Slim Riser - A Cost-Effective Tool for Ultra Deepwater Drilling

Until recently, operators have often been faced with a stark choice when planning their deep and ultra deep (over 5,000 ft) water drilling programs. They either paid the high cost of major Mobile Offshore Drilling Unit (MODU) upgrades or the high day rates associated with new, 5^th Generation MODUs. Recently, the Surface BOP (SBOP) Concept appeared as an alternative with attractive advantages under certain circumstances. A third alternative is herein presented. Namely, the Slim Riser Concept that can be used to bring older 3^rd and 4^th Generation rigs into the deepwater arena on a favorable economic basis, but still maintain many of the advantages of the standard, Conventional 21-inch riser, 18-3/4 inch BOP subsea system.

The key features of the slim riser concept presented are:

• 16 inch riser, which substitutes the Conventional 21 inch riser.

• The rig retains its existing 18 ¾ inch BOP stack and lower riser assemblies (LMRP) such as the flex joint.

• The rig retains its 21 inch upper riser assemblies such as the diverter, telescoping joint, etc.

• Several options, including a preinstalled wellhead sleeve in a conventional 18 ¾ inch housing, can be chosen to reduce the effective wellhead bore.

By reducing the riser size from 21 inch to 16 inch OD a rig’s water depth capability is improved significantly due to:

• Reduced tensioner loads

• Lower variable deck loads (VDL) requirements due to weight savings from the reduced volume of stored mud and reduced riser tensions

• Reduced required mud pit space due to the smaller riser volume

• Reduced riser storage requirements due to the smaller riser

This paper will fully explain the Slim Riser Concept including:

• A comparison to Conventional 21 inch riser systems and SBOP systems

• A review of the 16 inch riser design

• BOP control system considerations

• Exploration of casing program considerations with slim hole drilling

• Exploration of slim hole wellhead systems with standard 18-3/4 inch housings

• Exploration of mooring system considerations, including the use of conventional spread moored or taut/semi-taut line systems

• Exploration of guide line versus guideline-less reentry with Slim Riser

• Field installation considerations

In addition, the paper will compare the tools available for ultra deepwater drilling including Conventional 21-inch riser drilling, drilling with SBOP and drilling with the slim riser concept. Guidelines will be presented for selecting the "right" system for the "right" application.