HSE information sheet
Jack-up (self-elevating) installations: rack phase difference
Offshore Information Sheet No 4/2007
Contents:
Introduction ................................................................................................................................... 2
Background .................................................................................................................................. 2
Actions.......................................................................................................................................... 2
Relevant legal requirements ......................................................................................................... 2
Further information ....................................................................................................................... 2
Introduction
This information sheet draws the industry's attention to previous findings on rack phase difference
(RPD) in truss legs of jack up (self-elevating) installations. This information sheet replaces Safety
Notice 4/2002 (SN4/02) (issued August 2002) which has now been withdrawn.
Background
Several incidents have occurred on the United Kingdom continental shelf (UKCS) over the past
two years, which have resulted in damage to bracings of truss framed legs of jack-up installations.
To date this is only known to have occurred on those using fixation systems.
On at least two occasions the initiating cause was eccentric spudcan support due to hard, uneven
seabed conditions. However, more importantly, the common cause of failure was the introduction
of an imbalanced loading in the leg chords, producing a racking effect in the bracings (commonly
referred to as RPD), which subsequently caused their failure in compression. The full extent of the
imbalance was only realised during jacking.
Actions
Duty holders must ensure that on-board operating procedures prescribe appropriate limits to which
the installation can be operated to prevent this phenomenon. Such limits should address all
relevant failure modes. The limits may be established from the original design specification;
however duty holders must ensure that they remain appropriate for each specific site.
Duty holders must ensure that the prescribed limits are not exceeded, and that suitable monitoring
arrangements are in place, such as pinion load monitoring or measurement of RPD.
Relevant legal requirements
The relevant legal requirement is:
• The Offshore Installations and Wells (Design and Construction, etc) Regulations 1996,
regulation 7.
Further information
Any queries relating to this notice should be addressed to:
Health and Safety Executive
Hazardous Installations Directorate
Offshore Division
Lord Cullen House
Fraser Place
Aberdeen
AB25 3UB
Tel: 01224 252500
Fax: 01224 252648
This information sheet contains notes on good practice which are not compulsory but which you
may find helpful in considering what you need to do
