Voyage Acceleration Evaluation of Design Climate: A Comprehensive Statistical Method for the Evaluation of Design Loads for Offshore Transport
Authors: Aalbers A.B. , Leenaars C.E.J. , Quadvlieg F.H.H.A. 
Published: October 2008

The paper presents a new procedure for the calculation of realistic values for the design loads for heavy lift and vulnerable cargo transports. The presented work is the result of a joint effort of 6 surveying companies, 5 heavy transport companies and MARIN.

Traditionally, design accelerations for ocean transports were calculated by the use of a so called design wave. Now, a tool is produced which takes the motion climate during a complete voyage into account. During a voyage, a ship meets a variety of wind and wave conditions called the wind and wave climate. The ship and the crew react on these wind and wave conditions. This results in a motion climate for that particular voyage. The most probable maximum load during a transport is derived from the motion climate and on the time actually spent at sea.

The final goal of the participating companies is that the method will develop into an international industry standard.

Introduction

A user friendly computer program called VAC (Voyage Acceleration Climate) was developed. The ship motion calculations are based on striptheory, using 2-D diffraction theory to solve the hydrodynamic potential per section. This makes the method applicable to all monohull ship shapes.

In this paper, the general methods used in the program to derive the motion climate on arbitrary voyages of the ship are treated. Several aspects are elaborated: the voyage modelling, the wave climate, the calculated motion behaviour, the scenarios for ship operation, the sustained speed, voyage duration and acceleration statistics. In the second part of the paper, the results of the program are demonstrated in a number of different cases, some of which have been used for validation.

The application of the method is not restricted to heavy overseas transport design, but also for offshore operations like dredging and installation workability. As many of these type of operations make use of weather forecasts, to select a weather window, the VAC computer program allows to apply a wave height limit. Furthermore, for the design of ships, the program can be used to assess design values for e.g. container lashings or to assess the fatigue life of structural parts or cargo. To this purpose it is possible to define a linear combination of signals in a user interface. This can be used to compute loads or stresses at locations of interest.

New in this method are the following aspects:The probability of all wave heights on the route with their range of different periods are taken into account. So, it is no longer necessary to select a design wave.The effect of the actions of the crew can be taken into acount by applying various scenarios. In this way, a much better fit is obtained between the actual motion climate as experienced and the calculated motion climate.The effect on roll of water on deck can be taken into account. This is applicable for flat top barges only. The effect on the roll motions was already demonstrated in the Noble Denton Barge Research, Ref. (1)