Decommissioning of WSH-3 well and removal of Wardie-1 Conductor.
In accordance with Section 5 of the OPGGS(E), this EP applies to a defined ‘petroleum activity.’ Based on the WOMP and for the purposes of this EP, the petroleum activity is defined as:
‘Non-production operations phase Decommissioning of TA (suspended) wellhead on the seabed’.
Given the Wardie-1 well was fully plugged and abandoned, and the conductor has been cut (at least in part) there are no petroleum activities associated with the well other than retrieval of the conductor pipe in conjunction with future activities on WSH-3. The WSH-3 activity area during decommissioning operations is defined as:
All areas within the WSH-3 Petroleum Safety Zone (PSZ), which covers a 300 m-radius area around the WSH-3 wellhead.
This 300-m radius around the well equates to an area of 28.3 ha (0.283 km2). The Wardie-1 conductor lies within the WSH-3 Petroleum Safety Zone. As they are both abandoned there are no credible hydrocarbon spill scenarios from the wells and the extent of the PSZ can be reasonably used to define the activity area during decommissioning operations.
During the proposed Vessel-based decommissioning activities there is a very low chance of a minor hydrocarbon (Marine Diesel Oil) spill in the event of collision with another vessel. Although a spill would be restricted to less than 150m3, previous Oil Spill Trajectory Modelling (OSTM) was undertaken for a 200m3 MDO vessel spill incident at the West Seahorse location during operations. This modelling has been utilised for this activity as the increased volume would have a greater impact than any potential ‘actual’ spill. The modelling further represents a conservative approach given that mitigating strategies (such as tank lightening) would further limit the spill volume.
A spill of this volume is defined as a Level 2 Spill under the National Plan for Maritime Environmental Emergencies (NATPLAN). Modelling (as shown in Section 5) shows localised impact only, with some potential shore contact. The Environment Impact from this Level 2 oil spill is minor isolated local impacts with natural recovery expected within days. Management of such a spill is to be conducted under the VIC/RL17 Oil Pollution Emergency Plan (OPEP) for West Seahorse Decommissioning, and the vessel SOPEP.
