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PREPUBLICATION COPYâUncorrected Proofs 19 3 Lightship Weight Change Tracking Section 206(a) of the Save Our Seas Act of 2018 directs the U.S. Coast Guard (USCG) to promulgate regulations that require operators of inspected freight vessels to maintain records of all incremental changes in vessel lightship weight (sometimes referred to as lightweight), and to track total changes in lightship weight over time. Such tracking would presumably be accompanied by requirements for vessel operators to recalculate or retest a vesselâs stability characteristics when certain thresholds of lightship weight change are met. The USCG has asked the committee to advise on options for establishing such weight-tracking requirements for application to U.S.-flag freight vessels. In response to this request, the committee: 1. Reviewed past efforts by the USCG and International Maritime Organization (IMO) to require lightship weight change tracking for both passenger and freight vessels. 2. Reviewed existing USCG guidance pertaining to the tracking of lightship weight by operators of U.S flag vessels. 3. Expanded on the sensitivity analysis conducted in Phase 1 of the study that shows how changes in lightship weight can affect the stability characteristics of several smaller passenger vessel types. 4. Conducted the same sensitivity analysis of lightship weight changes on a selection of freight vessel types that shows how weight changes have differing effects on the stability characteristics of these vessels compared with passenger vessels. 5. Identified several potentially desirable qualities of a lightship weight-tracking program for freight vessels. For purposes of this task item, a freight vessel is defined (see 46 U.S. Code Section 2101 (15)),22 as a self-propelled vessel (greater than 15 gross tons) that carries freight for hire and not passengers, excluding an oceanographic research vessel or an offshore supply vessel. PAST EFFORTS TO REQUIRE LIGHTSHIP WEIGHT CHANGE TRACKING The IMO and the USCG have made several efforts since the 1990s to require lightship weight change tracking accompanied by periodic testing (or surveys) to verify lightship weight. The IMO has focused more on passenger vessels when applying these tracking and stability testing requirements, while the USCG has also proposed such measures for freight and other vessels in addition to passenger vessels. The IMO first established damage stability requirements for ocean-going passenger vessels that included requirements for lightship weight tracking and periodic stability testing in response to the loss of the British-flag roll-on/roll-off (ro-ro) passenger ferry Herald of Free Enterprise in 1987. It capsized from loss of stability after car deck spaces were flooded through a bow door not closed prior to the vessel getting under way. The IMO requirements, which were applied to new passenger vessels, were implemented as amendments to the 1990 Safety of Life at 22 See https://www.law.cornell.edu/uscode/text/46/2101.
PREPUBLICATION COPYâUncorrected Proofs 20 Sea (SOLAS 90) convention.23 Notably, when the IMO later established damage stability requirements for ocean-going freight vessels, it did not require lightship weight tracking and periodic stability testing as required for new passenger vessels. Concerned about the potential for many types of vessels to experience changes in lightship weight that could adversely affect vessel stability characteristics, the USCG issued a Notice of Proposed Rulemaking (NPRM) in February 199024 that would require weight change tracking and periodic stability testing by operators of a wide range of inspected vessels, including container ships, tankers, mobile offshore drilling units (MODUs), nautical school ships, and oceanographic research vessels. The proposal generated many comments from a cross- section of the maritime industry. Most of the commenters opposed the requirement, especially for periodic testing of stability, because of the high cost of taking a vessel out of service for the testing and the lack of a demonstrated need based on casualty data. After reviewing the NPRM comments and holding a series of public hearings, the USCG issued several rulemaking notices that curtailed the proposed requirements. The final rule published in 1997 removed the requirement altogether. (Ocean-going, U.S-flag passenger vessels with SOLAS certificates were still required to carry out the periodic lightship weight surveys in compliance with SOLAS requirements.) The need for lightship weight verification for passenger vessels was considered again by the USCG in response to the capsizing of the small pontoon passenger vessel Lady D in Baltimoreâs Inner Harbor in 2004. After investigating the casualty, the USCG determined that the average passenger weight values used for the stability calculations required by Subchapter S were unrealistically low. During the regulatory update prompted by this determination, the USCG sought to include an annual stability information verification requirement and a stability verification test (deadweight survey) at 10-year intervals for passenger vessels. The comments from industry were again largely negative due to concerns about the high cost of taking vessels out of service during testing as well as claims about the questionable need for such a measure based on casualty data. Lacking evidence of passenger vessels incurring significant lightship weight growth over time that had led to casualties, the USCG withdrew the proposed stability information and testing requirement from the final rule that changed the values of average passenger weight to be used for making stability calculations. In a 2006 amendment, the IMO consolidated its passenger and freight vessel damage stability requirements into a unified set of requirements, commonly referred to as SOLAS 2009.25 SOLAS 2009 retained the requirement for periodic lightship weight verification that applied to passenger vessels, but did not adopt a similar requirement for freight vessels. The USCG subsequently adopted these international standards for U.S. flag vessels holding SOLAS certificates. Currently, towing vessels are the only non-SOLAS domestic vessels required by the USCG to track lightship weights, as mandated in the recently implemented Subchapter M regulations (Part 144.315) (see Box 3-1). Since towing vessel stability can be sensitive to weight 23 See http://www.imo.org/en/OurWork/Safety/Regulations/Pages/RO-ROFerries.aspx. 24 See âStability design and operational regulations,â NPRM, 55 FR 5120, February 13, 1990. https://www.govinfo.gov/content/pkg/FR-1990-02-13/pdf/FR-1990-02-13.pdf. 25 See SOLAS 2009; Harmonized damage stability, resolution MSC.216(82), adopted December 2006. http://www.imo.org/en/KnowledgeCentre/IndexofIMOResolutions/Maritime-Safety-Committee- (MSC)/Documents/MSC.216(82).pdf.
PREPUBLICATION COPYâUncorrected Proofs 21 changes, the weight-tracking requirements of Subchapter M could be a model for other vessel types. BOX 3-1 Subchapter M Weight-Tracking Requirements for Towing Vessels Â§ 144.315 Weight and moment history requirements for a vessel with approved lightweight characteristics. (a) A weight and moment history of changes to the vessel since approval of its lightweight characteristics (displacement, Longitudinal Center of Gravity (LCG) and Vertical Center of Gravity (VCG)) must be maintained. All weight modifications to the vessel (additions, removals, and relocations) including a calculation of the aggregate weight change (absolute total of all additions, removals, and relocations) must be recorded in the history, along with a description of the change(s), when and where accomplished, moment arms, etc. After each modification, the lightweight characteristics must be recalculated. (b) When the aggregate weight change is more than 2 percent of the vessel's approved lightweight displacement, or the recalculated change in the vessel's lightweight LCG is more than 1 percent of the LBP, a deadweight survey must be performed to determine the vessel's current lightweight displacement and LCG. Use the following table to determine when the deadweight survey results or the vessel's aggregate weight change requires the vessel to undergo a specified stability test: Table 144.315 If - Then - (1) The deadweight survey results are both within 1 percent of the recalculated lightweight displacement and within 1 percent LBP of the recalculated lightweight LCG, the recalculated lightweight VCG can be accepted as accurate. (2) The deadweight survey results do not meet the criteria of paragraph (b)(1) of this section, the vessel must undergo a stability test in accordance with 46 CFR 170, subpart F. (3) The aggregate weight change is more than 10 percent of the vessel's approved lightweight displacement, the vessel must undergo a stability test in accordance with 46 CFR 170, subpart F. NOTE: LBP = length between perpendiculars. SOURCE: 46 CFR Subchapter M â Towing Vessels, https://www.law.cornell.edu/cfr/text/46/chapter- I/subchapter-M. EXISTING GUIDANCE ON LIGHTSHIP WEIGHT TRACKING Although it is not in the Code of Federal Regulations, various guidance on when operators of inspected domestic passenger (non-SOLAS), freight, and other vessels (except Subchapter M towing vessels) should record lightship weight (weight and center of gravity) changes and use this information to assess stability impacts can be found in certain USCG policy and technical notes. The guidance references clear situations (e.g., when major vessel modifications are made) in which vessel operators should notify the USCG about lightship weight change. Absent from the regulatory requirements, however, are specific numerical weight change values that would trigger a notification.
PREPUBLICATION COPYâUncorrected Proofs 22 The most applicable guidance triggering a notification is contained in the USCG Marine Safety Center (MSC)-issued Marine Technical Note (MTN) 04-95,26 which states that the owner of a vessel with USCG-approved stability documentation should report any vessel modifications that lead to weight being added, removed, or relocated. The guidance further states that any reported weight change information should include the center of gravity of each weight item. If the sum of all the reported weight changes and the respective weight moments are less than 2% of existing lightship values and/or the change in lightship longitudinal center of gravity (LCG) is less than 1% of length between perpendiculars, the operator should make a calculation of the effects on stability. If they are greater than these threshold values, a new stability test may be required. When an owner submits modification drawings to the MSC for review or USCG inspection reveals that modifications are being made or have recently been made to a vessel, it is normal practice for the USCG to invoke the requirements of MTN 04-95. It is regular practice for the USCG to request that the weight changes from the modifications be added to any known previous changes since the last incline test or deadweight survey.27 This information is to be used to determine whether the threshold for undertaking a lightship weight verification has been reached. It would be unconventional, in the committeeâs view, for a vessel owner carrying out modifications to not make such a calculation when the modifications create a lightship weight change approaching these threshold values. Furthermore, when the changes from the modifications exceed these threshold values, it is widely understood in the industry that a deadweight survey or incline test should be performed. Even if the weight change values associated with a vessel modification do not exceed these thresholds, the vessel owner would be expected to include the changed weight and center of gravity when checking stability going forward. It is important to point out, however, that such weight change recording may not include the weight growth (or weight creep) that can occur over many years as the result of a series of minor additions and other incremental changes to a vessel. For example, small changes in the vesselâs draft or trim could go unnoticed by the master or crew, potentially creating undetected reductions in a vesselâs stability. SENSITIVITY OF PASSENGER VESSEL STABILITY TO LIGHTSHIP WEIGHT CHANGES Vessel types can differ significantly in their vulnerability to stability impacts from lightship weight changes, thus complicating the setting of general requirements for weight change values that should trigger reporting and stability testing. One way to demonstrate this difference is to apply a range of weight and center of gravity changes to a selection of vessel types and then calculate the stability impacts. As previously noted, the USCG requires a vessel to undergo a new stability test if its lightship weight changes by more than 2% or the LCG shifts more than 1% of the length between perpendiculars. By using these weight change values as starting points, a sensitivity analysis can be performed to evaluate when weight changes begin to pose a risk of 26 See https://www.dco.uscg.mil/msc/mtn. 27 The request to calculate weight change from a proposed modification and to add this to a summary of all weight changes from the last stability verification test is a standard requirement in modification approval letters issued by the Marine Safety Center (MSC). For example, the typical wording in letters from the MSC to a ship owner submitting modification drawings for review often includes the following: âA record of all changes to machinery, structure, outfitting, and equipment that affect the lightship data (weight and centers of gravity) should be maintained in a lightship data alterations log and be taken into account in daily operations. Aggregate weight changes outside the tolerance in âMTN 04-95â may require the submission of additional information.â
PREPUBLICATION COPYâUncorrected Proofs 23 stability noncompliance (i.e., no longer being in compliance with intact and damage stability requirements). Phase 1 of this study conducted such a sensitivity analysis to demonstrate the impact of lightship weight changes on a 280-ft overnight river cruise boat. A similar analysis is therefore conducted for the following selection of smaller passenger vessel types: â¢ 65-ft, 179 passengers, tour boat. â¢ 90-ft, 340 passengers, excursion boat. â¢ 120-ft, vehicle ferry barge. Table 3-1 summarizes the results and also includes the results from the earlier analysis of the 280-ft cruise boat from the Phase 1 report. Appendix D provides a full description of the vessels and the details of the sensitivity analysis. TABLE 3-1 Effect on Stability Compliance of Selected Passenger Vessel Types with Lightship Weight Changes Vessel Type Approximate Lightship Displacement (Long Ton (s)) Lightship Weight Percentage of Full Load Displacement Added Weight as a Percentage of Lightship Displacemente 2% 3% 5% 10% 65-ft Tour Boat 27 56% FAIL FAILa FAIL FAIL 90-ft Excursion Boat 117 76% FAIL FAILb FAIL FAIL 120-ft Vehicle Ferry Barge 128 46% PASS MARGINAL c PASS FAIL 280-ft Overnight Cruise Shipd 1,842 85% PASS N/A PASS FAIL a Fails intact righting energy criterion in 90% burned-out condition. b Fails requirement for positive GZ (Righting Arm) range in damaged full load departure condition. c At +5% lightship weight added, the ferry barge barely misses the intact criterion of at least 15 degrees heel to the downflooding point. However, it continues to exceed all other intact and damaged criteria. d Results for the 280-ft Overnight Cruise Ship are from the committeeâs Phase 1 report. e In all cases, the weight was added at the VCG of the original lightship weight. Because these small passenger vessels have low lightship weights, weight changes of up to 10% were assessed where applicable (i.e., when lower weight changes did not cause failure). The committee recognizes that lightship weight changes of 10%, because of weight creep rather than a planned modification, may not be realistic; however, checking for such a value does illustrate the range of sensitivity that some vessel types experience due to weight changes. The analysis shows how lightship weight changes of only a few percent can lead to stability vulnerabilities due to noncompliance. A rating of âpassâ indicates that the vessel continued to comply with all stability requirements after the percentage weight change. A rating of âfailâ indicates that the vessel would be out of compliance with the stability requirement. The specific reasons for the fail ratings, which are provided in Appendix D, differ by vessel type. Some vessel
PREPUBLICATION COPYâUncorrected Proofs 24 types fail because they no longer comply with intact stability criteria, while others fail because they no longer comply with damage stability criteria. In most cases the noncompliance stems from the vesselâs inability to retain an adequate range of stability to down-flooding points because of the deeper draft created by the larger lightship weights. By way of example, the 120- ft vehicle ferry barge barely misses the 15-degree heel to the down-flooding point with a 5% weight increase. It is rated as being in âmarginalâ compliance at this weight change value because it readily meets other stability criteria due to its high initial GM. The other passenger vessels, particularly the 90-ft excursion boat, have little margin to withstand a lightship weight increase of even 2%. These examples illustrate how smaller passenger vessels, in particular, can operate with a low stability margin that can make them vulnerable to stability losses because of lightship weight changes. For vessels that operate with full passenger loads at their maximum approved drafts, incremental lightship weight changes should lead to reductions in allowable passenger loads. To account for the effect of lightship weight change on draft limits and allowable passenger loads, the vessel operator will need accurate information on weight changes. SENSITIVITY OF FREIGHT VESSEL STABILITY TO LIGHTSHIP WEIGHT CHANGES As demonstrated next, similar lightship weight sensitivity analyses for freight vessels show that these vessels do not have the same potential for stability noncompliance as small passenger vessels because lightship weight accounts for a relatively smaller share of their total full load displacement. Again, the vulnerability is assessed based on whether the vessel would remain in compliance with intact and damage stability criteria after the weight changes.28 Table 3-2 contains the results of the analysis (detailed in Appendix C) for the following five types of freight vessels, all present in the U.S. flag fleet. 1. Car carrier. 2. Ro-ro trailership. 3. Naval auxiliary built to commercial standards. 4. Product tanker. 5. Mid-size container ship. In each case, weight increases of 2%, 4%, and 6% were added to the vesselâs lightship weight. For each vessel type, the sample loading condition in the vesselâs stability book29 with the lowest GM margin (achieved GM minus required GM) was used for the analysis. In one set of analyses, the weight additions were applied to the upper deck or uppermost level where a weight of this magnitude could be added to a vessel, such as if a large structure or liquefied natural gas (LNG) fuel tank were to be mounted on the upper deck. In a second set of analyses, the weight was added at the existing lightship vessel center of gravity (VCG), which would be more typical of instances of weight creep caused by adding a variety of small weights such as additional spares in the engine room, reinforcements at the tank-top level, and new equipment in the deckhouse. 28 It is assumed that no actions are taken to improve stability (such as taking on ballast) because the weight changes are presumed to be unknown to the crew. 29 As per 46 CFR Â§ 170.110, a stability booklet approved by the MSC or a Recognized Organization must be prepared for each vessel and contain information that allows the master to operate the vessel in compliance with applicable regulations contained in Subchapter S. See https://www.law.cornell.edu/cfr/text/46/170.110.
PREPUBLICATION COPYâUncorrected Proofs 25 TABLE 3-2 Effect on Stability Compliance of Selected Freight Vessel Types with Lightship Weight Changes Vessel Type Approximate Lightship Weight (metric ton) Lightship Weight Percentage of Displacement Added Weight as a Percentage of Lightship Weight Weight Added at Upper Deck Level Weight at Existing VCG 2% 4% 6% 2% 4% 6% Car Carrier 12,500 50% PASS MARGINAL PASS FAIL PASS PASS PASS Ro-Ro Trailership 25,000 54% PASS MARGINAL PASS FAIL PASS PASS PASS Naval Auxiliary 25,000 56% MARGINAL PASS FAIL FAIL PASS FAIL FAIL Product Tanker 11,000 19% PASS PASS PASS PASS PASS PASS Mid-size Container Ship 15,000 32% PASS PASS PASS PASS PASS PASS The sensitivity analysis shows that all of the selected vessel types remain in compliance with intact stability requirements under a 2% lightship weight change scenario, even when this weight change is applied to the upper deck level. When the weight change is 6%, all vessels except the naval auxiliary ship remain in compliance when the weight is added at the existing lightship VCG. In fact, for all the other vessel types the margin of the attained GM continues to be significantly above the required GM. Vessels whose lightship weight is less than 50% of the total loaded displacement, such as tankers and container ships, pass under all of the lightship weight change scenarios. For these vessel types, cargo loads are such a large percentage of displacement that changes in lightship weight create a relatively small percentage change in overall displacement and center of gravity, which are key inputs to a stability calculation. In contrast, the vessels in which lightship weight accounts for 50% or more of total loaded displacement, such as the car carrier or ro-ro trailership, are more susceptible to stability losses. For these vessels, lightship weight changes can have a significant impact on total vessel displacement and center of gravity, which can lead to difficulty with sustaining large lightship weight increases while retaining stability compliance. DESIRABLE QUALITIES OF A LIGHTSHIP WEIGHT-TRACKING PROGRAM FOR FREIGHT VESSELS The intent of a lightship weight-tracking program is to record the inevitable weight changes that will occur over a vesselâs service life since its last incline test or deadweight survey. To yield accurate information, such a program would need to track lightship weight changes resulting from vessel modifications that are well defined (e.g., that require MSC plan review), as well as modifications to the vessel that can go unnoticed because they are small or not associated with a substantial modification. In addition to having potential applicability to operators of towing vessels subject to lightship tracking requirements in Subchapter M, such a weight-tracking program could aid operators of other vessels who must report cumulative weight changes to the
PREPUBLICATION COPYâUncorrected Proofs 26 USCG since the last deadweight survey or incline test as part of their submission for a proposed modification. It is important that a weight-tracking program have certain qualities to ensure that it records weight changes with a level of accuracy and timeliness so that it is used confidently, appropriately, and when needed. Two main qualities of a weight-tracking program that the committee believes are important for these purposes are the ability to: â¢ Provide a sufficiently accurate and timely record of both the small and large changes to a vesselâs lightship weight and center of gravity over its lifetime, so operators are alerted when the cumulative weight change is approaching the threshold values at which stability impacts must be reviewed. This quality implies that weight change data can be backed up and retained in a safe place for periods of time, at least until the next lightship verification of the vessel. â¢ Be readily usable by the crew and shore-based personnel responsible for a vesselâs operation, based on a technology that can be made available on board (or in the operatorâs office) and that is comparable to other technologies used by personnel and appropriate to their level of training and stability knowledge. This quality includes an ability to be used in a manner that allows for corrections to weights (weights to add, remove, or relocate) to be made on board the vessel to allow the crew to carry out informal deadweight surveys to confirm that the calculated changes in lightship weight are reflective of the actual lightship weight of the vessel. An explanation of why the two qualities are desirable is given next, and discussed in more detail in Appendix E. Although specific reference is made to how these qualities pertain to the tracking of lightship weights by operators of freight vessels (as requested by the USCG), they are applicable to any weight-tracking program for use by all vessel types. Accurate and Timely Weight Change Recording Accuracy is critical to the effective tracking of lightship weight changes that occur over a vesselâs lifetime. The level of accuracy of a vesselâs weight-change tracking program will be affected by the timing of when weight changes of different forms and magnitudes are recorded. Some changes are likely to be well documented, such as the very large weight changes that occur when a new adjustable car deck is added to a ro-ro vessel. Other smaller changes may not be recorded right away because they are made as part of routine maintenance or operations, such as switching out machinery components or switching out a shipâs mess chairs. When considering the accuracy needed for tracking these weight changes, the vessel type (e.g., tanker, ro-ro, container, car carrier), physical size, and interval since its last stability test (i.e., inclining test or deadweight survey) will be factors.30 Freight vessels have many different designs and levels of complexity to meet the wide variety of cargoes carried, ranging from relatively simple tankers and bulk carriers to more complex container ships and ro-ro carriers. Accordingly, determinations about how to strike the right balance between accuracy and timeliness of lightship weight tracking will depend in large part on the vessel itself. 30 If the interval between deadweight surveys or inclinings is short, then inaccuracies in the tracking of smaller weight changes will not lead to a large accumulated error simply because there is insufficient time for the errors to accumulate to become an issue. However, if the interval is a longer period or a deadweight survey/inclining is not performed on a fixed schedule (i.e. only performed when a weight tracking program triggers an action), the accuracy of the smaller weight changes can become critical.
PREPUBLICATION COPYâUncorrected Proofs 27 As previously explained, incremental changes in lightship weight will have greater effects on the stability of some vessel types than others. Thus, a weight-tracking program designed for a tanker may not be suited to a ro-ro carrier, and vice versa. Vessel types also differ in their likelihood of being modified substantially during their service lives. The probability of significant changes being made to the lightship weight of a tanker or bulk carrier is small compared to that of a ro-ro carrier or container ship, both of which are routinely modified to adapt to changing market conditions. In the case of some specialized vessels, such as heavy lift submersibles and vessels used exclusively for certain cargoes, there may be no need for a weight-tracking program. Although these vessels are often significantly modified for each job, with potentially significant changes in their lightship characteristics, these modifications tend to be well planned and engineered. As a practical matter, such vessels undergo a detailed stability review for each transit. Ease of Use by the Crew or Shore Personnel A weight-tracking program that is difficult to use may not be kept up-to-date, leading to unrecorded changes in lightship weight that may eventually place stability compliance at risk. Likewise, a program that is difficult to use may be applied incorrectly or not employed frequently enough to assess stability when needed. Usability is therefore important for both program accuracy and appropriate implementation. To be readily usable for operators of freight vessels, one would expect a weight-tracking program to be (1) suited to the training levels and stability knowledge of the vesselâs crew and shore-based personnel and (2) based on technologies that are in keeping with those typically used on board the vessel. Aligning the program with the stability knowledge and training levels of personnel can be challenging because crew expertise can differ widely. Although licensed officers of freight vessels are required by USCG regulation to undergo some stability training, the level of training is limited to understanding basic concepts. Shore-based personnel may not have any stability knowledge. Thus, such personnelâlacking such knowledgeâare less likely to use weight- tracking programs appropriately, especially if they require advanced knowledge of factors that affect vessel stability. Indeed, one concern about such a mismatch is that the vesselâs crew or shore-based personnel may believe weight tracking is being recorded correctly when it is not, putting the vesselâs stability at risk. Ensuring that the weight-tracking program technology is in keeping with the other technologies used on board the vessel can also be important for usability. Most freight vessels are required to have both stability books and cargo loading manuals for the shipâs crew and shore-based personnel to use to maintain the vesselâs stability levels and structural integrity. The stability books and cargo loading manuals are required at a minimum to be in paper form. In addition, many vesselsâbut not allâhave paper books and manuals that are supplemented by computer-based programs. These programs can range from simple spreadsheets to interactive graphic-based programs with some parameters such as vessel drafts and tank levels input automatically. A weight-tracking program that is similar in concept, form, and operation to these other onboard documents and technologies used for routine tasks, such as the stability books and cargo loading manuals and programs, would seem to be more likely to be used correctly and when needed than a program that is based on other, less familiar technologies. For example, vessels that use paper-based stability books and cargo loading manuals might be better suited to a paper- based weight-tracking program that does not require additional specialized equipment or
PREPUBLICATION COPYâUncorrected Proofs 28 computer programs to be installed, even if the latter technology offers some marginal advantages in terms of recording accuracy. Alternatively, computer-based tracking programs may be more in keeping with the technologies and practices commonly used by the vesselâs personnel, in which the use of a paper-based programâeven if simpler in some respects than a computer programâ would be out of the crewâs normal operating routines and less likely to be maintained and used. SUMMARY Currently, the only inspected vessels for which the USCG requires tracking of lightship weight changes are passenger vessels with SOLAS certificates and towing vessels subject to Subchapter M requirements. Nevertheless, the USCG requires that vessel operators report weight changes that arise because of vessel modifications and retain these records to know when stability calculations and tests may be needed in the future. While the USCG recently proposed requirements for lightship weight tracking and periodic stability testing for a wider range of passenger and freight vessels, it has withdrawn these proposals because of concern about the high cost of implementation (especially for periodic testing) and the absence of casualty data indicating that changes in lightship weight have caused vessel losses. Analyses in this chapter suggest that passenger vessels, much more than most freight vessels, are subject to degradations in stability levels as a result of increases in lightship weight. The former vessels would appear to be more pressing candidates for lightship weight-tracking programs than the latter, whose lightship weights tend to account for a smaller percentage of vessel-loaded displacement. To be effective in gauging stability impacts, it is important that a lightship tracking program, whether applied to passenger, freight, or other vessel types, have certain characteristics that ensure both timely and accurate recordings and a high degree of usability. Although an accurate recording of weight changes is essential, the timing of when incremental changes in weight should be recorded will depend in large part on the vessel type and its stability sensitivity to changes in lightship weight. As the sensitivity analysis in this chapter indicates, changes in lightship weight of freight vessels, particularly those with large cargo displacements, have a relatively smaller bearing on stability than changes in lightship weight of passenger vessels. As such, there is less of a need to record smaller weight changes on freight vessels; rather the focus for freight vessels can be on recording when larger weights are added, moved, or removed. For any vessel type, an important consideration in designing and implementing a weight- tracking program is its ease of use by crew and onshore personnel. In this regard, a program with complexity suited to the levels of stability training and knowledge of the crew will be essential to ensuring the program can and will be used as needed to ensure that weight gains that create stability risks are monitored and assessed as needed. Likewise, to facilitate use, the programâs format (e.g., paper- or computer-based) and operations will need to be familiar to the vesselâs crew and other users and compatible with other technologies and practices commonly used to operate the vessel safely. If the USCG deems it necessary to track lightship weight changes for freight vessels, strong candidates for the requirement would be those vessel types that have the greatest susceptibility to noncompliance because of weight changes as revealed by the kind of sensitivity analysis conducted in this chapter. One might consider, for instance, whether an increase of 4% (or double the threshold value of 2% for known weight changes) when applied at the highest likeliest location (such as upper deck level) would cause the ship to fail its intact or damage
PREPUBLICATION COPYâUncorrected Proofs 29 stability criteria based on the sample loading condition with the lowest stability as determined from the vesselâs trim and stability booklet. Vessels that do not fail that test could be considered to be at low risk for loss of stability from incremental lightship weight changes over time and would not be required to track incremental, small weight changes.
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