A Step Change in Safety: Drilling Deepwater Wells with Riser Margin
Authors: Ziegler Robert F. 
Published: April 2012

Riser Margin had to be abandoned when the fluid column in the riser became toolong with increasing water depth, accessed when floating drilling units withsubsea BOP systems moved to deepwater. Removing the riser in a disconnect eventmeans the well goes underbalanced, and relies on the BOP to contain wellborepressure. With the advent of a simple retrofit dual gradient system based onpartially evacuating the riser by pumping the mud returns back to surface froman outlet in the riser, "riser margin"in the form of an overbalanced well, even if the riser is disconnected, can be reintroduced in many cases. The mudused for drilling is also already at "kill mud"weight, so any pressure ramp-upcan be controlled with the mud system already in the well. The paper describessuch a dual gradient system retrofitted to an existing 6th generation semi andbeing used for deepwater drilling. This system also gives the opportunity tointroduce novel well control practices, which aim to preserve primary wellcontrol via drilling fluid overbalance in all cases, and greatly simplify andreduce the risk of deepwater well control operations. The benefits fordeepwater well design that come with a dual gradient system are brieflydiscussed as well. Lastly, the author proposes to introduce 3 pieces ofequipment that allow the safe handling of gas in the riser.

Introduction

Deepwater Drilling has evolved rapidly in little over a decade. Where in themid/late 1990ies drilling in some 3000ft of waterdepth was still considered amajor challenge, we now consider more than double this water depth as routine. Only when we push to some 9000ft of water and beyond this is still consideredcutting edge. With the great success stories of (ultra)deepwater discoveries ofthe recent years the economic drivers are there, and this industry must becommended for having pushed the technical limit in an effort that is onlycomparable to putting a man on the moon. But the question must be asked if allprocedures and practices really been regularly reviewed when we went intodeeper and deeper water, or have we simply size-up equipment and volumes andnever really stood back and asked ourselves if not something fundamental haschanged by going into water depths as remote to previous human activity asouter space was before Apollo? And most importantly, when the recent rapid "6thGeneration"rig building program was in full swing, did the designers reallyanalyze what capability changes are required when these rigs are marketed as"capable of drilling in 12,000ft of water depth"? There is great danger inrapid development of such complex systems, with the best example the SpaceShuttle Program, where the designers completely underestimated the complexityof the undertaking, uneconomically leading to about one launch a year per uniton average (instead of one every two weeks), and all but one shuttle destroyedin horrific accidents due to failure of some small components. With Macondo, wehave had the dramatic demonstration of what can happen in worst case deepwateraccidents, and it is the job of all of us to make sure we modify equipment andprocedures so we make a reoccurrence increasingly unlikely. This paper willendeavor to give a high level view of what can be done different in(ultra)deepwater drilling, and propose a solution that allows a significantimprovement in well control safety and efficiency, while also having a verypositive effect on deepwater well design.

Setting the Scene

Deepwater drilling has a multitude of challenges unique to the environment wehave to operate in. In this paper I would like to address specifically thefollowing 3 of these challenges:–related to the water depth, comprising cold temperatures combined with longfluid columns with high pressures at the seabed most often sitting squarely inthe hydrate formation zone in the phase envelope, mechanical stresses on theriser system due to METOCEAN forces, and most importantly, operating a highlycomplex hydraulic system, the Subsea BOP, remotely in a very hostileenvironment.