With drilling operations moving to ever-harsher environments, there is an increased need to ensure drilling riser integrity in unplanned incidents such as loss of station keeping.

Working with a major drilling contractor, MCS has developed an on-board vessel/riser drift-off simulator for use on the latest generation of dynamically positioned (DP) drilling rigs. The DeepDrift system predicts the vessel drift-off path in the event of power failure and determines the location of the alert offsets for the riser Emergency Disconnect Sequence (EDS). The simulator enables rig personnel to run "what-if" scenarios on prevailing and forecast metocean conditions.

Shore-based drift-off analysis

In recent years, DP drilling rigs have been active in a variety of well sites ranging from shallow water (1,500 ft to 2,500 ft or 457 m to 762 m) to extremely deep water (10,000 ft or 3,048 m). These sites also have also included a variety of metocean challenges. Because of differing requirements, well-specific operating criteria (WSOC) are developed for each well site.

Prior to operating at a well site, the WSOC are derived for the location and anticipated operations. Typically these will be created with input from the client to ensure that all parties agree upon when it is safe to conduct operations and when operations should be suspended. The WSOC provides guidelines to the offshore operations personnel that document when these decisions should be made. DP alert offsets are a critical part of the WSOC and are derived using metocean design conditions typically provided by the client for the well site.

Prior to arriving on location, a drift-off analysis is performed to determine the alert offsets for the well. During the drift-off analysis, the vessel position and heading is computed and parameters such as telescopic joint stroke are recorded. Allowable limits are established for each of these parameters and are in line with API RP16Q guidelines. The governing allowable limit is determined based on which parameter hits its limit first during the simulation.

The time and distance from the initial vessel position to the governing limit is termed the Point of Disconnect (POD). Once the POD has been determined, the red and yellow alert offsets can be calculated based on the EDS time and an operator-specified time between yellow and red alert offsets.

Once the drift-off analysis is completed, the calculated alert offsets for the connected drilling and connected non-drilling modes are subsequently provided to operations personnel for use in the WSOC. At this stage, these alert offsets are generated by the shore-based drift-off analysis.

The intention of the WSOC is to provide guidance to the crew so that members can determine when it is safe to conduct normal drilling operations with regard to drift-off. Since the shore-based alert offsets are calculated for a specific combination of co-linear wind, waves and current, it can be difficult to assess the alert level in which the vessel should be operating while in realistic metocean conditions.

The actual conditions experienced by a vessel can deviate significantly from the combination that has been used by the shore-based simulation. This can lead to situations where, using shore-based results, the vessel would either move to a higher state of alert or suspend normal drilling operations until conditions have been reduced below the levels set in the WSOC.

Onboard drift-off analysis

In order to reduce the conservatism associated with the shore-based analysis, onboard analysis is used to perform drift-off analysis for prevailing and forecast metocean conditions. Taking the drift-off simulation to a user-friendly onboard system presents several challenges:

•The onboard simulator must add value and not hinder daily operational decisions;

•The simulator should be intuitive and easy to use so that simulations can be performed with only a limited knowledge of the riser model;

•The simulator should allow personnel to perform drift-off simulations with minimal risk of error; and

•The results should be clear and easy to interpret.

The riser model used by the onboard simulator is generated in the office and sent to the rig electronically. The analyst defines the riser stack-up in terms of the riser joints deployed and specifies other details such as soil structure properties, conductor and casing details, and certain vessel-specific information. The riser model for use by the onboard drift-off simulator is then automatically generated by the shore-based software. This is then e-mailed to the vessel, where it is imported into the onboard program prior to running the riser and BOP on each well.

The onboard simulation is typically performed every 12 hours or more frequently if metocean conditions change. Once the simulation has been completed, the yellow and red alert offsets for the prevailing or forecasted metocean conditions are displayed.

The onboard drift-off simulator has an intuitive operator interface that provides a compass-like display of the vessel position relative to the wellhead along with a display of the predicted vessel drift-off path and the location of the yellow and red alert offsets. Wind, wave and current conditions are displayed in both graphical and text format. The display provides the operator with succinct information regarding vessel drift-off behavior.

The operator can specify all input data relevant to the drift-off simulation. Inputs are kept as simple as possible in order to minimize operator workload and the risk of error in specifying the input data.

The onboard drift-off simulator uses a fully coupled time-domain finite element (FE) model of the drilling riser. Figure 2 shows the upper part of the FE model. During onboard drift-off simulation, vessel forces and moments such as current loads, wind loads and wave drift loads are computed and applied to a node located at the vessel's center of gravity. In this way, the drift-off simulator solves for position and heading of the vessel at each simulation time step.

For computing the wave drift forces, the drift-off simulator requires information regarding the ambient sea state. All of the standard wave spectra models are incorporated into the drift-off simulator - the appropriate model for a particular location is selected during the shore-based analysis and specified in the FE model that is e-mailed

to the vessel.

Once the drift-off simulation is complete, the results are displayed on the operator's interface. This includes predicted drift-off path, location of yellow and red alert offsets and time taken to reach these, and values of the critical riser parameters at each of the alert offsets and the POD. The operator can print a one-page summary showing all these results in addition to the metocean conditions used in the simulation and the appropriate limiting values. This feature permits hard-copy records of all simulations to be kept, which allows specific metocean conditions to be recreated and simulations re-run in the office onshore.

In addition to these summary results, the operator may also view a detailed report file that presents key results in graphical format. These include time histories of all critical riser parameters during the drift-off simulation as well as snapshots of the bending stress and bending moment in the conductor/casing at the time of the POD.

An onboard drift-off simulator to supplement WSOC guidelines is proving to be valuable to operations, especially in the challenging locations where DP drilling rigs work. This simulator provides a tool to make more informed decisions regarding alert offsets in metocean conditions exceeding the WSOC guidelines. The simulator also provides assurance that the vessel should have sufficient time to perform an EDS when required, without damage to equipment or the well.