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g Hawsers
TTI offers specialist services to operators of Single Point
Mooring terminals with particular
emphasis on optimisation of mooring hawser replacement
strategies.
Mooring system design / re-design Hawser selection and
specification
Establishment of hawser retirement criteria
Fatigue life and residual strength prediction
Hawser failure investigation and analysis
Inspection and testing of hawsers
Deep Water Moorings
The engineering of vessel moorings in very deep waters is
challenging and requires innovative
solutions. Novel arrangements and synthetic materials
characterise deepwater mooring solutionsand TTI are at the
forefront of this technology having lead or contributed to most of
the recent
collaborative research projects in this area.
Fibre Tethers 2000. DEEPSTAR Fibre Rope Mooring Trial.
Engineers Design Guide for Deepwater Fibre Moorings.
Large Scale Testing of Fibre Mooring Ropes
Durability of Polyester Mooring Ropes
There are two particular concepts, which adopted separately or
combined, have proved to offer
considerable scope for mooring in very deep waters:
Taut Leg Moorings
Synthetic Rope Moorings
Taut Leg Moorings
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TTI defines a taut leg as one where the line rises from the
anchor under normal pretension, so
there is no grounded line to lift off the seabed as line tension
increases. Because of this, taut
moorings work in a fundamentally different way to catenary
moorings, their compliance derivingmainly from the elastic stretch
of the lines, whereas a catenary mooring derives its compliance
from the change in suspended line weight.
A taut mooring leg will usually have an angle of between 30 and
45 degrees to horizontal at the
vessel (determined from design considerations) and will exhibit
fairly linear load-excursioncharacteristics which are related to
this angle and the stiffness of the line.
Because taut moorings have a much more linear stiffness than the
progressively stiffening
catenary systems, offsets under mean load can be better
controlled and vessel motions do not
induce such a high proportion of total line tensions.
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A further advantage is the better load sharing between adjacent
lines than is typical of a catenary
array, so improving overall efficiency of the system.
The taut mooring also overcomes another limitation of deepwater
catenarys by having muchshorter lines. Scopes of under 2, compared
to 5 or more with catenary systems, save material and
give a more compact seabed footprint.
The lines of a taut mooring need to have sufficient elasticity
to absorb the vessel wave motionswithout over loading. Where
motions are low either because of benign wave environment (eg.
West Africa or Brazil) or because of the vessel characteristics,
(as a SPAR for example), wire
ropes may be appropriate, particularly in deep water where there
is the line length to developenough stretch. However, the weight of
steel mooring wires in very deep waters may become too
much of a penalty on the vessel payload and, furthermore,
strength utilisation is reduced as more
of the line tension capacity will be used in keeping the wire
taut.
Synthetic Rope Moorings
Synthetic fibre ropes offer a potential solution to the weight
problems of using steel mooringlines in deepwater as they have a
very low weight in water.
Also, compared to steel, there are a vary large number of
synthetic fibre material compositions
with a wide range of material properties. A synthetic rope can
therefore be designed to have
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properties that match the mooring requirements.
Materials that have potential for mooring line application
include:
Nylon Polyester
Aramid (Kevlar)
Liquid Crystal Polymer
High Modulus Polyethylene (HMPE)
Yarns of these synthetic materials may be built into ropes using
a number of constructions, some
of which are suited to particular fibres.
Some of the issues which affect choice of yarn material and the
rope construction are:
Load-extension behaviour Internal heating
Durability & fatigue endurance
Termination & end fittings
Wear & abrasion
Handling & installation
Synthetic ropes can be used as a lightweight insert in a
catenary mooring (usually having a chaincatenary segment touching
down on the seabed). However, their real potential lies in taut
moorings where ropes combining low weight and low elastic
modulus with good durability
characteristics, will enable efficient mooring systems to be
designed for a whole range of water
depth and environmental conditions.
The currently favoured synthetic material is polyester for
reasons of cost and good fatigue
endurance. Polyester has also been used in rope constructions
for several years, and the issues of
manufacture and end termination for large rope sizes have been
satisfactorily resolved. Polyesterropes also have elasticity in the
range most suitable for taut mooring of vessels with typical
wave
motions in very deep and ultra deep water depths up to around
1500 metres (5000 feet).
OPTIMOOR
TTI have developed an advanced computer program OPTIMOORfor
analysing the complex
behaviour of moored vessels under any environmental conditions.
The software enables rapidassessment of moorings to OCIMF
requirements and is licensed to port operators worldwide.
Both quayside and offshore moorings are handled by OPTIMOOR.
http://www.tensiontech.com/tools_guides/rope_constructions.htmlhttp://www.tensiontech.com/software/optimoor.htmlhttp://www.tensiontech.com/software/optimoor.htmlhttp://www.tensiontech.com/software/optimoor.htmlhttp://www.tensiontech.com/software/optimoor.htmlhttp://www.tensiontech.com/tools_guides/rope_constructions.html
