Seabed preparation design and construction for the Malampaya Phase 3 Depletion Compression Platform

Opkar, J. and Lorenti, L. and Grime, A. (2016) Seabed preparation design and construction for the Malampaya Phase 3 Depletion Compression Platform. In: ICSE 2016 (8th International Conference on Scour and Erosion), 12-15 September 2016, Oxford, UK.

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Abstract

The Depletion Compression Platform (DCP) was conceived by Shell Philippines Exploration B.V. (SPEX) to provide additional gas compression to account for the future expected decrease in well pressure from the Malampaya field. The Malampaya project is very important to the ongoing prosperity of the Philippines, with the exported gas feeding three power stations which have a combined output providing up to 45% of the power needs of Luzon, the largest and most populous island in the Philippines. During the concept phase of the project SPEX selected Arup’s ACE Gravity Base Structure (GBS) as the substructure solution for the DCP. The DCP substructure was designed to be supported on four linked regular hexagonal pad footings with dimensions of 18 m x 20.8 m x 4m deep. These pad footings were founded on individually prepared support pads composed of engineered rock fill material of approximately 1 m in thickness. It is standard practice to include a larger sized scour blanket around the footing perimeter to prevent local erosion, undermining and loss of support. However, during design development we were challenged by the project team to omit this element altogether by selecting a founding engineered rock fill material which was itself suitably sized to prevent scour. Given there is no analytical approach available to accurately determine the local flow enhancement around the apices of the individual hexagonal pad footings at seabed level, Computational Fluid Dynamics (CFD) modelling and Wave Tank Testing (WTT) were employed. The effect of the existing adjacent Concrete Gravity Structure (CGS) was also considered as part of this assessment. The results of the CFD modelling and WTT was used to calculate a minimum size range for the engineered rock fill which is capable of resisting the enhanced flow velocities in the 100-year return period cyclonic storm event. Following calculation of the required size of engineered rock fill material, this information was presented to the potential seabed preparation Contractors for agreement on the achievable surface profile. The agreed local surface profile of the seabed preparation support pads was then used to assess the structural performance of the pad footing. Once structural performance was shown to be adequate this confirmed that the dedicated larger sized scour protection layer could be omitted. This yielded savings to the project by minimising material procurement, vessel modification and offshore working time. The construction of the seabed preparation support pads using the larger sized engineered rock fill was performed successfully within the tolerances established and agreed through early consultation with the potential seabed preparation Contractors.

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