Comprehensive assessment targets feasibility of using 7th-gen floaters to go beyond the 12,000-ft frontier

Maersk Drilling analyses indicate that only limited rig modifications are needed to drill in 13,000 ft of water if working in benign areas, but experience is key

By Konstantin Puskarskij and Kristian Hansen, Maersk Drilling

As reservoirs become depleted and demand for oil and gas remains stable, the offshore industry is pushing to go deeper and deeper. By going into new frontiers, at unexplored water depths, operators see the opportunity to get more from existing assets.

A considerable amount of ultra-deepwater oil and gas prospects are split by the 12,000-ft water depth frontier, due to limitations in drilling vessel and equipment design. Even bigger discoveries potentially lie just across the 12,000-ft water depth demarcation line. Consequently, more operators are starting to investigate how to unlock these resources and extend their portfolio without investing in new drilling rigs and equipment.

However, to do this, they need more than a rig with the right specifications.

Assessing the Requirements for a New Frontier

When assessing wells to be drilled in up to 13,000-ft water depths, it becomes important to hold in-depth knowledge of both equipment and operational limitations, which often go hand in hand.

Maersk Drilling is the current world record-holder for the deepest water-depth oil or gas well ever drilled. The record belongs to the Maersk Venturer drillship, which drilled the Raya-1 well offshore Uruguay for Total E&P in a water depth of 11,168 ft in 2016. More recently in June 2019, the Maersk Viking drillship performed a BOP run at 10,125 ft at Nyankom-1X offshore Ghana, an operation involving 110 riser joints being run safely and successfully in the first go.

Because of operator interest in drilling wells beyond the 12,000-ft frontier, Maersk Drilling has performed several assessments since 2017. The objective was to investigate the feasibility of drilling prospects at 12,500- to 13,000-ft water depths with existing seventh-generation drillships exposed to benign metocean conditions.

The assessments have also aimed to define a scope of upgrades required to drill such deeply located prospects.

Lessons Learned

When reviewing previous ultra-deepwater campaigns, the number one lesson is that proper planning is of paramount importance. For Maersk Drilling, such planning includes tasks related to engineering, logistics, operation readiness and project execution. After-action reviews involving the rig crew, mariners, engineers and client experts have proven equally important.

The experience of drilling the Raya-1 well offshore Uruguay emphasized the importance of understanding the interactions between hull design – in this instance, a ship-shaped hull – actual sea conditions, and the selection of optimum heading to minimize vessel motions, i.e. heave, roll and pitch.

As an example, the dynamic load of the riser and BOP system when running the equipment to seabed will be influenced by vessel heave. Visual monitoring of BOP oscillations prior to landing and latching on to the wellhead is also key, e.g. by verifying the BOP lineup both vertically and horizontally. So is checking the drawworks’ ability to stabilize the significant vertical load when operating in heave compensation mode.

Another lesson learned was the understanding of how water depth, mud weight and maximum anticipated wellhead pressure (MAWHP) may exceed the absolute pressure ratings of the equipment. Under certain circumstances, it may be not possible to test the BOP to the MAWHP unless the API Technical Report (TR) 12 approach is applied.

Other aspects should also be assessed, like seabed temperature and effect to BOP rubber goods, and the availability of dynamic positioning ancillary equipment, including the communication properties of this equipment through more than 12,000 ft of water.

Generic and Location-specific Riser Analyses

In preparation for ultra-deepwater drilling, it is necessary to perform location-specific riser studies, based on relevant metocean data.

This is required to build an optimal riser stack-up in order to minimize the static weight without affecting any other key parameters (utilizing premium buoyancy modules, minimizing amount of slicks, etc).

The riser studies must also include a check of top tension requirements given the maximum mud weight as per the drilling program, as well as a confirmation of tension system capacity, taking into account potential failures of tension cylinders or wires.

Further required checks concern riser burst utilization and riser collapse strength for a partial mud loss scenario in an emergency disconnect event.

In addition, the extreme water depth makes it necessary to confirm that main pipe von Mises stress and axial forces will stay within the allowable limits for a number of load cases with pressurized auxiliary lines.

The worst-case dynamic loads when running the BOP and riser must be calculated, taking into account relevant tolerance factors and damping properties of the string. It must be confirmed that the running loads will stay within the rig’s topside capability, as well as within the riser main pipe and connector design limits.

Further, for BOP running, potentially limiting sea states, such as waves and current, should be examined. By doing so, safe running conditions can be determined for the area of operation. Riser recoil and hang-off conditions should also be assessed, and it may be necessary to run checks on watch circles and drilling envelopes, although these are not considered restrictive in ultra-deepwater.

Testing Topside Equipment

During the course of topside assessment, the load path was reviewed to ensure that forecasted static and dynamic loads would not exceed the rig’s equipment specifications when running the BOP.

Naturally, rigs with 3 million-lb hook-load capabilities are better positioned compared with rigs that have 2.5 million-lb capacity. However, analyses showed that with proper planning and under benign conditions, it is not required to upgrade the rig to 3 million-lb capacity to land a BOP in 13,000 ft of water.

Next, the heave motion compensation system was assessed, requiring either crown-mounted active heave compensators or active heave drawworks for BOP landing operations at the government weather scenario. Additionally, the rig’s tensioner system was assessed against overpull requirements, tensioner equipment failure cases and riser recoil scenarios. In this way, it was ensured that the riser could be supported and lifted in an emergency disconnect scenario, with all given environmental loads and mud weights.

Finally, it was important to review rig arrangements to identify where to store additional riser joints. Other storage scenarios could also be considered, e.g. by using a supply vessel or barge.

Third-party Equipment to Support the Job

When considering the third-party equipment required to construct an exploration well at water depths of 12,500 to 13,000 ft, the main question is whether the remotely operated vehicle (ROV) to be used is sensitive to extreme water depths.

During the course of the exercise, Maersk Drilling approached main ROV suppliers to ensure that they are able to deliver a model rated for use at 13,000-ft water depth.

Operational Readiness for Ultra-deepwater

When making preparations to drill at extreme water depths, a number of additional points have to be considered to ensure full operational readiness.

A major consideration is the risk of gas in the riser. Due to the extended marine riser column, the likelihood of getting gas trapped in the marine riser increases substantially compared with operations conducted under less extreme conditions. This is driven by the time gap between influx detection at the surface and annular BOP shut-in, which means that the influx may already have entered the riser. The gas will then reside above the closed annular BOP before it eventually migrates to the surface.

In such a scenario, it is important to have the capacity to isolate the gas in the riser but also to be able to release the gas volume in a controlled manner. This can be done by employing a riser gas handler, which is a riser joint with an integrated annular and flow spool where both components allow the driller to isolate the riser and divert the flow to the outside.

To minimize the time between kick detection and annular BOP shut-in, it is recommended to employ an early kick detection system that employs a mass-flow meter on the return line and sometimes additional mass-flow meters on the mud pumps. Such an arrangement helps the drillers to detect even the smallest influxes, which enables them to enact a timely response to minimize both influx volume and intensity.

The use of managed pressure drilling can improve safety even further because it adds the ability to apply backpressure to minimize influx. However, it is important to understand the system’s limitation in terms of riser burst capacity. A conventional burst check is part of the riser analysis and will help to identify the border line for mud weight vs applied backpressure. By applying the API TR 12 approach and including external pressure in the assessment, it should be possible to extend the operational window for mud weight vs backpressure.

Similarly, the high precision acoustic positioning (HiPAP) system normally used for dynamic positioning becomes less effective at 12,500- to 13,000-ft water depths due to the signal lag between the seabed and topside, reducing the dynamic positioning system’s reliability. In the tests performed by Maersk Drilling, it was concluded that deployment of the Hydroacoustic Aided Inertial Navigation system developed by Kongsberg offset HiPAP signal risks, as well as the risk of losing the GPS signal.

The good news is that ultra-deepwater depths, besides the many natural challenges, also come with a potential for efficiency gains for rigs with dual derricks. The added capacity means that the second well center can be used to build and run a casing string, for example, while the primary well center is occupied with drilling activities, supporting a smoother 24/7 operation. Hence, it is important to review the complete operation and identify where the water depth can be used to unlock opportunities.

A New Frontier to be Conquered

To summarize, the results of studies by Maersk Drilling’s Technical Organization, supported by several engineering companies, indicate that it is feasible to drill in 13,000 ft of water with a seventh-generation floater. This is conditional on working in benign areas defined by a riser analysis window and requires only limited modifications to rig equipment and systems. BOP qualifications currently cover 12,500 ft, and Maersk Drilling expects that 13,000-ft assessments will be done by OEMs in the foreseeable future.

In short, a new frontier is ready to be conquered with only limited modifications to existing high-end floaters, but it requires the right level of knowledge and experience to go there. DC

This article is based on a presentation at the 2019 IADC Advanced Rig Technology Conference & Exhibition, 22-23 October, Amsterdam.