Ship Construction
structure, but the correct placing of a suitable preventer should overcome
this problem.
INITIAL TESTS AND RE-TESTS OF DERRICK RIGS To comply with
the national and class regulations ships’ derricks designed to operate as single
swinging derricks are initially tested with a proof load which exceeds the
specified safe working load of the derrick by the following amounts:
SWL Less than 20 tonnes—25 per cent in excess of SWL.
SWL 20 to 50 tonnes—5 tonnes in excess of SWL.
SWL over 50 tonnes—10 per cent in excess of SWL.
Heavy lift derricks are tested at an angle of not more than 45° to the horizontal
and other derricks at an angle of not more than 30° to the horizontal.
During the test the boom is swung as far as possible in both directions,
and any derrick intended to be raised by power under load is raised to its
maximum working angle at the outermost position.
Before the test for a heavy derrick it is usual to ensure that the vessel has
adequate transverse stability. Before, during, and after all tests it is necessary
to ensure that none of the components of the rig show signs of any failure;
and it is good practice to have a preventer rigged during the test as a precaution
against any of the span gear carrying away. On completion of the
test the heel of the derrick boom is clearly marked with:
(a) Its safe working load in single purchase.
(b) Its safe working load in double purchase, if it is designed for that purpose.
(c) Its safe working load in union purchase, if it is designed for that
purpose, the letter ‘U’ preceding the safe working load.
e.g. SWL 3/5 tonnes SWL (U) 2 tonnes
and a certificate of test and examination is issued in an approved form.
Re-tests are required if the rig is substantially modified or a major part is
damaged and repaired.
The International Labour Organisation (ILO) Convention, 152 adopted,
25th June, 1979, requires thorough examination by a competent person
once in every 12 months and re-testing at least once in every 5 years.
Shipboard Cranes
A wide range of standard shipboard general purpose cranes with safe working
loads (SWL) from 5 tonnes to 60 tonnes are found on general cargo and