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cluded that a cylindrical-shaped grouted funded by the Germany Ministry for the has involved carrying out physical test-

EPIC connection design, without shear keys or Environment, Nature Conservation and ing in DNV GL’s laboratory, in addition to structural analyses, feld monitoring, additional support arrangements for axial Nuclear Safety (BMU). It refects latest and sharing experience. load, or grouted conical connection with research results and supports the indus- shear keys, are possible design solutions The laboratory testing is intended to try in avoiding past failure. Guidance on for large diameter grouted connections. obtain more reliable data for design and design calculation and structural details

New solutions

The industry partners also came up guidance on the use of shear keys in as well as requirements for manufactur- with new design solutions. In addi- monopile solutions. After the testing, a ing, quality control, installation, and tion, solutions for existing installations, design methodology was developed to monitoring is given.

for grouted connections including support arrangements, were account for the dynamic bending moment Both DNV GL offshore standards (DNV- presented. and the vertical force in connections with OS-J101 or GL-IV-2) now refect latest the temporary supports as a result of shear keys. industry solutions and state-of the art

Shear keys settlement in some grouted connections The results have been compared with recommended practices.

Shear keys are circumferential weld beads has led to concern about fatigue cracking laboratory test data from simulated capac-

Dr. Claus Fridtjof on the outside of the monopile and the in the structures which would lead to ity of large diameter connections. It was

Christensen is inside the transition piece, in the grouted repair needs. During a thorough review found that the design procedure provides regional manager for section. The shear keys’ purpose is to of the DNV-OS-J101 standard in 2009, design data that are in good agreement

Europe and North increase the sliding resistance between we discovered that some scale effects with the measured laboratory data.

America, the grout and steel so that no settlement were not properly accounted for in this

Looking for guidance

Renewables occurs. standard or in other standards for similar

Certifcation, at DNV

Existing design standards for such con- types of connections. The revised DNV GL design standard

DNV-OS-J101 was launched for an exter-

GL – Energy. He has nections were based on limited test data

Joint Industry Project nal hearing, which was due to be held in worked for the company for 13 years on small diameter connections for alternat-

A Joint Industry Project (JIP) has been February 2014. This was due to include and has a background in oil and gas, ing dynamic loading. Therefore, before initiated by DNV GL in order to look results of the JIP. within the feld of structural reliability this solution could be recommended, a design practice for shear keys had to be further into the issue, with the aim of In December 2013, DNV GL published and safety analysis for structures developed and properly incorporated in a solving an industry-wide challenge. a technical note for the certifcation of subjected to various types of load. revised design standard.

Conical grouted connections have been grouted connections, which is a supple-

Christensen also has experience in risk developed and tested as an alternative ment to the latest version of the offshore management and technical risk assess- ment, especially related to ship collision

Physical testing design. In addition, the industry partners guideline GL-IV-2. This technical note

The collaboration between DNV GL and risk analysis on offshore platforms, have been looking at grouted connections is based on the research project Grouted owners, operators, grout producers, uni- transformer platforms, and offshore with shear keys. Joints for Offshore Wind Turbine versity, certifying body, and designers, wind farms.

The joint industry project has con- Structures (GROW), started in 2007, and

Conical shaped connection

Grouted connections

The monopile and transition piece are fabricated with a small cone angle in the grouted section. If the bonds between the steel and

A grouted connection connects the transition piece to the monopile. A transition piece is placed on top of the monopile, resting on grout are broken, some slight settlement of the transition piece will occur. This will introduce compressive contact stresses between temporary supports. During installation, the transition piece is then jacked up before grouting. After curing, the jacks are removed, the steel and grout which, together with some friction, will provide sufcient resistance against further settlement. leaving a gap of a few centimetres between the tempworary supports and the monopile. Settlement down to the temporary supports

Images: ©DNV/Max & Co may result in a diferent force fow in the structures. oedigital.com May 2014 | OE 79 078-OE0514-EPIC4-DNV_v4.indd 79 4/18/14 9:38 PM