Stability and Anchor Handling Operations

7/24/2019 Stability and Anchor Handling Operations

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1 IntroductionIn April 2007 the AHTS vessel Bourbon Dolphin, figure 1, capsized outside the Shetland Islandswhile working with the deployment of an anchor. Out of the crew of 15 persons only 7 could berescued, the rest was either found dead or lost in the sea. As a consequence a commission wasput together to find out the cause of the accident, analyse it and contingently propose

appropriate measures. According to the commission there were several human errors involved inthe process leading to the incident, but the capsizing was due to inadequate stability. Thecommission proposes several measures to prevent similar accidents in the future. Some of thosemeasures are directly connected to the stability of AHTS vessels and the regulation systemtreating this aspect.

Current regulations are not designed to cover anchor handling operations, why the NorwegianMaritime Directorate investigates whether a new set of national rules regarding such vessels shall beimplemented to prevent similar accidents in the future and to improve the safety of these vessels.The proposals from the commission lie as a basis for such contingent rules. However, thecommission worked under a lot of time pressure, and further investigation is desirable to makesure the new rules are relevant and compliant with their purpose.

Simultaneously with the directorates investigation DNVwants to evaluate whether a new classnotation are to be made specifically for AHTS vessels, or if an amendment to the existing tugnotation is enough. This master thesis aims to validate the rules proposed by the commission andto further investigate the stability aspects of anchor handling operations. The purpose is toprovide a decision basis for the outworking of new rules for AHTS vessels.

The course of event of the accident and information about Bourbon Dolphin are described inappendix A. Existing stability regulations are described in appendix B.

Figure 1The Bourbon Dolphin


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1.1 Anchor Handling Tug and Supply vesselAn AHTS vessel is typically between 50 and 90 meters long, with a breadth between 15 and 20meters. The bollard pull varies from 65 tonnes and reaches up to 250 tonnes1. At the momentthere are even larger vessels under construction with an expected bollard pull of up to 350tonnes, reaching almost 100 meters in length and 25 meters in breadth.

Typically an AHTS has a high deckhouse in the stem with a high bow in front of it. The deck islow and located aft of the deckhouse, reaching approximately between 2/3 and 3/4 of the totallength of the ship. The tugs are equipped with several winches with pulling capacities between140 and 650 tonnes dependent of the size of the vessel. Consequently the ships can have awinching power that is far more powerful than its bollard pull. This could be a problem in thedesign process since it cannot be designed based on the bollard pull, which is a conventionaldesign parameter. It is also equipped with one or two cranes, to move heavy equipment on thedeck. All this equipment is mostly located right behind the deckhouse. The winches are locatedhigh above the deck, often with a KGbetween 10-18 meters above baseline. When full with wireand chain the total weight of this equipment will be heavy, giving the ship a high centre ofgravity located fore amidships. In the aft part of the deck a stern roller is placed to lower thefriction between the mooring line and the deck. The stern roller is either one unit or divided in

two parts and reaches between 2-3 meters out from the longitudinal centreline. Most AHTSvessels have one or more main propellers, and one or several side thrusters. Some also have anazimuth propeller, which can raise the stated maximum bollard pull during short sessions when

necessary. The 360 rotatable azimuth propeller and the side thrusters increase the vesselsmanoeuvrability and the ability to maintain a course while subject to strong side currents. Figure2 shows an AHTS with a layout typical for these vessels.

Figure 2An AHTS vessel from Farstad Shipping. The layout is typical for these vessels with the highdeckhouse in the forward part of the vessel as well as the highly located winches. This particular vessel isunder construction and will be 88 m long, 21 m wide with a maximum winch power of 500 tonnes and a

bollard pull of 250 tonnes [1].

1 Figured based on a random selection of approximately 20 vessels classed as Tug and Supply vessels ofDNV.


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1.2 Anchor-handlingAnchor handling is all kind of situations including an anchor. It could e.g. be anchoring a cruisingship only for a short period. It can also refer to anchoring some kind of installation in the water,an installation that is removable but must be made secure in one spot. The purpose of theanchor/anchors is to prevent the installation or boat to move in the water. In this report anchor-

handling operations will only refer to anchoring floating platforms or other permanentinstallations. Information about different mooring systems can be found in appendix C.

1.2.1 Normal procedure of anchor handling operationsAnchor handling operations are not very standardized and before every operation a thoroughplan has to be carried out by the operator. The plan should include both the anchor handling andthe movement of the rig. The seabed, depth and required length of the mooring line etcetera areconsidered. All necessary equipment is chosen and respective vessels specific task is decided ifthere is more than one vessel involved which is common in these operations.

Deploying the anchorsThere are several ways to deploy the anchor. Common in all cases is that the AHTS hauls the

mooring line from the rig to the anchor position. When the ship reaches the anchor spot themooring line is connected to the anchor and it is lowered down to the seabed using theanchoring winch and a working wire with the length of about 1.5 times the water depth [2].During this operation the ship can be exposed to very large forces from the mooring line,depending on how far away from the rig the anchor is to be deployed. Should the weather be badwith wind, waves and currents, the forces can be even greater and it is important that the ship hassufficient stability to handle the current situation. During the last part of the operation it iscommon that another AHTS vessel assists by grappling the mooring lines a few hundred metersfrom the operating ship to reduce the weight. According to [3] Bourbon Dolphin had anapproximate speed of 0.25 knots while moving away from the rig.

Recovering the anchorsThe recovering of the anchor is basically a reversed procedure of the deploying procedure. TheAHTS heaves out 1.5-2 [2] the water depths of working wire and tugs the anchor loose from thebottom of the sea [4]. When the anchor is clear from the seabed the rig starts to pull in themooring line while the ship slowly winches up the anchor and reverses towards the rig.


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2 ObjectivesRemovable rigs are placed where the gas and oil are, often in areas with unfriendly weather.Because of this AHTS-vessels must be able to operate in these conditions. When a tug handlesheavy anchors and mooring lines the forces acting upon the craft can be very high. BourbonDolphin was not designed for the forces she was exposed to, and as a consequence she sank. As

described above this thesis aims at providing a decision basis for a contingent new class notationor additions to the existing tug notation. This will be carried out by describing the mechanicsinvolved in anchor handling in a lucid way. The following aspects are to be considered:

1. Determine which forces that are dimensioning for the ships stability when performinganchor-handling operations and choose the point of attack and lever arm for those.

2. Analyse the influence of propulsion when deciding upon dimensioning forces.3. Evaluate the commissions suggestions for additional stability rules.4. Determine what stability parameters to be chosen as part of the dimensioning demands.5. Determine appropriate loading conditions to be included in the book of stability.6. How to handle the weather and the sea state in the dimensioning and how this should be

implemented in the set of regulations. Can the IMO Weather Criterion be modified tobe suitable for considering the combined effects of the mooring line and waves?

7. Evaluate if KGMAXcurves can be developed to be useful aboard the vessels.8. Evaluate other possible risks that should be taken into account when dimensioning these

vessels.

2.1 DispositionThe first part of the report, chapter 3 8, will deal with the first 5 points above. It will start witha description of the influential forces on the ship and the resulting heeling moment duringanchor handling, chapter 3 and 4. It will also describe how they can be calculated and where theywill affect the ship, i.e. their points of attack. The following chapter, 5, will give a shortdescription of stability rules and how they are based on the ships GZ curve. It will also describewhat a loading condition is and why it is important. Chapter 6 will in short render the part of the

commission report dealing with stability and its conclusions. The following chapter, 7, willevaluate the commissions suggestions, the heeling moment, importance of loading and therespective influential forces. Finally a discussion about the results will be held in chapter 8.

The second part of the report will deal with the last three points above. In chapter 9 the IMOweather criterion will be presented, and an evaluation will be carried out whether the criterioncan be modified to analyse the combined effect of waves and the mooring line. In chapter 10minimum GM and maximum KG curves will be described and their potential usefulnessdiscussed. The eighth point will not be handled separately, but will be included in the finaldiscussion of the last three points in chapter 11.

In the last part of the report suggestions on how to deal with the discussed aspects will bepresented, chapter 12. Finally the whole master thesis will be concluded in chapter 13.


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-".+ /

3 Influential forcesThe first part of this chapter will describe the ships intact stability, which is the ships ability towithstand heeling moments. Thereafter the forces affecting the ship during anchor handling willbe identified and their respective impact analysed. The purpose is to create a model that takes allmajor internal and external forces into consideration.

3.1 Surface ship intact stabilityThe intact stability of a ship can, a bit simplified, be described as its capability to straighten itselfback into upright position when exposed to a disturbance. The lifting force of a ship is calledbuoyancy, B. When the ship is in upright position the action line of the buoyancy is the same asfor that of the gravitational force, G. However, when the ship is exposed to a disturbanceresulting in a heel, the hull shape makes B moving towards the heeling side of the ship. If theship is correctly loaded and the cargo is secured G should be constant, resulting in the actionlines of B and G running parallel instead. Figure 3 shows a principal sketch of a box shaped ship

that has been exposed to a disturbance and consequently has a heeling angle,

!.

Figure 3A box shaped ship with a heel seen from behind. As the ship heels the buoyancy, B, will movetoward the heeling side of the ship creating a righting moment by coupling with G.

As can be seen in the figure B has been moved towards the deeper immersed side of the ship,and consequently changed the lifting forces line of action so that it is no longer overlapping G.The two forces create a force couple with corresponding lever arms resulting in a rightingmoment. The lever arm is called GZ and is one of the most important components whenconsidering stability. GZ is calculated as

(3.1)


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The first half of the equation is dependent on the hull shape and the other half is dependent onG. The larger GZ is the larger the righting moment will be. As can be seen in the figure GZwillgrow with increased heel due to B being moved further away sideways from G. If the heelinggrows too large, however, B will start to move back and eventually cross Gs action line andconsequently create an added heeling moment instead of a counteracting moment. How muchthe ship can heel and still attain a positive counteracting moment is dependent on the hull design,

which varies dependent on the purpose the ship is meant to serve.

The metacentre, MC, is the point where the action line of B crosses the ships centreline, as canbe seen in figure 3. Dependent on the heeling angle the position of MC will vary but for small

angles, !< 10, it is typically considered constant [5]. The distance between G and MC is calledmetacentric height, GM. As can be seen in the figure a positive GM will always give a positiveGZ resulting in a moment counteracting any heeling moments. Since MC can be consideredconstant for small heeling angles, so can GM,which is then called the initial metacentric height,GM0. Thus for small angles, !< 10

(3.2)

which is constant. Furthermore for small angles

(3.3)

Consequently GZfor small anglescan be calculated as

(3.4)

Thus there is a linear relationship between GZand the heeling angle for small angles. The largerGM0is the larger the initial stability will be. Since all vessels contain liquids there is a free surface

movement that has to be considered. It will not be explained here, since it is not necessary forthe analysis. However, hereafter GM0will be referred to as GM and will then be corrected forfree surfaces.

Obviously the initial stability is highly dependent on where G is located. From a stability point ofview an important measurement of G is the vertical distance from the keel, KG. A low KG willgive more stability than a high value, which is demonstrated by (3.1).

3.2 Internal forcesThe following sections will describe the forces that occur from within the ships boundaries, i.e.forces that are not directly connected to the ships external environment.

3.2.1 Mooring lineThe mooring line is heavy and long and therefore most definitely has an influence on the ship.The force is not constant though, but will increase the more line that is paid out. The mooringline will influence the ship in several different aspects like trim and heeling. It will also act as abackward force, increasing the demand on the ships available bollard pull. More informationabout different types of mooring line can be found in appendix C.

Effective componentsFigure 4 illustrates the ship seen from the side, in the XZ-plane, where all essential forces,components and points of attack are marked in relation to G.


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Figure 4A principal sketch of the side of an AHTS vessel. The mooring line is running over the sternroller with a vertical point of attack at the top of the stern roller. The dimensional relationships in the

figure are not consistent with the reality.

(3.5)

That is the total force from the mooring line in the three dimensional room. "XZis in the XZ-plane, which gives

(3.6)

(3.7)

Figure 5 illustrates the ship from above, in the XY-plane, where the effective components can befurther extracted. Again the figure also presents the essential points of attack and their respectiverelationship to G.

Figure 5 A principal sketch of an AHTS seen from above. As can be seen in the figure the verticalcomponent of the force from the mooring line, and the transverse horizontal effective component do nothave the same point of attack. The dimensional relationships in the figure are not consistent with thereality.

Since the stern is flat and smooth the friction is neglected why there will be no transversal

component acting on the stern roller. The force components can then be described as


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(3.8)

(3.9)

Points of attack

With equation (3.5)-(3.9) the force components along the different axis are determined. However,the points of attack of the forces are not at the same spot, as seen in figure 5. Thus the followingequations are added, where consideration is taken both to the direction of the effectivecomponent, as well as its point of attack.

(3.10)

where F1is acting on the top of the stern roller at (xAFT, ySRzDECK). ySRis determined by

(3.11)

#can be assumed to be approximately the same as the yaw angle. ySR,MAXis at the outer edge ofthe stern roller.

(3.12)

where F2is acting at (xWINCH, 0, zWINCH)

(3.13)

where F3 acts at (xTP, yTP, zDECK) and F4 at (xWINCH, 0, zWINCH). Thus FML,Y is acting at twodifferent locations, meaning that it will be divided between these. However, as F4is dependent on

F2and $

(3.14)

it can be neglected if $is small enough. $can be determined as

(3.15)

Considering the towing pins are only some meter from the centre line and located in the aft part

of the ship, while the winches are commonly in the first 1/3 of the ship, $can be expected to be

very small and thereby F4can be neglected. Thus

(3.16)

3.2.2 PropulsionThe propulsion system of the ship limits its bollard pull. Therefore the propulsion has a directlink to how large anchors and mooring lines the ship can handle. However, the maximumpossible bollard pull is not always possible to attain due to use of side thrusters. The bollard pullis dependent on the available engine power as well as the propellers.

Bollard pullThe bollard pull is an indication of the maximum pulling force the ship can exert on anotherobject or ship. It is commonly stated in tonnes, even though it is actually representing a force [6].


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Hence all forces in this report are stated in tonnes. A ships bollard pull is determined by a test,which is normally carried out under the supervision of a Classification Society.

Since the bollard pull is equivalent with pulling force, efficient in the XY-plane, it can be directlyconnected to FML,X and FML,Y. The bollard pull test is performed with the propulsion pointedalong the x-axis. Thus the maximum allowable force from the mooring line will be limited by the

bollard pull according to

(3.17)

A loss of pulling power might result in the vessel being dragged backwards. AHTS vessels havevery low sterns, which will be even lower due to the weight of the mooring line, and open decks.If the vessel is dragged backwards it might result in water flooding the deck. Naturally this willaffect the ships stability and might in worst-case lead to a full capsize if the flooding is largeenough. Since the flooding will push the aft deck further down the streaming water will create anegative lifting force, due to the flat deck acting as a lifting surface. A bad spiral is under waywhere more water on deck will result in even more water on deck [7]. This principle is illustratedin figure 6.

Figure 6AHTS being pulled backwards. As the wave in the stern is growing there might be water on deck,which will increase the displacement. Eventually the stern can be entirely submerged resulting in capsizing.

Side thrustThe side thrust is mainly used to counteract side drift during anchor handling operations. It isattained through the rudder, the side thrusters and the azimuth if there is one. The side thrust isefficient along the same action line as FML,Y. Studying blue prints of AHTS vessels shows thatthe side thrusters and the aft propeller generally is efficient at about the same level along the z-axis. Thus the side thrusters can mostly be assumed to act along a mutual line parallel to the x-

axis, zTHRUST. A contingent azimuth will, however, be efficient at a lower height, zAZIMUTH.

There are no exact figures to be found about the magnitude of the side thrust and neither isthere any test carried out to determine this. In DNVs class notation of tugs the applied forcefrom the side thrust is 60% of the maximum bollard pull [8], though without any specification ofthe reason. According to the bollard pull certificate of Bourbon Dolphin the use of azimuth willincrease the total bollard pull approximately 10% [9].

3.3 External forcesThe following sections will describe the major external forces that the ship is exposed to, i.e.forces that origin from the ships environment.


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3.3.1 WindAs the mooring lines have to run in all directions from the rig it is not always possible to avoidside winds. According to [10] the wind-induced force can be calculated as

(3.18)

The coefficient CD,AIR is commonly not known without experimental tests, but is generally in theorder of magnitude between 0.5 1.5 [10]. VR,AIR is calculated as a function of VWIND, VSHIP,

and %XY,WINDaccording to

(3.19)

To calculate the projected area &WINDhas to be calculated

(3.20)The projected area is calculated as

(3.21)

FWINDacts in the centre of AP,AIRwith coordinates (xP,AIR, yP,AIR, zP,AIR).

3.3.2 CurrentsAs with wind it is sometimes unavoidable to sail with the current coming from the side. A sidecurrent will act both upon the exposed side of the immersed part of the hull and the mooringline attached to it. According to [11] the force can be calculated with the drag equation

(3.22)

The equation is basically the same as (3.18), but all parameters are calculated for water and thesubmerged part of the hull instead. VR,CURRENT can be calculated according to (3.19), where

VWIND is changed to VCURRENT and %XY,WIND is changed to %XY,CURRENT. In a similar way

&XY,CURRENTand AP,WATERcan be calculated according to (3.20) (3.21).

To calculate the force due to currents CD,WATERfrom (3.22) has to be estimated. In 1976 a studywas carried out for the US Coast Guard to evaluate the stability criterion for towing vessels [12].During this evaluation model tests were performed to determine CD;WATERfor the vessels beingdragged in different directions during towing. Figure 7 is from the report and shows CD,WATER,for small heeling angles, depending on where on the vessel the towing point is located. CD,WATERis in the figure called C1.


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Figure 7The graph illustrates the drag coefficient, CD,WATER, for tugs being dragged in different angles.Dependent on where the longitudinal location of the tow pins is, the tug will be dragged in different

angles. The graph is based on experiments carried out on tugs by the U.S. Coast Guard during theseventies. C1is the same as CD,WATER.

CD,WATER should represent the situation where the vessel is being dragged sideways. Thecorresponding situation from the diagram above should be when the towing pin is somewhere inthe middle of the ship since this is most likely to result in a strictly sideways dragging.Consequently CD;WATERis in this case assumed to be approximately 12.

FCURRENTacts in the centre of AP,WATERwith coordinates (xP,WATER, yP,WATER, zP,WATER).

2The error margin can be expected to be fairly high for this coefficient, since the model tests were actuallycarried out with models of towing vessels and not AHTS vessels. It is also assumed to have a constant

value, even though the coefficient will alter with the relative direction between the ship and the current.However, drag test to determine CD,WATERis normally carried out by dragging the ship from the front, andnot from the side. Consequently there is very limited information available about side drag test of anykind. To make a more thorough calculation of the side forces model experiments or computer analysis for

AHTS vessels has to be done. Because of the complexity of the parameter, and the difficulty to getrelevant values, a simple and conservative approach is chosen with the highest value being constantregardless of angle.


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4 Heeling momentThe most important consequence of the large forces involved is the heeling moment. Themagnitude of the heeling moment is dependent on the magnitude of the various forcecomponents effective in the YZ-plane and their respective points of attack.

To calculate the heeling moment a few assumptions has to be made. The magnitude of the forcecomponents from FMLare considered constant in the YZ-plane, which is a plausible assumptionconsidering the length of the mooring line in relationship to the ship. Further their respectivepoints of attack are considered constant. The heeling moment will not be constant, but vary withthe heel as a consequence of altered lever arms. The calculations below are primarily valid forfairly small heeling angles. Figure 8 illustrates a sketch of a ship with a heel seen from behind.

Figure 8A sketch of an AHTS vessel seen from behind. The ship has a heel and the respective lever armsare included. The dashed blue line illustrates where the water line would be without the heel. FCURRENTandFWINDare pictured approximately at their presumed point of attack. The heel in the figure is exaggeratedto clarify the lever arms, as described above the model is mainly valid for relatively small heeling angles.

The lever arms to the force components from the mooring line can then be expressed as

(4.1)


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(4.2)

Thus the mooring lines contribution to the heeling moment can be calculated as

(4.3)

As the thrust will always act at the same distance from G its contribution to the heeling momentwill be

(4.4)

Naturally wind and current will influence the heeling moment as well. However, the respectivelever arms are somewhat more complicated to determine than those above since it is not obviouswhere the projected area centre is located. While the centre of the submerged projected areaoften can be assumed to be located at about half the draught the same simplification is harder todo on the area above the water. This is due to the fact that the projected area is hardly a rectangleseen from the side, which it more or less is under water. Consequently zP,AIRhas to be calculated

manually for every vessel.

zP,WATERwill vary with the heeling angle as the projected area will change, which is due to the factthat the draught will change and not be the same on both sides on the ship. It is calculated as

(4.5)

where T0 is the initial draught, i.e. when the ship is in upright position. ZWL is assumed to beconstant for small heeling angles. Naturally it can be wise to estimate whether it is a fairassumption to calculate it as half the draught.

The following equations will calculate their respective addition to the heeling moment.

(4.6)

(4.7)

Thus the total heeling moment is calculated as

(4.8)

Of course the direction of respective added moment has to be considered.

Angles

As can be seen M is dependent on three different angles, "XZ, #XY, and !YZ. As described before

#XYcan approximately be considered the same as the yaw angle and is thus a known variable.However, this model is only valid as long as the mooring line is on the stern roller. Thus the

maximum #XY,MAXfor which this calculations are valid can be expressed as

(4.9)

!YZis always known, but as described before the model has a better accuracy for relatively smallheeling angles. However, "XZis not as obvious to predict. To estimate "XZwith good accuracy a


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quite complicated calculation has to be performed, where consideration has to be taken todifferent density of different parts of the chain, their respective length, the speed of the vesseletcetera. Even so currents or sudden waves might change the angle why a more conservative

approach is preferable. Thus "XZwill be estimated as the angle giving the largest possible MML.This can be done by differentiate (4.3)

(4.10)

which leads to

(4.11)

This is the "XZthat will be used through the rest of the report.


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5 Stability criteriaThe rules on ship stability is regulated in the Code on Intact Stability(IS Code. More informationabout the IS Code and the regulatory system in general can be found in appendix B). The ISCode is based on different criteria that the vessels needs to fulfil. There are general criteriaapplicable to all vessels exceeding a certain length, and specific criteria applicable for ships with a

specified task. The different general criteria can be found in appendix B.

5.1 GZ curveThe stability criteria are generally based on the GZ curve since it describes the ships ability torestore from a heel. The following section will describe how the criteria are put into relation tothe GZ curve. Figure 9 shows an example of a GZ curve of an example ship.

Figure 9A GZ curve and added heeling moment. The curve is from Ship 4 with T = 5.6 m, Trim = -2.6

m (trim by the bow), KG = 6.1 m and all consumables are full. The added heeling moment is 1500tonnes'm, which is converted (5.1) to a lever arm, 0.25 m, in the figure.

From this curve all necessary information whether the ship fulfils the criteria or not can beextracted. The continuous black curve is the GZ curve. The curve displays the maximum GZvalue, h, and at what angle this occurs. These are both parameters that criteria are based on. So isthe area below the curve. The area is also elucidated by the area curve, the dotted curve in thefigure. The area describes the ships restoring potential energy at different heeling angles. Thedash-dot line is the tangent of the GZ curve at its origin. From this line GM can be extracted,which is the height of the line at 1 radian (not seen in the picture).

In this figure there has also been added an external heeling moment on the ship. It is converted

to a lever arm that is equivalent to the GZ curve by the following equation3

(5.1)

An added heeling moment alters the equilibrium to the intersection between the GZ curve andthe heeling moment lever arm. This will, obviously, reduce the area below the curve andconsequently the potential energy at all angles. As today there is no criterion involving a heelingmoment for AHTS vessels, in spite of the fact that there is a great chance they will be exposed toit due to the mooring line.

3 The equation is based on the assumption that the heeling moment is expressed in metertonnes. In SI-units a subtraction of 1000g has to be added.


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5.2 Loading conditionsAs the criteria are based on the GZ curve, it is basically dependent on the ships KG and GM asseen in (3.1). Thus the loading of the ship is essential, and also a factor that has to be consideredwhen determining whether a specific ship fulfils the criteria or not.

Loading condition describes how the ship is loaded in terms of ballasting, cargo andconsumables (fuel, drink water and oil etcetera) etcetera. It influences the ships draught and trimas well as KG and heeling. Every vessel has several standard loading conditions where ballastingand cargo are to correspond to a situation the ship is likely to encounter during normaloperation. Each loading condition shall be fulfilled for both arriving condition and departingcondition. Arriving condition means all consumables are to be calculated as 10% of maximumcapacity and departing condition means 100%. All standard loading conditions are to bepresented in a ship specific stability book that shall be onboard the ship at all times.


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6 The commission report

6.1 The commissions conclusionsAfter the accident a commission was put together to investigate all the circumstances around theaccident. The investigation resulted in several proposals for new rules regarding anchor handling.

Many of those suggestions do not involve stability, why those will not be represented in thisreport. The following list summarizes those proposals having to do with stability and is more orless a direct translation of the commissions report [3]. The translated text is presented in italictext and where it is not obvious what is meant the authors interpretation will be described.

All loading conditions shall be calculated with 10% respective 100% bunker.

All winches shall be full of the heaviest possible mooring line type.

External4force with the following characteristics:

o Vertical load: the full winch capacity shall be used between the outer towing pins. The winchesonly have full pull on first layer but as a matter of safety margin the winch is assumed to befully loaded with chain at the same time. The lever arm shall be calculated from the centreline

of the ship to the fore-edge of the stern roller and the vertical load with a point of attack atthe top of it. During this vertical load the ship shall have a heeling angle corresponding with aGZ value of no more than 50% of maximum GZ.

This situation would correspond to the vessel performing a very heavy lift, orbreaking the anchor from the bottom. It is supposed to be an extreme casewhere the mooring line is acting on the outer edge of the stern roller. It is to beimplemented as a stability criterion formulated as where the lever arm curveintersects the GZ curve, the value shall not be larger than 50% of GZMAX. Thecalculation is to be carried out as

Criterion 1:

Where GZ = lML

(6.1)

Calculations:

(6.2)

(6.3)

(6.4)

(6.5)

4

In this report the mooring line is defined as an internal force, since it is acting within the shipsboundaries. The commission calls it an external force, but it is assumed that it is the same force that isreferred to.


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o Mooring line payout: While paying out the mooring line the maximum force from the mooringline shall be calculated. The maximum force shall be based on static as well as dynamic loads.This force shall be decomposed to a vertical force and a horizontal athwardships force. Thelever arm for the horizontal component shall be counted from the height of the working deckby the towing pins to the centre of the forward thrust or the centre of the aft side thrust if thisis deeper. The lever arm of the vertical component shall be calculated from the centre line of

the ship to the fore-edge of the stern roller and with the vertical point of attack at the top ofthe stern roller. The mooring line shall have an angle of at least 25 from the ships centrelinein the horizontal plane. The angle related to the vertical plane shall be calculated so that itgives the largest possible heeling moment. The force from the mooring line shall not result in aheeling moment that gives a lever arm larger than 50% of the maximum GZ. The maximumforce from the calculations above will be the ships maximum capacity for these kinds ofoperations.

The second situation is more or less to correspond with the worst possiblesituation during an anchor handling operation. The mooring line has anunfavourable angle to the ship, both vertical and horizontal, and the thrust isdirected to increase the heeling moment rather than to decrease it. The

commission suggests that the angle between the longitudinal centreline of theship and the mooring line shall be at least 25. There is no specific reason given

why it should be 25. It is to be implemented as a stability criterion.

Criterion 2:

(6.6)

Calculation:

(6.7)

and equivalent to

(6.8)

Consequently FML,2can be solved as

(6.9)

o If ballasting is necessary to attain the calculated maximum force this condition should bedescribed as an instruction for ballasting during anchor handling operations.


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7 Evaluation of influential forces and thecommissions conclusions

This chapter will evaluate the impact of the various forces and the heeling moment. The analysisof the heeling moment and the force from the mooring line will emanate from the commissionsconclusions, while the focus otherwise will be on determining which forces that has the largestinfluence on the vessels stability and how they affect it. As it is not only external and internalforces that affect the ships stability, but also how it is loaded, an evaluation of the impact ofloading will be presented as well.

7.1 Type shipsThe following four type ships will be used throughout the rest of this report when a ship isnecessary of any reason. The vessels are existing ships DNV classed as tugs5, but will ofconfidentiality reasons not be mentioned by name. The tugs are chosen so that they representseveral different sizes of AHTS vessels.

Table 1The four type ships with relevant parameters to carry out the calculations necessary for theanalysis in this chapter.

Ship LPP b D T! BP

FWINCH,MAX

mML,MAXLCGWINCHKGWINCHySRyTP

1 51.5 15.0 5.5 4.7 2700 69 150 300 33.0 7.50 2.0 0.32 60.5 15.5 7.0 6.0 4300 143 300 400 39.4 13.5 2.5 1.53 68.2 17.2 8.3 6.8 5900 185 400 438 39.8 14.3 3.0 1.64 76.3 18.0 8.0 6.6 6800 237 500 600 37.1 11.2 3.0 1.3

As can be seen in table 1 no vessel has a bollard pull that is more than 50% of the maximumwinch pulling capacity.

7.2 Heeling moment

As the heeling moment is so important when considering the stability of the vessels it is essentialthat it is handled correctly in the regulation system, and presented as such a realistic value aspossible. The following section will present the effect on the vessels of calculating the heelingmoment as presented by the commission. It will also evaluate what effect the heel will have onthe total heeling moment, i.e. if it will influence the ships capability of handling the mooring linein any way. Finally the influence of the loading condition on the ships ability to handle theheeling moment will be analysed.

7.2.1 Criterion 1To evaluate the first criterion suggested by the commission the four test vessels GZMAX arecompared with the lever arm corresponding to the calculated heeling moment according to (6.2) (6.5). The results are presented for arriving condition in figure 10 and departing condition infigure 12. The loading conditions and all exact values can be found in appendix D.

5Particulars from DNV Exchange


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Figure 10The first bar is representing GZMAXand shall, for the criterion being fulfilled, be at least twiceas high as the other bar for each vessel. The second bar represents the lever arm calculated as suggested by

the commission. The calculations are done for arriving condition.

As can be seen only Ship 1 fulfils the criterion. Ship 2 and 3 would capsize during the presentcondition, since the lMLis actually greater than GZMAX. Ship 4 will not capsize, but neither will itfulfil the criterion, since the heeling angle will be larger than the one corresponding to GZMAX/2.The bars presented in figure 10 are based on a constant heeling moment, i.e. no consideration istaken to altered heeling moment due to heel. Figure 11 illustrates the corresponding heelingmoments calculated according to (4.3).

Figure 11The heeling moment calculated as suggested by the commission suggests but with considerationtaken to heel. The curves are calculated for arriving conditions.


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As can be seen in the figure the heeling moments will vary a great deal due to the heel. It caneither be a strict declining curve, or it can start to incline to a certain degree and then decline.Either way it will have impact on the criterion suggested by the commission. As the lever arm

curve is likely to intersect the GZ curve somewhere around 5- 206that is the most interestinginterval to study. Translated to the bars in figure 10 this will result in the second bar being shorter

for Ship 2 and 4, and longer for Ship 1 and 3.

Figure 12 shows the corresponding bars from figure 10 for departing condition.

Figure 12The first bar is representing GZMAXand shall, for the criterion being fulfilled, be at least twiceas high as the other bar for each vessel. The second bar represents the lever arm calculated as suggested by

the commission. The calculations are done for departing condition.

As can be seen in figure 12 Ship 2 - 4 are closer to fulfil the criterion for departing condition thanfor arriving condition. Ship 1 will have a smaller marginal than for arriving condition, but will stillfulfil the criterion with good marginal. For no ship lMLwill actually be greater than GZMAX, as forarriving condition. Just as for arriving conditions no consideration is taken to heel. Figure 13presents the respective heeling moment curves for the ships in departing conditions, calculatedaccording to (4.3).

6Studying several GZ curves indicates that this is a common interval for the curves to intersect. It is not agiven fact though, why every curve has to be studied individually.


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Figure 13The heeling moment calculated as suggested by the commission suggests but with considerationtaken to heel. The curves are calculated for departing conditions.

Just as for arriving conditions the heeling moments will vary a great deal due to the heel. For Ship2 4 the shape of the curve will be the same as for arriving conditions, while Ship 1 will, fordeparting condition, have a strict declining curve instead. Consequently the second bar in figure12 will be lowered for Ship 1, 2 and 4, where Ship 4 can actually be expected to pass the criterionshould the heeling be considered. Ship 3, whose lMLis almost exactly as long as GZMAXin figure12 would probably capsize since the heeling moment will increase while the ship heels.

7.2.2 Criterion 2The second criterion suggested by the commission is supposed to represent a more dynamicsituation, where the mooring line is not running vertically between the ship and the bottom buthas an angle to the ship. It is supposed to determine the largest allowable force from the mooringline that the ship will be able to handle no matter its direction in comparison to the ship. Todetermine FML,2(6.7) (6.9) are calculated so that (6.6) is fulfilled. The results are presented intable 2. The loading conditions and more specific values are found in appendix D.

Table 2The table presents the maximum allowable force the ships are allowed to handle calculatedaccording to the commissions second criterion.

ShipFML,2

Arriving conditionFML,2

Departing condition

1 150 150

2 83 128

3 112 185

4 240 400

As can be seen in the table all but Ship 1 will have a considerably lower capacity to handle forcesthan their winches allows. The forces are based on the heeling moment being constant, as


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suggested by the commission. Figure 14 and 15 presents MML,2 for each ship, whereconsideration is taken to the heel and the initial value is the one required by the commission.

Figure 14Theheeling moment calculated as suggested in the second criterion where consideration to heelis added. The calculations are for arriving condition.

As can be seen in the figure all vessels but Ship 1 has a strict declining curve, indicating that theywould actually be able to handle a slightly greater force and still fulfil the criterion. Thedifference, however, are fairly small why the importance of the heel can be considered quitesmall in this case.


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Figure 15Theheeling moment calculated as suggested in the second criterion where consideration to heelis added. The calculations are for departing condition.

Again all but one vessel have a strict declining curve but with small variations. However, all caseswhere the heeling moment curve is declining means that the vessel would be able to handle agreater force and still fulfil the requirement, since the lever arm will be shorter where the curveintersects the GZ curve.

7.2.3 LoadingAccording to the commission the heeling moment tests above are to be carried out with theships winches fully loaded with mooring line. Naturally this will influence the ships possibility ofpassing the criterion, since it affects KG and thereby GZ (see equation (3.1)). Table 4 presentshow the load on the winches affects KG.

Table 3The table presents how KG varies for the vessels dependent on whether the winches are full ofmooring line or empty. KG is presented both for arriving and departing conditions.

Ship KG for arriving condition KG for departing conditionFull winches Empty winches Full winches Empty winches

1 4.2 3.8 4.5 4.0

2 6.9 5.9 6.0 5.1

3 7.3 6.5 6.5 5.8

4 7.7 7.2 6.5 6.1

As can be seen in table 3 KG will be reduced between 0.4 1.0 m for all vessels in bothconditions with empty instead of full winches. Assuming that GZMAXwill occur somewhere in

the interval720 30heel it will increase between 0.14 0.5 m (Equation (3.1). It is assumed

7Based on several GZ curves for the ships in different loading conditions.


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that KG will be located along the ships centreline why TCG is 0). Studying figure 10 and 12 itcan easily be understood that this might very well be the difference between fail and pass for theships.

7.3 Remaining forcesThe following section will analyse the main effects of the remaining forces presented in chapter3. Since the mooring line is integrated in the heeling moment, and thus evaluated above, therewill be no further analysis of the mooring line. Even though all forces will influence the heelingmoment they might also affect the ship in other ways, which will be presented and analysed here.

7.3.1 Propulsion and currentThe forces due to currents and the propulsion are closely linked and will consequently beanalysed together. The forces will be presented in two sections, first the actual force and itspotential magnitude and then how the assumed force will influence the heeling moment.

Force

As soon as there are side currents use of the side thrusters are likely to be necessary. Figure 16presents the force, calculated according to (3.18) (3.21), for three different current speeds andthe relationship to the available bollard pull. The ships speed is approximated to 0.25 knots andthe value represents the side force for the worst possible angle between the ship and the current.In appendix D a more thorough presentation of the side force for different angles are presented,as well as for a higher VSHIP. The respective draught is assumed to be design draught and the trimzero.

Figure 16The blue bars in the bar chart illustrate the respective vessels maximum bollard pull. The threeother colours represents the side force on the hull due to three different current speeds.

The results in the figure are naturally dependent on the loading condition of the ship. It does,however, indicate that the necessary side thrust will make out quite a large part of the totalbollard pull for all four vessels at high current speed. At 2 m/s Ship 2 4 will need a side thrustof at least 50 % of the available bollard pull. For Ship 1 a current speed of 2 m/s would require75 % of the total available bollard pull to its side thrusters. Since maximum bollard pull is

synonymous to full use of the ships machinery, use of the side thrusters will reduce the availablebollard pull. Thus the ships manoeuvrability with heavy weights will be significantly reduced.


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When BD used 100% of the side thrusters the bollard pull was reduced from 180 tonnes to 125tonnes [3]. Even though that does not mean that all vessels have the same reduction in availablebollard pull due to use of side thrusters, it indicates that the difference might be distinct.

Heeling momentFCURRENTand FTHRUSTcan mostly be assumed to counteract each other since the side thrust is

mainly used to prevent side drift. Therefore the resultant of the two forces can be assumed to besignificantly smaller than the forces calculated in 7.3.1. FCOMBINEDcan be calculated as

(7.1)

Consequently their respective contributions to the heeling moment can also be assumed tocounteract so that MCOMBINED can be calculated as

(7.2)

The resulting heeling moments from the heeling moments calculated according to (7.2) are

presented in figure 17. FTHRSUTis for all cases equal to and directed opposite to FCURRENT. Theirrespective values and lever arms are presented more thoroughly in appendix D. Since MCURRENTis not a constant value, but a variable dependent on the heeling angle, a mean valuecorresponding to 10heeling is chosen for the calculation. How it varies for small heeling anglescan be seen in appendix D. MTHRUSTis constant and independent on the heeling.

Figure 17The resulting heeling moment from the current and the thrust for three different current speedsfor the four test vessels. A positive value means the thrust induced heeling moment is larger than the

current induced.

As can be seen in figure 17 all situations for all ships result in a positive moment, meaning thethrust induced heeling moment is larger than the current induced. This is due to the fact thatzTHRUST will always be longer than zP,WATER which is assumed to be half the draught for allheeling angles. Noticeable is that the heeling moment presented in this figure is unavoidablewhen there are side currents, since the alternative is to simple let the ship drift, which obviously isnot an option. Thus this heeling moment will always occur when there are side currents.


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7.3.2 WindForceFWIND is calculated according to (3.18) (3.21). However, as FWIND is not expected to beimportant at low wind speeds, relevant VWINDis so much higher than VSHIPso that VR,AIR can be

assumed equal to VWIND. Thus the largest FWINDwill occur when &is approximately 90. This is

proven in appendix D. Figure 18 presents FWIND for three different wind speeds, 10 m/s, 20 m/sand 30 m/s.

Figure 18 FWINDattacking from the side for three different wind speeds on the four test vessels.

As can be seen FWINDat 30 m/s is comparable to FCURRENTat 1.5 m/s. Even though the wind byitself only corresponds to a small part of the maximum bollard pull, the combined effect of thewind and the current will make out a large part of it. Wind speeds of up to 20 m/s results insmall side forces compared to the bollard pull, and can thus be considered insignificant from aside drift point of view.

Heeling momentJust as the current induced moment the wind induced heeling moment is dependent on theheeling angle. Figure 19 illustrates the moment curves for the four test vessels calculated with thethree wind speeds from figure 17. Again the curves are presented up to 20since the relevance isconsidered highest at low heeling angles.


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Figure 19The wind induced heeling moment on the four test vessels for three wind speeds. The graphs

are presented as a function of the heeling angle, between 0- 20.

As can be seen in figure 19 the variations in the heeling moments due to heeling are quite smallfor such small heeling angles. However, the heeling moments, if VWIND = 30 m/s, arecomparable with the resulting heeling moment from the current and the thrusters and thus notnegligible.

Naturally all heeling moment curves in this analysis can easily be transformed to lever arm curvesby using (5.1).


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8 Discussion part 1

8.1 Heeling momentNo matter how one calculates or models the situation around the ship the heeling moment mustbe considered. It is the heeling moment that will cause the ship to capsize, even though other

aspects might reduce the ships capacity to respond to the heeling moment. Therefore the heelingmoment is the most important aspect to consider when creating a stability rule for AHTS. Sinceit in a contingent rule must be represented by some kind of maximum value, it is important thatthe heeling moment is as realistic as possible. Too conservative and the working capacity of thevessel will be reduced below its actual potential, and the risk is ships will be over dimensioned.Too soft and the ship will be at risk of capsizing.

The tests of the commissions suggested criteria in the previous chapter resulted in three of thefour vessels failing Criterion 1. Criterion 2 showed that two of the vessels would only be able tooperate with mooring lines far lighter than their maximum winch pull force. No consideration istaken to heel even though it apparently will influence the heeling moment. The commissionsuggests that the heeling moment, in Criterion 2, shall be calculated with all force components

interacting to create the moment. That is indeed a worst case scenario and should be possible toavoided through cautious planning, i.e. make sure the mooring line is always resting on a towingpin on the down stream side. It is also worth mentioning that the heeling moment due to thethrust will likely be reduced due to the currents that brought the ship to use the side thrust in thefirst place. Thus the heeling moment will not be as large as suggested by the commission.

8.2 LoadingThe commission requires that all calculations shall be carried out with the winches full of theheaviest possible mooring line, in spite of the fact that the winches will not be able to pull theirmaximum force if full with mooring line. FWINCH,MAXis only for the first layer of mooring line,and the force will be reduced as the winch is filled up. It is thus questionable whether this is arelevant situation for these vessels at all. To attain the kind of forces the winches are constructed

for there has to be a lot of mooring line paid out and already in the sea. Consequently the ship islikely to be positioned on, or close to, the anchoring point. Thus there will be no need for a lot ofadditional mooring line being loaded on the ship at this moment. As could be seen in theprevious chapter the loading resulted in a significantly reduced ability to respond to a heelingmoment. Consequently it is a very conservative suggestion and its consequences should becarefully considered before implementing it in a rule.

8.3 ForcesBollard pull and side thrustAs today the marine authorities or a classification society tests the ships available bollard-pull. Atthe same time no test is carried out to establish the maximum side thrust, or the reduction inbollard pull due to side thrusters and the winch system. The maximum bollard pull is based on

100% machinery being available for the forward thrusts. During normal anchor handling thewinches are in constant use, why part of the available power will be used for those. Consequentlythe maximum bollard pull will basically never be available during anchor handling.

Equally important is the available side thrust. Not only because it will reduce the bollard pull, butalso because the side thrust is of outermost importance when the ship is exposed to sidecurrents. There where strong side currents when Bourbon Dolphin sank. It was the side currentthat forced her off course, and further created the increased angle between the mooring line andthe ships centreline. Probably the whole situation could have been avoided if the side thrustswould have been powerful enough, or would it have been stated that the ship did not havesufficient power for operating in such weather condition. Neither were there any accuratemeasurements of the currents at the time according to [3].


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WindAs shown before wind can result in forces too large to be ignored. However it would be pointlessto require all vessels being able to handle all kinds of forces while carrying out anchor handlingoperations. Naturally the wind may not be neglected, but neither should it limit the shipscapability unless it is necessary, i.e. if the accurate weather situation is windy. Thus the wind shallbe considered in the planning stage when the current weather situation is known.

8.4 Stability criteriaThere is nothing in the analysis performed in this report that indicates the suggestion of GZMAX

>2'lMLwhere they intersect should be bad. However, as the lML curves will vary with the heel,this requirement might not be enough. Even if the ship fulfils this requirement, nothing is saidabout how the lMLcurve continues. Dependent on the shape of the curve the remaining potentialenergy, the area between the GZ curve and the lMLcurve, might differ much from that should thelMLbe constant. Thus it might be wise to implement an area criterion as well.


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-".+ 0

9 Weather The weather will most definitely influence on the ships stability. An external effect on the shipthat has not previously been discussed is waves. Waves can e.g. create sudden changes in theheeling angle or alter the direction of the ship. Because of the periodicity and the shape of thewaves they will also constantly vary the ships displacement and accordingly its centre ofbuoyancy and trim. One important thing to consider is the combined effect of waves and aheeling moment due to the mooring line.

To describe the effect of waves is very complex, and is not made easier by adding the heelingmoment due to the mooring line. Consequently the most practical way to do this would be to usea recognized method to describe ships motions in waves. The IMO Weather Criterion is anexample of such a method. The weather criterion describes the vessels ability to withstand thecombined effect of beam wind and waves.

9.1 IMO Weather CriterionThe severe wind and rolling criterion describes a ships ability to withstand the combined effectsof beam wind and rolling. As for the entire IS code it is applicable to all vessels with a length of24 m or more, unless otherwise stated [13]. Its purpose is to ensure that a ship is able towithstand a predefined wind gust while rolling due to a beam wave and constant wind. Theassumed scenario is when all those parameters interact to heel the ship in the same direction.

The principle of the criterion is to measure the ships ability to restore to its equilibrium whenexposed to the disturbance mentioned above. It is basically an energy balance, where the totalamount of energy used to roll the ship from windward to leeward shall not exceed the shipsremaining potential energy. The principle is illustrated in figure 20, which can help give a betterunderstanding of the criterion, its principle and how it can be modified to serve the purposedescribed above.


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Figure 20The weather criterion is based on the curve illustrates in the picture. The principle is that the

ship is assumed to have an initial heel, (0due to a steady wind represented by a lever arm, lw1, of the

resulting heeling moment. The ship is then rolling windward due to waves to (1. Finally the ship rolls backwith the combined effect of the waves and a wind gust represented by the lever arm, lw2, of the resulting

heeling moment. (2is the maximum allowable angle the ship may roll back to, which is either 50, angle ofdown flooding or a major opening, which ever is less. The criterion is that b shall be greater or equal to a.

The curve in figure 20 is the GZ curve. In the weather criterion the ship is assumed to have aninitial heel, "0, because of a steady beam wind. This beam wind results in a heeling moment,which is represented by a lever arm, lw1. The equilibrium is where the heeling moment due to thewind is equal to the restoring moment of the buoyancy due to the submersed volume,represented by the GZ curve. Consequently lw1is equal to GZ where the moments intersect andis calculated as

(9.1)

where P is the wind pressure, A is the projected lateral area of the ship above the water line, Z isthe vertical distance from the centre of A to the centre of the underwater lateral area or at

approximately 50% of the draft, g is the gravitational constant and ) is the displaced water intonnes.

From "0the ship is assumed to roll towards the wind to "1 as the result of a beam wave. "1issupposed to be the ships most vulnerable condition and is calculated as

(9.2)

The calculated rolling angle is based on a simplified nonlinear roll theory and takes several ship

particulars into consideration. The first three parameters, X1, X2and k, are damping coefficientsdependent on the ships particulars. The first parameter, X1, is dependent on the beam anddraught relation B/d according to table 4.


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Table 4The table presents the damping parameter X1dependent on the beam/draught relation.

B/d X1

*2.4 1.0

2.5 0.98

2.6 0.962.7 0.95

2.8 0.93

2.9 0.91

3.0 0.90

3.1 0.88

3.2 0.86

3.4 0.82

+3.5 0.80

The second parameter, X2, is dependent on the ships block coefficient, CB, according to table 5.

Table 5The table presents the damping parameter X2dependent on the block coefficient.

CB X2

*0.45 0.75

0.50 0.82

0.55 0.89

0.60 0.95

0.65 0.97

+0.70 1.00

The third damping coefficient, k, is determined by calculating the relationship between the

projected area of the bilge radius, AC, over the length beam area, LWL'B. The following valuesshall be used for k:

k= 1.0 for round-bilged ships having no bilge or bar keels k= 0.7 for ships having sharp bilges k= as shown in table 8 for a ship having bilge keels, a bar keel or both.

Table 6The table presents the damping coefficient, k, dependent on the shape of the keel.

k

0 1.0

1.0 0.981.5 0.95

2.0 0.88

2.5 0.79

3.0 0.74

3.5 0.72

+4.0 0.70

For all damping coefficient the intermediate values in the tables shall be determined by linearinterpolation.


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The second two parameters in equation (9.2) consider the wave force and motion and the shipsrolling period. r is the effective wave slope and sis the wave steepness factor. The effective waveslope is calculated as

(9.3)

where OG is the distance from the waterline to the centre of gravity calculated as

(9.4)

The wave steepness factor is dependent on the ships natural rolling period, T roll, which is givenin equation (9.5)

(9.5)

GM is the metacentric height and C is a coefficient for the radius of gyration calculated as

(9.6)

The relationship between the steepness factor and the rolling period is presented in table 7.

Table 7The steepness factor dependent on the rolling period of the vessel.

Troll s

*6 0.100

7 0.098

8 0.09312 0.065

14 0.053

16 0.044

18 0.038

+20 0.035

Just as with the damping coefficients the intermediate values shall be obtained by linearinterpolation.

The next step is the combined effect of the wave roll back and a gust wind from "1to "2. A leverarm, lw2, represents the gust wind, which is to be 50% larger than lw1

(9.7)

"2 or "c, whichever is smaller, is the limiting parameter in the criterion. When the ships rollsfrom "1 to the leeward side the heeling angle shall not exceed the limiting angle. The limitingangle is either 50, down-flooding (openings in the hull) or the second intercept between the GZcurve and lw2.

As mentioned above the principle is an energy balance, where the energy of the total roll backshall not exceed the potential energy at the limiting angle. The energy is in figure 20 representedby the areas a and b, where a is the roll back energy and b is the potential energy. Consequently

area b shall be larger than, or equal to, area a.


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The criterion is based on an extensive experimental research of how vessels with different hullshapes respond and roll in waves. It was developed by the Japanese and the Russians in themiddle of the 20thcentury, both working on a national stability criterion which could later bemerged into one more universal criterion.

9.2 Modified weather criterionThe purpose of a modified criterion is, as mentioned before, to predict the combined effect ofthe force from the mooring line and waves. The useful part of the weather criterion is the rollingprediction, since it is a recognized way of describing how a ship with certain particulars will rollin waves. The modification is basically the added moment, which in the original criterion are thesteady wind and the wind gust. Instead of the wind induced moments a single mooring linemoment is used. Since the rolling prediction in the criterion is entirely separated from the addedmoments no other alterations must be done.

The principle is simple, and more or less the same as with the original criterion. The onlydifference is that there will only be one moment, and it is due to the force from the mooring lineinstead of wind. The lever arm of the added moment is calculated according to (5.1).


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Figure 21The figure illustrates the limiting KG-curve for which Ship 4 still fulfils the commissionsCriterion 1.

Figure 22The figure illustrates the limiting GM-curve for which Ship 4 still fulfils the commissionsCriterion 1.

A strong pattern can be seen in figure 21 and 22, where a small draught and trim by the bow isclearly preferable. Considering the high bow on these vessels this is quite understandable since


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there is so much displacing area in the front. However, too much trim by the bow and too littledraught might result in reduced steering capacity, which naturally is very negative. Thus there hasto be a balance between stability and drift when ballasting the ships. Nevertheless a curve like thisis a very good basis for how to ballast the ship for anchor handling and the same analysis of theother test vessels shows the same pattern for them.


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11 Discussion part 2

11.1 Modified weather criterionEven though the weather criterion is an existing method of predicting the response of ships inwaves it is under a lot of discussion at the moment. The discussion is about its validity on

modern ships, while the criterion is based on statistics of ships from the first part of the 20 thcentury. Consequently there are a lot of research and experimenting going on right now, todetermine whether the criterion needs to be updated with more information about modern shiphulls [14] and [15]. Thus it is possible the weather criterion does not give a perfect prediction ofan AHTS vessels response to waves. However, as long as the weather criterion is in use as it isnow it must be considered just as good as any other method. Should the criterion be updated, socan the method being presented here.

Another aspect to consider here is that no analysis has been carried out of the effect of thecriterion and when it should be used. To determine that more thorough studies has to be carriedout, at which significant wave height the criterion is necessary and in what extent it shall beimplemented in the regulatory system. The analysis presented in this report is merely to

determine whether the modification could be carried out at all, i.e. analyse the criterion to see ifthe wind induced heeling moment can simply be replaced by a mooring line induced heelingmoment. As it turned out there is nothing that implicates this could not be done. Since it is onlyrelevant when waves occur, there is no reason to make it a general criterion.

11.2 Limiting curvesThe commission suggests that a KG curve for onboard use shall be mandatory for every vessel.It is not mentioned exactly how it should be done, but a possible variant would be to use thesame kind of KG limit curves as described in the previous chapter. However, at the sea when theship is exposed to a certain heeling moment there is not very much to do about KG even if thecrew knows the situation is critical and KG instantly has to be lowered. It is therefore preferablyto account for all possible scenarios in the planning stage, where the ship is to be loaded as

advantageous as possible for the specific task. The use of such curves for the crew seemsconsequently rather limited.

An alternative is to implement a computer that constantly calculates the heeling moment asdescribed in this report. The computer should be connected to all variables presented, ballast

condition, consumables, force from the mooring line, thrust and direction, currents, wind, KG, #and draught etcetera. By constantly comparing the heeling moment to the accurate GZ curve thecrew would be constantly updated about the current risk for the ship. Even though the computercannot replace papers it will provide a very good compliment.

11.3 Avoid over dimensioningWhen design criteria are created a lot of aspects have to be considered, they have to be fairlysimple, easy to apply, be general and they shall serve a purpose. Three out of four tested vesselsfailed Criterion 1. Criterion 2 resulted in the same vessels having a significantly lower workingrange than they have winch capacity. Yet all three vessels are in operation, and have as today notcapsized. Loading all winches full of mooring line during anchor handling operations is basicallyvery bad loading, just as it would be bad loading to put all oil in the starboard tanks on an oiltank or all containers at the port side of a container vessel. Yet it is possible to do so, but it is notdone. The regulation system does not have to, and cannot be, fail-safe. Thoroughly stressedremarks about acceptable loading of the winches are probably a far better and more dynamic wayto go.

Another factor to consider is the over lapping of safety margins and its effect on the final result.The commissions suggestions are conservative in several ways. The moment is calculated for an

extreme condition, the effective components from the mooring line are effective at the mostunfavourable angles creating the highest possible heeling moment. In addition to this the loading


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condition of the ship is assumed to be unfavourable. On top of that the safety margin of theratio between the moment and the GZ curve is 100%. The risk is that this will require very largeships for quite small operations. That is not only inefficient and expensive, but also bad for theenvironment since larger ships makes larger resistance in the water and therefore consumes morefuel.

As with all safety aspects there are always several aspects to consider. After a terrible accident,like the Bourbon Dolphin, it is always easy to think one-dimensional and see only to the perfectsafety of the seamen. That is, of course, a natural way of responding to such an accident, and atlarge also a correct response. The final outcome, however, must take more aspects intoconsideration, where the safety of the ships has to be put in relation to the efficiency of the ship,its environmental friendliness, the total cost of the ship and a general realism of how acontingent rule can be formed.

In times of global warming and financial crises it is impossible not to mention those aspectshere. Even though money can never be compared to human lives, it is a fact that companies, andconsequently potential ship owners, are dependent on earning money. Over dimensioned shipsare definitely more expensive to build and buy, and will also be more expensive in use. Of course

this is not a reason to build unsafe ships, but things has to be put in relation to each other. If therules are so restrictive so that those ships are getting financially indefensible costs has to be cutelsewhere and that could mean a downturn in the industry, with unemployed people as aconsequence. Alternatively cost can be saved in other building or operating related aspects thatare not regulated, meaning the safety could be reduced in another end instead. The result wouldthen in fact be contra-productive.

An even more tangible consequence of over dimensioning is the environmental aspect. A largervessel requires more fuel to operate, more material to build and in the end more disposals. Eventhough the aggregated emission from the worlds AHTS fleet is probably quite modest it cannever be neglected. The ships are powered by diesel and thus contribute to the green houseeffect. Larger ships require larger engines, which results in more emissions. The increased cost,described above, can also result in a lowered interest from the buyers and builders to invest inmore expansive environmental friendly technology.

The conclusion is not that new stability rules will automatically cause the scenarios describedabove. There naturally has to be regulations to guarantee the safety of the seamen workingaboard these vessels. The conclusion is simply that all aspects have to be considered, and theregulations must stand in proportion of the potential risks.


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-".+ 1

12 SuggestionsIt may be obvious that the best way to avoid capsizing is to avoid heeling. Thus it is preferable toprevent any situation that gives root to any great heeling moments. Consequently great careshould be taken to preventively actions, which minimizes the risk of the ship being exposed togreat heeling moments. Considering the large forces involved in anchor handling it will never bepossible to fully avoid this, why the ships still need sufficient stability. It is, however, preferable tohave a regulation system that both include stability criteria and mandatory operational proceduresto avoid dangerous situations.

The following part will present some suggestions how the safety during these operations can beraised. It will include recommendations both on how to control the ships suitability and how tohandle the vessel. The focus will, however, still be entirely on stability. Thus the operationalrecommendations will only be such that is directly connected to stability.

Heeling moment

The contingent largest heeling moment shall determine the largest allowable force fromthe mooring line.

The heeling moments shall be calculated so that the heeling angle is accounted for. FTHRUST,Yshall never to be pointed away from F3. The mooring line shall always run along the downstream side of the vessel so that at

least one force component will always counteract the others considering the heelingmoment.

Propulsion and currents Establish the maximum side thrust and the remaining available bollard-pull at full use of

the side thrusts during the bollard pull test. Mandatory analysis of the accurate sea state, including the direction and force of the

currents. The current situation shall also be constantly updated during the operation.

Loading The maximum allowable heeling moment shall be determined for several different

loadings on the winches. E.g. for 0%, 25%, 50% and 75% full winches. The ballasting should be based on limiting curves, such as presented earlier in this

report.

Wind The wind induced heeling moment shall be added to the total heeling moment when the

wind exceeds a wind speed of 20 m/s.

Stability criteria

GZMAX> 2'lMLshall be complemented by a criterion with a minimum area requirement.

Modified weather criterion The modified weather criterion shall be considered when the weather is rough and the

significant wave height exceeds a predetermined limit. More analysis has to be carriedout to determine this wave height.

Limiting curves Requirements of onboard KGMAX curves are unnecessary if sufficient planning is

accomplished. Recommend a computer that constantly calculates the stability situation of the ship.


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13 ConclusionsThe eight objective points can be concluded as

1. The major forces that will act upon the ship during anchor handling are FML, FTHRUST,FCURRENT8 and FWIND. The largest force acting on the ship is the mooring line.

Consequently it is the force from the mooring line that is the largest contribution to thecontingent heeling moment the ship can be exposed to. However, all forces will affectthe ship, and FCURRENTand FTHRUST,Ywill always create a heeling moment.

2. Use of the side thrusters reduces the bollard pull, why the test of the available bollardpull should include a test of the available side thrust at the same time. No matter howgood stability a ship has, the best thing is always to avoid extreme situations. Withsufficient bollard pull and side thrusters, the ship has a lot better chance of avoidingsituations where the heeling moment grows large since it can avoid side drift due tostrong currents.

3. The commissions suggestions are altogether rather conservative and should, as shown inthis report, lead to several vessels not being allowed to carry out the kind of operationsthey are designed for. The loading requirement shall be more flexible and the heelingshould be considered.

4. In addition to the suggestion of the commission about GZMAX> 2'lMLthere should be acriterion based on minimum area below the GZ curve. This is due to the fact that theshape of the moment curves can be very different and thus an area criterion is necessaryto guarantee a certain remaining potential energy for the ship.

5. The loading of the ship has a great influence over its capability of handling heelingmom

Stability and Anchor Handling Operations

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