Development and Implementation of a Top-Tensioned Riser for Drilling, Completion and Producing Operations
Authors: Craik K.M. , Ju G.T. , Peterson J.L. , James B.R. , PCastleman K.D. 
Published: October 2008

For the Brutus tension-leg-platform (TLP), a total of eight wells were required for field development. Five were predrilled to total depth before the TLP was installed at the site. These wells are referred to as "pre-drilled" wells, and they will be completed and produced through 10-¾" production risers. The remaining three wells, referred to as "batch-set" wells, were pre-drilled to an intermediate depth, and their 13-5/8" casing strings were set. Final drilling for these three wells will be performed by the TLP rig.

In a departure from Company practice on the Auger, Mars, Ram/Powell, and Ursa TLPs, drilling to total depth on these three batch-set wells will not be performed through a 21" drilling riser system. Instead, a new concept was developed and approved for this project. 13-¾" top-tensioned risers will be used to drill, complete, and produce these wells. This combination or "Combo Riser" results in significant project cost savings by removing the need for a purpose-built drilling riser system. Furthermore, the rig time requirement for drilling and completion of the three batch-set wells from the TLP is reduced, since there is no drilling riser to install and remove.

Since the Combo Riser is used for all TLP drilling, completion, and producing operations, the risk of riser damage resulting from normal drilling operations and potential drilling hazards is greater than for typical production risers. Thorough studies were therefore conducted in order to identify the hazards associated with this concept, and mitigation strategies were formulated and implemented to minimize risk. In addition to documenting these hazards and the risk mitigation strategies, this paper also addresses design considerations, potential benefits, and limitations of this concept.

Introduction

Brutus is a Tension Leg Platform (TLP) installed in 2985' of water on the Green Canyon 158 block in the Gulf of Mexico, approximately 165 miles southwest of New Orleans (Figure 1). Gross recoverable reserves are in excess of 250 million bbls of oil equivalent. Owned 100 percent by Shell Exploration and Production Company (SEPCo), the hub class TLP is designed to accommodate eight platform wells and has payload and deck space for up to five future subsea development tie-ins. The dual train facilities can process up to 130 MBOPD and 165 MMCFD. The oil (20") and gas (20") pipelines tie into existing pipeline infrastructure 23-25 miles away.

Initial design work on the project began in 1997 during a period of severely depressed oil and gas prices. In order to meet minimum acceptable profitability hurdles the project focussed on three objectives: Reducing project equipment costs, time to first oil, and well drilling and completion costs. The use of Combo Risers helped permit the project to meet all three objectives.

Conventional Drilling/Completion Program

The well plans for the Brutus wells were based on experiences from the discovery well and two appraisal wells. The site selected for the TLP, close to the first appraisal well, was not expected to have severe shallow water flows.