Stability and Global Strength

8/3/2019 Stability and Global Strength

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TMC (Marine Consultants) Ltd. Stability & Global Strength

Telephone: +44 (0)20 7237 2617, email: [email protected]

www.tmcmarine.co.uk

Ship Stability & Global Strength

SEAMASTER (Program Suite)

SEAMASTER was originally written in 1979 by the founding partners of TMC, and after

continuous upgrading over the years this hydrostatic program remains one of the core in-house

programs. The program is used for retrospective investigations on opinion work and also as a pro-

active tool during salvage consultancy. The program is also sold to clients as an onboard loading

instrument and has been installed on many vessels over the last 20 years.

TMC carry out intact and damaged stability and longitudinal strength analyses using their in-house,

hydrostatic program SEAMASTER, which has been class approved as a loading instrument on a

case by case basis for specific ships. The program performs stability and strength analyses,

calculated using trimmed hydrostatics.

Figure 1: SEAMASTER Program screen view showing vessel profile with stability and

longitudinal strength results. Stability criteria may be presented in terms of actual values (shown)

or conventional criteria requirements.


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Program Modules

TMCs own additional modules are integrated seamlessly within SEAMASTER depending on

vessel type and options required to be considered. The modules are listed below:

SEACOC cargo operation control which simulates any proposed loading/ballast sequenceand reports automatically on the stability and strength at all intermediate stages;

SEAFLOOD immediate access to the effect of flooding any combination of compartments; SEADAM investigates stresses in the hull, damaged or intact, including wave loading effects

and also the consequences of grounding;

C-PLAN a stowage planning tool which stores container details in database, automaticallystows containers, and allows the rapid assessment of loading options;

T-PLAN for planning the loading of tankers to be partially discharged at a number of portsand also generates Tanker Forms in accordance with commercial practice;

COMLASH analyses all container and lashing forces. This is an autonomous module, butmay be used in conjunction with C-PLAN, above (see separate download for COMLASH).

Data Required

The amount of plans (as-built and approved) and supporting documentation required depends on

the level of modelling complexity required, and also how comprehensive various sources of dataare. Accordingly, some or all of the following data will normally be required:

- Lines plan / body plan / offsets / other approximate source (e.g. docking plan);- General arrangement;- Capacity plan;- Profile and Decks plan;- Midship Section;- Shell expansion;- Stability book and longitudinal strength information (including comprehensive tabulated

hydrostatics, lightship weight distribution, standard loading conditions);

- Actual scantlings (e.g. thickness gauging report);- Loading condition;- Draught surveys;- Damage status report;- Sea wave data (for specific wave loads);

In cases where there is no data available, it is often possible to construct a model using an existing

model from our extensive library, as TMC has a database of some 400 models of various ship types

created using SEAMASTER, from which a geometrically similar, or even sister, ship may be

derived.


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Modelling

Hull Form the SEAMASTER program initially requires a hull form to be generated which is

defined using transverse sections. Input data is required in the form of hull offset data obtained

from either lines plan, body plan or tabulated values (other sources such as docking plan are alsouseful). Plans can also be inputted manually, imported via spreadsheet or via digitiser. Shell plate

thickness can be attributed at the end in order to distinguish moulded and extreme dimensions.

Hull Appendages external appendages such as skeg, rudder, bar keel, etc and hull cut-outs (e.g.

tunnel thrusters) that influence buoyancy or which may be required for graphical or geometric

purposes can also be defined. The program has been proven by validation of hydrostatics with

published design data over the years, together with class approval procedures on a case-by-case

basis. Usage onboard as a loading instrument, as well as for salvage cases and for retrospective

analysis has confirmed the high degree of accuracy of the program in cases where the hull form and

internal compartment geometries are known.

Internal compartments - in most cases, internal spaces are required to be defined, either for

geometrical reasons alone or because of complex filling of bulk cargo/liquid and associated

permeability levels. Internal tanks and spaces are defined by boundaries comprising flat or

horizontal plane surfaces, or more complex polygons where boundaries are irregular, and in any

orientation.

Loading either by geometrical compartment loading (see above) or by non-geometrical point

loads which can, if required, be assigned a distributed length to simulate a uniformly distributed

load over the boundary defined. Also, a free surface moment of a non-geometrical point load can

be assigned. Calculated volumetric capacities and free surface moments may be validated against

vessel data, where available.

Figure 2: SEAMASTER Program water ballast and break bulk cargo summaries.


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SEAMASTER enables access of principal loading conditions, entry of deadweight items and

analysis of other intact/damage loading with vessel draught, trim, heel, stability and longitudinal

strength continuously updated and shown. The program provides detailed stability and strength

results giving a comparison between the calculated condition and the required or observed (draught

survey) condition.

Figure 3: SEAMASTER Program stowage plans.


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Figure 4: SEAMASTER Program screen view showing stability and longitudinal strength results.

In summary, SEAMASTER has the following functionalities and calculates:

Draughts (at end perpendiculars, amidships, LCF, draught marks or at user defined locationson the hull sides/centreline);

Trim and list (dimension/angle); Displacements and hydrostatics, with sounding tables; Lightship weight breakdown; Deadweight items and groups breakdown; Intact/Damage compartments breakdown; Geometric loads (% compartment filling, or by weight); Non-geometric loads (varying length and specified centre of gravity); Free surface effects; Grain stability; Density variations; Suspended (crane) loads; Longitudinal strength (shear force, bending moment, torsion moment); Deflection amount (hog/sag).


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SEACOC (Cargo and Ballast Control)

Cargo operation control is a feature only available with the SEACOC module. This checks all the

intermediate stages of any complex loading/ballast operation. Entries for various compartments or

tanks are independent. To simulate loading (or discharge), start and finish times are required for

every hold. Ballasting and de-ballasting operations may be run simultaneously. Minimum steptimes are in 0.1hour intervals (6 minute intervals).

Other instances where SEACOC may also be used include transient voyage conditions, after

allowing for consumables, and also as part of a salvage tool in SEADAM after incorporating

aspects such as flooding rates and grounding.

Figure 5: SEACOC Module cargo operations control.


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SEAFLOOD (Flooding)

The flooding module in SEAMASTER allows for the ships waterline to move well above, or sink

well below, the assigned draught limit with excessive trim and list. Free-flooding of any space is

computed including the flooding of normally dry spaces such as the engine room, cargo spaces,

cofferdams and void spaces. A time simulation of the effect of water ingress can be chosen as analternative to the free-flooding option, which gives the state of the ship at any given time after the

size and position of the damage is input. Time-simulated flooding can be interrupted at any point in

time, which is useful for retrospective investigation, and can be combined with features such as air

pressurisation or applied pump rates for salvage.

Figure 6: SEAFLOOD Module damaged tanks and flooding.

In summary, the SEAFLOOD module has the following functionalities and calculates:

Flooding of compartments (partially filled, filled, stage flooding by step/time) andassociated draught, trim and list;

Observed draughts option; Damage hole size (within external or internal boundaries);

Flooding modes (down-flooding, up-flooding, cross-flooding; Hole size definition (exterior boundary / interior boundary); Floodwater ingress rate (time domain, cargo miscibility); Liquid cargo outflow rate; Permeability (volumetric); Pump rate option; Air pressurisation option; Geometric loads (% compartment filling); Non-geometric loads (unit/point loads); Grounding support type (single/two point, line); Tidal effects; Damage longitudinal strength (shear force, bending moment, torsion moment); Deflection amount (hog/sag).


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SEADAM (Wave Induced Loading)

Although the permissible hull bending moment and shear force for sea-going conditions take into

account the effect of waves, these values are assigned by Class in accordance with their rules (i.e.

maximum wave amidships). Wave loading for this purpose are arbitrarily determined for design

purposes. However, it is sometimes necessary to know what the actual loads are for a given waveheight, and heading. This may be particularly important following grounding, or if hull damage has

been sustained resulting in flooding resulting in reduced bending strength and stiffness (and/or

reduced shear and torsion strength).

Figure 7: SEADAM Module wave induced loading.

In order to determine wave-induced applied loading, SEADAM requires two of the main parameters

to be inputted: i.e. wave height, wave length and wave period. Accordingly, the wave shape

including slope is obtained. The crest location along the hull and incident angle may also be defined

and the influence on longitudinal vertical bending determined.

The ship is poised on a 2nd

order Stokes wave which is similar to a trochoidal wave. The wave

parameters can be altered as described above, and the related parameters are adjusted automatically.

Vessel draught and trim are automatically updated to obtain static equilibrium, and although this

ignores dynamic effects, any error involved is generally not considered large in the context of

uncertainties surrounding wave definition. Limitations of linear motion theory apply concerning

very high waves and the varying shape and size of hull.


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SEADAM (Grounding)

The seabed is defined either as Rock (i.e. force concentrated at the two specified points) or Sand

(force distributed over the whole length specified). If the water depths specified are less than

calculated free floating draughts, then the vessel is grounded.

All draught, stability and strength parameters are calculated based upon the location and value of

the ground reaction. If the water depths are less than the draught at one end or at a specific location

along the hull, then the vessel will trim (heel) about this location as a function of the longitudinal

(transverse) centres of buoyancy and ground reaction, calculated by the program using combined

moments of the displacement and the ground reaction.

If the water depths specified at the draught marks are all greaterthan the calculated free floating

draughts, then the vessel will automatically float free and the ground reaction is displayed as zero.

The calculated draughts in this case will simply be those obtained by hydrostatics in the normal

way. Tide may be incremented by gradual amounts (+/- 1 cm) and the stability and strength is

updated automatically.

Figure 8: SEADAM Module - screen view showing vessel during grounding analysis, with stability

and longitudinal strength results.


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SEADAM (Scantlings and Stresses)

SEADAM calculates the bending stress in still water or waves (any incident heading and wave).

Regarding structural response, stresses are calculated for each of the longitudinally continuous

structural members based on classic beam theory.

For an upright and intact hull, or in cases of symmetrical port/starboard damage and/or symmetrical

flooding, the resultant stresses calculated are about the horizontal axis with an accompanied vertical

shift of the neutral axis.

Damaged or wasted structure is represented in SEADAM by changing the efficiency (% loss) of

individual plates and stiffeners. For asymmetrical damage or flooding, the hull girder bending

characteristics change, with the neutral axis shifting vertically and to one side (i.e. vertically moved,

offset to the centreline and angled to the horizontal). In this case, the resultant stresses calculated

allow for combined bending about both the horizontal and vertical axes.

In summary, SEADAM calculates the residual strength capability of the vessel and this is used inthe context ofapplied loading by taking into account the loading condition and free surface effects,

and any flooding or grounding as determined in the SEAFLOOD module.

Figure 9: SEADAM Module intact/damage structure and calculated bending stress.


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Figure 10: SEADAM Module damaged midship section and revised neutral axis.

In summary, the SEADAM module incorporates the following functionalities and calculates:

Geometrical definition of transverse section scantlings (continuous structural members); Reduction of scantlings due to wastage; Missing/deformed structure due to damage; Wave induced loading; Residual bending strength and stiffness calculated (i.e. applied stresses in the individual

structural members resulting from the longitudinal, vertical bending of the intact/damaged

hull girder);

Damage longitudinal strength (shear force, bending moment, torsion moment); Deflection amount (hog/sag).


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Figure 11: SEADAM Module damaged section and calculated residual strength.

Documents

Stability and Global Strength