Downhole maintenance or logging on subsea wells in the North Sea has typically required the operator to give inhouse budget approval for the large amounts of money needed to mobilize and operate a semisubmersible drilling rig with its attendant towing, anchor handling and standby vessels. The rig has to be supplied by the operator with a workover marine riser and tree control package that itself has required a capital outlay running into six figures Putting all these costs on one side of an equation and the actual results achieved on the other, it usually looks hopelessly lopsided from an economic point of view. This is one of the key factors that has historically mitigated against the development of subsea completed wells.
During the era of high oil prices and huge recoverable North Sea reserves, the large operators' clear choice was for seabed-supported platform production facilities. Apart from some high-cost visionary projects by these large operators, it was mostly only the smaller operators in the North Sea who opted for simple subsea completions and the resultant improved cash flow. Many factors have combined in recent years to change the preferred production method to one that predominantly involves the use of seabed equipment. The majority of planned field developments in the northern North Sea are for subsea trees The drop in oil prices has marginalized many fields to the point where subsea completions offer the best economics for overall field development, especially in an area like the North Sea that already has a substantial infrastructure of production facilities. This change has stimulated the need to develop cost-effective methods of re-entering subsea wells.
Cost-effectiveness of well re-entry is primarily pursued at present by using semisubmersible drilling rigs, because the demand is so low that the dayrates are rock bottom. Many opinions are expressed about how long these low rates will continue, but one thing is certain: if the dayrates for semisubmersible drilling rigs do rise substantially then the costs of well re-entry will also rise, perhaps prohibitively, unless alternative methods are developed. It is in the operators' economic interests to support the development of alternative methods to enable cost-effective servicing in future on the large number of subsea wells presently being installed or planned. Development of alternative well re-entry methods does not simply mean creating the prototype hardware to do the job, it means developing the operational capability that will primarily reside in the service companies. The service companies can only create an operational capability and all that implies in terms of track record, contracting experience, management structure, trained personnel, equipment inventory, maintenance and spares, if the operators actually make use of the emerging new services now, while the development momentum still exists If not, the experienced people will disperse, no offshore personnel will be trained, and the equipment is likely to be broken down into its component parts in an attempt to achieve some utilization for a return on investment
A cost comparison between a semisubmersible drilling rig and a diving support vessel (DSV) equipped with subsea well-servicing equipment and crew does not, at first glance, favour the latter for well re-entry at this time, especially considering the natural preference to use what is known rather than what is new. However, there are several ways in which subsea well re-entry methods can be used economically now. Money can be saved by doing the job quicker or by carrying out more than one operation simultaneously from the same vessel, or by reducing capital expenditure.