Snorre Riser and Well Systems
Authors: Alfstad O. , Rooney P. , Dankertsen O. , Manning B. 
Published: October 2008

The paper presents the Snorre well systems including the production/injection risers, the drilling riser and the workover assembly. Design bases are presented with particular emphasis on fatigue, fire protection, installation and operability requirements as well as overall safety requirements. The installation sequence and monitoring and maintenance equipment are also presented.

INTRODUCTION

The well systems on the Snorre Tension Leg Platform (TLP) comprise the many components of the production/injection risers, connections to the platform production facilities and the subsea well template and the well completion.

The Snorre TLP is designed with 44 wellslots and to accommodate up to 36 production/injection risers. Six wells will be pre-drilled. The well spacing is 4 m in either direction.

SYSTEM DESCRIPTION

The wells are drilled through a seafloor well template using a high pressure drilling riser. The BOP is located at the surface in the drilling substructure. Subsea wellhead equipment is used to land the casing strings at the well template. Each well is then tied back to the platform with a production/injection riser.

The Snore production/injection risers (Fig. 1) consist of 9 5/8-inch pipes running from the subsea wellhead on the seafloor well template to the Xmas trees in the wellbay, thereby extending the 9 5/8-inch diameter production casing string back to the platform for well completion. The risers are equipped with remotely installed tie-back connectors at their lower end for connection to the subsea wellhead. The production/injection riser string consists of tapered joints at the lower end, 12 m long coupled riser joints and a tensioner joint at the top extending up into the wellbay. The tensioner joint incorporates a tubing head to suspend the 5 1/2-inch production tubing string and the attachment for the monoblock Xmas tree.

The production/injection trees are connected to the platform's piping manifold with flexible jumpers. The risers are supported by tensioner assemblies located on the tree deck structure.

A workover assembly (Fig. 1) is used for workover and well completion operations. It is mounted on the surface wellhead and consists of a connector, two flex joints, workover spool piece and tensioner spool piece.

The drilling riser (Fig. 2) is a 17 1/2-inch I.D. X-70 grade pipe. It guides the drill string and tools into the well, and conducts circulation returns to the drill rig substructure where the BOP stack is installed. It is connected to the subsea wellhead equipment on the drilling template and is kept in tension by drilling tensioners. It also includes flex joints at the lower end and at the lower deck level.

Four one-inch guidelines provide guidance for installation and retrieval of the risers. Special procedures are developed for installing the guidelines between the platform and the well template.

The Snorre well template (Fig. 3) is a simple space frame structural design utilising tubular construction with mainly simple joints. The structural arrangement mirrors the wellbay layout on the TLP with the well receptacles designed to support both pre-drilling and TLP drilling operations.