Prediction of Weather Downtime for Derrick Barges

Abstract

ABSTRACT As oil is found in more hostile environments, economic and rapid completion of offshore production facilities will depend more heavily on the sea keeping ability of construction vessels. However, the relationship between vessel motion characteristics and the percentage of time that the vessel will be able to work is not generally understood. The net effect of the vessel motions on lifting operations can be reduced, in fact, to a single value represented by the vertical motion of the crane boom tip (or sheave centerline). The explanation for this is quite simply that the corresponding vertical motion of the hook controls the ability to safely pick-up or set-down a load. This single assumption presupposes that the lateral or swinging motions of the load are not a direct function, but rather an indeterminate consequence, of the lateral or horizontal motions at the boom end. Thus, vertical hook or boom end motion can be used as a parameter to qualify the operational tenacity of a derrick barge. Barge operation reports and corresponding measured wave data were analysed to yield sea-states when work could be accomplished. Further analyses provided the maximum vertical boom end motions with which piling work or module lifts could be carried out. These maximum vertical motions can now be used as criteria todeterm1ne the expected number of working days for a particular vessel at a given location, according to the operation required. A computer program has been developed to carry out the process, which is described in the paper. The method is based on the vessel motion characteristics, statistics of the expected weather and limiting criteria of vertical motion. The paper discusses the influence that equipment aids, such as heave compensators and tag-line .systems, might have on the predictions. A comparison is shown of the expected performances of typical heavy lift vessels in the North Sea. INTRODUCTION Model tests have been used for many years to determine the sea keeping characteristics of floating vessels. More recently computer programs have been developed which will perform the same task with equal reliability, but less expensively and much more rapidly. However, it is not immediately obvious from the fundamental motion responses derived by either method as to how efficiently a proposed vessel will operate in practice. All that can be done, provisionally, is to compare the motions of the proposed vessel with those of existing vessels. This procedure is clearly insufficient, even if motions in corresponding sea-states are compared, because it fails to recognize the effect of variations between the arrangement or geometry of different vessels. It also completely ignores the practical factors of the operation to be performed by the vessels. For a derrick vessel the ability to work is primarily a function of the relative motions between the package being lifted and the foundation on which it is to be set down. The ability to work can also be controlled by relative motions at the time a package is to be picked up.