A mooring consequence view on a monitor screen.
Drilling in deepwater presents rig personnel with a number of challenges. Among them is the requirement to position the rig such that the riser always operates within allowable limits. In deepwater, currents can have a greater (and potentially less predictable) influence on the riser than in shallower water. Even in shallow water, an accurate prediction of the riser response to changing conditions or vessel position is useful to onboard personnel.
The questions a riser management system should answer are: (1) where are we now? (2) what's going to happen next? (3) do we need to do anything now? Where are we now is most easily answered by using visual displays showing riser angles, slip joint stroke, rig position, and stationkeeping performance. Stationkeeping performance can cover both dynamically positioned (DP) and moored systems.
What happens next depends largely on weather and current conditions. The use of weather forecasts can be used to determine future influence of wind and waves. Current is more difficult to predict but recent experience using a type of management system developed by Global Maritime in the West of Shetlands corridor has shown that knowledge of recent conditions allows the operator to anticipate future trends.
With experience a rig operator will know where to move the rig to prevent excessive riser angles, though the best time to do this may not be obvious. A simulation tool would give an operator confidence that the actions are correct and allow what-if tests to be run before committing to an actual repositioning.
Riser behavior
A mooring status report summary on a monitor screen.
The behavior of a marine drilling riser is determined by the mean vessel offset, the drilling parameters (mud weight and riser tension), and the actions of current and waves upon the riser.
Vessel position and riser tension have to be controlled in such a way that the riser angles (particularly the lower flexjoint rotation) are within limits dictated by the operation in progress. It is also advisable (mandatory for DP vessels) to check what the consequences of a system failure may be in terms of extreme angles and riser stroke.
The failures may be sudden mooring line breakage or thruster/power failure. The latter is more critical in the case of a DP vessel. If these consequences are sufficiently serious drilling operations may need to be altered, suspended or, in extremis, the riser disconnected.
Drilling operations cannot normally be suspended instantaneously so decisions need to be taken ahead of any time-consuming changes in status or procedures (hanging off, disconnecting, etc). Thus, it is particularly advantageous to be able to anticipate the detailed effects of changing weather and ocean current conditions.
With appropriate consequence analysis and forecast/simulation software, a riser management strategy can be implemented which both enhances the safety of operation and reduces downtime by identifying the optimum vessel position and maximizing warning of the requirement for operational adjustments.
Riser modeling
A 3D plot of the deflected shape of a riser.
The horizontal forces exerted by currents acting over the length of the riser cause the riser to adopt a "bowed" shape. The distance between the upper and lower flexjoints (measured along the riser) is then greater than when straight and the curvature thus causes the slip joint to extend (stroke out).
In common with offset, riser bowing always extends the slip joint - only heave or tidal reduction in water depth can compress the slip joint. The bowing may be in quite a complex shape if the current changes direction appreciably over the depth.
The factors affecting the bowing of the riser are: top tension, mud weight, and current velocity profile over the riser/water column. It is possible to measure current velocity profiles using an ADCP (acoustic doppler current profiler). The riser management system does not require that current profiles are measured by sophisticated equipment, but can incorporate them if available.
An accompanying example shows the potential benefits of stationkeeping management. Placing the vessel in an optimum position can prevent excessive slipjoint stroke in the event of a failure. The example is for a moored semisubmersible. The DP benefits of this kind of operational strategy are in maximizing the time before offset would become critical.
The Skaas system
Global Maritime's onboard riser management package, known as Skaas (Stationkeeping Alert & Advisory System) can provide rig operators with advance information on riser performance in order to maximize drilling time. The system is the latest in a series of advisory computer systems developed for stationkeeping and stability monitoring.
The system monitors existing conditions and predicts how changes will affect the riser and DP/mooring.
The system was developed from earlier units deployed on the Amerada Hess production vessel AH001 and on the Sedco 706 which is providing tender assist drilling to Total's Dunbar platform. The latest system was fitted to the Jack Bates on behalf of Mobil North Sea and has been in operation since May 1998. The new system is able to provide the following:
- Data required for decision-making is displayed in one location.
- Trends in weather and riser angles provide for timely repositioning.
- Early repositioning can reduce drilling downtime.
- Black box logging of data allows for later examination of unusual incidents and events.
- Web access provides office-based engineers and managers with real time data.
