Plimsoll Lines- A Detailed Synopsis

Fig.1: Plimsoll Line and 
various loadlines
(Courtesy: www.rhiw.com )

Surely you know about the different kinds of ships in existence today and how the cargo or the method of loading or unloading determines the class of ship concerned.

 Among merchant ships today, the largest are the Containers, the ones with the bulk cargo (Bulk Carriers) and the tankers. Speaking in terms of deadweight, these ships range from somewhere around 50,000 DWT in Handymax Carriers to a whopping 5,50,000 DWT ULCC’s (Probably among the largest man-made objects around). 

Having said about the magnitude of things here, let us concentrate on a part of the ship, or rather a certain mark on the ship hull having a centre-dashed circle and vertical lines with branches on both sides and a horizontal line on top.

A SLICE OF HISTORY

Fig.2: Samuel Plimsoll
(Courtesy: Getty Images )

In the early 19^th Century, there used to be ships which were loaded to such great draughts and hence such less freeboard that they were at great risks of being drowned in rough seas. Such ships carried crew who risked their lives every time they went to the sea. Actually these ships were worth more to their owners if they drowned than they would have been had they actually completed their journey! This was because these ships were ‘over insured’ and the owners who would gain from the insurance money, took advantage of the situation during that time.

 It was not until this Gentleman named Samuel Plimsoll, a coal merchant who had written a popular book on disasters of ship overloading, started campaigning for safety at seas. Ultimately, his efforts paid off and the load lines became compulsory in British ships and spread worldwide over. Today they have been standardized and are visible on the ship hull as given in this photo. Let us try to understand these markings.

First of all, think about it, when large ships travel very long distances, they pass through oceans, the largest of seas, and sometimes, through freshwater passages in between. Now, if you remember, this means that they will be passing through different climates and different densities of water, also the salinity in sea water is also a factor to be considered. The ship’s weight, for all normal purposes would not change as much, but the different densities mean that the pressure would vary from seawater to freshwater, and so the immersed volume and draughts would change accordingly. The vessel would float to deeper depth in fresher water than in seawater. Now from certain calculations, we can find out the depths for each climatic or geographic condition directly from one primary load line. This primary load line as in almost all cases is the Summer load line. Together these are a shorthand representation of the freeboard of the same ship in different seas on Earth.

OF LINES AND DISCS

Fig.4: Screenshot from our Prezi on Plimsoll Lines.

Take a look at this screenshot from our Prezi on Plimsoll Lines, we will take each of them at a time. First, starting from the top, we have the deck line, which by convention is of 300 mm length and 25 mm breadth, for that reason all such lines are of same thickness. But the other horizontal lines are of 230 mm length. These marks are inverted on the other side of the ship hull.

DECK LINE

The deck line is placed at exact intersection of the freeboard deck with the outer shell of hull plating. In case you want to mark the deck line, which serves as a reference to be place somewhere else, you will have to correct the freeboard calculation accordingly.

LOADLINE MARK AND PLIMSOLL DISC

Immediately below the Deck line, we have what is called the load line mark which passes through the disc of outer diameter 300 mm, called the Plimsoll Disc. The upper edge of the line passes through the centre of the disc. The vertical line is placed a distance of 540 mm from the center of the plimsoll disc. From this, the load lines stretch on both sides to a length of 230 mm each.

SUMMER LOAD LINE (S)

This is the primary load line from which the other load lines are derived, the International Maritime Organization under the Load Lines Convention specifies certain rules for calculation of freeboards and their implementation under supervision of classification societies and flag states. This load line mark’s position depends on many factors such as length of ship, superstructures, terms linked with overall raking of the fore body and so on. These have been standardized and can be obtained from Freeboard Tables which look somewhat like this.

Fig.5: Freeboard Table for ships of 'A'-type

(Courtesy:Load Lines, 1966/1988 - International Convention on Load Lines,1966,as Amended by the Protocol of 1988)

Certain formulae are used to correct this freeboard in case values of draught slightly deviates from assumptions (T>L/15) or similar corrections accounting for block co-efficient, height of superstructures, etc.  For the purpose of such calculations, ships have been classified as type A & B. There are certain factors which decide this like the type of cargo, watertight spaces, permeability of cargo compartments, etc.

Now here's the interesting part. Refer to Fig.4 or for that matter, to any Plimsoll Lines on a ship. How are each of those loadline marks obtained? Are the vertical distances between each of the different draughts different for different ships? Or are they same? Well, read on.

TROPICAL LOAD LINE (T)

The tropical load line is obtained by an addition from the summer draught (considered T hereinafter) measured from keel to the centre of Plimsoll Disc by amount 1/48^th of T. That is, it is T/48 above the Summer Load Waterline (S).

FRESH WATER LOAD LINE (F)

This is marked above the Summer Load Waterline (S) by the following amount:

Δ is the mass displacement in salt water (in tonnes) at the summer load line.

T is the tonnes per centimeter immersion in salt water at the summer load waterline.( The TPC for any draught is the mass which must be loaded or discharged to change a ship’s mean draught in salt water by one centimeter)

When it becomes difficult to find out whether freshwater and tropical freshwater are the same things, the position of the latter line relative to former is found in same manner as that of summer load line and tropical summer load line.

TROPICAL FRESHWATER MARK (TF)

The tropical freshwater mark (TF) is always marked at (T + F) above the Summer Load Waterline (S).

WINTER LOAD LINE (W)

The winter load line is obtained this time by a deduction from the Summer Load Waterline (S), an amount of T/48. That is, it lies T/48 below the Summer Load Waterline.

WINTER NORTH ATLANTIC LOAD LINE (WNA)

When a vessel is bound to enter any part of the North Atlantic Ocean during its winter period an additional load line called the WNA load line is assigned 50 millimetres below the winter mark. By default, it is same as the winter mark (W) for other ships. A separate WNA mark is present only on vessels that donot exceed length of 100 m.

ADDITIONAL LOAD LINES (Used on Ships with TImber Freeboards)

Fig.6: The Timber Load Lines for vessels 
with deck timber cargo

(Courtesy: 1873 issue of Vanity Fair(edited))

Now, take a look at the left side of the vertical line, there another set of load lines with an additional ‘L’ prefixed to them, these are called the Timber Load Line Marks or ‘L’ for Lumber Load Line Marks. These are additional load lines assigned to certain vessels which carry timber deck cargo and are granted additional freeboard as this ship will have greater buoyancy and protection against the sea and waves. These are analogous to normal load lines and are calculated similarly from the Summer Timber load draught (This value is supplied in the table from the convention), the only exception that Winter Timber load line is 1/36^th of the Summer Timber Load Draught below the Summer Timber load line. The displacement used in the formula is that of the vessel at her Summer Timber Load Draught.

Some vessels like Ro-Ro ships and Passenger Vessels have sub divisional load lines which are nothing but load lines for different loading conditions based on passengers and cargo, in any case, the these should not be above the deepest load line in salt water.

There is one more thing which you must have seen on the load line mark passing through the disk (450 mm in length) which seems to bear the initials ‘NK’, this is called the ‘Mark of assigning Authority’. They tell you which Classification Society has surveyed the load line. The initials used include AB for the American Bureau of Shipping, LR for Lloyd's Register and IR for the Indian Register of Shipping and so on.

Such Load Line Convention rules do not apply to certain kinds of vessels like the warships, new ships of 24 length or less or those existing ones of less than 150 GT, even the yachts not engaged in trade and for that reason fishing vessels. Certain Geographical regions are free from the observations of the Convention. Definitely, these lines have made our life safer at seas and international trade fairer.

PREZI ON PLIMSOLL LINES (A Clearer View)

It often becomes tough for us to memorize the Plimsoll Lines unless we have a good on-ship experience or good experience in working on a ship design project. For Naval Architects, it is highly important to have a clear view of the Plimsoll Lines, and the following Prezi is an attempt to make that easier for you:

Wait till the Prezi loads. Once loaded, you need to click on the arrow or use your arrow keys to watch the Prezi. It is recommended you watch it in Full Screen Mode for the best view. LSD

    Article By: Sudripto Khasnabis

MOL Comfort- What Happened? (Part 2)

Flashback

In Part 1, we focused on the position of the crack on the hull girder, following which we saw how Class NK estimated the wave induced loads on the ship during the accident scenario, considering required uncertainties in the parameters. Based on the above obtained wave loads for different sea states, the maximum and minimum wave induced vertical bending moments were estimated.Having estimated the wave induced load, it was now required to estimate the hull girder strength (of the considered three hold model). This article is about to discuss the methods involved in calculating the strength of the structure, and obtained results of the same.

3-Hold Model for Strength Analysis

The region of failure was identified by field investigations. So for the finite element analysis, a three hold model was considered. Further conditions taken during the analysis were obtained from Class NK Guidelines for Container Carrier Strength (Guidelines for Direct Strength Analysis, 2012) as shown in Table 1.

Table 1: Conditions for 3-hold model analysis.
(Courtesy: Class NK)

Fig. 1: 3-hold model used for analysis. (Photo edited)
(Courtesy: Class NK)

Estimation of Ultimate Strength

The strength of the three hold model was estimated considering uncertainties as shown in Figure 2.

Fig. 2: Factors affecting uncertainty in strength of the double bottom structure.

How was the yield stress of the structure calculated? The value of yield stress of the members were obtained from their respective mill sheets. The average of all the yield stress values of the different member materials were calculated and regarded as the mean value of hull girder ultimate strength (μ).

It is important to understand what was done next. Given the fact that the strength of a marine structure follows a probabilistic nature (that can be represented by a Probability Density), it is evident that consideration of mean value alone for determining the ultimate structure is not a valid thing to do. What if the strength of the structure at any point of time, reduces from its mean value? Therefore, it is necessary to determine the minimum ultimate strength of the structure to consider the worst case scenario.

Class NK adopted two different methods to determine the minimum hull girder ultimate strength, an the strengths obtained through each of the two methods were categorized as Case 1 and 2 (will be referred by the same hereinafter).

Case 1- The standard deviation (σ) of the yield strength of the bottom shell plates were calculated from the mill sheet values. The minimum yield stress of the hull girder was defined as the value that was less than the mean by three times the standard deviation, i.e. Minimum yield stress = μ-3σ (Refer to Figure 3)
The hull girder ultimate strength was then evaluated corresponding to the above minimum yield stress of the bottom plating. This ultimate strength was regarded as the minimum hull girder ultimate strength.

Fig. 3: Graphical representation of Case 1

Case 2- The hull girder ultimate strength was evaluated corresponding to the minimum yield stress of the bottom plating specified in the mill sheets. The obtained ultimate strength was then regarded as the minimum hull girder ultimate strength.
The obtained values of yield stress (for both the cases) were as shown in Table 2.

Table 2: Yield stress for Case 1 and Case 2.

Now, in order to find the ultimate strength, a very simple method was adopted: The loads at the time of the accident were known and categorized into the following:

1. Hull weight corresponding to the double bottom structure (known before analysis)
2. Hydrostatic pressure corresponding to the full draught (known before analysis)
3. Container Loads (known before analysis, based on the loading information at the time of the accident)
4. Allowable still water bending moment for hogging (calculated before the analysis, from loads 1, 2 and 3)
5. Wave-induced pressure (priorly calculated from Class NK Direct Strength Analysis, 2012)
6. Wave-induced vertial bending moment (calculated from IACS UR S11)
7. Additional vertical bending moment (due to uncertainties)

The interesting part is how these loads were applied to the model for analysis. Initially, loads 1,2, and 3 were gradually increased every one second until they reached their known values. Then loads 4, 5, and 6 were applied in turn and increased every one second until their known values were attained. At last, load 7 was gradually increased every second until the stress in the structure exceeded the Von Mises Stress of the structure. (Graphical representation in Figure 4). The stress at which the structure failed, was regarded as the Hull Girder Ultimate Strength.

Fig. 4: Sequence of application of load on the model.
(Courtesy: Class NK)

The above method was followed for three conditions:

1. When yield strength of the structure was corresponding to the mean value (μ)
2. Case 1: Yield strength = μ-3σ
3. Case 2

The hull girder ultimate strength was also obtained for three different conditions and the corresponding vertical bending moments were obtained, as shown in Table 3.

Table 3: Obtained values of Vertical Bending Moments when the hull girder fractured.

Fig. 5: Time vs. Bending moment at the section that suffered failure in the case of average yield stress. (Picture edited)
(Courtesy: Class NK)

Fig. 6: Von Mises stress at the time of peak load.
(Courtesy: Class NK)

Fig. 7: Equivalent plastic strain at the time of peak load.
(Courtesy: Class NK)

Fig. 8: Von Mises stress at the time of peak load.
(Courtesy: Class NK)

Fig. 9: Equivalent plastic strain at the time of peak load.
(Courtesy: Class NK)

What's in Part 3?

Certain factors were multiplied to the obtained values of bending moment, in order to compensate for the factors of local deformations and residual stresses due to welding. Inclusion of these factors, reduced the strength further. It is on the basis of the then obtained strength values, that the probability and extent of damage will be discussed in the next part of this series.LSD

Article By: Soumya Chakraborty

Integrated Masts-The Next Generation Masts

Fig. 1: UNIMAST : The Integrated Mast Family
(Image Courtesy: www.selex-es.com)

                                         

The heading truly lays an emphasis on the need of using integrated masts .Why do we need it? The use of conventional masts with dozens of antennas, doesn't it make a naval ship communication system more complicated?

Let’s discuss how this integrated masts will prove beneficial in near future!

We can call it a housing that accommodates all the radars, sensors and antennas of a naval vessel. Gone are the dozens of antennas and sensors found on practically every flat topside surface of a modern naval vessel. The presence of all these systems, however sophisticated and advanced they individually may be, on one ship creates several problems.

As we know, the best position for a sensor is on top of the highest mast. There's only one system that can benefit from this position; all the others will be blocked to a certain extent by this mast. All antennas, so close together will affect each other. On most naval vessels it is necessary to switch one system off before another antenna can be used. This has been the cause of some serious incidents.

Features

Integrated mast reduces electromagnetic interference and physical obstructions between electronic sub-systems, and improves across the board performance through the provision of a single operation centre.

UNIMAST represents the Selex ES’ solution to the need of enhanced air, surface and sub-surface defence effectiveness in the naval domain.Let us discuss some of its benefits

Fig. 2:Main Systems Antennas Positions
(Image Courtesy: www.thalesgroup.com)

UNIMAST enhances operational effectiveness across all present and future scenarios:

• Anti-aircraft and anti-missile defence.
• Counter-fire.
• Improved search and track capabilities, against asymmetric threats like small manned or unmanned aircraft at low altitudes and at low speed.

• Reduced ship radar cross-section.
• Improved flexibility for different operating conditions, such as littoral surveillance or blue water operations.

In order to meet ever more demanding operational needs, the integrated mast includes:

• Surveillance radar and air and surface tracking by means of multifunctional AESA 3D four fixed face radars ,operating in C-band (two versions: MFRA and KRONOS) and X-band (two versions: 2D and 3D)
• A phased array IFF using a conformal antenna and operating up to Mode number 5.
• An optronic system.
• Integrated communication system, including tactical data links.
• Electronic Warfare system integration.

The Selex ES UNIMAST Integrated Mast Features

• Surveillance radar and air and surface tracking by means of multifunctional AESA 3D four fixed face radars, operating in C-band (two versions: MFRA and KRONOS) and X-band (two versions: 2D and 3D).
• Electro-optical system. Passive air and surface surveillance and tracking. Infra-red (IR) mapping to support threat evaluation and classification.
• Communications. Data links  and satellite communications system, line-of-sight VHF and UHF communications, Link 11 and Link22 UHF, Link16 Rx and satellite communications.

Let’s know the radars- In The Thales Integrated Mast eliminates these problems. All radars and antennas not only have a full 360° field of view; they are also developed so as to operate simultaneously without interfering each other.

Fig. 5: Integrated Mast tracking features

(Image Courtesy: www.thalesgroup.com)

The radars in the Integrated Mast are non-rotating, four-faced active phased array radars, which in itself is a major performance enhancement. As the four faces operate simultaneously, the radars achieve four times the time on target achieved by a rotating radar. The surface surveillance radar (Seastar) was developed especially for this purpose and it is capable of detecting and tracking small objects (e.g. divers' head) between the waves, contributing enormously to situational awareness in littoral environments.

How is it installed?

The Mast is tested as one system. Not before it fully complies with the customer's specifications is it transported to the shipyard. There, the Integrated Mast is simply bolted or welded to the ship, hooked up to the power supply, coolant system and data transmission and is operational in only two or three week time. Compared to the one year that is necessary to install, integrate and test all the separate systems, this is a huge time and money saving option, for Navy as well as shipyard.

The system’s support has also been simplified, providing access from within the mast, and protecting much of the electronics and cabling from wind, and corrosion.

Fig. 6: Holland-Class Offshore Patrol Vessel / OPV
(Image Courtesy: www.seaforces.org)

This system has been installed on the Patrol Ships for the Royal Netherlands Navy .The first one was scheduled to be operational in late 2010. The I-Mast 100, introduced in September 2009, is the second member of the I-Mast family. This system is designed for smaller, corvette-sized vessels. The type of systems in the Mast is completely up to the customer. Although the Integrated Mast for the Holland class OPVs for the Royal Netherlands Navy contains mostly Thales systems, it will be possible to use customer-furnished or third party systems in a Thales Integrated Mast

Below are some videos about the Thales Family of i-Masts and their integration in ships and benefits.LSD

Article By: Siddhi Indulkar

Recommended Visuals: 1.) Thales presents: The i-Mast Family

                                2.) Thales Integrated Sensor Mast

MOL Comfort- What Happened? (Part 1)

On 17th June 2013, MOL Comfort, a post Panamax container ship owned by Mitsui OSK Lines suffered a crack amidships and split off into two off the coast of Mumbai. It turned out to be one of the biggest structural failures in the history of container shipping, and the reason behind the same remained unknown unless a dedicated investigation was carried out into the matter by Class NK, who recently released their final investigation report on the case. 

This provides a great chance for designers to make reconsiderations and learn ship structure design from a different point of view. This series of articles are a detailed analysis on the investigation carried out by Class NK. The aim of launching this series is to give an interactive and informative insight into the entire investigation which fosters a better way of learning structural design than just what books and professors can do. It is also assumed that you have a basic touch up on ship structures before you read on.

The fracture generated from the bottom shell of the double bottom structure of the ship. The condition shown in Figure 1 is one of a later stage when the crack had propagated above the waterline. But the origination of the crack was investigated and found to be at 200 mm fore of Frame 151 (as shown in Figure 2), where there was a butt weld (remember this throughout the entire analysis). Also, the entire analysis was carried out on the double bottom structure at the hold corresponding to the area of crack generation and half of each hold fore and aft of the hold where the crack generated.

Fig. 1: The crack originated in the bottom shell and propagated above the waterline.
(Image Courtesy: Google Images)

Fig. 2: Actual position of generation of crack.
(Image Courtesy: Class NK)

The load on a ship not only depends on its cargo loading conditions but also on the environmental factors, which we collectively call as Sea State. The cargo loading conditions were obtained from the shippers (weight of each container along with the container loading plan). However, it can never be affirmed that the data obtained was correct, given the fact that many shippers practice in overloading the containers, which is often not a design standard in terms of the ship's strength. 

Estimation of Wave-Induced Loads (Considering Uncertainties)

The environmental conditions (wave induced loads) during the accident were recorded. But as ships always operate in an environment of periodically varying parameters, it is a common practice to consider certain deviations in the recorded data for a wider analysis. Class NK has pretty much done the same in their investigation. Considering certain deviations in the parameters of wave and wind induced loads, Class NK proceeded with the following data as shown in Table 1.

Table 1: Deviated values of wave induced load parameters during the accident.
(Image Courtesy: Class NK)

But how did they get to these values? Based on the recorded data during the accident, the sea state and load response on the ship was simulated probabilistically for 27 different scenarios. Since each load condition would subject the hull to a specific wave-induced vertical bending moment, a probability distribution was plotted against the possible values of the wave induced vertical bending moment and the frequency of occurrence of each, as shown in Figure 3. For each of the 27 conditions, 1000 waves (short term sea state) were considered for the simulation. Note how the frequency of occurrence of extreme maximum and minimum bending moment are lower than that of the occurrence of an averagely medium value.

Fig. 3: Frequency of occurrence of various wave induced vertical bending moments.
(Image Courtesy: Class NK)

For a better understanding, Table 4 shows the obtained parameters when the wave induced vertical bending moments were maximum and minimum. This is how the wave load parameters were considered with estimated deviations for more accuracy.

Table 2: Wave induced load parameters in case of maximum and minimum wave induced vertical bending moments.

(Image Courtesy: Class NK)

Uncertainty in Strength

Fig. 4: Factors that definitely affect the strength of the double bottom structure.

Fig. 5: Factors affecting uncertainty in strength on the structure.

What's in Part 2?

In Part 2 of this series, we will see how these uncertainties were used to estimate the strength of the double bottom structure probabilistically, rather than deterministically. The importance of probabilistic determination lies in the fact that ship structural failures may not occur in load scenarios which occur very frequently. It is the less frequent but most adverse conditions that lead to such failures. This makes it necessary to analyse the failure from a probabilistic point of view.LSD

Article By: Soumya Chakraborty