Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
PREPUBLICATION COPYâUncorrected Proofs 9 2 Background This chapter provides a brief review of vessel stability as needed to understand the concepts and terminology presented in subsequent chapters. The review repeats the discussion on this subject matter from the committeeâs Phase 1 letter report.15 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 the USCG require lightship weight tracking and review the effectiveness of stability regulations. VESSEL STABILITY CONCEPTS AND TERMINOLOGY16 Vessel stability is the ability of a vessel to return to an initial upright position after being disturbed by an outside force. A stable vessel is one that has sufficient righting forces at that moment to counter the forces working to capsize it, so it remains upright. The buoyant forces acting on the vesselâs hull as it heels (i.e., lists, leans, or tilts) create a vesselâs 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 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 vesselâs stability and its ability to remain upright (see Figures 2-1a and 2-1b). FIGURE 2-1a A vessel with sufficient righting forces. FIGURE 2-1b A vessel with insufficient righting forces. SOURCE: Phase 1 letter report. 15 See http://www.trb.org/Main/Blurbs/178088.aspx. 16 The discussion that follows draws heavily from, and in some cases repeats, the committeeâs discussion on vessel stability concepts and terminology from its Phase 1 letter report.
PREPUBLICATION COPYâUncorrected Proofs 10 A vesselâs stability is not static, and the interaction between a vesselâs righting forces and any capsizing forces is dynamic and will constantly vary throughout a vesselâs 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 vesselâs entire lifetime. Properly assessing a vesselâs stability requires measuring its dynamic stability, which is currently accomplished using two methods: (1) the vesselâs metacentric height (GM), which is a pure static stability measurement; and (2) the vesselâs righting arm curve, which is a quasi-static method. The metacentric height for vessels, first understood in the mid-1700s, was used for the initial stability criteria because it was a practical and simple stability calculation to perform. A vesselâs hull is a complex shape that required a lengthy series of calculations to analyze its true dynamic stability characteristics. However, the vesselâs metacenter (M) could be calculated from only the vesselâs 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 vesselâs weight centered at the center of mass (G) and the upward force of the buoyancy centered at the center of the immersed volume. As a vessel heels, the location of the center of buoyancy (B) moves and the path of its motion can be determined by the center of curvature of this curve. The metacenter (M) is this center of curvature (see Figure 2-2). 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. The metacentric height is calculated as the distance between the shipâs center of gravity (G) and its metacenter (M), where a larger metacentric height suggests a greater initial stability. As long as the center of gravity is below the metacenter, a vessel will have positive initial stability. A vesselâs 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 vesselâs hull. The buoyant forces are pushing upward
PREPUBLICATION COPYâUncorrected Proofs 11 over the vesselâs entire hull surface situated below the waterline. By calculating the center of these buoyant forces for each heel angle, the center of buoyancy (B) is determined. The righting arm then is the horizontal distance between the center of buoyancy pushing up and the center of the vesselâs weight, or center of gravity (G), pushing down (see Figure 2-3). 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 buoyancy [B]). SOURCE: Phase 1 letter report. At each heel angle there is a static snapshot taken of the vesselâs stability. When multiplied by the vesselâs 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 metacentric height only measures vessel stability at the upright condition, the righting arm curve calculates stability over a broad 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 vesselâs available righting energy, providing a useful measure of the vesselâs stability at the critical higher angles of heel that the metacentric height method cannot do reliably.
PREPUBLICATION COPYâUncorrected Proofs 12 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. Research and experience with shipâs 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 vessel, a variety of stability criteria were developed that represented well 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 stormâs high winds and large beam seas (see Figure 2-5).17 17 See http://www.imo.org/en/KnowledgeCentre/IndexofIMOResolutions/Maritime-Safety-Committee- %28MSC%29/Documents/MSC.267%2885%29.pdf.
PREPUBLICATION COPYâUncorrected Proofs 13 FIGURE 2-5 Righting arm curve approximating severe wind and rolling on a vessel. SOURCE: Phase 1 letter report. Similar heeling arm overlay methods are used to determine the effects of operational capsizing forces on a vesselâs 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 todayâs vessels are covered by the current generation of quasi-static stability criteria. U.S. FLEET SUBJECT TO COAST GUARD STABILITY REGULATION To evaluate U.S. stability regulations requires an understanding of the various 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 operating 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).
PREPUBLICATION COPYâUncorrected Proofs 14 TABLE 2-1 Number of U.S.-Inspected Vessels by Type and Length 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 Information for Safety and Law Enforcement (MISLE) data, such as missing or incorrect entries. SOURCE: USCG, MISLE database (data as of May 16, 2019). TABLE 2-2 U.S.-Inspected FleetâCargo Vessels by Propulsion Cargo Vessel Type Inspection Subchapter Totals D 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 Totals 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). Tables 2-3 and 2-4 provide more detailed breakouts of vessels subject to 46 CFR tabulated by inspection category (applicable subchapter), domestic versus international service, and route type.
PREPUBLICATION COPYâUncorrected Proofs 15 TABLE 2-3 U.S.-Inspected Fleet by Route and Subchapter Passenger Vessels Non-Ocean-Going Ocean-Going Totals Subchapter H 130 20 150 K 379 34 413 R 8 17 25 T 4,553 1,172 5,725 U 0 22 22 Totals 5,070 1,265 6,335 Cargo Vessels Non-Ocean-Going Ocean-Going Totals 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 Totals 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, Coastwise, 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).
PREPUBLICATION COPYâUncorrected Proofs 16 TABLE 2-4 U.S.-Inspected Fleet by Route and SOLAS Passenger Vessels Non-Ocean-Going Ocean-Going Totals SOLAS 16 14 30 Non-SOLAS 5,054 1,251 6,305 Totals 5,070 1,265 6,335 Cargo Vessels Non-Ocean-Going Ocean-Going Totals SOLAS 0 335 335 Non-SOLAS 4,530 1,266 5,796 Totals 4,530 1,601 6,131 NOTES: Passenger vessels in the 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 represents 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 damage stability regulations. TABLE 2-5 Number of All Active, Inspected, U.S. Flag Vessels by Decade Built 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 database (data as of April 2, 2019). In requesting a study focused on stability regulations âinward of the Boundary Line,â 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)18 separate the navigable U.S. waters considered to be âopen seasâ from the inland, protected, and partially protected U.S. navigable waters. Vessels operating in the more sheltered 18 See https://www.dco.uscg.mil/CG-ENG-2/BoundaryLine/ and https://www.law.cornell.edu/cfr/text/46/part-7. 1860â 1869 1870â 1879 1880â 1889 1890â 1899 1900â 1909 1910â 1919 1920â 1929 1930â 1939 1940â 1949 1950â 1959 1960â 1969 1970â 1979 1980â 1989 1990â 1999 2000â 2009 2010â 2019 All Years Number of Vessels 1 2 6 2 6 11 28 27 122 181 538 1,281 1,630 2,113 3,024 3,259 12,231 Decade Built
PREPUBLICATION COPYâUncorrected Proofs 17 waters inward of the Boundary Lines are normally subject to stability regulations less restrictive than those regulations 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. 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, including 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 vessels of the same type.19 Over the next few years, more of these uninspected vessels, primarily towing vessels, will need to comply with Subchapter M of Title 46 CFR.20 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 USCGâs 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 address 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 excessive heel, loss of propulsion, and eventual capsize. Congress subsequently passed the Save Our Seas Act of 2018.21 Title II of the act specifically addresses the maritime safety improvements recommended in the MBI and NTSB reports. The act contains several mandates for the USCG, including a mandate to institute ârecord-keeping requirements for small weight changes to freight vesselsâ and a âreview of the effectiveness of U.S. regulations, international conventions, recognized organizationsâ class rules, and USCG technical policy regarding intact and damage stability requirements, fire dampers, ventilators and lifesaving equipment for mariners.â Although the MBI and NTSB reports did not conclude that small changes in lightship weight was a probable cause or 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 Actâs 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 19 See https://media.defense.gov/2017/Mar/29/2001723819/-1/-1/0/CIM_16000_9.PDF. 20 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. 21 For the full text of the act, see https://www.congress.gov/bill/115th-congress/senate-bill/3508/text.
PREPUBLICATION COPYâUncorrected Proofs 18 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 useful exercise for informing more general inquiries into the need for a more consistent and uniform safety regime.