Jack-up vessel preload reliability study

Abstract

The offshore wind market has become a substantially growing industry to meet the worldwide increasing energy demands. This results in heavier crane operations on offshore jack-up vessels due to the increasing size of wind turbine components. To ensure the seabed can support the loads on the jack-up legs, a temporary foundation for the leg's footings is established before crane operations through a process known as preloading. Where the two diagonal opposing leg pairs are loaded alternatively by the weight of the vessel until a stable condition is reached. Traditionally, the capacity of those foundations is determined based on storm loads adopted from offshore oil and gas jack-up guidelines.

The main objective of this thesis is to develop a robust method for analysing the applied preload in past jack-up crane operations conducted by Jan de Nul's Vole au vent. This analysis aims to provide a better understanding of the effectiveness of traditional offshore jack-up guidelines within the rapidly growing offshore wind industry, where heavy crane operations are now performed on a daily basis. Such understanding is crucial, as these guidelines were not originally intended for the advanced state of the current offshore wind sector and are not specifically calibrated for its unique demands.

To accomplish this objective, a method is designed to assess the preload safety factors of past jacking operations and determine their optimal value for heavy lifting operations. This safety factor provides the ratio between operational leg reactions and applied preload. The developed models are then applied to a case study using measuring data from jacking operations of the Vole au vent to validate their effectiveness.

This method is based on reliability analyses which evaluate the probability of failure by assessing if a certain limit state is exceeded. Failure for jacking operations can occur when the leg reactions during crane operations become larger than the applied preload. From the acquired measuring data, certain probability distributions of the leg reactions can be obtained, which are used by a Monte Carlo Simulation to assess the probability of preload exceedance through the defined limit states. This probability of preload exceedance quantifies the reliability of the applied preload in different soil types at three offshore wind farm sites.

This research then defines optimal targets for the annual probability of preload exceedance based on the consequences of failure of operations in both low-risk and high-risk soil profiles. These targets provide a balance between operational efficiency and safety. From those targets, an optimal preload safety factor is obtained and compared to what was originally applied during the jack-up operations.

The findings of this thesis indicate the need to evaluate and improve the standards to better align with the industry's evolving requirements.

It is shown that the currently used preload safety factor from traditional offshore jack-up guidelines is not yet correctly calibrated for heavy crane operations on jack-up vessels. To achieve an optimal balance between operational efficiency and safety, the applied preload, with respect to the experienced loads from heavy crane operations, should be slightly lowered compared to what is currently applied.

In addition, this research observed that the measured conditions during jack-up operations are not correctly estimated, leading to operational and preload uncertainties.