During drilling and well intervention (DWI) operations today operating limits are normally given as limiting wave height, and sometimes wave periods. The resulting diagrams are often not directly comparable with weather information received on the rig and the final decisions are often based on subjective assessment of wave height and period. The paper will present how BP, on the newly developed Skarv field in the Norwegian Sea, through thorough planning in the engineering phase has implemented a system where operating limits are specified based on directly measurable parameters such as rig heave and upper and lower flexjoint angles. How weather forecasting can be translated to give the rig crew direct forecasting of the limiting vessel or riser responses (e.g. flexjoint angles or heave), will also be presented. It will be shown how this allows for improved operational planning and support from onshore. Over the last years requirements for oil companies to be able to document the structural integrity of their subsea assets, including wells, has increased. On the Norwegian Continental Shelf (NCS) there has been a particular focus on fatigue loading in the wellhead structure, including the upper sections of casing and conductor, due to loads induced by the riser and BOP during DWI operations. There have been cases where the design fatigue life of a wellhead system limits the number of days one can perform operations with a rig on a given well. This in term affects future oil recovery rates as the well fatigue life may not be sufficient to allow for side step drilling or intervention work required to maintain an optimal production from the well. The paper continues to present how BP on the Skarv field, stores and utilizes the measured lower flexjoint response to track and document well integrity. It will be demonstrated how the return on investment of a drilled well can be improved by documenting actual fatigue loading from each operation on a well compared to conservative design calculations. BP has addressed the above issues in a way that is likely to set a new standard for drilling and intervention operations in the North Sea in the future. 4Subsea AS has provided the engineering and instrumentation services that formed the basis for this paper.

Operation limits for temporary riser system are determined according to some probability of exceedance of a relevant variable. Accordingly, consistent statistical analysis and probability modelling of the data is required. The common industry approach is to rely on the classical narrow-banded Gaussian process assumption when considering time series of variables of interest. Thus, the time series peaks are characterized by means of the Rayleigh distribution and the relevant extreme values are estimated based on this. However, non-linearities present in riser systems may yield non-Gaussian (wide-banded) processes, rendering the classical approach inappropriate. In the present work, an approximate and practical method is presented to address above issue. It is demonstrated that the approximate method is capable of consistently estimating the relevant extreme values, even where the classical method comes short.