Comparison for MSC CJ62 rig of ISO versus SNAME for Clay and Sand site

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Sibyl Jennings
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1 Phase 2 ISO Benchmarking study GustoMSC 21 Oct 2010 Comparison for MSC CJ62 rig of ISO versus SNAME for Clay and Sand site

2 Basic data Jack-up: MSC design CJ62 type drilling rig Some numbers in the analyses: t elevated (survival) weight Triangular shaped hull, spacing 62 m 3 truss legs, leg length m, chord spacing 16 m, X-brace Spudcan (traditional shape), 250 m^2 Pre-load capacity per leg t Fixation systems 9500 BLM rack-pinion jacking system type C170 (54 pinions) Chords tubular with racks

3 Basic data Spudcan / penetration data: Some numbers in the analyses: Pre-load at seabed applied: MN Effective diameter B = m Maximum bottom area 250 m^2 Tip to maximum area = 1.60 m

4 Basic data site conditions: Site description North Sea (dense) sand GoM (deep) clay General Water depth (LAT) Airgap SWL Metocean Hmax Tass Wind Current Soil Dense sand Soft clay

5 Basic data Wave kinematics & wave period: Site description North Sea sand - fixity GoM (deep) clay - fixity Dynamics Intrinsic wave period / DAF 15.8 s / s / 1.16 Apparent wave period / DAF 15.2 s / s / 1.18 Applied in assessment Wave kinematics 1.28 for ISO 1.25 for SNAME Latitude or region North Sea TRS (GoM) Directional spreading factor Kinematics reduction factor Applied in assessment 0.86 on kinematics ISO 0.86 on wave height SNAME 1.16 for ISO & SNAME 0.86 on kinematics

6 Basic data soil conditions sand site: Site description North Sea (dense) sand type - Medium dense (silica) sand Internal friction angle [deg] 34 Effective subm weight [kn/m 3 ] 11 Steel-soil friction angle [deg] 29 = ( 5) Relative density Dr [%] 65 Poisson s ratio [-] 0.2 Shear modulus G D50/D90 particle size 23765* sqrt(vswl/(101.3a) 0.095mm / 0.15mm

7 Basic data Foundation data: Site description North Sea (dense) sand Leg penetration estimate [m] 1.4 Spudcan contact diameter [m] / [%] 14.0 / 80% Foundation fixity Shear modulus G [MPa] 50 (soil data) Applied G in fixity [Mpa] 46 MPa, based on recommendations Rotational stiffness [MNm/rad] Capacity VHM - Standard sand, VL0 = MN

8 Basic data soil conditions clay site: Site description GoM (deep) clay - fixity type - Very soft to stiff calcareous clay Shear strength [kpa] See table Shear modulus [MPa] See table OCR [-] (see table) Poisson s ratio [-] 0.5 Sensitivity ratio [-] 2.7

9 Basic data Foundation data: Site description GoM (deep) clay - fixity Leg penetration estimate [m] 45.6 Gross capacity [MN] 190 (pre-load = 153) Backfill (after pre-loading) [MN] 8 (8 m) Shear modulus for fixity Shear modulus G [MPa] 43 (soil data) => G/su = 730 Applied G in fixity [Mpa] 34 MPa, limited to G/su 600 in line with recommendations Embedment factor [-] 2.2 **), ISO gives 2.4 Rotational stiffness [MNm/rad] MNm/rad, ISO = Yield surface (ISO, gross cap) V=190, H=20, M = 401, a = 0.99 Yield surface (SNAME, net cap) V = 153, H = 19.2, M = 428, a = 0.99 **) Tables in ISO to be added to include effects of poisson s ratio > 0

10 Summary of results sand site North Sea site sand - pinned sand fixity ISO SNAME % ISO SNAME % Leg penetration [m] % - - SDOF-DAF % % External OTM [MNm] % % Max/ Min V reaction [MN] 144.7/ / 5.2 3% /NA 131.6/ / 16.0 Chord P u [MN] % % Utilisations: Holding % % 3% /NA Chord % % Overturning stab % % Pre-load (1.05) (1.02) 3% (0.96) (0.93) 3% VH bearing % % Sliding (50% variable load) Leg lift 3.7 NA 1.36(1b) 0.98(1b) 39% Sliding (100% variable load) > NA %

11 Summary of results sand site Effect of apparent wave period in DAF (with fixity = 1.28) is 6% more inertia load, 1% more total external loading and 0.5% more vertical soil reaction Effect of H = 0.86*H (SNAME) iso on wave kinematics (ISO) is 7% on w/c load, 5% on total external loading and 2.5% on vertical soil reaction Increased UC s in general due to increased external loading (see above). Chord improved. VH bearing check worse. Two variations to see effects: ISO using the exact external loading as in the SNAME case ISO using the = 0.80 (new formula)

12 Summary of results sand site North Sea site ISO load variations SNAME ISO = 0.86 SNAME ext loads = 0.80 SNAME External OTM [MNm] Utilisations: Holding Chord Pre-load (0.96) (0.93) (0.92) (0.93) VH bearing Sliding (50% variable load) (2a) 0.98 (1b) Sliding (100% variable load) (2a) 0.88 (1b) 1.04 (2a) 0.89 (1b) 1.00 (2a) 0.77 (1b)

13 Summary of results clay site SNAME external loading and ISO external loading is based both on: k = 0.86 on wave kinematics (not Height) and DAF on intrinsic period. Effect of apparent wave period in DAF is 15% more inertia load, 1% more total external loading and 0.5% more vertical soil reaction Effect of H = 0.86*H (SNAME) iso on wave kinematics (ISO) is 3% on w/c load, 2% on total external loading and 1.5% on vertical soil reaction

14 Summary of results clay site Gulf of Mexico site Deep clay SNAME rev 3 orig ISO SNAME % SNAME Leg penetration [m] % - SDOF-DAF % % External OTM [MNm] % % Max reaction [MN] %(net) %(net) Fixity [%] 31% 35% 12% 20% 50% Chord P u [MN] % % Utilisations: Holding % % Chord % % (Pre-load) (0.91)net (0.90) 1% (0.97) -6% VH bearing % % Sliding % %

15 Summary of observations In general the assessment progresses very similar: Leg penetration agree quite well External loading (except apparent/intrinsic discussion) Fixity (if SNAME has deep clay method and embedment updated) Response Holding, overturning The step 1a pre-load check agrees, but is hardly ever allowed (H/V ratio exceeded or when fixity included) The step 1b sliding check agrees The structural check of the chord provides some significant improvement in the results (upto 20% due to safety and buckling load) Foundation checks were found to be governing (by far) The foundation (bearing) check level 2 needs some careful as it deviates from SNAME and adversely affects the outcome upto some 30 % in a step 2a/2b assessment (step 2c or step 3 has not been assessed) in terms of: The way to apply the material coefficient (on yield surface versus on capacity vector) Excluding side resistance (a slice of the VHM surface) Gross capacity versus net capacity (deep clay) The foundation sliding for windward legs in SNAME is a pure step 1b check only, in ISO it is also the (relevant) VH bearing check, but here excludes side resistance

16 Chord strength In general the chord strength check is similar, except: the material coefficient for axial loading is 1.10 (compared to 1.15 (1/0.85) for SNAME) thechord axial strength Pn is based on a formula for Fy > 450 Mpa which gives more capacity

17 External loading: Wind load is identical Wave/current loading affected by: Discussion points Application of kinematics reduction factor (method and value) Inertia loading affected by use of apparent wave period i.s.o. intrinsic Application of kin red fact directly on wave kinematics is the right way. Allowance to reduce the wave height underestimates OTM slightly. Both methods are presently allowed in both ISO and SNAME ISO defines formula to calculate appropriate kin red fact for each rig&site specific case. The underlying method is in principle what is allowed to do (time domain simulation of wave/current loading in an irregular sea) in SNAME and ISO. The result may both be below and in excess of 0.86 In the present assessment cases using the appropriate kin red factor (0.80 NS and 0.77 GoM) in combination with DAFapparent leads to a reduction of external load: 2.5% for NS 2508/2685 =>7% for GoM

18 The foundation (bearing) check level 2 Discussion points Governing the assessment results upto step 2b assessment Is more conservative than SNAME due to: The way to apply the material coefficient (on yield surface versus on capacity vector) Excluding side resistance (a slice of the VHM surface) Gross capacity versus net capacity (deep clay) In the present (basic) cases the VH bearing check found was more conservative by: 3% for sand (North Sea with same external loading) 30% for clay (GoM) The pre-load check (net capacity) is in agreement, but inappropriate because of fixity in the calculations The foundation sliding for windward legs in SNAME is a pure step 1b check only, in ISO it is also the (relevant) VH bearing check, but here excludes side resistance. Including the VH step 2a is relevant but should side resistance be included?

19 Sand check - comparison VH & sliding comparison ISO - SNAME SAND - shallow penetration ISO - factored yield ISO - unfactored Factored storm reactions [MN] 240 deg - leg deg - leg deg - leg deg - leg 1 Sliding line (step 1b) SNAME - unfactored SNAME - allowed loads

20 Clay check - comparison VH & sliding comparison ISO - SNAME CLAY deep penetration ISO - unfactored ISO - Factored VH yield surface Step 1b Sliding check for windward legs SNAME - yield unfact SNAME - allowed loads SNAME - sliding step 1b vector loads vector yield

21 Panel 4 update Hamburg The foundation (bearing) check level 2 with the proposed Panel 4 change as of 19 th October 2010: For sand, with fixity, the UC was Using the new definition and m = i.s.o for partial contact, we arrive at UC = 1.11 It agrees properly with SNAME results UC = 1.05, the difference being load. It is shown by the UC calculated using the exact SNAME loading F = 7.59, V = => UC = 1.05 (SNAME = 1.05)

22 Panel 4 Hamburg update The foundation (bearing) check level 2 with the proposed Panel 4 change as of 19 th October 2010: For clay, with fixity, the UC was Using the new definition and m = 1.10 i.s.o for full contact, we arrive at UC = SNAME = 0.80 VH slice of VHM surface for M= unfactored gross capacity yield surface Factored VH yield surface, gross capacity by 1.15 vector loads vector yield Step 1b Sliding check for windward legs Oct update of factored yield surface, factor 1.10 on net capacity

RECOMMENDED PRACTICE FOR SITE SPECIFIC ASSESSMENT OF MOBILE JACK-UP UNITS

RECOMMENDED PRACTICE FOR SITE SPECIFIC ASSESSMENT OF MOBILE JACK-UP UNITS GULF OF MEXICO ANNEX Revision 0 September 2007 Rev Issue Date Details 0 September 2007 Submitted to SNAME OC7 for Adoption Introduction: