6 Lessons Learnt During Suction Anchor

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<ul><li><p>Lessons learnt during suction anchor and templateinstallations from a geotechnical viewpoint</p><p>Eelco van RaaijMetocean, Geotechnics and Earthquake department</p></li><li><p>2Contents Seabed topography and seabed survey accuracy Landing velocity Levelling procedure Erosion and wash-out Instrumentation Levelling equipment Wet storage location Removal</p></li><li><p>3StatoilHydro Technical Requirement (TR) TR1229 Subsea Production System - Technical and Professional </p><p>Requirements</p><p> TR1230 Subsea Structures and Manifolds - Technical and Professional Requirements</p><p> 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</p></li><li><p>4Seabed topography and seabed survey accuracy TR1230 It shall be possible to level the template within 0.3 degrees. For other </p><p>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.</p><p> Review seabed topography on location and back-up location</p><p> Important to understand survey (in-)accuracy</p><p> For a HPS structure being 29 m wide, 1 degree gives 50 cm height difference!</p></li><li><p>5HOST installation with seabed inclination 4 attempts to get inside the 2 degree </p><p>installation tolerance</p><p> but jacks allow for large tolerance... Important to take survey (in-)accuracy </p><p>into account</p></li><li><p>6Landing velocity TR1229 Subsea Production System - Technical and Professional </p><p>Requirements</p><p> P</p><p>Shallow soil failure</p></li><li><p>7Example 1; result of too high landing velocity DVD movie</p></li><li><p>8Difficult suction penetration due to:</p><p> High landing velocity resulted in wash-out</p><p> Initial seal destroyed Soil having more gravel and </p><p>boulders than anticipated</p><p> Suction pump operation problems Light structure gave small self </p><p>penetration</p><p>Result additional rock dumping</p><p>Example 2; Difficult suction penetration</p></li><li><p>9Levelling procedure, what not to do... Seabed 1.4 deg</p><p>S4 S3 Self penetration gave 5.4 deg Suction on S4 Resulted in crack outside S3 Should have been combination of </p><p>pumping up S3 and suction S4</p><p> Important with levelling procedure and geotechnical competence</p></li><li><p>10</p><p>Crack due to wrong levelling procedure</p></li><li><p>11</p><p>Optimal levelling procedure Self penetration</p><p> No full self penetration (making levelling difficult)</p><p> But sufficient self penetration for sealing</p><p> Close hatches </p><p> Level within criteria After self penetration but before full penetration</p><p> Levelling by small and monitored steps</p><p> Careful not overstressing structure</p><p> Full penetration Patiently to full penetration</p><p> Avoid grouting</p></li><li><p>12</p><p>Erosion and wash out TR1230 For template structures </p><p>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.</p><p> Batch drilling is not acceptable even for ITS</p></li><li><p>13</p><p>17. May... Rig calls and reports a 10 m diameter crater...</p><p>Important understand dependency between design, fabrication, installation, drilling, operation...</p></li><li><p>14</p><p>Instrumentation in TR1230 Facilities to monitor inclination and offsets on the structure shall be available </p><p>on the structure or the ROV panel with sufficient resolution/accuracy to comply with installation tolerances</p><p> 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</p><p> Critical horizontal planes shall be identified for each structure Fixed high precision water pressure gauges (e.g. Digiquartz) or inclinometers</p><p>shall be the primary tool for determining structure inclination</p><p> Necessary interface arrangements for central monitoring (e.g. ROV panel) of installed or planned instrumentation shall be made</p><p> The depth gauges and locations shall be calibrated/surveyed and documented in a measurement dimensional control survey report</p></li><li><p>15</p><p>Instrumentation failure is a common problemExample of a recent template installation </p><p>with major instrumentations problems:</p><p> 1 gyro / inclinometer package fell of 1 gyro / inclinometer package was filled </p><p>with water</p><p> Subsea display did not work Black-out of the positioning system when </p><p>the template was at 100m water depth</p><p> Template was installed using 3rd back-up, i.e. orientation by transponders and levelling with digiquartz</p><p> ROV ripped off digiquartz cable due to bad ROV access</p><p> ROV suction hose imploded twice...</p></li><li><p>16</p><p>Instrumentation, some issues to consider Selection of required instrumentation</p><p> Dimensional Control Survey and Yard Metrology Structure Survey </p><p> Understand readings</p><p> Interpretation of readings</p><p> Continuous on-line readings</p><p> Signal transfer</p></li><li><p>17</p><p>Levelling equipment TR1230 For the suction skirt option one ROV controlled levelling panel shall </p><p>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... </p><p> Hatches Levelling panel ROV interface / hot stab Hoses / piping Pumps</p></li><li><p>18</p><p>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: </p><p> Circuit 1: High pressure, low flow rate Circuit 2: Low pressure, high flow rate</p><p> 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.</p><p> 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 </p><p>litre/min. </p><p> 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.</p><p> 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.</p></li><li><p>19</p><p>Wet storage location Proposed wet storing location with </p><p>natural seabed slope of 6.3 degrees</p><p> Wet storing locations need the same level of documentation as a final location, e.g. sufficient soil investigation and full design</p><p> Placing a gravel berm (for easy removal) difficult with this topography</p></li><li><p>20</p><p>Removal TR1230 For suction skirt foundations, the skirts shall, with the exception of the </p><p>skirt marking and the upper 50cm of the skirt wall, be unpainted in order to achieve maximum friction between the skirts and the soil</p></li><li><p>21</p><p>Removal 10 cm clay sticking to a can with diameter of 5 m and height = 5 m gives 16 m3</p><p>soil and 100 tonnes extra weight !!</p><p> Does the structural design and procedures include this additional soil load during retrieval?</p><p>Concerns:</p><p> Eccentric loads</p><p> Clay falls off uncontrolled during removal resulting in eccentric and dynamic loads</p></li><li><p>22</p><p>The devil is in the details...</p><p>Hatch fell off during towing</p></li><li><p>23</p><p>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</p><p> Involve geotechnical competence</p><p>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</p></li></ul>