Various methods for installing the Otway Basin Thylacine wellhead platform in 100m of harsh environment water (100 year Hmax = 24m, with Tp = 16 sec) were reviewed and screened. A gravity base structure option was also considered. A base case installation method was adopted.  This involved a rig installed single piece jacket that was to be fabricated, loaded out and transported horizontally to the field.  The jacket was buoyant, and was to be offloaded at a sheltered location into the ocean, near the installation site.  It was to be towed to the rig, and upended and placed with the assistance of buoyancy tanks.

The piles were designed to be drilled and grouted, due to the geotechnical conditions.  Drilling was to be completed using reverse circulation. Pile segments were designed to be made up and installed though the derrick and piles run through the jacket legs. The topsides was designed to be installed by the reach over method. The jacket installation details were as illustrated below.

The most significant cost reduction with the rig-installed method was the elimination of the heavy lift crane barge from the project. The number of lifts was kept to a minimum and the drilled and grouted pile engineering was studied in detail.

The risk weighted cost ranking for the installation options was found to be as follows (From best to worst):

• Option A – Single piece jacket; Wet tow & floating upend
• Option B – Two piece jacket; Wet tow & floating upend of base; Dry lift upper jacket
• Option C – Two piece jacket; Dry lift & buoyant upend of base; Dry lift upper jacket
• Option D -  Single piece jacket; Dry lift & buoyant upend

Option A jack-up rig install was adopted as the base case for the project. Only one marine lift for the jacket was on rig critical path time was required.  The lift hook load was well within the capability of all the rigs under consideration, allowing for the widest possible rig choice.

The topsides were designed to suit the reach over method.

The EPIC phase of the project was awarded to Tier 1 contractor and ICON worked for Woodside during the installation. ICON had proposed in the FEED that the jacket be designed for float off by a transport ship. The contractor elected to instead launch the jacket from a cargo barge, which resulted in significant weight changes. The need for launch rails on the jacket resulted in a redesign of the jacket and substantial increase in the weight of the jacket and buoyancy required.

The Thylacine Platform was the deepest water depth jacket completed using a jack up rig to date.

Platform Data

Water depth                               100 m

Jacket Weight                            1,400 t

Topsides Weight                         550 t

Legs/Wells                                 3 legs/ 6 wells   

Lifting the Thylacine wellhead platform topsides into place on top of the jacket, using the drilling rig and eliminating the heavy lift crane barge from the budget and reducing the risk of construction delays due to Otway Basin sea state conditions.