February 2, 2016

Offshore crane

Two standards that offer guidance in developing minimum offshore crane requirements are API Spec. 2C and EN13852-1. They represent Gulf of Mexico and the North Sea respectively, while the rest of the world employs one of these two to size an offshore crane up.

API is widely used in benign environments, outside of Gulf of Mexico, which the standard considers to be of “very mild sea and wind conditions” (cf. §5.4.4 Legacy Dynamic Method), excluding hurricanes, of course. Notwithstanding this self-assessment, there have been projects and locations that continue to use API, when it may be more sensible to use the EN13852-1 instead in harsher environments — tropical or otherwise.

I have been poring over the two standards last week in order to understand how comparable they may be for a general application, and I thought, the quickest way to test the two would be to look at their off-board lift prescriptions, and in particular, their recipes for minimum required hoisting velocity. This, as API standard explains, as “the hoisting velocity at the elevation where lift is initiated (i.e., supply boat deck level) shall be fast enough to avoid re-contact after the load is lifted.” So, over the weekend, I wrote a little code (see below) to produce the following graph.

Crane (offboard) hoisting velocity

A couple of observations:

  1. API prescription for Vhmin covers supply vessels, while EN offers two additional options, viz., barge, and sea surface. (Graphs for barge or sea surface are not shown in the above, but can be separately generated, if desired, using the code I’ve shared.)
  2. EN is explicit about reeving efficiency — note the reduced velocity requirements in the case of multi-fall reeving. Looking at the two graphs above, i.e., for API and for EN, the multi-part reeving option appears to be somewhat close to API prescription in terms of Vhmin. It could be that API considers multi-part reeving efficiency in its prescription, even though it does not mention in §5.4.5.2.

I am curious to know, in general, how crane vendors cater to North Sea-like or worse environments with API’s prescriptions. Do they simply go for a faster motor, apply reeving efficiency, or do they also review crane’s spring factor in combination to ensure optimum crane performance? What else?

Here’s that code:

#!/usr/bin/env python
# -*- coding: UTF-8 -*-
"""
crane.py: Offshore crane: min. hoisting velocity
2016 ckunte

Feb 10: Conditional API curve fixed
Feb 16: Minimum code retained for graphs
"""
import numpy as np
import matplotlib.pyplot as plt

# -- OFF-BOARD LIFTS --
# x corresponds to significant wave height (Hsig)

def minhvelo():
    # -- API Spec. 2C 7th Ed --
    # Min. (required) hoisting velocity, m/s (§5.4.5.2)
    cf = 0.3048 # ft to m conversion factor
    x1 = np.arange(0.0, 1.83, 0.001)
    x2 = np.arange(1.83, 3.0, 0.001)
    Vhmin_api1 = (0.033 * cf) + 0.098 * x1
    Vhmin_api2 = 0.067 * (x2 + (3.3 * cf))    
    x = np.arange(0.0, 3.0, 0.001)
    # -- EN 13852-1:2013 --
    # Table B.2: Load supporting deck velocity, Vd (m/s)
    Vd = [0, (3.2 * x / (x + 13.5)), (4.0 * x / (x + 7.0)), \
    (6.0 * x / (x + 8.0)), (5.3 * x / (x + 5.5))]

    # Table B.3: Crane boom tip velocity, Vc (m/s)
    Vc = [0, (0.25 * x), (0.5 * x)]

    # Table B.4: Velocity factor K_H
    # K_H structure: [NL & SFR, RC & SFR, NL & MFR, RC & MFR]
    K_H = [0.65, 0.50, 0.40, 0.28]

    # Hook velocity 
    # Crane on bottom supported structure
    ## Lifting to/from: Supply vessel (At rated capacity, Single fall reeving)
    VH_f_sv_rc_sfr = K_H[1] * (Vd[3]**2 + Vc[0]**2)**0.5
    ## Lifting to/from: Supply vessel (At rated capacity, Multiple fall reeving)
    VH_f_sv_rc_mfr = K_H[3] * (Vd[3]**2 + Vc[0]**2)**0.5

    fig, ax = plt.subplots()
    # -- EN13852-1 --
    # Crane on bottom supported structure
    # Legend: NL: No load; [S,M]FR: [Single,Multiple] fall reeving 
    ax.plot(x, VH_f_sv_rc_sfr, label="EN: From supply vessel (RC, SFR)")
    ax.plot(x, VH_f_sv_rc_mfr, label="EN: From supply vessel (RC, MFR)")
    # -- API --
    ax.plot(x1, Vhmin_api1, color='black', label="API: From supply vessel")
    ax.plot(x2, Vhmin_api2, color='black')
    # ---
    ax.legend(loc=0)
    ax.set_xlabel('Hsig (m)')
    ax.set_ylabel('Vhmin (m/s)')
    plt.savefig("Vhmin.png")
    plt.show()

if __name__ == '__main__':
        minhvelo()

Update (Feb 18): Certain operators override minimum provided API/EN hoisting velocities in their engineering practices, such as 1.0m/s, which is off the chart above. Crane power source, hydraulic pumps and winches (to be sized for hydraulic flow rate) are cited as factors influencing minimum hoisting velocity.

What is not clear to me though is if there is room for improvement in suitably sizing the above, while ensuring adequate hydraulic flow rate, which could eventually result in cost savings.