Abstract
The modern oil rig uses the dynamic position system (or DP System) to maintain position and provide ways for the rig to accomplish its mission. Although the DP System has improved its reliability over the years, it can eventually fail and produce a critical incident called a drift-off. In this situation, the riser, wellhead and well casing will be subject to a fairly large amount of stress that must be properly evaluated.
It is well established in the offshore industry to perform this assessment by performing a time domain coupled analysis simulating the behavior of a riser, wellhead and well casing in the event of drift-off. This is commonly called drift-off analysis.
Over the past few years, significant improvements have been achieved in the mathematical models of these analyses, but even today we add many conservative factors to cover forthcoming operation uncertainties. Major factor are environmental conditions used on the analysis. Seeking to cover all scenarios, extreme colinear values are used as analysis input, resulting in narrow unfeasible operational watch circles for shallow waters or sensitive wells.
In this article we present the practical methodology and the results of our experience using commercial software Optima™ that performs drift-off analysis using on-site conditions. By using actual and short forecasted conditions, on-site analysis results provided valid feasible watch circles, allowing operations to take place in safely manner.
