P Delaforce and P Vinton, Rolls-Royce plc, UK SUMMARY
The use of composite materials offers the potential benefit of weight reduction and the associated increase in range, endurance, and payload. The classification societies have been reluctant to classify composite pressure hulls for manned submersibles, so where classification society approval is required traditional submarine metals are mandated. The classification societies rules in addition to the existing design standards and codes for industrial composite pressure vessels could provide the foundation for composite pressure hull standards. This paper discusses a number of proposals for the construction standards of composite hulls for small submersibles.
1. INTRODUCTION
Fibre reinforced composites materials have been adopted in many industries, from automotive to aerospace, in the pursuit of component weight reduction, utilising their high strength to density ratios. For small submersibles weight saving in the form of the minimisation of the hull weight to displacement ratio is crucial to increasing their range, endurance, and payload. A significant weight reduction of the submersible offers the potential to reduce the size of the launch and recovery systems.
Many parts of the exostructure, such as hydrodynamic fairings and floodable structures are already manufactured out of composite materials. The largest single component which offers the greatest opportunity of weight reduction from the utilisation of composite materials is the pressure hull.
There has been significant interest over the years in the design and production of prototype submersibles with a composite pressure hull [1-2]. However, the classification societies have been reluctant to classify composite pressure hulls for manned submersibles so, where classification society approval is required construction has reverted to known submarine steels or certain aluminium or titanium alloys [3-6].
Small submersible designs could be developed to utilising Acrylic plastic for windows and viewports, using a single domed viewport which makes up large proportion of the pressure hull. However, there is a limit by which Acrylic viewports can be used before a fibre- reinforced composite pressure hull is required for further weight reduction.
The purpose of this paper is to review the existing design rules and codes mandated by the classification societies for manned submersibles and present a set of proposals for classification society approval. Using existing design rules and safety factors used for submarine and composite pressure vessel construction.
2. CURRENT RULES AND STANDARDS The foremost shipping classification societies mandate the use of pressure vessel steels, Aluminium and Titanium alloys for the construction of the pressure hull, and permit the use of Acrylic plastic for windows and viewports [3-6].
In the United States, the American Bureau of Shipping (ABS) Rules for Building and Classing Underwater Vehicles [3] incorporates both the Boiler and Pressure Vessel Code (BPVC) for construction [7], and the Safety Standards for Pressure Vessels for Human Occupancy [6]. Both of these standards are produced by the American Society of Mechanical Engineers (ASME).
One section of the ASME Boiler and Pressure Vessel code covers the design and construction of fibre- reinforced plastic pressure vessels, Section X [8]. This design code for composite pressure vessels is the obvious starting point for developing composite pressure hull design code and acceptance criteria.
Examination of Section X reveals that two different design methodologies are permitted, design qualification through the destructive test of a prototype (Class I, composite vessels) and the use of mandatory design rules and acceptance testing by non-destructive examination (Class II, composite vessels). Class II vessels permit a relatively low maximum internal and external pressures are, were the maximum external pressure must not exceed 100 kPa (1 bar). This is due to the relative limited experience and data available for composite vessels to produce a robust design using a design by rule approach.
Design qualification through destructive testing of a prototype provides a strong base to demonstrate sufficient safety factors for pressure vessels subjected to large internal and external pressures. For Class I vessels the maximum external pressure is limited by the manufacturing method. Table 1 summaries the maximum pressure for the allowed manufacturing method for Class I vessels and the qualification test and production test acceptance requirements.
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects Table 1 Requirements for Class I Composite Pressure Vessels, from ASME BPVC, Section X
Parameter Requirement Code Ref
Internal or
External Dependent on fabrication method.
1 MPa (10 bar) For bag-moulded, centrifugally cast and contact- moulded vessels.
10 MPa (100 bar) For filament-wound vessels, or one-sixth of the bursting pressure.
Design Pressure
20 MPa (200 bar) For filament-wound vessels with polar boss openings or one-fifth of the bursting pressure.
RD-111, RD-120
a) Prototype subjected to 100,000 cycles of pressure ranging for max external and internal design pressure.
b) Prototype shall withstand an external pressure of twice max external design pressure without buckling.
RD-311, RT-223.2
Internal and External Pressure Service
c) Prototype shall withstand a hydrostatic pressure of at least six times the max internal design pressure.
RD-160
Vessel will be designed for a min internal pressure of 100 kPa in addition to external design pressure.
a) Prototype shall withstand an external pressure of twice max external design pressure without buckling.
b) Prototype subjected to 100,000 cycles of pressure ranging from max external to the internal design pressure of 100 kPa. (1 bar)
RD-312, RT-223.3
Qualification Test Requirements
External Pressure Service Only
c) Prototype shall withstand a hydrostatic pressure of at least six times the internal design pressure of 100 kPa. (1 bar)
RD-160
Thickness check To be within 10% of ½¥(R×t), where R = radius of
the shell, t = nominal specified thickness. RT-420 Vessel Weight To be within 95% of the weight specified in the
qualification test report from the prototype. RT-430 Barcol Hardness
Test
Within the range specified by qualification test
report. RT-440
1.1 times the internal or external design pressure for vessel without welded metal components.
Production Test Requirements
Hydrostatic Leakage Test
1.3 times the internal or external design pressure for vessel with welded metal components.
RT-450
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects
Table 2 Comparison of the Qualification and Acceptance Requirement from the Standards and Codes ASME Boiler &
Pressure Vessel Code, Section X
ASME Safety Standards Pressure Vessels for Human Occupancy
ABS Rules
Underwater Vehicles Systems and
Hyperbaric Facilities External
Pressure
2 times max external design pressure without buckling.
(RD-312 & RT-223.3)
Acrylic window:
Shall not exceed 1380 bar (138 MPa)
(Section 2, para 3-1.2)
Acrylic window:
Shall not exceed 1380 bar (138 MPa) (Section 7, para 19.13) Internal
Pressure
6 times the internal design pressure.
(RD-160)
- - Qualification
Fatigue
Prototype subjected to 100,000 cycles of pressure ranging from max external to the internal without leakage.
(RD-312 & RT-223.3)
Acrylic window:
10,000 cycles pressure cycles or 40,000 hrs, respectively.
(Section 2, para )
Acrylic window:
10,000 cycles pressure cycles or 40,000 hrs, respectively.
(Section 7, para 7)
Hydrostatically proof tested to 1.1 times the internal or external design pressure for vessels without welded metal components.
(RT-450)
All external pressure hulls hydrostatically tested to 1.25 of the design pressure.
(Section 1, para 1-7.13.6)
All external pressure hulls hydrostatically tested to 1.25 the design depth for two cycles.
(Section 3, para 3.1) Hydrostatically proof
tested to 1.3 times the internal or external design pressure for vessels with welded metal components.
(RT-450)
Strain gauges are to be applied at hard spots, discontinuities, high stress regions etc.
(Section 1, para 1-7.13.6)
Triaxial strain gauges are to be fitted in way of hard spots and discontinuities during proof test.
(Section 3, para 3.3) Pressure
Testing
- - - Acrylic window:
Shall not exceed 1.5 times the design pressure or 138 Mpa, whichever is the lesser value.
(Section 7, para 19.13) Acceptance
Test
Dive - -
Final test dive to design depth.
(Section 3, para 3.3)