A new onboard vessel/riser drift-off simulation for dynamically positioned drillships reduces risk and waiting-on-weather time.

DP drilling operations have been conducted for about 30 years now, with significant advances in equipment and controls resulting in low failure rates for the latest generation of dynamically positioned (DP) vessels. Even with these advances, the major concerns with DP vessels are associated with the possible consequences in the event of loss of station keeping. Unless the proper planning is performed and both the operator and drilling contractor agree to well-specific operating criteria (WSOC) that are put in place before the well is drilled, the consequences of loss of station keeping could be high.
When taking DP vessels into offshore locations such as the deepwater Gulf
of Mexico, West Africa, Brazil, Australia, or others, a realistic basis for the WSOC becomes very critical to successful drilling and completions operations. It is in these situations that realistic numerical modeling of the well drilling system becomes important to ensure low risk and low waiting on weather time.
Until recently modeling the complete well drilling system which includes the vessel, riser, blowout preventer/lower marine riser package (BOP/LMRP), conductor and soils model using a fully-coupled, time-domain solution was an extremely time consuming exercise. Performing such an analysis prior to drilling each well was not feasible. This type of analytical solution is now becoming routine and essential to the DP operation. DP alert offsets can now be calculated onboard based upon the prevailing or forecasted environment and drilling rig operating condition.
Drift-off simulator
To meet this need, GlobalSantaFe and MCS have developed an onboard vessel/riser drift-off simulator called DeepDrift, and designed for use with DP drillships.
DeepDrift provides a fully coupled riser and vessel analysis capability that simultaneously determines the drift-off path of the vessel and the riser drift-off response. This approach eliminates the requirement for separate vessel drift-off and riser drift-off simulators and the associated iterative simulation procedure, thereby significantly reducing the simulation times and increasing the solution accuracy.
The program predicts the vessel drift-off path after power failure and determines the location of the alert offsets for initiating riser disconnect, based on allowable limits of the riser (such as upper and lower flex joint angles, telescopic joint stroke, riser and conductor stress).
The program does not communicate directly with vessel and/or riser monitoring equipment. The simulator has been designed to be easy and simple to use, so as to minimize the chances of input errors and ensure correct alert offset predictions. The following input parameters are required for each simulation:
• Wind speed, current profile and wave height;
• Vessel position (heading, offset from wellhead) and heave motion; and
• Riser operational condition (initial slip-joint stroke, mud weight and top tension).
These parameters were selected so that user estimates and inputs are kept to a minimum. For example, the user can either specify the sea state or allow the program to infer it based on the heave period and motion measured by the DP system.
The graphical user interface (GUI) is straightforward and intuitive and is used to specify the inputs discussed earlier and to display the drift-off limits and alert offsets. An example of the operator interface is shown in Figure 2.
DeepDrift in operation
A process flow diagram (Figure 3) shows how to get from the initial site information to predicting the alert offsets offshore. Before moving to any location, site-specific data are provided by the operator to generate the riser stack-up. For each location a fully coupled time-domain drift-off analysis is performed onshore for drilling and non-drilling scenarios, using the MCS DeepRiser engineering tool. In these simulations, the metocean conditions are considered co-linear and the initial vessel position is assumed to be directly over the well. Under realistic offshore conditions, this is rarely the case. As a result, these design calculations are typically conservative estimates of the alert offsets.
Once the design calculations have been performed, the results provide the basis for the DP alert offsets that are used in the WSOC. After agreement with the operator and operations team, these alert offsets are considered the minimum allowable to be used for the well.
A riser, vessel and conductor model, developed onshore using DeepRiser, is provided to the drift-off simulator so that it can predict a drift-off path for the vessel. This model is built-up onshore using DeepRiser, whose analytical engine also forms the core riser analysis program for the DeepDrift simulator. When the model is built, it's then e-mailed to the drilling rig and imported into DeepDrift. It should be noted that this model can only be edited onshore using DeepRiser.
Once the model is imported into the program at the ship, the DP operators can start to perform drift off simulations based on prevailing or forecasted conditions and compare them against the minimum alert offsets specified in the WSOC. Should a simulation provide alert offsets that are lesser than the minimum specified in the WSOC, there will be consultation with the operations team to determine the proper course of action.
The main distinction between the design alert offsets and the onboard predicted alert offsets is that the metocean conditions in the onboard simulation are typically not co-linear. This will typically lead to larger alert offsets predicted onboard. Another key feature of the simulator is the ability to provide alert offsets when the rig is positioned up- or down-weather from the well. By having the simulator offshore, the actual conditions for the drift-off scenario can be analyzed and improved guidance can be provided to the DP operators.
Operational experience
DeepDrift was initially installed on all GlobalSantaFe DP Drill Ships in January 2004 and has been utilized successfully during several drilling campaigns. Most recently, GlobalSantaFe's new-build "Development Driller" Class DP semisubmersibles have installed the simulator for use in day-to-day operations.
Conclusion
An onboard drift-off simulator to supplement WSOC guidelines is providing a significant reduction in WOW time, especially in the more challenging locations where DP drillships have been working. When used as part of a risk assessment process, the DeepDrift simulator provides operations personnel with a tool to make more informed decisions regarding alert offsets in metocean conditions close to WSOC guidelines. The simulator provides assurance that the vessel has sufficient time to perform an emergency disconnect if and when required without damage to equipment or the well. This, in turn, reduces well costs by minimizing time spent waiting on weather in a market with high day rates.