Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019


is the maximum moisture content of a cargo that is considered safe for transportation in ships. It is calculated as 90% of the Flow Moisture Point (FMP) (Bureau Veritas, 2018).


Under the influence of the ship’s movement, motions and vibrations, the cargo compacts and the space volume between the particles reduces, which causes the pore water pressure to rise, reducing the shear strength of the particles(Gouvernec, 2018). The total quantity of cargo in one or all of the holds starts to behave as a dense liquid and reduces the ship’s stability by the free surface effect (see Figure 1). In the worst case the ship might even capsize (Lee, 2017).


1.3 TRANSPORTABLE MOISTURE LIMIT (TML)


Cargoes that have been identified as those ‘which may liquefy’ according the IMSBC code must have a signed certificate of moisture content provided by the shipper to the ship’s master or his representative, including a signed certificate of the transportable moisture limit (TML) as required by Section 4, regulation 4.3.2 of the IMSBC Code.


The TML should be tested and certified according to one of three methods prescribed in the regulatory guidelines by a recognized laboratory.


Following SOLAS Chapter VI, Part A, Regulation 2(1) “The shipper shall provide the master or his representative with appropriate information on the cargo sufficiently in advance of loading…” and “Such information shall be in writing…”.


The IMSBC Code, Section 4, Regulation 4.1.4: stipulates


that “The various properties of a solid bulk cargo… shall be determined… in accordance with the test procedures approved by a competent authority in the country of origin…”


As required by the IMSBC Code, a certificate of moisture content must be provided by the shipper to the master stating the current moisture content with the interval between testing and loading being not more than seven days.


The master must satisfy himself that the moisture content of the cargo is not more than the transportable moisture limit. If the moisture content or MC of the cargo is smaller than the TML, loading may proceed.


The liquefaction of certain types of bulk cargo gave rise to concern with the shipping industry. BIMCO or the Baltic and International Maritime Council, representing the ship owners interest, developed on 25 July 2012 some new charter party clauses for use in time- and voyage charter parties which may involve the carriage of bulk cargoes such as nickel ore and iron ore fines which are at risk of liquefying (BIMCO, 2012).


2.


LIQUEFYING CARGO AND THE STABILITY OF BULK CARRIERS


Some more details regarding the stability of bulk carriers carrying liquefying cargo are provided below.


WL: Water Line M: Metacentre G: Centre of gravity of the ship Gv: Virtual centre of gravity of the ship K: Keel g: centre of gravity cargo GZ: righting arm of stability moment GvZv: righting arm of stability moment after free surface correction B: Centre of buoyancy AA1: Original liquid surface TT1: Liquid surface after cargo shift


Figure 1. The free surface effect


GZ is the righting arm of the stability moment that brings a ship back to its original position when inclined by an external force. The GZ is reduced to GvZv due to the presence of free moving masses on board. This effect is called the free surface effect (FS) (Figure 1).


The magnitude of this free surface effect on the GZ curve can be formulated as follows:


δG = ρ∑LB³ 12∆ (1)


Gv is the virtual center of gravity, ρ is the cargo bulk density (t/m³), Δ is the ship’s loaded displacement (t), L is the length of the free surface (m), B breath of the free surface (m) and ∑LB³/12 = total volumetric heeling moment of the cargo free surface (m4). α is the surface angle of repose, or for cargo that is transported above the safe TML α = 20° (Clarck, 2008).


The result is a serious reduction of the righting arm of the stability moment,


= 0 − sin (2) Where G0Z is the initial GZ, and θ is the angle of heel. A-420 ©2019: The Royal Institution of Naval Architects


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