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Introduction
Seadrill West Prospero
- Rig-moves, which must be completed with a minimum loss of time and without risk to the integrity of the rig
- Drilling operations, where the stability of the rig is important and punch-through" of one or more of the legs is a risk
- Long distance moves (wet or dry tows) where "leg-bending" moments can cause damage to the legs and fatigue consumption determines the integrity of the legs.
During rig-moves the vertical motion (amplitude and period) of the rig is often the limiting factor.
Based on theoretical calculations (motion response) a limiting wave height (Hs) is determined and moves are not allowed when this wave height or forecast is exceeded.
However, the actual (heave) motions of the rig are usually different from the theoretical motions and wave heights (Hs) are nearly impossible to observe offshore.
This often results in larger than necessary safety margins, loss of time and discussions between rigmaster, Client and Warranty Surveyor.
All jack-up rigs are fitted out with level gauges, modern rigs with electronic level gauges in addition to the traditional "bubble"
The SiriMarine Motion Monitoring system combines all of the above and has added features:- Heave monitoring
- Angle display with accuracy of 0.05 degrees, including alarm functions
- Logging of all motion data and subsequent reporting possibilities
- Hull deformation / Hull Integrity module (2 sensor configuration)
- Fatigue consumption calculations can be performed, based on 6 DoF acceleration data
Figure: typical display of SiriMarine system with angle and heave module screens active
Siri Marine Proposal
For Jack-up rigs a SiriMarine Motion Monitoring system is proposed as following:A complete system, including one motion sensor, can be installed in the jacking-control room.
This will provide the following information:
- Angle display
- Heave monitoring
- Logging and alarm function
- 1 x DimMax 3.0 motion sensor
- USB converter and cabling
- PC desktop + 19" monitor– HP 7600 SFF or similar
Alternatively a system with 2 sensors can be provided. This will then include the Hull Deformation / Hull Integrity module and will give insight on the global deformation of the rig during drilling operations and during tows.
A dual sensor system will also provide full redundancy in data collection and display. The sensors can be installed as indicated on the figure below. Data communication between sensor and PC is with standard shielded Cat 5 data cable.
The Siri Marine Motion Monitoring and Hull Integrity system consists of one ore more DimMax 3.0 sensors, connected to a PC with dedicated software. Connection between sensors and PC can be done with standard Cat5 data cable (shielded twisted pairs, 4x2, braided strands preferred, 0.2 mm²)
The DimMax 3.0 sensors have the following specification:- 3-axial linear accelerations with an accuracy < 0.1% of the scale (resolution 1 mg) ,
- range 0-6 Hz
- 3-axial rotational accelerations with an accuracy < 0.05 deg/sec
- Roll, Pitch. Accuracy : static conditions < 0.05 degrees, dynamic conditions < 0.2
- degrees or 10% of amplitude(freq. DC-1Hz) , whichever is the bigger
- Yaw : Accuracy < 0.5 degrees or 10% of amplitude (freq 0.1 – 1 Hz) , whichever is the bigger>
- Sample rate : 25 Hz
- Signal : RS 485
- Logging : at sample interval
1 – Roll, Pitch and Yaw angles and periods of the vessel and cargo
2 – 6 DoF accelerations (linear and rotational) in the sensor positions of the cargo
3 – 6 DoF accelerations (linear and rotational) in other positions than the sensor position, by means of inter-/extrapolation, based on 'rigid body motions'
4 – Autoreports: maximum, minimum, mean accelerations, standard deviation, zero crossing periods and 3 hrs extreme values, based on 30 minutes interval summary data.
5 – Autoreports: maximum, minimum, mean angles, standard deviation, zero crossing periods and 6 hrs extreme values, based on 30 minutes interval summary data.
6 – Structural deformation (Deflection and torsion) between 2 or more sensors Cat5 data cables shall be delivered and installed from the proposed sensor positions to the platform control room by Client.
Siri Marine shall also deliver a dedicated PC (HP 7600 SFF or similar) with monitor and UPS.
The PC shall have SiriMarine software installed and the complete system of PC, software and sensors shall be shop-tested before shipment to Malaysia.
Service agreement, reporting and data analysis
Siri Marine BV offers periodical reporting and an annual visit to the rig for maintenance.
Standard reports on rig condition will be issued 4 times per year. Intermediate reports can be issued on request. The reports are based on the Autoreports that are generated by the system. Once per year a service engineer from Siri Marine will visit the rig for system maintenance. Software upgrades are free when a service agreement is in place.
Visits to the rig for service and maintenance shall be done during one day while the rig is in dock or when helicopter travel can be completed within 1 day.
A typical voyage report, generated from the auto reports, is shown below.
Cumulative fatigue calculations
The Autoreports also provide 6DoF acceleration data, which can be used for (cumulative) fatigue calculations and reporting.
The motion data, which is generated by the system, enables fatigue life calculations. Transfer functions for these calculations shall be developed in cooperation with Client
A typical fatigue consumption graph, which was developed on board during a previous transport with sensitive cargo, is shown below.
