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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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Suggested Citation:"2 Background." National Academies of Sciences, Engineering, and Medicine. 2019. Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. Washington, DC: The National Academies Press. doi: 10.17226/25565.
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13 2 Background This chapter provides a brief review of vessel stability as needed to under- stand the concepts and terminology presented in subsequent chapters. The review repeats the discussion on this subject matter from the committee’s Phase 1 letter report.1 This background is followed by an overview of the U.S. flag fleet that is subject to U.S. Coast Guard (USCG) stability regulations and that operates inward of the Boundary Lines. The chapter concludes with a brief discussion of the October 1, 2015, sinking of the SS El Faro that prompted the Save Our Seas Act of 2018 and of specific provisions in the act that state that the USCG require lightship weight tracking and review the effectiveness of stability regulations. VESSEL STABILITY CONCEPTS AND TERMINOLOGY2 Vessel stability is the ability of a vessel to return to an initial upright posi- tion after being disturbed by an outside force. A stable vessel is one that has sufficient righting forces at that moment to counter the forces work- ing to capsize it, so it remains upright. The buoyant forces acting on the vessel’s hull as it heels (i.e., lists, leans, or tilts) create a vessel’s righting forces. Capsizing forces can originate from many sources, including natural forces such as wind, waves, and the accumulation of ice, or operational and loading conditions such as improperly stowed and shifting cargo, the free 1 See http://www.trb.org/Main/Blurbs/178088.aspx. 2 The discussion that follows draws heavily from, and in some cases repeats, the committee’s discussion on vessel stability concepts and terminology from its Phase 1 letter report.

14 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE surface effects of slack tanks, the lifting of gear and cargo over the side, or the tripping forces from a towed vessel. The interaction of these many forces will determine a vessel’s stability and its ability to remain upright (see Figures 2-1a and 2-1b). A vessel’s stability is not static, and the interaction between a vessel’s righting forces and any capsizing forces is dynamic and will constantly vary throughout a vessel’s voyage. Although a vessel may start a voyage with sufficient positive stability (i.e., remaining upright), this is no guarantee that a vessel will maintain sufficient positive stability for the entire voyage. A stable vessel must have sufficient righting forces to counter all of the forces that are working to capsize it not only throughout an entire voyage, but also throughout a vessel’s entire lifetime. Properly assessing a vessel’s stability requires measuring its dynamic stability, which is currently accomplished using two methods: (1) the vessel’s metacentric height (GM), which is a pure static stability measurement; and (2) the vessel’s righting arm curve, which is a quasi-static method. The GM for vessels, first understood in the mid-1700s, was used for the initial stability criteria because it was a practical and simple stability calcula- tion to perform. A vessel’s hull is a complex shape that required a lengthy series of calculations to analyze its true dynamic stability characteristics. However, the vessel’s metacenter could be calculated from only the vessel’s FIGURE 2-1a A vessel with sufficient righting forces. SOURCE: Phase 1 letter report. FIGURE 2-1b A vessel with insufficient righting forces. SOURCE: Phase 1 letter report.

BACKGROUND 15 displacement and waterplane inertia, allowing a designer or shipyard of that period to perform the required initial stability calculations. Vessel stability results from the association of the downward force of the vessel’s weight centered at the center of mass and the upward force of the buoy- ancy centered at the center of the immersed volume. As a vessel heels, the location of the center of buoyancy moves and the path of its motion can be determined by the center of curvature of this curve. The metacenter is this center of curvature (see Figure 2-2). The GM is calculated as the distance between the ship’s center of grav- ity and its metacenter, where a larger GM suggests a greater initial stability. As long as the center of gravity is below the metacenter, a vessel will have positive initial stability. A vessel’s righting arm curve is a graphical display of its righting arms, ranging from the initial upright position to larger angles of heel, which covers all conditions a vessel is expected to encounter over its lifetime. As previously noted, the forces working to right the vessel are created solely by the buoyant forces on the vessel’s hull. The buoyant forces are pushing upward over the vessel’s entire hull surface situated below the waterline. By calculating the center of these buoyant forces for each heel angle, the center of buoyancy is determined. The righting arm then is the horizontal distance between the center of buoyancy pushing up and the center of the vessel’s weight, or center of gravity, pushing down (see Figure 2-3). At each heel angle there is a static snapshot taken of the vessel’s stabil- ity. When multiplied by the vessel’s displacement, the area under the curve for each heel angle determines the energy required to heel the vessel to that given heel angle. Although the GM only measures vessel stability at the upright condition, the righting arm curve calculates stability over a broad FIGURE 2-2 Metacentric height, as a measure of initial static. NOTE: B = center of buoyancy; G = center of gravity; M = metacenter. SOURCE: Phase 1 letter report.

16 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE FIGURE 2-3 Righting arm (RA), also referred to as GZ (the horizontal distance between the center of gravity [G] and the vertical line through the center of buoy- ancy [B]). SOURCE: Phase 1 letter report. range of heel angles, from the initial upright condition to larger angles of heel (see Figure 2-4). The righting arm curve also provides a means to calculate a vessel’s available righting energy, providing a useful measure of the vessel’s stability at the critical higher angles of heel that the GM method cannot do reliably. Research and experience with ship stability over the years indicates that the real-life, highly dynamic stability interactions between the heeling moment and the righting moment can be modeled using practical quasi- static methods. By overlaying curves that represent the forces acting to capsize the vessel on the righting arm curve, which is acting to right the ves- sel, a variety of stability criteria were developed that represented the ability of a vessel to withstand heeling moments. An example is the severe wind and rolling criterion. This criterion was developed to measure the stability of a vessel hove to in beam seas during an extreme storm. This criterion uses three overlays on the righting arm curve to approximate the dynamic motions a vessel would experience in a storm’s high winds and large beam seas (see Figure 2-5).3 Similar heeling arm overlay methods are used to determine the effects of operational capsizing forces on a vessel’s stability, such as towing a barge or lifting a weight over the side, or the effects of water on deck from a boarding sea. Most stability-related concerns associated with today’s vessels are covered by the current generation of quasi-static stability criteria. 3 See http://www.imo.org/en/KnowledgeCentre/IndexofIMOResolutions/Maritime-Safety- Committee-%28MSC%29/Documents/MSC.267%2885%29.pdf.

BACKGROUND 17 FIGURE 2-4 Righting arm curve. NOTE: Righting Arm (RA) (or GZ) is the horizontal distance between the center of gravity (G) and the vertical line through the center of buoyancy (B). SOURCE: Phase 1 letter report. FIGURE 2-5 Righting arm curve approximating severe wind and rolling on a vessel. SOURCE: Phase 1 letter report.

18 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE U.S. FLEET SUBJECT TO COAST GUARD STABILITY REGULATION Evaluating U.S. stability regulations requires an understanding of the vari- ous types of vessels regulated by 46 CFR and the services they provide. The stability regulations that apply to the inspected vessels categorized in the tables that follow are the focus of this report. The tables are based on information provided by the USCG for the Phase 1 study, which were updated for this second phase. Table 2-1 shows the number of inspected vessels by length and basic type (cargo and passenger). Of the 12,118 total vessels in the fleet as of May 2019, slightly more than half are passenger type, while the remainder are cargo vessels. The vast majority of the vessels in Table 2-1 are smaller (shorter length) ships, barges, and boats serving domestic routes and op- erating in coastwise, protected-waters, or inland service. The traditional, longer, ocean-going ship is a small part of the U.S. vessel fleet, accounting for fewer than 10% of vessels. Of the more than 5,500 cargo vessels, only 908 are self-propelled because most are barges used in the towing industry (see Table 2-2). Tables 2-3 and 2-4 provide more detailed breakouts of vessels subject to 46 CFR tabulated by inspection category (applicable subchapter), do- mestic versus international service, and route type. TABLE 2-1 Number of U.S.-Inspected Vessels by Type and Length Cargo Total Length (feet) Number Passenger Vessels under 65 4,518 65 to 79 480 79 to 328 1,110 greater than 328 24 Passenger Total 6,132 Cargo Vessels under 79 374 79 to 262 2,919 262 to 328 2,142 328 to 492 237 greater than 492 314 Cargo Total 5,986 NOTE: Discrepancies in the tables and charts are due to inconsistencies in the raw Marine Informa tion for Safety and Law Enforcement (MISLE) data, such as missing or incorrect entries. SOURCE: USCG, MISLE database (data as of May 16, 2019).

BACKGROUND 19 TABLE 2-2 U.S.-Inspected Fleet—Cargo Vessels by Propulsion Cargo Vessel Type Inspection Subchapter TotalD I I-A O OD OI L Self-propelled 74 473 1 0 42 6 312 908 Non-self-propelled 948 234 4 6 3,256 139 12 4,599 Total 1,022 707 5 6 3,298 145 324 5,507 NOTE: Discrepancies in the tables and charts are due to inconsistencies in the raw MISLE data, such as missing or incorrect entries. SOURCE: USCG, MISLE (data as of May 20, 2019). TABLE 2-3 U.S.-Inspected Fleet by Route and Subchapter Passenger Vessels Non-Ocean-Going Ocean-Going Total Subchapter H 130 20 150 K 379 34 413 R 8 17 25 T 4,553 1,172 5,725 U 0 22 22 Total 5,070 1,265 6,335 Cargo Vessels Non-Ocean-Going Ocean-Going Total Subchapter D 699 323 1,022 I 33 674 707 L 2 322 324 O 5 1 6 OD 3,202 96 3,298 OI 144 1 145 M 445 179 624 I-A 0 5 5 Total 4,530 1,601 6,131 NOTES: The Non-Ocean-Going category includes vessels with the following Certificate of Inspection (COI) routes: Rivers, Lakes, Bays, and Sounds; Lakes, Bays, and Sounds Plus Limited Coastwise; Lakes, Bays, and Sounds Plus Limited Great Lakes; and Limited Coastwise. The Ocean-Going category includes vessels with the following COI routes: Great Lakes, Coast- wise, Coastwise and Great Lakes, and Oceans. Discrepancies in the tables and charts are due to inconsistencies in the raw MISLE data, such as missing or incorrect entries. SOURCE: USCG, MISLE (data as of May 20, 2019).

20 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE TABLE 2-4 U.S.-Inspected Fleet by Route and SOLAS Passenger Vessels Non-Ocean-Going Ocean-Going Total SOLAS 16 14 30 Non-SOLAS 5,054 1,251 6,305 Total 5,070 1,265 6,335 Cargo Vessels Non-Ocean-Going Ocean-Going Total SOLAS 0 335 335 Non-SOLAS 4,530 1,266 5,796 Total 4,530 1,601 6,131 NOTES: Passenger vessels in the International Convention for the Safety of Life at Sea ( SOLAS) category are those that have a valid Passenger Ship Safety Certificate. Discrepancies in the tables and charts are due to inconsistencies in the raw MISLE data, such as missing or incorrect entries. SOURCE: USCG, MISLE (data as of May 20, 2019). Table 2-5 is a summary list of the date of build for the U.S.-inspected fleet. The years shown in Table 2-5 are decade years and the number repre- sents the number of vessels built during that decade. The number of vessels by decade and service, decade and route, and decade and subchapter are listed in Appendix B. 8,396 of the 12,231 total vessels (69%) were built since 1990, when Subchapter S was created in its current format and dam- age stability regulations came into force for vessels with SOLAS (Interna- tional Convention for the Safety of Life at Sea) certificates. In requesting a study focused on stability regulations “inward of the Boundary Line,” the USCG is placing an emphasis on vessels that operate domestically and have stability regulations little impacted by international stability regulations. The Boundary Lines (defined in 46 CFR, Part 7)4 separate the navigable U.S. waters considered to be “open seas” from the inland, protected, and partially protected U.S. navigable waters. Vessels operating in the more sheltered waters inward of the Boundary Lines are normally subject to stability regulations that are less restrictive than regula- tions applicable to vessels operating on the high seas. Of concern is that the domestic stability regulations that govern this fleet, consisting of primarily smaller passenger vessels regulated by Subchapter T (and Subchapter S), are not updated as frequently as the regulations that apply to larger ocean- going vessels subject to international convention. 4 See https://www.dco.uscg.mil/CG-ENG-2/BoundaryLine and https://www.law.cornell.edu/ cfr/text/46/part-7.

21 T A B L E 2 -5 N um be r of A ll A ct iv e, I ns pe ct ed , U .S . Fl ag V es se ls b y D ec ad e B ui lt D ec ad e B ui lt 18 60 – 18 69 18 70 – 18 79 18 80 – 18 89 18 90 – 18 99 19 00 – 19 09 19 10 – 19 19 19 20 – 19 29 19 30 – 19 39 19 40 – 19 49 19 50 – 19 59 19 60 – 19 69 19 70 – 19 79 19 80 – 19 89 19 90 – 19 99 20 00 – 20 09 20 10 – 20 19 A ll Y ea rs N um be r of V es se ls 1 2 6 2 6 11 28 27 12 2 18 1 53 8 1, 28 1 1, 63 0 2, 11 3 3, 02 4 3, 25 9 12 ,2 31 N O T E : D is cr ep an ci es i n th e ta bl es a nd c ha rt s ar e du e to i nc on si st en ci es i n th e ra w M IS L E d at a, s uc h as m is si ng o r in co rr ec t en tr ie s. SO U R C E : U SC G , M IS L E d at ab as e (d at a as o f A pr il 2, 2 01 9) .

22 USCG VESSEL STABILITY REGULATIONS AND GUIDANCE It merits noting that in addition to the vessels in Tables 2-1 to 2-5, the domestic fleet contains thousands of additional uninspected vessels, includ- ing towing, fishing, work, and recreational boats. Other than the larger fishing vessels, the USCG generally does not directly regulate the stability of the uninspected vessels. However, certain vessels that obtain a Load Line certificate are subject to the same intact stability criteria as inspected ves- sels of the same type.5 Over the next few years, more of these uninspected vessels, primarily towing vessels, will need to comply with Subchapter M of Title 46 CFR.6 THE SS EL FARO SINKING AND THE SAVE OUR SEAS ACT PROVISIONS The U.S. flag roll-on/roll-off (ro-ro) cargo vessel SS El Faro sank off the Bahamas during a hurricane in the morning hours of October 1, 2015. The 33 lives lost in this casualty were the most on a U.S. flag vessel in more than 30 years. Both the USCG’s Marine Board of Investigation (MBI) and the National Transportation Safety Board (NTSB) investigated the sinking. Their reports (cited in Chapter 1) contain several recommendations to ad- dress safety issues, including improving weather alerts and information, and planning and response to severe weather. Probable related causes included failure to keep the vessel clear of the track of a hurricane and the loss of watertight integrity of the hull, leading to flooded holds and creating exces- sive heel, loss of propulsion, and eventual capsize. Congress subsequently passed the Save Our Seas Act of 2018.7 Title II of the act specifically addresses the maritime safety improvements recom- mended in the MBI and NTSB reports. The act contains several man- dates for the USCG, including a mandate to institute “record-keeping requirements for small weight changes to freight vessels” and a “review of the effectiveness of U.S. regulations, international conventions, recog- nized organi zations’ class rules, and USCG technical policy regarding intact and damage stability requirements, fire dampers, ventilators and lifesaving equipment for mariners.” Although the MBI and NTSB reports did not conclude that small changes in lightship weight was a probable cause or 5 See https://media.defense.gov/2017/Mar/29/2001723819/-1/-1/0/CIM_16000_9.PDF. 6 When briefing the committee at its first meeting, the USCG estimated that approximately 5,000 vessels could be affected by Subchapter M. More information on Subchapter M is provided at the following link: https://www.dco.uscg.mil/Our-Organization/Assistant-Commandant-for- Prevention-Policy-CG-5P/Traveling-Inspector-Staff-CG-5P-TI/Towing-Vessel-National-Center-of- Expertise/SubIRegulations-Copy. 7 For the full text of the Act, see https://www.congress.gov/bill/115th-congress/ senate- bill/3508/text.

BACKGROUND 23 contributing factor in the SS El Faro loss, the MBI report recommended tracking of weight changes (see Recommendation 6 of the MBI report). With regard to the Save Our Seas Act’s requirement for the USCG to assess the effectiveness of its stability regulations in conjunction with international conventions, class rules, and technical documents, it merits noting that the MBI report (in Recommendations 28 and 29) does note the importance of having a more integrated, uniform, and consistent safety regime generally. In the case of vessel stability, the International Maritime Organization (IMO) has been the main driver of modifying regulations, primarily for ocean-going vessels. For U.S. vessels with IMO convention certificates, such as SOLAS, the USCG normally requires compliance with the IMO intact and damaged stability regulations. These international stability regulations, therefore, have guided the development of some of the regulations in Subchapter S for domestic-only vessels. Examining the effectiveness of the USCG stability regulations in the context of the broader set of international requirements and their influence can therefore be a use- ful exercise for informing more general inquiries into the need for a more consistent and uniform safety regime.

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Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance Get This Book
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U.S. Coast Guard (USCG) ship stability regulations governing the ability of a vessel to return to an upright position after being disturbed is the focus of a new TRB publication, Review and Update of U.S. Coast Guard Vessel Stability Regulations and Guidance. The authors advise the USCG on how it can make its stability regulations more usable and complete in meeting the requirements of different types of vessels and those vessels that have undergone weight changes that can affect their stability characteristics.

The USCG has safety regulatory jurisdiction over vessels registered in the United States. One of its oldest regulatory functions is to ensure these ships, boats, and other floating vessels remain upright as they encounter both expected and unexpected loading, operating, and weather conditions, including wind and wave conditions and unexpected failure of watertight integrity.

Stability standards have been improved over time - particularly in the past 30 years - and the USCG remains keenly interested in making sure the regulations are kept updated based on the latest technical knowledge, well aligned with international standards, and organized and presented in a manner that facilitates compliance and enforcement. The recommendations in the report are intended to further these aims. The USCG earlier commissioned a National Academies study to identify options for improving vessel stability regulations, and after receiving that study in September 2018, the USCG asked for this second study to provide more in-depth advice on applying these options.

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