Load Testing of Lifeboat Davits
Offshore oil and gas platforms are dangerous places to work, and accidents can happen very easily. To alleviate any accidents due to failure of mechanical lifting equipment, these components are load tested using weights at percentages above their Safe Working Load (SWL). These weights are known as the Proof Load.
The equipment to be tested consists of runway beams and padeyes which are static load tests and lifeboat davits and pedestal cranes which are a combination of both static and dynamic load tests. These are all tested according to Safety of Life At Sea (SOLAS) and Lifting Operations and Lifting Equipment Regulations (LOLAS)
All tests are carried out by a qualified person and witnessed by the Certifying Authority and sometimes by the platform’s client.
The tests are carried out during the construction phase when there is plenty of room to maneuver the large, heavy weights around the structure.
This article is another in the series of Marine Offshore Oil and Gas and is written from my experience as a mechanical engineer in the offshore construction yards, from which I retired in 2000.
It may therefore be a bit dated but the methods used to load test and certify mechanical equipment will not have changed that much, although the legislation may well have, so it is best to check the current versions of these documents.
We shall start by looking at the procedures involved in the load testing of lifeboat davits, the test weights used and how these are fabricated and certified.
Installation of Davits
These are a vital component of the lifesaving equipment on board an offshore structure.
Normally the davits are delivered with the winches sheaves and wire ropes, these being stored until required.
Once the deck has been fabricated and all supports in place the davits can be fitted.
This is carried out by welding the steel pads on the bottom of each davit section to under-deck supports after cutting away the surrounding deck plate, rather than welding them just to the steel deck plate.
The winch which includes the motor, gearbox and drums is welded to the deck supports by the same method that was used for welding the davits down.
The wire ropes are then reeved onto the winch drums. They run from here through several sheaves which have been fitted to the deck. These sheaves are used to guide the wire rope horizontally along the deck to the bottom of the davit arms. The wire ropes are then fed upwards along the davit arms through another set of guide sheaves, to the end sheave where the ropes drop downwards over the last sheave hanging vertically. This final section which hangs down is known as the lifeboat davit falls.
The bottle screws are fitted to the wire rope end rings to enable final adjustments when the lifeboat is fitted. The ring on the other end of the bottlescrew is used to connect the falls to the lifeboat quick-release mechanism.
Certified testweights can be hired or fabricated and certified at the construction yard. I used a steel trough filled with concrete with lifting eyes at each end, one testweight for the Safe Working Load and one for the Proof Load. The weights of the testweights were checked using a load cell, which must have a current calibration certificate.
The whole operation is witnessed by the Mechanical Engineer, Lloyd’s or DNV surveyor then certified.
Before any testing is carried out, a thorough examination of the sheaves, wire ropes, winch and attachments should be carried out to ensure all these components are in working order and well lubricated and greased.
The supporting welds between the deck and the davits are tested using a Non-Destructive Test (NDT)
Once these examinations have been carried out, and welds have been passed the relative positions of the davits are measured using a theodolite, testing can commence in the following sequence,
- The Safe Working Load (SWL test) weight is attached to the davit ropes and the winch operated to raise the weight to the normal height that the lifeboat would be stowed.
- The winch brake is released and the weight allowed to free-fall a few meters before being applied again. The weight should stop and not creep downwards. Any deviation from this will require the winch brake to be adjusted.
- The positions of the davits are again measured and any deflections recorded.
- The SWL test weight is removed
- This procedure is repeated, this time using the proof load weight. (The proof load is 2.2 times the weight of a fully laden boat including equipment, plus the weight of the max number of persons it can carry.)
- The weight is removed from the davit ropes.
- The positions of the davits are again recorded using the theodolite and any deflection noted.
- The supporting welding is then subjected to post test NDT, and the data recorded.
- It usually takes 24 hours for the certificates of the various tests to be completed, and once they are obtained and checked they will be placed in a davit load-test certification package which will form part of the lifeboat and davit commissioning package. This will go offshore with the structure at sailaway.
Certification Package Contents
This consists of the following,
- A dynamic load-test certificate for the davits, signed by the mechanical eng and surveyor.
- The deck structural drawing showing the location and Equipment I/D. of the davits.
- The certification for the testweights and load cell
- A sketch showing position of davits before, during and after load-test
- A NDT report showing the condition of the supporting welding before and after the load-test.
A certain amount deflection of the davits is expected but they should return to their original position once the loads have been removed. The NDT report should confirm no cracks in the support welds, and a thorough visual check of the winch, sheaves, and ropes should be carried out.