Section 5 Design analysis

5.1.1 A comprehensive analysis will be required in all cases and model tests are normally to be performed for ship shape units or unique designs. Validation will be required for each part of the analysis process, by correlation with model tests or other proven method.

5.1.2 Analytical procedures and numerical methodologies and models used in the analyses are to be described and shown capable of capturing the physical phenomenon pertinent to the specific design. Industry recognised proprietary software or in-house software may be used for the analyses. The original developer is expected to have performed adequate validation and verification of the software, and to readily provide evidence of such validation. In-house software needs to be shown to have been adequately calibrated and validated against model tests data, field measurements, or the results of other already validated industry-recognized software. Indicative accuracy of analytical and numerical tools used in the design analyses of the unit's response are to reported.

5.1.3 The use of validated numerical tools and software does not generally exempt the design from the need to calibrate and validate the project specific models.

5.2.1 Consideration may be given to dispensing with specific model test requirements when the design is shown to be similar in all design and environmental parameters to that of an existing unit which has undergone model testing. The designers must provide evidence of this and justification for the request, as well as report the alternative methodology proposed for calibrating and validating the project specific numerical models. Any scaling techniques used for this must also be detailed. The request is to be submitted to LR for review and consideration for acceptance at an early stage of the design.

5.2.2 In general model tests are to address both sea keeping and station keeping aspects. The model test programme and test facilities are to be to LR’s satisfaction. The model test programme and specification are to be submitted for review by LR and acceptance prior to the test campaign. Model test specifications and reporting for wave basin, wind tunnel and ice tank testing are to be in accordance with Pt 4, Ch 1, 4.6 Model test specifications and reporting.

The purpose of the model tests is to be well defined and generally is to enable calibration of key input parameters to the positional mooring system numerical model, validation of numerical modelling results, identification of unpredicted motion phenomena, assessment of complex loadings which are difficult to model numerically and evaluation of wind and current coefficients.

5.2.3 It is recommended that preliminary analyses be performed prior to the start of the model test programme, in order to understand and clarify the conceptual design, and to help focus the model testing on the most important design parameters.

5.2.4 Estimating wind forces and moments for design input into analysis or model basin wind fields should preferably be done on the basis of wind tunnel tests using an accurate project-specific model. see Pt 4, Ch 1, 4.6 Model test specifications and reporting.

5.2.5 The design philosophy of units intended to be moored in regions subject to sea ice or icebergs is required to be defined, including any quick-release mooring system arrangements.

5.2.6 The requirements for units intended to be moored in regions subject to seismic events, such as earthquakes or tsunamis, will be subject to special consideration.

5.3.2 The mooring line dynamic behaviour is to be accounted for in the station keeping analyses, taking into account the components mechanical and hydrodynamic characteristic properties such as mass (where appropriate inertia), drag and added mass (where appropriate added inertia) and elasticity.

5.3.3 Weight and elasticity properties of anchor lines are to be obtained from chain, wire or fibre rope manufacturers. While the mooring chain elasticity can be expected to be linear that of ropes may not be, especially fibre ropes. The non-linear stiffness properties are to be accounted for in the model.

Tolerances of these characteristics are be established and the information is to be documented and included in the submission. For chain parts of the mooring lines, properties are to be based on the total line diameter including corrosion allowance, see Table 10.8.1 Chain size corrosion and wear margins.

5.3.4 The sensitivity of the simulated positional mooring system and the response of the Offshore Unit to these tolerances on line properties (inclusive of expected variations of these over the service life, effect of corrosion, and marine growth) are to be carried out to ensure the resulting responses remain within acceptable limits (e.g. Offset Limit, factor of safety on mooring line strength, clearances). Similarly the analyses are to investigate the sensitivity of responses to variations in assumed drag and inertia coefficients of the mooring lines.

5.3.5 The effect of mooring line interaction with soil is also to be taken into account in the station keeping analyses. Consideration is to be given to the local bathymetry, sea-bed slope (or specific profile), sand wave phenomena (and associated changes in mooring line seabed support and embedment), friction (in-line and lateral) between the line and potential in service scouring or dig-in in the touch down region.

Sensitivity of the simulated positional mooring system and the response of the Offshore Unit to these soil-mooring line interactions (that may occur over the service life) is to be carried out to ensure the resulting responses remain within acceptable limits (e.g. Offset Limit, factor of safety on mooring line strength, clearances).

5.3.6 The offshore unit station keeping analyses is also to take into consideration manufacturing (e.g. length, stiffness) and installation tolerances (e.g. anchor location, potential remaining slack in the line after installation or in inverse catenary of the buried line section close to the anchor) as well as precision and accuracy of survey/inspection techniques intended to be deployed in service to confirm the positional mooring system configuration and integrity.

The positional mooring system design is also to ensure that the simulated positional mooring system and the responses of the Offshore Units remain within acceptable limits (e.g. Offset Limit, factor of safety on mooring line strength, clearances ) when these uncertainties are considered.

5.3.7 When, after installation, the positional mooring system, Offshore Unit, structure and equipment etc. are found to significantly differ from what was accounted for in the design, as-installed station keeping analyses will need to be carried out to confirm compliance with these Rules.

5.3.8 For positional mooring systems using fibre ropes, analyses methodologies are to be submitted to LR for acceptance. The recommendations of API RP 2SM are to be taken into account in analyses methodologies to ensure conservative estimates of mooring line tensions and the offsets of the offshore unit. Due attention is to be paid to the non-linear dynamic behaviour of the ropes, frequency dependent stiffness characteristics and the delayed elastic stretch and delayed elastic recovery characteristics of the ropes. The analyses are to investigate the sensitivity of the responses to these input parameters.

5.4.2 Hydrodynamic analysis is required to establish the six degrees of freedom motion response amplitude operators (RAOs) of Offshore Units.

The response amplitude operators (RAOs) of the six degrees of freedom motions should be determined, covering a range of frequencies encompassing the wave spectra pertinent to the project (with sufficient refinement in increment around natural periods of responses) and headings covering 360° (unless symmetry can be used).

In general at least three different drafts or loading conditions should be considered taking due account of the site specific water depth. Table 10.5.1 RAO Parameters illustrates such practice.

The six degrees of freedom motion RAOs are input to heading, motions and mooring analyses.

Generally the hull of large offshore units should be modelled with 3D-diffraction elements and validated first order radiation-diffraction numerical software can be used in the derivation of the RAOs of the six degrees of freedom motions of the offshore unit. While for simple catenary mooring line configurations in shallow to medium water depth configurations, the positional mooring system can generally be assumed to not significantly affect the first order motions of the offshore unit, such assumptions may not apply to offshore units in moored in deep water or when using semi-taut to taut mooring lines configurations. Thus the validity of such assumptions are to be checked and, when necessary, coupled analysis be used.

5.4.3 Heading analysis is generally used in load response analysis in the structural assessment of weathervaning ship type offshore units as part of the LR ShipRight Procedure for Ship Units to establish response parameters and design waves. It may also be used in support of motions and mooring analyses to assess the mean heading of the unit relative to environment parameters to be used in the station keeping analyses and fatigue analyses.

It requires a set of hindcasted environmental data (see Pt 3, Ch 10, 3.3 Metocean data 3.3.2).

The mean heading of the unit is to be calculated for each sea-state considering the action of the wind sea, swell, current and wind. The hull is to be modelled with 3D-diffraction-radiation elements at a minimum of three draughts representative of all loading conditions. The effects of current, drag loads and wind loads on the hull should be represented by current force coefficients and wind force coefficients. The current force coefficients should be derived from model tests (or the OCIMF data [Mooring Equipment Guidelines] when applicable). The wind force coefficients should preferably be based on values from model test results (for ship shape hulls preliminary analyses may use wind coefficients from the OCIMF data [Mooring Equipment Guidelines] corrected as appropriate for topsides structures).

For offshore units with thruster assisted heading control, both fully operational and single failure is to be considered.

5.4.4 Motion analyses of the moored unit focus on assessing the characteristic motion response of the Offshore unit within envelopes of design environmental conditions, see Pt 3, Ch 10, 4 Design aspects.

The analyses are to investigate a large set of stationary (typically 3 hours) environmental conditions to enable the estimation of maximum offsets (horizontal motions primarily associated with surge, sway and yaw of the unit) but also the maximum heave, roll and pitch.

As may be required by the specific positional mooring system and offshore unit design and operations, the motion analyses may also need to focus and investigate motions in relation to specific criteria such as clearance criteria (e.g. for external turret moored units potential overshoot in surge motion requires special consideration).

The motion analysis should generally be based on time domain simulations. Frequency domain analyses may be acceptable when non-linear or coupling effects are not significant, subject to sufficient model test or field data calibration confirming the validity of the analysis and agreement with LR.

When linearization techniques are used they should be fully documented and shown to have insignificant impact on the motion responses for the environmental conditions considered.

The effect of the mooring system on the first order wave frequency motion responses may generally be ignored (for positional mooring systems using loose catenary mooring line configuration in shallow to medium water depth). Similarly the effect of riser and umbilical systems on the first order wave frequency motion responses may generally be ignored for traditional compliant configurations in shallow to medium water depth). These effects may become significant in deeper water in which case, coupled analyses should be conducted to verify the motion responses for the estimate of maxima. When part of the hull of the offshore unit is slender, small in comparison to wave lengths to be considered, or presents sharp edges, viscous effects are to be considered and included in the analysis. For example on ship shape hulls, linearised roll damping should be calculated for each sea-state using a published method and the results verified with model tests.

Low frequency motion responses occurring close to the natural frequency (e.g. surge motions of ship-shaped FPSO) are quite sensitive to damping contributions from mooring lines and risers. These should be accounted for in the analyses, generally including the effect of line dynamics to the analysis. Damping input to the analyses model, its calibration and validation against pertinent model tests or full scale data should be reported in detail.

Local constraints to the sea water or wind flow or obstacles in the vicinity of, or attached to, the offshore unit or its moorings that may cause interferences should be given special consideration.

5.4.5 Mooring load analyses are to address both loads acting on mooring lines (and their components) and loads the mooring lines impart on support structures of the offshore unit and mooring line attachment points.

The resulting loads for each stationary environmental condition considered should be described in terms of their steady mean component, low-frequency component and wave frequency component.

The oscillatory component should be statistically described with standard deviations and distribution of peak responses and enable the estimate of maximum (or minimum values).

Results should include in-line tensions, but also where necessary at component interfaces forces and moments. The analyses should enable derivation of the loads (forces and moments) acting on components, as required for input to the detailed design of the components.

Mooring load analyses are often combined with motion analyses of the moored unit as the characteristic motions of the mooring lines attachment points on the unit are required to be modelled to the same extent as can be derived from the motion analyses of the moored unit.

Mooring load analyses should provide all necessary load characteristics for the verification of the mooring lines global performance and detailed design of the mooring line components and support structures. The mooring load analyses are generally to be carried out in time domain to capture the non-linear dynamic behaviour of the lines.

Frequency domain analyses may be acceptable when non-linear or coupling effects are not significant, subject to sufficient model test or field data calibration confirming the validity of the analysis and agreement with LR.

When linearization techniques are used they should be fully documented and shown to have insignificant impact on the load responses for the environmental conditions considered.

The mooring lines model should be representative of the weight and buoyancy, geometric, mechanic and hydrodynamic properties of the various components and their assembly.

The mooring line layout should take into consideration the location of the anchor points to the sea bed, as well as the location of the attachment point on the unit and mooring line pretensions, the unit’s draft, water depth and seabed morphology.

The mooring lines component drag and inertia characteristics can generally be modeled using a Morison formulation.

Due consideration should be given to potential onset of vortex shedding along the line and associated loads and vibrations arising from these. This can significantly affect drag characteristics of the mooring lines.

5.4.6 For offshore units operating in areas subject to squalls special consideration should be given to the transient nature of the load and motion responses. Generally squalls are considered to reach the moored offshore unit from any direction at any time during otherwise stationary environmental conditions. The analyses should investigate a sufficient number of squall cases (for various squall time traces) to enable to establish maxima of responses. While such analyses require substantial number of cases to be considered, the analyses duration needs only to be sufficient to capture the transient squall wind loading and associated response of the moored unit. Care should be taken to ensure that the peak responses are captured.

Thruster damping may also be applicable in relevant cases and the basis for the damping terms used in the analysis is to be documented and submitted.

5.4.8 Tensions due to low frequency, and wave frequency excitation can be computed separately. The effect of line dynamics is to be accounted for in wave frequency analysis. Low frequency tension can be based on quasi-static catenary response. Wave frequency dynamic line tension is to be computed at alternative low frequency offset positions, see Pt 3, Ch 10, 5.5 Combination of low and high frequency components – Design values 5.5.4.

5.4.9 For dis-connectable positional mooring systems, analyses are required to simulate the transient connection and disconnection operations to ensure the responses (e.g. loads, slack, motions, clearances, potential overshoot or run-up etc.) are within design envelopes (See also Pt 3, Ch 10, 4.3 Design combinations of return periods of environmental parameters 4.3.6). Similar model as for motion or mooring analyses can be used. Generally the analyses should capture the various stages and configurations of the positional mooring system during such operation, cover a representative range of environmental conditions in which the operation may be initiated at any time. The transient nature, speed and duration of the operation should be taken in consideration in the analyses, as well as the level of controls (e.g. uncontrolled quick disconnect or control disconnect, re-connect), the load transfer, progressive coupling, decoupling of the Offshore Unit and its positional mooring system etc.

5.5.1 Maximum design values for offset and tension are to include nominal pre-set static values, steady component, and wave and low frequency contributions derived from combined wave frequency and low frequency dynamic response analyses. The time domain simulations are to be of sufficient length to establish reasonable confidence levels in the predictions of maximum response. When squalls are considered, the approach for selecting the design values of tensions and offsets is to be agreed with LR.

5.5.2 Symmetry of the positional mooring system can be accounted for in the estimation of maximum design values of offset and mooring line tensions to reduce the number of maximum design values to be considered in the design verification.

5.5.3 The most probable maximum values for tension and offset can be determined from the distribution of peak loads. The statistical basis and probability distribution (Rayleigh, Weibull, Gumbel, etc.) fitted to the peak responses from the analyses to derive the design maximum values is to be documented and submitted for review. For each response considered the expected average of maxima of multiple simulations and associated standard deviation, and the probability distribution used in the derivation of the most probable maxima are to be demonstrated to provide a good fit to the peak values.

Sensitivity of the maximum design values to the underlying assumptions (number of peaks, threshold etc.) should be documented.

When fitted distributions are not well defined or assumptions are not verified (e.g. narrow banded process assumption) a robust estimate expected maximum value (derived from multiple seed analyses) should be referred to in the design.