Over the last decade Heerema Marine Contractors (HMC) has successfully performed multiple installation campaigns of large sized in-line structures (ILS) with Deep Water Construction Vessels (DCV) Aegir and Balder. Nowadays steady increase in size and weight of ILS have made these special operations even more complex. Presence of large sized ILS and accompanying buoyancy modules in the catenary have proven to play a dominant role in pipeline integrity. Originally hydrodynamic force formulations in finite element analysis are solely designated for the pipeline itself. These computations comprehend the application of the Morison equation using constant hydrodynamic coefficients of basic shapes in steady flow.
Therefore hydrodynamic forces acting on the ILS, characterized by irregular relative motions of a complex shaped and perforated structure, are highly simplified while playing a dominant role in the analyses. Validity of applying the standard Morison equation is debatable, since large ILS cannot be assumed slender. Nonetheless Morison type formulations can provide reasonable results depending on the accuracy of the hydrodynamic coefficients. Deriving these coefficients for complex shaped structures using industry standards is a highly interpretive process involving an accumulation of assumptions. This approach yields varying coefficients, which are applied conservatively in installation analyses, resulting in an unnecessary reduction of DCV offshore workability.
To improve workability of these complex installations, HMC has implemented an ILS specific hydrodynamic profile from Computational Fluid Dynamics (CFD) analysis into the installation analyses. This is effectuated by the development of an enhanced methodology with a dedicated hydrodynamic formulation for large perforated ILS. Dependencies on Keulegan-Carpenter (KC) number and local angle of attack are addressed in this formulation to respectively cover the inertia dominated oscillating motions and complex geometric composition. The applied hydrodynamic formulation is based on work of Molin et al. which showed a good agreement to the CFD analysis performed for this study. Development and application of this methodology is initiated as a first assessment towards more accurate ILS installation analyses.
Analysis of a study case shows reductions up to 50% of maximum bending strain in a specific regular wave analysis. From the work presented it is concluded that the industry practice vastly overestimates hydrodynamic forcing on large sized ILS. Complementary research is needed on the topics of oscillations for low (<1.0) KC number, effects of relative fluid velocity and finally the implementation of irregular waves.
