Outfitting related structural defects
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This document discusses outfitting-related structural defects found on double-hull tankers. It notes that about 5 out of 6 defects in the upper deck plate and stiffeners were due to cracks caused by outfitting details being neglected. The document recommends shipyards develop standards for outfitting design and construction to avoid such failures. It also recommends outfitting inspections focus on compliance with outfitting details and be conducted at the block stage when possible. Typical design considerations are outlined such as using open support sections, avoiding pipe sections, ensuring proper backing structure and transitions, and limiting bracket toe heights. Examples of defects like fractures at bracket toes and cracks from lack of backing structure are also provided.
Outfitting related structural defects
- 1. 1 2010 Shipbuilders Meeting TSCF Outfitting Related Structural Defects 2010 Shipbuilders Meeting TSCF Outfitting Related Structural Defects “Structure Defects on Double Hull Tankers”, Philip G. Rynn, American Bureau of Shipping, TSCF 2007 Shipbuilders Meeting “The service history of existing double hull oil tankers has revealed the fact that outfitting details are not to be neglected. Since about 5 in 6 defects found in the upper deck plate and stiffeners were cracks due to outfitting, shipyards and others must consider developing the appropriate design standards to avoid such failures in the future double hull oil tanker.”
- 2. 2 2010 Shipbuilders Meeting TSCF Outfitting Related Structural Defects Considering the above, TSCF decided to set up a Working Group in order to produce an Information Paper detailing the problems encountered and their consequences, and establishing best practice and recommendations for design of outfitting details, and verification during construction. Final Information Paper is still to be approved by the TSCF Steering Committee, and will be issued by the end of the year The purpose of this presentation is to offer some insights into the final Paper 2010 Shipbuilders Meeting TSCF Introduction Shipyard Standard Design and Plan Approval Inspection Typical Design Consideration Examples of Defects Outfitting Related Structural Defects
- 3. 3 2010 Shipbuilders Meeting TSCF Introduction 2010 Shipbuilders Meeting TSCF 2010 Shipbuilders Meeting TSCF Outfitting reinforcements should be designed considering that: Dynamic loads have to be taken into account Any stress concentrations created by local reinforcement must be considered and addressed in the design Smooth transition is desirable Hard points have to be avoided Good workmanship is also a critical issue as: Poor alignment or poor welding can lead to structural failure even for a good design Outfitting works may not be done by same working force as structure work Introduction
- 4. 4 2010 Shipbuilders Meeting TSCF Shipyard standard 2010 Shipbuilders Meeting TSCF 2010 Shipbuilders Meeting TSCF Shipyard standard Outfitting is generally governed by the “Shipyard Standard”, which is: Specific for each shipyard: only few recognized industry standards Generally not submitted to the Classification society for Approval The shipowner should request the shipyard standards for review and comment, as part of his building contract. Owners should also discuss and agree the extent of drawings for approval with the shipyard and this should include reinforcements in way of outfit items. Specific analysis for each connection detail would be time consuming, and in most cases not worth the effort.
- 5. 5 2010 Shipbuilders Meeting TSCF According to class rules some main equipment reinforcements are subject to specific approval. With reference to CSR for Double Hull Oil Tankers Section 11 chapter 3 “Support structure and structural appendages”: 3.1.1.2 This sub-section includes scantling requirements for the support structure and foundations of the following pieces of equipment and fittings: (a) Anchor windlasses (b) Anchoring chain stoppers (c) Mooring winches (d) Deck cranes, derricks and lifting masts (e) Emergency towing arrangements (f) Bollards and bitts, fairleads, stand rollers, chocks and capstans (g) Other deck equipment and fittings which are subject to specific approval (h) Miscellaneous deck fittings which are not subject to specific approval The extent of the support structures covered by the sub-section is not obvious! Shipyard standard Application of Rules: 2010 Shipbuilders Meeting TSCF Design and plan approval 2010 Shipbuilders Meeting TSCF
- 6. 6 2010 Shipbuilders Meeting TSCF Design and plan approval While CSR requires a “Booklet of Standard Construction Details” to be provided for review, this typically does not include outfitting supports (CSR Oil Tankers Sect 4/3). During plan approval the shipowner should request the shipyard to supply booklets of outfitting details (“Outfitting Booklets”) such as piping supports, mooring fittings supports, deck attachments, penetrations, etc. 2010 Shipbuilders Meeting TSCF When is outfitting work performed? At block stage During erection When modifications are required… Reworking of drawing is more cost effective than modifying already built structure The building contract should have provisions for management of changes to design and approved plans. For example, comment such as “any modification of the approved drawing is subject to approval” can be made to give the owner the opportunity to review all yard changes during production. Design and plan approval
- 7. 7 2010 Shipbuilders Meeting TSCF Inspection 13 - 2010 Shipbuilders Meeting TSCF 2010 Shipbuilders Meeting TSCF Inspection Reinforcement below and in way of outfitting is not always on the shipowner's plans Inspection should focus on compliance with outfitting booklets Whenever possible, outfitting inspections should be done at the block stage. When outfitting items are installed at a later stage, final inspection of the outfitting items and associated reinforcement structure can be done at the outfitting inspection. Don’t forget patrol inspections!
- 8. 8 2010 Shipbuilders Meeting TSCF Inspection The inspectors should check that the deck outfit matches with the underdeck structure Shipowner’s outfitting inspections include checks for: material grade and scantling, missing structure, misalignments, missing and undersize weldings, interferences, mis-cuts, proper surface preparation and painting of outfitting 2010 Shipbuilders Meeting TSCF Typical Design Considerations 2010 Shipbuilders Meeting TSCF
- 9. 9 2010 Shipbuilders Meeting TSCF Typical design considerations The necessary improvement of detail design depends on the stress level in the deck structure, i.e. increases when higher tensile steels are used. Stanchions or supports: Consider reducing or eliminating pipe sections (internal corrosion). Use open type rolled / fabricated sections such as T, L, I, X, H, etc. Align with underdeck structure (largest member is preferable) Provide additional backing structure to supports if necessary Use doublers only when appropriate, when under compression only. Use collar plates on stiffener cutouts directly beneath supports Through Fittings (Pipes, radar stands, etc.): Where possible pass fitting through deck with sleeves Ensure sleeve pipe is either seamless or if welded pipe the external weld reinforcement is ground flush. Use ring support for larger pipe (e.g. 200+ mm diameter) Tie ring support into underdeck framing system. 2010 Shipbuilders Meeting TSCF Typical design considerations Deck openings (e.g. coamings): Ensure the plate edges of the deck opening are machine cut and smooth i.e. no notches. Welding of fittings and coamings to the deck to be well clear of opening edges. Manholes, access openings etc are to be avoided in way of concentrated loads and high stress areas. Miscellaneous: Bracket toe heights to be within 10-15mm. The purpose of the bracket “nose” is only to facilitate the effective wrap weld. Use soft toe terminations When outfitting dimension is large and its stiffness is high, consider splitting the item. Check hull girder loads when working with structures longer than 3 frame spaces. For tall structures (e.g. radar mast) consider vibration loads. Lack of access (for maintenance purposes) is to be avoided.
- 10. 10 2010 Shipbuilders Meeting TSCF Typical design considerations Underdeck Backing: Transitions to extend to stiffener/frame break. Partial transitions to be avoided. Use conservative load calculations (including global and local stresses) and assume no support from the deck plate. Use deck inserts where shear loads are high. Gutter bars : Continuous gutter bars should be treated like a sheer strake. Avoid connections to the upper edge of ship side gutter bars. 2010 Shipbuilders Meeting TSCF Examples of Defects 2010 Shipbuilders Meeting TSCF
- 11. 11 2010 Shipbuilders Meeting TSCF Examples of Defects: Framework 1. Location 2. Category 3. Type of defect 4. Cause 5. Corrective Action 6. Recommendation 2010 Shipbuilders Meeting TSCF Main deck cargo line supports i.w.o. manifold area
- 12. 12 2010 Shipbuilders Meeting TSCF Main deck cargo line supports i.w.o. manifold area Location: Main deck cargo area (0,4L) Category: Design / Construction & Installation Type of Defect: Fracture Cause: Bracket toes of 50 mm, possible misalignment Corrective Action: Radius brackets, reduce the toe height to no more than 10-15mm, and support toe of bracket if on unsupported panel Recommendation: Toe height to be no more than 10-15mm. Support toe of bracket with underdeck stiffening if on unsupported deck plate and check alignment. 2010 Shipbuilders Meeting TSCF Main deck cargo line supports i.w.o. manifold area Crack at the bottom of the pipe support
- 13. 13 2010 Shipbuilders Meeting TSCF Main deck cargo line supports i.w.o. manifold area Location: Main deck cargo area (0,4L) Category: Design Type of defect: Transverse crack developed through main deck at the fillet weld toe Cause: No backing structure provided for pipe support doubler Corrective Action: Backing structure added. Recommendation: Proper backing structure is to be provided below deck or improved structure above deck is to be provided to better distribute the load. The support structure should preferably be made with open sections making alignment with under deck structure easier. Align with underdeck structure and fit well rounded brackets with soft toes, aligned with longitudinal below, with maximum toe height 10- 15mm at connection of support to deck. If necessary additional under deck supports to be made to align with brackets. 2010 Shipbuilders Meeting TSCF Midship Manifold Drip Tray Supports
- 14. 14 2010 Shipbuilders Meeting TSCF Midship Manifold Drip Tray Supports Location: Main deck cargo area (0,4L) Category: Design Type of defect: In this vessel, the manifold drip trays are reported to be cracking where their supports are welded to the upper deck. The cracks have been located where the doubler is welded to the upper deck, but have not penetrated into the cargo tank. There are 9 pairs of supports for the drip tray, all landing on web frames or transverse stiffeners. The tray is 1.8 m wide and approx. 19 m long. Cause: It is believed that this is the result of the ship’s structure bending in waves and while loading. The spill trays are quite stiff therefore large forces are created as the spill tray attempts to prevent the upper deck from moving. Recommendation: The drip tray to be constructed in several sections along the length of the drip tray to make it more flexible. The support structure to allow flexibility, while still adequate to resist the weight, wave impact, and buoyancy forces. Arrange thick phenol resin pad between the flanges on the pipe supports, and the holes in the flanges to be slotted to allow movement. 2010 Shipbuilders Meeting TSCF Thrust Stopper
- 15. 15 2010 Shipbuilders Meeting TSCF Location: Pipe Thrust Stopper Category: Design Description: Cracks occurred at the toe of the brackets Cause: These brackets were originally designed with radii and 10-15 mm toes. The stoppers were not aligned with underdeck longitudinals and were instead supported by transverse stiffeners. As stoppers were not aligned directly with longitudinals, the brackets terminated on soft plate, leading to a stress concentration. Analysis after the incident indicated that the underdeck structure was sufficient for the loads induced by the pipes, however the toes of the brackets did increase the stress in the deck. This stress concentration quickly led to cracks in an already highly stressed deck. Corrective Action: The vessel in service was retrofitted with additional underdeck support and doublers were added in way of the toes. The doubler pads reduced the stress concentration in the deck sufficiently to eliminate cracking. For the next ships, it was recommended that the stoppers be aligned with longitudinals or integral doubler plates Recommendation: Align with underdeck structure and fit well rounded brackets with soft toes, aligned with longitudinal below, with maximum toe height 10-15mm at connection of support to deck. If necessary additional under deck supports in alignment with brackets. Thrust Stopper 2010 Shipbuilders Meeting TSCF Main deck cargo line stanchion
- 16. 16 2010 Shipbuilders Meeting TSCF Location: Main deck cargo area (0,4L) Category: Design Type of defect: Cracks developed on upper deck around bracket toes of pipe line stanchions. Cause: This was a fatigue crack. Corrective action: A doubling plate was provided to reduce the stress induced by longitudinal moment. Soft toe type is also preferable way to reduce the stress level Recommendation: The support structure should align with the underdeck structure and fit well rounded brackets with soft toes, aligned with longitudinal below, with maximum toe height 10-15mm at connection of support to deck. If necessary additional under deck supports to be made to align with brackets. Main deck cargo line stanchion 2010 Shipbuilders Meeting TSCF Main deck cargo line stanchion
- 17. 17 2010 Shipbuilders Meeting TSCF Location: Main deck cargo area (0,4L) Category: Design Type of defect: Cracking of the upper deck plating at the support connection to the deck plating. Corrective action: Soft brackets to be fitted on either side of the supports and these to be in alignment with the underdeck structure i.e. the brackets to be fitted to reduce the stress concentration where the pipe support on the top side of the deck crosses the longitudinal on the underside of the deck. Recommendation: Supports should always be in alignment with underdeck structure and if possible the supports to be of a flexible design. The solution shown is one option and so far has worked. As a general comment: pipe support arrangements should be flexible and this should include the correct securing of pipes to the supports to allow the flexible couplings to work. Main deck cargo line stanchion 2010 Shipbuilders Meeting TSCF MARPOL line deck crack
- 18. 18 2010 Shipbuilders Meeting TSCF Location: Main deck cargo area (0,4L) Category: Design Type of defect: Cracking on MARPOL line deck penetration Cause: Most likely the result of deck penetrations placed closely, together with pipe movement. There are two large (855 mm diameter for 600 mm pipe) penetrations in line with each other, with a small (315 mm diameter for 150 mm pipe) penetration in close proximity. Corrective actions: Considered long term repair possibilities include: Relocate the MARPOL line penetration through the upper deck aft by 1-2 meters. Insert the upper deck with similar thickness and grade of plating. Add a transverse stiffener approx. 600 mm aft of the center of the #3 drop line, spanning the upper deck longitudinals at 840 and 1680 mm off centerline Recommendation: Allow sufficient space between penetrations. Doublers should be avoided, especially when the deck is made of Higher Tensile Steel MARPOL line deck crack 2010 Shipbuilders Meeting TSCF Penetration on the upper deck
- 19. 19 2010 Shipbuilders Meeting TSCF Cargo Tank Radar Contents Gauge Pedestal / Butterworth hatches / Crude Oil Washing Machine penetrations … Location: Main deck cargo area (0,4L) Category: Design / Construction & Installation Type of Defect: Cracking of the upper deck plating at the edges of the deck openings. Corrective Action: Penetration extended through deck plating or ensure welding is well clear of deck opening. Ensure that possible cracks are removed by the grinding; in case of larger cracks, insert to be fitted. Recommendation: Wherever possible the fittings should penetrate through the deck plating and the penetration piece be full penetration welded to the deck above and under the deck. If the fitting stops at the deck level, the deck opening plate edges to be well clear of the fitting to facilitate good full penetration welding detail. Such openings to be machine cut and smooth i.e. no notches Penetration on the upper deck 2010 Shipbuilders Meeting TSCF Pipe Support
- 20. 20 2010 Shipbuilders Meeting TSCF Location: Main deck cargo area (0,4L) Category: Design / Construction & Installation Type of Defect: Fracture Cause: Excessive stress at the bracket toes. The problem was most severe near the Midship area where the relative displacement between the hull and cargo pipes is most extreme. Corrective Action: For the vessels in operation the brackets were modified to provide a gradual radius and a toe of 10-15 mm. The brackets were also fitted with doublers to further reduce the stress concentration in the deck. Recommendation: Expansion couplings to be provided where necessary. Care should be taken to ensure the “u” bolts are fitted to allow one end of each pipe length to be flexible. The doubler should only be fitted below the tubular pipe support, not below the new brackets. The support structure should preferably be made with open sections making alignment with under deck structure easier. Align with underdeck structure and fit well rounded brackets with soft toes, aligned with longitudinals below, with maximum toe height 10-15mm at connection of support to deck. If necessary additional under deck supports to be made to align with brackets. Pipe Support 2010 Shipbuilders Meeting TSCF Vent riser supports
- 21. 21 2010 Shipbuilders Meeting TSCF Location: Main deck cargo area (0,4L) Category: Construction & Installation Type of Defect: Fracture Cause: Misalignment Corrective Action: For this particular vessel the brackets were close enough to additional above deck structure that they were fitted with a bracket above deck that linked to the adjacent riser. Recommendation: Align with the underdeck structure Vent riser supports 2010 Shipbuilders Meeting TSCF Ship side gutter bars
- 22. 22 2010 Shipbuilders Meeting TSCF Location: Main deck cargo area (0,4L) Category: Design / Construction & Installation Type of defect: Cracking of the upper deck plating in way of the ends of expansion gaps in the gutter bar or in way of expansion swedges or knuckles. Cause: Toe nose too high Corrective action: Crop and renewal of deck plating. If there are expansions arrangements along the length of the gutter (P&S) the deck plating at the toes of the gaps in the gutter bar or in way of swedges / knuckles to be inspected and dealt with as necessary. Recommendation: There are numerous expansion arrangements for ship side gutter bars i.e. gaps, knuckles, swedges etc. They should generally be avoided, and extension of sheer strake is the most preferred option. Nota: Mooring fairleads and other outfit items connected to the upper edge of ship side gutter bars in general results in gutter bar cracks. Such cracks if left unattended will eventually propagate downwards and into the deck plating. Ship side gutter bars 2010 Shipbuilders Meeting TSCF Ladder connection to oil tank
- 23. 23 2010 Shipbuilders Meeting TSCF Location: Engine room oil tank. Category: Design. Type of Defect: Crack in engine room oil tank bulkhead. Cause: Vibration Recommendation: The ladder lug should be located in way of internal stiffening structure. A bolted connection between the lug and ladder would be preferable. Ladder connection to oil tank 2010 Shipbuilders Meeting TSCF Ladder Support
- 24. 24 2010 Shipbuilders Meeting TSCF Location: Cargo area, tank bottom Category: Design / Construction & Installation Type of Defect: crack develop at the ladder connection to tank bottom Cause: No support Recommendation: Ladder should be welded on a doubling plate with backing structure below deck if the preferred bolted structure is not used. Ladder Support 2010 Shipbuilders Meeting TSCF Staging lug left from new building stage
- 25. 25 2010 Shipbuilders Meeting TSCF Location: Fresh water tank in engine room. Category: Design / Construction & Installation Type of Defect: Crack in way of the fresh water tank stiffener that propagated into the bulkhead plating resulting in a leak. Cause: Vibration induced crack. Recommendation: Such lugs should have a “softer” shape and although not structural the welding condition should be improved by a smooth and correct profile. Removing the lug should also be considered, especially at locations where excessive vibration is expected. Staging lug left from new building stage 2010 Shipbuilders Meeting TSCF Instrument pipe support in cargo tanks
- 26. 26 2010 Shipbuilders Meeting TSCF Location: Cargo area (inside tank) Category: Design Type of Defect: The transverse BHD had a crack around fillet welding to pipe support. Cause: Doubling plate had not been provided at the connection. The cause of damage is vibration. Recommendation: A doubling plate should be provided and number of supports increased. Stiffeners to be considered, especially at locations where excessive vibration is expected. Instrument pipe support in cargo tanks 2010 Shipbuilders Meeting TSCF Pipe Penetration at Pump Room Bulkhead
- 27. 27 2010 Shipbuilders Meeting TSCF Location: Cargo area (aft) Category: Design Type of defect: Crack developed along with fillet welding of pipe penetration at a bulkhead. Cause: Vibration Corrective action: Making the bulkhead stiffener arrangement “softer” did not work. The pipes were supported inside the pump room deckhouse and the pipe securing arrangement outside the deckhouse made more flexible. Recommendation: To avoid vibration damages, additional stiffener on BHD plates and additional pipe supports were recommended. The pipe supporting arrangement should be more flexible (couplings). Pipe Penetration at Pump Room Bulkhead 2010 Shipbuilders Meeting TSCF Lower corners of deckhouses on the upper deck
- 28. 28 2010 Shipbuilders Meeting TSCF Location: Main deck cargo area (0,4L) Category: Design Type of Defect: Cracking in way of the fillet welds at the corners of the deckhouse where the deckhouse connects to the upper deck and such cracks propagating into the upper deck plating. Corrective Action: The corner fillet weld to be gouged out and the location MPI tested. The fillet weld at the corners to be re-welded or if cracks are found in the upper deck plating the deck to be inserted the deck. Brackets fitted at the corners. Recommendation: Deck house sides should be aligned with under deck structures and brackets to be fitted at the corners of deckhouses / accommodation blocks etc. Lower corners of deckhouses on the upper deck 2010 Shipbuilders Meeting TSCF Submerged pump supports
- 29. 29 2010 Shipbuilders Meeting TSCF Location: Cargo area Category: Design Type of Defect: Fracture Cause: Stiffener fitted on submerged pump trunk created hard point (high distance between stiffener toe and trunk flange). Cracks developed through the trunk due to vibration of the trunk and the pump. Recommendation: Stiffener to be fitted with a soft toe bracket with bracket toe connected to trunk flange. Submerged pump supports 2010 Shipbuilders Meeting TSCF And more …. THANK YOU!