MOORING - seaways.net.auseaways.net.au/wp-content/uploads/2015/11/Jetty-Safety-book.pdf · 3 Mooring The operations associated with berthing a ship alongside a berth or jetty are

MOORING

a sample chapter taken from:

TANKER JETTY SAFETYM anagement of the Ship/Shore Inter face

www.seamanship.co.uk

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Tanker Jetty Safety – Management of the Ship/Shore Interface


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3 Mooring The operations associated with berthing aship alongside a berth or jetty are generallyregarded as the ship’s responsibility.

A properly moored ship is the mostimportant requirement for safe terminaloperations as securing the ship alongsidefor the duration is fundamental to preventany strain on the cargo transfer equipment.

Securing alongside is affected by theexternal forces on the ship and this chapterof the book looks at them in more detail.

The mooring layout for all classes of ship,that the berth is designed to accommodate,is the responsibility of the terminal.

The terminal will understand the designcharacteristics of the mooring equipmentinstalled, its location and the loads it can

withstand and they will also be aware of theprevailing environmental conditions.Therefore, it is the terminal’s responsibilityto give the arriving ship all necessaryinformation to optimise the distribution,heading and number of mooring lines used.This ensures a fully balanced mooringpattern, with loads evenly shared by allmooring lines.

The terminal will prepare an optimummooring layout for each class of shipexpected to visit. It describes the standardmooring requirements and possiblealternatives for circumstances where themooring equipment of the ship does notmeet the standards.

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Figure 3.1 – LNG Carrier Approaching the Berth at Cove Point


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Figure 3.2 – Tanker Jetty Arrangement

There are a number of general principlesthat govern the restraint capability of amooring pattern at a conventional pier.These, along with some guidelines forachieving an efficient mooring arrangement,are outlined in the following sections.

3.1 Mooring ForcesIn the past, mooring arrangements weregenerally devised on the basis of operatingexperience. It was, and still is in manyinstances, common practice to run out thefull complement of mooring lines carriedonboard the ship.

Ships rarely broke out of these moorings,not because the mooring pattern wasefficient, but because the ship was over-moored.

A ship’s moorings must resist the largenumber of forces that are exerted on theship, both environmental and operational.Failure of the moorings can result in

It is the Master’s responsibility toensure that the ship is moored to thesuggested pattern. However, it isalways a joint ship/shore effort toensure that the ship remainssecured alongside for the duration.

damage to the ship and berth and injury topersonnel.

The goal when berthing is the optimisationof the mooring arrangement to resist theapplied forces. This problem is resolved byconsidering the following:

● What forces will the ship be subject to atthis particular berth

● what will be the effect of these forces onthe mooring lines

● what mooring arrangement is required tocounteract these forces?

The forces acting on a moored vessel areboth environmental and operational.Environmental forces are caused by naturalphenomena such as wind, waves, currentsand tides. Operational forces include thosecaused by passing ships, changes in thevessel trim, freeboard or draught andmooring line over-tension. The feature thatdistinguishes the operational forces from theenvironmental forces is that ship’spersonnel will generally have some level ofcontrol over the operational forces. However,a ship must be moored to resist whateverseverity of environmental forces exist, andthese are not controllable.

3.1.1 Wind and CurrentIn protected harbours the major sources ofenvironmental force are the wind and

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Figure 3.3 – Shows an Optimised Mooring Arrangement

current. Normally, in such areas ships aremoored at conventional pier facilities.

The magnitude of the wind force acting on aship is influenced by the velocity of the windand the area of the ship that is exposed it.The force effect of the wind will double asthe exposed ship area doubles. The ship isleast susceptible to wind forces when itheads into the wind and is low in the water,such as when fully laden.

A tanker is exposed to the highestwind forces when the wind strikesthe ship abeam while it is in a ballastor light condition.

3.1.1.1 WindThe force effect of wind is greater on alarge ship than on a small ship in a similarlyloaded condition as it has more exposedarea.

Figure 3.6 demonstrates how the wind forceof a tanker varies with wind velocity anddirection. Wind forces on a tanker can bebroken down into two components:

● Longitudinal force acting parallel to thelongitudinal axis of the tanker

● transverse force acting perpendicular tothe longitudinal axis.

The magnitude of wind force effect on theship correlates to the square of the velocity.If the wind velocity doubles, the force due to


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Figure 3.4 – Mooring Load MonitoringReproduced with permission from Harbour & Marine Engineering www.harbourmarine.com

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Figure 3.5 – Quick Release Schematic

wind will be four times greater. If the velocitytriples, the wind force will be nine timesgreater.

Other factors that contribute to themagnitude of the wind force include theshape of the area on which the wind isacting and the angle at which the windstrikes the surface. However, such effectsare complicated and, while important to theterminal designer, are of limited useonboard a ship when evaluating the effectof wind.

Though wind velocity can disturb aship at a berth, it is the suddensquall increase of force associatedwith a dramatic change of directionthat can be most dangerous.

3.1.1.2 CurrentWater current force considerations aresimilar to those of wind force. Themagnitude of current forces on a shipdepends on the velocity of the current, thehull area exposed to the current and theunderkeel clearance of the vessel.

As with wind, current forces are directlyrelated to the area of the ship exposed tothem. The maximum force of the current willbe experienced when the vessel is in aloaded condition and the current is actingdirectly on the beam. The force is minimisedif the ship is light in the water and its bow isheaded into the current.


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Figure 3.6 – Wind and Current Forces

Current force increases with the square ofthe current velocity. If the current velocitydoubles, the current force is four timeslarger. If the velocity triples, the force is ninetimes larger.

Since current forces act on the submergedportion of the ship, they are likely to be mostcritical when the ship is loaded.

While it is usually evident when the wind isblowing at or near gale force, high currentvelocities are not as noticeable to the ship’spersonnel. Only a review of the currentinformation for the terminal is reliable.

It should be noted that it is possiblefor the subsurface currents to have adifferent velocity and direction thansurface currents, especially atoffshore terminals.

The depth of the water under thekeel greatly affects current forces.As the clearance under the keeldecreases, the forces due tocurrents increase. The magnitude ofcurrent force can be three times asgreat on vessels with very smallunderkeel clearances than forvessels in deepwater.

3.1.2 WavesWaves are a major force on vessels atexposed mooring locations. In such areas,ships are generally moored at sea-islands,single point moorings or multiple buoymoorings.

Wave direction and frequency (period) aretwo factors that influence the effect ofwaves on a moored ship. Whether the shipresponds by surging, swaying or yawing willdepend on whether the waves are strikingthe moored vessel head-on, beam-on orquartering, the frequency of the waves andthe manner in which the tanker is moored.

Ships do not usually respond to a singlewave but to a system of waves. It is thecumulative effect of each wave in the wavetrain that causes the tanker to move. It ispossible to observe individual waves havinglittle or no effect on the moored vessel, yetthe vessel is moving slowly in response tothe entire wave system. This behaviour isnoticeable by observation of the rise andfall of the mooring line tension catenaries ata sea-island.

In harbours, there are sometimes very longperiod waves present, which are verydifficult to detect visually. These waves areknown as seiches and they are potentiallydangerous because of their ability todisturb moored vessels. They are capable offorcing a moored vessel to move slowly in a

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Figure 3.7 – Effect of Wind/current Forces on a Ship in Light/loaded Condition

cyclic manner, causing high mooring loads.Movements with periods of one to threeminutes are typical and are best observedby noting the rise and fall of the mooringline catenaries. If a moored vessel isresponding in this manner, the amount ofship motion can be modified by changingthe tension of the mooring system (either byslacking-off or heaving-in mooring lines).Actual measurements of line loads formoored VLCCs show that harbour seichescan cause tanker mooring loads to increaseby 15 to 20 tonnes.

3.1.3 Tidal ForcesVertical forces due to the tidal rise and fall(not including the effects of tidal current)are predictable, as the variation in tidal patterns is well understood at mostterminals. Changes in line loads are not inresponse to increasing or decreasingexternal forces but instead to changes inthe elevation of a vessel relative to a jetty orpier. Forces caused by tidal rise and fall cantherefore be controlled by slacking-off orheaving-in vessel mooring lines. However,without line tending, increased mooringforces due to tidal rise can be quite severeat some terminals.

3.1.4 Tanker Loading andDischarge

Mooring forces caused by tanker loadingand discharge (ie sinkage while loadingand rise during discharge) are similar tothose caused by tidal elevation as they arecreated by a change in the height of thetanker deck relative to the pier. Line tendingby ship personnel can minimise or eliminatethese forces.

3.1.5 Forces Exerted byPassing Ships

A ship moving through the water exertsforces on moored ships and other objects inthe vicinity. The magnitude of these forcesdepends on a number of conditions, themost important of which are:

● Clearance under the keel of the mooredvessel

● separation distance between the passingship and the moored vessel

● sizes of the passing and moored vessels

● the speed at which the ship passes themoored vessel.


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Figure 3.8 – Vessel Motion

A moored vessel is at its most vulnerable tothe passing of another vessel when:

● It has little under keel clearance

● the separation distance between theships is relatively small

● the passing vessel is travelling at arelatively high speed

● the passing vessel is of a comparativelylarger size.

This can be particularly severe if mooringlines have already lost their pretensionthrough reduced elevation as the ship fallswith the tide. The most critical time for aloaded tanker (when moored) would occurat low tide, at which time the underkeelclearance would be at its minimum.However, a passing ship cannot bedisregarded any time that a vessel ismoored.

A report on an LNG carrier that was pulledoff of the berth is detailed in the casestudies section.

3.2 Factors AffectingLoad Distribution

The factors affecting load distribution canbe divided into the following categories:

● Overall mooring pattern

● orientation of the hawsers

● elasticity of the hawsers.

3.2.1 Overall Mooring PatternThe overall mooring pattern affects the loaddistribution to individual lines. Generally, the

The water to landward of a ship at ajetty that has restricted underkeelclearance cannot flow easily underthe ship to replenish the suctioneffect caused by a passing vessel.

mooring pattern should be as symmetricalas possible, at about the ship’s mid-point toensure a uniform load distribution amongthe lines for varying wind and currentconditions. Although an unsymmetricalpattern may be viable for some situations,such as where wind and current approachfrom one direction, there is always a risk ofa limited number of lines resisting the entireload.

3.2.2 Orientation of theMooring Lines

The effectiveness of a mooring line isaffected by two angles, the vertical anglebetween the lines and the pier deck, andthe horizontal angle between the mooringline and the parallel side of the ship.

The steeper the orientation of the mooringline, the less effective it is in resistinghorizontal loads. As an example, a hawseroriented at a vertical angle of 45º is only75% as effective in restraining the shipagainst wind forces as a line oriented at a20º vertical angle. Similarly, the larger thehorizontal angle between the parallel side ofthe ship and the mooring line, the lesseffective the line is in resisting a longitudinalforce.

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Figure 3.9 – Vertical Angle Mooring LineReproduced with permission from Harbour &Marine Engineering www.harbourmarine.com

3.2.3 Elasticity of the HawserThe elasticity of a mooring line is a measureof its ability to stretch under load. The effectof hawser elasticity is often overlooked,even though the differences between themooring line elasticities can be very large,and where mooring lines of differingelasticity are connected at the same point,the stiffer mooring line will always take thegreater load stress.

The hawser’s material, diameter and lengthare the primary determinants of its elasticity.

Figure 3.10 demonstrates the significanceof each factor on load distribution.

The effect of mooring line material on loaddistribution is generally recognised.Although “mixed moorings”, consisting ofvarious types (material) of mooring lines inthe same service are generally condemned,within the industry they are still commonlyused.

At times, the effect of line length on loaddistribution is overlooked in mooringarrangement evaluation. Line elasticityvaries directly with line length and has asignificant effect on line load. An 80 metrelong wire line will take half of the load if it isused with a 40 metre line that is of the samesize and material and is both parallel andadjacent to it. However, if fibre ropes areused, the longer line will carry less than halfthe load of the shorter line (possibly 25% fornylon) since the elongation curve for fibresis not linear.

If a wire mooring rope is run outparallel to a fibre mooring rope, thewire will carry almost the entire loadwhile the fibre rope will carrypractically none.Note: This also applies for differenttypes of fibre ropes (Figure 3.10)


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Figure 3.10 – Elasticity of Mooring Lines

The elasticity of each type of line alsovaries with its diameter. Usually, this factor isnot an important consideration as ropescarried on board ships are of a uniformdiameter

3.2.4 Mooring DistributionSummary

● Mooring lines should be symmetricallyarranged on the transverse centre line ofthe ship to ensure a good loaddistribution

● where it is possible, breast lines must beperpendicular to the side of the ship

● spring lines should be arranged parallelto the ship's side— a mooring system that adheres to the

above three principles is shown Figure3.10. By proper orientation of bothbreast and spring lines an efficientload distribution and tenability isassured. Normally, no more than twogood spring lines are required toresist longitudinal wind, current andsurge forces

● the vertical angle between the mooringline and the pier deck should be as smallas possible.

● bow and stern lines, oriented at an angleof 30º-60º off the bow and stern, are notrequired for mooring when adequateberth facilities are available— because of their long length and poor

position, bow and stern lines arenormally not very effective inrestraining a ship in its berth, althoughthey can be useful however formanoeuvring purposes

● mooring lines of the same size and typeshould always be used for all leads usedin the same service, ie breast lines,spring lines, bow lines, etc

● mooring lines should be arranged so thatall lines in the same service areapproximately the same length betweenthe ship and shore bollards (with breastlines for a VLCC normally at about35-50 m).

3.3 Mooring EquipmentVLCC’s have in the past been involved in anumber of mooring incidents, some ofwhich resulted in damage to terminalloading arms.

These incidents were attributed to winchslippage, broken mooring lines or tails andexcessive ship movements permitted by thehigh elasticity of synthetic mooring lines.

The following section deals with some of thefactors affecting the design andmaintenance of mooring equipment.

3.3.1 All Wire Mooring andSynthetic Lines

The main reason for recommending wiremooring lines for VLCC’s is because theyprevent excessive ship movement, which isthe usual cause of damage to shore basedhardarms. A secondary reason is that whenmixed moorings are used, the wire linestake most of the load and so early linefailure can occur. Figure 3.10 illustrates theexcessive elongation of synthetic (manmade) fibre lines at relatively low load levels.New synthetic lines can have even greaterextensions.

Some of the materials used for syntheticfibre lines are sensitive to weatherexposures, particularly sunlight. Shockstresses applied to some types of syntheticfibre lines could significantly reduce theirbreaking load without any apparent visibledamage to the line itself. Synthetic lineshave a relatively short service life.

Many modern ropes are a blend or mixtureof several fibres. Combinations of polyesterand polypropylene fibres in ropes arecommon. Some, but not all, compositeropes made of polypropylene and polyesterare as strong as ordinary polyester ropes.

Careful inspection should be carried outwhen an ‘all synthetic lines’ mooring isused. Attention should also be paid to

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ensure that all of the synthetic mooring linesused (or at least these positioned in thesame direction) are made of the samematerial.

3.3.2 Selection of WireMooring Lines

To keep line handling and tending withinmanageable proportions for the larger sizes,ship mooring lines should be selected whileconsidering the maximum breaking loadcompatible with ease of handling andreasonable flexibility. It is generallyaccepted by VLCC operators that wires of42-44 mm diameter meet this criteria. Thefactors that contribute to the strength ofwire mooring lines are construction and wiretensile strength. For mooring VLCCs it isrecommended that, as a minimum, a 42 mm(13⁄4") diameter 6 x 37 class I.W.R.C.preformed, heavily galvanised wire line(minimum tensile strength of 180 kg/mm2)with an MBL of 115 tonnes is used. Theminimum lengths of wire should be275 metres, to allow for berthing at multi-buoy moorings, piers and sea island berths.

Splices, other than eye splices, are notrecommended and wires that are splicedbetween the mooring point ashore and theship should not be accepted by theterminal.

3.3.3 Synthetic TailsWire mooring lines are generally fitted with alength of synthetic rope, normally nylon, onthe shore end. The additional elongation ofthe mooring line system permitted by the tailreduces the risk associated with poor linetending, particularly in berths with largetidal variations and high loading/unloadingrates. They are also valuable at berths thathave short breast or spring leads.

Model tests and field measurements, as wellas experience, confirm the effectiveness ofthe additional elasticity provided by thetails. This additional elasticity reduces theloads induced in wire mooring lines under

dynamic loads, by permitting the ship torespond more favourably to variouscombinations of wind, waves and current,as well as to ships passing at slow speed inclose proximity. Testing has shown asubstantial reduction in breaking strength innew synthetic tails in a relatively shortperiod of time.

An incorrectly designed tail could introducetoo much elasticity. The size of the ropechosen should be capable of easy handlingand be of sufficient size to ensure that thesynthetic rope has a breaking strength atleast 25% greater than that of the wire lineto which it is attached. It is most importantthat the tail is attached to the wire by use ofa patent design such as a ‘Mandal’ or‘Tonsberg’ mooring link.

3.3.4 Mooring WinchesTypical deck mounted reel stowing mooringwinches are shown in Figures 3.12 and 3.13

Automatic tension winches have beendesigned so that a specific line tension canbe preset, allowing the winch to pay outwhenever the value is exceeded and thenheave-in when the line tension falls below it.However, use of such winches should notbe accepted at the tanker jetty as extremewind/current forces can cause winchrelease at the weather end of the vessel,followed by heaving of the winches at theopposite end of the vessel. Such self-tensioning wind effects would allow the shipto ‘slide’ along the berth, disturbing the


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Figure 3.11 – Tonsberg Mooring Link

perpendicular angle desired for hose orhard arm connections to the ship’s manifold.Mooring management is, therefore, amanual operation.

If automatic winches are installed,the winch should be placed on amanual brake while the vessel ismoored alongside, as fore and aftspring lines and head and stern linescan work against one another whenexposed to wind and currentsituations.

Winch brakes should be set to hold aminimum load of 60% of the minimumbreaking load of the wire. The winch brakeholding capacity must be regularly tested.OCIMF recommends that this test is made,and the results recorded, at least once ayear.

A torque incorrectly applied to the brakecould result in a sharp reduction in brakeholding capacity.

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Figure 3.12 – Deck Mounted Mooring Winch

Figure 3.14 illustrates the correct method ofreeling. Mis-reeling could reduce theholding load capacity to 30% of thecapacity of a properly reeled wire.

3.4 Mooring SystemManagement

The objective of good mooring systemmanagement is to provide for the safety ofthe ship and terminal, preventing damage toeither.

Good mooring management requires theapplication of sound principles, wellmaintained equipment, trained personneland proper co-ordination and interactionbetween the ship and shore.

The terminal can reduce the possibility ofship break-out in a number of ways. Theseinclude:

● Development of guidelines and amooring layout for each class of ship thatis acceptable at the berth

● ensuring that information about the ship'smooring equipment is obtained before itsarrival

● after berthing, inspection of the ship’smooring equipment to decide if anymodifications must be made andsubsequent periodic inspection of linetending

● by having good contingency plans forprompt cessation of cargo transfer, therelease of cargo transfer equipment(hoses/hardarms) and the safe removal

Care should be taken when reelingthe wire onto the winch drum ascases of mis-reeling have beenreported. Band brakes are designedfor the wire to pull directly againstthe fixed end of the brake strap.

of the ship from the berth should therebe a failure in mooring.

3.4.1 Operating LimitsOperating limits establish the environmentalconditions within which the loading armsand other shore based equipment canoperate.

Should the wind, wave or current forces(either individually or combined), beexcessive, the moorings run the risk ofbeing overloaded. Should the movement beexcessive and not controllable, the ship maybe required to vacate the berth to avoiddamage to the ship or jetty.

3.4.2 Joint Terminal Meetingand Inspection

Once the ship has berthed the terminalrepresentatives will meet the Master or othersenior officer. At this meeting they willreview and confirm the following:

● Freeboard limitations

● ballasting

● conditions for emergency disconnection

● actions where there are changes to themooring loads

● weather forecasts

● complete the ship/shore safety checklist

● complete any oil pollution avoidancechecklist

● obtain agreement on details of cargo,bunker and ballasting arrangements.This will include agreement aboutsimultaneous cargo/ballast handling,

Movement of the ship alongsidemay, in addition to suspendingloading/unloading, requiredisconnection of cargo transferequipment and the gangway.


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Figure 3.13 – Deck Mounted Reeled-Stowage Mooring Winches

Figure 3.14 – Winch Brake and Reeling Arrangement

where and when it is required foroperational/environmental limitations

● assess under keel clearance limitations

● agree on procedures for draining anddisconnecting cargo transfer equipment.

3.4.3 Line TendingOnce all the mooring lines are made fast,they should be inspected regularly toensure that all lines are taut and that theship is hard against the fenders. Thisinspection will also allow all lines to assumetheir share of the loading on the ship. Iflines are slack, or if the ship is lying off anyfenders, the appropriate lines should behove-in immediately to correct the situation.

Even though a ship may initially be properlymoored and have lines that are adequatelypre-tensioned, changes in weather, tide orfreeboard will necessitate tending of thelines to prevent them from being overloadedor going slack.

The frequency of necessary line tending willbe situation dependent.

A special emphasis should be placed oninspection of the tanker’s mooring systemwhenever any of the following conditionsare present:

● Periods of high loading and/ordischarging rates, where the ship’sfreeboard changes rapidly

● a sudden increase in wind speed or achange in direction, whenever windspeed exceeds 15 m/sec, 30 knots, orwhere gusts are expected

● in swell conditions

● during periods of maximum tidal flow

● whenever underkeel clearance is low

● prior to, during and immediately after theclose passing of other ships.

3.4.4 Precautions Applicablein High Mooring LoadConditions

Overload of mooring lines is evidenced byeither direct measurement, observation ofthe mooring by experienced personnel orby winch slippage. The followingprecautions and actions are likely to apply:

● Harden-up on the winch brakes. Do notrelease brakes (or slacken from themooring bitts) and attempt to heave in

● discontinue cargo operations

● reduce freeboard by taking on ballast ifloads are due to high winds.

● disconnect cargo transfer equipment

● call crew, linemen, mooring boats, tugsand put ship’s engines on standby

● run extra moorings, as available, togetherwith any shore moorings available.

3.4.5 Mooring at a MultipleBuoy Mooring

A Multiple Buoy Mooring (MBM) usuallyconsists of between three and sevenpermanently anchored buoys and is arelatively simple and inexpensive type ofmooring facility.

This type of berth is rarely used for largetankers but can be installed where weatherand sea conditions are mild to moderate. Agraphic of a typical 4-buoy MBM is shownin Figure 3.15.

While it is the Master’s responsibilityto moor in a safe manner, accordingto the mooring layout supplied bythe terminal, and to ensure that linesare properly tended during the entirestay alongside, the terminalrepresentative must also satisfyhimself that good mooringmanagement is constantly followed.


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Note:The mooring anchor lines allow thebuoy sufficient elasticity so nylon tails arenot used for the mooring lines.

In many MBMs the bow of the vessel is heldin position by the ship’s own anchors whilethe ship’s mooring lines secure its stern tothe mooring buoys. In general, the samegood mooring practices that arerecommended at conventional piers arealso applicable to sea berths. However,shipboard and terminal personnel shouldbe aware of several additional points thatensure the safe mooring of the vessel andthe integrity of the cargo transfer system.

3.4.5.1 MBM Mooring ProcedureThe same procedures are normally used tomoor a tanker once it has entered the MBMberth. As soon as the tanker hasmanoeuvred into the berth a launch takesthe tanker’s lines, one at a time, and towsthem to the various mooring buoys. Themooring line may be placed on the buoyhook from the launch or the buoy may beboarded to secure the line. Some berthsuse “preventer wires”, which arepermanently attached to the buoy and aretowed to the tanker with the launch. At mostberths, it takes about 21⁄2 hours from thetime the tanker is cleared for berthing to thepoint where all mooring lines are made fast.

The order in which lines are laid out to thebuoys depends on the environmentalconditions. The lines are set to counteract

the effects of the prevailing winds andcurrent. As an example, lines on thewindward side are put out first.

When the tanker leaves the berth, themooring procedure is reversed. The ship’slines are slackened and slipped off thebuoy’s quick release hooks by the launch.Once the lines are cleared, the anchors areretrieved. If forward breast lines are used,they are usually released before the sternlines. It normally takes between 60 and 90minutes for the unberthing operation, oncethe hoses are on the seabed, depending onthe capabilities of the tanker’s mooringequipment.

3.4.5.2 Berth Layout and ProperAnchor Deployment

The primary restraint for a ship’s bow on anMBM is generally provided by the vessel’sown anchors. This is because the shiprequires unrestricted manoeuvring room inthe vicinity of the bow to be able to enterand exit from the berth, so buoys cannotusually be placed near the bow.

To provide adequate restraint for the shipwhen it is finally moored, the angle betweenthe anchor lines should be between 60-90º.If the angle between the anchor cables (orchains) is too small, insufficient resistanceto lateral sway may result. However, if theangle between the anchor cables is toolarge there may not be enough resistance toa sternward surge.

Sufficient anchor cable must be paid outfrom the ship for the pull of the anchorcable to be in a horizontal direction. If thepull of the anchor cable is not in a horizontaldirection on the sea-bed, the anchor will

It must be remembered that whilestern lines are in the water, theship’s engines should not beoperated

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Figure 3.15 – Multiple Buoy MooringReproduced with permission by APL Norway

tend to be pulled out under severe wind andwave conditions.

The pilot or berthing master, through hisexperience at a particular facility, will knowat which point each anchor must be let goto ensure an adequate length of cable andthe required angle between the cables. Theimportance of anchor release of anchorsand cable payout /retrieval being doneproperly is emphasised by the fact that theweak link at an MBM, even for a properlymoored ship, is the vessel’s own anchors.

Since the MBM system needs submergedhoses to be lifted and connected to transfercargo, ship movements while in the berthmust be confined to prevent damage toeither the hoses or the pipeline andmanifold. If the hose string is made toolong, it will chafe either on the ship or on thesea-bottom. The tolerances are relativelysmall and proper use of mooring lines isrequired to maintain the vessel within thedesigned berth arrangement. Use of all-synthetic lines could result in excessivemovements, with subsequent damage to thehoses.

3.4.5.3 Pre-Tension Lines to the FullCapacity of the Winches

The buoy legs of an MBM allowconsiderable drift when under little or noload. To prevent excessive drift of a shipand damage to hoses and/or pipeline andmanifold, mooring lines should be pre-tensioned to the full heaving capacities ofthe winches.

3.4.5.4 Use of Preventer Lines Preventer lines are steel wire linespermanently attached to the mooring buoys.The preventer lines are hauled to the tankerand tied off to bitts on the vessel’s deck.They are intended to act as backup lines incase the ship’s line should fail and they willprevent the excessive movements that occurwhen synthetic lines are used to moor thetanker. Occasionally, they are used as theprincipal mooring lines if the ship’s lines are

either too short to reach the mooring buoysor are in poor condition. Preventer lineswould also be used to ensure the vessel’ssafety if the tanker has mixed moorings.

The use of preventer wires as principalmooring lines is not normallyrecommended. Since 38 mm-50 mmdiameter wires are difficult to handle, it willbe hard to adequately secure the wiresaround the bitts and they can never bemanually pre-tensioned to the same level asshipboard winch-operated lines. The shipshould ensure suitable wire stoppers areavailable to restrain the wires when securingthem around the bitts.

Use of preventer lines does not double themooring restraint capacity offered to theship since preventer lines, due to the lack ofpretension, will rarely share loads equallywith winch-operated lines. Wire ropehandling at an MBM berth incurs strenuouswork for the ship’s crew and great care andattention is necessary to manage this safely.

3.4.5.5 Operating LimitationsThe operating limitations of the berth andthe advice of the berthing master should befollowed when disconnecting the hoses anddeparting the berth. As weather and seaconditions deteriorate, the MBM is moredifficult to depart from than other offshoreberths as a launch is generally required torelease the ship’s wires from the buoys andthe ship’s anchors must be weighed.Simultaneously, care must be exercised toprevent mooring wires from fouling thepropeller during the operation.

Consideration should be given tosuspension of loading/unloading operationsand getting underway when waves fromahead are approaching 3 m maximum orwhen waves off the bow are 1.5 mmaximum.

MBMs require more moderate windconditions than other offshore berths asthey can become untenable in beam or


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quartering winds greater than 25-35 knots.Limiting current conditions are normally1 knot from abeam and 2 knots for headcurrents.

Very little can be done by the ship to extendits berthing capacity in hostile situations atan MBM so care must be exercised whentaking the decision to disconnect hoses andmooring lines and depart the berth.

3.4.6 Single Point MooringSystems

At a Single Point Mooring (SPM), which isalso called a Single Buoy Mooring (SBM), amono-mooring or a bow mooring, the tankeris moored by the bow, using one or twohawsers, to a buoy or tower. The tanker isfree to rotate about this point under theinfluence of wind, waves and current.Because the tanker is free to align itself withthese forces, the total force is less than ifthe tanker were held at a fixed heading.

There are two basic types of SPMs at whicha tanker may berth, the Catenary AnchorLeg Mooring (CALM) and the Single AnchorLeg Mooring (SALM).

CALM

The Catenary Anchor L eg Mooring (CALM)(sometimes called a Single Buoy Mooring orSBM), illustrated in Figure 3.17, is the most

common. The CALM consists of a largebuoy held in place by four or more anchorcables that extend in catenaries to anchorpoints some distance from the buoy.

The ship to buoy mooring hawser(s) is/arefastened to a turntable or platform on thedeck of the buoy. The loading hose, floatingin the water, connects to a pipe or pipes onthe buoy turntable. This pipe is connectedthrough a fluid swivel unit in the centre ofthe buoy to an underbuoy hose that extendsdownward, and sometimes to the side, to asubmarine pipeline.

SALM

The SALM, shown in Figure 3.19, consists ofa deep-draught buoy, anchored by a singleanchor leg, which is tensioned to pull thebuoy against its buoyancy. The anchor leg isprovided with an anchor swivel, whichallows the buoy to rotate. The anchor leg isattached to a large base that is held to theocean floor by internal fill and/or piling. Afluid swivel, connected to the submarinepipeline, surrounds a shaft in the anchor legand is located either at the base or at apoint above the base. An underwater hoserises from the fluid swivel to the sea surfaceat a point some distance from the buoy anda floating hose extends from that point tothe midpart or bow loading connection ofthe moored tanker.

Mooring Tower

The third form of SPM that may beencountered is the mooring tower. Mooringtowers have a floating hose connected tothe turntable at a point near the watersurface.

The tower has a rigid loading arm thatconnects to the edge of the turntable, dropsdown and extends again alongside thetanker. The bow mooring tower consists of arigid or semi-rigid structure, held to the seafloor by piles and rising above the seasurface. A mooring turntable, with a fluidswivel at the centre, is mounted on the topof this structure.

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Figure 3.16 – Single Point Mooring SystemReproduced with permission from Harbour &Marine Engineering www.harbourmarine.com


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Figure 3.18 – VLCC Approaching an SPM

Figure 3.17 – Catenary Anchor Leg Mooring

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Figure 3.19 – Single Anchor Leg MooringReproduced with permission by APL Norway

Figure 3.20 – Mooring Tower


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Figure 3.21 – Bow-mounted Twin HawserHooks on Texaco’s FSO ‘Lombo Este’,

AngolaReproduced with permission from Harbour &Marine Engineering www.harbourmarine.com

Note: Figure 3.21 and 3.22 show a ‘quick release’bow stopper arrangement. They are useful forillustration purposes but differ from the tankermooring arrangement in that the chain is notpulled through the stopper, as can be seen infigures 3.23 and 3.24

Figure 3.22 – Quick-release Chain HawserHook Fitted with Hydraulic Remote-

Release, Load Pin and Hawser Fast LeadReproduced with permission from Harbour &Marine Engineering www.harbourmarine.com

3.4.6.1 SPM Mooring Equipment andFittings

At all single point moorings, the mooringropes are provided by the terminal. Usually,at older SPMs, they consist of two large-diameter nylon hawsers.

These hawsers are permanently attached tothe mooring buoy, or tower, or are storedelsewhere and attached to the buoy by thelaunch crew prior to the tanker’s arrival.Generally, flotation collars are placed on thehawsers to keep them afloat when not inuse.

Many tankers larger than 150,000 dwt areequipped with towing brackets and largePanama fairleads or chocks for mooring atan SPM (note: that the terms, “fairleads”and “chocks”, are used interchangeably)and some are now being equipped withbow stoppers, also known as ‘SmitBrackets’ or ‘AKD Stoppers’. However, mostsmaller ships have only bow fairleads andbitts for accommodating mooringequipment.

In general, ships of approximately200,000 dwt and larger will have 2 bowmooring stoppers. As a result, and becauseof the variety in shipboard equipment, manytypes of mooring arrangements areprovided by the terminals. At the older SPMterminals that cater for smaller tankers, atypical arrangement is as shown on Figure3.24. In this arrangement, a single chafingchain is brought through the tanker’s bowfairlead. Note that both hawsers are passedthrough the same fairlead wheneverpossible. Passing the individual mooringlines through widely separated fairleadscould lead to an extremely unequaldistribution of the loads taken by the twolines.

For terminals handling tankers of up to the250,000 dwt class, the “Buoy MooringForum” recommends an arrangement asshown in figure 3.24. In this arrangement,the twin mooring lines from the buoy areattached to a triangular plate, which in turnis connected to a three-inch chafing chain.The figure shows the connection for shipshaving a towing bracket and a sufficientlylarge panama chock (or fairlead).

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Figure 3.23 – Mooring Arrangement up to 500,000 dwt

Should the ship lack the proper sizedPanama fairlead or towing bracket, it will benecessary to shackle a smaller size chafingchain to the triangular plate. This chain canterminate with suitable sized wire or nylonsnotters in a figure of eight around the bitts.Care must be exercised by the ship andterminal since the ship’s bitts may fail atlower loads than the terminal’s hawsers.

The following are considered safe workingloads for various diameter mooring bitts:

Bitt Diameter* Safe Working Load

(metric tonnes)

500 155550 190600 250

* These loads are based on well designed,fabricated and installed bitts.

For tanker terminals designed for tankerslarger than 200,000 dwt, dual mooringarrangements (each with a safe workingload of 200 tonnes) are recommended.Some of these tankers will have a bowstopper, others will have towing brackets,see figures 3.21 and 3.22.

3.4.6.2 Preparations for Mooring at anSPM

Since special mooring gear such asshackles, strop ropes and messenger ropesare provided at some terminals, they areusually delivered to the tanker at the sametime as the Pilot. A boom should be riggedto lift this equipment onboard from thelaunch. Hose connection gear may also bedelivered at this time. The Pilot and gearshould be taken aboard on the side towhich the hoses will be connected to avoidcarrying the hose connection equipmentacross the tanker. The hose is connected tothe port side at almost all SPMs.

Any SPM mooring gear should be takenpromptly to the forecastle and preparationsshould be made for receiving and securingthe mooring lines. The arrangement offairleads, fittings, winches and obstructionsshould be selected to provide the bestmeans of securing the tanker to themooring.

3.4.6.3 Mooring Operations at an SPMWhile the mooring procedures employed bya Pilot (or berthing master) and launch crew

may vary, the following is a generaldescription of a typical SPM mooringprocedure that illustrates good practice.

1. A messenger rope should be riggedthrough the fairlead selected for the firstmooring rope and brought back over therail in preparation for lowering to thelaunch. The first mooring rope to bebrought aboard will usually be the portline. However, to avoid crossing themooring ropes, the instructions of theberthing master or pilot as to which ropeshould be rigged first should befollowed.

2. The messenger rope should not belowered over the centre of the bow, butshould instead be lowered over the sideof the forecastle. This ensures that thelaunch does not have to position itselfdirectly ahead of the tanker to receivethe messenger. This is especially

important for tankers fitted with largebulbous bows.

3. The other end of the messenger ropeshould be wrapped around the forwardwindlass (gypsy) on the winch inreadiness to heave up. As soon as thelaunch has made the messenger fast tothe pick-up rope and is clear of theropes, the messenger should bewinched aboard the tanker as rapidly aspossible.

4. As the mooring pick-up rope is winchedin, it should be flaked on deck. When thechafing chain passes through thefairlead it should be stopped.

5. Once the first mooring line is secured,the messenger rope should be riggedthrough the fairlead selected for thesecond mooring rope and then droppedto the launch as before. The second


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Figure 3.24 – SPM Mooring Arrangement to 250,000 dwt

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Figure 3.25 – Single Buoy Mooring

mooring rope is then brought aboardand secured.

3.4.6.4 Practices at SPMs to ReduceHigh Mooring Line Loads orBuoy Ride-Up

Once moored, the terminal may request theship to go astern slowly to prevent the ship

riding up on the buoy, or to keep tension onthe mooring rope, so that the ship mayweathervane with a change in tide. Caremust be exercised in agreeing to theserequests. On many ships, when less than10 rpm is requested over protractedperiods, damage to stern tube bearingsfrom lack of positive lubrication could occur.

Similarly, a number of terminals request25 rpm astern for protracted periods of timeto reduce the vessel’s yawing, therebyreducing mooring loads. Before agreeing tothis type of request, the ship’s personnelshould confirm that the ship actually hasthis capability.

In many ports where this is required, a tugis fixed to the stern of the vessel to keep itaway from the buoy.

3.4.6.5 SPM Load MonitoringLoad Monitoring will be achieved throughsystems that incorporate load pins into theconnector plate/joints or by audible andvisual alarms located on the buoy. Alarmstatus will be displayed remotely viatelemetry.

Power on a buoy for telemetry, navigationlight etc, is often generated by marinisedsolar panels or by using 12V DC batterieswith back up capacity.

Continuous telemetry monitoring, which istypically via a UHF radio link, is alsopossible and the range can be up to20 kilometres over water.

Remote Displays will display and recordsignals for:

● Hawser load dynamics

● wind speed/direction

● current

● line pressure

● temperature

● alarm status

● battery voltage

● navigational aids operation

● lighting status

● manned/unmanned indicator.

‘Intelligent’ load pins such as theSmartCell® shown in Figure 3.28, which areinstrumented to allow mooring line tensions

to be measured can replace the standardconnector swivel pin.

SPM monitoring instrumentation, such asshown in Figure 3.26, will provide load datato the operator. Alarm warnings shouldrequire operator acknowledgement.

3.4.7 Shore Based MooringEquipment

This equipment must be compatible withtanker moorings, be properly located andbe of sufficient capacity to ensure safemoorings and straightforward line handling.

3.4.7.1 CapstansLine handling units for pulling tankermooring lines ashore have vertical spindlesand should be located at each mooringpoint. They will be either integrally built intothe quick release mooring device or belocated immediately adjacent to it, in aposition that enables the eye of the mooringline to be slipped over the mooring hook.The unit should have a minimum loadedpulling capacity of approximately 2000 kgat a speed of about 24 m per minute.Conditions requiring long pulls over soft seabeds may require higher pulling capacity forgear operated capstans. The motors shouldbe the reversing type to allow unwrappingof a seized tag line or messenger. Capstansshould be able to hold the load with themotor stopped.

The use of capstans often results in surplusbights of messenger line and slippage,requiring greater operator care compared towinch operations.

3.4.7.2 WinchesWinches (ie with horizontal drums), arepreferred by some operators. They have thesame pulling and load characteristics ascapstans and are generally preferred foruse with a gallows, which permits the line tobe placed over the quick release hook morereadily.


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Figure 3.26 – SPM Monitoring SchematicReproduced with permission from Harbour & Marine Engineering

Figure 3.27 – SPM Handheld RemoteMonitor

Reproduced with permission from Harbour& Marine Engineering

Intrinsically safe handheld monitor. This receivesdata from the SPM every 5 seconds, displays up

to 4 pages of LCD data and features both audibleand vibrating alarms.

Figure 3.28 – SPM Load PinReproduced with permission from Harbour

& Marine Engineering

3.4.7.3 Fairleads for MessengersSuitably located fairleads should beprovided adjacent to the capstan or winchto facilitate the placing of mooring lines onquick release hooks.

3.4.7.4 Quick Release HooksThis type of mooring unit is recommendedfor the safe mooring of large ships (figure3.30). Each hook, whether single or part ofa multiple hook unit, should have a safeworking load of not less than the MBL of thelargest line anticipated.

One mooring line should be placed on eachquick release hook so sufficient hooks mustbe provided to allow this. All hooks shouldbe capable of separate release, safely fromthe mooring point area, under full to no loadconditions. Remote hook release facilitatesemergency sailing with minimum manpoweravailability.

3.4.7.5 BollardsFixed shore bollards require a line to belifted from the mooring point when gettingunderway under slack conditions and so arenot recommended for large tanker berths.

3.4.7.6 Strength of EquipmentAll of the equipment should be designedusing appropriate safety factors, andfabricated and installed to withstand theexpected maximum loads.

3.4.7.7 Increasing Restraint Providedby Ship’s Mooring

An alternative to the use of shore mooringsis the provision of a shore mooring pointattachment for the bight of the vessel’smooring wire. The end of the ship’s wire canthen be hauled back onboard and securedto the ship’s mooring bitts, after which thestanding part is hove taut by the mooringwinch and the brake is secured in thenormal manner. In this way one mooringwire can be used to provide a restraint ofapproximately twice its normal capability.

3.4.8 Laser and GPS DockingSystems

Laser and GPS docking and pilotingsystems provide real-time feedback of avessel’s position during approach and


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Figure 3.29 – CapstanReproduced with permission from Harbour


Figure 3.30 – Quick Release HooksReproduced with permission from Harbour


berthing at the jetty. Key information such asspeed, distance and angle relative to thejetty are displayed to the pilot, ship’sCaptain and jetty operators. Data is loggedand reports are available for later review oraudit.

3.4.8.1 Laser Docking Aid SystemA Docking Aid System is a tool used by jettyoperators and marine pilots during vesseldocking. The primary benefit is the provisionof real time data on the vessel’s positionand progress, relative to the jetty, bymeasuring distance from the jetty andspeed of approach in the critical 0 to

300 metres zone. With this data the vessel’sMaster and Pilot can better direct tug andshipboard personnel.

A display board that is mounted in aposition clearly visible from a vessel’sbridge will provide vessel speed anddistance in numbers, speed trend indicationand speed warning by using a red, amberand green light system. Speed and distanceunits are normally (see Figure 3.33):

Speed: 0.0 to 99 centimetres persecond

Distance: 0.0 to 199 metres

Laser Docking Aid Systems are a reliableand precise method of measuring vesselapproach over the final 200 metres to thejetty, with accuracy to 1 cm.

As an example, a system such as‘SmartDock®’ uses two laser sensorslocated on the jetty that measure distance tothe bow and stern sections of the ship. This,together with average speed, is captured ata jetty control unit and displayed to the shipand mooring crew on a wireless monitor,computer screen or jetty mounted displayboard. Laser sensors are the most reliabletechnology employed for vessel dockingand operate very effectively in poor visibilityand heavy rain.

The basic components of such systems are:

● Laser sensors, which may be fixed on thejetty head or provided on elevators

● jetty controller/interface which can belocated indoors or on the jetty

● computer workstation to monitor, displayand record information in the jetty controlroom

● handheld monitors, certified intrinsicallysafe to provide constant update ofdocking information to the pilot

● jetty mounted display boards, visiblefrom the vessel’s bridge to display speedof approach, distance from jetty andoptional angle

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Figure 3.31 – Horn BollardReproduced with permission from Harbour


Figure 3.32 – Pillar, Kidney, Horn and TeeDesign Bollards


Figure 3.33 – Docking System DisplayReproduced with permission from Harbour & Marine Engineering

● speed warning indication lights, whichcan be totally independent or integralwith the large display board.

3.4.8.2 GPS Docking SystemAn example of a new development utilisingthe latest in GPS technology is theSmartDock® PILOT. This system calculatesthe vessel’s position and displays it on anelectronic chart on the pilot’s laptop,


114

independent of the ship instruments. Thismakes Pilots more familiar with the systemand less reliant on the ships equipment,which may not be accurately calibrated.

As the laptops are self-contained andportable, the pilot will have access tonavigation data from all over the bridge.

3.4.8.3 OptimoorThere are also mooring analysis softwarepackages such as “OPTIMOOR” by TensionTechnology International Ltd. Thesepackages are designed for use by ship andterminal personnel to assess whether theship can safely moor at that berth before itarrives. The packages follow the mooringguidance issued by OCIMF and include theOCIMF wind and current coefficients fortanker moorings. At tanker jetties,OPTIMOOR can be used to assess theadequacy of the ship’s mooring equipmentfor the berth’s mooring arrangement. Windor current limitations and details of tidescan be used to predict mooring line tendingrequirements.

3.4.8.4 Vessel Drift-WarningMonitoring

Following the mooring of the vessel, thedocking system can be switched to drift-warning mode. Should the vessel drift a pre-determined distance away from the fenders,alarms will be raised to notify both jetty andship’s staff.

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Figure 3.34 – Bulk Carrier Docking UsingLaser Docking System


Figure 3.35 – SmartDock® DAS Screenfor an LNG Vessel


Figure 3.36 – SmartDock® Pilot ScreenReproduced with permission from Harbour


offering piloting assistance that is berthindependent.

To meet the demands of navigating anddocking large vessels, the measurement oflow speeds, precise heading and Rate ofTurn is of utmost importance to the pilot.SmartDock® PILOT derives thesemeasurements using GPS and Rate Of Turn(ROT) Sensors integrated via an advancedKalman filter to create an independentsystem that is more accurate than ship-installed gyros, ROT sensors, and speedlogs.

Data provided by systems such asSmartDock® PILOT system are completely

3.5 Tugs and the Safetyof Tankers

There are four areas where tugs are used tomaintain the safety of tankers:

Escorting tankers This is carried out from the tanker’sarrival position off the port, at speeds of7 to about 12 knots.

Ship-handling and berthing assistanceManoeuvring and berthing assistancewithin the port or harbour.

Offshore berthing Assistance in mooring and hose handlingoperations at offshore terminals withSingle-Point Mooring (SPM) or Single-Buoy Mooring (SBM).

Fire-fighting and stand by dutiesProvision of a rapid response to coverfire-fighting and emergency towing withinthe port area.

3.5.1 Escorting TankersTankers are escorted by tugs in high traffictanker routes and in narrow or dangerouswaters. The basics of ship-handlingcontinue to apply but the key areas for thetug are available speed, towline force andavailable time.

Tanker escorting is comparatively new andits need was proposed after a number ofsevere tanker accidents occurred. The casefor escorting is based on a risk analysis ofpossible and likely accidents and theirseverity. The primary benefit provided by anescort or safety vessel is immediateassistance to the vessel, achieved by


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Figure 3.38 – A Tanker in a Confined Environmentally Sensitive Waterway

Figure 3.37 – Laser Sensor Installed onElevator System to Accommodate

High/Low Tidal Range.Reproduced with permission from Harbour


connecting the tug to the tanker. The bestconnection position is at the stern of thetanker. Rudder or engine problems are afrequent cause of incident, as braking andsteering forces are exerted most efficientlyfrom here. In certain cases, such as fire orexplosion, connection to the stern of thevessel may be hampered and the alternativeis to make fast at the bow of the tanker.

While under escort a tanker must reduce toa speed that allows the tug to provideefficient assistance in the event of loss ofcontrol, taking into consideration the inertiaforces of the tanker. The tanker’s speedmust be sufficient to allow control of thevessel in the prevailing conditions.

An escort vessel should:

● Be able to quickly and safely make fast atthe stern of a tanker at escorting speed

● under normal circumstances, follow thetanker with as little interference to thebehaviour of the tanker as possible

● be able to immediately exert an effectivebraking force

● be able to immediately apply activesteering forces that exceed the tanker’sown capabilities under the prevailingconditions

● be able to make fast at the bow of atanker safely and at high speed, takinginto consideration the existing pressureconditions in this area

● be able to immediately fight any fire onboard the tanker while moving

● be able to take immediate appropriateaction in order to prevent any oil spillspreading.

3.5.2 Ship-handling and BerthingAssistance

The modern tug can deal with:

● The forces interacting between the tugand the vessel to be assisted at slow andhigh speeds

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Figure 3.39 – Tug Escorting a Tanker


118

Figure 3.40 – Tug Assisting a Chemical Tanker to Berth

● the two principle forces acting on the tugwhilst berthing, ie the propeller thrustand towline force. The aim is to minimisethe danger of capsizing under alloperating conditions.

3.5.3 Offshore BerthingAt offshore terminals with Single-PointMooring (SPM or SBM), oil is handled byfloating hoses.

Tugs are used to connect the hose lines tothe tanker. As the hose lines are easilykinked or squeezed, control must beextremely sensitive and precise. Heavy seasand swells in these locations frequentlymakes the task more difficult.

3.5.4 Fire-fighting andStand-by Duties

The modern tug is an ideal fire-fightingvessel as a vast amount of power can bediverted from the main engines to power thefire-fighting pumps without rendering theengines either ineffective or incapable ofkeeping the vessel under control. In thecase of escort vessels, effective fire-fightingcould even begin while the tanker is still onpassage or while reducing her speed.

At many loading and discharge berths it iscommon to have the tug acting on stand-byduties. This allows it to be quickly deployedif it is required for fire-fighting or emergencyduties. In the event of oil pollution, the tugcan be used to position skimmer arms,operate skimmer vessels without self-propulsion or to arrange oil spill booms.

3.5.5 Types of Tugs3.5.5.1 Conventional TugsStill operated in many parts of the world, theconventional tug is recognisable by thefollowing features:

● Its propulsion unit, which is generally asingle screw propeller and a standardrudder. This is similar to the configurationon many merchant ships today, thoughmore recently they may have beenreplaced by a controllable pitchpropeller.

● the location of the towing hook, which isgenerally fixed amidships, tends to limitthe tugs manoeuvrability and leaves it atrisk of capsizing (or girting) if it gets in toan awkward position, particularly if thevessel she is connected to is turning. Therisk of capsize to the tug is the same,regardless of whether she is connectedto the bow or the stern.

3.5.5.2 Tractor TugsA tractor tug uses multi-directionalpropulsion units that consist of controllablepitch rotating blades located below theconning position in the wheelhouse. Aleading manufacturer of this type ofpropulsion unit is Voith Schneider and, inthe hands of a skilled tug Master, theycreate a highly manoeuvrable vessel. Inaddition to the high degree ofmanoeuvrability that the propulsion unitoffers, the towing line is operated from awinch drum, which the tug master canoperate remotely from the wheelhouse, toincrease or decrease the length of towlineas required.

3.5.5.3 Azimuth Stern Drive TugThis type of tug combines the benefits ofboth conventional tugs and tractor tugs. Themain propulsion units are located aft, like aconventional tug, but these units are two

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Figure 3.41 – Modern Tug with Fire -fighting Capabilities

Figure 3.42 – Conventional Tug

Tugs are generally rated by “Bollard Pull”,This is the force produced by a tug, intonnes, when pulling against a staticbollard.


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rotating azimuth systems, similar to those inuse on a tractor tug. The towing positioncan be either located amidships like aconventional tug or located forward. Whensecured to the forward winch, this tug canpush or pull (tow) and, when pulling, gainsa huge lever effect from the distance thatthe propulsion units are from the forwardwinch position.

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Figure 3.44 – Azimuth Stern Drive Tug

Figure 3.43 – Tractor Tug


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Figure 3.45 – Azimuth Stern Drive Tug

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Documents

MOORING - seaways.net.auseaways.net.au/wp-content/uploads/2015/11/Jetty-Safety-book.pdf · 3 Mooring The operations associated with berthing a ship alongside a berth or jetty are