible waters. Estimates of both installations and removals could change as technology, the price of oil and gas, economic conditions, legislation, and regulations evolve during the next decade.

5. Deciding when to abandon or remove a platform is a business decision based on many factors. The most important factors include the operator 's perception of the potential productivity of the geologic structure from which the oil or gas is extracted; the condition of the platform; whether or not the platform can be used elsewhere; the cost of maintaining the platform; concerns about liability; whether the platform is being used for other purposes, such as an intermediate point on a pipeline; and the operator's financial constraints, business strategies, and objectives. The range of factors leads to considerable variation in the age at which platforms are removed.

6. Explosives were used to remove about 70 percent of the platforms that were taken out of the Gulf of Mexico during the past decade. Almost all of the remaining 30 percent, which were removed by nonexplosive means, were either located in very shallow water or were caissons. A small number of platforms located in deep water were also removed by nonexplosive means because doing so facilitated their reuse.

7. The installation of platforms in the Gulf of Mexico has significantly increased the amount of hard-bottom habitat available for reef fish and other marine lifesometimes in areas accessible to recreational and commercial fishermen. Some platforms have been important destinations for fishermen for decades, and a high percentage of the annual take of some highly valued species of fish, notably red snapper, is near platforms.

8. Fish that reside near platforms are subject to a very high probability of being killed when structural members of the platform or well conductors are cut with explosiveseven when the use of explosives conforms to current, best engineering practices and MMS and NMFS regulations and procedures.

9. Estimates of fish kill by the best methods currently available is uncertain at best, and some fundamental causal questions remain unanswered. The blast waves from buried detonations differ significantly from the blast waves of traditional in-water detonations used in fish kill experiments in the following ways:

   • The blast impulse lasts significantly longer because of velocity differences. This raises questions about the validity of existing kill criteria which are all based on observations from controlled, in-water detonation experiments. (Current kill criteria also apply predominantly to detonations and fish in relatively shallow water.)

   • The blast strength alternates with distance at about half the rate of in-water detonations. The attenuation rate is strongly affected by attenuation in the soil which varies significantly with location of the platform site.

   • There are no data on the lethal effects of a series of detonations as compared to a single detonation equal to the cumulative charge in the series, even for in-water detonations. In the absence of evidence to the contrary, it seems prudent to assume that multiple buried detonations are significantly more lethal than a single one.

10. Acoustic devices have been successful in keeping fish of 100 feet or more away from structures in very shallow water. Their effectiveness appears to be species-dependent, however, and has not been demonstrated for the resident species near offshore platforms in the Gulf of Mexico. Moreover, there are no experimental fish kill data relating fish kill to distance from a platform; nor can this distance be computed with reasonable confidence from available information.

11. Reducing the size of explosive charges by using shaped or specially configured charges, to half or a quarter of the currently used 50 pounds, would reduce the intensity of the blast but apparently not enough to significantly reduce fish kill. However, if the fish could be kept far enough from the platform (say by the acoustical technique) so that there would be an appreciable percentage of survivors, the foregoing strategies might become more effective. For example, reducing the size of the charge may then reduce the kill by, perhaps, another 10 to 20 percent, and increasing the depth of the charge may reduce the kill by another 10 percent. Using smaller charges may reduce the distance fish must be kept away from platforms.

12. Studies are scarce and specialized, but according to the available evidence the expert judgment is that:

    • The populations of highly valued and overfished species vary greatly around individual platforms throughout the year and are subject to migratory changes.

    • Under current law, the effects on fish populations from events outside the purview of existing regulations, such as fish killed as a consequence of platform removals, could be incorporated into fish management plans by adjusting the parameters that are under regulatory control (e.g., catch limits, fishing seasons, or allowable harvests). Adjusting regulatory parameters appears to be cost effective because these changes cost much less than prohibiting the use of explosives to remove

platforms. However, regulatory adjustments may cause economic l osses and hardships for commercial shrimpers or fishermen, thus raising questions about equity or fairness. These equity issues are essentially political rather than technical in nature and fall outside the professional expertise and purview of the committee.

13. The NMFS Observer Program helps to ensure that the efficient use of explosives as constrained by current NMFS, MMS, and Marine Mammal Protection Act regulations is not likely to harm either sea turtle or marine mammal populations or to violate any of the provisions of the Endangered Species Act. The observer program promotes strict adherence to NMFS and MMS regulations and facilitates intervention in scheduled operations if sea turtles or marine mammals are threatened.

14. Nonexplosive techniques involving mechanical or abrasive cutting, either by machines or divers, and torch cutting by divers are viable techniques that have been used in the Gulf of Mexico—particularly on simple structures or components located in shallow water. But grouted well conductors or grouted piles, either independent of or within the structural elements of a platform, may be difficult to cut using existing nonexplosive techniques. Limiting the use of explosives to well conductors and grouted piles and requiring the use of nonexplosive techniques for other elements of the structure would not appreciably reduce the damage to fish or other marine life because every explosion poses a risk to nearby marine life.

15. Research and development on a wide variety of nonexplosive techniques and advanced explosive techniques using smaller charges are currently supported by potential vendors, users, and governments. Motivation for this research and development is economic (especially when nonexplosive cutting techniques may make reuse of a platform more feasible) but has also been undertaken in anticipation of regulatory restrictions on the use of explosives for ecological reasons.

16. The prevailing judgment of platform owners and operators, as well as the engineering community, is that using either existing or reasonably prospective technologies, explosive cutting is more economical, safer, more flexible, and more reliable than nonexplosive removal techniques for most platform removals in the Gulf of Mexico. The premises on which this judgment rests include:

    • scheduling and cost uncertainty of nonexplosive cutting based on lack of field experience.

    • failure of nonexplosive techniques may necessitate the use of divers, who would be subject to additional risk

    • nonexplosive techniques require sophisticated and delicate operations (therefore expensive) that can potentially complicate the removal procedure when compared to explosive techniques

17. Leaving platforms in place, partially removing them, toppling them in place, using them to make artificial reef habitat, or disposing of them in designated disposal areas are options that some ocean-user groups believe are advantageous. Other groups, however, object to these alternatives and recommend complete removal and onshore disposal of all platforms.