# DNV - Subsea Structure

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DNV Marine Operations Rules for Subsea Lift Operations Simplified Methods for Prediction of Hydrodynamic Forces Tormod Be DNV Marine Operations 29th November 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 2 29. November 2011 Content Brief overview of relevant DNV publications DNV Rules for Marine Operations, 1996, Pt.2 Ch.5 Lifting Capacity Checks Simplified Methods for prediction of Hydrodynamic Forces o in Splash Zone, DNV-RP-H103 Ch.4 o in Deepwater, DNV-RP-H103 Ch.5 DNV Marine Operations' Rules for Subsea Lift Operations Slide 3 29. November 2011 Relevant DNV Publications Lifting- and subsea operations : Specially planned non-routine operations Routine operations DNV Standard for Certification No.2.22 Lifting Appliances October 2011 DNV Rules for Planning and Execution of Marine Operations 1996 and DNV-OS-H101 Marine Operations, General - 2011 Specially planned, non-routine operations of limited durations, at sea. Marine operations are normally related to temporary phases as e.g. load transfer, transportation and installation. DNV-OS-E402 Offshore Standard for Diving Systems October 2010 DNV Standard for Certification No. 2.7-3 Portable Offshore Units May 2011 DNV Marine Operations' Rules for Subsea Lift Operations Slide 4 29. November 2011 DNV-RP-C205 Environmental Conditions and Environmental Loads October 2010 DNV-RP-H101 Risk Management in Marine and Subsea Operations, January 2003 DNV-RP-H102 Marine Operations during Removal of Offshore Installations, April 2004 DNV-RP-H103 Modelling and Analysis of Marine Operations, April 2011 Relevant DNV Publications - Other DNV Marine Operations' Rules for Subsea Lift Operations Slide 5 29. November 2011 Relevant DNV Publications - WebSite Most DNV publications can be downloaded for free at: http://www.dnv.com The 1996 DNV Rules for Marine Operations is not in the DNV intranet site. DNV Marine Operations' Rules for Subsea Lift Operations Slide 6 29. November 2011 Content Brief overview of relevant DNV publications DNV Rules for Marine Operations, 1996, Pt.2 Ch.5 Lifting Capacity Checks Simplified Methods for prediction of Hydrodynamic Forces o in Splash Zone, DNV-RP-H103 Ch.4 o in Deepwater, DNV-RP-H103 Ch.5 DNV Marine Operations' Rules for Subsea Lift Operations Slide 7 29. November 2011 Capacity Checks - DNV 1996 Rules Dynamic loads, lift in air Crane capacity Rigging capacity, (slings, shackles, etc.) Structural steel capacity (lifted object, lifting points, spreader bars, etc.) Part 2 Chapter 5 Dynamic loads for subsea lifts are estimated according to DNV-RP-H103 DNV Marine Operations' Rules for Subsea Lift Operations Slide 8 29. November 2011 Capacity Checks DAF for Lift in Air Dynamic loads are accounted for by using a Dynamic Amplification Factor (DAF). DAF in air may be caused by e.g. variation in hoisting speeds or motions of crane vessel and lifted object. The given table is applicable for offshore lift in air in minor sea states, typically Hs < 2-2.5m. DAF must be estimated separately for lifts in air at higher seastates and for subsea lifts ! Table 2.1 Pt.2 Ch.5 Sec.2.2.4.4 DNV Marine Operations' Rules for Subsea Lift Operations Slide 9 29. November 2011 The dynamic hook load, DHL, is given by: DHL =DAF*(W+Wrig) +F(SPL) ref. Pt.2 Ch.5 Sec.2.4.2.1 Capacity Checks - Crane Capacity W is the weight of the structure, including a weight inaccuracy factor The DHL should be checked against available crane capacity The crane capacity decrease when the lifting radius increase. DNV Marine Operations' Rules for Subsea Lift Operations Slide 10 29. November 2011 The maximum dynamic sling load, Fsling, can be calculated by: Fsling = DHLSKLkCoGDW / sin ref. Pt.2 Ch.5 Sec.2.4.2.3-6 where: Capacity Checks - Sling Loads SKL =Skew load factor extra loading caused by equipment and fabrication tolerances. kCoG =CoG factor inaccuracies in estimated position of centre of gravity. DW =vertical weight distribution e.g. DWA = (8/15)(7/13) in sling A. =sling angle from the horizontal plane. Example : DNV Marine Operations' Rules for Subsea Lift Operations Slide 11 29. November 2011 Safe working load, SWL, and MBL, of the shackle are checked by : a) Fsling < SWL DAF and b) Fsling < MBL / 3.3 Both criteria shall be fulfilled (Pt.2 Ch.5 Sec.3.2.1.2) The sling capacity Minimum breaking load, MBL, is checked by: The safety factor is minimum sf 3.0. (Pt.2 Ch.5 Sec.3.1.2) Capacity Checks - Slings and Shackles sfslingslingMBLF 6 . 10A lower limit of Hmax=1.8Hs=/10 with wavelength =gTz2/2 is here used. There are two alternative approaches: Alt-1) Wave periods are included: Analyses should cover the following zero-crossing wave period range: Alt-2) Wave periods are disregarded: Operation procedures should in this case reflect that the calculations are only valid for waves longer than: DNV Marine Operations' Rules for Subsea Lift Operations Slide 20 29. November 2011 Alt-1) Wave periods are included: The wave amplitude, wave particle velocity and acceleration can be taken as: Simplified Method, Splash Zone - Wave Kinematics S a H = 9 . 0 ,g Tza w zdeTv2242 tt, ||.|

\| =g Tza w zdeTa22422 tt, ||.|

\| =s H d 35 . 0v esH g 30 . 0w = tsH d 35 . 0a e g 10 . 0w = tAlt-2) Wave periods are disregarded: The wave particle velocity and acceleration can be taken as: d : distance from water plane to CoG of submerged part of object DNV Marine Operations' Rules for Subsea Lift Operations Slide 21 29. November 2011 Slamming impact force Slamming forces are short-term impulse forces that acts when the structure hits the water surface. AS is the relevant slamming area on the exposed structure part. Cs is slamming coeff. The slamming velocity, vs, is : Simplified Method, Splash Zone - Hydrodynamic Forces 2 2w ct c s v v v v + + =g V F = o Varying buoyancy force Varying buoyancy, F , is the change in buoyancy due to the water surface elevation. V is the change in volume of displaced water from still water surface to wave crest or wave trough. vc = lowering speed vct = vertical crane tip velocity vw = vertical water particle velocity at water surface 2 2~ct a wA V q , o + =g V F = o a = wave amplitude ct = crane tip motion amplitude w = mean water line area in the wave surface zone DNV Marine Operations' Rules for Subsea Lift Operations Slide 22 29. November 2011 Mass force Mass force is here a combination of inertia force, Froude-Kriloff force and diffraction force. Crane tip acceleration and water particle acceleration are assumed statistically independent. Drag force Drag forces are flow resistance on submerged part of the structure. The drag forces are related to relative velocity between object and water particles. The drag coefficient, CD, in oscillatory flow for complex subsea structures may typically be CD 2.5. Relative velocity are found by : Simplified Method, Splash Zone - Hydrodynamic Forces 2 2w ct c r v v v v + + = vc = lowering/hoisting speed vct = vertical crane tip velocity vw = vertical water particle velocity at water depth , d Ap = horizontal projected area M = mass of object in air A33 = heave added mass of object act = vertical crane tip acceleration V = volume of displaced water relative to the still water level aw = vertical water particle acceleration at water depth, d ( ) | | ( ) | |233233 w ctM a A V a A MF + + += DNV Marine Operations' Rules for Subsea Lift Operations Slide 23 29. November 2011 Simplified Method, Splash Zone - Basics Forces: Weight [N] Buoyancy [N] Weight = M*gmoon Weight = M*g Buoyancy = *V*g Properties: Mass, M [kg] Volume, V [m3] Added mass, A33 [kg] DNV Marine Operations' Rules for Subsea Lift Operations Slide 24 29. November 2011 Simplified Method, Splash Zone - Added Mass Hydrodynamic added mass for flat plates b a476 . 0 A 233 = tExample: Flat plate where length, b, above breadth, a, is b/a = 2.0 : DNV Marine Operations' Rules for Subsea Lift Operations Slide 25 29. November 2011 Simplified Method, Splash Zone - Added Mass Added Mass Increase due to Body Height The following simplified approximation of the added mass in heave for a three-dimensional body with vertical sides may be applied : o 332233 A) 1 ( 211 A ((((

++ ~ppA hA+= where A33o = added mass for a flat plate with a shape equal to the horizontal projected area of the object h = height of the object Ap = horizontal projected area of the object and Added Mass Increase due to Body Height11.11.21.31.41.51.61.71.80 0.5 1 1.5 2 2.5ln [ 1+ (h/sqrt(A)) ]A33/A33o1+SQRT((1-lambda^2)/(2*(1+lambda^2)))DNV Marine Operations' Rules for Subsea Lift Operations Slide 26 29. November 2011 Added Mass from Partly Enclosed Volume Simplified Method, Splash Zone - Added Mass A volume of water partly enlosed within large plated surfaces will also contribute to the added mass, e.g.: The volume of water inside suction anchors or foundation buckets. The volume of water between large plated mudmat surfaces and roof structures. DNV Marine Operations' Rules for Subsea Lift Operations Slide 27 29. November 2011 Added Mass Reduction due to Perforation Simplified Method, Splash Zone - Added Mass No reduction applied in added mass when perforation is small. A significant drop in the added mass for larger perforation rates. Reduction factor applicable for p

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