A derrick pole failure investigation

Introduction

A derrick pole on board a boat hoists a lifeboat on and off the ship in open water. A winch attached to the rigid vertical member carries the cable allowing for hoisting of the lifeboat. The vertical member allows for rotation of the derrick pole i.e. to swivel between the deck and water (Figure 1). The derrick has a safe working load of 1 ton. During a hoisting operation, the derrick pole failed catastrophically, releasing its load.

Figure 1: Schematic of Lifeboat Derrick

Objectives

An investigation was required to determine the root cause of the failure of the derrick pole.

Investigation

The various components of the derrick together with the failed rigid vertical member were inspected (Figure 2). The swivel shroud locates with the swivel hub on the derrick base, shielding the swivel from debris. The corresponding fracture surfaces are shown in Figures 3 and 4. The surface is indicative of a unidirectional bending failure occurring under the action of a high nominal stress. The majority of the failure surface is rough, indicating fast fracture. No evidence of fatigue failure could be found on the fracture surface.

Figure 2: Lifeboat Derrick
Figure 3: Fracture surface and swivel shroud
Figure 4: Fracture Surface and base

The failure initiated at the point where the swivel shroud attaches to the vertical member. This is a stress concentration point due to the change in shaft diameter (Figure 5).  A cross section through the weld site revealed considerable porosity (Figure 6). This porosity increases the effect of the stress concentration.

Figure 5: Failure site cross section and weld copy
Figure 6: Failure site cross section showing weld porosity

Results

The material composition of the vertical member was analysed. The material was found to be an AISI1040 or EN8 designation (BS970080M40). This was used to determine the mechanical properties of the material, shown in Table 1, to be used for the stress calculations.  A hardness test confirmed that the material was in the normalised condition.

 

Table 1: Mechanical Properties of AISI 1040 Steel:

Yield stress (MPa) 290-525
Fracture Toughness (Mpa√m) 50-80

 

A lifeboat quick release system was in place to release the load before damaging connected equipment. Tests certificates for the lifeboat and quick release system show that the load limit for the quick release is double that of the derrick. This means that the derrick would fail before the quick release. The manufacturer’s specifications show a maximum allowable load in the boat of 900 kg. Taking into account the boat and motor mass, there is a potential overall maximum load of 1050 kg on the derrick.

 

The maximum load that the lifeboat could apply to the derrick pole by way of its normal weight is below the tensile strength of the derrick pole material. Therefore such a lifting operation would not break the derrick pole.

 

It was reported that the vessel listed under the action of a heavy swell.  This caused the strapping and hoisting rope to become slack as the life boat touched the water.  Further listing then created a shock load in the hoisting wires.  This in turn would transfer a shock load to the derrick pole. A shock load can translate as much as two to three times the load applied under static conditions. The derrick pole was therefore not conservatively designed to accommodate even the weight of the lifeboat particularly under conditions where shock loading may occur.

 

Given the fracture toughness and the flaws observed in the weld, calculations show that the derrick pole would yield before fracture. Therefore, the flaws in the weld are not significant to the failure.

Conclusions

  • The failure surface is indicative of a fast fracture due to the application of a high nominal stress, probably under shock loading conditions. Failure by fatigue is excluded.
  • The shock loading of a mass in excess of 275 kg is likely to result in failure of the derrick pole, leading to brittle fast fracture.
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