Thursday, August 16, 2007

Integrated management for a tension leg platform

Integrated management for a tension leg platform

Fig. 1. The TLP and its moorings are designed the ferocity of a ‘1000 year’ storm.

Acomplete PLC-based integrated marine management system, (IMMS), has been supplied by Strainstall UK Limited for a new tension leg platform, (TLP), to be deployed in the deepwater area of the Gulf of Mexico.
The standalone TLP will be in a water depth of approximately 4300feet, around 120 miles from the Louisiana coast, and will have a nominal production capacity of 100000barrels of oil and 50 million cubic feet of gas per day.
The IMMS consists of a distributed PLC system and Strainstall manufactured subsea load cells. The load cells were designed and manufactured by Strainstall.
Designed for a 25 year life at a nominal water depth of 114ft and the maximum tendon loads estimated in a 1000-year storm, the load cell design has been approved by the Classification Society, ABS. The sensors are installed in the tendon top connectors, and from these the strain gauge bridge networks are routed to the hull junction boxes via underwater mateable connectors and offshore underwater cables. The PLC system is located in the hull, where Strainstall developed PLC logic and HMI display software provides the operator with data from a number of parameters on the TLP.
All data inputs, and the control of systems are available through local touch-screen interface panels and remotely through fibre and wireless networks. The system features include the following:

  • Tendon data that incorporates load and bending moments from redundant strain gauges in the subsea load cells.
  • Reports on the height of the deck above the sea surface and wave conditions from installed air gap sensors.
  • Both permanent and temporary ballast tank levels which are monitored through a network of bubblers.
  • Additional bubblers and subsea pressure sensors that provide platform draft and calculated inclination. An on-board inclinometer is also included.
  • Ballast control using a software interface that coordinates the operation of pumps and valves to distribute ballast according to platform parameters. All information affecting ballasting decisions is available through the system, as are the controls themselves. The use of any control is subject to password protection, according to operator permissions.
  • Information on the absolute position of the platform using GcGPS (Globally corrected GPS). This feature provides sub-metre positional accuracy in offshore locations.
  • MMS compliant current data that is generated using Acoustic Doppler Current Profilers (ADCPs). A sideways-looking sensor provides the surface details, whilst a downwards-looking sensor provides the subsea details.
  • Reporting on a number of meteorological conditions, including wind speed and direction, air temperature and pressure.


Sandy Thomas is Marine Director, Strainstall UK Ltd, Cowes, Isle of Wight, UK. www.strainstall.com

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