Case history: Soil investigation for offshore suction anchors anchor foundations for tension and taut leg floaters in deep waters Alternative anchors for floating structures pile Over last 5 - years anchoring of floating and fixed structures have to an increasing extent been done with suction anchors much quicker installation cost effective reliable design procedures require reliable soil parameters for optimal design of suction anchor dimensions risers Tension Leg Platform risers tendons subsea manifold Taut Leg Platform upper chain fibre rope Driven pile Fluke anchor suction anchor suction anchor lower chain Illustration: Saga Petroleum ASA Driven Plate anchors Basic principle pile ground chain pump skid 1 Seabed installation of suction anchors Positioning of anchor monitoring heading, verticality & clearance slacking lifting wire and ground chain pumping out entrapped water monitoring verticality, diff. pressure & penetration Step by step Seabed installation normal duration -3 hrs recover pump skid by umbilical Pre-tension ground chain 3 recover lifting wire anchors in deep waters Main foundation problems penetration of skirts ( must get to required depth) bearing capacity for pull out forces ( including cyclic loading in some cases) Soil parameters required shear strength for penetration analyses shear strength for bearing capacity Depth for which soil information is required : suction pile length + 1 diameter
Floating structure with suction anchors Project information: Water depth : m Four clusters, each with anchors Overall anchor pattern diameter :,0 m Expected suction anchor dimensions diameter : 5 m ; length : 11-13 m Main geotechnical problems penetration of skirts to required depth below seabed bearing capacity in terms of pull out resistance Anchor pattern for North Sea Floating Production Unit 00 m Floating structure with suction anchors Soil investigation program: Nine boreholes to 15 m Intermittent CPTUs and sampling Laboratory tests including CAUC, CAUE and DSS Soil design parameters based on correlations between CPTU and laboratory tests Example borehole locations for one anchor cluster Down-hole insitu testing Investigation for floating structure - - example down-hole CPTU results 500 m Operation with umbilical, hydraulic cylinder pushes tool below bottom of borehole. Data aquisition through cable and real time display of test results. Depth limitation about 700 m. Anchor location Borehole location From Fugro brochure
Selection of N kt factors for CPTU interpretation Investigation for suction anchorsresults form combined borehole with CPTU and sampling Recommended design shear strengths for suction anchors Index shear strength From CPT U using N KT =13.5 CAUC triaxial test Deep-water sites around the world Mid Norway Exxon Diana - Pile anchors Exxon Diana - Pile anchors anchors, each with 1 500 ton holding capacity - Highest! ton weight,.5 m penetration and. m diameter - Largest! 1500 m water depth - Deepest! First permanent suction pile mooring in the Gulf of Mexico Gulf of Mexico Columbus NW Africa Amazon Campos Basin Deep Mediterranean West Africa East India Barong NW Shelf Tarajam Houston 1500 m water depth Diana field Gulf of Mexico m 1 m 1 m 1 m 1 m 1 m 1 m 1 m 1 m 1 m 1 m 1 m 1 New Zealand NGI's main office, Sognsveien 7, Oslo
Exxon Diana - Geotechnical design 1500 ton Depth 0 m Exxon Diana Platform Exxon Diana Mooring system Diana Spar floating storage and production platform 0 m 0 0 0 0 Downtown Houston Downtown Houston READY FOR INSTALLATION Exxon Diana - Pile installation Gulf of Mexico, October to November 1999 Marine Contractor: Saipem, Vessel: S-7000 Exxon Diana - Pile installation 1 1 pile installation contractor: NGI-FRAMO Pile Pump skid m m Seabed at 1500 m depth
Exxon Diana - Pile installation Weight penetration with free water evacuation Exxon Diana - Pile installation Positioning and landing of Pile Exxon Diana - Pile installation Slack lifting wire and close valves 1 1 Deviation from target heading (deg) 0 - - - - Touch Down Target heading 1 1 1 1 - m m - -3 - -1 0 1 3 Elapsed time after Touch Down (min) m m seabed m m Exxon Diana - Pile installation Exxon Diana - Pile installation Pump out entrapped water and penetrate by suction Maximum suction penetration depth Weight penetration with free water evacuation 1 m 1 m Penetration depth (m) 19 1 3 5 7 9 31 0 - -0-0 -0-0 -1-0 Differential pressure (kpa) 1 m 1 m 1 Petrobras P- Riser supports L/D =.5 driveability Weight penetration 1 Allowable suction L/D =.5 L/D = 5 L/D = Required suction Penetration depth (m)
ROV penetration response Pre-installed conductors 19 Cone penetration resistance, Qt (MPa) 0.0 1.0.0 3.0.0 3.0.0 1 Predicted upper bound 5.0 Penetration (m) 3 5 Predicted lower bound Penetration (m).0 7.0.0 CPT B 7 9.0 CPT A 9.0 CPT C Predicted under pressure for penetration 31 0 - -0-0 -0-0 -1-0 Differential pressure inside anchor (kpa) Soft homogeneous clay typical for deep-water sites 0-50 -0-150 -0-50 -0-350 -00-50 Differential pressure inside anchor (kpa) Layered glacial clay Haltenbanken Norway caisson Conductor