A riser fatigue monitoring strategy and implementation on a deepwater Gulf of Mexico Spar top tensioned riser is presented. The paper explains why a fatigue monitoring program is considered necessary to provide the operator with assurance of the riser system performance and integrity in service, the considerations that led to the selection of an appropriate monitoring system and describes in detail the standalone motion logger system adopted.
The principles and methods of measurements that permit the monitoring of motions at discrete locations on the riser are presented along with the methods of processing this data such that fatigue damage along the riser can be interpreted.
The paper describes how a standalone logger monitoring system has been successfully installed entirely using a ROV, eliminating the need to run the monitoring system during critical path riser installation activities.
A number of Spar floating platforms with top tensioned risers have been installed in the Gulf of Mexico in water depths up to 5,600ft. Top tensioned risers provide the production conduit from the seabed to the facility and allow workover activity on the well. The risers are subjected to dynamic fatigue loading from platform motions and ocean currents requiring a robust riser design with high quality components. A schematic of a top tensioned riser system for a Spar with a standalone logger monitoring system is shown in Figure 1.
Each top tensioned riser is fitted with tapered stress joints at areas of high bending moment, which are at the connection to the wellhead at the seabed and either side of the Spar keel. In addition, the conductor below the mud line and the upper riser in the hull structure can be highly loaded regions. These are critical locations that exhibit complex response having the lowest predicted fatigue life.
The uncertainties in riser fatigue life prediction come from various sources. The magnitude and persistence of environmental events such as loop currents and hurricanes may differ in service compared with those used in the riser design. The riser structural response to phenomena such as vortex induced vibrations (VIV) is complex, especially in deepwater where high multi-mode response is expected. It is recognized that the ability of current VIV analysis tools to predict the actual measured VIV response of a riser requires a re-adjustment of typical design input parameters such as the level of structural damping or lift coefficient . Further back analysis and calibration with full-scale riser measurements is required to better define input parameters and give confidence in predictions.
Consequently, monitoring the response of a riser around its critical fatigue locations allows fatigue damage rates and accumulation to be tracked. It also allows comparison between actual and predicted response for different operational conditions, allowing the long-term integrity of the riser to be confirmed and a project specific fatigue factor of safety to be calculated.
The main objectives of the monitoring program implemented on a deepwater Spar top tensioned riser in the Gulf of Mexico are to provide a means of measuring the following: