Jacket Structures

Background for Design Standards for Grouted Connections

Cylindrical grouted connections between piles and jacket legs have a positive track record, stretching back over many years, in jacket structures in the oil and gas industry. In older jacket structures, the piles were driven through the jacket legs and then welded to the top of the legs. Grouting was performed by filling the space between the outer surface of the pile and the inner surface of the leg. The grout was made as mix between water and cement. This methodology was used for the first platforms installed in the North Sea in the 1970s. In these structures shear keys were not required in order to achieve sufficient structural capacity. Shear keys are understood to refer to circumferential weld beads on the outer surface of the pile and on the inside of the sleeve in the jacket structure, which increases the shear capacity of a grouted connection when the pile is subjected to axial loading. In older design recommendations from the American Petroleum Institute (API) (1977) and UK Department of Energy (1977), no explicit guidance was provided on how to determine the capacity when mechanical shear keys were used. However, these recommendations did allow the use of shear keys, and the following section is quoted from the UK Department of Energy document (1977):

The following recommendations for design can be made:

1. Mechanical shear keys should be used on both the sleeve and the pile surfaces and should be spaced uniformly along the length of the connection.

2. Full-scale static tests (or reduced scale tests in which all geometries and material properties are accurately modelled) should be used as a basis for design.

3. To determine long-term design bond strength from static ultimate load tests, a safety factor of at least 6 should be used to allow for unknown effects such as cycling loading, both during the grouting operation and in the long term, and offshore construction conditions.

In the same document it was stated that the API specifications at the time were based on tests conducted on relatively small-diameter and radially stiff piles. Furthermore, it was noted that recent laboratory testing had indicated that there was a severe reduction in joint strength with large-diameter piles, such as those used for North Sea structures.