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McDermott

McDermott Australia: Ichthys Umbilicals, Risers and Flowlines, Design of Mid Depth Buoys & Gravity Bases

ICON Engineering was engaged by McDermott Australia Pty Ltd to design two (2) mid depth buoys (MDB) and gravity bases for operator, of the Ichthys Development, INPEX, in the Timor Sea in 250 m water depth.

The design had to cater for a 40 year operating life, the ability to survive 1:10,000 year cyclone events, as well as meet more traditional installation and operating criteria.

Mid Depth Buoy Design

The stringent INPEX functional specification, dictated that each mid depth buoy support five flexible risers / umbilicals. Furthermore, the minimum required 285 Tonnes net buoyancy was to be developed from syntactic foam. The buoy structure and arch system was prescribed to be largely composed of open steel sections, in order to minimise reliance on a significant volume of “closed pressure vessel buoyancy”.

Key buoy dimensions

  • Length                    24 m over all
  • Width                     17 m over gutters
  • Height                    10 m
  • Dry Weight              480 Tonnes
  • Buoyancy                285 Tonnes

 
The functional specification also dictated that each MDB system, be fitted with fully redundant secondary tethers of high performance polyester rope. The design was further enhanced with structural appurtenances to allow the fitting of replacement tethers while in the operating condition, using fully diverless techniques.

The arches and riser support systems were configured to allow flex installation by either the “lay to” or “lower over” techniques. A multi-function transportation cradle for each buoy was integrated into the design. These cradles supported the buoys in the inverted position, to allow safe and efficient loading of syntactic foam blocks, then supported the buoys and pre-installed tether components, in the upright position during loadout, sea transport, and also during rig up on the offshore installation vessel.

Gravity Base Design

The design of the gravity bases was based on the worst case installation load, extreme event survival condition load and the failed tether / replacement tether condition. Padeyes were provided for each of the primary, secondary, and replacement tethers as well as for loadout and end of life retrieval. The gravity bases also include perimeter and internal skirts and a fully plated mudmat as determined by geotechnical design.

Key gravity base dimensions

  • Length                   28 m over all
  • Width                     15 m
  • Height                    1.3 m
  • Dry Weight             680 Tonnes
  • Submerged weight   460 Tonnes [plus 0.6 m skirts]
     

Please read more:

ICON Principal Engineer and co-founder Garrick Aberle's OTC paper on constructing the MDBs: ICON designs world first subsea buoys

Authors
G. E. Aberle (ICON Engineering Pty. Ltd.)
L. Christensen (McDermott International, Inc.)

DOI
https://doi.org/10.4043/26849-MS

Document ID
OTC-26849-MS

Publisher
Offshore Technology Conference

Source
Offshore Technology Conference Asia, 22-25 March, Kuala Lumpur, Malaysia

Publication Date
2016

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