ACCESS NUMBER: 85237

STUDY TITLE: Explosive Removal of Offshore Structures - Information Search and 

Synthesis 

REPORT TITLE: Explosive Removal of Offshore Structures - Information Synthesis 

Report

CONTRACT NUMBER(S): 85237 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREAS: Eastern Gulf of Mexico; Central Gulf of Mexico; 

Western Gulf of Mexico; Southern California; Central California; Cook Inlet 

FISCAL YEAR(S) OF PROJECT FUNDING: 2002, 2003 

COMPLETION DATE OF REPORT: April 2004 

COST(S): FY 2002: $15,762; FY 2003: $229,689 

PROJECT COST: $245,451 

PROJECT MANAGER: Stephen T. Viada 

AFFILIATION: Continental Shelf Associates, Inc. 

ADDRESS: 759 Parkway Street, Jupiter, Florida 33477 

PRINCIPAL INVESTIGATORS: Roberto Racca, David B. Snyder, M. John Thompson, 

Denis Thomson, Stephen T. Viada 

KEY WORDS: explosive structure removal; marine fishes; marine turtles; marine 

mammals; acoustics; impact assessment; platform; endangered species; protected 

species; synthesis; mitigation; monitoring; data gaps 

BACKGROUND: MMS requires that offshore oil and gas structures be removed from 

the U.S. outer continental shelf (OCS) within 1 year of lease termination. MMS 

regulations require that structures must be severed at least 5 m (15 ft) below the 

surface of the seafloor. Offshore structure (e.g., platform) removal typically involves the 

use of explosives to sever structure-associated components – wellheads, piles, etc. 

Historically, offshore development and production of oil and gas reserves (and most 

platform removals conducted to date) have occurred in waters of the continental shelf 

(at depths of 200 m or less). Recent interest in deepwater development, however, has 

raised the issue of future platform removals in deeper OCS waters over the continental 

slope. The presence of listed (i.e., endangered or threatened species listed or 

proposed for listing under the Endangered Species Act [ESA]) and/or protected species 

(i.e., marine mammals protected under the Marine Mammal Protection Act [MMPA]) in 
ACCESS NUMBER: 85237

marine waters, coupled with the mandate to minimize or eliminate the potential for 

impact to these species, underscores the need to fully understand the environmental 

impacts associated with explosive removal. Topics of interest in this regard include a 

thorough characterization of explosive removal technologies and techniques, a 

description of the physics of underwater explosions, and environmental impacts 

resulting from these underwater demolitions. While the expected application of this 

knowledge may include U.S. waters (e.g., Gulf of Mexico, California, and Alaska OCS), 

the scope of this effort is worldwide. 

OBJECTIVES: (1) To provide a summary of available information on the explosive 

removal of offshore structures by topic; (2) To identify information and data gaps that 

could be filled by subsequent research activities; and (3) To recommend areas of 

research to meet MMS information needs.

DESCRIPTION: This project focuses on the collection and summarization of existing 

information on four topics associated with the explosive removal of offshore structures: 

explosive structure removal methods; the physics of underwater explosions; the effects 

of underwater explosions on three faunal groups: marine fishes, turtles, and mammals; 

and mitigation and monitoring of these effects. The goal was to assemble published 

and unpublished literature and compile these into a preliminary bibliography database in 

an electronic and searchable format (ProCite, Access). Collected information was then 

reviewed and summarized in an information synthesis report. 

SIGNIFICANT CONCLUSIONS: (1) The most commonly used technique for explosive 

cutting of piles and conductors is with bulk explosive charges; (2) Increased use of 

techniques that involve smaller charge sizes could reduce potential impacts on marine 

life; (3) The physical principles of underwater detonations and of the propagation of 

shock and sonic waves in the surrounding medium are well understood, but there are 

significant gaps in applying this knowledge to actual removal of offshore structures; (4) 

There are important differences among fishes, marine turtles, and marine mammals 

with respect to the types and adequacy of available data. In general, fishes are the 

best-studied group, and marine turtles the least studied. For predicting impacts on 

fishes, the empirical data from observations during actual structure removals would 

seem to be more useful than any attempt to calculate impacts based on experiments or 

mechanistic models. Years of experience using the 3,000 ft (914 m) “safety range” 

monitored under the “generic consultation” suggests it has been effective in preventing 

most deaths and serious injuries of marine turtles and marine mammals. In all three 

groups, there is relatively little information about sublethal impacts, particularly on the 

auditory system; and (5) While mitigation measures appear to be effective in preventing 

death or injury of mammals and turtles, it is uncertain to what extent sublethal effects 

may be occurring beyond the safety range. 

STUDY RESULTS: The most common technique for explosive cutting of piles and 

conductors is with bulk explosive charges, which can be separated into two main 

categories: configured bulk charges and cutting charges. Configured bulk charges 

(including ring charges and focused charges) are designed to collide or “focus” the 
ACCESS NUMBER: 85237

detonation front to concentrate more energy along the fracture line, and thus reduce the 

size of the charge needed to cut a piling. Cutting charges include linear-shaped 

charges and “cutting tape.” Linear-shaped charges and flexible cutting tape use highvelocity explosive energy to accelerate a v-shaped band of cutting material, usually 

copper, in a high velocity jet that penetrates through the steel of the piling. Potential 

future explosive cutting techniques include contact plaster charges, shock-wave 

focusing charges, and radial hollow charges. Generally, these techniques are designed 

to more effectively focus the explosive pressure wave against the piling or other 

structure to be cut, thus requiring smaller charge weights.

The explosive reaction is the chemical breakdown of oxygen within unstable molecules 

in the explosive material into more stable derivatives, a process accompanied by the 

release of large amounts of heat. Detonation is the term for the rapid movement of a 

pressure front ahead of the chemical transition front. In a detonating explosive, a 

physical shock front rapidly compresses the explosive material and advances 

significantly faster than the sonic velocity of the material. Beyond a short distance from 

the blast, the main sources of impact are the shock wave and the expanding gaseous 

reaction products. The original shock wave is the primary cause of harm to aquatic life 

at great distances from the shot point; the expanding gases, if they do break into the 

water column from the substrate where the explosion occurs, can set up a pulsating 

bubble whose recurring pressure waves may also contribute significantly to damage. 

The manner in which shock and acoustic waves propagate from the source into the 

ocean is strongly influenced by the ocean environment. Noise propagating in shallow 

water (i.e., depths less than a few hundred meters) can reflect many times from the sea 

surface and bottom. In these cases, sea surface roughness and sea bottom 

characteristics are often very important. Fewer surface and bottom interactions occur in 

deeper water. However, refractive effects due to differential temperature and salinity 

profiles can cause sound to be trapped in small depth channels and can lead to sound 

focusing. Sound level metrics are parameters that quantitatively describe the 

characteristics of sound pressure waves at a given spatial location, and their values are 

used to gauge the degree of impact that underwater sound signals have on marine life. 

Standard thresholds for the metrics have been established in reference to the minimum 

levels at which specific impacts have been observed to occur for given species. The 

most common metrics for impulsive sounds are peak pressure; peak-to-peak pressure; 

impulse; root-mean-square (rms); energy flux density; and sound exposure level. A 

modified version of the peak pressure metric, identified as dBht(Species), also has been 

developed that accounts for the frequency-dependent hearing sensitivities of specific 

species. This metric is based on the same principle as frequency weighting schemes 

used for determining impacts of noise on humans. 

The lethal effects of underwater explosions on numerous species of fishes have been 

studied in laboratory and field experiments and have been modeled extensively. These 

data and models have been used to calculate effect ranges. In addition, there have 

been monitoring studies specifically designed to estimate fish kills associated with 

structure removals in the Gulf of Mexico. These studies have documented that most of 

the fishes recovered from explosive removal activities were found within 25 m of the 
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structures, with numbers decreasing with distance out to 100 m, the greatest extent to 

where divers searched for dead fishes. Therefore, it seems that most fishes associated 

with platforms are close enough to be killed or injured by a typical underwater 

detonation associated with structure removals. There are many variables affecting the 

fate of individual fishes, including their size, shape, vertical position in the water column, 

and orientation relative to the detonation source. For predicting impacts, empirical data 

from observations during actual structure removals may be more useful than any 

attempt to calculate impacts based on experiments and models. Huge variations in the 

fish population itself, including numbers, species, sizes, and orientation and range from 

the detonation, would make it very difficult to accurately predict mortalities at any 

specific site. 

The effects of an underwater explosion on marine turtles are dependent upon several 

factors: the size, type, and depth of the explosive charge; the size and depth of the 

turtle in the water column; overall water column depth; and the standoff distance from 

the explosive charge to the turtle. Potential impacts to marine turtles include noninjurious effects (acoustic annoyance and mild tactile detection or physical discomfort), 

non-lethal injuries (“minor” injuries to the turtle's auditory system and certain internal 

organs), and lethal injuries. There have been no laboratory studies of explosive impacts 

on sea turtles, and only limited field observations and experiments. In several 

instances, turtle injuries and mortalities (and in some cases, strandings) have been 

noted following underwater detonations. Only one field experiment has been conducted 

in which sea turtles were exposed at known distances from a structure removal 

detonation; however, that study did not include concurrent pressure measurements to 

estimate the magnitude and duration of the shockwave received by the caged turtles. 

Further, there have been no mechanistic models developed specifically to estimate 

impacts on sea turtles. Rather, it has been assumed that models developed for other 

vertebrates are reasonable approximations. An equation developed for a turtle “safety 

range” was based on field observations of three turtles following an open-water 

detonation and on the criteria for platform removal established by the National Marine 

Fisheries Service (NMFS) – i.e., it was not independently derived from observations or 

experimental data. Data suggest the NMFS “generic consultation” has been effective in 

preventing most deaths or serious injuries of sea turtles. 

For many years, the only data available for predicting blast impacts on marine mammals 

were extrapolations from experiments on terrestrial mammals submerged in ponds. 

Only recently have experimental studies of explosive impacts on marine mammals been 

conducted, using animal carcasses and opportunistic post-mortem examinations of 

stranded animals following detonations. There have been numerous attempts to model 

explosive impacts on marine mammals, mostly focused on larger explosions associated 

with military testing such as ship shock trials. There has also been considerable 

research into sublethal auditory effects such as temporary threshold shift (TTS) as well 

as behavioral responses to underwater noise, primarily from seismic surveys using 

airguns. As with marine turtles, the 3,000 ft (914 m) range monitored under the “generic 

consultation” suggests it has been effective in preventing most deaths or serious injuries 

of marine mammals. Auditory effects such as TTS and permanent threshold shift (PTS) 
ACCESS NUMBER: 85237

may occur beyond the “safety range” monitored during structure removals. These 

effects are of particular concern for marine mammals because of the regulatory 

implications of the MMPA, which prohibits “harassment.” The National Oceanic and 

Atmospheric Administration – Fisheries Division (previously NMFS) has accepted TTS 

as a criterion for marine mammal harassment. 

STUDY PRODUCT(S): Continental Shelf Associates, Inc. 2003. Explosive Removal of 

Offshore Structures – Information Synthesis Report. U.S. Department of the Interior, 

Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS 

Study MMS 2003-070. 236 pp.