Well Intervention using MSV/DSV
Authors: Engebretsen Cato 
Published: October 2008

Abstract
Well intervention work through a rigid monobore riser from a dynamically positioned monohull vessel puts new challenges to the industry to ensure a safe and efficient intervention to the increasing number of subsea wells. Improved DP systems and modified riser systems and workover control systems are the key issues.

Introduction
Over a period of years we have seen the development of new and old oilfields applying an increasing number of subsea well solutions. Further, the development of 'smart wells' and a general decrease in production average of existing wells, an increasing demand for well intervention services in the future is anticipated. This is supported by several studies which conclude with a significant volume increase in well intervention services required in the years to come.
More cost effective solutions than using anchored drill rigs will have to be established to reduce the total cost of servicing subsea wells. The solution which has been paid most attention and seems to be the most interesting alternative today is performing the intrevention work using a monohull vessel, like the Botnica concept, which has been developed to provide Operators with an alternative to both the conventional semisubmersible rig market and to other potential monohull solutions.
This paper will briefly addresse the commercial aspects of a monohull vessel versus a semi submersible rig, a market driven situation which should be well known to the market. Then the paper will focus on the operational / technical challenges of (1) vessel motion and its impact on a rigid riser system when connected to a well and (2) dynamic positioning of a monohull, a subject which will be supported by testing data from the MSV Botnica which have installed and tested a new generation DP system.
Well intervention work through a Subsea Wireline Lubricator provides less challenges to the monohull vessel concept than riser operations. Such operations has successfully been undertaken by other vessels and will not be addressed in this paper.

Commercial aspects - monohull vessel versus semisubmersible rigs.
A few months back it was acknowledged that well intervention services from a monohull vessel was likely to be provided at a significantly lower cost than from a semisubmersible rig. Comparative cost schedules between monohulls and semis are however depending on how the Operators calculate the cost of using a semi which is already on charter. Cost / benefit scenarios are also subject to fluctuating rates due to both short and long term changes in oil prices and the corresponding level of oilfield activities which impacts the utilisation of vessels and semis. Today we experience a change where rig rates are dropping and rigs are being laid up due to Oil Companies' cut back in activities.
The total cost picture in a cost comparison between the two alternatives should include below cost elements:

Rates.
Monohull vessels traditionally outclass semisubmersible rig alternatives when it comes to dayrates for the units itself, mainly due to the lower investment and thus less capex.

Associated rig cost -1.
A large potential component of the associated costs will normally be caused by taking a drill rig away from a drilling programme to undertake well intervention work.

Associated rig cost -2.
One of the most visible associated cost elements for a drill rig may typically be the direct associated costs of employing a number of anchor handlers for a number of days.

Associated rig cost -3.
Monohull vessels will offer a flexibility which will provide significant time savings due to reduced transit time between locations and less time spent to establish p