Page 278 - Practical Design Ships and Floating Structures

Practical Design of Ships and Other Floating Structures 253
You-Sheng Wu, Wei-Cheng Cui and Guo-Jun Zhou (Eds)
0 2001 Elsevier Science Ltd. All rights resewed

ESTIMATING THE RISK OF CARGO SHIFTING IN WAVES
- METHODOLOGY AND RESULTS

A. Ryrfeldt' and T. K&lIstam*

'0, Ericson) Naval Architecture, KTH, SE-100 44 Stockholm, Sweden
'Center of Safety Research, KTH, SE-100 44 Stockholm, Sweden

ABSTRACT
An important cause for ship casualties is cargo shifting in rough seas. According to statistics
approximately 10% of the founderings of general cargo ships are related to the shifting of cargo.
Damage to cargo also means large costs to ship and cargo owners and insurance companies. This paper
presents a methodology to calculate the long-term risk of initial cargo shift onboard a merchant vessel
carrying unit cargo. Important applications of the methodology are the evaluation of new ship and
cargo concepts, as a support for operational decisions and improvement of ship safety through risk
estimations and assessment. In the methodology, models for the cargo shifting process, wave
representation and calculation of ship motions have been combined with statistical models. The
methodology has been used to perform case studies, indicating that the influence of factors such as
stability, speed and lashings have a large influence on the risk of cargo shifting. The results confirm
that the suggested methodology can be used to increase the understanding of factors influencing the
risk of cargo shifting, which can affect design practice and operational aspects.

KEYWORDS
Cargo shifting, Risk analysis, Ship motions

1 INTRODUCTION

One of the major causes for ship casualties is the shifting of cargo, as can be seen from Lloyd's World
Casualty Statistics (1999). According to these statistics approximately 10% of the founderings of
general cargo ships are related to the shifting of cargo. Apart $bm being a threat to the vessel, cargo
shifting causes damage to the cargo and thus costs to ship and cargo owners as well as insurance
companies. Therefore cargo is secured to the vessel to prevent it from shifting, and much focus has
been put on the lashing loads (e.g. Andersson et al. (1986)). However, ship owners and operators also
need a tool for comparing risk levels, for example of different ships and different types of cargo on
different routes. Although several studies have provided tools for evaluating the risk of unwanted
events such as capsizing, grounding and collision, no such tools exist for estimating the risk of cargo
shifting. The problem of cargo shifting is similar to that of estimating the risk of capsize as described