Abstract

Off-shore operations and installations can be occasionally delayed or interrupted due to the arrival of severe metocean conditions which could put human safety or structural integrity of the work at risk. In particular, the motion and loads induced by wind, waves and current on the vessel and on the installing structure could exceed the allowable limit for a safe and correct installation.

Whatever the operation that is being carried out, it is necessary to have a favorable window with a duration longer than the time necessary to complete the job or to interrupt the operation in order to carry out the work safely, e.g. pipe abandonment for pipe laying.

Predicting when these windows will occur is not an easy task and the decision-making process shall be correctly managed by a reliable balance between safety, first of all, and efficiency, to not unduly increase time and the cost of operations. These two aspects, safety and efficiency, respectively, tend to cause unnecessary/anticipated interruptions to the work with consequent delays or to cause late interruptions of an operation with risks to assets integrity and/or human safety.

The available information for managing the decision-making process is the monitoring of the actual conditions, both weather and installation vessel motions, and the weather forecast bulletins. Valuable information can be found also in the metocean hindcasted time series, mainly wind and waves, nowadays available for any off-shore area which the oil&gas industry is interested in with a typical duration of 30 years, for the determination of the typical characteristics of the metocean conditions of the area of interest.

This paper presents a comparison between the performances of different criteria for interruptions of long duration operations based on monitoring and on forecast, proposing an improved method based on the combination of the two kinds of information. A preliminary analysis of the metocean conditions of the area based on hindcasted time series is considered as well.

It should be noted that the same principle can be adopted for short operations to be completed within favorable weather windows. In this case the time for completing the operation is equivalent to the time necessary to safely interrupt the operations.

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