Surface BOP with mudline shut-off device: a deepwater well construction alternative: design and application of industry's first mudline shut-off system to make surface BOP drilling environmentally safer with the accompanying high-pressure riser
World Oil, Jan, 2005 by Armel Simondin, Dougie MacPherson, Nicolas Touboul, Gerald Ragnes
Described here is the background of deepwater drilling using a BOP stack on the surface, rather than the previous conventional application of the BOP at the mudline, and the design and first application of a shut-off device located at the bottom of the riser to make the surface BOP system environmentally safe in the event of riser failure.
The application is based on Total's experience in its Donggala deepwater block in Indonesia in 2003. The shut-off device called the Environmental Safe Guard (ESG) was developed by Total and Cameron, with contribution of a riser design study by 2H Offshore Engineering. The deepwater rig used for the described case history ESG application was supplied and upgraded by Transocean. This successful application was the first time, worldwide, that a combination surface BOP with a mudline shut-off device was successfully run in 2,000m water.
Most RecentEnergy Articles
INTRODUCTION
In recent years, operators have been drilling wells in ever increasing water depths. These deepwater wells are drilled under specific conditions, namely a combination of extreme thermodynamic conditions (low temperature and high external pressure at mudline level) and a narrow margin between pore pressure and fracturing pressure. These conditions lead to a number of drilling difficulties, especially in terms of well control.
Traditionally, these challenging wells are drilled using a subsea BOP stack connected at the mudline with a low-pressure riser to surface. From a well-control point of view, this configuration presents several limitations: BOP closing time may be long enough to allow part of the influx to migrate inside the riser; long kill and choke lines add significant friction losses which sometimes jeopardize kick control; and BOP components are subject to extreme thermodynamic conditions and, therefore, are prone to frequent failures which drastically increase rig non productive time (NPT).
To overcome the drawbacks of using a subsea BOP stack and reduce costs associated with an expensive rig package, industry developed a technique using the BOP stack on surface in a "jack-up type" configuration and a high-pressure drilling riser. Primarily used in the Far East, this technique presents certain limitations, the main one being absence of a secondary barrier at, or just below, the mudline in case of riser failure.
Total considered using this technique in 2,000-m (6,560-ft) water on its Donggala deepwater block, in Indonesia, off-shore Mahakam Strait, Kalimantan Timur. A quantitative risk assessment (QRA) on surface BOP stack drilling was performed. This highlighted the necessity to have a shut-off device at mudline level to reduce the environmental impact in the event of riser failure. Total, in association with Cameron, designed and developed this subsea shut-off device (hereafter referred to as ESG for Environmental Safe Guard) and, with Transocean, upgraded a rig to allow safe and efficient drilling on Donggala.
This article outlines development of the ESG in parallel with the required riser design and well architecture, as an alternative to deepwater well construction. An operational feedback on the successful running of the ESG is provided.
SURFACE BOP BACKGROUND
Over the past several years, industry has developed a technique allowing deepwater wells to be drilled with a semisubmersible rig with the BOP located at surface. Drilling with a surface BOP stack presents several advantages compared with traditional subsea drilling, such as:
* Cost saving" Drilling within 2,000 m of water and a subsea BOP stack usually requires a 5th-generation rig with typical dayrates up to $200,000/day. This generates high total well costs. The surface BOP technique allows use of 3rd-generation rigs with lower dayrates (typically $80,000/day), thus the potential for lower well costs. Reductions in high mob/demob costs are also possible, especially for deepwater locations remote from the traditional 5th-generation rig operating areas.
* Optimize drilling operation." The potential is there to save rig time as no time is spent running and pulling the BOP together with the riser in deep water.
* Reduce Non Productive Time: Having the BOP at surface actuated by a simple control system reduces the risk of BOP failure, which can result in significant downtime to pull the BOP to surface for repair, and re-run it to the mudline.
* Well control." The kick or well-control situation is simplified by not having several kilometers of choke/kill line (and their corresponding high frictional pressure losses) while applying the simpler surface-BOP well control techniques.
Having acquired the deepwater Donggala block in 2001, Total's objective for using the surface BOP stack technique in 2,000-m water was not only to drill its prospect in Indonesia in a cost-effective manner, but also to assess possible use of the technique in other subsidiaries worldwide. A quantitative risk assessment (QRA) on Donggala was conducted to evaluate potential risks of using surface BOP techniques in this specific case, and propose workable solutions to &iii safely with this method.
READ MORE HERE
No comments:
Post a Comment