Disclaimer: This study is a preprint posted on SSRN and has not yet undergone peer review.

Title: Numerical Analysis of Vibration and Fatigue Characteristics of a Deep‑Sea Mining Riser Subjected to Irregular Waves and Current

Author / Sponsor: Yan Li and colleagues, Dalian Maritime University

Date: May 2024

Report Length: 40 pages

BLUF: Computer simulations indicate that irregular waves and surface currents can magnify a deep‑sea mining riser’s side‑to‑side motion up to nine times and its bending loads more than six times compared with calm water. As weather worsens, the spot where metal fatigue accumulates most shifts hundreds of meters down the pipe, meaning sensors and reinforcements must move with it. Good heave compensation on the vessel and smart weather planning extend riser life more effectively than simply adding wall thickness.

A deep‑sea mining riser is a vertical pipe that lifts ore‑rich slurry four to five kilometers from the seafloor to a surface ship. The paper models a 500‑meter upper section that connects the ship to a subsea buffer tank. This segment is critical because it experiences both the ship’s motion and ocean forces.

The authors developed a time‑domain finite‑element model in ANSYS and validated it against two other solvers and published laboratory data. Natural frequencies match within about 0.3 percent, which gives confidence that the model can represent real behavior.

Under calm conditions the riser behaves much like a hanging string. Lateral displacement increases toward the subsea end, while tension and stress peak near the ship. When a current of about 0.95 m s⁻¹ (roughly 1.85 knots) is added, drag forces push the pipe sideways and raise stress. Stronger currents increase these lateral motions but also create some downward pull, which slightly reduces top tension.

Wave forces dominate the response. At a moderate sea state with 2 m significant wave height and an 8 s peak period, the pipe’s lateral swing roughly doubles. When significant wave height grows to 10 m, lateral motion increases by about six times, bending by more than four times, and von Mises stress a little more than doubles, rising roughly 110 percent. Shorter wave periods amplify these effects even further. A 5 s swell can raise the lateral amplification factor to about nine and bending to just above six.

Fatigue life follows the same trend. In calm seas, the highest Palmgren‑Miner cumulative fatigue damage occurs within the top 50 m where tension is greatest. As wave height exceeds 6 m or peak period falls below 6 s, the damage hotspot moves between 65 m and 475 m along the riser. Under very rough seas with 10 m significant wave height, the peak shifts to around 140 m. These shifts occur because large waves create standing‑wave patterns that increase bending well below the ship.

Hardware choices matter. Increasing the subsea buffer mass from 5 t to 30 t reduces lateral swing and bending but increases axial displacement, tension, and stress at the top. Designers must balance reduced side loads against greater straight‑line stretch when sizing the buffer.

Overall, the study shows that dynamic loading is driven mainly by the ship’s heave motion and the local wave spectrum, not by riser wall thickness. Operational improvements such as narrowing weather windows or upgrading heave‑compensation systems are more effective for extending riser life than adding steel.

Key recommendations drawn from the findings include placing strain gauges and accelerometers along the full riser instead of only near the vessel, using mid‑string buoyancy or damping modules to reduce bending during rough seas, selecting a subsea buffer mass that controls swing without overstressing the top joint, scheduling heavy lifting and slurry pumping for calmer sea states, and investing in high‑performance heave‑compensation systems to minimize riser stretch and fatigue.