Section 1 Site and Environmental Conditions

Ch 3

101. General

The provisions of this section are intended to establish the method of defining the location of the SPM, the environmental conditions which will affect operations at the SPM and which are to be considered in establishing design criteria, and the bottom soil conditions which affect the anchorage of the SPM.

102. Mooring location

1. Site chart

A complete chart of the mooring area is to be submitted. This chart is to show depth soundings and obstructions within the swing circle, the maneuvering area, and where applicable, the approach channel from deep water or an established navigation channel. The chart may be based on local charts published by government agencies or on hydrographic surveys conducted by a marine consultant. In case of charts based on hydrographic surveys, a survey report is to be submitted de- scribing the surveying method, equipment and personnel employed to conduct the survey. The exact location and water depth of the mooring base or pipe line end manifold (PLEM), and each anchor point is to be indicated on the chart. The route of the submarine pipeline and of all other pipelines and cables is to be indicated on the chart. If the mooring is associated with other SPMs in the area, or with a pumping or control platform, these features are to be indicated on the chart. All other features and water use areas which may present potential navigational hazards are to be identified. All existing and planned navigation aids such as lights, buoys, and shore markers which will be used in conjunction with the mooring are to be indicated and identified on the chart.

2. Bottom topography

All depths on the chart are to be referenced to the datum of the local navigational chart. The chart is to be based on depth soundings taken at 15 horizontal intervals or less. The chart is to show

bottom contours at a vertical interval of 1.5 . Where the bottom is irregular,

soundings is to be reduced. Where side scan sonar or wire drag is employed, soundings may be increased.

the

the

spacing of

spacing of

All obstacles, such as sunken wrecks, rocks, and pinnacles are to be identified

and their clear

depths indicated. Where such obstacles are encountered, wire drag at a depth beneath the required

water depth or a side scan sonar survey is to be conducted throughout. Where it is shown that wa- ter depth is far in excess of the required water depth, the survey may be appropriately modified.

3. Maneuvering area

The maneuvering area is to be indicated and captioned on the site chart. The maneuvering area is defined as the area through which a vessel is to maneuver in making an approach to, or a de- parture from, the SPM. The shape and size of the maneuvering area are to be established based on pertinent local conditions. The radius of the maneuvering area abut the mooring is to be at least three(3) times the length of the largest vessel for which the SPM is designed.

Where it can be shown that the prevailing environment(wind, waves, current and tides) favorably influences the mooring maneuver, and that the vessel can always maneuver to and from the SPM without danger, the maneuvering area may be appropriately modified. Where tugs will always be used to assist in mooring, the maneuvering area may be appropriately modified. Where mooring maneuvers are to be made in extreme environment, the minimum radius is to be increased.

Fixed obstacles such as platforms or buoys, other than the mooring, are not to be anywhere within the maneuvering area. The route of the submarine pipelines may be marked by a buoy at the edge of the maneuvering area. It is suggested that no other pipelines exist in the SPM maneuvering area.

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4. Swing circle

The swing circle is to be indicated and captioned on the site chart. The radius of the swing circle is the sum of the horizontal excursion of the SPM from its center position under operating hawser load and minimum tide, the horizontal projection of the length of the hawser under operating haw- ser load, the length overall of the largest vessel for which the SPM is designed, and a safety al- lowance of 30 .

5. Water depth

The water depth at any place within the maneuvering area is to be such that no vessel which may use the SPM system will touch the sea bottom or any protrusion therefrom in any sea condition under which such a vessel is expected to be present as outlined in the design premiss within the maneuvering area.

The designer may elect to specify limiting drafts for various vessel sizes when the proposed water depth is not sufficient to allow a vessel of the maximum size to be moored in the maneuvering area under the design operating environmental condition.

The determination of the required water depth is to be based upon calculations, data from ship model tests or full scale trials, designers' experience or other available sources of information.

The designer is to submit evidence to demonstrate to the satisfaction of the Society that in de- termining the required water depth, the following effects have been considered:

(1) Vessel's dimensions and other relevant characteristics

(2) Wave height, wave period, and compass direction with respect to the vessel.

(3) The prevailing wind and astronomical tides.

(4) The expected vessel's heaving, rolling and pitching and adequate vessel under keel clearance.

(5) The

consistency of the sea bottom material or the character of any protrusion from the sea

bottom.

(6) The level of accuracy of the depth survey data.

103. Soils data

1. Bottom soil condition

The general character of the soil on the sea floor throughout the maneuvering area is to be in- dicated on the site chart. The presence of a rock bottom or of rock outcroppings is to be clearly indicated. Where soil movement such as soil slides, excessive erosion or deposition of soil, or an active fault are suspected, an analysis by a soils consultant of the nature and degree of this hazard is to be submitted.

2. Sub-bottom soil conditions

Soil data should be taken in the vicinity of the mooring site; an interpretation of such data is to be submitted by a soils consultant. In the case of a mooring having a piled or gravity base, a bor- ing or probing is to be taken at the location of the base to the depth of any piles or to a depth sufficient to establish the soil characteristics of the site.

For mooring systems with anchor piles, gravity boxes, or drag anchors, borings or probings are to be taken at all anchor locations to the depth of any piles or to a depth sufficient to establish the soil characteristics of the site. As an alternative, sub-bottom profile runs may be taken and corre- lated with at least two (2) borings or probings in the SPM vicinity and an interpretation may be made by a soils consultant to adequately establish the soil profile at all anchor pile locations.

104. Design environmental condition and data

1. Design environmental conditions

The design of an SPM system is to consider the following two environmental conditions :

(1) Operating condition

The operating environmental condition for an SPM is defined as the maximum seastate in which a vessel is permitted to remain moored to the SPM without exceeding the allowable loads and

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stresses required in Ch 3 and 4 of this Guidances. Wind, waves and the associated current used in the design shall be based on site specific data, as determined by marine and meteorological consultants.

(2) Storm condition

The storm condition for an SPM design is defined as the environmental condition with max- imum wind, waves and associated currents based on a 100 year recurrence interval. At this con- dition no vessel is moored to the SPM system, unless the SPM system is specifically designed for this environmental loading. The wind, waves and the associated currents are to be estab- lished by marine and meteorological consultants.

2. Waves

(1) Operating wave

The characteristic of the wave for the operating environmental condition described in Par 1 is to be established. The wave characteristics are to include wave height in terms of significant wave height (the average of the highest one third wave heights), associated wave spectrum and associated mean spectral period.

(2) Design storm wave

The wave characteristics representing the storm condition as described in Par 1 for the design of an SPM and its anchorage are to be established based on not less than a 100 year re- currence interval. The characteristics to describe the storm wave are to include: the significant wave height and the maximum wave height, the maximum wave in terms of maximum crest el- evation above mean low water, an indication if the wave is expected to be a breaking wave, the wave spectrum, associated mean spectral period corresponding to the maximum wave, and the tide surge associated with the maximum wave. When component parts are designed for a wave representing lesser recurrence interval, they are to be noted in the design document.

(3) Wave statistics

A report is to be submitted presenting wave statistics for the mooring area. The statistics are to be based on wave data analyzed and interpreted by a marine consultant. The statistics are to in- clude; a table showing the frequency distribution of wave height, period and direction, and a ta- ble or graph showing the recurrence period of design storm waves. It is recommended that data be obtained from a wave recorder operated in the general vicinity of the SPM for a period of time sufficiently long to establish the reliability of the wave statistics. lf the site of the wave

recorder is in a different water depth or different exposure from the mooring site,

an inter-

pretation to transfer the data to the mooring site is to be performed by a marine consultant. Alternatively, data may be based on wave observation records for a period of time sufficiently long to establish the reliability of the wave statistics from a local shore station or from pub-

lished references. The bias of such observations against design storms and therefore against ex- treme wave heights is to be accounted for. The statistics for the maximum wave are to be

based on wave records for a period of time sufficiently long to establish the reliability of the wave statistics performed by a marine consultant.

3. Wind

(1) Operating wind

The wind characteristics for the operating condition described in Par 1 are to be established. The wind velocity is to be specified at a height of 10 above the ocean surface, and averaged

over a one minute period. A one-hour wind with appropriate wind spectrum may be used as an alternative approach.

(2) Storm wind

The wind characteristics for the storm condition described in Par 1 for design of SPM, are to be established based on not less than a 100 year recurrence interval. The wind velocity is to be specified at a height of 10 above the ocean surface, and averaged over a one minute period. A one-hour wind with appropriate wind spectrum may be used as an alternative approach.

(3) Wind statistics

(A) A report is to be submitted presenting wind statistics for the mooring area. The statistics are to be based on wind data analyzed and interpreted by a marine consultant The statistics are to include following;

(a)

a wind rose or table showing the frequency distribution of wind velocity and direction,

(b) a table or graph showing the recurrence period of extreme winds,

(c)

the percentage of time which the operating wind velocity is expected to be exceeded during a year and during the worst month or season.

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(B) Where possible, statistics are preferably to be based on data from an anemometer operated in the general vicinity of the mooring for a period of time sufficiently long to establish the reliability of the wind statistics. If the site of the anemometer is influenced by terrain or is inland, or if the mooring site is far offshore, an interpretation to transfer the data to the mooring site, performed by a marine consultant is to be submitted. Alternatively, the sta- tistics may be based on wind velocity determined from synoptic weather chart pressure gra- dients for a period of time sufficiently long to establish the reliability of the wind statistics performed by a marine consultant. If synoptic weather charts are not available, the statistics may be based on observations from published references. These records are to be reviewed and interpreted for the site by a marine consultant. The bias of such observations against extreme storms and therefore against extreme wind speeds is to be accounted for.

4. Current

(1) Operating current

The current characteristics for the operating condition described in Par 1 are to be established. The operating current is defined as the maximum current associated with the maximum wind and waves in which a vessel will remain moored. The current velocities at the sea surface and

seabed are to be included. If the current profile is not linear, the velocities ber of intermediate depths are also to be included.

(2) Storm current

The storm current characteristics for the storm condition described in established. The current velocities at the sea surface and seabed are to be

at a sufficient num-

Par 1 are to be included. If the cur-

rent profile is not linear, the velocities at a sufficient number of intermediate depths are to be

included.

5. Seiche

The location of the mooring site in relation to seiche nodal points is to be investigated by a ma- rine consultant if the site is in a basin or other area known for seiche action. Seiche is defined as long period oscillation of the water in a basin as excited by a disturbance such as wind, waves, at- mospheric pressure, or earthquake. Mooring sites located at or near seiche nodal points may be in- fluenced by currents not otherwise predicted. If the mooring site is at or near a seiche nodal point, currents induced by seiche are to be reflected in the operating current and maximum current, and the influence of the period of the current on the dynamic response of the moored vessel is to be considered.

6. Tidal data

Tidal data is to be based on astronomical tides and storm surge. The astronomical tidal extremes and tidal means for the mooring site are to be established. Sufficient data is to be submitted to es- tablish the validity of the tide data. Tide levels may preferably be determined from records of a tide gauge in the vicinity of the site or from published tide tables for a location in the vicinity of the site. If the location from which the tide data is obtained is from a remote mooring site, a

transformation of the tide data maximum storm surge for the

to the mooring site is to be performed by a marine consultant. The mooring site is to be established if the mooring is in a coastal or

estuary location. Sufficient data is to be submitted to establish the validity of this storm surge.

Maximum storm surge may preferably be determined from tide records taken near the location. If the location from which the tide data is obtained is remote from the mooring site, a transformation of the tide data to the mooring site is to be performed by a marine consultant. Storm surge hind- casts for design (extreme) storms performed by a marine consultant may be submitted.

7. Temperatures and ice

Where drift ice may be a hazard to a mooring or to a vessel navigating to or moored at a moor- ing or to floating hoses at a mooring, an analysis of the nature and degree of this hazard is to be submitted. When air temperature and precipitation, spray, or tidal action may combine to cause sub- stantial ice formation on the mooring, an analysis of the degree to which ice may form and how this ice may affect the performance of the mooring is to be submitted. The structure, equipment, hoses/flexible risers, component parts and their respective material which may be affected by low temperatures are to be examined.