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 seriesbulk carriers (about 30,000 DWT) to wozmax seriesbulk carriers (about 230,000 DWT). In which, the handysize seriesbulk carriers, handymax seriesbulk carriers (about 50,000 DWT), panamax seriesbulk carriers(about 80,000~90,000 DWT), aframax seriesbulk carriers (about 11,000 DWT) and capsize seriesbulk 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, andmeet 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 are always less than the calculation results based on the formula lleg = fyd fweld f2 tp-grs + tgap
- 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 correspondingscope 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 increasedcorresponding for the tank testing load cases?
4）
In beam sea load cases, the transverse loads acted on the FE model areunsymmetrical, 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 againother 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”