The Riser Management System, a Dynamic Operational Approach for Drilling in Extreme Conditions
Drilling operations in extreme, ultra-deepwater environments with harsh metocean conditions are a challenging proposition, with high risks to asset integrity.
But Total met the technical and financial challenges when drilling the Raya-1 well offshore Uruguay by using a Riser Management System upgraded to our specifications. These included the ability to redraw watch circles in real time and calculate anticipated operating envelopes several times a day in line with metocean forecasts. As a result of these innovations, the crew was able to make decisions in real time, continuously adjust drilling operations, and set a depth record.
Drilling the impossible: Start where others stop
In adverse metocean conditions, the ship’s heave, surge and sway can break the riser. In such circumstances, drilling operations sometimes have to be shut down to avoid compromising the integrity of drilling equipment. This is known as Waiting on Weather (WOW) and can result in significant delays and extra financial costs.
The Maersk Venturer and a platform supply vessel (PSV) on the Raya-1 site
The Raya-1 wellhead
The Limitations of a Standard Approach in a Complex Environment
All offshore drilling operations are governed by Well Specific Operating Guidelines (WSOG) that are used to determine whether drilling can be safely continued or needs to be stopped. The tool is based on a key factor: the vessel’s positioning. And with good reason. Because the elastic limit of drilling equipment is low, it is important that the rotary table is in a position where stress is as low as possible. That usually means vertical with regard to the well; in the case of strong currents, offset in the direction of the least load on the riser. Depending on the degree of drift-off, which can even result in loss of position, the riser can be disconnected as necessary.
The standard approach is based on a simple method involving static and geometric limits. Time-to-Go (TTG), i.e. the time remaining in which the riser can be disconnected before a component fails, is determined by computing the drift rate in the event of loss of position under historically extreme meteorological conditions. In other words, a single calculation of watch circles, which are fixed and centered on the well position. This method is very conservative, since no system can indicate a better position in the event of currents in the depth range. (The least load position is not necessarily vertical to the well in these cases.) When the limits of the watch circles are reached, operations are immediately halted, which can result in significant extra costs even though project safety was not actually jeopardized.
Hence the need to develop a new, more dynamic approach that maintains good operability during challenging drilling operations, takes into account the real operating envelope, reduces shutdowns related to metocean conditions as much as possible, and decreases the risk of riser disconnection.
Taking Drilling to the Next Level with the Riser Management System
Drilling the Raya-1 well offshore Uruguay was expected to be one of the most difficult of such operations ever undertaken, combining a record depth of more than 3,400 meters and metocean conditions similar to those in the West of Shetlands area of the North Sea. The WOW stoppages that would have been entailed by using the standard method would have put us significantly behind schedule and increased the equipment’s exposure to the harsh prevailing conditions.
Already used by our contractors to monitor certain parameters, the Riser Management System (RMS) is a computer connected to sensors on the drilling rig and to the vessel’s dynamic positioning system, interfaced with the geolocation systems that include acoustic receivers and a Differential Global Navigation Satellite System. We decided to optimize the RMS so that it could also manage operating envelopes and adjust them in real time. After specific improvements, the system can now be used to:
- Identify the optimum position for the drillship to reduce the loads and stress on the drilling rig as much as possible and, as a result, cut WOW and riser disconnections.
- Determine the TTG using a virtually live computation of the vessel’s potential drift-off in the event of a failure, based on the data from the dynamic positioning system, the wind, the waves and the current.
- Produce dynamic watch circles centered on the drillship’s optimum position, which is determined by calculating drift and taking into account the main stresses on drilling equipment.
This new calculation method, patented as DynWSOG, was used for the first time on Raya-1, in close cooperation with the drilling company (Maersk Drilling), the RMS supplier (Kongsberg) and the riser design provider (Wood Group Kenny).
The RMS offers crews fast and efficient decision support. More broadly, this innovation expands operating envelopes while ensuring maximum safety and even making possible certain deepwater projects. The technical success of the Raya-1 well, which set a new record in terms of water depth, also consolidated our position as a leading subsea operator and allows us to optimize drilling operations in any challenging geographical area.
A comparison between standard watch circles (complete circles) and the dynamic limits of the RMS (the arcs). The limits are reached when the riser tensioner, or RT, (the small circle in the center of the ship) intersects with the arcs. The arcs are less conservative than the circles.
The tools developed: Results of computing the anticipated operating envelope according to the metocean forecasts (decision support).