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.
A couple of observations:
- 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.)
- 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 §18.104.22.168.
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?
Update (Feb 2016): 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.
Update (Dec 2018): IOGP specification S-6181 issued in Dec 2018 now fixes the low velocity in API’s offboard lift recipe, which stuck out like an anomaly2 (see plot above), and is now in step with that of EN 13852-1, which can be seen in the updated plot below.
Code: Scripts to generate these plots are at pyplots/crane.
IOGP S-618 Supplementary Specification to API Specification 2C Offshore Pedestal-mounted Cranes. ↩
To be sure, my interest in API Spec 2C is now academic, and I am not judging it for its merits or demerits, since API’s application domain is primarily the US. However, due to the fact that API standards are generally good to very good that the engineering community tends to adopt them across domains (outside USA) — sometimes without pausing to consider whether or not cross-domain application is appropriate. ↩