Derrick Pole Failure

Introduction

A derrick pole on board a boat hoists a lifeboat on and off the ship in open water. The vertical member allows for rotation of the derrick pole. During a hoisting operation, the derrick pole failed catastrophically, releasing its load.

Investigation

The various components of the derrick together with the failed rigid vertical member were inspected (Figure 1). The fracture surface (Figures 2) 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. 1 (Custom)

Figure 1: Lifeboat Derrick

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. 2 (Custom)

Figure 2: Fracture Surface and base

What We Found

Results

Compositional analysis of the vertical member found the material to be an AISI1040 or EN8 designation (BS970080M40). A hardness test confirmed that the material was in the normalised condition. The load limit for the life boat quick release system is double that of the derrick i.e. the derrick would fail first. 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. It was reported that the vessel listed under the action of a heavy swell, creating a shock load which is transferred 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.

Conclusion

  • 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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