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Concrete In Australia : September 2008
Construction of the various types of concrete structure applications including CGS and other forms of floating subsea platforms is a multi-million dollar industry that has the potential for creation of several hundreds of jobs. An understanding of the critical tow-out issue could provide support to the industry by way of decreased capital expenditure. For example, a concrete structure designed to carry a given payload may have a large footprint with a smaller draft thus enabling ease of tow-out in a shallow channel. Alternatively, the footprint may be reduced and air-lift may be used to conduct a safe tow-out in shallow channels. In a sample calculation for a generic CGS, it was found that by using the air-lift technique an order of magnitude savings of up to $17 million in lower base sub-structure concrete volume could be achieved. The same principle could apply to other types or application of concrete structures (eg: caissons, and fl oating or fi xed support structures). Previous studies conducted under the aegis of COSI Future CGSs for the North West Shelf are likely to be larger as they are adopted for large gas fi elds in various water depths. As CGS requirements get larger in weight, their depth of submergence during tow-out increases. In combination with long skirts, available water depth becomes a critical criterion. In such a situation, a compressed air cushion may need to be provided beneath the CGS fl oor and into the skirt chambers in order to artifi cially lighten the structure during tow-out. With a view to increasing the capability of local construction yards, COSI has been a participant in a series of studies aiming to understand the feasibility and design consequences of airlifting a range of concrete structures which would require fl oat out including caissons and subsea support facilities. A brief overview of these studies is given below. Pilot study on air-lifted CGS In 1997-1998 Maunsell Groner Multiconsult Group and Centre for Oil & Gas Engineering of UWA conducted a pilot study on airlifted CGS in shallow waters. The work was supported by Minerals and Energy Research Institute of WA (MERIWA), COSI, WA Petroleum (WAPET) and Woodside Energy. Findings of the study and various aspects of the problem were given by Hill and Sow (1997), Hill and Ronalds (1998), and Thiagarajan et al (2000). In the pilot study a model of CGS with 80 air cushions was tested in a shallow wave tank to measure it’s response in regular waves. Heave and pitch amplitudes were found to be attenuated substantially with decreasing under-keel clearance. At the same time the measured behaviour of the model was noted to be somewhat different from that predicted numerically using existing software, in particular at small under-keel clearances (UKC). The overall ability of a CGS to clear a shallow channel in head seas is given by the lowest clearance between the seabed and the bow or stern of the structure. One can obtain some engineering information on the dynamic seabed clearance at the bow by combining the vertical motion information with the corresponding UKC and plot in a three-dimensional chart. Thiagarajan et al (2000) explains the minimum clearance as a Typical CGS model tested in waves. function of still water UKC and incident wave period. For UKC between 1m and 2m, the smallest seabed clearance occurs in the range of 15s to 19s periods. The usefulness of this plot can be illustrated by assuming future CGS towout on the Australian west coast uses a UKC of between 1m and 2m. Towing the CGS in wave periods in less than 15s to 19s would be desirable to provide adequate seabed clearance in such conditions. Literature, user and software survey on air-lifted CGS In 1998-1999 the Centre for Oil & Gas Engineering conducted a literature, user and software survey (Thiagarajan and Sow (1999)) on air-lifted CGSs and similar applications. This showed that phenomena associated with the behavior of airlifted CGSs and similar fl oating structures such as caissons in shallow water are generally unknown to the industry and clear guidelines are not available for critical design parameters, such as the minimum under-keel clearance and the water plug height (WPH). Air cushion scaling studies Scaling of air cushion experiments to full scale prototypes is a well known problem in scientifi c literature. This arises because the atmospheric pressure is not scaleable - ie: it has the same value at both prototype and model scales. Consequently, all model studies with air cushions should be accompanied by expensive supporting infrastructure, such as a cavitation tunnel or a volume reservoir. The pilot study sponsored by COSI and others presented an innovative idea of using lenticular balloons to scale air cushions. The concept was proved mathematically by Thiagarajan (2004) and presented to an international audience. The volume to pressure relationship was found to be largely linear for a lenticular balloon as opposed to a non- linear relationship for a regular balloon. Shallow water tow-out of CGS with and without air-lift This was a three-year project sponsored by COSI along with the Minerals and Energy Research Institute of WA (MERIWA), Woodside Energy and Ove Arup and Partners. Concrete in Australia Vol 34 No 3 41