Abstract: Offshore floating structure under the various environmental conditions maintains a fixed position by mooring system. Environmental conditions, vessel motions and mooring loads are applied to mooring lines as the dynamic tension. Because global responses of mooring system in deep water are specified as wave frequency and low frequency response, they should be calculated from the time-domain analysis due to non-linear dynamic characteristics. To take into account all mooring loads, environmental conditions, added mass and damping terms at each time step, a lot of computation time and capacities are required. Thus, under the premise that reliable fatigue damage could be derived through reasonable analysis method, it is necessary to reduce the analysis cases through the sensitivity studies and appropriate assumptions. In this paper, effects in fatigue are studied for spread mooring system connected with oil FPSO which is positioned in deep water of West Africa offshore. The target FPSO with two Mbbls storage has 16 spread mooring lines (4 bundles x 4 lines). The various sensitivity studies are performed for environmental loads, type of responses, vessel offsets, mooring position, loading conditions and riser behavior. Each parameter applied to the sensitivity studies is investigated from the effects of fatigue damage through fatigue analysis. Based on the sensitivity studies, the following results are presented: Wave loads are more dominant in terms of fatigue than other environment conditions. Wave frequency response causes the higher fatigue damage than low frequency response. The larger vessel offset increases the mean tension and so it results in the increased fatigue damage. The external line of each bundle shows the highest fatigue damage by the governed vessel pitch motion due to swell wave conditions. Among three kinds of loading conditions, ballast condition has the highest fatigue damage due to higher tension. The riser damping occurred by riser behavior tends to reduce the fatigue damage. The various analysis results obtained from these sensitivity studies can be used for a simplified fatigue analysis of spread mooring line as the reference.
Abstract: Motion response of floating structures is of great
concern in marine engineering. Nonlinearity is an inherent property
of any floating bodies subjected to irregular waves. These floating
structures are continuously subjected to environmental loadings from
wave, current, wind etc. This can result in undesirable motions of the
vessel which may challenge the operability. For a floating body to
remain in its position, it should be able to induce a restoring force
when displaced. Mooring is provided to enable this restoring force.
This paper discusses the hydrodynamic performance and motion
characteristics of an 8 point spread mooring system applied to a pipe
laying barge operating in the West African sea. The modelling of the
barge is done using a computer aided-design (CAD) software
RHINOCEROS. Irregular waves are generated using a suitable wave
spectrum. Both frequency domain and time domain analysis is done.
Numerical simulations based on potential theory are carried out to
find the responses and hydrodynamic performance of the barge in
both free floating as well as moored conditions. Initially, potential
flow frequency domain analysis is done to obtain the Response
Amplitude Operator (RAO) which gives an idea about the structural
motion in free floating state. RAOs for different wave headings are
analyzed. In the following step, a time domain analysis is carried out
to obtain the responses of the structure in the moored condition. In
this study, wave induced motions are only taken into consideration.
Wind and current loads are ruled out and shall be included in further
studies. For the current study, 2000 seconds simulation is taken. The
results represent wave induced motion responses, mooring line
tensions and identify critical mooring lines.
Abstract: Cyprus- offshore aquaculture industry has promising
prospects taking into account that Cyprus is an island. Its production
trend is increasing overtaking bigger countries such Greece and Italy.
However, current mooring systems seem to be under-performing
acting as obstacles for its future development. Furthermore, shallow
coastal waters scarcity due to competing industries dictates future
development to come by moving further from shore exposing fish
farms and subsequently mooring systems to harsher environmental
loadings. It is, therefore, of paramount importance to design mooring
systems based on engineering and scientific principles and leave
behind the present “trial and error" methods. This paper presents the
current state of Cyprus- offshore aquaculture industry and focuses of
its mooring designs by proposing a new methodology for designing
more reliable systems, hence ensuring its future.