Response Analysis of a Nonstationary Lowering Operation for an Offshore Wind Turbine Monopile Substructure

Abstract

This study addresses numerical modeling and time-domain simulations of the lowering operation for installation of an offshore wind turbine monopile (MP) with a diameter of 5.7 m and examines the nonstationary dynamic responses of the lifting system in irregular waves. Due to the time-varying properties of the system and the resulting nonstationary dynamic responses, numerical simulation of the entire lowering process is challenging to model. For slender structures, strip theory is usually applied to calculate the excitation forces based on Morison's formula with changing draft. However, this method neglects the potential damping of the structure and may overestimate the responses even in relatively long waves. Correct damping is particularly important for the resonance motions of the lifting system. On the other hand, although the traditional panel method takes care of the diffraction and radiation, it is based on steady-state condition and is not valid in the nonstationary situation, as in this case in which the monopile is lowered continuously. Therefore, this paper has two objectives. The first objective is to examine the importance of the diffraction and radiation of the monopile in the current lifting model. The second objective is to develop a new approach to address this behavior more accurately. Based on the strip theory and Morison's formula, the proposed method accounts for the radiation damping of the structure during the lowering process in the time-domain. Comparative studies between different methods are presented, and the differences in response using two types of installation vessel in the numerical model are also investigated.

Operability Analysis of Monopile Lowering Operation Using Different Numerical Approaches

“…The distributed wave forces fW,s are integrated along the MP to obtain the total wave forces and moments, FW. The added mass coefficients for different strips along the MP are chosen according to Li et al (2015c), where the excitation forces calculated using Morison's formula at different drafts were compared with those from panel method and good agreement was achieved.…”

“…Li et al (2015c) concluded that the radiation damping on the large diameter MP is of importance especially for short wave conditions in incident waves. However, the hydrodynamic interaction between the HLV and the MP was not included in Li et al (2015c), and the influences of the radiation damping of the MP in short waves for the coupled case were unknown. This study compares the effects by implementing the radiation damping on the MP for the coupled HLV-MP case.…”

“…A detailed explanation of the modes and corresponding natural periods refer to Li et al (2015c). The gripper is excluded when calculating the natural periods because the gap between the MP and the gripper allows relative motions.…”

“…Recommendations regarding the heading angles of the vessel during the lifting operations of the two OWT substructures were given. Moreover, the importance of radiation damping of the MP during the nonstationary lowering operation were examined by Li et al (2015c). A new approach was proposed to implement the radiation damping effects into the timedomain simulation of the nonstationary lowering process.…”

“…In those studies, some simplifications were made in the numerical model, e.g., the hydrodynamic forces on the vessel were simplified by only considering the first order wave excitation forces in Li et al (2013Li et al ( , 2014Li et al ( , 2015c, and the hydrodynamic interaction between the floating installation vessel and the MP was not included when studying the influences of MP radiation damping in Li et al (2015c). Wave short crestedness is another factor of influence for lifting operations (DNV, 2014) which has not been taken into consideration in the mentioned references.…”