Determination of Transportable Moisture Limit of Bulk Cargoes

Tamaki URA Institute of Industrial Science, University of Tokyo 7-22-1, Roppongi, Minato, Tokyo 106 Japan Tel:+81-3-3402-6231, Fax:+81-3-3402-5078

 

SUMMARY

 

When solid bulk cargoes, such as concentrates and coals, contain high moisture and are subjected to cyclic forces, pore water pressure successively rises resulting in abrupt loss of shear strength. To minimize the risk of this phenomenon, i.e. liquefaction, the BC Code introduces the upper bound of moisture content of cargo, which is defined by the Flow Moisture Point (FMP). The Flow Table Test has been introduced in the BC Code as a recommendable method to measure the FMP, but it is widely recognized that its reliability is not adequate since the resulting value considerably depends on the ability of the operator. Besides, this test is not applicable to coarse materials such as coals.

The Penetration Method has been developed as an alternative test method in Japan. A test sample in a cylindrical vessel is subjected to vertical vibration and a penetration bit is placed on the surface of the sample. When a depth of the bit exceeds a criterion for six minutes, it is concluded that the liquefaction occurred, i.e. the sample contains water more than FMP. On the basis of the test results of several materials, it is concluded that the Penetration Method can be applicable to wide variety of materials.

 

INTRODUCTION

bulk-fig1

 

*********Fig. 1 Dangerous Shifting of Bulk Cargo *****

 

Due to cargo shifting, there have been a number of serious casualties which resulted not only in the loss of the ship but also in loss of life. Since shifting develops abruptly, it is difficult to prevent the ship from heeling when the master of the ship recognizes cargo shift. It is substantially important, therefore, that the property of the material has been examined prior to loading and the cargo in the hold is reasonably trimmed to the boundary of the cargo space.

Bulk cargo moves in a hold in two ways, i.e. sliding failure and liquefaction (cf. Fig. 1 ). To prevent grain from sliding failure, it is regulated in the Chapter 6 of the SOLAS Convention (International Convention for the Safety of Life at Sea ) that "All necessary and reasonable trimming shall be performed to level all free grain surfaces and to minimize the effect of grain shifting." Though this chapter will be revised to "The International Code for the Safe Carriage of Grain in Bulk" in 1994, the basic idea will not be changed. To prevent other cargoes in bulk from shifting, "Code of Safe Practice for Solid Bulk Cargoes ( BC Code ) " was published in 1969 and currently revised on the basis of discussion at the BC Subcommittee of the International Maritime Organization ( IMO ) .

 

When solid bulk cargoes with fine grain size, such as concentrates and coals contain high moisture and is subjected to cyclic forces, pore water pressure rises with a consequent reduction in the effective stress. In case that magnitude of the external force and number of cycles exceed certain limits, they abruptly lose shear strength and flow like fluids. This phenomenon is called "liquefaction" and causes dangerous cargo shift. To minimize the risk of liquefaction, the BC Code introduces the upper bound of moisture content of cargo called the Transportable Moisture Limit (TML). The TML is defined as 90% of the Flow Moisture Point (FMP), which depends on the characteristics of cargo and should be measured experimentally.

 

bulk-fig2

 

**********Fig. 2 Flow Table Method**********

 

For determination of the FMP, the Flow Table Test (cf. Fig. 2 ) has been introduced as a recommendable test in the BC code. It is widely recognized, however, that the reliability of this method is not adequate since the resulting value depends considerably on the ability of the person who conducts the test. It should be also emphasized that this test is not applicable to coarse materials such as coals.

The Shipbuilding Research Association of Japan has organized a committee in order to investigate an alternative test method, which is less dependent to the operator, and applicable to coals and similar materials, and developed a new method called the Penetration Method.

 

PENETRATION TEST

bulk-fig3

 

**********Fig. 3 Basic Idea of the Penetration Method **********

 

Since liquefaction is accompanied with loss of shear strength, occurrence of liquefaction of a test sample can be readily verified by putting a weight on the surface (cf. Fig. 3). When a flow state developed, the weight sinks into the material.

The basic idea of the Penetration Method has its origin in this phenomenon. A test sample in a cylindrical vessel is subjected to vertical vibration of acceleration X in 50/60 Hz exerted thereon. To judge liquefaction, a penetration bit of P gf/cm2 as the weight is placed on the surface of the sample. When the penetration depth of the bit exceeds a criterion D mm for T minutes, it is judged that the liquefaction occurred. Otherwise, the moisture content of the sample is less than the FMP.

 

bulk-pictu

 

********Fig. 4.1 Apparatus for the Penetration Method.*********