An Innovative Workover Riser System for 3000m Water Depth
Authors: Moreira J.R.F. , Oliveira M.F.D. , Paulo P.C.S. , Branca M. , Mateus F.J. 
Published: October 2008

This paper describes the development of an innovative workover riser system for 3000 m water depth, which has been accomplished through a technological agreement between two major companies, a deepwater operator and subsea equipment manufacturer. The main purpose was to thoroughly investigate the pros and cons of all existing workover risers and devise a new concept that would meet all challenges associated with the installation and retrieval of both subsea trees and tubing hangers in ultra deep waters. The experience with the Drill Pipe Riser, designed for 2000m water depth, showed that the umbilical was one of the critical points, particularly in the tubing hanger mode, which requires running of clamps inside the marine riser, which is always a risky operation, and the use of a hang off equipment. A 3000 m long umbilical would require an enormous and very heavy hose reel and would delay significantly the running time. Accordingly, the system has been designed to operate umbilical-less while being as reliable or even more reliable than the conventional ones. Both open sea and in-riser subsea HPU have been developed to meet the umbilical-less requirement. A reliable communication system to allow subsea HPU remote control was a key point. Lab tests have been successfully carried out to demonstrate that. A communication back up system was also devised.

A special riser connector was developed to meet NACE and significantly reduce running time. Riser string weight was a major concern due to limited motion compensator capacity. To cope with this problem, the use of a special material joints were deemed one of the best alternatives while minimizing maintenance costs.

Introduction

The development of a workover riser for 3000 m is included in the project portfolio of the PROCAP 3000, a Petrobras program which objective is eliminating the technological bottlenecks for production in ultradeepwaters up to 3000 m water depth. Obviously, ultradeepwater exploitation is always associated with very expensive rigs and high flowrate subsea wells thus making Workover riser efficiency to play an important role in reducing both CAPEX and OPEX. For instance, subsea tree and tubing hanger deployment time at 3000 m water depth, combined, can be as high as 10 days, considering conventional workover riser running time.

Accordingly, the Workover Riser for 3000 meters should be devised aiming at not only making feasible completions at 3000m water depth but also to do it in a most efficient and reliable manner.

Petrobras has been using the so-called Drill Pipe Riser to perform completions and workovers at water depths up to 2000 m. Although far more efficient than the conventional completion riser the operational experience showed that it was worthwhile improving some components. The weak point is the control umbilical and related equipment such us hang-off equipment and clamps, which showed problems deemed to be critical in 3000 m. Control umbilical requires careful handling, particularly in the tubing hanger mode when it has to be deployed inside the marine riser. This significantly delays the running time and poses risk of dropping clamps inside the well, which can lead to lengthy and complicated fishing operations.