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Quality requirements for an FPSO hull and marine piping fabrication (August 2004)

 
John Still, Lochead Still Associates (formerly of Amerada Hess London), Julian Speck, TWI and  Marcos Pereira, TWI

OMAE-FPSO'04-0088

Proceedings of OMAE-FPSO 2004, OMAE Specialty Symposium on FPSO Integrity, Houston, USA, 30 Aug - 2 Sept 2004

Abstract

To guarantee that a Floating Production Storage and Offloading (FPSO) vessel operates successfully during the life of the field, it is recommended that an inspection regime over and above the Class requirements be introduced during construction. The enhanced inspection programme would be implemented by the shipbuilder to address the operator's concerns about conventional construction standards for vessels. In this way FPSOs will be installed to the same high standards as other offshore structures. Any additional requirements specified by the operator are intended to ensure that the FPSO hull will operate without incident during the field life with the minimum of disruption to production.

Construction of a new build FPSO involves the control of materials, shot blasting and priming, fabrication of hull panels, fabrication of blocks, and the erection of blocks within a dry dock. Several of the above activities involvethe use of a variety of welding processes, which have been selected for a specific reason, such as high productivity and low cost.

The fabrication methods for marine piping systems, for example, are driven by these reasons. The inspection requirement for the majority of marine pipe welds is simply visual inspection. If the fabrication inspection methods are not suitable for the detection of fabrication flaws, it is possible that structurally unsound welds will be installed in the vessel. (Welds in the above condition may be considered critical in the context of fatigue during service).

To accommodate the additional requirements from the operator regarding the integrity of the vessel and its systems, it is essential to apply realistic standards to ensure that the desired quality is achieved. This paper outlines the experiences of an operator during the conversion of an intercept hull to a FPSO.

Introduction

The service conditions experienced by an FPSO are more rigorous that a conventional trading tanker. A typical FPSO can remain semi-permanently on station anything from 5 to 20 years, whereas a trading tanker is dry docked at regular intervals for inspection and repair.

FPSOs can either be constructed as a new purpose built unit, converted from an existing tanker, or as an 'interception' where the vessel is purchased from a third party prior to, or during building (initially as a trading tanker,eventually modified to become an FPSO).

Prior to construction, the vessel operator has the opportunity to review the service conditions and request a change to both materials and welding consumables for the hull to a grade that would provide improved structural integrity,through enhanced mechanical properties such as greater fracture initiation toughness and crack-arresting properties. [1]

Although vessel hulls are constructed in accordance with class rules, when requested the shipyard will allow critical and fatigue sensitive areas to be inspected over and above class requirements (by the operator's site inspection team). This additional inspection, in conjunction with the improved mechanical properties, would effectively extend the safe life of critical structures provided the in-service inspection is carried out at pre-determined intervals.

With regard to marine piping systems, in the majority of vessels they are designed, constructed, and inspected by the shipyard in accordance with Class rules. Inspection requirements imposed on marine piping systems are either visual or radiography (where the level of inspection of the latter can vary from zero to 10% and 100%).

Offshore operators contemplating the construction or conversion of a tanker to an FPSO seldom question the standard of weld quality of marine piping. (Based on one operator's experience it is doubtful if Class requirements are suitable for FPSO marine piping at all).

Hull structures

Hull Materials

Selection of steel grades for vessel hulls is dictated by the classification society's rules, although service and environmental conditions will influence the selection. In a recently completed 'Intercept' FPSO, originally designed as a trading tanker with a double hull, the steel grades originally specified were Grade A and AH32. Since the vessel was being converted to an FPSO, the environment and service conditions were investigated.

The specification for the outer hull materials was subsequently changed to Grade D and DH32. In addition to changing the material grades, the thickness of the deck and bottom outer hull plates was increased from 16.5 to 19.5 mm. All other plates remained as per conventional tanker design. This change in material grade and increased thickness was to provide better crack arrest properties for fatigue sensitive areas.

The specification requirements for chemical composition and mechanical properties for Grade A, D, AH32 and DH32 are presented on Table 1 for comparison. Location of the new materials in the hull and deck structure is illustrated in Fig.1.

Table 1 Chemical and mechanical properties of base materials

GradeChemical CompositionMechanical Properties
CSiSPMnThickness
mm
UTS min
N/mm 2
Yield min
N/mm 2
Elongation
min%
(All values quoted are maximum unless otherwise stated)
A 0.21 0.50 0.035 0.035 2.5 x C min <50 400 235 22
AH32 0.18 0.10/0.50 0.035 0.035 0.90/1.60 <50 440 315 22
D 0.21 0.10/0.35 0.035 0.035 0.6 min <50 400 235 22
DH32 0.18 0.10/0.50 0.035 0.035 0.90/1.60 <50 440 315 22

Table 1 Chemical and mechanical properties of base materials continued

GradeCHARPY Impact Properties
Test Temperature
°C
Longitudinal JoulesTransverse Joules
(All values quoted are maximum unless otherwise stated)
A - - -
AH32 Zero 34 24
D -20 27 20
DH32 -30 34 24
Fig.1. FPSO Hull Materials
Fig.1. FPSO Hull Materials

Plate materials are shot blasted and primed prior to profile cutting to a variety of shapes for a particular part of the hull structure. Forming of plates is either carried out cold by pressing, or by line heating. The line heating process involves heating one surface only and rapidly cooling predetermined areas on the plate surface with water.

This heating and cooling process is such that the microstructure is modified creating an isolated area where the resulting surface residual stresses are in tension. By applying this technique in a predetermined sequence the plate is gradually formed into the desired shape.

Welding Processes, Consumables and Properties

Welding processes used for fabricating the hull structure involve the following welding processes:

  • Gravity welding
  • Shield Metal Arc welding (SMAW)
  • Gas Shielded Flux Cored Arc Welding (GSFCAW)
  • Submerged Arc Welding (SAW)
  • Electro Gas Welding (EGW)
  • Gas Metal Arc Welding (GMAW)

Figure 2 illustrates where welding processes are used for pre-assembled structures, and block assembly and erection of the outer hull structure. The most common welding process used for hull construction is GSFCAW. This process deposits the largest volume of weld metal in the vessel hull. EGW is an automatic process which is restricted to the joining of block structures. (Welding robots using either GSFCAW or MAGW have a limited use on high volume repetitive fabrications depositing fillet welds). The use of ceramic tiles for semi-automatic and automatic processes for single side joints is widely used at every stage during hull construction.

Fig.2. Welding processes used for block assembly and erection
Fig.2. Welding processes used for block assembly and erection

The selection of welding consumables for hull construction is based on the Classification Society's list of approved consumables. Welding consumable for hull steels are classified or graded 'numerically' on the basis of toughness,e.g. 1 to 5. The grading system also defines the minimum yield strength levels with the use of an 'alphabetical indicator', for example:

  • N (normal strength steels, minimum yield strength 235N/mm 2 ).
  • Y (higher strength steels, minimum yield strengths 355N/mm 2 ).

For extra high strength steels the 'alphabetical indicator' is followed by a number representing the yield strength of the steel grade, e.g. 3Y40, where '3' indicated the toughness grade, 'Y' indicates the base minimum yield strength, and '40' indicates additional minimum yield strength requirements, i.e. 400N/mm 2 .

The selection of welding consumables is based on the weld metal Charpy impact toughness requirements matching the minimum Charpy impact toughness levels specified for hull materials. Table 2 summarises the mechanical properties requirements for consumables used for hull material grades.

Table 2 Examples of the consumables grades used for fabricating Grade D and DH32 steels

Welding ProcessClass Filler GradeAWS GradeWelding PositionWire Diameter
mm
Gas ShieldPolarityAll Weld Metal PropertiesMaterial Grades
Yield
N/mm 2
CHARPY Impacts
Temp
°C
Joules
(Average)
GSFCAW 2Y A5.29 E71T1 All positions and vertical down 1.2 CO 2 DC + ve 470 -18 80 AJ,DH,EH
  3Y A5.29 E80T1-K2 All positions and vertical down 1.2 CO 2 DC + ve 550 -20 118 AJ,DH,EH
SMAW 3Y E7016 All positions 3.2/4.0   DC + ve 490 -29 118 AJ,DH,EH
(Gravity-Welding) 3Y E7028 Horizontal 4.5/5/6.4/7   AC 510 -20 100 AJ,DH,EH
SAW 3Y FA(P)4
EL8(EL12)
Flat 2.0/4.8   DC + ve 440 -20 150 AJ,DH,EH
  3Y P7A(P)2
EH14
Flat 4.8   DC + ve 460 -20 120 AJ,DH,EH
EGW 3Y A5.26 EG70T-2 Vertical 1.6 CO 2 DC + ve 470 -20 62 AJ,DH,EH

Classification Societies specify either Grade 2Y or Grade 3Y consumables for welding Grade D and Grade DH 32 steels. The Charpy impact toughness requirement for 2Y consumables is 34 Joules at zero degrees Celsius which meets the requirements for Grade A and Grade AH 32 steels. Grade 3Y consumables specify Charpy impact toughness requirements of 34 J at -20°C, which satisfy the toughness requirements for Grade D and Grade DH32 steels.

Some concerns were expressed about the suitability of specified Grade 2Y consumables for fillet welds (in preference to Grade 3Y consumables). These concerns were addressed by undertaking fracture toughness testing. A subsequent assessment was based on the CTOD values measured in accordance with BS 7448 [2,3] from test specimens extracted from a simulated fillet weld. [4] The procedure involved manufacturing specimens from a butt-welded test plate with joint dimensions identical to those of production fillet welds.

It is recommended that operators of FPSO's should consider applying fracture mechanics to develop an Engineering Critical Assessment (ECA) to establish a realistic fracture control program. [1]

Construction of FPSO Hulls

Construction of the vessel hull involves three stages: assembly, pre-erection and erection. To expedite the vessel construction, machinery and marine piping are installed within the block assemblies where piping joints between blocks are either butt, flange, socket or sleeved welded. [5]

The assembly stage involves pre-fabricating plate or structural sections in suitable sizes for pre-erection into blocks or part blocks. The erection stage involves moving the blocks or part blocks to a predetermined location with in a dry dock for final erection and welding. The final stage in the building of a hull structure involves transferring block fabrications to a dry dock where the units are located in a predetermined position (A to E) ( Fig.3) and erected and welded into a hull structure. Painting of the structure is carried out at all stages from pre-assembled, block assembly and final erection and welding.

Fig.3. Block erection sequence (Section A to E)
Fig.3. Block erection sequence (Section A to E)

Installation of the Moon Pool

A critical part of the modification of this tanker for the FPSO is the moon pool in the deck. This structure supports the turret containing the risers and mooring chains that allows the vessel to weather vane freely in all sea conditions. Installation of the moon pool structure into the vessel hull can be accomplished in one of the following ways:

  • Fabricated as part of a block fabrication.
  • Installed as a separate fabrication into the hull structure.

For the former route, the moon pool is fabricated as part of a block fabrication where welding is identical to that used for the hull structure. The latter route is dependent on whether the shipyard can accommodate the fabrication and installation of the moon pool within their building program. If the yard declines to fabricate the moon pool structure, the operator must arrange for an alternative facility to undertake this.

Weld Inspection During Construction

The fabrication NDT requirements are in accordance with Class Rules, and limits the radiography, ultrasonic and magnetic particle inspection to specific areas within the hull and deck structure. This involved a total of 126 weld joints and 119 welds to be examined by radiography and ultrasonic techniques respectively. Shipyards have an internal quality program and may carry out additional inspection to ensure quality is being achieved. The inspection requirements for hull structures imposed by the Classification Society, for a recent 'intercept' FPSO, are summarized in Table 3. In order to meet the Class requirements, the shipyard must (amongst other things) carry out radiographic and ultrasonic examination at predetermined locations, on a sample basis, which are defined by the Classification Society. The shipyard followed a recognized quality management system, i.e. ISO 9001 [7] , and carried out a limited number of inspections of the hull welds in accordance with the shipyard's quality standard. (This internal document detailed the shipyard's welding fabrication acceptance criteria, which were simply based on the requirements of Classification Societies).

Table 3 NDT requirements in accordance with class rules

LocationNDT processNumber of weld joints
Shell expansion Radiography 60
Ultrasonics 136
Upper deck Radiography 66
Ultrasonics 59

All the radiographs viewed by the Classification Society's surveyor were also examined by the vessel operator's inspectors to verify that the quality of the welds and radiographs were satisfactory. In some instances it may beadvisable for the operator to introduce additional inspection requirements involving their own inspection personnel as outlined in Table 4. [6] In this instance the operator of a new build tanker destined for conversion to an FPSO examined over 37 % of the outer hull weld joints using ultrasonic techniques.

Table 4 Inspection techniques applied to selective areas of the vessel hull

Additional ultrasonic examination of the outer hull welds was also carried out on the instruction of the operator. The technique applied simply involved the use of both 60-degree or 70-degree angle probes for detecting planardefects, and compression probes for checking and recording plate thickness (as 'fingerprints' to assist with future operational in-service inspection planning). A limited number of critical areas were also subject to magnetic particleinspection (MPI) as outlined in Fig.4. The welds selected for MPI were considered potential areas for in-service fatigue cracking, i.e. weld details that would require inspection throughout the life of the vessel.

LocationFabrication defectInspection technique
Hull block welds Misalignment
Root gap
Undercut
Buried linear defects
Visual and dimensional survey
Visual and dimensional survey
Visual
Longitudinal stiffener butt welds Alignment
Undercut
Visual and dimensional survey
Visual and magnetic particle
Rat holes in longitudinal and bulkheads along the hull shell plate Undercut
Weld Profile
Visual and magnetic particle
Visual
Fillet weld profiles of critical weld areas such as:

-Stringers to bulkheads and internal cargo tank brackets.

-Deck stiffening under support stools.

-Longitudinal stiffeners and bulkhead intersection.
Alignment
Weld profile
Undercut
Visual and dimensional survey
Visual
Visual and magnetic particle
Weld profile
Undercut
Weld distortion
Visual
Visual and magnetic particle
Dimensional survey
Weld profile
Undercut
Visual and magnetic particle
Visual and magnetic particle
Fig.4. Magnetic particle inspection of critical toe areas
Fig.4. Magnetic particle inspection of critical toe areas

Weld Quality

By adopting this rigorous inspection regime the standard of welding improved by an order of magnitude. Table 5 outlines the final defect analysis of the weld inspection carried by the offshore operator on the butt welds in the outer hull. The final repair rate of 0.09% is considered excellent. An analysis of the defect types encountered shows that slag inclusion was by far the main cause of repair, particularly with GSFCAW process).

Table 5 Results of ultrasonic examination of outer hull

Ultrasonic examination - assembly, pre-erection and erection stages
Welding processThickness
(mm)
Length of weld examined
(m)
Length of slag
(m)
Length of lack of side wall fusion
(m)
Length of lack of root fusion
(m)
Length of porosity
(m)
Total length repaired
(m)
GSFCAW 14 74 0.234        
GSFCAW 16.5 - 18 759 0.33 0.028      
GSFCAW 19.5 - 21.5 1520.8 1.97 0.4 0.385    
GSFCAW 20 - 35 30          
GSFCAW 27.5 - 34 60 0.035        
              3.382
GSFCAW/SAW 18.5 - 20 364 0.205 0.086      
GSFCAW/SAW 16.5 119          
              0.291
GMAW/SAW 16 - 19.5 143 0.655        
GMAW/SAW 19.5 - 21.5 280 0.95        
              1.605
SAW 15.5 40          
SAW 16 - 19 1167          
SAW 19.5 - 21.5 854          
               
EGW 17.5 19          
TOTAL   5429.8 4.387 (0.08%) 0.514 (0.01%) 0.385 (0.01%) NIL 5.278 (0.09%)

Marine piping

Marine Piping Standards

Example of marine systems for a conventional AFROMAX tanker converted to an FPSO are shown in Table 6, where the NDT requirements are defined in terms of pipe wall thickness. The Class rules state that any systems with a pipe wall thickness up to a maximum of 9.5mm require 100% visual inspection and all systems greater than 9.5mm thick require 100% radiography.

Table 6 Examples of marine piping inspection requirements to ABS rules

Vessel systemDiameter
(mm)
Thickness
(mm)
Inspection requirements
Vessel pipe work
GROUP I      
Steam Service 300 (Sch 40) 9.5 Visual
Flue Gas 500 (Sch 40) 9.5 Visual
GROUP II      
Cargo Handling 600 12.7 Visual
Tank Cleaning 250 (Sch 40) 9.3 Visual
Fire Fighting 65 (Sch 40) 5.2 Visual
Engine room
GROUP I      
Steam Service 65 (Sch 160) 9.5 Visual
Boiler Blow-Off 125 (Sch 80) 9.5 Visual
Fuel Oil 200 (Sch 80) 12.7 100% Radiography
GROUP II      
Steam Service 125 (Sch 80) 9.5 Visual
Heating Coil 250 (Sch 40) 9.5 Visual

Welding of Marine Piping

Welding processes used for joining marine piping include GTAW (GAS Tungsten Arc Welding) and GSFCAW. Root runs are deposited using GTAW and filled with GSFCAW. (Temperature control is unexceptional; minimum preheat temperature =ambient, maximum interpass = 250°C).

The majority of piping systems consist of pipe spools flanged at either end. This method has been adopted by shipyards to reduce pipe welding within the vessel and allow items to be installed with the minimum of effort.

Weld Quality

In the absence of adequate inspection equipment such as optical aids, the chance of weld root defects going undetected increases. [8] Individuals in shipyards have been known to take advantage of the above loophole, knowing that the Class surveyor's workload is such that he may not have adequate equipment (or the time!) to carry out a thorough inspection. In this new build tanker for FPSO service, the operator requested additional radiography to be carried out on marine piping systems that did not warrant radiography in accordance with the Class rules. The result of the additional inspection revealed that over 60% of the butts examined contained lack of root penetration defects.

The extent of the defects encountered in marine system that do not warrant radiography must bring into question the fitness of Class rules when applied to FPSO's. Based on the above findings, offshore operators must decide whether Class piping rules are adequate or an alternative standard should be applied for FPSO service. Several routes can be considered to achieve a standard of pipe quality suitable for a 20-year life.

Future In-Service Inspection

Corrosion monitoring of marine piping systems is not widely practiced. For an FPSO it is advisable to establish an effective corrosion monitoring program [9] if the vessel is destined to remain on station for the duration of the field life which can be up to 15 to 20 years without having to dry dock.

The corrosion-monitoring technique applied should involve determining the wall thickness at strategic locations within the marine systems prior to operations commencing. Regular in-service monitoring is advisable to ensure that thepiping systems are free from corrosion or fatigue cracks where weld quality is questionable.

Summary

The use of FPSO for the processing of hydrocarbons from offshore reservoirs is becoming a popular alternative to fixed offshore structures. It is advisable that the operator introduces a fabrication hull inspection program carried out by a third party under his control. In conjunction with the above it is advisable to use the results of any pre-service engineering assessments, to develop the in-service inspection programme.

One may argue that if a shipyard is qualified to BS EN ISO 9000/1, quality is achieved. We have shown in this paper that additional inspection and assessment is essential if the vessel has to remain on station with the minimum of major repairs being carried out.

Designing marine systems for FPSO applications must take into account the service and the ability to inspect and repair piping systems during the operational life of the vessel.

Several routes can be considered to achieve a standard of pipe quality suitable for prolonged service offshore. These include an alternative standard such as ASME B31.3, [10] where the operator can select the welds to be inspected at random. Also, the use of Engineering Critical Assessments (ECAs) is recommended to develop suitable flaw acceptance criteria, to provide assurance that any weld containing known defects can be monitored at a convenient opportunity, preventing the possibility of weld failure in future.

References

  1. Still, J. R., Speck, J. B., and Razmjoo, G. R., 2000, 'Integrity of FPSO Hull Structures', The Naval Architect, pp. 28-36.
  2. BS 7448 Part 1, 1991, 'Fracture Mechanics Toughness Tests. Method for the Determination of K IC , Critical CTOD and Critical J Values of Metallic Materials', British Standards Institute.
  3. BS 7448 Part 2, 1997, 'Fracture mechanics toughness tests. Method for the determination of K IC , critical CTOD and critical J values of welds in metallic materials', British Standards Institute.
  4. Speck, J. B., and Still, J. R., 2003, 'Preventing Brittle Fracture in Hull Fillet Welds', The Naval Architect, pp.34- 38.
  5. Still, J. R., and Speck., J. B., 2000, 'Making a Turn Around the Deck', Welding and Metal Fabrication, pp.19-23.
  6. Still, J. R., and Speck, J. B., 2000, 'Hull Weld Quality Critical for Offshore Oil Production Vessels', AWS, Welding Journal, pp.33-37.
  7. BS EN ISO 9000/1:1994 Quality Management and Quality Assurance Standards: Guidelines for Selection and Use.
  8. Still, J. R., and Speck, J. B., 2002, 'Determining Marine Pipe Quality', AWS, Welding Journal, pp.54-58.
  9. Still, J. R., and Nelson, P., 1992, 'Development of Corrosion Monitoring in Offshore Production Industry', British Institute of NDT, Vol. 34, No. 7, pp. 336-340.
  10. ASTM B31.3: 1996, Code for Process Piping, New York N.Y: American Society of Mechanical Engineers.

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