PRINCIPAL HAZARDS

Hydrocarbons

Oil and Gas Escape

The main hazard in scientific ocean drilling with respect to pollution prevention and safety is the possibility of encountering a charged reservoir, thereby allowing oil or hydrocarbon gas to escape in large quantities into the sea and atmosphere. Because natural submarine seeps of both oil and gas exist in many parts of the world with little apparent deleterious effect on the environment, it is difficult to determine what amount of oil or gas release during drilling operations should be termed hazardous. Certainly, as a pollutant, oil must be considered more serious than gas. However, as a hazard to personnel and property, gas is more dangerous than oil because of its mobility, high flammability, and negative effect on water buoyancy and because of the difficulties in controlling its pressure.

Hydrocarbon Origin and Occurrences

The term "petroleum" is applicable to any hydrocarbon substance, although it is popularly reserved for crude oil, natural gas, and asphalt. Mixtures of petroleum hydrocarbons exist as gaseous, liquid, and solid phases depending on temperature, pressure, and composition of the system. Under Earth surface conditions, C1–C4 hydrocarbons (methane, ethane, propane, and butane) are predominantly in the gaseous phase, whereas C5 and heavier hydrocarbons are predominantly liquid.

Hydrocarbon gases, largely methane (C1), may be generated in significant quantities from organic matter in sediments (Pimmel and Claypool, 2001), either under near-surface conditions by bacterial action (Claypool and Kaplan, 1974) or at greater depths by thermochemical action (Schoell, 1988). Liquid petroleum (oil), however, is almost exclusively the product of thermochemical generation from hydrogen-rich organic matter in deeply buried sediments. This generation appears to become quantitatively important only as temperatures reach 50°–150°C (typically at burial depths of 1500–5000 m for average geothermal gradients). Hydrocarbon gases are generated with the oil, and, although they consist largely of methane, they usually include quantities of ethane, propane, butane, and heavier hydrocarbons. Thermochemical conversion of organic matter to hydrocarbons continues at accelerating rates with increasing depth and temperature until all organic matter including the oil itself has been converted largely to methane and carbon-rich residues. It should be stressed that, although biogenic hydrocarbons are generated at relatively shallow depths and thermochemical hydrocarbons at relatively greater depths, either may be found at any drilling depth because of migration, subsequent burial, or exhumation.

Biogenic methane is commonly found in swamps, where it is known as marsh gas, but it is also formed in marine sediments that contain sufficient concentrations of organic matter. Biogenic methane can usually be distinguished from thermochemical methane by means of isotopic ratio mass spectrometry; the biogenic form has a distinctly greater abundance of light carbon isotope 12C relative to the heavy carbon isotope 13C. Although thermochemical methane is formed along with ethane and heavier hydrocarbons in the early stages of hydrocarbon generation, the ratio of methane to ethane gradually decreases as hydrocarbons of thermochemical origin become more abundant. More complete discussion of geologic factors involved in the origin and occurrence of petroleum can be reviewed in Tissot and Welte (1984) and Hunt (1979).

Both biogenic and thermochemical methane may be found in many ODP boreholes. There is no appreciable difference in their physical and chemical properties. Both are flammable and can cause blowouts. Both can be associated with ethane and can occur in substantial quantities at shallow depths. The only significant difference is that the conditions that produce thermochemical methane may also produce liquid oil, whereas oil of microbial origin is unknown.

A common misconception is that methane that is identified as biogenic can be disregarded as a safety hazard. A serious blowout occurred in offshore drilling in Cook Inlet, Alaska, apparently due to biogenic gas. One of the world's largest gas fields and >20% of the world's gas reserves are apparently biogenic. It has been wrongly suggested that if methane/ethane or 12C/13C ratios exceed certain values, gas dangers can be dismissed because it is only "marsh gas," not true thermogenic gas. It is the quantity of gas present in reservoir strata rather than its origin that is of primary concern.

The PPSP and TAMUSP (usually referred to as the Safety Panel) strongly favor obtaining all information possible on the character of hydrocarbons in ocean sediments. However, because of the multitude of geological, geochemical, operational, and experience factors that enter into decision-making concerning safety, PPSP considers it a menace rather than an aid to safety to set "magic numbers" as substitutes for balanced judgment. Arbitrarily imposed numerical guidelines for safety decisions are dangerous because numerical guidelines may be used blindly as crutches to obscure sound and reasoned judgment.

Blowouts

In oil well drilling operations, formation fluids (water, oil, or gas) flow into the well bore when the pressure of the fluid in the reservoir exceeds the pressure in the drill hole. If the fluid entering the well bore is less dense than the drilling fluid, it moves upward in response to buoyancy.

When the formation fluid is gas, gas-charged water, or gas-charged oil, it may permeate the drilling fluid, causing it to be filled with gas bubbles (gas-cut), thus diminishing the drilling fluid's density and ability to exert pressure on surrounding formations. Gas entering the well bore undergoes rapid expansion because of pressure reduction while traveling up the hole. Because of the confinement of the narrow borehole, increasing expansion of gas in the drilling fluid as it moves upward causes a flow of displaced drilling fluid from the hole mouth, further reducing the weight and pressure of the fluid column in the hole. The consequence is a chain reaction. Gas enters the hole at ever-increasing rates as the pressure differential between the gas-bearing formation and hole is increased. If not promptly controlled, the process results in violent ejection of drilling fluid, a wild, unrestrained flow of gas or gas-charged formation fluid at the surface. Such an event is called a blowout and is extremely dangerous to life, property, and the environment.

ODP Operations vs. Petroleum Operations Blowout Risks

Elaborate measures are employed by the petroleum industry to prevent blowout occurrences: weighting of drilling muds, application of backpressure with pumps, use of mechanical BOPs, etc. The drilling equipment used for ODP operations is very different from that used in customary oil and gas drilling, mainly because of lack of means for return circulation, use of seawater rather than heavy drilling mud as a drilling fluid, lack of a riser and BOP, and generally greater water depths involved. In ODP operations, gas encountered under pressure sufficient to cause it to enter the hole, permeate the seawater drilling fluid, and move upward is confined by the hole walls only until it reaches the ocean floor or soft, soupy fluid sediment that often underlies the seafloor. Gas continuing upward would dissipate from the borehole into seawater and reach the ocean surface in lower, perhaps imperceptible, concentrations over a broad area, with dimensions proportionate to the water depth the gas traversed.

Considering the great water depths usually involved in ODP drilling, there is relatively less danger of violent discharge of gas at the sea surface. However, means of mechanically controlling gas flow into the hole in ODP operations are limited. Moreover, even though the escape of gas or oil at the ocean surface from holes drilled in water depths of thousands of meters might be so diffuse as not to be readily discernible, total pollution of the ocean by hydrocarbons might be substantial over time.

A gas blowout imperils the vessel and crew in several ways: releasing toxic gases, triggering fires, and causing a loss of buoyancy as a result of gas bubbles charging the surrounding seawater. The shallower the water at the drill site, the greater the potential of danger of buoyancy loss, which could destabilize the ship.

The greatest fire danger on the JOlDES Resolution would result if a blowout occurred through the drill pipe. In relatively shallow water, gas escaping to the surface from around the drill pipe may present a fire hazard. ODP drill crews are trained in standard oilfield practices to avoid and control these possibilities. Buoyancy impairment is unlikely in water depths usually encountered at ODP sites. However, buoyancy problems have occurred at least twice in commercial drilling for oil in shallow waters and cannot be ignored at shallow ODP sites.

Intercommunication between Reservoirs/Fluid Exchange

Situations can occur where formation fluids flowing up the borehole from deep, overpressured zones encounter shallower, lower-pressure zones. Under these conditions, the higher-pressured fluids (oil, gas, or water) may enter a zone that opens to the seafloor via fractures or permeable beds, resulting in an uncontrollable leak. The higher pressured fluids may charge shallower zones with fluids having greater than normal hydrostatic pressures, thus making even shallow future drilling in the area hazardous. It is also possible, though not likely under most ODP conditions, that deep saline formation water might contaminate shallower freshwater offshore aquifers in this way.

Drilling Active Ridges

High-temperature hydrothermal systems close to magma chambers present special hazards for scientific ocean drilling. The behavior of water in hydrothermal systems is governed by pressure-volume-temperature (PVT) relationships. When the specific volume of water at constant temperature is plotted as a function of pressure, there is an abrupt change of slope below the critical temperature of water (374°C) corresponding to the phase change resulting from boiling. Above the critical temperature, the rate of change of volume with pressure is gradual. Cold and thus denser seawater pumped down the drill pipe provides a hydrostatic overpressure that suppresses flow into the pipe. Steam flow to the surface through a cold drill string is extremely unlikely, especially if some seawater is being pumped periodically. Cold (2°–4°C) seawater cools the hole near the bit by as much as 90°C, which can cause thermal stressing and sloughing of rock chips into the hole. Gradually cooling the hole by circulation every 500 m while tripping in the hole can reduce thermal shock.

Bottom-Simulating Reflectors and Hydrates

The known presence of bottom-simulating reflectors (BSRs), hydrates (clathrates), gassy sediments, and H2S should be considered at the precruise meeting, and special precautions should be reviewed with Transocean and noted in the Scientific Prospectus operations plan. Operations may be slowed to permit adequate evaluation and handling of the cores. Operations may be terminated if liner failures or unsafe levels of gas or H2S are detected in the core handling area, laboratory cutting room, or enclosed ship areas.

There are several hazards that could occur from a combination of these effects. Hydrates and authigenic (biological methanogenic) carbonates can form an effective pressure seal and free gas can accumulate under the seal (Leg 164). PCS data have indicated that the biogenic gas pressure can be 350 psi above seawater hydrostatic pressure (i.e., it is overpressured) at 450 mbsf; however, no gas flow has been noted to date in BSR penetrations. Poor permeability in silty clays under hydrates may have restricted flow thus far, although this may not always be the case. Hydrates have been analyzed as 98.5% methane and 1.5% carbon dioxide. Typically, hydrates are not composed of thermogenic or liquid hydrocarbons; nevertheless, BSRs and hydrate sections should be penetrated carefully (see Sassen et al., 1998, for an exception).

Hydrogen Sulfide

Hydrogen sulfide (H2S) is the principal noxious gas that could be released during ODP drilling operations. H2S is easily detected in extremely low concentrations by its characteristic odor and by using commercially available monitors. It is a transparent, colorless, flammable, heavier-than-air gas that is lethal in concentrations measured at a few hundred parts per million. Below 100 ppm, this gas is characterized by its rotten egg odor. However, over a period of a few minutes at concentrations approaching 100 ppm, ability to smell this gas is lost. The threshold limit, 10 ppm, is the concentration at which it is believed that all workers may repeatedly be exposed, day after day, without adverse affects.

Concentrations of 250 ppm are considered hazardous and may cause death with prolonged exposure. Concentrations of 700 ppm are considered to be lethal and will cause death with short-term exposure.

Geochemical considerations, together with past drilling experience, direct observations, and sampling from research submersibles, have shown that excessive H2S may interact with high temperature to further complicate active ridge drilling. H2S solubility in water is a function of PVT conditions. This fact dictates a safety approach in which depths and temperature anticipated at specific drill sites determine safety measures to be taken for a given active ridge drilling leg. This approach was followed in drilling on the Juan de Fuca Ridge and Escanaba Trough (Legs 139 and 169) and the Trans-Atlantic Geotraverse (TAG) massive sulfide deposits (Leg 158). Extensive safety procedures for avoiding H2S-related accidents were spelled out for Leg 139 in the Hydrogen Sulfide–High Temperature Drilling Contingency Plan, ODP Technical Note, 16 (Howard and Reudelhuber, 1991). ODP Technical Note 16 was updated in 1993 and became Revised Hydrogen Sulfide Drilling Contingency Plan, ODP Technical Note, 19 (Foss and Julson, 1993). ODP Technical Note 19 was recently updated to Hydrogen Sulfide Drilling Contingency Plan, ODP Technical Note, 33 (Mills et al., in press).

Unusual isolated concentrations of H2S gas are possible, especially in cases of active hydrological downflow or sulfate-rich upflow in faults or in carbonate-rich sediments where H2S is not quantitatively precipated as iron sulfides (e.g., pyrite) because of low iron contents. H2S concentrations to 150,000 ppm (vacutainer) were handled safely in core sections from one site during Leg 182. However, coring operations should be suspended when H2S concentrations in the ambient air on the core-handling deck exceed 10 ppm until proper safety measures can be implemented. Operations should be terminated if necessary core handling procedures on the catwalk and in the laboratories cannot be completed in a safe manner.

Most gas hydrates encountered by ODP have contained mostly C1 with a small amount of CO2; however, H2S was noted in the presence of hydrates during Leg 146. Therefore, hydrates should be treated with extreme caution because of the potential for sudden high-volume releases of H2S. If H2S is noted in the presence of hydrates, a full H2S alert should be declared and coring should be halted pending an evaluation of the situation.

ODP Technical Note, 16 (Howard and Reudelhuber, 1991) reviews extreme safety procedures for a heavy hydrogen sulfide leg; however, experience in handling H2S cores and new safety equipment (such as air dilution fans, hose-fed air packs, and gas evacuation fans) has improved H2S safety procedures and permitted safe handling of degassed (<10 ppm) H2S cores.

If the potential for H2S is known or suspected in an operating area, H2S precautions should be reviewed before the leg, a training program should be conducted for all personnel, an H2S evacuation drill should be conducted, general H2S precautions should be in effect, safety equipment should be serviced and staged, laboratory personnel should receive safety equipment training, and monitors should be calibrated and in operation (Mills et al., in press). H2S concentrations are normally <50 ppm in the normal near-seafloor sulfate reduction zone, which is fed by seawater (to ~40 mbsf). Cores containing H2S are quickly degassed outside on the core-handling deck by drilling holes in the liner and sectioning the cores. The H2S is diluted by normal airflow mixing aided by the fan on the core-handling deck. The suction fan in the core-cutting room should be used to further vent gas liberated by cutting the cores. Marine Laboratory Specialist (MLS) personnel may need to wear air packs when handling and cutting the cores. It is prudent to allow core sections with H2S concentrations >10 ppm to degas on the outside core storage rack.

Environmental Hazards

Weather

Transocean is required to provide trained personnel for weather-related duties. ODP is responsible for providing and maintaining the weather equipment and providing training in its operation. The vessel's deck officers are responsible for copying and interpreting weather maps and satellite photos, as well as recording and transmitting routine weather observations. The Transocean Offshore Installation Manager (OIM), ODP Operations Manager, and Co-Chief Scientists should stay informed about the approach of storms or other weather conditions that could affect operations; however, the Master's (i.e., Captain's) decision is final in weather-related matters concerning the safety of the vessel and/or onboard personnel. This includes course changes to avoid or minimize weather effects, tripping or hanging-off the drill string, departing the area, etc.

The JOIDES Resolution may encounter extreme weather conditions such as cyclones, otherwise known as typhoons or hurricanes, and storm tracks and frequencies that are likely to threaten the ship's safety. The Master is required to follow policies for dealing with and avoiding tropical cyclones as set forth in the Transocean Hurricane/Cyclone Contingency Plan. The provisions of the document are highly conservative in terms of lead time to abandon site operations and depart the area.

Currents

DSDP and ODP operating experience indicates that deepwater ocean currents can be more complex and capricious than generally believed. Subsurface currents may exist with velocities and directions in complete disagreement with published charts and they also may come or go completely without warning or on a diurnal cycle. Major currents, such as the Gulf Stream or Arctic currents, can produce strong and deep-running eddies and spin-off vortices of surprising velocity and direction.

A strong current is defined in this document as a sustained general movement of subsurface water mass at a speed of 2.0 kt or more, which m ay induce swirling water motion by movement through, over, and around obstacles, or by interaction with tidal surges. Currents are deep running and distinct from transitory water-mass motion induced solely by surface waves or swells or wind action. Whereas current information on nautical charts and publicly available data can be characterized as generally accurate, the JOIDES Resolution is fixed at a specific site and experience has demonstrated that current effects vary locally and hourly.

Current velocities estimated as high as 3 kt (Kuroshio Current off Japan) and 4 kt (Gulf Stream in the Florida Straits) have been encountered during ODP operations. The current forces were handled by the JOIDES Resolution's propulsion power, but stationkeeping and vessel-motion limits were exceeded when 50-kt winds and 20-ft swells developed at right angles to the Kuroshio Current. The strength of the current forced the vessel to maintain its heading into the current and to lie in the trough. Off northeast Australia and in the Florida Straits, the strong current extended to the seafloor in relatively shallow water and physically tilted the positioning beacon downcurrent, hampering the ship's stationkeeping capability.

The design/contractual capabilities for the JOIDES Resolution include the ability to maintain horizontal position within 3% of water depth with wind limits of 45 kt (gusts to 50 kt), maximum wave height of 27 ft, and surface current of 2.5 kt (with the prevailing environmental forcing function within 30° of the bow or stern).

ODP operations in areas with strong currents (of >2.0 kt) have been affected to a limited extent. Pipe has audibly and visibly vibrated (e.g., strummed by current like a taut string at 20–60 cycles/s) when used in strong current areas. BHAs, drill pipe, casing, and guide bases have been tilted off-vertical in excess of 5° by the force of the surface current against the sail area of the object, which resulted in problems making up the next (vertical) joint and latching the dual elevators. Running pipe can be difficult because the pipe and tool joints are pressed against the upper and lower guidehorns. Vibration-isolated television (VIT) frames have noticeably vibrated, tilted, and "weather vaned." VIT coaxial cables have vibrated, been wrapped around the pipe by a weather-vaning frame, and been pushed against the edge of the moonpool, thereby damaging the cable.

Successful coring operations were conducted in the Gulf Stream and Kuroshio Current where deep-running current velocities were >2.5 kt. On a few occasions, the crew has experienced an inability to maintain the ship position during operations conducted under the following conditions:

  1. A high angle of divergence between strong wind and strong current forces (such as sudden strong wind gusts from canyons or glaciers, storms approaching from the side of the ship, and sudden tidal surges in channels);
  2. Swirling vertical and horizontal vortex-type currents that rapidly changed direction and force (such as Arctic eddy currents over underwater obstructions and sills in the Yermak Straits); and
  3. A rapidly changing interaction of tidal surges and high current in shallow water, such as on the New Jersey Shelf in 100 m water depths, where tidal and (Gulf Stream) current eddies combined to produce strong and rapidly changing environmental forces.

High- and Low-Latitude Operations and Ice

The drillship is adapted for high-latitude operations, with winterized and heated work areas and an ice-strengthened hull to Class 1B for navigation in medium ice conditions. Successful operations have been conducted in both Arctic and Antarctic waters under hostile environmental conditions. Winterization of the ship includes adding special additives to lower the pour point of the fuel, changing to low-temperature coolants and lubricants in topside machinery, rigging windwalls around exposed work stations, activating the special boilers that circulate heated water to various locations, and adding antifreeze to tool storage shucks.

In areas where free-floating ice or other objects may be encountered, "alert zone" and "safety zone" radii are calculated in accordance with procedures jointly developed by ODP and Transocean:

  1. An alert zone radius will be calculated once on site based on the time required to suspend operations, pull out to 50 mbsf, and set the safety landing sub or 500-ton elevators, plus contingency time. The alert zone is dependent on depth and expands as the penetration depth increases. If free-floating ice or other objects enter the alert zone, operations will be suspended and the bit will be pulled to 50 mbsf while the situation is evaluated.
  2. A safety zone radius is also calculated based on the time required to terminate operations and pull above the seafloor far enough for safe maneuvering plus contingency time. The safety zone is also dependent on depth and expands as the penetration depth increases. If free-floating ice or other objects enter the safety zone, operations in the hole will be terminated and the bit will be pulled as far as required to clear seafloor obstacles and permit the ship to move.
  3. If time permits, the Master should keep the Transocean OIM, ODP Operations Manager, Staff Scientist, and Co-Chief Scientists informed of the situation to make a joint decision on the suspension or termination of operations.
  4. A drill-string landing sub below the top drive and/or 500-ton elevators should be used anytime an emergency drive-off situation may occur. If time permits, the compensator will be locked and the 500-ton elevators will be landed on the rotary. All personnel will be restricted from the rig floor.

Shallow-Water Operations

Operations in water depths of <75 m are not permitted at present, and operations in 76–650 m of water require special operational guidelines to ensure safety for the crew and equipment. ODP and Transocean management and supervisors, Master, and Co-Chief Scientists should reach agreement prior to the leg on detailed limitations, operational procedures, etc., that will be imposed to reduce the risk of stuck pipe and operational problems. The operational agreement should be reviewed on the ship prior to arrival at each site so that all personnel are aware of the limitations.

General Guidelines for Shallow-Water Operations

Positioning control is especially critical in shallow-water situations because the short drill string provides less flexure and elasticity if the ship moves off the hole. If a substantial loss of positioning or a horizontal excursion is anticipated, coring should be suspended and the core barrel should be withdrawn.

Guidelines for water depth ranges are as follows:

Positioning Control Considerations and Beacons

Positioning control is especially critical in shallow-water situations because the short drill string provides less flexure and elasticity if the ship moves off the hole. If a substantial loss of positioning or a horizontal excursion is anticipated, coring should be suspended and the core barrel should be withdrawn. As a practical matter, the standard yellow (2% of water depth) and red (3% of water depth) warning lights are overly cautious when operating in shallow-water areas with soft sea-floors because they represent horizontal (lateral) excursions off the hole that are relatively minor. It is advisable in shallow-water areas with soft seafloors to increase the yellow and red warning-light tolerance (Table T5) so that frequent positioning warnings for minor lateral excursions do not unnecessarily shut down operations. The suggested yellow and red warning light tolerances are

In areas with soft seafloor sediment, the pipe can pull into the soft wall of the hole near the seafloor and the curved 350-ft radius on the guide horn reduces severe bending angles at the ship. Excursions of 5%–20% have occurred without drill string damage, and coring was resumed without tripping the pipe. In areas with hard seafloors, a more cautious approach is required to avoid drill pipe damage or getting stuck in key seats.

Beacons for shallow-water operations should have lower power output (i.e., reduced from the standard 208–199 dB) to avoid multi-pathing, which is bouncing sound signals back and forth between the bottom of the ship and the seafloor. Low-powered beacon tests in shallow water and good weather have demonstrated that the narrow transmission angle of a standard beacon transducer can be acquired even with substantial ship excursions and thruster noise (±20% displacement in 200 m water depth with 80% thruster power rating).

Currents in shallow water are often stronger at or near the bottom and may cause the tethered beacon to sway; therefore, it has been necessary in some instances to fix the beacon to a frame (e.g., Leg 133). Operating primary and backup beacons shall be deployed in shallow water, especially where operations could be impacted by confined locations, shipping lanes, potential high currents, severe weather, hard seafloors, or deep-penetration (long term) operations.

Logging in Shallow Water

Logging should not be attempted in shallow water (0–650 m) unless hole conditions are good. The conical side-entry sub (CSES) should not be used to log holes in shallow water. This reduces potential exposure to stuck pipe (especially while handling and rigging the CSES) and the added danger of sticking a logging tool because of bad hole conditions. Holes in shallow water that are logged should be loaded with sepiolite mud after the wiper trip as a precaution to provide the best hole conditions for logging. Upper hole sections (0–250 mbsf) may start to react and swell into the hole after 3–5 days. Upper hole sections down to 400 mbsf tend to wash out to progressively larger diameters and become unstable with extended drilling.

The best hole conditions are normally obtained by logging the upper hole sections as soon as practical; therefore, if time permits, drilling a dedicated logging hole should be considered in reactive formations that require 5 or more days to core. A dedicated logging hole usually provides a fresh and more in-gage hole that has not had time to react or become unstable. This is especially true in shallow water because the trip for a drill bit requires less time and logging operations in unstable holes are more risky.

High-Temperature Formations

Operations have been successfully conducted in 316°C high-temperature hydrothermal zones; however, in high-temperature formations there is a potential danger of steam flash problems, swabbing in corrosive (pH = 2–6) wellbore fluids, and/or H2S. When retrieving core barrels or when a core barrel is in place (holding the float valve open), circulation should be maintained at low pump rates (50 gpm) to prevent swabbing or prevent fluid from U-tubing (see below) up into the drill string. It is sometimes possible to cool high-temperature holes by stopping every 500 m on trips to circulate at 500 gpm. The primary danger of getting stuck in a high-temperature hole is that the temperature limit of the Schlumberger explosive severing devices might be exceeded, especially if the pipe and hole are plugged or cannot be cooled by circulation.

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