CN30-8 (Classification Notes - Strength Analysis of Hull Structures in H...
CN30-8 (Classification Notes - Strength Analysis of Hull Structures in H...
CN30-8 (Classification Notes - Strength Analysis of Hull Structures in H...
No. 30.8
STRENGTH ANALYSIS OF
HULL STRUCTURES IN
AUGUST 1996
DET NORSKE VERITAS AS (DNV AS), a fully owned subsidiary Society of the Foundation, undertakes classification and
certification and ensures the quality of ships, mobile offshore units, fixed offshore structures, facilities and systems, and
carries out research in connection with these functions. The Society operates a world-wide network of survey stations and is
authorised by more than 120 national administrations to carry out surveys and, in most cases, issue certificates on their behalf.
Classification Notes
Classification Notes are publications which give practical information on classification of ships and other objects. Examples of
design solutions, calculation methods, specifications of test procedures, as well as acceptable repair methods for some
components are given as interpretations of the more general rule requirements.
An updated list of Classification Notes is available on request. The list is also given in the latest edition of the Introduction-
booklets to the "Rules for Classification of Ships", the "Rules for Classification of Mobile Offshore Units" and the "Rules for
Classification of High Speed and Light Craft".
In "Rules for Classification of Fixed Offshore Installations", only those Classification Notes which are relevant for this type of
structure have been listed.
August 1996
August 1996
reference to drawings
description of model including boundary conditions
description of loads
description of results
conclusion.
2.2 Procedure
Figure 2-1 Frame or compartment for web frame
2.2.1 analysis
Calculations of the transverse web frame strength should
generally be performed for a typical frame in the midship
2.3 Load conditions
region for vessels less than 50 m. For larger vessels, and for 2.3.1
vessels with unusual arrangement, several sections along the
length of the vessel should be considered. Load Condition 1, sea pressure, maximum load on decks
(LC1)
2.2.2 This load condition is shown in Figure 2-2, and may be
When maximum response for a transverse web frame is to be decisive for side and deck structures.
established, a frame in the middle of a compartment is
normally analysed, see also Figure 2-1. The design pressures due to cargo loads (including structure)
are to be taken as :
Acceptable calculations may be performed by
p v = H ( g 0 + 0,5a v ) (kN/m2)
2-dimensional beam element framework analysis
3-dimensional framework or H = 0,35 t/m2 for accommodation decks, see also the Rules
finite element calculations. Pt.3 Ch.1 Sec.2 C.
August 1996
For wheel loaded decks, the actual wheel loads should be 2.3.2
applied. The worst combination of wheel loads on one frame Load Condition 2, symmetric bottom slamming (LC2)
should be analysed.
This load case is shown in Figure 2-3 and
In addition to a load case with point loads representing wheel
loads, an equivalent evenly distributed load should be Figure 2-4, and may be decisive for the bottom structure.
considered (minimum 4 kN/m2). The load case investigates the effect of symmetric impact
pressure on one frame, using the average impact pressure
Forces transferred to the analysed frame from surrounding values as given by the Rules.
structure should be applied as point loads (e.g. from
longitudinal girder when considering frame in way of The deck load distribution is the same as in LC1.
pillars).
The bottom slamming pressure is to be found from the Rules
Design sea pressures are to be taken in accordance with the Pt.3 Ch.1 Sec.2 C200-C300, and be taken as the greatest of
rules Pt.3 Ch.1 Sec.2 C500, and should be applied on all bottom slamming and pitching slamming. For frames
external surfaces. positioned where the forebody side and bow impact pressure
is largest, the loads must be applied up to main deck or
vertical part of craft side.
Figure 2-2 Load condition 1 (definition of point loads Figure 2-3 Load condition 2. In midship region, bottom
from surrounding structure indicated schematic for one slamming pressure applied. In foreship area bow impact
pillar on one deck) pressure applied
August 1996
0 .8 (m)
H L = 0 ,2 2 L ( k c L)
1000
2.3.3
Load condition 3 and 4, asymmetric bottom slamming (LC3
and LC4)
This load case may be decisive for the bottom structure. The
load case investigates the effect of asymmetric impact
pressure on one frame, using the average impact pressure
values as given by the Rules. In the bottom area design loads
are applied on only one side at the time (inside slamming or
outside slamming).
The design load area is taken as half of the area used in LC2.
In cases where a 3-dimensional analysis is performed, the
slamming pressure only needs to be applied to one frame
(sea pressure on other frames). Figure 2-5 Load condition 5
2.3.4 2.3.5
Load Condition 5, flat cross structure slamming (LC5) Load condition 6, transverse racking (LC6), monohull
vessels only.
This load case is shown in Fig.2.5, and may be decisive for
the cross structure of a multihull vessel. The load case This load case represents the vessel in heeled condition, and
investigates the effect of impact pressure on the wet deck, may be decisive for the lower side frames of a monohull. If a
using the average impact pressure values as given by the global racking calculation has been carried out, the
Rules. transverse and vertical displacements of decks and side
should be given as input, see also 3.3.10 regarding procedure
The deck load distribution is the same as in LC1, and the sea for global racking calculations.
pressure from water line to wet deck is taken from the Rules.
A simplified check may be performed as indicated below.
The tunnel top slamming pressure is to be taken from the
Rules Pt.3 Ch.1 Sec.2 C400. Design sea pressure is applied The deck vertical design load is taken as:
from centre line to design water line on the outside of the
p v = H g 0 co s (kN/m2)
hull.
The design load area is taken as the frame spacing times the
distance between the hulls. The deck horizontal design load is taken as:
If the height from the water line to the wet deck is more than p h = 0 ,5 H a t (kN/m2)
August 1996
at = design transverse acceleration (m/s2), to be taken from model transverse frame at pillars or between pillars
the rules Pt.3 Ch.1 Sec.2 B302 for multihull vessels in forced model from side to side or half-model
roll, and from Rules for Classification of Ships for monohull longitudinal position(s) of modelled frame(s).
vessels. See also 3.2.10.
2.4.2
= maximum roll inclination.
It is assumed that correlation between individual 2-
2.3.6 dimensional models is proven to be satisfactory (e.g. deck
and transverse frame models).
Load condition 7, asymmetric deck load (LC7)
The symbols used are described in Figure 2-8.
This load case is shown in Figure 2-6 and Figure 2-7, and is
only relevant for deck grillage including pillars.
2.4.3
Deck load as for LC1. Figure 2-9 and Figure 2-10 show typical models of
transverse frames for a monohull and a multihull. Racking is
not considered critical for a multihull vessel, and a half-
model with symmetry conditions at centre line is normally
modelled, while a full web frame is modelled for a
monohull.
2.4.4
Areas of the web frame with large curvature should be
modelled with increased number of elements.
August 1996
EA
Figure 2-9 Transverse web frame model of monohull,
ls = (mm)
K
complete frame modelled between pillars.
A = cross-sectional area of spring element
2.4.7
Transverse frames are often connected by longitudinal deck
girders, bottom girders and longitudinal bulkheads in the
cross structure of multihull vessels. In a 2-dimensional
model, this connection must be represented by springs. The
spring stiffness of slender elements may be calculated by the
following formula :
E
K= (N/mm)
( n + 1)l 3 2,6( n + 1)l
+
384 I 8 As
Figure 2-10 Transverse web frame model of catamaran, l = distance between effective transverse bulkheads
only half of the frame modelled at pillar row. Valid only (fixed box ends) or distance between pillars for
for symmetric load conditions deck beams supported by pillars
E
K= (N/mm)
2,6( n + 1)l
8 As
August 1996
2.5.2
The mesh fineness and element types used in finite element
models must be sufficient to allow the model to represent the
deformation pattern of the actual structure with respect to
matters such as:
2.5.3
In order to obtain sufficiently accurate results, the mesh
fineness should represent the true web frame structure. This
means modelling plating, webs and flanges as separate
elements. It is acceptable that only one frame is modelled
with a fine mesh (the highest loaded frame).
2.5.4
In order to properly consider shear and bending, 3 elements
should be used over the height of the web of the frame, and
with an element length to breadth ratio of 3.
Figure 2-11 Rigid element end
In areas with curved flanges, the element length should be
approximately equal the stiffener spacing.
2.5.5
In areas with discontinuities (ends of flanges, knuckles,
brackets), the model should represent the discontinuity with
increased mesh fineness An alternative solution is to perform
separate analysis by separate local models of such details.
2.5.6
Calculated stresses based on constant stress elements may
have to be considered with respect to the stress variation
within each element length.
2.5.7
Symmetry conditions are to be applied at each end of the
model. If the model only covers half of the breadth of the
vessel, symmetry conditions should also be applied at centre
line. Boundary conditions representing vertical support
should be added as vertical shear forces at the end nodes of
Figure 2-12 Effective flange consideration the model, in order to obtain a balanced model.
August 1996
2.6.2
The allowable stresses given above assume that appropriate
considerations and conditions are taken with respect to the
model definition and result analysis. In particular the
following should be noted:
August 1996
3.1.4 This load case is based on the difference between weight and
buoyancy in still water condition at design draught. A
General guidelines given by this Classification Note are only loading manual is to be prepared to document the various
applicable for High Speed Light Craft, and references are still water conditions. The load condition should be
given to the Rules for Classification of High Speed and Light accurately modelled to avoid trim of the model due to
Craft. different position of Longitudinal Centre of Gravity and
Longitudinal Centre of Buoyancy. The correct transverse and
3.2 Design loads longitudinal mass distribution should be used.
3.2.1 The load case is a supplement to LC2, LC3 and LC4.
The load calculations should be based on the Tentative Rules
for the Classification of High Speed and Light Craft 1996. 3.2.5
Design values should be agreed between designer and DNV Load Condition 2, longitudinal hogging moment (LC2)
prior to final analysis.
This load case is shown in Figure 3-1 and will be decisive
3.2.2 with regard to allowable longitudinal stresses and buckling
Alternative loads, i.e. from direct hydrodynamic capacity in the bottom area.
calculations, may be used for design calculations. Alternative
The longitudinal hogging moment may be derived from :
design load formulations must be agreed with DNV in each
case. Wave load analysis programs and their application will rule crest landing formula, Pt.3 Ch.1 Sec.3 A200
only be accepted on a case to case basis. rule hogging moment, Pt.3 Ch.1 Sec.3 A500
direct calculations of hydrodynamic loads
3.2.3
Table 3-1 describes typical loading conditions and their Only the largest needs to be analysed.
applicability with respect to type of design. Each load case is
described below. An example of acceptable modelling of the load case is
shown in Figure 3-1. The mass distribution of the vessel is
Additional load conditions may be considered relevant. given (go+acg) vertical acceleration, and this load is balanced
with buoyancy line loads around LCG.
August 1996
1 3
negligible reaction forces at supporting nodes. Fy 1 3
Fy x
2 4 2 4
Correct transverse and longitudinal mass distribution is to be 1
used. Fy
4
x+ y=h
Longitudinal and transverse mass Aft
distribution multiplied by (go + acg) 3
M split = Fy y + M s ,keel
4
LCG
1
M s ,keel = Fy {( x + y ) ~x}
4
Bouyancy applied at
Boundary conditions only bottom around LCG
to prevent rigid body movement x+ y =h
BASELINE
~
x
Figure 3-1 Load condition 2, only half of the vessel x~ = the mean offset line established by measuring the distance
between the keel and the baseline
shown
NOTE : - the sum of the horizontal forces is to act at 75% of the draught
- no mass is required to be modelled in this condition
3.2.6
Load Condition 3, global sagging moment (LC3)
Figure 3-2 Load condition 4, transverse split, split
This load case may be decisive with regard to allowable outwards shown
longitudinal stresses and buckling capacity in the upper
3.2.8
decks.
Load Condition 5, torsion moment / pitch connecting
Modelling of this load case may in principle be as for LC2. moment (LC5)
3.2.7 This load case is shown in Figure 3-3, and may be decisive
for the cross structure. As indicated in Figure 3-3, the torsion
Load Condition 4, transverse split force (LC4) and pitch connecting moments are combined in the same
load condition. The load condition may be modelled without
This load case is shown in Figure 3-2 (split force acting
a mass distribution.
outwards) and is decisive for the structure between the hulls,
the side and bulkheads for a multihull vessel. The load case A full structural model should be applied.
represents horizontal wave loads acting on the hulls.
August 1996
ph
qs
qb
August 1996
August 1996
4. WATERJET DUCTS
4.1 Introduction
4.1.1
The reaction forces from the waterjet nozzles need to be
transmitted into the hull structure in a manner for which
adequate strength and fatigue life of critical details can be
ensured through careful design.
4.1.2
For steerable jet units the reaction forces will typically arise
from acceleration (thrust) and manoeuvring actions.
For booster nozzles with no steering function reaction forces
arise from acceleration (thrust) forces only. Figure 4-2 Typical duct / jet nozzle configuration critical
areas
Additional to this vibration forces from impeller
pulses/cavitation, turbulent waterflow in duct and around 4.1.5
stator vanes, and various other possible sources (shaft
misalignment, shaft/impeller imbalance etc.) will be present. The duct and the structural details of the duct must be
considered to be experiencing high and low cycle loads and
Figure 4-1 shows a typical steering gear for a waterjet unit, will therefore have to be considered for fatigue strength. For
which through manoeuvring actions will transmit reaction this reason it is recommended to design with few welded
forces through the bolted connection at the transom to the details and attachments on the duct itself, and that due
duct and hull structure. attention is paid to the detail design and execution of welds
(e.g. grinding of welds).
LC 1 Crash stop
LC 2 Maximum loads from reversing
LC 3 Maximum loads from steering
LC 4 Waterjet unit weight accelerated as
cantilever in pitching
In addition, high cycle loads from impeller pulses should be
Figure 4-1 Typical waterjet steering gear
considered, if available from the manufacturer.
4.1.3 Guidance note :
The steering nozzle reaction forces should normally be High cycle loads and loadpaths are generally not specified by
transmitted into the hull structure in one of the following manufacturer and will vary with size and make, so a recommended
manners: way to take account for these loads in duct design is, for each
critical detail, to design to the best possible fatigue class (see part
through the duct and into transverse frames, bulkheads four for typical details and fatigue classes). Also very important is
and bottom plating shaft alignment at installation, balancing of shaft and impeller and
through additional stiffening structure at transom. condition of bearings during service.
August 1996
Load distribution 1, Reversing loads (LD1) The accumulated sum of stresses will have the profile as shown.
Load distribution 2, Steering loads (LD2) The steering load varies with the angle ranging from corrective
steering 5 (flat water) and full steering 30 (heavy seas).
August 1996
Load distribution 3, pitching load (LD3) The vertical accelerations at the stern will cause cyclic cantilever
bending loads from the waterjet.
Npitching= 2 x 107
The steering curve does not take into account the long term
distribution for angle of encounter of seas, nor seakeeping
characteristics of particular ship types.
4.3 Modelling
4.3.1
Due to the complex structure, Finite Element Method
analysis should normally be used for the assessment of the Linear unit load Sinusiodal unit load distribution
transom region.
August 1996
Ft
Fc
Figure 4-5 Aft part of duct, actual boundary conditions (Ft and Fc are tensile and compressive load respectively).
Ft
Fc
Figure 4-6 Aft part of duct, simplified boundary conditions (Ft and Fc is tensile and compressive load respectively, for
top and bottom)
August 1996
Ft
Fc
Figure 4-7 Aft part of duct, simplified boundary conditions (Ft and Fc is tensile and compressive load respectively, for
top and bottom)
Ft
Fc
Figure 4-8 Non-continuous duct, boundary conditions joining flange (Ft and Fc are tensile and compressive loads
respectively)
= n i / Ni
4.4 Results and stress analysis
4.4.1 Where;
Stresses should be taken from the model as principal stresses = fatigue damage ratio
along the local element axis.
ni = number of cycles at stress range i
For static analysis (LC1 to LC4), the allowable stresses are:
Ni = number of cycles to failure at stress range i.
Load condition Combined axial-and Shear
bending In general the damage ratio at one point is the sum of the
damage ratio from each of the load effects.
LC 1 180 f1 100 f1
LC2, LC3, LC4 160 f1 90 f1 total = rev + steer + pitch
For fatigue assessment the stress range at the detail, from
For water jets the steering loads will not act in the same
maximum tension to maximum compression should be used.
point as the others, therefore
4.4.2 steer = 0 when considering the top and bottom parts of
The analysis results should be used to establish areas of high the duct.
and low loading.
The sum of the damage ratio from each of the load effects or
Welded joints in the duct should typically be placed as far in the case of steering the sum for steering alone should be
away from stress hot spots as possible. Results from an axi- kept less than one.
symmetric FEA, where linear stresses can be read at the
actual position of the weld, should be used to position and i.e < 1
design the welded connection with respect to fatigue.
Guidance note :
- The fatigue check for welded aluminium details should be
4.4.3 based on the ECCS "EUROPEAN RECOMMENDATIONS FOR
ALUMINIUM ALLOY STRUCTURES FATIGUE DESIGN"
The fatigue assessment may be based on the Miner-Palmgren
method for accumulated fatigue damage. - Reference stress for fatigue check is the principal stresses in the
main load carrying member.
August 1996
4.4.5
In situations where two steerable jets are located next to each
other, the total relative displacement should be taken into
account when estimating the stresses. Sufficient distance
between such jets should be ensured to allow sufficient
flexibility in the transom plating usually having a thickness
dimensioned to take the vertical shear forces.
August 1996
August 1996
As a minimum the following should be documented from the A separate analysis should be performed as a verification of
yard : local strength in way of hull support for the foil system.
Forces acting on the hull, derived from the foil system
still water global bending moments
analysis, should be considered with regard to structural
longitudinal and transverse bending moments in foil-born
strength of the supporting structure. Acceptance criteria are
condition, subject to design vertical acceleration at
to be as given in the rules Pt.3 Ch.2 or Pt.3 Ch.3.
longitudinal centre of gravity.
5.4.3
5.3 Strength analyses Bolted connections between foil system components
5.3.1 Bolted connections should be considered separately. Forces
The builder should submit complete documentation for acting on the bolted connections are to be taken from the
calculations of forces, stresses and deflections for the foil analysis of the foil system. In the analysis of bolted
system. The quality and extent of the calculations may connections, the following items should be taken into
influence the settings of operational limitations for the consideration:
vessel. geometry of the bolted connection
(symmetry/asymmetry, stiffness of flanges, local stress
5.3.2 concentration)
Local strength analyses of each foil system, as well as pretension of bolts (stress in bolts, surface pressure
strength calculations of hull structure in way of support for below nuts/bolthead)
foil system are required. stiffness of bolts versus stiffness of bolted material (note
that gaskets or similar will reduce the stiffness of the
5.3.3 bolted material, and hence increase the loading of the
Based on the structure in question, simplified analysis (such bolts)
as beam element analysis) may not be sufficient for a proper distribution of forces through the bolted connection.
evaluation of the stress distribution. Finite element analysis
of the complete foil structure or parts of the structure may be 5.4.4
required. Highly stressed areas in foil structure
5.3.4 Highly stressed areas should be specially considered with
Stress analysis respect to local stress concentration and evaluation of stress
concentration factors for fatigue assessment.
The results from the strength analyses should identify:
5.4.5
deflections
distribution of shear forces and bending moment, and Vibration and/or buckling analysis
reaction forces at all boundary nodes for the analysis
Vibration analysis should in general be performed. The
stress distribution and identification of areas with
safety of local as well as global buckling of the foil system
maximum values of stresses
should be documented.
direction and size of principle stresses for areas later
subject to fatigue assessment
all forces acting on bolted connections. 5.5 Acceptance criteria
August 1996
6. APPENDIX A
TYPICAL STRUCTURAL DETAILS
The appendix shows some selected typical structural details
where a good design is found important for life time of
detail. Alternative solutions may be proposed, and the detail
solutions shown in the appendix are to be considered as
guidelines.
August 1996