The work we have finished
For the research work of the common structural rules for double hull oil tank, we have
studied all types of typical ship structures from handymax series oil tanks (about
40,000 DWT) to VLCC (about 300,000 DWT). In which, the handymax series oil
tanks, aframax series oil tanks (about 11,000 DWT) and VLCCs have been designed
and constructed.
For the research work of the common structural rules for bulk carriers, we have
studied all types of typical ship structures from handysize series bulk carriers (about
30,000 DWT) to wozmax series bulk carriers (about 230,000 DWT). In which, the
handysize series bulk carriers, handymax series bulk carriers (about 50,000 DWT),
panamax series bulk carriers(about 80,000~90,000 DWT), aframax series bulk
carriers (about 11,000 DWT) and capsize series bulk carriers (about 18,000
DWT)have been designed and constructed.
We think the common structural rules are transparent and consistent rules in
general, and it give a clear link between structural requirements for new buildings and
ships in operation through the net thickness approach, and meet the current and future
expectations from regulatory bodies (IMO, EU etc) to structure.
We found several technical problems in the applications of CSR, the followings
are the problems we found in JTP rule.
1 Welding
1) The weld factors (fweld) of for the connection of Primary Support Members to its
face plates are a little big especially for the Primary Support Members of the
biggish (DWT) oil tanks. Should these factors be decided only based on their
gross face areas?
The weld factors of for connection of Primary Support Members to its face plates
2) There are no the weld factors about hull structures to some key structures such as
rudder horn, aft stern casting, hawse pipe, anchor bed and others.
3) For the welding of end connections of primary support members, the calculation
results based on the formula lleg  1.41 f yd
hw t p  grs
l dep
are always less than the
calculation results based on the formula lleg = fyd fweld f2 tp-grs + tgap
2. load
1) In the calculation the green sea load, the formula shown as following is the
most crucial: Pwdk-dyn=f1-dk(fWLfopP1-WZ-10zdk-T). For the calculation of P1-WL, it
is described in JTP as following: “P1-WL: pressure at still waterline for
considered draught, in kN/m2, sees 3.5.2.1”. Because the still waterline is
always at most outside end of ship breadth, it seems that the P1-WL should be a
fixed value at certain draught. But at the middle part of ship in ship length
direction, the green sea load is deduced mainly by maximum roll of ship and
the load should be alterable at the different transverse locations of upper deck.
1）
The bottom slamming press calculation formula in JTP is pricked off from LR
rules, but the formula about minimum requirement of structures is taken from
DnV. Though the bottom slamming press calculation formula of LR is little
conservative, the corresponding allowable stress is also bigger than the other
rules. LR’s bottom slamming press + DnV’s minimum requirement formula
of structures will lead to more conservative calculation result.
The above formula In JTP rules are pricked off from LR rules,
2）
About the design slamming draught at the F.P., TFP-mt is defined as following:
“is not to be greater than ……any loading conditions with tanks inside the
bottom slamming region that use the “sequential” ballast water exchange
method.” We think this definition of design slamming draught is a little
conservative compared with JBP rule. Because the shipmen will always
exchange the ballast water in “sequential” method in a good sea condition as
the weather forecast can be received from secondary planet.
The corresponding definition in JBP rule
3）
The longitudinal reinforcement scope of bow impact loads is a little extensive.
This formula is taken from ABS rule, and the corresponding scope in ABS
rule is from “the forward end to the collision bulkhead”.
2．(FE Load Cases for Oil Tankers)
1）
As we know, the maximum hogging condition is always found in ballast
condition. If more oil is loaded in the oil cargo tanks, the hogging
displacement will be smaller and sagging displacement will be bigger. When
all the cargo oil tanks are filled fully, the ship hull girder will be in sagging
condition. For the A1, A2, B1, B2, B3 load patterns, when the draught is
0.9Tsc, even one or two tanks are empty, most of ship hull girder will be in
sagging condition. Even there is a little hogging in certain part, the hogging
displacement and moment are very small compared to the maximum hogging
moment in ballast condition. We advise that for these load patterns, the
maximum SWBM on the middle part of FE models are taken as 50% of the
permissible SWBM or smaller value.
2）
For the A14 and B11 load patterns, when two tanks are empty, in no case the
draught of ship can be push down to scantling draught. Why not taken the draught
as 0.9Tsc in the A1 and B3 load patterns?
3）
For the allowable stress of structures in the harbour and tank testing load
cases, it is about 0.8 time of the allowable stress of structures in the sea-going
load case. For the harbour load cases, this allowable stress is reasonable. But
for the tank testing load cases, it is carried out when ship is delivered, and the
corrosion of plate’s thickness is zero. May the allowable stress be increased
corresponding for the tank testing load cases?
4）
In beam sea load cases, the transverse loads acted on the FE model are
unsymmetrical, the reaction forces will be found at the two boundary ends of FE
model. If we apply the transverse acceleration not only on the cargo oil but also
on the FE model according to D’ alembertian’s principle, this unbalanced reaction
forces will be smaller as there are no transverse adjusting forces are applied at
frames.
The followings are the problems we found in JBP rule.
1．
We hope that the allowable buckling stresses on the hull shell plate (such as out
shell, bottom shell and inner bottom plates) can be considered again other than the
primary supporting members because the increased plate weights are noteworthy
in these areas. Since in the actual bulk carriers, there are lesser buckling wrecks in
these areas.
2．
About the stress assessment of hatch corner, some designers think that the
longitudinal warping stress at the hatch corner induced by torsion moment should
be calculated based on the ships cross section without cross deck area.
3．
In the FE load cases, when the maximum vertical shear forces are analyzed, there
are no clear definitions about the doubtless positions of maximum vertical shear
forces and their directions. The following descriptions in JBP rules are inexplicit.
“In vertical shear force analysis the target hull girder loads are the maximum
vertical shear force which may occur at one of the transverse bulkheads of the
mid-hold in the FE model”