MATERIALS & MATERIAL GRADE SELECTIONS

Transcription

1 MATERIALS & MATERIAL GRADE SELECTIONS Oleh: Ir. Murdjito, MSc.Eng Dosen Jurusan Teknik Kelautan Fakultas Teknologi Kelautan Institut Teknologi Sepuluh Nopember (ITS) Surabaya

2 Structural Material Carbon Steel most important material for offshore & marine material used for: pile, tubular and cans, rolled beam and plate girders, flat plate, etc Stainless Steel Quite expensive material Restricted used as roof and wall cladding, fastener, plating in splash zone Common type AISI 304 & 316, yield: MPa Grout A structural material for steel designer Application: grouting of annulus between pile sleeve and pile, internal grouting of tubular joint

3 Continue.. Glass fibre reinforced (GFRP) Very strong but lack stiffness, the fatigue resistant is excellent Not corroded excellent marine environmental stability Jointing techniques is particularly important, welding is not an option Repair & inspection procedure will yet require considerable effort to obtain offshore acceptance Offshore applications: Furniture and internal cladding of LQM Grating, Life boats Ducting, Small covers and enclosed, Piping for water GRFP gives weight saving over carbon steel of 40 70% Cost: CAPEX level : %,OPEX level lower Cast steel For offshore application has progressed by effort of the industry in UK, Germany and japan Aluminum Major application: helideck, roof and wall cladding, platform bridges, stairs and platform, telecommunication mast, cable trays, walkway, hand railing etc Development to be considered as platform deck flooring

4 STEEL PROPERTIES Yield and tensile strength Ductility Through thickness properties Fatigue strength Suitability for fabrication (welding, forming) Corrosion resistance

5 YIELD STRENGTH AND TENSILE STRENGTH Tensile strength is required to be not too close to the yield strength Material selection specify: σy/ σt API/AISC <= 0.83 The ultimate strength of a structural is dependent on tensile strength and ductility σy/ σt maximum value is not appropriate anymore and not directly relevant to weld structure ultimate strength of welded structure is dependent on the geometrical form and stress concentrations resulting fracture mechanics can provide models for ultimate member strength based on tensile strength and ductility high-strength material require higher ductility values to achieve high ultimate member strength high-strength material with a high σy/ σt value cannot provide fro gross section member yielding prior to net section member failure

6 DUCTILITY & THROUGH THICKNESS PROPERTY Ductility phenomenon of failure after considerable deformation the risk of brittle fracture is influenced by speed of straining temperature of material thickness of material amount of strain and reliability of its prognosis basic tools for providing ductile behavior are: fracture mechanics analysis fracture mechanics test Through thickness properties problem in welded structures: lamellar tearing occurs in fabrication lamellar tearing can be very costly to repair

7 FABRICATION PROCESS impact of fabrication process to material properties: thermal cutting hot or cold forming welding

8 PROBLEM OF STEEL MATERIALS Fabrication process: Weldability of material Cutability Corrosion resistance Fatique resistance Stiffness according to the temperature change lamellar tearing, fracture Quality control & assurance

9 MATERIAL GRADE SELECTION FOR OFFSHORE STEEL STRUCTURES extreme importance to the fitness of the structure material selection procedures: the material selection of each member or category members is justified in the design report the material grade is precisely specified on the drawing

10 API RP 2A WSD Section 8: Material Minimum strength level, group and class specified by designer Mill test report provided by fabricator in accordance with ASTM A6 or A20

11 Steel Group Grouped according to strength level and welding characteristics: Group I: mild steel with yield strength <= 40 ksi (280 MPa) Carbon equivalent 0.40% or less Welding processes described in AWS D1.1 Group II Intermediate strength steels yield strength : Ksi ( MPa) Carbon equivalent ranges: > 0.45% Require the use of low hydrogen welding processes Group III Yield strength > 52 ksi Each application is investigated with regard to: Weldability and special welding procedures required Fatigue problem Notch toughness fracture control

12 Steel Classes Class C steel Applicable to primary structural members involving limited thickness, moderate forming, low restraint, modest stress concentration, quasi static loading and redundancy For piling, jacket braces & legs, deck beam, etc Class B steel Suitable use where thickness, cold work, restraint, impact loading, lack of redundancy the need for improved notch toughness Charpy V-Notch energy: Group I: 15 ft-lbs (20 J) Group II: 25 ft-lbs (34 J) Charpy test temperature: 10 0OC) Impact test in accordance with ASTM A 673, Frequency H Class A steel Suitable for use at subfreezing temperatures Charpy test temperature: (-)20 (-)40OC Impact testing in accordance with the specification under which steel ordered Heat lot testing may be used

13 table

14 Table

15 Steel for Tubular Joint Underwater joint Notch toughness criteria: NRL Drop-Weight Test no break performance Charpy V-notch energy: Group I: 15 ft-lbs Group II: 25 ft-lbs Group III: 35 ft-lbs Above Water Joints Expose to lower temperatures and possible impact from boat or critical connections at any location: Class A steel considered Critical Joints: use steel having improved through thickness (Z-directions) properties e.q: API Spec 2H, Suplements S4 and S5 Brace ends: same class as joint cans or one class lower

16 DNV-OS-B101: METALLIC MATERIALS rolled steel for structural application requirements for weldable normal strength, high strength and extra high strength hot rolled structural steel plates and sections, seamless steel tubes and pipes intended for structural application applicable to steel products with a thickness not exceeding 150 mm.

17 STEEL GRADES The steel grades strength groups: normal strength steels (NS) high strength steels (HS) extra high strength steels (EHS) Parallel series of steel grades steels of normal weldability steels of improved weldability.

18 DnV STEEL GRADES Table A1 Definitions of steel grades Strength group Normal weldability Impact testing Symbol x Improved weldability Test temperature ( C) Symbol y Tensile properties Minimum yield stress(n/mm2) A - - NS B2) BW DW Omit-ted D DW -20 E EW 40 A AW HS D DW E EW F A D DW EHS E EW F ) For steels of improved weldability the required minimum yield stress is reduced for increasing material thickness. 2) Charpy V-notch tests are required for thickness above 25 mm but is subject to agreement for thickness of 25 mm or less.

19 Normal Strength Steel Table B3 Condition of supply for normal strength (NS) steel Grade Thickness, t (mm) Condition of supply 1) NV A t?50 Any 50 < t?150 AR2) CR, N, TM NV B t?50 Any 50 < t?150 AR2), CR, N, TM NV D t?35 Any 35 < t? 150 AR3), CR, N, TM NV E t?150 AR3),CR3),N, TM 1) Condition of supply: AR: As rolled. N: Normalised. CR: Controlled rolled. 2) Grades NV A and NV B may be supplied as rolled (AR) subject to spe-cial consideration. 3) Subject to special consideration, sections in grade N V D m ay be sup-plied as rolled (AR) provided satisfactory results are consistently ob-tained from Charpy V-notch impact tests. Similarly, sections in grade NV E may be supplied as rolled (AR) or controlled rolled (CR).

20 Mechanical properties Table B4 Mechanical properties for normal strength (NS) steel of normal weldability Yield Tensile Elongation Test stress strength A5 temperature Longitu Transve Longitud Grade ReH Rm minimum minimum (N/mm²) (%) ( C) (N/mm²) dinal rse inal NV A NV B NV D NV E ) ) Averageimpact energyjminimum t < t < t ) ) Transv erse 24 2) Longit udinal 41 2) Transve rse 1) Charpy V-notch impact tests are generally not required for grade NV B steel with thickness of 25 mm or less. 2) Impact tests for grade NV A over 50 mm thick are not required when the material is produced using fine grain practice and furnished normalised or thermo-mechanically controlled processed. 3) For full thickness flat test specimens with a width of 25 mm and a gauge length of 200 mm the elongation shall comply with the following minimum values: 10 < t 15 < t 20 < t 25 < t 30 < t 40 < t Thickness, mm t 5 5 < t Elongation )