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Subject on This Issue:
* Steels & Properties
* Heat Treatment


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Material Selection for Leg Guides

The leg guides are submerged in seawater during the offshore operation. It was reported that severe wear on the lift legs rather than on the leg guides was observed. It is undesirable because the leg guides can be more easily replaced than the lift legs. The objective of material selection was to identify a material for the leg guides to meet the following specific requirements:

  • Less wear resistance than the lift legs
  • Appropriate corrosion-resistant property

Criteria for selection

In order to meet the above requirements, the material selected should be:

  • softer than the lift leg
  • appropriate corrosion-resistant in seawater
  • low material and processing costs consistent with adequately long wear life under application conditions
  • adequate strength and toughness to prevent premature failure
  • available in local market

Metal hardness is one of the most important characteristics to consider. Comparisons of hardness values can provide an approximate guide to relative wear behaviour among different metals, especially in metal-to-metal sliding and abrasive application. The lift leg is made of EH-36 steel and its composition is given in Table 1.

Considerations for material selection

A.            Hardness test of a lift leg

Hardness test was conducted on a lift leg sample and the test results are shown in Table 2.

B.            Seawater

Galvanic corrosion in seawater is quite possibly a major concern because two dissimilar materials were used and the corroding medium has a fairly high conductivity. A practical galvanic series for metals and alloys in seawater is given in Table 3.

In a structural joint or contact, the ratio of the areas of two dissimilar metals has enormous influence on the corrosion rate of one of the numbers of the joint. In general, similar material coupling results in less corrosion.

Material selection

Based on the hardness test results, the corrosive behaviour of metals in seawater and the above criteria for material selection, the materials in Table 4 are recommended in order of selection priority. 

Table 4  Recommended materials in order of selection priority

 
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