Woodside Angel AP4 Umbilical Free Span Assessment

Background

The Woodside Angel project includes a number of umbilicals that facilitate control of the subsea wells from the Angel Platform. The umbilical touchdown locations from the J-tube were stabilised post installation with sandbags. Scour has resulted in the sandbags at the AP4 umbilical touch down sinking into the seabed, leaving a significant scour hole and elongated span at the J-tube exit. Woodside engaged Atteris to asses the integrity risks associated with the loss of the touch-down stabilisation and elongated span.

Work Performed by Atteris

Atteris performed a risk assessment together with Woodside to identify the potential threats to umbilical integrity. Key risks identified were further scour at the umbilical span touch down, umbilical instability leading to over-bending of the umbilical at the J-tube exit, and risk of damage to the J-tube seal. The analysis concluded that for the as-surveyed span configuration the risk of damage to the umbilical at the J-tube exit was acceptable, however scour studies concluded that there was a risk of ongoing scour further exacerbating the risk.

Potential options to stabilise the situation were evaluated and scored, including use of gravel dumping, mattressing and frond mats, with the preferred method being frond mats due to the simplicity and low risk associated with installation.

Benefits

Atteris employed in-house stability analysis software to assess the risk of umbilical over-bending at the J-tube bellmouth and developed cost effective mitigation measures to minimise the risk to umbilical integrity.