More than three years after installation of the Girassol Hybrid Riser Towers, this paper aims at presenting the choices made in terms of inspection and monitoring on such equipment and their inspection results.
The access for inspection is very restricted and the riser tower design is based on a zero inspection and maintenance concept: FMEA studies conducted during the design phase have led to incorporate changes in the final design and the project has worked to eliminate the areas of concern.
However, external cathodic protection measurements, general and close visual inspection by ROV are regularly performed.
These inspection activities are implemented within the framework of a general sub-sea inspection strategy defined through a Risk-Based Inspection approach.
The results of inspection are compiled in a dedicated Computerized Inspection Management System (CIMS).
So far, the riser towers have not shown any significant defect or anomaly.
The temperature of the production spools at the base of the towers is monitored with sensors. The data recovered from these sensors, together with data collected from topside instruments, allows monitoring the thermal performance of the towers. This is critical in ensuring that the risk of hydrate formation in the system is minimized.
Each of the three Girassol towers connects the sub-sea system to the topside FPSO carrying production, water or gas injection, gas-lift and service fluids. It is principally made of six main parts:
buoyancy tank
taper joint / transition pieces
current / foam section
umbilical arches
bottom of tower
suction anchor
spool pieces from bottom tower to the horizontal bundles
flexible jumpers from the buoyancy tank top to the FPSO
Fig. 1: Overall tower view (Available in full paper)
Each riser tower comprises the following pipes (see figure 2):
1 carbon steel central core pipe ensuring a mechanical link between the riser base and the buoyancy tank
6x8" carbon steel pipes dedicated either to oil production, gas or water injection
4x3" carbon steel gas-lift lines
2x2" stainless steel methanol service lines
Insulation material
Fig. 2: Riser cross-section (Available in full paper)
This set-up was the first development in such depth. Many new technologies were used with significant technical challenges. At the time of the design, there was no existing monitoring philosophy of such deepwater system. Thus the monitoring strategy was only defined during the development process.
As for the inspection strategy, it was defined at the very beginning of the field operation.
The methods and organization implemented with respect to Girassol Hybrid Riser Towers are part of a general sub-sea equipment inspection plan.
