Grouted connections have been a challenge to the offshore wind industry. The sector is now looking for class society guidance so that the latest applications for grouted connections are reflected in the latest design standards.
Wind energy is a fast developing industry, which means independent certification bodies like DNV GL have to ensure that widely used standards for the design of offshore wind turbine support structures like DNV-OS-J101 or GL-IV-2 are based on the most recent technical knowledge, methodology, experience and test results.
The most well-known offshore wind turbine support structures are monopile, gravity based, jacket, and tripod structures. Monopiles are the most widely used design and grouted connections are used to connect the transition piece to the monopile. Grouted connections have been successfully used in the offshore oil and gas industry on drilling and production platform jackets for the last 40 years.
However, the major difference for a monopile offshore wind turbine structure is that, while the grouted connection in jackets is mainly intended for small diameter piles, with large axial forces, the wind turbine monopile grouted connection is large diameter and predominantly subjected to bending.
An unintended force transfer through
the temporary supports as a result of settlement in some grouted connections has led to concern about fatigue cracking in the structures which would lead to repair needs. During a thorough review of the DNV-OS-J101 standard in 2009, we discovered that some scale effects were not properly accounted for in this standard or in other standards for similar types of connections.
Joint Industry Project
A Joint Industry Project (JIP) has been initiated by DNV GL in order to look further into the issue, with the aim of solving an industry-wide challenge. Conical grouted connections have been developed and tested as an alternative design. In addition, the industry partners have been looking at grouted connections with shear keys.
The joint industry project has concluded that a cylindrical-shaped grouted connection design, without shear keys or additional support arrangements for axial load, or grouted conical connection with shear keys, are possible design solutions for large diameter grouted connections.
The industry partners also came up with new design solutions. In addition, solutions for existing installations, including support arrangements, were presented.
Shear keys are circumferential weld beads on the outside of the monopile and the inside the transition piece, in the grouted section. The shear keys’ purpose is to increase the sliding resistance between the grout and steel so that no settlement occurs.
Existing design standards for such connections were based on limited test data on small diameter connections for alternating dynamic loading. Therefore, before this solution could be recommended, a design practice for shear keys had to be developed and properly incorporated in a revised design standard.
The collaboration between DNV GL and owners, operators, grout producers, university, certifying body, and designers, has involved carrying out physical test- ing in DNV GL’s laboratory, in addition to structural analyses, field monitoring, and sharing experience.
The laboratory testing is intended to obtain more reliable data for design and guidance on the use of shear keys in monopile solutions. After the testing, a design methodology was developed to account for the dynamic bending moment and the vertical force in connections with shear keys.
The results have been compared with laboratory test data from simulated capacity of large diameter connections. It was found that the design procedure provides design data that are in good agreement with the measured laboratory data.
Looking for guidance
The revised DNV GL design standard DNV-OS-J101 was launched for an external hearing, which was due to be held in February 2014. This was due to include results of the JIP.
In December 2013, DNV GL published a technical note for the certification of grouted connections, which is a supplement to the latest version of the offshore guideline GL-IV-2. This technical note is based on the research project Grouted Joints for Offshore Wind Turbine Structures (GROW), started in 2007, and
funded by the Germany Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU). It reflects latest research results and supports the industry in avoiding past failure. Guidance on design calculation and structural details as well as requirements for manufacturing, quality control, installation, and monitoring is given.
Both DNV GL offshore standards (DNV- OS-J101 or GL-IV-2) now reflect latest industry solutions and state-of the art recommended practices.
Dr. Claus Fridtjof Christensen is regional manager for Europe and North America, Renewables Certification, at DNV GL – Energy. He has worked for the company for 13 years and has a background in oil and gas, within the field of structural reliability and safety analysis for structures subjected to various types of load. Christensen also has experience in risk management and technical risk assessment, especially related to ship collision risk analysis on offshore platforms, transformer platforms, and offshore wind farms.