ANALYSIS CONSIDERATIONS FOR JACK-UP EXPLORATION WELLS
Exploration wells are drilled by oil and gas exploration companies to locate reserves of recoverable oil and gas and to determine whether or not the area should be developed further. In water depths of up to 120 m, these exploration wells are usually drilled using jack-up rigs.
Jack-up rigs are supported by legs that pass through the hull of the rig (above the surface of the water) and into the seabed. These rigs are mainly used in shallow water areas because you need to ensure that a large enough air gap can be achieved between the water level and the bottom of the hull. This is obviously limited by the length of the jack-up’s legs. Jack-ups can also withstand relatively large waves and strong currents from adverse weather conditions – unlike a semi-submersible rig that often has restricted operating windows in shallow water depths
The legs of a jack-up prevent the rig, and therefore the top of the conductor system, from significantly moving off-centre from the well. It’s because of this that jack-up wells do not require flex joints, and therefore the riser and conductor analysis considerations for a jack-up drilled exploration wells are different to those for other types of well e.g. a semi-sub. A surface wellhead is also usually used for exploration wells, distinguishing it from HP riser systems, which tend to use subsea wellheads, and the conductor is not laterally supported by guides, unlike a platform drilled development well.
Looking at the below picture, in jack-up rig systems, you’ll see that the conductor top is above sea level, with inner casings hung from a surface wellhead (shown in green). A surface BOP (shown in red) and overshot (shown in orange) are all above the wellhead. The entire system is often supported around the top of the conductor by a tensioning system (this is indicated by arrows), and will sometimes be laterally restrained by the rig at this point as well (e.g. by a Texas deck).
As a member of Aquaterra Energy’s in-house riser and conductor analysis team, our main focus when performing analysis for this type of well is on strength and stability, as well as VIV (vortex-induced vibration).
I’ll go into why in a little more detail in the sections below:
In simple terms, VIV causes fatigue damage on a conductor system due to rapid movements of the conductor. The likelihood of VIV increases if the conductor system has a long span with no restraints is a narrow diameter conductor or experiences a stronger current. As jack-up exploration wells usually only have one lateral restraint at most, and currents and conductor sizes can vary, the possibility of VIV lock-on can be a concern for these types of analyses. As calculating the effects of VIV fatigue damage can be time-consuming (and therefore expensive for our clients), our approach begins with a screening check to determine whether VIV lock-on is likely. A range of applied tensions is considered, as VIV can often be designed out of the conductor system by increasing the tension. If VIV is predicted by the screening check, clients are provided with recommendation options to use either VIV suppression devices that would reduce the likelihood of VIV lock-on, or perform the more involved calculations to determine whether VIV fatigue damage will be a problem. We’ve found this approach to be the most time and cost-effective, as it means that our clients do not need to spend money on a more expensive VIV fatigue analysis or VIV suppression devices if there is no VIV predicted, or if simply increasing the primary tension can prevent VIV altogether.
Strength and Stability
We carry out strength and stability analysis to assess the response of the conductor system to an extreme instance of loading, for example, from a very large incoming wave. As exploration wells are generally only operational for a matter of months, strength and stability analysis is more relevant than fatigue analysis, which would be best used to assess the application of small loads over a longer period of time. Our strength and stability analysis enables us to inform our clients if their conductor system is over- or under-designed for the proposed conditions it will be operating in, and therefore can make recommendations for how the design can be optimised. This not only provides peace of mind in terms of knowing that their well design is fit for purpose but can also save money by informing them if more cost-effective options can be used without compromising on safety or performance.
Areas of concern in the conductor system from a strength point of view are usually around the point of fixity (usually a few metres below mudline) and around the lateral restraint if this is used. Strength calculations are dominated by bending moment caused by wave and current loads, which can generally be reduced by increasing the tension in the system. Each component in the system will have its own bending and tension capacity, so our analysis will measure the tension and bending loads at relevant elevations in the system, and then compare them to the capacities of the components at those elevations. Our analysis can, therefore, be used to recommend a suitable applied tension that will fulfill the system’s VIV and strength requirements, while also being large enough to keep the system in tension to minimise the risk of stability issues, such as buckling.
The type of wellhead used can also have a big effect on the system response. By looking at the way the conductor and surface casing interface with the wellhead (i.e. if the wellhead is rigidly attached to the conductor, and whether a landing ring is used) it’s possible to accurately determine where the conductor and surface casing will experience the most bending, allowing us to reduce unnecessary conservatisms.
The main considerations for analysis of jack-up drilled exploration wells are VIV and strength and stability. This is due to the relatively long span without restraint, which allows the conductor to vibrate at lower resonant frequencies (VIV), experience more bending (strength), and potentially buckle if in compression (stability).
Because of this, applying an appropriate tension load is very important for these wells, as a sufficient tension load can increase the modal frequency to prevent VIV lock on, increase the conductor stiffness to reduce conductor bending, and keep the conductor in tension so that it cannot buckle. Our jack-up exploration well conductor analysis can recommend an appropriate range of applied tension loads based on the capabilities of the primary tensioner used in the well design, providing comfort to our clients in knowing that their well design is appropriately optimised for the operations it is needed for and ultimately that their project will run smoothly and successfully.
At Aquaterra Energy, we pride ourselves on using our extensive riser and conductor analysis experience to make things easier for our clients wherever we can. For example, by recommending design improvements or alternative products that will optimise the conductor system as much as possible for the operating conditions in which it will be used. By reducing unnecessary conservatisms, and maximising efficiency in the analysis itself, we ensure that any cost savings our analysis discovers will be passed directly onto the clients we work with.