Dynamic analysis of a hang-off drilling riser considering internal solitary wave and vessel motion

Abstract

Hang-off mode of a drilling riser is occasionally needed during subsea installation/platform relocation operations or evacuation after an emergency disconnection. A suspending riser without any restriction at its bottom is more flexible and more dangerous in complex sea states than a connected riser with excess axial tension. Internal solitary waves (ISWs) can particularly exert a sudden impact and shearing force on risers, and vessel motion can expand horizontal dynamic responses of the risers. In this paper, considering vessel motion and the combined excitation of ocean currents, surface waves and ISWs, a dynamic model is constructed based on the Euler-Bernoulli theory, in which ISW is simulated by the Korteweg-de Vries (KdV) equation with a two-layer seawater model. Then, the structural governing equation is numerically solved by the Wilson- θ method and preconditioned generalized minimal residual method (GMRES) with a self-developed MATLAB program. Case calculation shows that ISW can largely increase the envelopes of riser properties in the upper seawater layer and dramatically expand the horizontal deviation of the bottom of a hang-off riser during ISW spreading. Particularly, the dynamic responses of a riser will be larger with ISW amplitude augmentation, and larger with an increase in the density difference between the two seawater layers. In addition, vessel motion can increase the horizontal deviation along the entire length of a riser with a range that is nearly the same as that of the vessel motion amplitude, and increase the envelopes of bending moment and shearing force at the lower section of the riser near the riser bottom. Therefore, limiting the vessel motion amplitude, optimizing the vessel towing speed, maintaining a lower marine riser package (LMRP) at the riser bottom to strengthen axial tension, and using slick joints with larger wall thicknesses in the upper depth may be effective engineering considerations. More importantly, much attention should be paid to avoid the riser bottom/LMRP striking other subsea equipment in oceans in which ISWs occur frequently.