42
LNG
INDUSTRY
MARCH
2016
The solutions were designed using proven technology and
components of conventional onshore loading arms, supported
by 50 years of LNG transfer experience.
Side-by-side offloading
Technology challenges
When moving to nearshore and offshore locations, the loading
arms are installed between two floating structures, requiring
the replacement of traditional stress analysis with complex
non-linear methods to design and ensure the integrity of both
the loading arms and the LNG carrier manifolds during operation.
The fatigue caused by dynamic motions is a key consideration
in the design of the loading arms. It impacts the design of the
structural bearings and swivel joints, as well as the sizing of the
welding nets.
Assistance must be provided to allow full capability and
availability of the transfer system during the connection and
disconnection of the manifolds to the LNG carrier. The patented
targeting system developed by FMC Technologies consists of a
rope stretched at constant tension between the base riser of the
loading arms and the LNG carrier manifold. This guides the
loading arm, which is placed in passive mode during the
connection phase. A winch fitted at the end of the loading arm
drives the loading arm up to its final connection to the LNG
carrier manifold.
The control system has been rethought with care. In
conventional onshore LNG transfer, the loading arms use
proximity switches to detect the maximum extension and the
slewing limits. Meanwhile, a SIL 2 PLC manages the Emergency
Shutdown (ESD) sequences. A Position Monitoring System
(PMS) is used in parallel to monitor the position of the loading
arm coupling flanges.
The control system developed for offshore application
enforces a SIL 3 PLC and analogue sensors replace the proximity
switches. As a result, the operating envelope shape can be fully
customised in the Constant PMS (CPMS) and reflect exactly the
operating envelope defined during the engineering phase, which
comes from the outputs of the naval studies performed by the
engineering, procurement and construction (EPC) contractor. The
design of the loading arms is validated in stress and fatigue at
every location of the operating envelope. Thus, such CPMS
set-up ability is key to ensuring that the loading arms will never
remain connected to the LNG carrier manifold when outside of
this operating envelope. This system also enables the
implementation of safeguards to limit human errors.
Finally, the involvement of classification societies at an early
stage of the project has an impact on the design and execution
schedule.
FLNG: a new milestone
The transition from onshore to offshore operations has been
successful for the first FSRU/GBS projects and is close to
reaching a newmilestone with the upcoming deployment of
the first floating LNG (FLNG) projects. FMC Technologies has
supplied loading arms for Shell’s
Prelude
, Petronas’
FLNG
1
and
FLNG 2
and the Golar
Hilli
projects. The loading arms have
been mechanically completed and entered the start-up and
commissioning activities for the
Prelude
and Petronas
FLNG 1
projects whose first gas is planned for the next 12 to 18 months.
These floating facilities will operate in exposed
environmental conditions (typically up to 2.5 mHs), under which
it has been demonstrated that side-by-side offloading remains a
safe and viable operation. The excellent track record of marine
loading arms in the LNG industry – as well as the ability to
distribute LNG to conventional LNG carriers – naturally led to the
development of offshore loading arms.
Offshore systems and
maximised operability
Such specific operations require particular attention with regards
to the handling and maintenance of loading arms.
A variety of solutions are available to help conventional LNG
terminal owners install the loading systems, including the
following:
Training: almost every incident reported in the last 10 years
has highlighted the responsibility of personnel. Perfect
understanding and control of the equipment is key to
ensuring safe and continuous operations. Classrooms
and hands-on training sessions are proposed for loading
arms operators, as well as for electric, instrumentation and
mechanical maintenance supervisors.
Spare parts and maintenance: the maintenance programme
of the loading arms shall be defined at an early stage of the
project execution phase. On some projects, a fourth loading
arm has been considered for redundancy purposes. The
maintenance activities have to be coordinated by skilled
supervisors.
Such aftermarket programmes are managed through regular
specialist site interventions on import/export terminals, FSRUs
and GBSs, to maintain equipment performance and operator
competence.
When producing LNG offshore (FLNG), the consequences of
loading arm downtime can become critical in a short period of
time, because of continuous operations and limited LNG storage
capacity. The increased duration required to reach the floating
unit must also be taken into account. Full time assistance
onboard the floating unit is a new approach to the operation and
maintenance management that is seen in upcoming FLNG
projects. FLNG operator companies are particularly interested in
maintaining the offloading function along the FLNG operation
life, hence encouraging the loading arm supplier to remain fully
involved in the maintenance of the equipment after delivery.
Figure 2.
Offshore loading arms of Adriatic LNG.
