Perhaps one of the most important ventures in the area of high-cost technologies for deepwater challenges is the development of dual gradient drilling systems (DGDSs). DGDS is often referred to as riserless drilling. It is generally accepted that DGDS is required in water depths of 5,000 ft. There have been a number of unpublished examples, however, in which application of the technology was needed in water depths as shallow as 3,000 ft. The need for DGDS is caused by the reduced fracture gradient of formations below the mudline, resulting from the reduced weight, or gradient (0.5 vs. 1.0 psi/ft), which, itself, is a result of water above the mudline as viewed from a drillship operating at sea level.
The seafloor temperature in deepwater locations is approximately 40 F, but it can approach 32 F. The temperature downhole can exceed 300 F. The drilling fluid should exhibit the appropriate rheological properties throughout this wide range. In the riser near the mudline, the fluid is apt to thicken excessively from exposure to the cold seafloor temperature. SBFs that contain little or no commercial clay appear to remain the most stable under these conditions. These clay-free and low-clay systems rely on emulsion characteristics to achieve the desired rheological properties and provide sufficient barite suspension. Seismic data can help operators to predict and evaluate the risk of encountering an SWF on a given well.
Gedge, Ben (Regional Marine & Engineering – PVD Well Service Co Ltd) | Yaw, Norman (Regional Marine & Engineering) | Postel, David (Regional Marine & Engineering) | Mardon, Jeff (Regional Marine & Engineering) | Frohlich, Herbert (Regional Marine & Engineering) | Nguyen, Bot Viet (PVD Well Service Co Ltd) | Thanh, Ban Nguyen (PVD Well Service Co Ltd) | Hong, Duc Vu (PVD Well Service Co Ltd) | Barbee, Brian (Ensco) | Santos, Helio (Safekick) | Gouldin, David (Seadrill)
There has been a lot of discussion around rigs with some capability to perform Managed Pressure Drilling - MPD - operations, with rigs having different permanent equipment installed. But all the rigs, somehow, would all be classified as ready to perform MPD operations. In order to clarify a bit this subject, this paper clearlt differentiates rigs with some permanent equipment installed, but the specialized MPD equipment and service are still provided by a service company, which is herein called MPD-Capable-Rigs, from rigs which have installed all the equipment needed for MPD, and the drilling contractor is the one offering the service, which is called Complete-MPD-Rigs. The objective of the paper is to discuss and illustrate what is involved in preparing an offshore and even onshore Complete-MPD-Rig. The paper also discusses the evolution of MPD, initially provided by service companies, with equipment needed to be rigged up and down on rigs, all the way to current stage where drilling contractors are providing the service directly. To achieve this stage the drilling contractors are equipping their rigs with the full MPD package and providing the service directly to the operators. The paper describes the evolution of equipment installed and owned by the rig, from a few lines in the very beginning to a complete packge today. The movement towards the drilling contractor owning and offering the MPD service started with the deepwater rigs, but evolved to onshore rigs also.
New technology to address the fundamental challenge of accessing remote Arctic (i.e. overhead hazard) deepwater reserves, with no requirement for near-source separation, no requirement for production platforms (i.e. fixed or floating ‘islands’), no requirement for ultra-long pipelines over complex submarine topography (hence also no power-boosters), and no requirement for the release of burnt gases damaging to the environment. May be likened to a string of sausages – each a storage unit – comprising a multitude of casings all so linked that a long vertically-aligned string is formed where gas separated from oil up exits all units down. System as a whole at its lowermost, stabs, lubricates and locks into a permanent subsea station serving also as a multi-subsea well hub. Station has separate inlets and outlets. Outlet gas fills another set, or is reinjected, creating a complete hydrodynamic reservoir communication cycle. Filled strings are replaced with towing to and from market. Elegant, flexible, replaceable (‘one-size-fits-all’), and immediately repairable; the compartmentalisation of the interlinked units and enhanced turning-circle providing additional environmental transit security against technogenic catastrophe. Could theoretically be installed even prior to drilling for blowout fluids containment. Eliminates fundamentally therefore also the limitations of the profitability criteria applicable to wells within the platform context.
Gas influxes are common during drilling operations. If not handled properly, in open loop systems gas can expand and unload the riser, however in closed loop systems, unless gas is depressurized, the top section of the riser will become over pressured. Riser unloading has not been accurately modeled due to uncertainties in gas expansion. This paper presents studies on gas expansion and unloading in a lab scale model at Texas A&M University.
We have performed experiments in a scaled riser at the Dual Gradient Drilling laboratory. We ran experiments using water and air as the gas phase. We recorded the change in volume as gas expands and liquid height changes. We mitigated the disproportion of the atmospheric pressure and the scale of the model by using vacuum pumps. We also measured overflow by allowing liquid level to rise to determine the final gas volume.
The experiments provide information on gas expansion and riser unloading. During the expansion process the top of the bubble travels rapidly building momentum, which carries a large volume of mud to the top of the riser model. This occurs through the rapid change in gas volume and the energy released as the pressure decreases. By allowing gas to expand in a controlled environment we measured the impact of gas expansion.
This paper provides information on controlled and uncontrolled gas expansion, impact on riser unloading, and benefits of a closed system.
In this study, both experiments and numerical simulations have been performed to study sinusoidal oscillations of an identical pair of circular cylinders in a side-by-side configuration for various gaps in the still fluid. The key parameter of Keulegan-Carpenter (KC) number in the experiment is chosen between 0.5 and 20, Strokes number (β=Re/KC) values are selected from 350 to 2810 and gap ratio is selected from 0.5 to 3 in the experiments. Compared to the single cylinder cases, a large drag coefficient increase has been observed for gap ratios from 0.5 to 1.0. This phenomenon has later been confirmed by numerical simulations (in a smaller fixed Reynolds number of 120) using Lily-Pad, a solver built on boundary data immersed method (BDIM). In the numerical results, wake visualization shows that vortices shed from the cylinder pair will induce a jet between the gap, forming a vortex pair and accelerating the fluid particles away. This jet motion helps to expel energy from the structure into the fluid, and is confirmed by the energy flux calculation on the control volume around the cylinder pair, thus explains the enhancement of the drag coefficient.
Compared to the fluid structure interaction problem of cylinders open to the uniform flow that has been widely investigated, cylinders in the oscillatory flow has attracted less attention (Xu, 2013). However its significance cannot be undermined for its rich physics as well as its prevailing existence in all kinds of engineering scenario, especially in the ocean engineering field. As Fan (2016) pointed out that examples can be found in the offshore field such as the wave induced oscillatory flow around the risers, mooring lines, point wave energy generators, pump towers in the LNG ship experiencing sloshing load in the liquid tank induced by ship motion and blow-out preventers (BOP) forced to vibrate under the influence of upper riser motion, etc. In all these scenarios, the hydrodynamic model of the problems can be sufficiently simplified as fluid structure interaction in the oscillatory flow.
The subsurface formations and reservoir conditions encountered in certain areas of the Barents Sea offer some unique challenges for the operators and the drilling industry. These areas of the Norwegian Barents Sea consist of naturally fractured and weathered carbonate formations which may incorporate open karsts. Drilling into large open fractures or karsts will result in total losses and mud cap drilling practices must be employed to enable further progress. Such locations may require some unique solutions to drill effectively and safely while conforming to local rules and regulations; including deploying winterized rigs capable of year-round operations in Arctic environment. This paper describes the process of selecting a viable solution for drilling in such an area where potential mud cap drilling practices could be required, the rig integration process, class notation process, development of operational procedures, risk assessment, training and testing of equipment prior to commencing the operation. This was a fast track project which incorporated developing new mud cap drilling procedures and processes when operating the Controlled Mud Level (CML) system. Since mud cap drilling practices from a floater on the Norwegian Continental Shelf (NCS) is a rare occurrence, it required close cooperation between the operator, the drilling contractor and the service provider. To facilitate the process and assure sound practices a third party with experience on mud cap operations was also engaged. The paper will also briefly describe the Controlled Mud Cap Drilling (CMCD) principle and the results from drilling the well.
The main objective of this work is to study the dynamic behavior of gas expansion in marine risers. The term ‘riser equilibrium' is used to provide a more rational explanation of gas expansion and its relation to riser unloading. While handling a gas kick situation with the blowout preventer closed, the gas within the riser expands until its pressure equals the hydrostatic pressure of the mud column above, plus any applied back pressure. Buoyancy or slip will cause the gas to migrate, which in turn will cause further expansion, if allowed to, or an increase in pressure. But after a certain point, termed as ‘Riser Equilibrium Point', any small decrease in the hydrostatic pressure would trigger the gas to expand rapidly until enough back pressure is applied or the top of the gas reaches surface. This could cause an explosive unloading. Complete dissolution of gas in oil based muds and the relative delay in noticing any surface indications for the gas influx makes influx detection more complicated with oil based mud as opposed to water based mud. Estimation of various parameters to study these phenomena and their correlation to the gas expansion is studied through an analytical and iterative approach. All schemes are implemented in Matlab and give a basic understanding of the severity of the situation. Calculation of the riser equilibrium point is beneficial to understand the risks related to riser unloading and riser collapse through proper estimation of collapse load. Since the conventional ‘free gas' approach to calculate surface volume of gas during kill operations tend to overestimate the risks involved, this study is expected to provide a more comprehensive understanding of the situation providing a safer operational conditions to handle gas in risers.
Many people believe in the benefits of incorporating Managed Pressure Drilling (MPD) into well planning, which has been done onshore for a long time. In recent years this technology has been transferred to offshore drilling vessels. However, there appears to be some hesitation in widely implementing MPD solutions offshore, due most likely to perceptions regarding complexity and cost of integration. This paper has been written to communcate the scope associated with implementing a Controlled Mud Level (CML) MPD technology onto an offshore drilling vessel in order to allow Operators and Rig Contractors to evaluate the level of complexity involved. This paper emphasizes that the technology has now been integrated onto several rigs, allowing the CML provider, Rig Contractors and Operators to work out best practices for technology implementation. Integration of a CML system is now accomplished with standard products, a well-defined scope and experience-based understanding of what is required.
The aim of this paper/presentation is to highlight the recently introduced alternative drilling methods that are now being field trialed in ultra deep water. The objective of the new methodologies is to ‘drill the undrillable’. A number of underlying principles are utilized to achieve safe and cost effective bottom hole management to enable the drilling industry to realize target horizons that hitherto, in many problematic well cases, have been considered unreachable. There is no doubt that the Macondo disaster has been earmarked as a serious catalytic ‘wake up call’ which has resulted in accelerated funding into new innovative and enabling technologies to provide more safe and cost effective oil & gas exploration in ultra deep water field developments.
A number of supplementary systems have been introduced in parallel with adaptive drilling methods. Their application, whether an integral part of an adaptive method or else an optional stand-alone system is identified and discussed.
Four processes are presented, commencing with Conventional Riser Drilling (CRD). The other processes describe the principles of Managed Pressure Drilling (MPD), Dual Gradient Drilling (DGD) and the Pumped Riser System.
Further, to both compliment and supplement the above systems, a number of recent technology advancements are described. For example:- Early & Deep Water Kick Detection (EKD & DKD), Riser Gas Handling (RGH), Continuous Circulation System (CCS), Downhole Shut-Off Valves (DSVs) & Riserless Mud Recovery (RMR).
Throughout the latter quarter of 2014 and currently through this year (2015), the adaptive drilling methods being discussed in this paper are being field trialed. Such aspects, as economic drivers and safety assessments, remain in their infancy but trends of absolute proof of enhanced safety in drilling using one of the described methods is undeniable. All the upside/downside aspects are discussed, inclusive of operational continuity practical issues in the use of such systems throughout an extended drilling campaign (3-5 years).
CRD technology is historically rooted in the drilling industry, dating back to the ‘50s for offshore drilling. The methods are deeply entrenched in the offshore drilling culture with few refinements through the years, in spite of the advancement of exploration into deeper and deeper offshore acreages. The adaptive drilling methods described in this paper represent, in terms of drilling culture and established mindsets, a radical step change approach to drilling safely and effectively in ultra deep water. The new mental approach required for drilling crews using an adaptive drilling method makes the topics presented in this paper topical, necessary and illuminating.