Gunnar Lian

Long Term Analysis of TLP Extreme Tendon Tensions Using a Coupled Model and Comparison With the Contour Line Approach

This paper presents a full long term analysis of a TLP extreme tendon tensions using the all seas approach, and its comparison to the results estimated by the contour line approach.The analysis of the TLP tendon tension response is performed in the time domain using a coupled model where the floater is modelled in the software SIMO, while the tendon system is represented by a Finite Element Model in RIFLEX, including therefore the effects of non-linear restoring from the tendon system and bending and deformations of the tendons. The characteristic tendon tensions with q-annual probability of exceedance are estimated from a full long term analysis where both the short and long term variability are considered. These results are then compared to those obtained through the long term estimate from the contour line approach when assuming the 90th percentile for the worst sea state with q-annual probability of exceedance.The results from the full long term analysis will allow us to verify the adequate percentile level to be used with a contour line approach when estimating extreme TLP tendon tensions.Copyright

Impact Loads From Drop Test of a Circular Section With 42 Force Transducers

Results from previous model test campaigns of various large-volume platforms indicate that wave impact loads on vertical platform columns can become high in extreme sea states. Moreover, column slamming is a highly non-linear and complex problem and reliable estimation of Ultimate Limit State (ULS) and Accidental Limit State (ALS) design loads is a challenge. A model test campaign dedicated to investigate column slamming has been performed on a large volume platform at Marintek. Special effort was put into designing a model and instrumentation package that could capture the complex phenomenon of slamming due to breaking or near breaking waves as accurately as possible.As part of the validation of the instrumentation for this test, drop tests were performed on a circular section with 42 force transducers. In the model test, this section was mounted on one of the platform columns for measuring wave impacts. In the present drop tests, the same section was dropped in still water in a small basin. Different impact velocities and impact angles were investigated. High-speed video recordings were also used to document the tests.This paper presents the setup used in the drop tests. The results from the drop tests are discussed and compared to theoretical solutions.Copyright

Estimating Long Term Extreme Slamming From Breaking Waves

Characteristic loads for design are defined in terms of their annual exceedance probability, q. For ultimate limit state (ULS) q = 10−2, while q = 10−4 for accidental limit state (ALS). In principle a full long term analysis is required in order to obtain consistent estimates. This is straight forward for linear response problems, while it is a challenge for non-linear problems in particular if they additionally are of an on-off nature. The latter will typically be the case for loads due to breaking wave impacts.The Contour line approach is an alternative convenient method to estimate the long term extreme response, based on short term statistics from an appropriate sea state. The consequence of very large short term variability (large coefficient of variation for 3-hour extreme value) on the application of the contour method will be discussed.The long term integral is carried out over all sea state combinations. The lowest sea states will of course not affect the extremes. However, for the impact problem the short term variability is much larger than for most response cases. The coefficient of variation of the 3-hour maximum impact pressure is often between 0.5 and 1, while for a typical response process it is between 0.1 and 0.2. Due to the large variability, lower sea states than normal will contribute to the long term response.In this paper the irregularity of the response surface, and the uncertainties related to the number of seeds used in each sea state is looked into. The focus is on slamming loads from breaking waves, and some results from a model test are presented. The uncertainties in long term response from slamming loads are compared to a more common response process. The effect on the long term response when integrating over a reduced area of sea states in the scatter diagram is discussed.© 2015 ASME

Long Term Analysis for Estimation of Wave Slamming Pressures for Spar Design

A long term analysis was performed to determine extreme wave slamming loads on the Aasta Hansteen Spar, the first production and storage Spar to be installed in the Norwegian Sea. The Spar will experience high slamming pressures on the hull due to harsh environments in the field. Extensive model tests were performed to measure the wave slamming pressure which is one of challenging design parameters.The slamming loads were measured with a 3×3 array of force transducer panels attached to the Spar hull. The extreme slamming loads were estimated from 3-hour simulations of the 100-yr and 10000-yr wave environments at the Aasta Hansteen field in the Norwegian Sea. The wave simulations included fourteen sea states, and each sea state was represented by as many as 20 realizations.Based on model test data, short term analysis of 3-hour extreme pressure at each tested sea state was performed using the Gumbel distribution. Due to high variability of 3-hour maximum pressures, a long term analysis was required to investigate the proper percentile level to be used in the design.The paper presents a long term statistical methodology for extreme wave slamming loads that is used to calculate long term slamming pressures corresponding to a specified annual exceedance probability of q (e.g., q = 10−2 and q = 10−4). The paper also derives the appropriate non-exceedance probability for a short term wave environment that reproduces the long term pressures of a specified annual exceedance probability, q.Various sensitivity analyses (e.g., on the two Gumbel parameters, number of realizations, etc.) were performed to validate the short term target percentiles and associated extreme pressures derived from this approach.Details of the model tests and methodology to define the design pressure profile above mean water level (MWL) are presented in a companion paper of this Conference.Copyright