PFN305 - Lashing Forces 1

LASHING FORCES - EASEACON LASHING MODULE

Prepared by : Capt.Goran Belamaric, Mag. Eng.

Master Mariner

Split, listopad.2011.

Faculty of Maritime Studies - SPLIT

SUMMARY

> 1. Theoritic Approach of ship‟s motion and

lashing forces.

> a. Review of the basic parameter & forces based on

BV rules

> b. Deck securing

> c. Cargo Holds Stowage

> 2. Lashing module on Easecon

> a. Presentation of calculation display

> b. Example of optimisation of bay plan

Capt.Goran Belamaric, Mag. Eng.

EASEACON LASHING MODULE

Container loss at sea

THE BASIC CAUSE of many collapsed container stows

and the loss of containers overboard can be

attributed to inadequate or incorrect securing,

resulting from either lack and/or misunderstanding of

instructions/information on-board concerning

applicable stowage and securing arrangements



Common reason why stows collapse(source “West of England P&I”)

> Container stacks being too heavy and too high overall, exposing the lower containers to excessive transverse racking and compressive forces due to the tipping effect.

> Containers at the top of the stack being significantly heavier than those below.

> The wind force acting on the outboard stacks not being taken into account.

> Loose lashings allowing the stacks to tilt and arrest sharply as the slack is taken up.

> Two 20 foot containers placed in a 40 foot slot, leaving insufficient space for lashing rods to be fitted to the inward facing ends.

> Out of gauge containers, such high cube boxes, mixed randomly with standard units

> High cube boxes stowed cumulatively

> Securing equipment not applied correctly

> Portable and/or fixed securing equipment in damaged or worn condition

> The movement of inadequately secured heavy cargo within a container resulting in damage to the container walls



Ship motion

Ship at sea may move in six different direction simultaneously.

Rolling, pitching and heaving generate the highest forces



Pounding

Heavy pounding at

bow and stern

increases considerably

vessel motion and

lashing forces



Ship movement and forces on

containers and lashing

> Rolling

> affects container corner posts, twistlocks, hatch covers and the deck by inducing compression and tension forces.

> The motion also creates transverse racking forces which, if excessive, may distort the walls and ends of container frames

> Deck cargo racking forces are resisted primarily by lashing rods and turnbuckles

> Pitching

> Forces are similar to forces created by rolling, but acts longitudinally

> Heaving

> Increases tension and compression forces on twistlocks and container posts



Acceleration forces> When considering the shipboard stowage location of

cargo items, acceleration forces should be borne in mind:

> Lower accelerations forces occur in the mid-ship sections, lowest cell position under deck and as close to the centerline as possible

> Higher accelerations forces occur at ship's extremities, high on deck and in the outrigger slots by the ship's side.

> The higher is GM the greater is righting moment when ship is rolling, consequently transverse acceleration forces intensify with increase of GM

v = speed (knots)

L = Length between perpendiculars (m)

Acceleration correction for ship length and speed



Forces effecting container stack

Standard ISO 20’ 40 ’

Racking force door end 150 kN 150 kN

Racking force doorless end 150 kN 150 kN

Racking force side walls 75 (150*) kN 75 (150*) kN

-Corner post compression 864 kN 864 kN

*closed box containers



CLASS REQUIREMENTS

The following forces are to be considered:

> static (gravity) forces, (20’ – 24 T, 40 ‘ – 30, 48 T)

> inertial forces due to ship motions (roll and heave,

pitch and heave, GM=0.07B)

> wave impact forces (forward of 0.25L, lashing strength

increased for 20%)

> wind forces, (20’ – 10kN, 40’ – 20kN)

> forces imposed by the securing arrangements.



Lashing bridges

Advantages:

> shorter lashing bars

> easier and safer lashing

> Possibility to secure fourth tier

> Heavier containers may be loaded on higher tiers

> Lashing system is simple

> Easy access to reefer containers on 2nd tier

Disadvantage:

> Lashing bridge is fixed structure while containers are moving with hatch covers, resulting effect is that lashing become slack or takes excessive load

> Difficult maintenance of lashing bridge - painting

> Additional weight on deck, reduces stability

> 45‟ container can not be loaded on 1st and 2nd tier



Lashing in holds is done with cell guides



Lashing system

Container vessel

should be equipped

with approved

lashing system.



On deck lashing


15EASEACON LASHING MODULE

Capt.Goran Belamaric, Mag


DUAL FUNCTION TWISTLOCK

TURNBUCKLE

LASHING BAR

ISO LASHING GAP

BASE TWISTLOCK

LOCKABLE STACKING CONE



Cargo securing manual

> The International Convention for the Safety of

Life at Sea (SOLAS), 1974, Chapter 6 (Carriage

of Cargoes), Regulation 5 and Chapter 7,

Regulation 5 (Carriage of Dangerous Goods),

requires that cargo units, including containers,

shall be loaded, stowed and secured throughout

the voyage in accordance with a Cargo Securing

Manual approved by the Administration.



CSM permitted vertical container weight

distribution


Vertical distribution in CSM

GM < 1 m GM < 2.5 m



Mixed stowage vertical weight

distribution

20 FEET -> 2 tiers 20 FEET -> 3 tiers



CSM - loading in holds



Mixed stowage stack calculation

16

22

18

24

14

26

MAX STACK LOAD 40’

MAX POINT LOAD 40’

CURRENT POINT LOAD 40’ AFT

120 MT

30 MT (max stack load / 4)

(16+22+18+24) / 4 = 20 MT

CURRENT POINT LOAD 40’ FWD (16+22+14+26) / 4 = 19.5 MT

AFT FWD

19.5



Effect to on stack due to shifting of

mass, wind force, missing lashing



Easeacon – „Bay planner‟ operates in

two modes: (Ctrl+L switches Lash mode while focus is on Bay Planner window)

STACK MODE LASH MODE

When Easeacon is in STACK MODE, stack alarm (red color) appears if

stack weight is exceeded. In LASH MODE red color means that at least one

lashing force is excessive.



Bay 11, lashing / no lashing

NO LASHING WITH LASHING



Bay 11, Red flag on GAP end

LASHED SIDE GAP END

When 20’ containers are loaded with ISO gap then lashing is from one side of

container only. Unlashed side shows increased racking force. If container is lashed

from both sides then Easeacon warning for GAP forces may be ignored.


Bay 11, Wind or no wind

WIND NO WIND

Identical stack when loaded in inner slot will have smaller lashing forces then stack

loaded at a ship side due to wind effect. FORCE BY WIND = 1kN/m2


EASEACON and max stack

Easeacon often shows some

forces to be 100 to 105% although

stack is loaded as per cargo

securing manual.

Small differences are acceptable

due to different hydrostatic data

used in calculation.

Even Easeacon lashing test

condition exceed lashing forces in

some parameters.

CSM VALUES



Change of GM and lashing forces

With change of GM acceleration forces changes, consequently all

lashing forces will change accordingly



Change of container COG

COG at 45% COG at 60%

Easeacon default container vertical center of gravity is at 45% of container height.

Value might be changed to 50 or 60% for safer more pessimistic GM calculation.

Change of VCG reduces calculated GM consequently lashing forces are lower.



Warnings

> Exceeding the permissible stack mass may result in:

> overstressing hatch top construction,

> overstressing stowage and securing devices,

> damaging containers,

> loss of containers overboard.

> Neglecting permissible sequences of masses in stacks may result in:




> Failing to properly lock Twistlocks may result in:


> Failing to apply lashings as lined out in the relevant bay plans may result in:

> overstressing Twistlocks,

> damaging containers (racking),


> Exceeding the recommended pretension of 5 kN in lashings may result in:

> overstressing these lashings,



> Exceeding of the max. GM-value in the stowage plan result in:

> higher transversal acceleration,


> damaging containers.


PFN305 - Lashing Forces 1

Documents

compression forces

tension forces

compressive forces

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transverse racking forces

lower accelerations

highest forces capt

basic parameter forces