Kjell Larsen

Coupled analysis of floater motion and mooring dynamics for a turret-moored ship

Abstract Traditional global response analyses of moored floating structures are calculated in two separated steps: calculations of floater motions; and dynamic response analysis of moorings and risers using the top end motions estimated in the first step. Typical shortcomings in the traditional separated approach are neglection or simplification of current forces and low frequency damping contribution from moorings and risers. In a coupled analysis procedure where floater motions and mooring and riser dynamics are calculated simultaneously, these drawbacks are avoided. Motions and mooring line tensions from model tests and simulations using coupled and separated analysis procedures are compared. Illustrations are given by extensive case studies of a turret-moored ship operating in 150 m, 330 m and 2000 m water depth. The main conclusions are that the traditional separated approach may be severely inaccurate, especially for floating structures operating in deep waters. Coupled analysis should be applied for deep water concepts, at least as a check of important design cases. The agreement between model test results and results from coupled analysis is very good.

Challenges in Wave Force Modelling for Mooring Design in High Seas

(18/11/2019) Challenges in wave force modelling for mooring design in high seas Line breakage events have been experienced on moored structures during recent years. These are often occurring in heavy weather and overload is one of the reasons pointed out. The present paper identifies posible physical phenomena that may lead to wave forces higher than predicted by state-of-the-art hydrodynamic tools and procedures, and thereby higher mooring lineloads, in high and steep waves. In particular, a need to re-explore wave-group induced slowly varying, low-frequency (LF)drift forces has been identified. Both mobile offshore units (MODU’s) and permanently moored floaters are considered, semisubmersibles and FPSOs. Empirical corrections are sometimes being applied in design of mooring lines, while not ingeneral, and there is no established common industry practice on such corrections. More advanced tools and knowledge do exist in research communities, while they still need further development for robust engineering use. A brief overview is given of state-of-the-art methods and tools in modelling of the hydrodynamic forces on large-volume floaters, with particular focus on slowly varying wave forces. Full scale experiences from real sea events and from a variety of earlier case studies including model tests are reviewed. It is found that several items may be critical in the proper prediction of LF wave forces in high seas and combined current and should be investigated further, in particular: Wave-current interaction Viscous wave drift forces Large and nonlinear wave -frequency vessel motions. Based upon these preliminary investigations, the paper gives recommendations for actions and further developments for improved predictions in industry practice. Copyright 2015, Offshore Technology Conference

Global Performance of Synthetic Rope Mooring Systems: Frequency Domain Analysis

For deep and ultra-deepwater applications, synthetic fibre ropes are considered an enabling technology due to their higher strength-to-weight ratio as compared to steel wire ropes and chains and due to their superior station-keeping performance. The advantages of synthetic fibre rope mooring systems include: • A higher floater payload and reduction in structural costs due to lower vertical load from mooring lines. • A reduction in vessel offsets and associated riser loads due to taut mooring system. • A potential reduction in installation costs due to lighter installation and handling equipment. • Superior endurance under cyclic loading compared to steel moorings. Synthetic fibre ropes have visco-elastic stiffness and stretch characteristics. The change-in-length response of a fibre rope is non-linear, load-path dependent (different unload-reload stiffness), and the length varies with the rate and duration of loading (due to elongation and contraction). The commonly accepted analysis approach is a simplification where a lower-bound and an upper-bound stiffness is used. This practice is primarily based on two factors: 1. The industry at large does not at present have a common, well-defined understanding of fibre-rope change-in-length performance. 2. There is a lack of commercially available mooring analysis programs with the capability to simulate the non-linear change-in-length response of the synthetic fibre rope. Individual designers may however have more advanced analysis procedures, but these are not commonly accepted yet. This paper presents results from the Syrope pilot study, Ref. /5/ and /6/, which has used rope testing to determine the characteristics of the elements in the spring-dashpot model. On this basis a strategy for software implementation in the frequency-domain has been proposed. A case study was performed for a semi-submersible production unit in deep water and harsh environment. The paper focuses on the differences between a commonly accepted, hereafter called traditional analysis approach and the proposed new frequency domain approach. The results show that there are large differences in extreme tensions and offsets as well as fatigue results. Hence, the new approach is considered to represent a significant improvement.Copyright

Development of Time Domain Model for Synthetic Rope Mooring Systems

Mooring of offshore structures in very deep water has been made possible through the use of lines made of fibres of synthetic material. The mechanical behaviour of synthetic ropes is considerably more complex than that of steel wire rope and chain, due to the visco-elastic and visco-plastic properties of the synthetic material. In particular, the gradually developing permanent increase in rope length will affect the offset and motion of the moored structure and make its characteristics change from one storm to the next.For design and analysis of offshore mooring systems incorporating synthetic ropes it is valuable to have good models that can be used for response simulation in the time domain. The paper describes the development of a time domain model for synthetic rope. The model structure and the values of the parameters are determined from experimental data using a system identification technique. The resulting model is implemented in an existing computer program for analysis of mooring and riser systems. In particular, the permanent elongation of the synthetic rope appears to be well represented.Copyright

On the Probability Distribution of Mooring Line Tensions in a Directional Environment

A joint probabilistic model of the metocean environment is assembled, taking account of wind, wave and current and their respective heading angles. Mooring line tensions are computed in the time domain, for a large set of short-term stationary conditions, intended to span the domain of metocean conditions that contribute significantly to the probabilities of high tensions. Weibull probability distributions are fitted to local tension maxima extracted from each time series. Long time series of 30 hours duration are used to reduce statistical uncertainty. Short-term, Gumbel extreme value distributions of line tension are derived from the maxima distributions. A response surface is fitted to the distribution parameters for line tension, to allow interpolation between the metocean conditions that have been explicitly analysed. A second order reliability method is applied to integrate the short-term tension distributions over the probability of the metocean conditions and obtain the annual extreme value distribution of line tension. Results are given for the most heavily loaded mooring line in two mooring systems: a mobile drilling unit and a production platform. The effects of different assumptions concerning the distribution of wave heading angles in simplified analysis for mooring line design are quantified by comparison with the detailed calculations.Copyright