Lessons learnt during suction anchor and templateinstallations from a geotechnical viewpoint

Eelco van RaaijMetocean, Geotechnics and Earthquake department

2Contents Seabed topography and seabed survey accuracy Landing velocity Levelling procedure Erosion and wash-out Instrumentation Levelling equipment Wet storage location Removal

3StatoilHydro Technical Requirement (TR) TR1229 Subsea Production System - Technical and Professional

Requirements

TR1230 Subsea Structures and Manifolds - Technical and Professional Requirements

The objective of this document is to address StatoilHydro specific requirements and recommendations for subsea structures and manifolds; all within the frameworks set forth by recognised and accepted industry specifications and standards

4Seabed topography and seabed survey accuracy TR1230 It shall be possible to level the template within 0.3 degrees. For other

structures such as cluster manifolds and PLEMs the final inclination shall be less than 1.0 degrees. The foundation shall be designed for a seabed slope of minimum 3 degrees or as specified for each project.

Review seabed topography on location and back-up location

Important to understand survey (in-)accuracy

For a HPS structure being 29 m wide, 1 degree gives 50 cm height difference!

5HOST installation with seabed inclination 4 attempts to get inside the 2 degree

installation tolerance

but jacks allow for large tolerance... Important to take survey (in-)accuracy

into account

6Landing velocity TR1229 Subsea Production System - Technical and Professional

Requirements

P

Shallow soil failure

7Example 1; result of too high landing velocity DVD movie

8Difficult suction penetration due to:

High landing velocity resulted in wash-out

Initial seal destroyed Soil having more gravel and

boulders than anticipated

Suction pump operation problems Light structure gave small self

penetration

Result additional rock dumping

Example 2; Difficult suction penetration

9Levelling procedure, what not to do... Seabed 1.4 deg

S4 S3 Self penetration gave 5.4 deg Suction on S4 Resulted in crack outside S3 Should have been combination of

pumping up S3 and suction S4

Important with levelling procedure and geotechnical competence

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Crack due to wrong levelling procedure

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Optimal levelling procedure Self penetration

No full self penetration (making levelling difficult)

But sufficient self penetration for sealing

Close hatches

Level within criteria After self penetration but before full penetration

Levelling by small and monitored steps

Careful not overstressing structure

Full penetration Patiently to full penetration

Avoid grouting

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Erosion and wash out TR1230 For template structures

erosion/wash out due to drilling shall be accounted for. If the distance between skirt foundation and the well is short and soil conditions are sensible to erosion/wash out, 25% of the circumference of one skirt foundation shall be considered eroded when drilling through the same conductor.

Batch drilling is not acceptable even for ITS

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17. May... Rig calls and reports a 10 m diameter crater...

Important understand dependency between design, fabrication, installation, drilling, operation...

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Instrumentation in TR1230 Facilities to monitor inclination and offsets on the structure shall be available

on the structure or the ROV panel with sufficient resolution/accuracy to comply with installation tolerances

Alternative primary and secondary (back up) instrumentation and methods shall be evaluated, designed and planned for based on structural deflection measurements and installation water depth, to achieve above mentioned installation criteria

Critical horizontal planes shall be identified for each structure Fixed high precision water pressure gauges (e.g. Digiquartz) or inclinometers

shall be the primary tool for determining structure inclination

Necessary interface arrangements for central monitoring (e.g. ROV panel) of installed or planned instrumentation shall be made

The depth gauges and locations shall be calibrated/surveyed and documented in a measurement dimensional control survey report

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Instrumentation failure is a common problemExample of a recent template installation

with major instrumentations problems:

1 gyro / inclinometer package fell of 1 gyro / inclinometer package was filled

with water

Subsea display did not work Black-out of the positioning system when

the template was at 100m water depth

Template was installed using 3rd back-up, i.e. orientation by transponders and levelling with digiquartz

ROV ripped off digiquartz cable due to bad ROV access

ROV suction hose imploded twice...

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Instrumentation, some issues to consider Selection of required instrumentation

Dimensional Control Survey and Yard Metrology Structure Survey

Understand readings

Interpretation of readings

Continuous on-line readings

Signal transfer

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Levelling equipment TR1230 For the suction skirt option one ROV controlled levelling panel shall

be installed on the template or the separate protection structure and each skirt shall be able to operate individually. The panel shall be connected by piping to each skirt compartment and...

Hatches Levelling panel ROV interface / hot stab Hoses / piping Pumps

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Requirements for suction pump The suction pump shall be tested onshore at suppliers yard prior to delivery to Contractor One HPU skid with two independent pump circuits with the following specifications:

Circuit 1: High pressure, low flow rate Circuit 2: Low pressure, high flow rate

Maximum suction capacity of the high pressure pump 250 kPa (2,5bar), i.e. the pump should be able to provide 250-kPa relative over-pressure or under-pressure (suction) in the skirt compartments.

The suction/pressure shall be able to be increased linearly. There shall be a visible and clear indicator for when pressure is applied and when suction is applied. Reasonable levelling velocities should be aimed at, thus the pumping capacity should be circa 2000

litre/min.

A function test shall be performed during mobilisation to document that the WROV is able to supply required hydraulic pressure and flow to the pump. The test shall be performed simultaneously with manipulator operation and normal WROV thrust. The performance of the function test shall be documented through a flow-meter and manometer.

The ROV tool design shall assume a hydraulic supply from ROV of variable pressure 0-207 bar and a maximum flow of 40 l/min.

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Wet storage location Proposed wet storing location with

natural seabed slope of 6.3 degrees

Wet storing locations need the same level of documentation as a final location, e.g. sufficient soil investigation and full design

Placing a gravel berm (for easy removal) difficult with this topography

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Removal TR1230 For suction skirt foundations, the skirts shall, with the exception of the

skirt marking and the upper 50cm of the skirt wall, be unpainted in order to achieve maximum friction between the skirts and the soil

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Removal 10 cm clay sticking to a can with diameter of 5 m and height = 5 m gives 16 m3

soil and 100 tonnes extra weight !!

Does the structural design and procedures include this additional soil load during retrieval?

Concerns:

Eccentric loads

Clay falls off uncontrolled during removal resulting in eccentric and dynamic loads

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The devil is in the details...

Hatch fell off during towing

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Summary Understand seabed topography and accuracy Control landing velocity Importance of levelling procedure Erosion and wash out problem Good instrumentation Focus on levelling equipment, including pumps Document wet storage location Design for removal

Involve geotechnical competence

Lessons learnt during suction anchor and templateinstallations from a geotechnical viewpointContentsStatoilHydro Technical Requirement (TR)Seabed topography and seabed survey accuracyHOST installation with seabed inclinationLanding velocityExample 1; result of too high landing velocityExample 2; Difficult suction penetrationLevelling procedure, what not to do...Crack due to wrong levelling procedureOptimal levelling procedureErosion and wash out17. May...Instrumentation in TR1230Instrumentation failure is a common problemInstrumentation, some issues to considerLevelling equipmentRequirements for suction pumpWet storage locationRemovalRemovalThe devil is in the details...Summary