Optimised design of a seabed monopile gripper

Optimised design of a seabed monopile gripper

Thangaraju, Deepakraj (TU Delft Mechanical, Maritime and Materials Engineering)

Metrikine, A. (graduation committee)
Hoving, J.S. (mentor)
Luiken, R.J.N.J (graduation committee)

Delft University of Technology

Offshore and Dredging Engineering

2021-01-28

To increase the benefit of wind energy, the next generation wind turbines are becoming larger and are moving into deeper waters. This significantly increases the size and weight of the corresponding monopile support structures. In current practice, the majority of monopiles are installed using jack-ups or floating vessels using so-called monopile gripper frames. Both vessels have their advantages and disadvantages. Jack-ups are limited by weather conditions and lifting capacity depending on the jack-up legs. For floating vessels, it is difficult to achieve stability and verticality of the monopile during installation, because the gripper frame moves with the vessel. This reduces the operating weather condition for installation of monopiles using a floating vessel. To operate in larger weather windows and to be able to lift larger loads, Van Oord is investigating the use of a monopile gripper on the seabed, so that the gripper frame is no longer subject to the motions of the vessel. To install a monopile support structure using a seabed gripper, the monopile gripper frame is installed first. The monopile is then slewed into the gripper frame, which is able to hold the monopile in position and vertical while hammering it. After the installation of the monopile, the seabed gripper structure is removed and transported to the location where the next monopile is to be installed. This thesis covers the optimized design of such a re-usable seabed monopile gripper. First, the different options for the gripper structure and its foundation currently available in practice are identified from a literature study. Suitable concepts for the gripper structure are generated based on the requirements, of which the two optimal concepts are selected based on a Multi Criteria Analysis (MCA). A four-legged Jacket and a Tripod are chosen as the best concepts for the gripper structure using either Helical piles or Suction buckets for its foundation. A preliminary design of both the gripper structure and the foundation is done to identify the initial member sizing and reactions of the piles at the seabed. A FEM model of the gripper structure including a soil model is created in ANSYS and analysed for both members and joints in ULS conditions. Due to the required opening on one side of the gripper structure (to slew in the monopile), the stresses at the joints of the gripper frame are higher than anticipated and strengthened accordingly. Based on the soil-structure interaction, the four-legged jacket structure is selected as the best concept, because its piles are further apart than for the tripod structure, thereby leading to minimal interaction between the piles of the gripper structure and the installed monopile. Still, both helical piles or suction bucket can be used as the foundation for jacket. A fatigue analysis shows that the gripper frame is subjected to cyclic loads that lead to high bending stresses at the joints leading to possible failure. Based on this, certain modifications have been made to the gripper structure to properly distribute the stresses. A case study has been performed comparing the monopile installation by a floating vessel with and without seabed gripper. This case study shows that the time taken and cost associated with monopile installation using the jacket gripper structure is most economical using suction buckets. However, modification of the installation using helical piles may lead to time- and cost-reduction compared to using suction buckets. However, installation without using a seabed gripper is more economical compared to with using a seabed gripper. From this thesis, it is thus concluded that a four-legged jacket structure with either helical piles or suction buckets is an optimal seabed gripper structure. However, it is not an economical solution to use seabed gripper for monopile installation for Van Oord.

Monopile Installation
Seabed gripper

http://resolver.tudelft.nl/uuid:8f93a0b6-ed9e-4c46-b554-7a5b6e5a5482

2023-01-27

Student theses

master thesis

© 2021 Deepakraj Thangaraju