Vessel Mooring Force Analysis
Most important for a maritime engineer to understand is that a moored vessel1 essentially represents a mass-spring-dashpot system. Once the mass of the vessel’s body is set into motion (by external forces), the mooring system will respond resulting in the generation of time varying forces in the mooring system. The way the mooring system responds is highly depending on the dynamics of external forces and the response characteristics of the vessel. Typically, a large vessel will not respond dynamically to high frequency load variations (for example: a 200,000 DWT vessel does not respond to wind waves of less than (say) 5 seconds wave period). For the vessel to respond dynamically a critical period needs to be exceeded, but then the moored vessel may respond to a certain extent to this dynamic load depending the natural response of the vessel. For waves this range of periods is quite wide: roughly between 5 and 300 s,
depending on vessel size. Finally, a vessel will respond statically to very slowly varying forces (for example: tide). When addressing the assessment of vessel motions and mooring forces, a maritime engineer should always keep this in mind. Having said this, the available methods to assess mooring forces (and vessel motions) are:
Hand calculations to BS6349; Optimoor analysis;
Termsim analysis; Quaysim analysis
Each method is discussed further below. Hand calculations to BS 6349
In this static method, the total horizontal force on the vessel is calculated based on the wind force and current force on the vessel. There are normally 3 components, the transverse force at the bow, the transverse force at the stern and the longitudinal force. From this the designer
estimates how this force is shared between bollards, and sizes the bollards accordingly. There is some conservatism in the method; if the bollard load is estimated at 105 tonnes, one might provide a 150 tonne bollard.
The structure should be designed for the rated bollard capacity, not the calculated load. However where a structure supports several bollards, the structure is not usually designed for every bollard to be loaded at the same time but the designer makes an assessment.
This method is simple and quite quick, say a day’s work. It is appropriate for continuous quays where the berths are relatively sheltered from waves. It is also usually appropriate for island berths where the vessels are up to say panamax size.
For any mooring situation, hand calculations should be carried out to prepare the initial design, which can then be used as the basis for a more sophisticated analysis where the latter is appropriate.
Optimoor analysis
Optimoor is a programme that allows for the relatively quick analysis of a moored vessel sitting on a berth or moored using catenary lines. It allows for the influence of wind, waves,
seiches/surges, currents and tides to be considered.
Optimoor includes “models” for a range of standard vessels, including bulker carriers and
container vessels. If the vessel under consideration doesn’t fall into the same category as one of the models, a new model will be required. The availability of alternative models is very limited. The programme carries out a static analysis. There are “add-ons” for Optimoor that allow
dynamic effects to be taken into account, but the version held by RH does not include these add-ons.
In addition to the basic vessel characteristics, information on winch and fairlead positions, mooring line arrangements and properties, and winch settings is also required. This information can be difficult to obtain. Winch and fairlead positions can be scaled off main deck drawings, but the mooring line information and winch settings are dependent on the whim of vessel’s Master. The usual approach is to take one of the standard arrangements in BS 6349 as a starting point, and adjust it to suit the mooring bollard and fairlead positions.
The output is basically line loads for given environmental conditions. The programme can also give vessel movements, winch slippage, and bollard and fender loads. The output can be presented in a number of different forms. One of the most useful is a “rose” which gives the limiting wind speed (based on allowable loads in lines) from any given direction for the mooring arrangement under consideration.
The programme isn’t particularly user-friendly. For example, some of the input data relating to berth and vessel geometry, and the manual and automatic adjustments to line tension, requires some thought to understand. However, it is usually possible to set up the model and produce initial answers within 2-3 hours. Once the model is set up making adjustments to berth and mooring line arrangements, environmental conditions etc is very quick.
Reproducing answers can be difficult if careful attention isn’t given to balancing the mooring arrangement before the analysis is run. Large vessel movements usually indicate that the arrangement isn’t balanced.
The latest version of the programme is held by the Newcastle office (contact Mat Greaves) and requires a USB key to initially install the programme and then to run it. An older version, which uses an installation CD and dongle, is held by the Peterborough office (contact James Morley). For further information/advice contact Mat Greaves in Newcastle or Julian Hodnett in
Peterborough.
TERMSIM II analysis
TERMSIM II is a software program developed by MARIN in The Netherlands that carries out a dynamic mooring analysis in time domain. It calculates how a floating body interacts with the environmental loads acting on it and the forces generated in the mooring and fendering arrangement. It can be used not only for a vessel but also for other floating bodies such as a pontoon. An extensive description of the TERMSIM II capabilities can be found in the Dynamic Mooring Analysis Capability Statement2.
Fields of application
Three types of mooring arrangements can be addressed using the TERMSIM II program: Single Point Mooring (SPM)
Multi Buoy (spread) Mooring Jetty/Quay
Input
The program requires detailed information of the environmental conditions at the terminal, the mooring arrangement and the vessel. Below, a list of input parameters for the three types of mooring arrangements is presented.
Environmental Conditions
Vessel Mooring Arrangement
Water depth
Wave spectrum (sea) Wave height (sea) Wave mean period (sea) Wave direction (sea) Swell spectrum Swell height Swell mean period Swell direction Wind direction Wind speed Wind spectrum Current direction Current speed Vessel type Length between perpendiculars Moulded breath
Displaced moulded volume
Draught
Loading condition
Projected side area
Projected front area
Bow shape
Hydrodynamic data file
Additional damping for surge/sway/yaw
Number of fairleads
Fairlead positions (x,y,z)
Number of mooring legs Number of bollards Number of fenders Bollards positions (x,y,z)
Fender positions (x,y,z)
Mooring line type and
characteristics
Output
The output of the TERMSIM II dynamic mooring analysis programme typically consists of time series of vessel movements in six degrees of freedom of the Centre of Gravity and a predefined reference point (e.g. manifold) of the ship, forces in all mooring legs, forces in all fenders and wind, wave and current forces, separately and in total in six degrees of freedom.
Having available these time series, spectral analysis techniques may be applied to obtain a better understanding of the motion of the vessel and the generated mooring line forces. By comparing the spectra of vessel motions and spectra of mooring line forces, the motion giving cause to the occurring loads can be identified. Comparing the spectra of vessel motions to spectra of incident wind and/or wave conditions may reveal the cause of the occurring vessel motions.
Plotting the x- and y-position of the manifold gives an impression of the required envelope of the loading arm manifold (obviously also x, z and y, z positions may be plotted).
Limitations
Albeit that the TERMSIM II software provides that information expected to result from a dynamic mooring analysis program, the standard application is limited to a number of conditions:
The TERMSIM II program originally has been set-up to analyse the combined effects of gusting wind, irregular waves and current. The user defined wind, wave and current parameters are converted into time series of wind, wave and current forces acting on the vessel and subsequently the motions of the moored vessel and the mooring line and
fender forces are computed as explained above. However, forces from other sources (e.g. by a passing vessel) originally could not be accounted for.
The TERMSIM II software originally has been set-up (and verified) for open sea conditions. Reflections from any (nearby) structures are not accounted for.
Being developed for open sea conditions, the TERMSIM II software adopts a certain correlation between the shorter sea and swell waves and the second order infragravity waves. Due to different wave processes this correlation is lost when the sea and swell waves reach shallow water or harbour entrances. Hence a method needs to be defined to study ship motions for these situations.
Recognizing the limitations of the original TERMSIM II software, MARIN modified it. By means of a separate input file, a time series of external forces and moments acting of the vessel can be (user)defined. Through this external force file the above limitations of the TERMSIM II software have been alleviated as is explained in the Dynamic Mooring Analysis Capability Statement. TERMSIM II should be used where it is expected that the moored vessel will respond
dynamically. In most situations where the vessel is affected by sea and swell waves and gusting wind this will be the situation, albeit that the magnitude of the response may still be acceptable. Where this magnitude is not acceptable (in terms of too large vessel motion or too high mooring line forces) the vessel might need to leave the berth and downtime results. Indeed TERMSIM II offers a means to assess the limiting environmental conditions resulting in downtime and is often used for this purpose.
For the dynamic analysis of moored ship motions it is required to determine the hydrodynamic coefficients and wave forces on the ship. These can be obtained by performing 3D
diffraction/radiation calculations around the hull of the vessel. If required, the submerged surface of any nearby structures (quay wall) can be included as well. At present such calculations are performed by external parties on an as needed basis. The result of each computation is stored in a “hydrodynamic file” or “hyd-file”. Such hyd-file is used by TERMSIM II as input to the dynamic calculation. Each hyd-file includes the vessel displacement, hydrostatic restoring, added mass and damping coefficients, first order and second order wave forces and drift forces.
Each hyd-file is applicable to a certain vessel (characterized by its hull shape), draught and water depth. Hence, in principle, for different vessels, draughts and water depth different hyd-files should be applied. Having carried out quite some studies in the past, at present a series of such hyd files is available, including tankers (60), LNG carriers (25, membrane and spherical), LPG (5) and container vessels (2). But each situation will be different and hence may require a site specific hyd-file to be prepared. Typically the cost for an additional hyd-file lies in the order of € 2500.3
The software is in Rotterdam and is not subject to licence cost. Cock van der Lem is the main contact, but there are other staff experienced in using the software including Benno Beimers, Justin Cross, Perry Groenewegen, Bas van Son and Gijs Bosman. It is recommended that the work is carried out by the Rotterdam AG as TERMSIM runs on a stand alone computer only (fixed installation) and is not particularly user friendly.
A typical study could take from 2 man weeks of input (single vessel, simple input) to multiple weeks (complex cases, various vessels etc.). Contact Cock van der Lem for advice.
3It is under investigation whether it is worth buying a software package capable of calculating such hyd-file.
QUAYSIM analysis
Quaysim is developed by CSIR (Wim van der Molen, PhD) and in principle has the same functionality as TERMSIM II. It is limited to the analysis of vessels moored at jetties and quays, but (contrary to TERMSIM II) it allows the dynamic analysis of vessels moored with Cavotec Moormasters®.
Comparison OPTIMOOR / TERMSIM II / QUAYSIM
Assuming OPTIMOOR to perform dynamic calculations similar to TERMSIM II and QUAYSIM it is the intention to carry out a comparison between the three tools in the near future. This to judge the quality of the three packages in terms of results, and to advise which method to apply accordingly.