Chapter 8. Falling Structures - Risk Associated to Non-Conventional Connections of Tertiary Structures Available to Purchase

This paper reports methods of and experiences with "nonconventional" joining of tertiary outfitting structures to identify associated risk in the oil and gas industry. With nonconventional we mean other methods than fusion welding and bolted connections (bolt and nut) between similar materials. These methods can be self-tapping screws, explosion welding and blind bolts.

The objective is three-fold. Firstly, to address the risk awareness associated with other parts of the structure than main structure, based on reported incidents. Secondly, report on methods for these components. And thirdly discuss need for standardization.

We have identified and categorized different nonconventional joining systems, described the methods and identified typical experience, damage, and integrity challenges in the industry. The non-conventional joining methods can be divided in welding and non-welding.

A review of selected standards and best practices shows that there are limited requirements towards non-conventional joining of tertiary structures. The focus for an operator is mainly on structural bolted connections and traditional welding.

NORSOK M-001 “Material selection” Amendment has some requirements towards selection of material and how to avoid galvanic corrosion between steel and aluminum by electrical insulation however, these are mainly for through-bolts.

The review of industry experience and selected incidents from Petroleum Safety Authority (Norway) Incidents database and investigation reports has identified that galvanic corrosion between the fastener and the tertiary structures and overload/fatigue due to wind are the most common threats leading to, structure associated, dropped objects. Another identified challenge is that components can be joined/fastened to structural items, and blind bolts have been used in RHS profiles leading to water ingress and internal corrosion of the structural steel member. This can on a long term threaten the structural integrity.