As the industry continues to push the boundaries of offshore exploration into deeper water depths and harsher environments there is an increased need to ensure the integrity of both the drilling riser and wellhead in the event of loss of station-keeping in dynamically positioned (DP) rigs. In order to determine vessel limiting operating/alert offsets it is necessary to carry out a vessel drift-off analysis which predicts the vessel drift-off speed and direction in the event of a DP failure.

This analysis is typically performed for a limited combination of environmental conditions (usually collinear wind, wave and current) and will generally assume a vessel position that is directly above the wellhead. Whilst the results of the standard drift-off analysis, and associated drift-off plots, are useful in determining expected drift-off speed and feasibility of the emergency disconnect sequence (EDS), the outputs are of little operational use to the rig personnel in determining allowable operational/alert offsets for day to day operations. As the vessel is rarely located directly above the wellhead and environmental conditions can vary significantly in both direction and magnitude over the drilling campaign a more comprehensive analysis methodology and set of load cases is required in order to develop useful inputs to the well specific operating guidelines (WSOG).

The current standard industry approach is to generate drilling riser operability envelopes independently of drift-off curves and thus recommended operating offsets do not account for drift-off speed, reaction times or EDS times. This can lead to a potential lack of conservatism in WSOG. In order to avoid this lack of conservatism in calculation of operating offsets a coupled steady-state/transient response analysis approach is required to ensure that a DP failure event at limiting operational offset does not lead to damage to the wellhead or riser system.

A new revised methodology for the calculation of allowable operating windows and associated alert offsets is presented in this paper. This new methodology couples both the steady state operating limits of the vessel and the transient system response during a drift-off, due to DP failure, in order to present a comprehensive go/no-go operability chart of allowable offset vs. environment for use as input to the WSOG.

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