Monday, March 12, 2007

Position & Integrity Monitoring System

BPP Technical Services provides a specialist range of engineering expertise, products and services to the offshore oil and gas industry and to the insurance industry.

"A First in Mooring Position and Integrity Monitoring"

An innovative position and integrity monitoring system has been incorporated into the Chevron ALBA turret moored FSU providing enhanced safety.

Developed by BPP Technical Services Ltd for Single Buoy Moorings Inc., who were responsible for the Bottom Mounted Integral Turret (BMIT) and mooring system, it uses chain angle measurements to determine the turret excursion from the calm water reference position and mooring line integrity.

The BMIT is part of the ALBA Floating Storage Unit (FSU) of approximately 120,000 tonnes dwt moored in the central part of the North Sea, 138m deep, west of the Alba North Platform, ANP. The BMIT provides permanent mooring of the FSU which remains free to weathervane and supports the entry of the fluid product transfer into the FSU. Crude oil is loaded into the FSU through a fluid product swivel mounted on the BMIT. Also diesel oil may be transferred. These fluids flow from the ANP through a pipeline and a flexible riser up to the BMIT. For production to continue, it is essential that the mooring integrity is maintained so that the flexible risers remain intact. The FSU must remain on location within its prescribed maximum excursion circle.

The FSU is maintained on location by an array of twelve composite anchor lines. The catenary angle of each chain is measured using an underwater inclinometer mounted on the chain hawse articulation shaft, which has only one degree of freedom. These are monitored by a computer, which uses complex algorithms to derive the BMIT location. This is compared with position information from an Artemis microwave range and bearing unit also forming part of BPP's supply.

The FSU surge oscillations have a period of 100 to 150 seconds and wave periods vary from a few seconds to 17 seconds. The nominal chain angle is 47.5¡ã to the horizontal. A single measure of the mooring system integrity is provided by the amplitude of the FSU excursion, i.e. the offset of the BMIT from its neutral position in the absence of external loads (i.e. loads caused by wind, wave and current). By monitoring the chain angles, it is possible to determine at all times both the excursion and the individual line tensions so that any abnormality can be detected.

Similarly the BMIT position can be monitored using a line-of-sight ranging system, D-GPS or other positioning system to warn of any problems causing unacceptable excursions. In this case the Artemis system is used.

The receiving Artemis antenna mobile station is placed on the radar mast above the FSU control room. The positioning system is equipped with all necessary sensors such that the position of any point on the FSU offset from the receiving antenna can be determined. This is includes roll and pitch sensors mounted near the FSU's pitch and roll centres of rotation to adjust for the FSU movements and relative location of the receiving antenna. The location of the FSU is thus determined relative to ANP on which the fixed Artemis station is located approximately 3km away. The data is further processed to present the position of the BMIT relative to the neutral reference position.

Position and integrity are displayed on dual screens in a variety of formats. The system allows for recovery of long term statistical information and time series gathering which is of benefit in analysing the performance of the FSU and its mooring system, potentially offering data which may be used for improved future designs. The installed system also provides for extensive interfacing with other systems, including alarm reporting and logging, as well as exporting pitch and roll data which may be used in assisting other operations such as with helicopters.

In addition to the above capabilities, the mooring line monitoring system proved of great worth in enabling the installation contractors access to the otherwise unavailable chain angle values to assist in the tensioning of the mooring lines at the time of installation.

It is anticipated that many more of this type of system will see duty throughout the world.

For more information on this subject or related subjects please contact us.

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