Coupled Transient CFD and Diffraction Modeling for Installation of Subsea Equipment/Structures in Splash Zone

Abstract

In development of deep water oil and gas fields, successfully and economically installing subsea equipment and structure is critically important. This paper presents a state-of-the-art methodology for predicting the motions and loads of subsea equipment/structure during such operations basing on time domain simulations of the combined installation vessel and subsea equipment/structure. The time domain diffraction simulation of the moving lifting vessel is coupled with multiphase CFD simulation of subsea equipment/structure in splash zone. Transient CFD model with rigid body motion for the equipment/structure calculates added masses, forces and moments on the equipment/structure for diffraction analysis, while diffraction analysis calculates linear and angular velocities for CFD simulation. This paper has many potential applications, such as, installation of pile, manifold, subsea tree, PLET/PLEM, or other subsea equipment/structure. This coupled approach has been successfully implemented on a cylindrical structure. The results show that total load level, and dynamics of the subsea equipment/structure due to waves in splash zone are predicted. Current practice of installation analysis in accordance with the recommendations from DNV-RP-H103 [1] cannot determine in detail the wave loads either during the passage through splash zone, or added mass and damping when the equipment/structure is submerged. In order to determine wave loads in detail, model tests are needed. In the absence of tests, simplified equations or empirical formulations have to be used to calculate/estimate these hydrodynamics coefficients as recommended in DNV-RP-H103. Steady-state CFD simulations on a stationary equipment/structure are usually used to predict drag and added masses on submerged structures. However the steady-state assumption in CFD ignores the resonating motion of equipment/structure in calculating hydrodynamics coefficients, which can severely affect the accuracy of these predictions. The above methods often give overly conservative results for allowable sea state which results in uneconomical vessel time or inaccurate results for installation. The methodology of this paper gives more accurate results, and provides potentially economical vessel time during installation. The intent of this paper is to demonstrate the solution and methodology.