APPENDIX M Summary of Questionnaire Responses from Owners and Operators of Double-Hull Tank Vessels1
I. Operation of double-hull tankers
1 a. What is your experience with operational safety of double-hull (DH) tankers in regard to:
Stability during loading and discharging
A. No stability problems. It is important to build DH vessels with center bulkheads.
B. No stability problems.
C. Not perceived as a problem. Officers must be aware of the limitations.
D. Modifications to generic tanker specifications necessary for the company's special cargo trade. Structure added into center tanks. "Caution posters" displayed in Cargo control room; information on any restrictions documented when duties are handed over.
E. No stability problems. However the Trim and Stability Booklets including any restrictions must be complied with. Stability of DH hull tankers is an issue. Early designs largely ignored the trade-off in intact and damage stability characteristics.
F. No significant problems. DH tankers have centerline (CL)heads that reduce free surface moment. OBOs (Oil-bulk-ore [vessel]) do not have CL bulkheads: Masters cautioned of hazardous stability conditions.
G. Special precautions have to be taken with regard to stability.
H. Satisfactory experience. Design requirement for vessels to remain stable during loading or discharge. Concerned about stability problems for ships without longitudinal bulkheads.
I. No specific problems. Officers and crew well trained in cargo operation of double hulls. Sizing ballast pumps in relation to cargo discharge capability important.
J. Problem maintaining stern trim due to discharging patterns in parcel chemical tanker trade.
K. This is an issue but can be dealt with easily.
L. Free surface effect has increased dramatically.
M. No problems with company tankers. A problem for tankers built without a centerline bulkhead in cargo tanks and DB (double bottom) tanks.
N. Stability problems during ballasting and de-ballasting on a 90,000-deadweight ton (DWT) tanker with no centerline bulkhead in DB tank; operational procedures required.
1 b. What is your experience with operational safety of double-hull tankers in regard to:
Safe access to ballast spaces
B. Access through hatches and inclined ladders strictly controlled. Procedures follow Chapter 10 of the International Safety Guide for Oil Tankers and Terminals (ISGOTT). Horizontal stringers and larger longitudinals provide access in tanks.
C. To guarantee safe access, warning signs have to be posted.
D. Access through openings on main deck using steel ladders. Number of bays without access opening minimized. Horizontal decks provide access for inspection and maintenance. Large openings in intermediate decks for direct access to other levels.
E. Stringent safety regulations in effect: no problems experienced. Classification requirements for design do not provide good access for inspection. In the absence of permanent access some inspection methods (rafting, etc.) are more difficult in DH spaces.
F. Complexity of structure may increase safety risks compared to single-hull (SH) tankers. More openings on deck may be required for adequate ventilation.
G. Access to ballast spaces good.
H. Satisfactory experience. DH vessels designed with built-in walkways and adequate access. Space entry procedures enforced.
I. Vertical ladders in lieu of inclined ladders: not unsafe but harder to use. Independent rescue hatches in every tank for direct access to main deck in case of emergency.
J. People should not go into ballast spaces unless necessary. Current requirements for access sufficient.
K. Same care required as on entry into any ballast space.
L. More difficult access; longer distance to escape; complex construction requires knowledge of the configuration.
M. Only one entry into ballast tanks: access both fore and aft convenient but not necessary. Large openings for emergency access require provisions to prevent falls.
1 c. What is your experience with operational safety of double-hull tankers in regard to:
Ventilation of ballast spaces
A. Portable fans at hatches. Flexible hoses if needed.
B. Ventilation through ballast lines. Air supply from inert gas main connected to a flexible hose. Ventilation time (4,000 m3 in DB and 5,000 m3 in side) on order of 3.5 to 4.5 hours. Alternative: fill tanks with water and then empty them.
C. Ventilation via airpipes at forward and aft end of tanks, if necessary using portable water-driven fans.
D. Ventilation by mushroom ventilators. Opening sizes adequate for air intake and exhaust.
E. Flooding relief vent head and hatch opening for natural vent during ballast or de-ballast operation. Purge pipes from deck to centerline bulkhead. Mechanical ventilation and vapor testing prior to entry. Forced ventilation to double bottom difficult.
F. See response to I b.
G. Ventilation of ballast spaces good.
H. Satisfactory experience. Sufficient venting facilities provided. Ventilation on DH ships requires more attention than on SH vessels.
I. Difficult to ventilate ballast tanks. Cross-connection from inert gas line to ballast line can provide good circulation even in DB tanks.
J. Can be done adequately with existing fixed and portable units. Testing and ''safe entry" procedures are important.
K. As in any other vessel design.
L. Pockets without oxygen may exist. In case of oil leakage, pockets of flammable gases may exist even after ventilation.
M. Flexible hose used to provide air; discharge through the tank opening. Safety always a major concern. Risk of cargo leakage to DH spaces overemphasized: must always be vigilant. Instrumentation may lead to complacency.
1 d. Any other safety issues that need to be addressed.
A. DH tankers are always capable of ballasting to a safe draft. Inerting of ballast tanks possible using flexible hose connections to inert gas plant.
C. It is important to have a capability to inert all ballast and void spaces using emergency connections to the ballast pipe in the double bottom.
D. Modifications made: ballast pump designed to trip when tank 98 percent full; hydrocarbon detection sensors in ballast spaces; inert gas system (IGS) can be used for forced ventilation; portable probes to test ballast tank atmosphere and treat water for microorganism.
H. Must have means to promptly detect hydrocarbons in ballast spaces and identify structural problems.
I. Accumulation of sediment in double bottom requires good drainage (design requirement). Fixed IGS (for ballast spaces) should be a requirement.
J. Product tankers should not be allowed to carry noxious liquid substances (personnel do not have proper experience, and construction standards are insufficient).
L. Permanent IG piping should be considered for all segregated ballast tanks (SBTs) and void spaces in case of oil leakage. More stringent check of tank atmosphere prior to entry required.
N. No difference in shiphandling.
2. Are there significant differences in cargo operations between double-hull and single-hull tankers?
A. DH tankers are more flexible in ballasting and de-ballasting: More operational flexibility. Can discharge part cargoes in almost any sequence.
B. Cargo operations generally easier on DB tankers: stripping and cleaning easier. Stripping can normally be done by main cargo pumps and time for tank washing is reduced.
C. Cargo tanks on DH tankers easy to discharge and clean. Unloading operation is faster on DH tanker or OBO (quicker discharge, tank washing, and stripping).
D. Enforced on DH tankers: understanding of arrangement (subdivision, piping, etc.) ; monitoring and constant awareness of stability and restrictions in concurrent ballast-cargo operations; listing moments; shear and bending stresses; cargo heating control.
F. No significant differences. Cargo operations easier on DH tankers.
G. Stability has to be carefully monitored during cargo operations.
H. DH tankers have better cargo outturn and tank washing characteristics.
J. No difference in cargo operations. Significant differences in design, maintenance and life expectancy (depends on planning and maintenance).
K. Monitoring of stability.
L. Accurate stability calculations required prior to and during cargo operations. No ballast in cargo spaces.
N. Discharge of DH ships better than SH ships.
3. Have you established operational procedures specifically for double-hull tankers?
A. Cargo and ballast operations on double-hull tankers are comprehensively described in vessels' operational procedures.
B. Monitoring of ballast tanks emphasized. During loaded passage, all ballast spaces are monitored weekly using portable gas detectors and checked with a sounding rod. Visual check after ballasting. All checks recorded.
D. DH tankers: ventilation of ballast spaces; stability instructions; IGS operation for ballast spaces; restrictions in ballast or cargo handling documented for "hand-over"; coating inspection and maintenance; detection of hydrocarbons in ballast spaces.
E. No special operating procedures established except requirements in Trim and Stability Booklets for ballasting and cargo handling sequence.
F. No; except cautionary advice as required.
G. Procedures for cargo operations with regard to stability.
H. Existing operational procedures adopted from those for DB tankers.
J. No. All ships have double bottoms and/or double sides.
K. Yes, to address stability with free surface in cargo tanks.
L. Stability procedures and procedures in case of oil leakage to ballast tanks have been established.
II. Inspection and maintenance of double-hull tankers
1. Please provide information on structural and tank coating inspection frequencies and practices on double-hull tankers.
A. Ballast and cargo tanks are inspected at least once a year.
B. Each laden voyage, ballast tanks inspected. Coating and structure inspected in two tanks each laden voyage (i.e., all tanks inspected every year). Minor coating repair during inspection.
C. Crew inspects ballast and void spaces every three months and repairs paint damage when necessary. Detailed inspection by an independent surveyor approximately every 2.5 years.
D. Coating and structural inspection at least once a year by technical inspector. Safety inspection every 120 days. "Guidelines for Enhanced Survey" and "Standard Coating Condition Inspection Guidelines" followed. American Society for Testing and Materials (ASTM) rust grade principles applied.
E. Crew inspects ballast tanks annually. Outside contractors monitor coating and structure on a schedule that follows survey schedules: new vessels, five-year cycle; older vessels, 2-3 year cycle.
F. Structural and coating inspection of coated tanks every other voyage. Ballast tanks inspected at least every other month.
G. Ballast and cargo tanks inspected annually by superintendent or ships' officers and by classification society as required.
H. All tanks are inspected on six-month schedule.
I. Visual inspection every three months.
J. Structure and coatings inspected every six months.
K. Nearly 100 percent sound coating should be maintained.
L. Coating inspection every six months. Possible damage repaired after each inspection.
M. Eggcrate-type structure improves quality of inspection on DH ships. Areas not easily accessible inspected using video camera, portable staging, and rafting. If additives are used for mud removal, surfaces become very slippery.
2. What is your experience with different types of coating in ballast spaces? Have you encountered significant corrosion problems? If so, please describe.
A. 27,000 DWT DH tankers since 1988: routine maintenance during yearly inspections; no significant corrosion problems. Coating in ballast tanks light colored. One of the 299,000 DWT DH tankers has had coating damage due to poor work at the yard.
B. Tried soft coatings with limited success. Proper protection with epoxy paint system (properly formulated and applied). No severe corrosion problems in coated, regularly inspected or maintained tanks.
D. High built coal tar epoxy coating. Good surface preparation essential. Experience limited to four years; no significant breakdown or corrosion. Quality control during construction is key to good coating.
E. Five vessels built in 1970s required considerable attention between 15 and 20 years of age. Vessels that carried heated cargo had far worse failure rates than others. Some coating failure on new vessels due to poor quality control during construction.
F. Coal tar epoxy used. No major corrosion problems; touch-ups made as required.
G. Tar epoxy. No special problems encountered.
H. Corrosion in ballast spaces of SH and DH vessels wherever coatings not properly maintained.
I. Used both coal tar epoxy and light-colored epoxy systems. Sporadic failure of coal tar systems on two vessels.
J. Experience with coatings from soft tar to pure epoxy. Soft epoxy tar-type, poor for wet spaces; pure epoxy, best. Significant corrosion in DH spaces in the past. Corrosion problems contributed to scrapping six ships in past five years.
K. Careful initial coating application and adequate coating thickness give effective corrosion control.
L. Prefer to answer question after company has more experience with OBOs built 1992-1994. Coating to date in excellent condition. No corrosion.
M. Importance of surface preparation emphasized. Continuous coating inspection and maintenance is key to success. Light-colored coatings preferred. Spot maintenance extended life of coating to 15 years.
N. Combination of epoxy coating and anodes gives a long life.
3. What are your current practices with regard to ballast tank coatings (include type, number of coats, thicknesses)? From your experience, what is the expected life of the coatings?
A. 27,000 DWT, built 1986—2 × 150 microns of coal tar epoxy; 299,000 DWT, built 93/95—1 × 150 microns surface tolerant epoxy, 1 × 150 microns modified tar epoxy (light color); with proper maintenance and initial application, coating will last vessel's life.
B. Surface preparation and two coats of tar epoxy. Stringent inspection and quality control during building. Effective lifetime of coating system 1520 years.
C. Coal tar epoxy 2 × 125 microns plus anodes (pitguard anodes on bottom). With proper maintenance painting system lasts vessel's life. Next generation of OBOs will use light-colored coatings for ease of inspection.
D. High built type coal tar epoxy. DFT minimum 200 microns. Aluminum and/or zinc anodes. With ideal surface preparation, 25 years; realistically, 15 years.
E. Experimenting with coating suppliers and blasting methods. Primary method for large-scale maintenance work: dry grit blast, dehydrating, and coating with Devoe 235 Epoxy, two coats plus stripe.
F. Coating thickness of coal tar epoxy 250-300 microns. Expected life approximately 10 years.
G. 2 × tar epoxy, each 125 microns. Grinding of edges and strip coating. Life expectancy 10-20 years depending on workmanship.
H. Two coats and two stripe coats of light-reflecting, light-colored, modified epoxy (not coal tar base), minimum 250 microns total DFT. 100 percent anode system. With normal maintenance, life expectancy at least 15 years, provided quality work at construction.
I. Light color epoxy 2 × 150 micron dry film thickness. Expected life 10 years.
J. Minimum three coats (100-150 micron thickness each) pure epoxy. Life expectancy 20 years with minimal maintenance.
K. Tar epoxy, 500 microns in three coats. Maintenance and cathodic protection extend life of coating to useful life of ship.
L. Surface preparation to SA 2.5, coal tar epoxy, 2 × 125 microns, plus three stripe coats. Expected life 15-20 years.
4. Do any of your maintenance and inspection practices for single-hull tankers differ from those used on double-hull tankers?
B. More effort in monitoring ballast tanks on the first-generation DB tankers.
D. Inspection on DH tankers simple compared to SH tankers. More stress concentrations in DH tanker structures due to higher rigidity; more frequent inspection warranted.
E. Coating inspection and maintenance more critical in DH vessels: high cost of coating replacement, low life-time expectancy for replaced coating, if steel replacement required, may force vessel to early retirement.
G. Operates only double-hull tankers.
H. Periodic inspection and maintenance of DH spaces.
I. Flushing of sediment from double bottoms. Increased inspection requirements.
J. Not applicable.
K. Maintenance and ballast tank coatings.
L. Same as far as coated tanks are concerned.
N. Cost of maintenance less for DH than SH tankers due to improved accessibility (egg-crate structure).
III. Design of double-hull tankers
1. Have you had any structural problems on double-hull tankers?
A. Heavy weather damage on a very large crude carrier (VLCC). Ballasting more forward prevented similar problems.
B. So far no structural problems.
C. A small number of leakages into upper stool spaces due to faulty welds.
D. No significant problems (five-year operational experience).
E. No structural problems on new tankers. On older vessels, coating more of a problem than structure.
F. No major problems. Detail design and welding sequence important.
G. No problems.
H. No problems to date.
I. No problems in newbuild DH tankers. Company strongly supports efforts of American Bureau of Shipping (ABS) to modernize ship structure analysis.
J. No significant problems.
L. So far no problems, but complex design details increase possibility of fractures.
M. Some structural details are areas subject to fracture. Structural modifications have been carried out.
N. Two minor incidents of leakage into double hull; three minor incidents of leakage into double bottom.
2. What is your experience with high-strength steel construction?
A. No particular problems.
B. Good experience. Attention paid to details and workmanship. Too often necessary to increase scantlings above rule requirements. Buckling and fatigue criteria checked. Careful in defining loading conditions.
C. No problem as long as good coating system protects against corrosion.
D. Excellent experience. Corrosion protection key to long life: reduction of plating thickness from 19-32 mm in 1970s to 17-19 mm enforces this. Exposure to excess heat should be prevented. Dedicated crew and committed technical support important.
E. No experience worthy of comment. New vessels built with approximately 70 percent high tensile steel (HTS), limited to Grade 32.
F. Limited experience. Company believes in use of mild steel. High tensile steel kept to a minimum in newbuildings.
G. Cracks in high tensile side shell longitudinals between bilge keel and ballast load line.
H. Localized high stresses and fatigue can lead to accelerated corrosion and cracks. Location, amount, and type of HTS and shipbuilder's experience with HTS important.
I. Company's ships have larger percentage of mild steel than other tankers. DH fleet too young to show problems associated with high-strength steel.
J. Currently not using high-strength "black" steel in new designs.
K. May not be available everywhere for repairs.
L. High-strength steel construction requires good surface preparation and good-quality coating, as scantlings are reduced.
M. Experienced high number of cracks in 165,000 DWT class due to use of high tensile steel.
3. What design changes would you suggest in future double-hull tankers?
B. Normally minimum class requirements for longitudinal bending moments insufficient. Deflections of secondary members important. Ballast tank amidships to reduce bending moment is a possibility. Details according to Tanker Structure Cooperative Forum (TSCF) recommendations.
C. Better accessibility to ballast tanks and light color paint in ballast spaces.
D. High-tensile steel should be restricted to internal structure. Reduction in steel thickness if coating thickness increased not to be allowed. Certification of outer shell structural welding inadequate and inconsistent. Surface preparation rules needed.
E. Standards for access, staging fittings, coatings should be mandatory rather than at option of owner.
F. Double hulls may be useful for smaller tankers (order of 45,000 DWT), less so for larger vessels. Perhaps a gradual double hull replaced by double bottom for larger vessels.
H. Requirement for inherent positive stability throughout ballast or cargo handling; light-colored coatings; high-volume, continuously monitoring hydrocarbon detection system; shipyard design and practices to be certified by class if high tensile steel used
I. Requirements for redundancy, alarm, and automatic changeover for steering gear in event of single failure. Increased powering requirement. Requirement for emergency propulsion.
J. Design specific to trade and size. What is applicable for an ultralarge crude carrier (ULCC) or VLCC is not applicable to smaller ships. In general, U and L tanks should be avoided for small size ships.
L. Longitudinal center bulkheads would improve stability.
M. Easy access for inspection should be included in structural design. Coating regulations, which inhibit development, should not be established.
IV. Fleet Information
1. Based on your experience, what are the advantages of double-hull tankers compared to single-hull tankers?
A. Always capable of ballasting to safe draft for immediate departure in case of emergency.
Capable of ballasting to heavy weather ballast without cargo tanks.
No water in cargo tanks—practically no corrosion; increases expected lifetime.
Easy access to frame structures that are mainly in ballast spaces.
Almost complete discharge of cargo.
Easy tank cleaning.
Increased environmental protection.
B. Added protection against cargo outflow in case of low-impact casualty.
Efficient stripping and tank washing; good cargo turnout.
C. Faster cargo unloading (discharging, tank washing, stripping)
D. Psychological shield in low-impact groundings. (However, due to structural rigidity may cause fracture of shell plating.)
Politically acceptable design.
E. Greater cargo outturn.
Fewer tank washing machines.
Greater protection from minor contact damage or oil spill.
F. Easier to load and discharge.
Good protection in low-impact collisions and groundings.
G. Safety in groundings or collisions.
Easy to clean cargo tanks.
Easy to empty cargo tanks.
H. Pollution protection for certain types of casualties.
Better cargo outturn and pumping performance.
Superior tank washing results.
Better access to inspect ballast tank structure.
Meets legal requirements.
I. Safer than single hulls.
Cleaner than single hulls.
J. Regulations will hopefully force scrapping of older ships.
L. Greater SBT capacity.
Reduced risk of pollution in case of grounding or collision.
Reduced risk of pollution.
Better heating performance.
Better stripping ability.
M. Eggcrate structure in way of side shell and bottom structure resistant to fatigue- related failures.
Inspection-friendly structure if intermediate stringers provided in wing ballast tanks.
N. Double hulls eliminate piping leaks as major source of pollution (no cargo pipes in ballast tanks).
2. Based on your experience, what are the disadvantages of double-hull tankers compared to single-hull tankers?
A. Larger, lightweight, beam and draft.
More expensive to build.
More expensive canal and port expenses.
Today's market offers no compensation for higher costs of DH tanker.
B. Ballast tanks have large surfaces coated with sophisticated and expensive coating: need continuous monitoring and maintenance.
Cleaning of ballast space after a possible leakage.
Higher building cost.
D. Excessive cost for no gain in safety or environmental preservation.
Reduced cargo capacity.
Increased ballast (non-earning).
Increased port dues and insurance costs due to increased gross registered tonnage (GRT).
Increased coating areas in ballast spaces.
Heavier (not necessarily stronger) hull structure.
Potential for hydrocarbon leakage to ballast spaces. Potential for explosion.
Increased longitudinal forces.
Increased transverse free surface.
Poor accessibility for inspection and maintenance in double bottom.
Poor initial, static, and dynamic stability.
Extra maintenance costs.
Structure will not withstand forces due to collision or grounding.
Alternate design should be considered.
No return on higher cost. Most oil majors continue to embrace substandard tonnage at low freight cost.
E. DH vessels need more resources to properly manage them. Inadequate coating maintenance, structural problems if vessels built to class rules only, and stability problems may lead to problems for the industry.
More critical stability.
Higher construction cost.
Ballast tank coating critical issue.
Ballast tank ventilation difficult.
Difficult to salvage after hard grounding.
Greater beam or freeboard.
F. More equipment required to monitor void spaces.
Reduced cargo carrying capacity.
More surfaces to maintain.
For larger vessels, not much advantage in way of environmental protection.
G. Stability problems if no centerline bulkhead.
Problems if leakage in inner hull.
H. Explosion risk in double-hull spaces if vapor detection system not fitted.
Increased construction and maintenance cost.
Stiffer hull structure may lead to localized cracking.
Increased vigilance required to ensure integrity of double-hull spaces.
Increased port and insurance costs due to greater GRT.
I. Approximately 10 percent increase in cost.
L. Stability problems with center bulkhead.
Risk of fractures and oil leakage into SBT could create dangerous atmosphere.
Difficult to clean, gas free, and repair.
Possible risk of local steel wastage or loss of strength due to deteriorated coating.
N. Cleaning mud from ballast spaces a bigger problem than on SH ships.