A Method for Predicting Weather Downtime For Semisubmersibles
Authors: Springett C.N. 
Published: December 2007

This paper presents a method for calculating the percentage of downtime resulting from weather that a semisubmersible will experience in a particular sea state. An example for a vessel operating in the South China particular sea state. An example for a vessel operating in the South China Sea is given.

Introduction
In recent months, the market for semisubmersible drilling vessels has been reduced significantly. This is partly because of the sharp increase in the number of semisubmersibles now available, and partly because many oil companies have cut back their exploration budgets. As a result, obtaining contracts for semisubmersibles has become a very competitive business. An important tool that the owner has at his disposal to help sell a particular semisubmersible is a method for predicting the percentage of time that a vessel will be unable to work because of percentage of time that a vessel will be unable to work because of the weather. The principal factor affecting weather downtime is vessel motion resulting from wave and swell action. To predict accurately this downtime, a knowledge of the vessel's response to the complete range of sea and swell conditions is needed. This response for the critical motions (heave, pitch, and roll) is usually available in the form of a response-amplitude operator. These responseamplitude operators are obtained from either theoretical calculations or model tests. This paper describes in detail one approach to predicting weather downtime and shows how it can be applied to any semisubmersible for which the motion responses are known.

Theory
The basic tool used to predict vessel performance is spectral analysis. Detailed weather information is usually gathered for each ocean area and is presented as the percentage of time that various wave height and period percentage of time that various wave height and period groups are experienced in a particular area for each month or season. A typical presentation of this data for the ocean area off Vietnam in January is given in Table 1. The wave height and period groupings are broad in their definition, but with the wave power spectrum they can be used very precisely. In this instance, the Bretschneider formula for precisely. In this instance, the Bretschneider formula for the typical wave spectrum has been chosen:
.........(1)
It should be noted that the spectrum is calculated so that the ordinate is twice the square of peak-to-peak wave height. This is not the usual method of presentation, but it has the happy result that the area under the spectrum curve is exactly the square of significant wave height. If any other ordinate definition is used, the area is some fraction of the square of significant wave height. The wave data for the example presented here were taken from Ref. 2.
The values for Hs and Ts that appear in Eq. 1 are taken as the mean values, in each case, of the particular wave-period grouping. For example, one common category of waves fell in the 10- to 11-second, 13- to 16-ft bracket. Hs = 14.5 and Ts = 10.5 are substituted in Eq. 1, which has been plotted in Fig. 1 as spectral density, to a base of frequency. A similar spectrum for each remaining wave height or period grouping can be generated similarly. The significant wave height for that grouping is the square root of the area under the curve. A particular vessel motion can be described in a manner similar to the wave.
JPT
P. 649