AHTS Non-welded Seafastening Grillage

Subsea Energy Australia – Technology & Innovation Award Winner – patent pending

The Scope

Offshore operators regularly transport heavy equipment to and from offshore locations/facilities for maintenance and construction activities.  To securely “seafasten” the heavy equipment to the deck of the supply (transport) vessel a bespoke steel grillage is usually welded to the deck then cut off and scrapped after a single use.  Going through this process for every voyage is expensive and wasteful because it generates sub-contract management, interface management, bespoke design engineering, site welding / hot-work, waste and safety issues.  Woodside recognised this inefficiency and led a competitive tender for the concept, design & manufacture of a re-usable seafastening grillage which could fit a range of AHTS vessels and did not need hot-work for installation i.e. installed by their own technicians on site.  Cube’s solution was to provide a generic design that could be installed on several vessel types and used mechanical seafastening to replace welded seafastening. Cube’s design was considered by Woodside to be the best in class and were engaged to design and supply the seafastening system.

The Outcome

Cube worked collaboratively with Woodside, ETeknology and DNV GL to finalise the concept, functionality and basis of design before completing detailed design, fabrication and testing.   The end result was a reusable, rapid deployment, non-welded seafastening grillage capable of supporting a range of subsea equipment (XT, EDP/LRP, THD, etc) on a range of AHTS vessels.  Woodside personnel and marine crew were able to mobilise & install the grillage system onto the deck of their AHTS vessel in 1 hour without the need for hot-work or dedicated construction crew.  The subsea equipment is then lifted onto the grillage and locked into position using mechanical fasteners. No lashings are required which reduces working at height safety concerns and large (removable) bumpers incorporated for hands free back-loading offshore.  Each seafastening grillage is designed to support up to 2 x subsea equipment items and multiple grillages can be used simultaneouusly.  The grillages developed for Woodside have the following attributes:

  • In-air weight of grillage c/w 2 x interface frames = 13.5t
  • Payload = 102t
  • Grillage mobilisation time onto AHTS vessel = 1hr
  • Designed for short transit between shore base and field (Hs=6m) resulting in grillage steel weight efficiencies compared to typical offshore transport criteria
  • Designed for greenwater on deck
  • Rated for offshore lift
  • Stackable for efficient storage & road transport
  • Colour coded interface frames to match equipment items
  • Mechanical securing devices at base level – no overhead lashings
  • Design approved by DNV

Exploded view of non-welded seafastening system

Road transport of grillage c/w 2 x interface skids

Grillage c/w 2 x interface skids being lifted onto AHTS deck & over the dedicated deck posts


Seafastening grillage installed.  Subsea equipment being lifted onto grillage.

Equipment landed & locked into seafastening grillage – ready for transit.