Draft Technical Background for Rule Change Proposal 1 to 01 JAN 2018 version

Transcription

1 Common Structural Rules for Bulk Carriers and Oil Tankers Draft Technical Background for Rule Change Proposal 1 to 01 JAN 2018 version Notes: (1) These Rule Changes enter into force on 1 st July Copyright in these Common Structural Rules is owned by each IACS Member as at 1st January Copyright IACS The IACS members, their affiliates and subsidiaries and their respective officers, employees or agents (on behalf of whom this disclaimer is given) are, individually and collectively, referred to in this disclaimer as the "IACS Members". The IACS Members assume no responsibility and shall not be liable whether in contract or in tort (including negligence) or otherwise to any person for any liability, or any direct, indirect or consequential loss, damage or expense caused by or arising from the use and/or availability of the information expressly or impliedly given in this document, howsoever provided, including for any inaccuracy or omission in it. For the avoidance of any doubt, this document and the material contained in it are provided as information only and not as advice to be relied upon by any person. Any dispute concerning the provision of this document or the information contained in it is subject to the exclusive jurisdiction of the English courts and will be governed by English law.

2 Contents CHAPTER 2 GENERAL ARRANGEMENT DESIGN... 3 SECTION 3 COMPARTMENT ARRANGEMENT... 3 CHAPTER 3 STRCUTURAL DESIGN PRINCIPLES... 4 SECTION 3 corrosion additions... 4 SECTION 6 STRUCTURAL DETAIL PRINCIPLES... 4 CHAPTER 7 DIRECT STRENGTH ANALYSIS... 6 SECTION 3 local structural strength analysis... 6 CHAPTER 10 OTHER STRUCTURES... 8 SECTION 1 FORE PART... 8 SECTION 3 AFT PART CHAPTER 12 CONSTRUCTION SECTION 3 DESIGN OF WELDED JOINTS SECTION 3 DESIGN OF WELDED JOINTS SECTION 3 DESIGN OF WELDED JOINTS RCP 1 TO 1 JANUARY 2018 PAGE 2 OF 14

3 Part 1 General Rule Requirements CHAPTER 2 GENERAL ARRANGEMENT DESIGN SECTION 3 COMPARTMENT ARRANGEMENT 2 DOUBLE BOTTOM 2.3 Height of double bottom The proposal is to clarify how to measure the height of double bottom for oil tanker. The measurement of double bottom height for tanker is based on MARPOL Regulation , Figure 1. At any cross-section, the depth of each double bottom tank or space shall be such that the distance h between the bottom of the cargo tanks and the moulded line of the bottom shell plating measured at right angles to the bottom shell plating as shown in figure 1 is not less than specified below: h = B/15 (m) or h = 2.0 m, whichever is the lesser. The minimum value of h = 1.0 m Figure 1 There is no impact on scantlings due to this change. RCP 1 TO 1 JANUARY 2018 PAGE 3 OF 14

4 CHAPTER 3 STRCUTURAL DESIGN PRINCIPLES SECTION 3 CORROSION ADDITIONS 1 GENERAL 1.2 Corrosion addition determination, Table 1 This proposal is made to clarify how to determine the corrosion addition for slop tank. In current designs, the slop tank is usually designed as a cargo oil tank and in actual operation, the slop tank carries cargo oil in most of the time. The corrosion environment is also considered to be the same as for cargo oil tanks and the same corrosion addition should be applied, even though it is used as a storage tank for washing oil/water in limited part of the time. Therefore, it is proposed to add a new description for the slop tank in Table. 1. Please also find the corrosion margin comparison for the inner bottom plating with two different approaches. Compartment 1 Compartment 2 Required corrosion margin Slop tank considered as heated Double bottom ballast tank = 4.5 mm cargo tank Slop tank considered as ballast tank Double bottom ballast tank = 3.0 mm There is no impact on scantlings due to this change since the slop tank is usually designed as cargo oil tank. SECTION 6 STRUCTURAL DETAIL PRINCIPLES 3 STIFFENERS 3.4 SNIPED ENDS The proposal is to clarify the application area for in the vicinity of engines or generators or propeller impulse zone in [3.4.1]. The application area of stiffeners with sniped ends is not clear with the Rules issued on 1 Jan 2018 and the rule change proposal clarifies that in the vicinity of engines or generators means in the machinery space and propeller impulse zone is in the stern area. There is no impact on scantlings due to this change. RCP 1 TO 1 JANUARY 2018 PAGE 4 OF 14

5 3.4.2 The proposal is to clarify the application of requirements. The Rules issued on 1 Jan 2018 requires that the tapering of sniped stiffeners are not to be more than 30. This requirement is intended to be applied for the load bearing members with relatively longer span to mitigate hard spot due to plate bending. For shorter span, the original rules require the stiffener to be triangle with the reduced height due to 30 tapering as shown in Figure 1 so an alternative solution need to be considered in this case. Figure 1: Sniped stiffener at the narrow space The following details may be considered as an alternative solution. One side taper Lap type connection There is no impact on scantlings due to this change. RCP 1 TO 1 JANUARY 2018 PAGE 5 OF 14

6 CHAPTER 7 DIRECT STRENGTH ANALYSIS SECTION 3 LOCAL STRUCTURAL STRENGTH ANALYSIS 2 LOCAL AREAS TO BE ASSESSED BY FINE MESH ANALYSIS 2.1 List of mandatory structural details List of structural details The proposal is made to make a mandatory fine mesh analysis for the bracket at the heel of horizontal stringer. The bracket at the heel of horizontal stringers in way of transverse bulkheads is not included in the list of mandatory fine mesh analysis in Pt 1. Ch 7, Sec 3, [2.1.1]. In addition, the screening factor in Table 6 is not applicable for brackets which cannot be modelled properly in the cargo hold FE model due to the size of bracket i.e 600 mm mm. Consequently, there is neither direct fine mesh analysis nor screening requirements defined in Pt 1, Ch 7, Sec 3, table 6, so the proposal is made to take into account the different shape and size of these brackets by direct fine mesh analysis as shown in Figure 1. If the size of bracket is able to be reflected in cargo hold FE model with suitable number of element along the edge of bracket then, the screening requirements in Pt 1, Ch 7, Sec 3, table 6 can be applied. The recommended brackets are defined in Pt.1 Ch.9 Sec.6 Table 9 Design standard I-transverse bulkhead horizontal stringer heel. Figure 1: RCP 1 TO 1 JANUARY 2018 PAGE 6 OF 14

7 The shape or scantling of bracket at the heel may need to be improved Bracket at the heel of horizontal stringer [2.1.7] is newly made to support rule change for [2.1.1] The selection of fine mesh analysis is described in this new paragraph [2.1.7]. The location with maximum yield utilization found in cargo hold coarse mesh analysis (See Figure 1) is to be chosen for the local fine mesh analysis with 50x50mm mesh size. If the geometry, scantling or mesh size are significantly different compared with the typical detail, then this location is to be analysed additionally. The recommended brackets can be found in Pt.1 Ch.9 Sec.6 Table 9 Design standard I-transverse bulkhead horizontal stringer heel. Figure 1: Cargo hold coarse mesh model The shape or scantling of small backing bracket may need to be improved. 3 SCREENING PROCEDURE 3.2 List of structural details RCP 1 TO 1 JANUARY 2018 PAGE 7 OF 14

8 3.2.2 Outside midship cargo hold region The proposal is made to make a screening requirement for the bracket at the heel of horizontal stringer outside midship cargo hold region. The bracket at the heel of horizontal stringer is newly added in [2.1.1] as a mandatory structural detail for fine mesh FEA in the midship cargo hold region and the screening requirement shall therefore be considered outside the midship cargo hold region. The screening procedure is the same as the connections of corrugation to adjoining structure so the screening stress concentration factor, K SC, can be found in Pt 1, Ch 7, Sec,3 Table 4. The shape or scantling of backing brackets outside midship cargo hold region may need to be improved. 3.3 Screening criteria Screening factors and permissible screening factors The proposal is made to specify the screening factors, λ sc, and permissible screening factors, λ scperm, for brackets at the heel of horizontal stringer outside midship cargo hold region. A requirement for bracket is newly added in table 4 to be in line with rule change proposal of [3.2.2] The shape or scantling of backing bracket outside midship cargo hold region may need to be improved. CHAPTER 10 OTHER STRUCTURES SECTION 1 FORE PART 3 STRUCTURE SUBJECTED TO IMPACT LOADS 3.2 BOTTOM SLAMMING Shell plating The Rule change is to clarify the requirements about bilge plating thickness within cylindrical part of the ship for the bottom slamming pressure. The extent of strengthening due to slamming pressure is significantly increased for bulk carrier (0.17L to 0.3L from F.P.) and now applicable even for transversely stiffened bilge plating within the cylindrical part of the ship. RCP 1 TO 1 JANUARY 2018 PAGE 8 OF 14

9 Rules issued on 1 Jan 2018 is applicable for the flat of bottom and adjacent plating with attached stiffeners (Longitudinally stiffened panels) up to a height of 500 mm above the base line so this rule change is a clarification for the transversely stiffened bilge plating. Please also note that transversely stiffened bilge plating within cylindrical part is most likely located between 0.23L and 0.3L from the F.P (See A below) and the slamming pressure in this area is not maximum. A direct fine mesh analysis with mesh size of 50 x 50 mm was carried out to prove that the capacity of bilge plating within cylindrical part of the ship has enough strength with respect to the bottom slamming pressure. A capsize bulk carrier was chosen since the large part of bilge plating within cylindrical part of the ship is still inside 0.3L from F.P (Slamming Zone). The local model was prepared in accordance with Pt 1, Ch 7 and includes all relevant structures within hopper tank as shown in Figure 1. The slamming pressure is calculated in accordance with Pt 1, Ch 4, Sec 5, [3.2] at the location where the maximum pressure is expected for the bilge plating as shown in Figure 1. The total surface stress including both bending and membrane stresses at the upper and lower plate surface together with the deformation were evaluated and plotted in Figure 2. The maximum stresses are found at the long edge of flat bottom plating due to the plate bending. However, the total stress level in way of the bilge plating is found to be much lower than that of the flat bottom plating since only the lower part of the bilge plating up to 500 mm from the base line is exposed to the slamming pressure and the plate bending stress is significantly reduced due to the curvature of the bilge plating, even if the membrane stress is increased slightly. RCP 1 TO 1 JANUARY 2018 PAGE 9 OF 14

10 Figure 1: FE local model and load application Figure 2: Stress plots for both surface Figure 3: Deformation RCP 1 TO 1 JANUARY 2018 PAGE 10 OF 14

11 There may be slight impact on material yield strength of bilge plating in some cases but the results are more rational. SECTION 3 AFT PART 3 STERN FRAMES 3.2 Propeller posts Gross scantlings of propeller posts The Rule change proposal is to clarify the requirements. The wording of those above is replaced by Table 1 and Table 2 to clarify the requirements. Scantling impact is not expected Section modulus below the propeller shaft bossing The Rule change proposal is to remove confliction between [3.2.1] and [3.2.2]. [3.2.1] is applicable for the typical current design of propeller post without sole piece and the second paragraph of [3.2.1] allows deviations in scantlings and proportions required by Table 1 & 2 provided the RCP 1 TO 1 JANUARY 2018 PAGE 11 OF 14

12 strength integrity i.e the section modulus of the propeller post is kept based on Table 1 & 2, while the paragraph of [3.2.2] allow section modulus deviation, conflicting with [3.2.1]. To keep consistency of the Rule, [3.2.2] is removed. Typical design has same thickness for both below and above the propeller shaft bossing so no scantling impact is expected otherwise there might be slight impact on scantling. CHAPTER 12 CONSTRUCTION SECTION 3 DESIGN OF WELDED JOINTS 2 TEE OR CROSS JOINT 2.5 Weld size criteria (Table 1: Minimum leg size) The proposal is made to clarify the minimum leg size for 3m below top of compartment. In Table 1, additional leg length of 0.5mm is required for areas within 3m below top of a compartment. This requirement is originally taking into account the excessive corrosion of this area. The corrosion addition specified in Pt.1, Ch.3, Sec.3, Table 1 has the same categorization of Within 3m below top of tank for cargo oil tanks and ballast tanks. The corrosion addition in CSR-OT and CSR-BC basically follow the same philosophy and it is understandable that the top part of tanks would be a more corrosive environment compared with Elsewhere in the tank due to the presence of high salty and humid vapour during hot weather (sun effects). On the other hand, the table for minimum leg size in this Chapter has categorization of Welds within 3m below top of compartment for welds both in water ballast/cargo tanks and also in fresh water tanks, fuel/diesel oil/other tanks, dry spaces and voids. In addition, it is not specified that the 3m below top of compartment are to be considered only for spaces located immediately below the weather deck. It is desirable to resolve the current discrepancy between these two requirements for corrosion additions and minimum leg sizes in terms of the types of compartments for which excessive corrosion is expected. It is proposed to amend the minimum leg length requirements so as to be in line with the requirements for corrosion addition, i.e. the application of additional requirement of Within 3m below top of tank of compartment is limited to cargo oil tanks and water ballast tanks with weather deck as tank top considering the technical background about this requirement. In Table 1, specific requirement for superstructures and deckhouses is missing, therefore it is proposed to add a new category based on the relevant requirements specified in CSR-BC and CSR-OT. Because this change affects only the size of fillet welding, there is no impact on scantlings. SECTION 3 DESIGN OF WELDED JOINTS 2 TEE OR CROSS JOINT RCP 1 TO 1 JANUARY 2018 PAGE 12 OF 14

13 2.5 Weld size criteria (Table 2: Weld factors for different structural members) The proposal is to limit the extent of full penetration welding of the connection of the longitudinal hatch coaming plating to the deck. Weld factors are provided for various connections in Table 2. For the connection of the longitudinal hatch coaming to deck plating at corners of hatchways, full penetration welding is required, and the extent of full penetration welding is 15% of the hatch coaming length. This is not in line with the requirement of connection of hatch coaming end bracket to the deck plating, which is 15% of the hatch coaming height in Pt 1, Ch 12, Sec 3, [2.4.5] (h) and the minimum extent of full penetration welding described in Pt 1, Ch 12, Sec 3, [2.4.4] and [2.4.7] A fine mesh FEM study is carried out to analyse the stress level of such connections in consideration of all relevant EDWs. The stress plots are shown in Figure 1. The connection of the longitudinal hatch coaming to deck plating is shown in a green line, and the connection of the transverse coaming to deck is shown in a red line. The maximum utilization factors, which is represented by λ f / λ fperm, refer to Pt 1, Ch 7, Sec 3, [6.2], of elements in way of connection are about , as listed in Table 1. Figure 1. Stress plots of hatch coaming connection to deck plating Max Stress of Seagoing condition Max stress of HSM load cases Max stress of FSM load cases Max stress of BSR load cases Max stress of BSP load cases Max stress of OST load cases RCP 1 TO 1 JANUARY 2018 PAGE 13 OF 14

14 Max Stress of Harbour condition Table 1. Maximum utilization factors of hatch coaming connection to deck plating Location Seagoing condition Harbour condition Longitudinal hatch coaming Transverse hatch coaming According to the study, the stress levels of these connections based on 50 x 50 mm mesh size are within a reasonable range. Therefore, full penetration welding can be limited to a local area only, and 15% of hatch coaming height is proposed to be in line with the principle of full/partial penetration welding for the local peak stress area in accordance with Pt 1, Ch 12, Sec 3, [2.4.4] and [2.4.7]. In addition, Non-destructive examination (NDE) is less reliable in fillet welding compared with a full penetration welding as crack propagation from the root is difficult to detect before the crack reach the surface so the fillet welding for such a critical locations i.e hatch corner should be avoided for better quality control. The remaining part of the connection within 15% of the longitudinal hatch length should follow the general requirement of watertight plate to boundary plating with welding factor of The extent of full penetration welding will be reduced. SECTION 3 DESIGN OF WELDED JOINTS 2 TEE OR CROSS JOINT 2.5 Weld size criteria (Table 2: Weld factors for different structural members) The proposal is to clarify the requirements. The requirement has been updated to clarify that the superstructure and deck house, as well as the bulkheads of these structures, are in the same category. Scantling impact is not expected. RCP 1 TO 1 JANUARY 2018 PAGE 14 OF 14