MONACO -- The offshore industry is paying insufficient attention to hydrodynamic issues associated with deepwater installations, according to a paper delivered today at the Deep Offshore Technology Conference.
The authors are representatives of a joint industry project in France led by Oceanide, in partnership with Stat Marine, Bureau Veritas, and Ecole Centrale de Marseille. Their work, which includes performing model tests at the BGO first offshore basin in La Seyne sur mer, southern France, is sponsored by Total, Doris Engineering, Saipem, and Technip.
A typical large subsea package, possibly weighing more than 100 tons (91 metric tons), comprises suction anchors, seabed manifolds, and other subsea equipment. As these packages grow in size, and water depth for the new fields becomes deeper, the engineering offices for the installation vessels are operating at the limit for lay-down design, the authors claim.
There is insufficient data, they add, concerning the hydrodynamic coefficients needed for this design, such as added mass, radiation and quadratic damping, and the slamming load in the splash zone. The lack of data has led to over-conservative increases in the lifting spread of the installation vessels, or over-compensation of the environmental operating conditions, which in turn impacts the project costs.
Large subsea `packages’ generally fall into two categories: cylindrical shape (i.e. suction anchors) at the base and partly open at the top; and parallel piped shape (manifolds, skids), which are usually supported by a mudmat to aid geotechnical stability. The size and shape of the mudmat is often the chief component of the overall hydrodynamic loads.
During the water entry phase of subsea installation, the package buoyancy and slamming loads can destabilize equilibrium. Following immersion, hydrodynamic damping is influenced by the free surface proximity. During the subsequent lowering phase, particularly in very deep water, the design should take into account the dynamic interaction between the package being lowered and the lowering cable. Close to the seabed, there is further impact on the system’s added mass and hydrodynamic damping.
The JIP examined conditions for three sea states, ranging from 1 m (3.3 ft) significant wave height and 4 seconds peak period to 1.5 m (4.9 ft) significant wave height and 8 seconds peak period, with two lowering velocities for the splash zone crossing.
Model tests were applied using scaled down mudmats comprising a 5-mm (0.2-in.) thick steel plate, either perforated or non-perforated, with four vertical skirts; and an ID motion generator. These tests measured various values such as impact velocity (slamming) on still and irregular waves, oscillation effects, and porosity hydrodynamic impact.
The results, the authors claim, are significant progress in the knowledge of hydrodynamic coefficients used for heavy lifting operations. This knowledge should help optimize future deepwater load deployments and reduce their cost.