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PREPUBLICATION COPYâUncorrected Proofs 106 Appendix E Cargo Ship Weight-Tracking Programs A weight-tracking program for cargo ships, or any ship, tracks the inevitable lightship weight changes that will occur over the shipâs lifespan from the last inclining test or deadweight survey. The weight categories to be tracked would need to include both those that are well defined as well as weight changes that often go unnoticed because they are considered individually to have no significant impact on a cargo shipâs stability. Based on the information derived from a functional weight-tracking program, the potential impact on a cargo shipâs stability can be periodically reevaluated, as needed, to ensure the cargo shipâs stability is adequate over its entire life. FUNCTIONAL CARGO SHIP WEIGHT-TRACKING PROGRAM REQUIREMENTS For a cargo shipâs (or any shipâs) weight-tracking program to be functional, it will need to meet two basic requirements. 1. The cargo ship weight-tracking program must be readily usable by either the crew and/or shore-based personal responsible for its upkeep. 2. The cargo ship weight-tracking program must be accurate enough to track the weight changes that occur in a cargo shipâs lightship condition over its lifetime and to determine when the cargo shipâs stability must be reviewed to ensure that the ship maintains adequate levels of stability. Failure to meet both of these interrelated requirements for any ship weight-tracking program could pose more of a danger than having no weight-tracking program. For example, if the cargo shipâs weight-tracking program is very cumbersome to use, it will not be maintained to a sufficient level of accuracy to reflect the actual changes in the cargo shipâs lightship. In this case, the cargo shipâs weight tracking could inform the shipâs crew that all is well when in fact, weight changes could have been missed, compromising the shipâs stability to unacceptable levels. Ease of Use by the Crew or Shore Personnel As noted in the previous example, any effective cargo ship weight-tracking program must be usable by the crew and/or shore based personnel. To do so, a weight-tracking program is required to meet the following guidelines. Type and Complexity of Ship The weight-tracking program must be suited to the type of cargo ship on which it is being used. Cargo ships exist in many different designs and levels of complexity to meet the wide variety of cargoes being carried. Cargo ships range in complexity from relatively simple tankers and bulk carriers to container ships to complex ro-ro carriers. Cargo ships also range in likelihood that changes will be made to the ship over the life of the ship. The likelihood of significant changes to a tanker or bulk carrier is small, while a ro-ro carrier or container ship will likely have modifications to meet ever-changing market conditions. Thus, a weight-tracking program suitable for a tanker may not be suitable for a container ship. Some vessels, such as heavy-lift submersible ships or cargo ships, used solely to carry specialized or unique cargoes, may not need a dedicated weight-tracking program. Even though these vessels are often significantly modified for each job with potentially significant changes in their lightship characteristics, these modifications are thoroughly planned and engineered.
PREPUBLICATION COPYâUncorrected Proofs 107 Essentially, they undergo a detailed stability review for each transit. Thus, these vesselâs lightship changes are already under constant and detailed review, and the vesselâs stability levels are being maintained at adequate levels. Training Level of Crew The weight-tracking program must be suited to the training levels of the shipâs crew and/or shore-based personnel using the program. Licensed officers of most cargo ships are required to receive some education on ship stability, but it is limited to the basic concepts. In addition, shore-based personnel may have no stability education at all. Thus, persons with little or no stability training are unlikely to effectively use any weight-tracking program that require advanced knowledge of ship stability or how changes to ship lightship affect its stability. Such programs may actually be dangerous to the vesselâs safety as the crew may believe they are maintaining the weight-tracking program correctly when in fact they are not. As previously discussed in this case, the cargo shipâs weight tracking could inform the shipâs crew that all is well when in fact, so many weight changes have been missed and the shipâs stability has been compromised to unacceptable levels. Technology Available on Ship The weight-tracking program could be suited to the technology available onboard the cargo ship. All cargo ships are required to have both stability books and cargo-loading manuals so the shipâs crew and shore-based personnel can maintain both the cargo shipâs stability levels and structural integrity. At a minimum, the stability books and cargo-loading manuals are required to be in paper form, but are normally supplemented by a computer-based stability program for ocean- going ships. These programs can range from simple spreadsheets to interactive graphic-based programs with some parameters such as vessel drafts and tank levels being input automatically. Any weight-tracking program could then be integrated into the available technology on board a particular cargo ship to increase the likelihood of it being effective in tracking any changes in the cargo shipâs lightship conditions. This is particularly critical as any cargo shipâs weight-tracking program is not going to be used as regularly as stability and cargo-loading programs. When a nonroutine task is similar in concept, form, or operation to everyday routine tasks, it is more likely to be completed and completed correctly. For example, if a cargo ship uses paper-based stability books and cargo-loading manuals, it would use a similar paper-based weight-tracking program. The paper-based weight-tracking program would require no additional specialized equipment or computer programs to be installed on board the vessel. The use of paper weight-tracking forms would be similar to other forms used by the crew to check their stability and cargo loading, in keeping with their normal operating routines, and thus more likely to be kept up-to-date. On the other hand, cargo ships that primarily use computer-based stability books and cargo-loading manuals would not use a paper- based weight-tracking program. The paper forms would be out of the crewâs normal operating routines and not as likely to be maintained. Accuracy in Tracking Weight Changes The second key requirement in any cargo ship weight-tracking program is its accuracy in tracking the weight changes that occur in a cargo shipâs lightship condition over its lifetime and in determining when the cargo shipâs stability should be reviewed. These weight changes to a cargo shipâs lightship condition occur in many different forms and magnitudes, and can range
PREPUBLICATION COPYâUncorrected Proofs 108 from very small, such as changing the chairs in the shipâs mess, to large, such as adding a new adjustable car deck in a ro-ro vessel. Some changes are well documented (i.e., the adjustable car deck), while others just occur as routine operations (i.e., shipâs mess chairs) and often go unnoticed with respect to their impact on the shipâs stability. The potential accuracy of any weight-tracking program is determined by the setting of two key threshold values. The first value is which changes should be recorded by the weight- tracking program and which changes it can safely ignore (i.e., weight change recording threshold). The second value is for determining when the impact of cumulative recorded weight changes is sufficient to have a potential negative impact on the shipâs stability and, thus, when the shipâs stability should undergo review (i.e., stability review threshold). . Both thresholds need to be unique to an individual vessel and carefully selected to maximize the effectiveness of any cargo ship weight-tracking program. Proper threshold configuration will ensure any potential negative impacts on a cargo shipâs stability are highlighted, while at the same time not being overly burdensome to the shipâs crew or shore- based personnel, thus ensuring effective implementation. The key threshold values determined for each cargo ship would be based on the following three factors: 1. Type of cargo ship. (e.g., tanker, ro-ro, container, car carrier). 2. Physical size of the ship. 3. Time between formal reviews of the shipâs stability. (e.g., inclining test or deadweight survey). The setting of the threshold values must be carefully done; one cannot simply set a very low value under the guise of being overly conservative to ensure safety. For example, setting too low a threshold value for when a weight change would be recorded can actually create two problems affecting a weight-tracking programâs effectiveness. The workload on the shipâs crew will be overly burdensome from the numerous small changes recorded that normally occur to most cargo ships. This, coupled with the fact that the crew may simply not believe that any weight change that small can have a significant negative impact on their vesselâs stability levels, can cause the shipâs crew to treat the weight-tracking program less seriously and ignore it. Conversely setting too high a threshold value can have the obvious problem of missing weight changes that may have a cumulatively significant negative impact on the vesselâs stability compliance. Cargo Ship Weight-Tracking Program Configurations As previously discussed, any cargo ship weight-tracking program would be tailored to its intended ship to maximize its potential effectiveness. All weight-tracking programs in essence are a simple database of records of the weight changes that have occurred to a vessel. The configuration of the programâs database would be customized for each cargo ship in the following four areas: â¢ Information to be recorded on each weight change being tracked. â¢ Paper-based or computer-based or a combination of both. â¢ Setup for use by the shipâs crew, by shore-based personnel, or a combination of both. â¢ Method for determining when the cumulative lightship weight change is at an actionable level.
PREPUBLICATION COPYâUncorrected Proofs 109 Information to Be Recorded In configuring any weight-tracking program database, the first area to determine is what information will be recorded for each weight change that exceeds a certain threshold. At a minimum, the following information would be recorded: 1. Description of the weight change that occurred. The description would include sufficient detail that allows for future verification of the weight change; for example, Added Convection Steamer, Hood, and Exhaust Fan/Ducting to Galley. 2. Amount of total weight change, for example, 12.5 long tons. 3. Description of where the weight change occurred. The description would include sufficient detail for the location to be verified in the future, for example, steamer located in aft port corner of the galley with exhaust ducting run to 03 deck port deckhouse side adjacent to weathertight door. 4. Longitudinal, vertical, and transverse center of the weight change from the shipâs common reference point, e.g., 245.6 ft aft of amidships (Frame 64), 42.8 ft above baseline, and 24.5 off centerline to port. 5. Date of weight change, e.g., March 04, 2012. 6. Optional: The person(s) entering the weight-change records if the information is needed to suit the particular cargo shipâs configuration, operation, or management. Paper- or Computer-Based The second area for configuring a weight-tracking program is whether it would be paper-based, computer-based, or some combination of the two. As previously discussed, the configuration chosen would be based on the technology available onboard the ship or at the shore-side operation. Paper-based weight-tracking programs typically will consist of one or two forms to record the individual weight changes and to keep track of the current cumulative change in the shipâs lightship condition. The simplest paper-based configuration would be a single form that records each weight change and tracks the cumulative change in lightship condition. This single form configuration is best suited for ships that have relatively small and simple weight changes and can be maintained by most crews. More complex weight changes are best handled by a two-form configuration, one form to record each weight change and one form to track the cumulative change in the shipâs lightship. In addition to being better suited to more complex weight changes, the two-form configuration has the advantage of allowing the weight-tracking program to be conveniently done by more than one group. For example, the shipâs crew can fill out the individual weight change forms onboard the ship, and then forward the forms to the operations office. The operations office would then fill out any missing technical information on the weight change not available to the crew, such as the numerical location of the center of gravity. The operations office would then complete the second form to update the cumulative impacts on the ship and make the determination if the shipâs lightship needs to be reviewed. Computer-based cargo ship weight-tracking programs would be similar to a paper-based weight-tracking program with the exception that a computer would be used to input, record, and analyze the information. As with the paper-based program, the computer-based program would consist of one or two âformâ input screens, with a âformâ such as a spreadsheet or PDF document. The key difference between the paper-based and computer-based programs is the number and complexity of the weight changes that can be handled.
PREPUBLICATION COPYâUncorrected Proofs 110 A combination of the paper-based and computer-based weight-tracking programs could be appropriate for a fleet of vessels that have low technology onboard or typically have simple weight changes, but a central operations office. In this configuration, the shipâs crew would complete a paper-based form to record basic information for each weight change that occurs. These forms would then be forwarded to the central operations office where the information would be entered into some type of computer-based program. With such flexibility, the mixed paper- and computer-based weight-tracking program has the advantage of being able to match the best of each configuration for the particular cargo ship(s) in question. Setup for Use by the Shipâs Crew or Shore-Based Personnel Responsibility for keeping the weight-change records needs to be assigned when configuring the weight-tracking program. Three groups can be responsible for gathering the required information and evaluating the impacts: the shipâs crew, the shore-based operations personnel, or the technical staff. Because a single group is not required to collect all the information, the tasks can be divided among the three groups depending on a particular shipâs operating management. For example, fleets with multiple vessels and a central operating office would likely have a setup where the shipsâ crews record the basic information for each weight change. The weight change records would then be forwarded to the technical staff at central operations, who would log and evaluate impacts of the weight changes. This division of responsibility minimizes the workload of nonnavigational tasks on the shipsâ crews and places the more demanding technology aspects of any weight-tracking program with the central operations staff, who are better suited to manage them. Single vessel operations, on the other hand, will likely have all of their weight tracking handled by the shipâs crew. This crew could be assisted as needed by ouside technical persons, such as naval architects, for more complex weight changes. Method for Determining Impact of Cumulative Weight Changes The last issue to be decided in the configuration of any cargo shipâs weight-tracking program is what method is to be used to determine the cumulative impact of weight changes on the cargo shipâs stability and when its stability would be reevaluated. It is important to note that this âcumulativeâ impact must be evaluated both as the ânetâ change in the vesselâs lightship characteristics (i.e., changes in lightship displacement and centers of gravity), as well as the âgrossâ level of changes that have occurred. The calculated net change lightship characteristics are the vesselâs lightship condition after totaling the weights added, subtracting the weights removed, and shifting any weights moved from one location to another on the vessel, and also adding the moments of these weight changes to enable determination of center of gravity changes from the last verified lightship condition. This calculated net change lightship condition would be used to determine if the vesselâs stability needs to be reevaluated and by what means that should be done: revised calculations, deadweight survey, or inclining. For example, if the net change in a passenger vesselâs lightship weight is less than 2%, only a mathematical reevaluation of the vesselâs stability is required. Exceeding the 2% threshold can trigger either a deadweight survey or inclining depending on how much above the 2% threshold the net change is.63 The calculated net change lightship weight condition, however, cannot by itself be used when determining if a cargo shipâs stability needs to be reevaluated. This is because the change in the calculated net change lightship condition from the last verified lightship weight condition 63 See Marine Technical Note (MTN) 04-95, https://www.dco.uscg.mil/msc/mtn.
PREPUBLICATION COPYâUncorrected Proofs 111 does not reflect the total magnitude or number of weight changes made since the last verified lightship condition. That is, from the calculated net change lightship condition one only knows the starting point of the lightship changes (i.e., the last verified lightship condition) and the ending point of the changes made to a vesselâs lightship (i.e., current cumulative net change lightship condition). What one does not know is how the cargo ship got from the last verified lightship condition to its current lightship condition, or in other words how much confidence there is in the accuracy of the calculated net lightship condition. If there have been few changes to the vessel, then the accuracy of the calculated net lightship condition would be good, as there are minimal places where errors can occur. However, if there were many small changes that in the end had little net change on the vesselâs lightship condition or instances in which several major weights were added and removed that canceled out their net impact on the vesselâs lightship, there could be more of an opportunity for significant error in the calculated net change lightship condition even if that value is very close to the last verified lightship condition. For this reason, the gross magnitude of the weight changes would also be calculated and a different set of threshold values used to determine when the cargo shipâs stability would be reevaluated. The gross magnitude of the weight changes is the sum of the absolute value of the total weight added, the absolute value of the total weight removed, and the absolute value of the total weight relocated. This is the approach required by MTN 04-95, which requires submittal of the total weight and center change data for an evaluation of what type of stability verification is required based on the magnitude of the weight changes and the accuracy of the method by which they were calculated. A similar approach is used when inclining a vessel to determine if the amount of weights to add, remove, or relocate is at an acceptable level to ensure the accuracy of the incliningâs results. Typically for an inclining, if the gross magnitude of the weights to add, remove, or relocate is less than 2% of the final lightship weight ,the inclining is acceptable as done. However, if the gross magnitude of the weights to add, remove, or relocate is between 2% and 10% of the final lightship weight, a confirmatory deadweight survey is required, and if in excess of 10%, another inclining is required. Similar values could be used in a cargo shipâs weight-tracking program to determine when the cargo shipâs stability needs to be reevaluated. Cargo Ship Weight-Tracking Program and Stability Review Threshold Determination As previously discussed, two key thresholds in any cargo ship weight-tracking program are the threshold value for when to record a weight change (weight change recording threshold) and the threshold value for when the cumulative weight changes are sufficient to warrant a formal review of the shipâs stability (stability review threshold). The two thresholds could be unique values determined specifically for each ship, as both values are interrelated. As such, the stability review threshold values must be done first, from which the weight change recording threshold can then be set. The U.S. Coast Guard (USCG) has determined a set of stability review threshold values for inspected vessels, including cargo vessels, that have either undergone modifications or are requesting sister ship status with an existing vessel. For a vessel that has undergone modifications, a formal stability review, such as a deadweight survey or inclining test, could be done if the gross cumulative weight change(s) exceeds 2% of the vesselâs lightship weight and/or the net change in the vesselâs longitudinal center of gravity (LCG) is more than 1% of the vesselâs length between perpendiculars (LBP).
PREPUBLICATION COPYâUncorrected Proofs 112 When sister ship status has been requested and a deadweight survey is being performed, the International Maritime Organization (IMO) Intact Stability Code applies a 1% weight change and/or a 0.5% LCG threshold value for cargo ships greater than 50 m in length and a 2% weight change and/or 1% LCG threshold value for ships less than 50m in length. USCG applies the 2% and/or 1% standard for all cargo vessels regardless of length. The 2% weight change and 1% LCG change threshold would likely be the mandatory threshold to notify the USCG when a reevaluation of a vesselâs stability is required due to cumulative weight changes. A potential trigger point for the vessel operator to investigate the effects of the cumulative weight changes could be approximately 75% of the mandatory threshold value, that is 1.5% of lightship weight or a 0.75% shift in LCG. Although there is no mandatory threshold for changes in vertical center of gravity (VCG), it is considered prudent to also calculate the change in VCG from the documented weight changes and apply a threshold value for when this could be a concern. One possible threshold could be when an increase in lightship VCG occurs, causing a reduction in attained metacentric height (GM) in any design load case so that the margin on the required value is below a minimum margin. Before taking action, the vessel operator determines the minimum margin based on the vessel type and anticipated accuracy of cargo and weight input into the stability calculation. This minimum margin could be approximately 0.5 ft, or some other value based on the operatorâs experience, but could be predetermined during the development of the weight- tracking program. Cargo Ship Weight-Tracking Program and Weight-Recording Threshold Determination As previously discussed, the weight-recording threshold is the other key component for any cargo ship weight-tracking program. This threshold will determine whether a weight change is significant enough to warrant recording in the shipâs weight-tracking program. The weight- recording threshold value could be unique to each vessel and based on the stability review threshold values determined previously. The basic concept in determining an appropriate weight- recording threshold is to calculate the total weight the cargo ship in question can experience before a formal stability review must be done and then spread that weight change over a set period of time. The first part, the total weight change, is relatively easy to determine. The second part will require the development of some assumptions on how weight changes typically occur for each type or class of cargo ship. The total weight change before a formal stability review must be done for each cargo ship is simply calculated by multiplying the cargo shipâs stability review weight change threshold percentage value by the cargo shipâs reference lightship weight. For example, using a vessel with a lightship weight of 23,000 tons and a 2% stability review threshold percentage, the total weight change before a stability review is required is 460 tons. The next step is to spread this weight change over a set period of time with an assumed number of individual weight changes. The period is an estimate of the time over which it is expected weight changes will occur, and the number of individual weight changes represents the average number of weight changes that will occur in the average time period. The time period and the total number of weight changes selected will be unique to each type or class of cargo ship and may require a systematic study by the USCG to determine their values. For passenger ships, the time period can be set at 5 years based on the International Convention for the Safety of Life at Sea (SOLAS) rules requiring a passenger vessel to undergo a deadweight survey every 5 years. Assuming a typical passenger ship will undergo an average
PREPUBLICATION COPYâUncorrected Proofs 113 of 10 weight changes every year means a total of 50 weight changes will occur in the 5-year period. Assuming the sample ship previously mentioned were a passenger ship, its average weight change would be 460 tons divided by 50, which equals 9.2 tons per weight change. The average weight change, however, cannot be directly used as the weight-recording threshold value. Because the value is an average, it will require one last step to convert it to a usable weight-recording threshold value. This last step will require a USCG systematic statistical study that varies the weight recording threshold value versus the number of weight changes to determine a threshold value that provides a level of accuracy, that is, a confidence interval, that is adequate to ensure the cargo shipâs stability is not unduly compromised before the stability- review threshold has been met. Regular Deadweight Surveys or Inclining Tests in Lieu of a Weight-Tracking Program As previously discussed, cargo ship weight-tracking programs consist of recording significant weight changes that occur over the lifespan of a ship and tracking potential cumulative impacts the changes may have on the vesselâs stability. For any weight-tracking program to be effective, it must be diligently applied throughout the lifespan of a ship. If applied haphazardly, a weight- tracking program will not provide reliable results given that the program may not be a routine part of the shipâs operation. Indeed, irregular implementation could lead to missed weight changes that potentially compromise the shipâs stability. The use of regularly scheduled deadweight surveys could be an alternate weight-tracking program. Such an approach is currently required for all SOLAS passenger vessels, which must have a deadweight survey at least every 5 years. Essentially, regular deadweight surveys track the cumulative weight changes and their impact on the vesselâs LCG that have occurred since the last inclining and stability review. Of course, regular deadweight surveys have advantages and disadvantages over the weight change recording type of weight-tracking program. The major advantage of regular deadweight surveys is that they are more accurate and reliable than a weight-tracking program. A deadweight survey will capture all of the weight changes that have occurred, large and small. In addition, a deadweight survey requires no involvement of the shipâs crew or operations department on a regular basis, removing a nonroutine task from their day-to-day concerns. Conversely, a deadweight survey requires suspending the cargo shipâs operation for a specific period to prepare the vessel for and perform the deadweight survey. Such a situation creates a cost for lost sailing time and for the required professional assistance to perform the deadweight survey and analyze the results.