Case history: Soil investigation for offshore suction anchors

Over last 5 -6 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

Suction anchor foundations Suction anchor foundations for tension and taut leg floaters in deep waters

Driven Plate anchors Driven Plate anchors

Fluke anchor Fluke anchor

Driven pile Driven pile

Suction pile Suction pile

Alternative anchors for floating structures

Basic principle

Suction pile

Suction pile

Step by step Seabed installation normal duration 2-3 hrs

Seabed installation of suction anchors

Suction anchors in deep waters

•Main foundation problems

–penetration of skirts ( must get to required de pt h)

–bearing capacity for pull out forces ( including cyclic loading in some cases)

• Soil param eters requi red

–shear strength for penetration analyses –shear strength for bearing capacity

Depth for which soilinformation is required : suction pile length + 1 diameter

Floating structure with suction anchors

Project in form ation: •Water depth : 340 m

•Four clusters, each with 4 anchors

•Overall anchor pattern diameter : 2,300 m

•Expected suction anchor dimensions –diameter : 5 m ; length : 1 -13 m

•Main geotechnical problems

–penetration of skirts to required depth below sea bed

–bearing capacity in terms of pull out resistance

Anchor pattern for North Sea Floating Production Unit

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

Operation with umbilical, hydraulic cylinder pushes tool below bottom of borehole. Data aquisition throughcable and real time display of test results. Depth limitation about 700 m.

Investigation for floating structure - -example down-hole CPTU results

Selection of Nkt factors for CPTU in terpr eta tion

Investigation for suction anchorsresults form combined borehole with CPTU and sampling

Recommended design shear strengths for suction anchors

Deep-water sites around the world Deep-water sites around the world

Exxon Diana -Suction Pile anchors Exxon Diana -Suction Pile anchors

Exxon Diana -Suction Pile anchors Exxon Diana -Suction Pile anchors

•12 anchors, each with 1 500 ton holding capacity -Highest ! •220 ton weight, 30.5 m penetration and 6.4 m diameter -Largest !

•1500 m water depth -Deepest !

•First permanent suction pile mooring in the Gulf of Mexico

Exxon Diana -Geotechnical design Exxon Diana -Geotechnical design

1500 ton

Diana Spar floating storage and production platform

Diana Spar floating storage and production platform

Exxon Diana Pl atf or m

Exxon Diana Pl atf or m

Downtown Houston Downtown Houston

Exxon Diana Mooring system

Exxon Diana Mooring system

Downtown Houston Downtown Houston

Exxon Diana -Suction Pile installation Exxon Diana -Suction Pile installation

Suction PilePump skid

Exxon Diana -Suction Pile installation Exxon Diana -Suction Pile installation

Exxon Diana -Suction Pile installation Exxon Diana -Suction Pile installation

Weight penetration with free water evacuation

Exxon Diana -Suction Pile installation Exxon Diana -Suction Pile installation

Positioning and landing of Suction Pile

Exxon Diana -Suction Pile installation Exxon Diana -Suction Pile installation

Slack lifting wire and close valves

Exxon Diana -Suction Pile installation Exxon Diana -Suction Pile installation

Weight penetration with free water evacuation

Exxon Diana -Suction Pile installation Exxon Diana -Suction Pile installation

Pump out entrapped water and penetrate by suction

Su cti on d riveab ility

Maximum suction penetration depth Maximum suction penetration depth

Petrobras P-18 Riser supports L/D = 8.5

Suction penetration response Suction penetration response

Soft homogeneous clay typical for deep-water sites

Layered glacial clay Haltenbanken Norway

Pre-in st alle d co nductors

Pre-in st alle d co nductors

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