IMO ANY OTHER BUSINESS. Verification of technical standards for ship construction. Submitted by the United Kingdom

Transcription

1 INTERNATIONAL MARITIME ORGANIZATION E IMO SUB-COMMITTEE ON SHIP DESIGN AND EQUIPMENT 48th session Agenda item 24 DE 48/INF.6 17 December 2004 ENGLISH ONLY ANY OTHER BUSINESS Verification of technical standards for ship construction Submitted by the United Kingdom Executive summary: Action to be taken: Paragraph 25 SUMMARY This document presents a review of a number of industry technical standards carried out by the United Kingdom as part of a continuing series to confirm that these achieve the outcomes anticipated by SOLAS and Load Line regulations for ship construction. Related documents: MSC 79/3/1, annex 1; and MSC 79/6/8 Introduction 1 Many of the detailed technical standards for ship construction and equipment are developed by recognized organisations or technical associations on behalf of national administrations. The United Kingdom Maritime and Coastguard Agency (MCA) have a continuing programme to verify such standards, in particular where these are adopted or recommended by the International Maritime Organization (IMO). This report covers the following standards developed and adopted by the International Association of Classification Societies (IACS): UR S18 (Rev. 5) UR S26 (Rev. 1) UR S27 (Rev. 2) UR S30 (Rev. 1) UR S31 (Rev. 2) Evaluation of Scantlings of Corrugated Transverse Watertight Bulkheads in Bulk Carriers Considering Hold Flooding. Strength and Securing of Small Hatches on the Exposed Fore Deck. Strength Requirements for Fore Deck Fittings and Equipment. Cargo Hatch Cover Securing Arrangements for Bulk Carriers not Built in accordance with UR S21 (Rev. 3). Renewal Criteria for Side Shell Frames and Brackets of Single Side Skin Bulk Carriers and Single Side Skin OBO Carriers not Built in accordance with UR S12 Rev. 1 or subsequent revisions. For reasons of economy, this document is printed in a limited number. Delegates are kindly asked to bring their copies to meetings and not to request additional copies.

2 DE 48/INF UR S18 Corrugated Transverse Watertight Bulkheads in Bulk Carriers 2 UR S18 applies to corrugated watertight transverse bulkheads, which form the main sub-dividing structure between dry cargo holds in a bulk carrier. A critical review has been conducted which included both general comments on this standard, and analytical work to evaluate the implied safety for a Cape-size bulk carrier. For this example, bending moments, stresses and deflections of the corrugated bulkhead were calculated for two scenarios. Firstly that with an iron ore cargo loaded in alternate holds and with flood water to the level specified in UR S18, and secondly with flood water to the damaged water line. Account was taken of the support provided to the corrugations by the lower stool mounted onto the double bottom and by the upper stool supported by the cross deck strip. 3 It was concluded from this analysis that the maximum bending moment occurred in the lower part of the corrugations and that this was accentuated by the rotation particularly of the lower stool. This rotation is about a transverse axis and is due to the heavy load of iron ore and flood water in the hold, combined with the buoyancy force acting on the double bottom of the adjacent empty hold. Resulting stresses at this position were found to be at around yield for the first scenario, based on a section using net scantlings, that is allowing for a prescribed diminution in plate thickness as specified in UR S18. However, stresses due to bending at around the mid height of the corrugations were found to be rather less, at around 60% of yield. This suggests that in terms of an ultimate collapse of the bulkhead there is some margin. 4 For the second scenario, stresses at the same positions were found to be a little less. Again the highest were in the lower part of the corrugations, with a compressive stress of about 85% of buckling strength, and that at mid height was less than one half yield. This suggests that an upwards dynamic acceleration of about 0.16g would be needed for some deformation to occur in the lower part, but that for collapse the reserve strength capacity at mid height of the corrugations indicates a somewhat higher value. All these results assume the corrugated bulkhead is worn down to the net thickness, and that it is constructed from steel for which the yield stress just complies with the minimum acceptable level. In practice most steels possess yield properties in excess of the minimum, which is not taken into account. 5 UR S18 is seen to be a considerable improvement over certain of the Classification Rules that were current circa 1990 for two main reasons. Firstly, a corrosion allowance of 3.5 mm is required which exceeds the earlier standards, and secondly the combined effect of a saturated cargo with the flood water is now included. However the earlier standards specified that the flood head was to be taken at least to the upper deck, which has been relaxed in UR S18 for all holds except the No. 1, but this only when there is no sheer. It is also considered that UR S18 complies with SOLAS chapter XII, Regulation 5 insofar that there is an implicit allowance for the dynamic effect of the flood water. UR S26 Strength and Securing of Small Hatches on the Exposed Fore Deck 6 UR S26 provides requirements for the strength and securing of small hatches located on the exposed fore deck. These are seen to effectively override the specification contained in ISO 5778 and other Industry standards for those small hatches situated within the deck area specified in UR S26. UR S26 is applicable to existing bulk carriers, related types and to general dry cargo ships. For new ships it applies to all ship types where the exposed deck is below a specified height above the summer water line. The UR requires increased strength based on wave pressures measured on the fore decks of two ship models tested at MARIN in

3 - 3 - DE 48/INF.6 the Netherlands. These tests, developed jointly by the United Kingdom Department for Transport and IACS, and supervised on their behalf by Lloyd s Register and the MCA, were an outcome of the Re-opened Formal Inquiry into the loss of the bulk carrier DERBYSHIRE. 7 The main criticism of previous small hatch covers was that wave pressures would force the cover down on its gasket, allowing the butterfly nuts to be washed off their position on the fork plate and hence the cover to come open. This should now be prevented by two measures. First, the cover is tightened onto a steel to steel contact provided by a bearing pad, such that the gasket cannot be further compressed. Second, the end of the fork is curved upwards so that the butterfly nut cannot be easily washed off its position. In the extreme event that the primary securing devices were to fail, the cover is further to be held in place by an independent secondary device, and the position of the hinges is specified to be such that the predominant direction of green seas will cause the cover to close. 8 It is considered that this standard should provide a real improvement in the strength and securing of those small hatch covers within its scope of application. UR S27 Strength Requirements for Fore Deck Fittings and Equipment 9 UR S27 provides general formulations to assess the strength requirements for air pipes, ventilator pipes, their closing devices, and for the securing of windlasses, all located within the fore deck as defined in the standard. The wave pressures used in developing the standard were also derived from the above-mentioned MARIN tests, combined with ship service experience. 10 The pressures specified in S27 are used to determine bending moments applied to pipes and reaction forces in bolt groups securing the windlasses which are compared to defined stress or strength criteria. In addition, to simplify calculation, bracket sizes required to reinforce standard Load Line air pipes of 760 mm height and standard ventilator pipes of 900 mm height are prescribed, based on these principles. Current pipe thickness standards are maintained for ease of retrofitting existing ships. 11 The standard also states that rotating type mushroom ventilator heads are unsuitable for application within the defined foredeck areas for new and existing ships. Such ventilators heads serving spaces wholly or partly forward of the collision bulkhead on existing ships are to be replaced. 12 It is to be expected that UR S27 should provide substantially improved requirements for air pipes, ventilator pipes, and their closing devices; also for the securing of windlasses mounted on ships contracted for construction after 1 January It should further be noted that upgraded survey requirements for automatic air pipe heads are contained in IACS Unified Requirement Z22 which also came into effect on 1 January Thus UR S27 in combination with UR Z22 should substantially lessen the risk of water ingress into the forward spaces. UR S30 Cargo Hatch Cover Securing Arrangements 13 UR S30 specifies upgraded requirements for the securing of main hatch covers fitted to bulk carriers whose covers were not built in accordance with UR S21 Rev. 3. The new S30 standard is applicable to the Nos. 1 and 2 hatch covers which are wholly or partly within the forward 0.25L on the main deck of all bulk carriers with either single or double side skins. It is to be implemented according to the same time schedule as UR S26, S27 and S31.

4 DE 48/INF Within this standard S30.2 provides requirements for the securing devices of main hatch covers, that is their holding-down cleats. This clause is seen to be quite similar to what used to be contained in IACS Recommendation No Basic requirements for the design of stoppers to restrain the forward main hatch covers in the longitudinal and transverse directions are stated in S30.3. It is seen that the intention of this clause is to prevent wave forces from laterally displacing these hatch covers from their correct position. Wave pressures acting on the side plates of the Nos. 1 and 2 main hatch covers are defined which are identical to those specified in UR S21 (Rev. 3). These pressures correspond to 80% of that applied to the coaming, which are seen to be derived from the MARIN model tests. This reduction evolves from the pressure gradient that can be expected to occur at the top of the combined vertical face presented by the hatch coaming and the cover side. It is also noted that prior to this recent knowledge of the forces acting on the sides of hatch covers, very little information was available on this subject. 16 IACS UR S30 is considered to represent a substantial advance in the securing arrangements for forward main hatch covers, and provided it is correctly applied should ensure that these covers are not displaced laterally from their correct position, even under very severe wave conditions. UR S31 Renewal Criteria for Side Shell Frames and Brackets 17 The side shell frames and brackets of a single side skin bulk carrier are directly exposed to the cargo, moisture from atmospheric condensation on the shell, water content from within the cargo, and occasionally sea water in the ballast hold. During unloading they may also be subject to impact from pneumatic hammers or other devices to dislodge cargo remnants. Not surprisingly in this harsh environment for a structure, several cases of corrosion and damage in the frames and brackets are known to have occurred. The understood intent of UR S31 is then to limit levels of web corrosion to satisfy four criteria and to provide countermeasures such as a bending strength check, sand blasting and coating, and the fitting of tripping brackets, in order to reduce the risk of such events occurring. 18 UR S31 is applicable to the single side shells of bulk carriers and OBO ships, which were not built in accordance with UR S12 Rev. 1 or later revisions. UR S12 Rev. 1 applies to ships contracted for construction on or after 1 July 1998 and is the base document to which UR S31 refers. Hence UR S31 would not apply to ships contracted after that date. It is noted that UR S12 Rev. 1 and subsequent revisions are essentially empirically based. For example the frame minimum thickness relates only to ship length. There are therefore no expressions in this standard relating frame bending and shear strength criteria to maximum applied wave forces. Thus requirements for the strength of side frames are to be according to those specified by each Classification Society. 19 In the review of UR S31, it is noted that the thickness of the renewed frame web and lower bracket is to be at least 90% of that required by UR S12. It is considered not unreasonable that a thickness slightly less than that of UR S12 may be fitted in view of the shorter expected life for the ship at the time when these repairs are performed, i.e. when the ship is 10 or more years of age. 20 Analytical work was also carried out to evaluate the implied safety of this standard for some example ships. For this an analytical model was constructed for the side frame member

5 - 5 - DE 48/INF.6 which allowed for the different end constraints provided at main web positions and in ordinary frames, and also for the effect of rotation of the hopper caused mainly by the cargo loading acting on the inner bottom and the sea pressure on the outer shell. Three bulk carriers were analysed, two Cape-size and one Handymax. In each case the hold side frame web was assumed to be corroded to the relevant renewal web thickness required by UR S31. The frame face plate was also assumed to be corroded to the same extent, but the side shell to one half this value. 21 From this work it was found that: i) Generally the forces on the side frame are most onerous when the ship is fully loaded with a dense cargo, such as iron ore, that provides little or no internal pressure to react wave loads. ii) Wave forces on the side shell are most severe for all holds aft of about 0.2L from the forward perpendicular when in quartering or beam seas. iii) Stresses in the side frames are usually worst when in a deep loaded condition, for those frames located in an empty hold. The reason for this is that the buoyancy force on the bottom shell is not reacted by a weight of cargo. This causes an upwards deflection of the double bottom and a consequent rotation of the hopper structure such as to add to compressive stresses in the frame face plate. Of these frames, those which are connected to a hopper web frame but no web frame within the topside ballast space are slightly more critical. For all these frames, stresses in the face plate are strongly compressive in their lower part and bracket and particularly for higher tensile steel frames the possibility of tripping associated with the curvature of the face plate into the lower bracket may need to be considered when the ship is subjected to severe sea states. iv) Stresses in those frames located in the loaded holds are seen to be less critical even when the cargo is not in direct contact. For these frames the higher stresses are tensile and tend to occur around mid height. Stresses in the lower frame and bracket are rather less than for those located in the empty holds. v) All calculations assumed that the connecting structures in the topside and hopper spaces are maintained in good condition. Should any backing brackets at ordinary frames, or the longitudinals to which they are connected, be allowed to deteriorate to a poor condition, then the stresses in the hold frame can be expected to rise. In particular if cracking or buckling were to occur in these backing structures then this increase in stress is likely to be appreciable. Similar comments may also apply for those hold side frame connections to topside or hopper web frames. It should be noted that UR S31 does not specify any standards for these backing structures. 22 In the sense that UR S31 could be expected to reduce the number of occasions when severe corrosion is found to occur in the side frames of bulk carriers not conforming with UR S12 (Rev. 1) or later, the standard should contribute to reducing the risk of side shell failure in existing ships. However, the measures and criteria proposed in UR S31 are not considered sufficient to completely eliminate this problem. In particular, recent information in regard to wave pressures in beam seas and at headings away from head seas suggests that there is a general need for side frame scantling requirements in new bulk carriers to be increased. To provide some improvement to UR S31, it is suggested that IACS should give consideration to the fitting of tripping brackets for those frames located in holds which are permitted to be empty, and in particular where the frames are constructed from higher tensile steel.

6 DE 48/INF As such it is considered important that UR S31 does not set a precedent for new ship design. In addition, now that double side skins are not mandated by SOLAS Chapter XII, there is an urgent need for UR S12 (rev 3) to be expanded to include frame bending and shear strength requirements that incorporate wave forces consistent with recent information, together with corrosion allowances reflecting measured diminutions and survey criteria, including thicknesses for steel renewal. 24 The draft IACS Common Structural Rules for Bulk Carriers does indeed set out wave pressures, corrosion allowances and side frame net strength requirements to be maintained throughout the ship s life, which may go towards resolving this situation. According to the examples analysed herein it would seem that although there is an increase in the required net modulus which may be adequate for some bulk carriers, this may not be sufficient for the smaller bulk carrier, where this is designed to operate with empty holds in the fully loaded condition. Further for cargo hold side frames constructed from higher tensile steel and located in holds which may be empty in the loaded condition there may be a need to consider frame stability in way of the lower bracket. Action requested of the Sub-Committee 25 The United Kingdom invites the Sub-Committee to note this information, and that it will submit comments on the draft common Rules for Bulk Carriers, including the above points, to IACS by the end of The full report is available from the United Kingdom delegation, or from the Research Manager, Shipping Safety Branch, Maritime and Coastguard Agency, Spring Place, 105 Commercial Road, Southampton SO15 1EG, Tel.: , anna.rothwell@mcga.gov.uk.