DISCONNECTION OF WORKOVER RISERS ON VERY DEEP WATER
for advanced riser analysis. Deep water oil recovery have forced a change in the
systems used to tension risers, the traditional wire-pulley systems are replaced
with direct acting hydraulic-pneumatic systems. In order to design these systems
to obtain the desired operability, analysis tools including the heave compensation
system is necessary. As a result a pipe in pipe RIFLEX model have been developed.
In this masters thesis the pipe in pipe model will be used to investigate drive-off
and weak link fracture. Both subjects investigated exposes the riser to large forces,
and will push the model to its limits. Another part of the thesis is focused around
batch execution of analyses with the use of MATLAB.
The focus of the drive-off study lies mainly in investigating the dynamic behaviour
of a deep water riser (3000[m] water depth) compared to a shallow water riser
(300[m] water depth). Results are presented for upstream and downstream drive
off scenarios. The maximum offset is 100[m] obtained during a 50[s] period. Driveoff
scenarios include cases where the vessel remains at the 100[m] offset, and cases
where the vessel returns to its original position.
Drive-off simulations revealed large differences in dynamic behaviour of the deep
water versus the shallow water system. When observing the lower riser angle
for shallow water simulations, the angle were closely related to the vessel offset
position. Deep water simulations showed a delay of almost one minute before the
lower angle responded with a rotation in the vessels movement direction. During
the first minute an initial effect that caused the riser to rotate away from the vessel
position was observed. Current had a limiting effect on the lower angle when
driving upstream, and increased the angle for downstream drive-off. By including
a return motion the riser angle increased more rapidly to large values. Variation in
return motion had little effect on the maximum amplitude. The results show that
it might be difficult to take advantage of the dynamic delay in the riser response.
Bear in mind that offsets of only 100[m] was investigated, the picture might change
for simulations including larger vessel offset.
Weak link fracture is of concern since it will release large amounts of stored energy
that is potentially harmful for personnel and equipment. Establishing analysis
methods for weak link fracture can help to better understand the dynamics of
the problem. In the present work a suitable analysis model was selected and a
parametric study on the effect of drag on response was performed.
A weak link fracture was simulated with 460 tonnes over pull. Weak link fractures
is a highly complex problem since the high pressure content of the riser is released
into the water, causing a rocket effect. The presented results are only accounting
for potential energy stored as strain in the riser and heave compensation system.
A parametric study of the tangential drag versus the maximum vertical amplitude
of the riser is presented. The results are meant to be used as a starting point in
an investigation of measures to limit the weak link fracture response.
The weak link simulations showed that large accelerations are involved during the
weak link fracture, and therefore added mass and mass can be of importance to
the response. Due to the rapid movement after fracture, it was found that special
care needs to be taken when selecting the time incrementation for the simulations.
Place, publisher, year, edition, pages
Institutt for marin teknikk , 2012. , 78 p.
ntnudaim:7734, MTMART Marin teknikk, Marin konstruksjonsteknikk
IdentifiersURN: urn:nbn:no:ntnu:diva-18542Local ID: ntnudaim:7734OAI: oai:DiVA.org:ntnu-18542DiVA: diva2:566062
Larsen, Carl Martin, ProfessorWærstad, Øystein