First Subsea’s ball and taper technology is ideal for handling tubular sections commonly used in the oil and gas industry, more recently however the technology has been developed to resist torque loads hence the opportunities for use of these products are now even more versatile.

The figure to the left shows a typical Diver-less Bend Stiffener System. With this type of connector there is quite often a requirement to resist torque, therefore, IDAC were tasked to carry out research into the degree of indentation with regard to differing pull loads. They were also required to determine the torque that the tool would withstand before the indented balls started creating a channel of yielded material.

IDAC carried out the evaluation and research of the indentation and torque transmission process for First Subsea Tools in two phases:

1. Ball indentation
2. Torque transmission

The model was made up of two annular rings one inside the other. Due to the presence of cyclic symmetry only one sector (15 deg) was modelled. The model consisted of three parts the Pocket, Ball and Pipe with frictional contact defined being between them. Non-linear elasto-plastic material was used for Pipe and Pocket parts; and the ball was treated as perfectly rigid.

The analysis was carried out in two steps. The first was to pre-load the tool and indent the pipe, the indent varied depending on the pre-load. The Pocket component was fully fixed on the outside diameter with the pipe component being pulled to cause the ball to roll and create an indent on the pipe. This step was carried out for eight different indentation depths.

After pre-loading and achieving the required indentation, the torque was applied to the tool. This was applied as a rotation of 1.2 degrees.

The graph right shows the Torque vs. Rotation behaviour for different indentation depths. A logarithmic behaviour was observed, the higher the indentation the higher the maximum torque that could be applied.

The graph below right shows Max Torque vs. Initial Indentation. A best-fit curve was fitted to the eight points had been calculated and a 3rd order polynomial below was found to give very good correlation to the calculated points:

It was also found that the pulling force needed to produce the initial indentation, did not exhibit linear behaviour and it too followed a 3rd order polynomial curve. With indentations greater than 1.2mm pulling forces tend to keep constant.

Design Benefit

IDAC has carried out several analyses for First Subsea on various tools that have been designed by them. The analyses provide the design engineers at First Subsea with confidence and insight into the performance of new designs. This research has enabled First Subsea to be able to more precisely specify loading capability of ‘special’ tools that are require to perform outside normal operating conditions, making First Subsea more responsive to its customers needs, maintaining its place as a world leader in ball and taper lifting technology.