For offshore drilling ships and semisubmersibles, a riser analysis is usually performed for each well to find out the operability, tension settings, operating limits (percentage of offsets) of the riser (and drilling operations) 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 analysis, drilling operation can be done for the target mud weight in environments up to the operate-able environment which an operating offset limit is obtained and shown from the riser analysis (for example, 1 year winter storm wave and current).But in reality, the sea state is changing every day. Sometimes, the current is much higher than that of the operate-able environment and wave is lower than that of the operate-able environment. In another time, the wave is higher than that of the operate-able environment but the current is lower than that of the operate-able environment. Drilling operation may still be able to be done in such environment but it is not included and considered in the riser analysis. This reduces operating time and increases the cost. Drilling contractors such as Ensco want much broader environments to be included in riser analysis, so they can perform drilling operation in those environments to reduce drilling downtime.
This paper has done some studies and riser analysis to find more operate-able environments for drilling operation for the riser system. A much broader environment range is used for riser analysis, and either wave upper limit or current upper limit is found for which drilling operation can still be done and an operating offset limit can be obtained. The result shows a broad operate-able environment range for drilling contractors including a wave vs current operate-able limit curve. By using this broad operate-able environment range and wave vs current operate-able limit curve, drilling operation time will be increased and operating downtime will be reduced.