เกร็�ดความร็ � ว ธี�การ็ออกแบบ PAD-EYES   An Example Of Padeye Thickness Calculation

Okay, in this occasion in I will describe an example calculation of how to determine the thickness of the pad eye. This article is a continuation of the previous, which was discussed about the formula on how to determine the thickness of the pad eye that we shall install on our basket offshore.

Calculations to determine the pad eye thickness is based on DNV 2.7-1 standard. Here I will take a sample calculation for offshore basket with a rating of 8000 kg, 4 lifting points with a lift angle of 45 °.

Based on the value rating 8000 kg then we get the value of WLL is 13 070 kg (Table 8.1 Standard DNV 2.7-1). Furthermore, from the value of WLL we can determine the value of working load limits for shackles (WLLs).

WLLs = WLL ÷ ((n-1) . cos θ)

= 13070 ÷ ((4-1) . cos 45°)

= 6158,5 kg

So, the shackles that shall be used is shackle with size 7 / 8 “(6.5 tons)

The next step is we set the value of RSL (Resulting Sling Load).

RSL = (3 . R . g) ÷ ((n-1) . cos θ )

= (3 . 8000 . 9,81) ÷ ((4-1) . cos 45°)

= 110988,5 N

Before going any further to calculate the tear out stress and contact stress, there are four parameters that we have to specify in advance:

1. Specify the material to be used for the pad eye. In this calculation example of the material used is the yield strength of A36 at value of (Re) 250 MPa.

2. Holes on the pad eye to enter pin Shackle (DH)

DH = B + (0,06 . B) , where B is the diameter of shackles pin

= 25,4 + (0,06 . 25,4)

= 26,924 mm , so we shall set the value up

= 28 mm

3. Padeye distance from the center hole to the top (H) we set in advance by trial and error. Later

we will know whether the numbers that we’ve set it in conformity with the criteria or not. For that we set H = 44 mm

4. For the thickness (t) we also specify in advance by trial and error. Numbers that we specify this formula will be inputted into the tear out stress and contact stress. Later we will know if we input the numbers it would correspond to the two criteria or not. For that we set t = 36 mm.

Once all the parameters we’ve got and we set, then we do the calculations to determine the tear out stress criteria and contact stress criteria.

Tear Out Stress Criteria

Re ≥ (3 x RSL) ÷ ((2 X H X t)-(DH X t))

Re ≥ (3 x 110988,5) ÷ ((2 X 44 X 36)-(28 X 36))

Re ≥ 332965,6 ÷ 2160

250 ≥ 154 MPa —-> OK

Contact Stress Criteria

Re ≥ 23,7 * (√(RSL ÷ (DH X t)))

Re ≥ 23,7 * (√(110988,5 ÷ (44 X 36)))

250 ≥ 248,7 MPa —-> OK

From the above calculation we can know that the criteria for tear out stress and contact stress with the material A36 is OK. Therefore padeye thickness 36 mm can be used for basket offshore with a rating of 8000 kg.

To simplify this calculation you can use Microsoft excel, especially in the determination of tear out stress and contact stress associated with H and t.

credited by....engineersknowledge.com