Monitoring of Rack Phase Difference on jack-up rigs

Rack Phase Difference (RPD) is a difference in the elevation between rack teeth of the chords of any single leg of a jack-up rig. This relative displacement creates an additional load on the leg members, which may results in failure of structure members in case RPD reaches more than the allowable value. (Wikipedia)

Monitoring of Rack Phase Difference on jack-up rigs


To serve the needs of the offshore industry, the Danish offshore supplier Semco Maritime, has launched a small to medium size control, monitoring and reporting system. The system is especially targeting the drilling rig and marine markets and will also serve as a platform for process optimisation and special application modules for the industries in these markets.
The system is labelled SemView and is designed as a traditional SCADA system. The system may utilize a PLC as front end towards the process but special interfaces are used to communicate directly with process equipment with own intelligence, and depending on the application it can therefore operate without PLC. Such interfaces have been implemented for: diesel engine control, generator monitoring and synchronisation, and tank monitoring systems.
For jack-up rigs, special application modules such as motor current balance and RPD (Rack Phase Difference) have been developed. When installed, these application modules will be fully embedded in the SCADA functions.
Rack Phase Difference is generally attributed to imbalanced loading on the rig leg chords during jacking. This imbalance may cause a racking effect in the leg bracings (commonly referred to as RPD). In worst case this effect may cause failure of these bracings. In a few cases, RPD has also been caused by eccentric spudcan support due to hard or uneven seabed conditions. In all cases the causes of RPD are nearly impossible to determine before jacking proceeds.
Further, even during jacking close attention is required to assure that RPD is not occurring and influencing the leg with excess stresses. With a SemView RPD monitoring module the jacking operator receives online information to determine when RPD is occurring and whether the RPD level is within acceptable tolerances for rig.
In the long term, this information will save both time and money since rig downtime attributed to RPD can be reduced if not totally eliminated.
It should also be noted that some countries have already issued safety notices regarding RPD and all notices require that a suitable monitoring system be in place. One such system is noted as being a RPD monitoring system.
The SemView monitoring system for RPD application modules provides a monitoring and alarm function for jack-up rig rack phase difference.
The module function is based on measuring the extension on each of the leg chords at each leg. See figure above. This is achieved through the use of encoders or other sources, which provide measurements to the software module in the SemView system. From these values the total leg cord extensions and RPD values are calculated. These calculated values are then displayed as part of a SemView operator station screen or picture.

Further, the software module displays leg inclination angle and direction derived from the RPD values together with physical leg constants. All values are displayed online to give the operator an overview of the system status during jacking.
Should a user-defined RPD limit be exceeded, an alarm will occur and the actual leg and chord causing the alarm will be shown in red. The RPD monitoring software continuously monitors and logs calculated data as well as events and alarms in the SemView system database system. From the logged values historical trend curves, fault logs and other reports or logs can be generated for display and printing.

• A visual overview screen, continuously showing the actual RPD values for each leg together with inclination angle, direction and leg extension.
• Visual and acoustic alarms when the predefined RPD alarm limit is exceeded with indication of which leg and chord have caused the alarm.
• Alarm for system failure, encoder failure, etc.
• Continuous logging of events and alarms.
• Historical trend facility.
• User set-up facility for alarm levels and system parameters.

Semco Maritime is able to provide a complete system from field survey through to final implementation including design, system integration, installation and offshore commissioning - with the RPD monitoring system as a part of an existing SemView system or delivered as the first module of a new SemView system for the jack-up rig monitoring needs.
Nguồn: http://www.offshoreenergy.dk/offsho...of-rack-phase-difference-on-jack-up-rigs.aspx