Outfitting Related Structural Defects

This post is a summary of a Tanker Structure Co-operative Forum (TSCF) paper which reviews the outfitting defects of tankers and provides an analysis on potential causes and repair proposals. Service history of 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. Outfitting reinforcements should be considered as part of the ship structure because a failure of this kind of local reinforcement may affect even the primary structure and cause a critical failure.

Generally outfitting reinforcements are designed taking into account static loads on outfitting items. Reinforcements fitted under deck are fit mainly for this purpose. Smooth transition is desirable. Any stress concentrations created by local reinforcement must be considered and addressed in the design. Hard points have to be avoided.

As reinforcement below outfitting are included in the hull structure they will also be subject to hull girder load, pressure load, vibration load, etc. Outfitting reinforcements are usually not checked against these loads and the normal practice is to apply “shipyard standard” and/or “recognized industry standard” so that they conservatively cope with ship loads. This includes proper connection of the reinforcement to the ship structure and/or effective adaptation and/or modification of the ship structure in way of the outfitting item.

Good workmanship is also a critical issue because 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 and education of the outfitting workers in term of steel structure quality is very important.

New-building and repair process are different challenges. Where design can be easily adapted at the design stage to select the most efficient as well as cost-effective configuration, compromises have to be done at repair stage to minimize changes and make repair reliable for working onboard where scaffolding, parts weight, etc are critical issues.

Basic hull structure design typically precedes outfitting design. Hull plans for approval may not include all reinforcements below and in way of outfitting. Moreover late changes (e.g. change of supplier) may not be reflected in hull structural design details (e.g. underdeck stiffeners). 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. Reworking of drawing is more cost effective than modifying already built structure.

As reinforcement below and in way of outfitting structure may not be on the shipowner’s plans, the only opportunity to check compliance with the “outfitting booklets” is during inspection. Inspection should not only focus on compliance with approved drawing but should also focus on compliance with the “outfitting booklets”. At the block inspection stage outfitting reinforcement may already be fitted, so the inspector has the opportunity to check compliance of the reinforcement with the “outfitting booklets”. 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. Whenever possible, outfitting inspections should be done at the block stage. The deck outfit must match with the underdeck structure. Patrol inspections are also a good way to check compliance. The shipowner’s outfitting inspections should include checks for:

Material grade and scantling,
Missing structure,
Misalignments,
Missing and undersize weldings,
Interferences,
Mis-cuts,
Proper surface preparation and painting of outfitting.

Below are a few examples of outfitting defects. The TSCF paper contains 27 examples of outfitting defects with information about the location and the type of defect, its category and what caused it, what needs to be done to correct the defect as well as additional recommendations.

Thrust Stopper

Location: Pipe Thrust Stopper

Category: Design

Type of defect: Cracks occurred at the toe of the brackets.

Cause of defect: 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. Fracture due to stress concentration. As stoppers were not aligned directly with longitudinals, the brackets terminated on soft plate, leading to a stress concentration. Analysis carried out 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 in this area. For the follow ships in this series, 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 to be made to align with brackets

Crack at the pipe stopper connection to main deck toe

High stress concentrations in way of damaged cargo pipe thruster stoppers caused by hull girder bending stress

Corrective action applied

Main Deck Cargo Line Stanchion

Location: Main deck cargo area

Category: Design

Type of defect: Cracks developed on upper deck around bracket toes of pipe line stanchions

Cause of defect: 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

Cracks that have been developed on upper deck around bracket toes of pipe line stanchions

Corrective action applied

Bracket Toe Heights

Location: Main deck cargo area

Category: Design / Construction & Installation

Type of defect: Upper deck plating cracks at the toes of miscellaneous brackets on the upper deck

Cause of defect: Excessive toe height. Bracket toes need only be a sufficient height to facilitate the bracket return welding

Corrective Action: The upper deck to be inspected and if paint disturbance is noted in way of bracket toes etc any attached scale to be removed and the deck plating NDT tested. If deck plate cracks are found this will require local inserts. The shape of brackets with excessive toe “nose” heights should be altered to reduce the height of the bracket “nose”

Recommendation: Bracket toe height to be as small as possible (10 – 15 mm). Connecting the bracket and the pump foundation by a common stiffener type bracket should be considered when as closely fitted as in this case. Bracket on the pump foundation should also be considered