A Practical Assessment of Existing Bulk Carrier Local Structural Strength in Relation to the Allowable Hold Mass Curves

Topics: Bulk cargo, Bulk carrier, Shear stress Pages: 34 (11076 words) Published: November 2, 2011
A PRACTICAL ASSESSMENT OF EXISTING BULK CARRIER LOCAL STRUCTURAL STRENGTH IN RELATION TO THE ALLOWABLE HOLD MASS CURVES K Chatzitolios, Bureau Veritas, Greece G de Jong, Bureau Veritas, France Dr JE Kokarakis, Bureau Veritas, Greece SUMMARY The allowable hold mass curves for vessels built after 1998 are mandatory in the loading manual & the loading instrument as per IACS Unified Requirements S1A. The majority of the bulk carriers in service have been constructed before 1998 and generally do not have allowable hold mass curves. Pre-1998 bulk carriers engaged in multi-port operations need to have allowable hold mass curves to control the local strength of the cargo hold structure for the envisaged loading conditions. The curves are produced according to the loading conditions of the approved loading manual as a function of the draught. For the case of an individual hold they are determined by examining bending and shear stresses in floors and girders, as well as buckling stresses in the associated plating. For the case of two adjacent holds the strength of the transverse bulkhead and cross deck is considered as well. The curves can be checked with finite element analysis or other methods to obtain the applicable safety margin. The paper presents a theoretical derivation of the hold mass curves as function of the draught and provides some comparisons with formulations by other class societies and IACS requirements. A practical methodology to determine the hold mass curves when not available is proposed. An interesting application, presented in a case study in the paper, is the determination of the maximum draught as a function of the static still water bending moment at the empty holds. The combination of a hogging hull girder bending moment and hydrostatic pressure at 60 to 70% of the scantling draught may cause severe buckling of the bottom plating and exceed its ultimate strength. A methodology on how to assess this loading condition for holds which are not usually empty is proposed. 1. INTRODUCTION the vessel is not overstressed during loading and discharging in port, which can happen due to faulty loading sequences or (de)ballasting operations. The sequence of loading the cargo holds, as well as the amount of cargo which is loaded in each hold in one time greatly influences the induced hull girder loads. This issue is still very actual, in particular due to high speed cargo loading at iron ore terminals (up to 16,000 tonnes per hour) [1]. For new bulk carriers (contracted for construction on or after 1 July 1998) of 150 m length and above, UR S1A requires the class approved loading manual to additionally include the following data: • Maximum allowable and minimum required mass of cargo and double bottom contents of each hold as a function of the draught at mid-hold position; • Maximum allowable and minimum required mass of cargo and double bottom contents of any two adjacent holds each hold as a function of mean draught in way of these holds. The values of maximum allowable and minimum required mass of cargo can be plotted as a function of the draught and are generally referred to as hold mass curves. The loading instrument is required to display whether the cargo hold mass is within permissible limits and therefore needs to incorporate the hold mass curves. This requirement for “new ships” effectively regulates that, for any given loading condition, the local strength capacity of the hull structure (strength of double bottom,

In 1998 IACS adopted Unified Requirement (UR) S1A, effectively introducing additional requirements for loading conditions, loading manuals and loading instruments of both new and existing bulk carriers1. UR S1A requires existing bulk carriers (that is, bulk carriers contracted for construction before 1 July 1998) with a length of 150 m and above to be provided with a class approved loading instrument in order to enable the ship’s master to check the envisaged loading conditions (whether at sea or...
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