Deep water Riserless Drilling
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This master thesis presents different techniques for riserless drilling in deep waters. The objective has been to see advantages and challenges as well as limitations concerning the different solutions presented and identify associated risks. In early stage of offshore development it was possible to solve problems associated with increasing water depth by increasing the size of the marine riser and subsea wellhead. However, the industry has reached a point where further enlargement of marine riser and associated equipment is impractical, and over the last decades, many alternatives to the use of the conventional marine riser for deepwater drilling have been investigated. A study on the motivation for developing riserless drilling techniques has been performed and is included together with basic information on how deep water wells are drilled today and how top holes are drilled riserless from floating rigs. An introduction to common equipment utilized in deepwater operations is given, in addition to presenting well control aspects and necessary barriers. The Riserless Mud Return system developed by AGR Drilling Services for riserless top hole drilling is described in detail in this thesis. In addition, AGR is currently developing a Riserless Drilling System for shallow water and a feasibility study for utilization of the new system on the Gullfaks Satellites has recently been performed. Both the RMR and the RDS system is addressed and current top hole drilling techniques in combination with theory on riserless drilling techniques for shallow water are used as a basis for some of the deepwater riserless solutions presented by the authors. The main focus regarding the task on working out viable solutions for riserless deepwater drilling has been equipment enabling these new techniques to be performed and well control. Specialized equipment required for the different solutions to riserless drilling is presented. Some of the equipment already exists in today’s market, but since many of the discussed solutions have only reached the planning stage yet, the authors has supplemented with their own suggestions to new equipment and its required properties. Technology gaps regarding the different methods are highlighted and solutions to existing problems have been suggested. In addition to discussing riserless drilling for deepwater applications, attention is paid to riserless completion. This topic is not much discussed in the literature yet since riserless drilling is only on the development stage. However, the authors of this thesis have used the available information in combination with innovative thinking to outline solutions and identify challenges regarding this topic. A chapter is also dedicated to riserless Managed Pressure Drilling. In addition to highlighting two different methods which still are only on the theoretical stage, new solutions for how to achieve pressure management without a riser has been put forward and compared to conventional backpressure MPD and dual gradient drilling. A simulation model for riserless drilling has been worked out in Matlab, and calculations on mud savings, friction loss and required mud weights and casing points are performed for two example wells. The calculations are performed for both riserless and conventional scenario and compared against each other. The results are presented in this report and the software is included electronically. Well control, as well as focus on secure barriers is of the uttermost importance when it comes to developing techniques for riserless drilling. Effort needs to be made to minimize the prominent risks associated with eliminating the marine drilling riser. The calculations performed in regards to the two example wells clearly support the claimed advantages, namely fewer casing points required, less mud volumes required and reduced circulation time. Deepwater pump technology and reliable and substantial annular seals needs to be further developed in order to enable riserless drilling in ultra-deep waters. Further development of autonomous chokes and intelligent completions is essential in regards to making completion procedures riserless.