GULF OF MEXICO DECOMMISSIONING—4: Explosives offer another option
Mark J. Kaiser, Allan G. Pulsipher
The cost of severing platforms with explosives during the decommissioning of Gulf of Mexico structures is time-invariant or nearly time-invariant. Also the fixed cost of explosive severance dominates the variable cost, indicating the variable cost is relatively minor for most jobs.
This fourth in a five-part series of decommissioning summarizes the historic use of explosives, current regulatory structure, and fundamentals of the technology.
The first part of the series was in OGJ, Sept. 27, p. 41.
History
The industry probably first used explosives in decommissioning and salvage operations in the mid-to-late 1950s to sever well conductors. During the early 1980s, about 10 companies, mainly diving and wireline subcontractors, performed this service. Today, only three contractors offer this service in the gulf: DEMEX, Halliburton Jet Research Services, and Explosives International Ltd.
In the mid-1980s, environmental issues concerning endangered species changed the use of explosives offshore. Before 1986, rules or regulations did not exist for platform decommissioning with explosives in the gulf, and the rule of thumb was that if 5 lb did a good job, then 10 lb would do a much better job.1
Since 1986, however, several rules and regulations have helped minimize the incidental taking of marine life and quantified the impact of explosives on sea turtles and marine mammals.
On Apr. 15, 1986, the US National Marine Fisheries Service (NMFS) sent a letter to the US Minerals Management Service (MMS) expressing concern regarding marine-life stranding events in 1985 and 1986.2
NMFS suggested that these stranding events correlated with explosives used to remove platforms off Texas. The MMS subsequently imposed an unofficial moratorium on using explosives, but nonexplosive removals continued while rules and regulations were enacted.
Under the current regulations in the gulf, platform operators have to submit to MMS a structure removal application and site clearance plan. Also if they use explosives for cutting, operators have to receive an Endangered Species Act Section 7 consultation.
The consultation is generic so that it reduces the administrative burden on the MMS and NMFS for conducting repetitive consultations on activities that may have similar impacts to listed species and that require identical mitigating measures to maintain adequate protection.3
A generic consultation permit considers only those removal operations that meet certain criteria pertaining to explosives size, detonation depth, and blasts per structural grouping. The specific criteria established to cover a standard removal are as follows:
- Use high-velocity explosives, detonation rates greater than 7,600 m/sec, such as C-4, composition-B, and cyclotron.
- Do not exceed 50 lb of explosives/detonation. A special consultation is required if more than 50 lb/charge are employed.
- Limit detonations to eight individual blasts per group with charges staggered at 0.9-sec intervals. Staggering limits the cumulative effect of the blast, and the short time interval between groups minimizes the chumming effect, which is the attraction of other fish.
- Set charges at a minimum 15-ft below the sediment surface. This involves jetting out the inside of the structure to at least 15 ft below the sediment floor to place the explosives. This depth decreases the impulse and pressure forces released into the water column upon detonation.
Detonation of explosives in the water column requires a special consultation. Also consultations continue on a case-by-case basis for structures that do not meet the standard established criteria.
Regulatory structure
To protect endangered species and marine mammals from potential injury or death, the industry must follow basic regulations when using explosives offshore.
The NMFS enacted an Incidental Take Statement for sea turtles in 1988 and one for bottlenose and spotted dolphins in 1995, which expired on Nov. 15, 2000.
These regulations include the following definitions:
- Take means "to harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect, or to attempt to engage in any such conduct."4
- Harass means "an intentional or negligent act or omission which creates the likelihood of injury to wildlife by annoying it to such an extent as to significantly disrupt normal behavior patterns which include, but not limited to breeding, feeding, or sheltering."5
- Harm means "an act which actually kills or injures wildlife. Such an act may include significant habitat modification or degradation where it actually kills or injures wildlife by significantly impairing essential behavior patterns, including breeding, feeding, or sheltering."5
On Aug. 1, 2002, the NMFS issued interim regulations for monitoring and reporting the taking of bottlenose and spotted dolphins effective through Feb. 2, 2004.6 The following take statements outline the measures required when using explosives:
- Qualified observers, as approved by the NMFS, must monitor the site for 48 hr prior to, during, and after the detonation.
- Pre- and post-detonation diver surveys must be conducted if sea turtles are observed in the area and are thought to reside at the site. Most turtle sightings are turtles in transit. In cases where turtles are believed to be resident to the structure, the decommission operation must either relocate the turtles or perform the work with nonexplosive methods.
- A 30-min aerial survey must be conducted 1 hr before and 1 hr after each blasting episode. This ensures that no marine mammals are within the designed 1,000-yd safety zone nor are likely to enter this zone prior to or at the time of detonation. The predetonation survey must encompass all waters within 1 nautical mile of the structure.
- The operation must delay blasting if sea turtles or marine mammals are observed within 1,000 yd of the structure prior to detonation. The delay must remain in affect until the sea turtles or marine mammals are beyond 1,000 yd of the platform. The aerial survey must be repeated prior to resuming detonations.
- If sea turtles or marine mammals are observed within 1,000 yd of the structure, an additional survey must be performed, involving either a diver survey dedicated to marine mammals and sea turtles within 24 hr after the detonation event or an aerial or vessel survey within 2-7 days after the blast. The aerial or vessel survey must concentrate down current from the structure.
The NMFS-approved observer may waive post-detonation monitoring if no marine mammal were sighted during either the aerial surveys before detonation or during the 48-hr predetonation monitoring period.
The operation cannot detonate explosives sooner than 1 hr after sunrise or no later than 1 hr before sunset. NMFS personnel on site may allow modifications to these times for special circumstances.
Sea turtle and marine sightings must be reported to NMFS personnel or the agent of the holder of the Letter of Authorization by:
- Divers upon surfacing from all diving operations.
- Remotely operated vehicle (ROV) operators, for operations in water depth of 150 ft or greater or in cases where divers are not deployed in the course of normal removal operations.
- Acoustic device detection monitors. Operations in water depth of 328 ft (100 m) or greater must employ passive acoustic detection prior to detonation.
The decommissioning operations need to recover either dead or injured animals after the explosives are detonated.
The Letter of Authorization requires the submission of a report within 30 days of the structure removal to NMFS Regional Administrator that summarizes the results of structure removal activities, mitigation measures, monitoring efforts, and other information as required.
Impact from explosives
The NMFS observer program collects data at all explosive removals in both state and federal waters of the gulf and its embayments. The NMFS data describes where and how explosives are used, turtle and marine mammal sightings, and any delays due to the presence of marine mammals.
Since 1988, the industry has removed more than 1,500 structures from the gulf and during this time, NMFS has recorded five documented takes of loggerhead sea turtles with one fatality.7 In each case, NMFS observers had given permission to detonate and the work followed all regulations.
Since 1995, the industry has removed more than 500 structures from federal waters and NMFS observers reported no indication of injury or death to bottlenose or spotted dolphins or any other marine mammal.6
It is clear that the regulations work and are effective in protecting sea turtles and marine mammals.
Explosives used to remove offshore platforms do kill and injure fish, but it is important to understand the magnitude of the kill relative to other sources of fish mortality. For all practical purposes, it is impossible to predict fish mortality at an offshore platform during an explosive removal. Some of the many variables affecting the fish kill include fish density at the structure, water depth, the season of the year, distance of the fish from the blast, amount of explosives detonated, and the types of fish present.
Scientific information on the impact of explosives on fish populations is scant but interesting. Between 1993 and 1999, the NMFS assessed the impact of explosives at 10 platform removals in the gulf. Total estimated mortality ranged from 2,000-6,000 fish/platform.8
Relative to shrimper by-catch, the fish kill due to explosives is generally considered negligible.
The annual shrimping season in Louisiana and Texas consists of two 90-day periods, and an average trawler by-catch is about 1,000 fish/trip. If we assume that half of the 55,000 registered trawlers in Louisiana and Texas complete 1 trip/day for one-quarter of the shrimping season, then the fish by-catch on average due to shrimper trawling is about 1.25 billion fish/year.
Relative to structure removals, if the industry removes 200 structures/year with explosives and if the analysis assumes kill is 50,000 fish/structure, an order-of-magnitude greater than the NMFS study, then the total kill is 10 million fish/year or less than 1% of the estimated shrimper by-catch.
Explosive severance technology
The following summarizes explosive severance technology. References 1-9 provide more details.
An explosive cut is sized according to the diameter and wall thickness of the member to be cut along with the number of strings. Single-layer targets have only one wall thickness to penetrate, while multilayer targets have more than one wall thickness to sever. Most piles are single-layer targets while well conductors with one or more inner casing strings are multilayer targets.
A standard bulk charge is the most common explosive method (Figs. 1 and 2a). Bulk charges have one single mass of explosive material detonated at a single point and are configured for the conditions that exist downhole. The process requires water jets to clear the interior of the piling of mud so that explosives can be lowered 15 ft below the mudline (Fig. 3).
In the operation, the explosives technician prepares and loads the charges into the legs and conductors (Fig. 4) before the derrick barge backs off to a safe distance (Figs. 5 and 6).
The derrick barge backs off a safe distance before detonation of the explosives (Fig. 5). Photos from NMFS
The field technician relies on the brute strength of the explosive to overcome the target. The force of the detonation creates directional cutting energy to sever the pile or conductor.
A double detonation bulk charge (Fig. 2b) creates more cutting power and is more effective than an ordinary bulk charge. Detonators at opposite ends of the charge create the double detonation. The force of the detonation at the ends moves toward the center of the cylinder and is dissipated radially outward perpendicular into the target material when the two explosives collide. For larger offshore tubular members, a configured charge uses explosives in closer proximity to the target material, which upon detonation, results in a higher average pressure on the target (Figs. 2c-2e). Multiple points of detonation on the inner periphery of the charge direct the explosive energy into the target material.
The ultimate combination of the best features of the configured double detonation and bulk charge is the shockwave enhancement-focusing device (Fig. 7). Because of its increased confinement and multiple point detonation, the shockwave enhancement device is the most reliable explosive device available on the market. Shockwave devices contain large-diameter plates in varying thickness, and as the plate diameter and thickness increases, so does the manufacturing and handling cost of the device.
Large tubular members require a configured charge in close proximity to the target material (Fig. 7). Photo from NMFS.
Shockwave devices cost about 5-10 times more than bulk charges to use.
Safety
Explosive severance operations are safe and only one fatality has occurred during the past 20 years in the offshore application of explosives.9
The procedure uses a detonator (blasting cap) attached to a detonation cord that is secured to each end of the explosive. High voltage initiates the detonation. The detonation cord usually is several hundred feet long so that the support vessel can move clear before the installation of the cap.
Standard contract
The severance subcontractor agrees to provide cutting services as defined in the terms and conditions of work. Explosive service contracts usually have a time and material basis with the following elements:
- Mobilization-demobilization personnel to and from dock site(s), $K1/mile.
- Mobilization-demobilization explosives to and from dock site(s), $K2/job.
- Personnel, $K3/day.
- Cutting charge, $K4/job.
- Manufacturing and special devices, $K5/job.
- Incidental, $K6/incident.
- Contingency, $K7/cancellation.
- Idle time, $K8/day.
Each contract has unique terms and conditions and the following discussion only highlights the primary terms.
The mobilization/demobilization rate covers the cost of the service provider to transport personnel ($K1/mile) and explosives ($K2/job) to and from the company facilities to dockside.
The value for K1 only depends on the distance between the service facility and the dockside location, while the value for K2 depends on the job, the size and weight of the equipment that needs to be transported, and the form of the transportation arrangement negotiated between the subcontractor and the trucking company.
The operations require that loading and offloading explosives be performed at an US Coast Guard approved dock.
The job charges personnel on a per day basis ($K3/day), and because of the nighttime restriction on the use of explosives, the operation only requires a 12-hr service crew. Explosive operations usually maintain a project manager and one explosive technician to provide the service.
The cutting charge ($K4/job) is computed according to the amount of explosives needed to perform the operation and the expected time involved to set the charges. Most jobs require 50 lb of explosives to cut each structural member, and therefore a reasonable estimate of the total amount of explosives used is simply the number of elements that need to be cut multiplied by 50 lb.
This is a lower-bound estimate because the cutting may require re-shots and more than 50 lb/element. Using less than 50 lb/shot does not occur frequently. The price structure for K4 depends on the element diameter and the manner of the explosive delivery. Equation 1 in the equation box expresses a typical cost function.
In the equation, K4BC(d(i)) represents a standard bulk-charge cost function and K4SWED(d(i)) represents a shockwave-enhancement-device cost function. The bulk-charge cost function is a linear and nearly flat function of d(i), while the shockwave cost function typically resembles a power relation.
Standard bulk charges have excellent reliability and performance rates for tubular members with a 42 in. or less OD, while larger diameter elements will require a shockwave device.
Explosives do not have an upper limit on the size of the element that can be cut, but for a tubular member with an OD greater than 60 in. will usually require three separate shots.
The contract will include a manufacturing and special device cost ($K5/job) if the job needs the manufacturing of a special device, either at the request of the contractor or because of job requirements. Most explosive jobs performed to date in the gulf have used bulk charges, but large, deep, and over-designed structures will require special devices.
Incidental expenses cover the cost of lost magazines and toolboxes ($K6/incident). A $K7/cancellation penalty covers cancelled jobs.
If the cutting crew moves from site-to-site without returning to shore, or if work conditions warrant, the contract also includes an idle time charge ($K8/day).
The contractor picks up the cutting crew at dockside, provides for their meals and board while on the barge, and then delivers them back to dockside (or to another site) upon completion of the job.
All rates commence at the point of departure and terminate upon return to land at the point of departure. The contractor typically ensures that the inside of all piles to be cut is free of mud, clay, and other obstructions. If piles are not jetted and cleared of obstructions prior to the arrival of the cutting crew, the crew will need to wait to begin their operation.
Changes in work scope, technical complications, and environmental conditions, such as hangers and/or bent stabbing guides that prevent the placement of the explosive charges and the presence of sea turtles and marine mammals will also impact the cutting time.
Total cost
The job specification includes the job location and the number, size, and type of piles and conductors that need to be cut.
The job location specifies the water depth of the cutting activities. Water depth is not a primary cost driver for explosive cutting because increasing water depth only adds to the time to go in and out of the hole and the amount of detonation cord required in the operation. In deep water, the explosive severance cost will increase as the charge size and weight of the device increases. The nomenclature in the equation box provides the notation for describing job J.
Equation 2 calculates the total cost, TC(J), of job J. If K41(d(i)) is a constant function of d(i), Equation 3 simplifies the total cost expression.
The operation incurs the fixed cost once the job is specified while the variable cost is determined once the job is complete and the crew has returned to shore. The fixed cost includes mobilization and demobilization and the charge to cut which usually is known before the job.
The variable cost includes the day rates to cover equipment, personnel and idle time, and incidental expenses. The relative contribution of the fixed and variable cost for a given job depends, among other factors, on the nature of the job, the success of cutting and the value of the contract parameters Ki, i = 1,…,8.
Table 1 provides a range of typical values for, Ki, i = 1,…, 8.
References
- DeMarsh, G., "The use of explosives in decommissioning and salvage," Paper No. OTC 12023, Offshore Technology Conference, Houston, May 1-4, 2000.
- Gitschlag, G.R., and Herczeg, B.A., "Sea turtle observations at explosive removals of energy structures," Marine Fisheries Review, Vol. 56, No. 2, 1993, pp. 1-8.
- Roberts, S.P., and Hollingshead, K., Marine mammal regulatory issues and the explosive removal of offshore structures: The small take authorization program, NOAA Fisheries, Office of Protected Resources, Marine Mammal Conservation Division, Washington, DC
- US Code, Title 16, Sec. 1532, No. 19, 2002.
- Federal Registry, Vol. 50 No. 17.3, 2002.
- "Taking and importing marine mammals; taking bottlenose dolphins and spotted dolphins incidental to oil and gas structure removal activities in the Gulf of Mexico," Federal Registry, Vol. 67, No. 148, Aug. 1, 2002, pp. 49869-75.
- Childs, J., EROS actions and ESA mitigation measures for EROS in the GOM, e-mail communication, Minerals Management Service, New Orleans, Dec. 24, 2002.
- Gitschlag, G.R., Schirripa, M.J., and Powers, J.E., Estimation of fisheries impacts due to underwater explosives used to sever and salvage oil and gas platforms in the US Gulf of Mexico, OSC Study MMS 2000-087, National Marine Fisheries Service and Minerals Management Service, New Orleans, 2000.
- Explosive Technology Report, TEI Construction Services DEMEX Division, October 2000.
