Abstract
This paper describes the installation scope for the Chevron-operated Wheatstone Project, including the following items:
A number of heavy subsea structures comprised of Pipeline Termination Structures, Subsea Isolation Valves (SSIV) and drill centre manifolds.
Fabrication, metrology and installation of a large number of clad tie-in spools of widely varying weight, length, shape and stiffness.
Tie-in spools at the Wheatstone Platform (WP) to connect the WP risers to the SSIVs and flowlines (and associated diver-assisted flanged connections).
A Dynamically Positioned Heavy Lift Vessel (DPHLV) was used for the subsea structures installation campaign. The DPHLV performed 13 heavy structure lifts in a range of water depths. The maximum structure weight was 527 tonnes (Te).
A Heavy Construction Vessel (HCV) was mobilised to perform the deployment of 68 spoolpieces, ranging in size from 4" to 44" internal diameter. A range of spool spreader-beam arrangements was incorporated depending on the spool geometry and weight. The maximum combined spool plus spreader weight was 188Te.
The subsea structures were installed within very tight seabed landout tolerances. Such tolerances were achieved using the vessel's Class 2 Dynamic Positioning capability, as well as the calibration of an acoustic seabed survey array which provided the necessary global subsea positioning reference.
The spools offshore lifting operation, including multi vessel-interactions, was a technically challenging and weather sensitive activity. Safe and efficient methods were developed for the transfer of personnel from HCV to barge, mooring the barges to the HCV, and tagging the spools during lift off barge. The HCV was also used to tie-in the spools at the drill centres using diverless connection systems.
The spools deployed in the vicinity of the Wheatstone Platform were tied-in using a Dive Support Vessel. Pipe handling frames and air bags were operated by the divers to assist flange tie-ins of the stiff and large diameter spools.
The structures were installed in seastates up to 1.8m Significant Wave Height, with the governing case for the installations being seabed landout. A high capacity Passive Heave Compensator (PHC) was incorporated into the rigging arrangement to dampen the load during seabed landout, hence allowing landout within the maximum allowable speed. The PHC also ensured that vessel roll was kept to a minimum by allowing for a controlled unloading of the crane, and that there was no structure re-impact with seabed during de-ballasting operations upon seabed landout.