For offshore drillship, a riser and drift off analysis is usually performed for each well to find out the operability, tension settings, operating limits (percentage of offsets) of the riser for certain target mud weights and environment (such as 1 year winter storm and 10 year winter storm including wave, current, and wind). Based on the results of riser and drift off analysis, drilling operation can be done for the target mud weight in environments up to the operable environment which an operating offset limit is obtained and shown from the riser analysis (for example, 1 year winter storm wave and current). For the drift off analysis, we usually do an analysis for a typical environment (such as 1 year winter storm) to get watch circles for that environment.

But in reality, the sea state is changing every day and is not the same as the typical environment used for the riser and drift off analysis. Most of the time (normal operation), the environment is calm sea conditions (with small waves, low wind speed, and low or high currents). Because watch circles change with environment, the watch circles for normal operation are much different from that of the typical environment. Drillers may need to know drift off limits in other environments (such as normal operation environment) to get a real picture for drift off and watch circle. Also, drillers want to know the drift off limits in high loop current environment to make operational decisions.

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