lifting appliance for submarine vehicle

[EricFrance]

Hello,
I am a french engeneer,
I must design a "gantry swivel" for submarine vehicle.
I want to calculate the "DAF" using a standard.
The load is 35 kN.
the customer asks for a sea state 3.
I saw IN DNV the following equation :
DAF = 1 + Vr (C / W.g)e0.5.
with Stiffness factor = 2333 kN/m
what is the value of H1/3 for sea state of 3 (0.5m or 1.
25 m)?
I find a DAF 3.5 (with H1/3 = 1.25); Is this normal?
Thank you for your response (sorry for my english).

 

[Ussuri]

Which DNV code are you looking at? Rules for classiifcation and certification of submersibles? Rules for Planning Marine Operations?

The lifting codes usually have a DAF of 1.3 for an in-air lift. For a subsea lift it is usally more than this. It is dependent on velocties and accelerations from a number of factors, the wave for the seastate you are deploying in, raising/lowering velocity of the crane, crane stiffness, vessel motion.

As a rule of thumb, I believe our ROV department use a DAF of 3 for designing ROV launch A-frames

 

[EricFrance]

Ussuri,
I have two codes :
- [1]Pr En 13852 Offshore Cranes,
- [2] DNV Certification of lifting Appliances.
I am agree with you for DAF >=1.3 (Offshore Crane, load lifted and moved on the shipboard, DNV page 25, B304).
But I need calculate the DAF when the vehicle is picked (vehicle filled of water, oscillating at the end of the umbilical,).
With DNV [2] I can calculate Vr (B304, page 25, relative velocity between load and hook) :
Vr = Vl + (Vin2 + Vt2)e0.5.
How calculate the good value for Vin (downward velocity) and Vt (velocity from motion of the crane) for seastate 3 ?
Is it correct to take H1/3 = 1m, and calculate with formula Vin = 0.6 Hsign (page 25, B306) ?
If ok, Vin = 0.6*1 = 0.6 m/s,
In the Code EN 13852-1, Vin is equal to :
- semi-submersible plateforme : Vin = (3.2*Hsign)/(Hsign+13.5).
If ok, Vin = (3.2*1)/(1+13.5) = 0.22m/s
Between DNV, and EN 13852, the value of "Vin" are very different ? this is normal ?
Thank you for your response.
Eric-France-Toulon

 

[Ussuri]

I'm not familiar with either of those codes, so not sure I can be of much help with the specifics of how to apply them.

Typically a DAF of 1.3 is used in air, and for checking the structure/rigging you are lifting. For designing the lifting device the DAF to be used is typically higher.

The subsea DAF is usually taken as (submerged weight + hydrodynamic loads)/submerged weight. This is calculated and depends on the object lifted. If you are designing the lifting appliance then you wont know this.

Is Vin to take account of the hydrodynamic effect or the operational, raising or lowering effect? The wave vertical velocity will vary with depth.

One thing to consider, who will be certifying the system (reviewing the design)? If it is DNV you should stick with their rules.

 

[lancer360]

I have been designing lifting appliances for ROV's for over 10 years now and I am currently a consultant. I have gone through the certification process numerous systems with ABS and with DNV. I would get your hands on the following three specifications as I have found that all three have some gaps. By understanding all three, you can use one as a primary based on your clients and/or government requirements and use the other two as a basis for educated decision on the missing design criteria.

1. DNV Certification of Lifting Appliances
2. ABS Rules for Building and Classing Underwater Vehicles, Systems and Hyperbaric Facilities Appendix 4: Certification of Handling Systems.
3. Lloyds Certification of Lifting Appliances in a Marine Environment

In the past DNV has not specified a specific DAF for use with lifting UNMANNED objects/vehicles out of the water with a dedicated device such as a gantry or a-frame. The formulas provided by DNV are typically used for lifting equipment on and off supply vessels. These calculations do not include entrained mass from the water. I typically use a DAF of 2 and DNV has accepted this. It will be stated on the design certification from DNV what the safe working load and the DAF are.

I do go a step further in an analysis and include vessel accelerations on the mass of the lifting structure. DNV also does not dictate this, but ABS does at X=+/-0.75G, Y=-0.25/-1.75, and Z= +/-0.75G. "Y" is vertical with a -1G being gravity and a variance of +/-0.75. Unless we have vessel specific acceleration data, these are the numbers we use. ABS however, does not differentiate between vessel accelerations and a DAF. In the past ABS has told me to use the same 1.75 as the DAF.

Lloyd's does specifically refer to DAF's for lifting equipment in and out of the water. If I remember correctly, it came up to something like 2.04 which confirmed our original thinking of a DAF of 2 was acceptable. However, you should still use some judgment if you are going on a smaller vessel in North Atlantic/North Sea type environment and consider going higher on the DAF.

 

[EricFrance]

Thank you for your response.
I continuing my study for design the A-Frame.
I seen the codes : ABS, DNV, and PR EN 13852.
Effectively, This is not obvious to use.
I confirm the DAF=2 for sea state = 3 when i use the code PR EN 13852 (offshore crane).
The horizontal and lateral forces are not given in the code DNV "Lifting appliances."
In the code ABS, acceleration values are: X = +-0.75g; But the sea state is not indicated (3,4,5 ?).
In the code EN 13852, acceleration values are : 0.5 at 1m/s2 for H1/3 = 2m.
I will continue my research.
Thank you very much for your guidance. They will be very useful to me.
-EricFrance-