Reducing the Cost of Deepwater Subsea Wells
Authors: Taylor Bill , Theiss David , Toalson Dave , Mowell Roger 
Published: October 2008

A major factor in the cost of deepwater wells is associated with the day rate of the late generation rigs required to drill such wells. In times of low demand, market forces control the day rates. But, in periods of high demand, as during the recent rig-building boom, these rates are predominately set by rig new-build and upgrade costs.

The cost of building or upgrading a fully capable, deepwater rig is driven in large part by the diameter of the marine drilling riser utilized. Variable deck loading, required riser tensioning, riser storage area, and liquid mud storage capacity, are all factors that are significantly linked to the drilling riser diameter selection.

The authors will describe several small drilling riser options that reduce rig cost and its operational requirements. The slender riser concept proposed permits drilling ultra deepwater wells with a smaller rig, but does not sacrifice any casing strings that can presently be considered in large bore riser and wellhead system, while still employing 18-3/4" subsea blowout protection. The use of this technology would also offer an additional reduction in through-life intervention/recompleting costs of deepwater wells over the life of the field.

To fully realize benefits of this technology, a second, less capable rig hoisting system is required. Also, while riser tensions are much lower than those imposed by standard marine risers, they are substantial, and many older rigs would require tensioning system upgrades. Technical challenges include methods for economically effecting these changes on older second or third generation rigs. For either conversion of existing rigs or new builds, this technology would result in significantly lower overall costs.

Introduction

It is now generally agreed that drilling, especially deepwater subsea wells (BOP's on bottom), using a reduced diameter and lighter drilling riser is a major cost saving alternative. The smaller and lighter drilling riser equates directly to reduced riser tension and deck load requirements, which allows smaller older MODU's to drill in deeper waters than originally designed for. Additionally, the substantially reduced mud fluid volume requirements of a slender riser, means savings associated with the rig's topside mud handling, storing, and processing needs. For conventional slender well drilling, the wellhead limits the size and number of casing strings but results in the removal of less drill cuttings, reducing the total time to drill the well.

The sacrifice of two casing strings that can presently be considered with the current, larger 21" drilling riser system, and 18-3/4" wellhead systems is a major consideration in drilling deep wells.

The evolutionary approach proposed here, involves a conventional, small bore riser that provides the cost saving benefits described above, but, obviates the I.D. restriction issue encountered when considering a slenderwell drilling program. The overall system design requirements include:Smaller, 2nd or 3rd generation rig, with dual hoisting SystemNominal 16" diameter slender drilling riserModified 18-3/4" subsea BOP and wellhead systemUnrestricted "big hole" casing program (30", 20", 13-3/8", 10-3/4", 7")