Instrumented Monitoring of Workover Risers
Authors: Osen P. , Johannessen B. , Strømsem K. , Wernø T.G. 
Published: October 2008

A Workover Riser Management System (WORMS1), has been developed for on-line instrumented monitoring of completion/ workover risers. The primary objective is to increase the weather window for safe well completion and workover operations. Additionally, for disconnecting the riser, the system can be used to position the rig such as to minimise the bending moments in the connector to be un-latched, Riser monitoring is based on the continuous measurement and analysis of riser strains. The vessel re-positioning which minimises structural stresses is displayed. Advice is given when shut-in or disconnect is required to avoid exceeding the permissible stress in the riser. The system has been demonstrated during completion operations in the North Sea during the winter season.

Introduction

In 1993 the well completion operations for the Sleipner Satellites and Statfjord Satellites fields were started. The Sleipner Satellites, comprising the Sleipner East and Loke locations were, at that time, to be completed with three wells in total. The Statfjord Satellites, Statfjord North and Statfjord East, added up to a total of 20 subsea completions. These operations were conducted from the semi-submersible Transocean Prospect. The subsea equipment, comprising the wellhead systems, Christmas tree, template structure, manifold and production control system, and the workover riser and workover control system, were supplied to Statoil by Kongsberg Offshore Services (KOS). As part of the supply, KOS provided operational procedures and operational limitations to the workover system. These limitations referred to rig motions, i.e. heave, pitch and roll, and wave height coupled with riser tension. Due to the nature of the problem, operational limitations had been established based on worst case assumptions in order to ensure operational safety. During the subsea completion operations it became evident that the operational limitations as stated in the operational procedures were too conservative and limited the effectiveness of the operations. Practical experience from observing the sea current relative to the ROV operations, the relative riser/rig motions, and the station-keeping capability of the rig relative to weather conditions, indicated that operations could be optimised, The first attempt was to evaluate the possibility of a procedure allowing adjustment of the operational limitations based on actual measured sea current, weather conditions, and the subsequent required top tension. The idea was to develop a graphical interpretation ofthe actual environmental data to establish a more correct operational limitation. It was however soon realized that this would require a very frequent updating of the graphs. Furthermore, this would require more accurate measurements of sea current at different depths, and more accurate sea state measurements, than was available with the present equipment onboard the rig. Some investments would be required. At the same time, it was realised that this approach aimed at monitoring the effects or processes causing riser stresses, rather than monitoring the riser stresses directly. The idea for the WORMS system was conceived during the dynamic analyses of the supplied workover riser, which were performed to verify structural strength and establish operational limitations. This third party verification was conducted by SeaFlex, who presented the WORMS concept to Statoil, who is preparing for deep-water operations in the near future.