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    New IMCA publication on diver-based concrete mattress operations

    Publications // December 7, 2007

    During the 1970s, the first pipeline support and stabilisation mattresses were installed subsea in support of offshore oil and gas operations.

    The design and composition of these early mattresses was very basic, as they were essentially large canvas bags filled with a bituminous material mixed with aggregates. Now, as a result of 30 years’ evolution and experience coupled with safety and environmental requirements, mould produced concrete mattresses are commonly used in the offshore industry for protection, support, separation and stabilisation.

    The International Marine Contractors Association has just published ‘Guidelines for Diver-Based Concrete Mattress Handling, Deployment, Installation, Repositioning and Decommissioning’ to provide reference material based upon ‘best industry practice’ and historical experience to a wide range of disciplines based both onshore and offshore when dealing with issues associated with concrete mattresses.

    “Design and specification; load out; mobilisation; over-boarding; deployment; installation; re-positioning; decommissioning; and recovery are all considered in the document,” explained IMCA’s chief executive, Hugh Williams. “It also includes the use of ‘roll out anti-scour fronded’ mattresses plus the essential associated safety and environmental issues. The guidance note assumes that the typical mattress installation worksite is supported by vessels equipped with cranes that have operational and calibrated line-out meters and load cells; and that installation work is supported by divers and that the diving operation permits two divers to be involved with the installation sequence and be working with helmet-mounted cameras and lights and supported by a monitoring remotely operated vehicle (ROV)."

    Lifting mattresses differs from many other subsea lifts because the mattresses are flexible and linked by ropes. This requires careful handing through the splash zone and then diver assistance to position the mattress correctly and disconnect the slings. The guidance acknowledges that visibility may not be perfect for the divers. The task is repeated many times to cover a large area, so any benefits in the safety and efficiency of the operations are multiplied many times over. Repeated operations can be a source of incidents if over-confidence or familiarity creeps in. That is why good practice guidance is especially important to keep all the parties aware of the risks and required safety measures.  

    “Mattresses have had a fascinating history and we cover this in the new publication too,” he added. “Reading this section of the guidelines will not only bring back many memories for some, but helps demonstrate the continual improvements made by the industry in relation both to safety and the environment.”

    There has been considerable development since mattresses of the 1970s that, when left exposed on the deck of an installation vessel, would often soften and become misshapen due to the warmth generated by the vessel engine rooms and transmitted through the deck or by the effects of direct sunlight exposure.

    Once immersed subsea, they would then become brittle, often crack and, due to the cold induced stiffness, fail to take the shape of the pipeline or spool they were laid over. Incidents involving lifting and moving these types of mattresses were frequent.

    In the early 1980s further development saw the introduction of the first concrete mattresses. These were designed to be more flexible and robust, were far more versatile and could support a greater range of applications. The early concrete mattresses were known as ‘link-lok’ mattresses. They were essentially a large sheet of pre-made plastic pots, joined together by a lattice work of connecting rope. The plastic posts were then filled with concrete and allowed to cure. The concept was that these sheets of rope-connected pots could be easily transported to a mobilisation port and then filled with concrete.

    In practice this proved impractical, the product was extremely expensive and this manufacturing process ceased in the early-1990s at about the same time that the continued use of bituminous type mattresses was prohibited due to the obvious problem of re-introducing a hydrocarbon based product into the environment. The demise of the bitumen and link-lok type mattresses paved the way for the evolution of the mould produced concrete mattress type now in use.

    Members of IMCA can download the new publication (IMCA D 042) free of charge from the members-only website. Printed copies are available at £2.50 (£5 for non-members) plus 20% delivery fee outside Europe, and can be ordered online at www.imca-int.com/publications/diving; or from IMCA at 5 Lower Belgrave Street, London SW1W 0NR, UK. Tel: 020 7824 5520; Fax: +44 (0)20 7824 5521; Email: publications@imca-int.com.

     


     

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