TECHNICAL STANDARDS COMMITTEE
SEABED AND SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOBILE OFFSHORE UNITS (MOU)
0016/ND
This document has been replaced by the standard DNVGL-ST-N002 which may be accessed through https://my.dnvgl.com/ This document may still be valid for some existing projects. This Guideline was updated as part of the first stage of the harmonisation between the GL Noble Denton and DNV heritage marine services requirements. Refer also to DNVGL-SE-0080 Noble Denton marine services – Marine Warranty Survey for further details. All references to GL Noble Denton apply to the legal entity trading under the DNV GL or GL Noble Denton name which is contracted to carry out the scope of work and issues a Certificate of Approval, or provides a marine related advisory or assurance service. Once downloaded this document becomes UNCONTROLLED. 23 Oct 16 14 Dec 15
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SEABED AND SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS
PREFACE This document has been drawn with care to address what are considered to be the primary issues in relation to the contents based on the experience of the GL Noble Denton Group of Companies (“the Group”). This should not, however, be taken to mean that this document deals comprehensively with all of the issues which will need to be addressed or even, where a particular matter is addressed, that this document sets out a definitive view for all situations. In using this document, it should be treated as giving guidelines for sound and prudent practice, but guidelines must be reviewed in each particular case by the responsible organisation in each project to ensure that the particular circumstances of that project are addressed in a way which is adequate and appropriate to ensure that the overall guidance given is sound and comprehensive. Reasonable precaution has been taken in the preparation of this document to seek to ensure that the content is correct and error free. However, no company in the Group shall be liable for any loss or damage incurred resulting from the use of the information contained herein or shall voluntarily assume a responsibility in tort to any party or shall owe a duty of care to any party other than to its contracting customer entity (subject always to the terms of contract between such Group company and subcontracting customer entity). This document must be read in its entirety and is subject to any assumptions and qualifications expressed therein as well as in any other relevant communications by the Group in connection with it. Elements of this document contain detailed technical data which is intended for analysis only by persons possessing requisite expertise in its subject matter. © 2015 Noble Denton Group Limited. The content of this document is the copyright of Noble Denton Group Limited. All rights reserved. Any reproduction in other material must have written permission. Extracts may be reproduced provided that their origin is clearly referenced.
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SEABED AND SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS
CONTENTS SECTION 1 INTRODUCTION 2 DATA REQUIREMENTS 2.1 General Requirements 2.2 Geophysical Investigations 2.3 Geotechnical Investigations REFERENCES TABLES Table 2-1 Table 2-2
PAGE NO. 4 6 6 6 8 16 11
Table 2-3
Foundation Risks, Methods for Evaluation and Prevention Details of Resolution and Approximate Penetrations of Different Geophysical Data Acquisition Methods In-Situ and Laboratory Soil Testing
FIGURES Figure 2.1 Figure 2.2 Figure 2.3
Site Investigation Requirements for Mobile Offshore Units (MOU’s) Jack-Up Site Survey Line Pattern Moored Vessel Site Survey Line Pattern
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1
INTRODUCTION
1.1
These guidelines describe the seabed and sub-seabed data required by GL Noble Denton to assess the suitability of locations for independent leg and mat-supported jack-up units, other mobile units operating on the seabed, and anchor installation and performance assessments for floating units. Good knowledge of the soil conditions is required for the design and assessment of the following types of mobile unit and drilling operations: Jack-up (independent leg and mat) units Submersible units and barges on the seabed Anchoring of moored units (or jack-ups moving near platforms or for stand-off locations) Conductor / casing setting for any MOU type Initial spudding of drilling equipment, including guidebase stability, from any MOU type Top-hole drilling.
1.2
1.3
1.4 1.5
1.6 1.7
It is not feasible to produce a rigorous procedure applicable to all current and future operating areas; and, for this reason, these guidelines are not intended to overrule or contradict local experience or knowledge. Additionally, there are from time to time likely to be exceptional cases in which the stated guidelines would be inappropriate. The purpose of the site survey is to provide data with which to evaluate potential foundation hazards. These guidelines are intended to lead, in part, to an approval by GL Noble Denton. Such approval does not imply that approval by designers, regulatory bodies, harbour authorities and/or any other party would be given. Geophysical data alone are insufficient to perform a site specific assessment of the foundation conditions and should be complimented by geotechnical data. These guidelines should be read in conjunction with current versions of the following: a. b. c.
1.8
1.9
1.10 1.11 1.12 1.13 1.14
0001/ND General Guidelines for Marine Projects, Ref. [1] 0009/ND Guidelines for Site Specific Assessments of Jack-Ups, Ref. [2] 0032/ND Guidelines for Moorings, Ref. [3].
Electronic versions of GL Noble Denton Guidelines are available on: https://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.html Care should be taken when referring to any GL Noble Denton Guideline document that the latest revision is being consulted. This guideline does not provide specific guidance on the seabed and sub-seabed data required for fixed offshore structures. However, it is noted that most of the requirements detailed herein are applicable for assessing the suitability of locations for the initial emplacement of fixed (non-piled) seabed structures. Revision 2 incorporated minor changes to the text. Revision 3 superseded and replaced Revision 2 dated March 2003 and incorporated minor changes to the text. Revision 4 superseded and replaced Revision 3 dated December 2007. This Revision included reformatting and removal of references to NDI (Noble Denton International). Revision 5 superseded and replaced Revision 4. The only change was rebranding as GL Noble Denton. Revision 6 superseded and replaced Revision 5. The changes were: Further details provided on the investigation and requirements in Section 2.3.1. Modifications to Table 2-1 and Table 2-3.
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SEABED AND SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS 1.15
Revision 7 superseded and replaced Revision 6. The main changes, were the adding of: the reference to OGP Geomatics Guidance note 18-1 “Guidelines for the conduct of offshore drilling hazard site surveys”, Ref. [4], in Section 2.1 guidance on validity of seabed surface surveys for moored units in Section 2.2.4.2. soil sampling depths for anchors in Section 2.3.1.5. foundation inspections after storms or settlement in Table 2-1.
1.16
This Revision 8 supersedes and replaces Revision 7. The main changes, marked with a vertical line in the right hand margin, are: New Section 2.2.1 on Geophysical Investigations Additional giuidance for shallow seismic surveys / sub-bottom profiling in Section 2.2.5. More guidance on survey line spacing in Section 2.3 New Ref. [7], ISO 19901-7:2013 “Petroleum and natural gas industries - Specific requirements for offshore structures – Part 7: Stationkeeping systems for floating offshore structures and mobile offshore units” in Sections 2.3.1.2 and 2.3.1.8 New Ref. [8] ISO 19901-8:2014 “Petroleum and natural gas industries - Specific requirements for offshore structures – Part 8: Marine soil investigations”, in Section 2.3.1.8; Additional risks in Table 2-1. Removal of the old Table 2-3 “Details of Possible Soil Surveys for a Jack-up installation”
1.17
Please contact the Technical Standards Committee Secretary at
[email protected] with any queries or feedback.
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2
DATA REQUIREMENTS
2.1
GENERAL REQUIREMENTS Figure 2.1 presents the general site investigation requirements for MOUs. Table 2-1 summarises various foundation risks relevant to MOUs that require investigation during a location assessment. Further hazard impacts are listed in OGP Geomatics Guidance Note 18-1 “Guidelines for the conduct of offshore drilling hazard site surveys”, Ref. [4]. Methods for evaluating these risks and procedures for minimising and/or preventing these risks from occurring are also summarised in Table 2-1.
2.1.1
LOCATION CO-ORDINATES The co-ordinates of the location expressed in terms of degrees, minutes and seconds of latitude and longitude are required. Latitude and longitude co-ordinates should be given to at least two, or preferably three, decimal places of precision and should also include details of the Datum and Projection used (e.g. International Spheroid, European Datum 1950 (ED50), WGS84, etc.).
2.1.2
LOCATION WATER DEPTH The water depth at the location, referred to Chart Datum (CD) or Lowest Astronomical Tide (LAT), is required.
2.2
GEOPHYSICAL INVESTIGATIONS
2.2.1 2.2.1.1
REQUIREMENT Geophysical site surveys are necessary for the reliable prediction of anchoring / foundation conditions and the identification of potential geohazards and seabed obstructions (such as wrecks, anchor scars, shallow gas pockmarks, mud volcanoes, etc.).
2.2.2 2.2.2.1
SURVEY LINE SPACING The survey line spacing is a function of the water depth, key objectives of the site survey and the nature of the intended MOU. Figure 2.2 and Figure 2.3 show typical survey line plans for a jack-up and a moored vessel. These line plans are for guidance only and should be modified to meet the survey needs and the local foundation conditions. Guidance on appropriate survey line spacing is provided in Table 3 of the OGP’s “Guidelines for the conduct of offshore drilling hazard site surveys” (Ref. [4]). For units where the seabed topography and/or shallow soils are complex then additional survey lines at a narrower spacing may be necessary in the immediate vicinity of the proposed mat or spudcan or anchor positions. For floating units, where anchors are to be deployed, the survey area should encompass the expected limit of any anchor position plus 1 km.
2.2.2.2
2.2.2.3 2.2.3 2.2.3.1
2.2.3.2 2.2.4 2.2.4.1
BATHYMETRIC SURVEY An appropriate bathymetric survey should be supplied for an area of approximately 1 km squared centred on the proposed location. Line spacing of the survey should be typically not greater than 100 metres x 250 metres over the survey area. For independent leg units with large foundations and mat units, interlining is to be performed within an area 200 metres x 200 metres centred on the location. This should be complemented by at least two orthogonal lines running 1 km in each direction from the proposed location. Interlining should have spacing not exceeding 25 metres x 50 metres. Interlining should be performed if any irregularities are detected. SEABED SURFACE SURVEYS Seabed surface survey shall be completed utilising side scan sonar, swathe bathymetry and high resolution echosounder techniques1. The survey should be of sufficient competency to identify obstructions and seabed features and should cover the immediate area of the intended location (normally a 1 km square). The slant range selection shall give a minimum of 100% overlap between adjacent lines. Each site survey for a jack-up should include a magnetometer survey to identify any buried pipelines, cables or other metallic debris located below the mudline. A magnetometer survey is considered less crucial for anchored floating MOU installations, however should the presence of any
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SEABED AND SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS pipelines, cables or other metallic debris be anticipated at a location, a magnetometer survey should be performed in the areas where the mooring system is in, on, or adjacent to, the seabed. 1
Swathe bathymetry is now available in portable units and is installed on most survey vessels so should be used as standard on all survey projects. Due to constraints imposed by calibration and processing requirements (single point obstructions may be removed in processing), conventional high-resolution bathymetry and side scan sonar should be run in conjunction.
2.2.4.2
2.2.4.3 2.2.5 2.2.5.1
2.2.5.2
2.2.5.3
2.2.5.4 2.2.5.5
2.2.5.6
2.2.5.7 2.2.5.8
2.2.6
Seabed surface surveys can become out-of-date, particularly in areas of construction/drilling activity or areas with mobile sediments. As a general rule, most site surveys will become out-of-date after six months and will require the sidescan sonar and swathe bathymetry to be re-run. For moored units in areas without seabed mobility and where there has been no known activity that could result in seabed hazards in the areas of interest a longer validity period may be considered, but not normally longer than two years. At locations where a jack-up will be operating in close proximity to existing installations, an additional seabed survey should be carried out immediately prior to the jack-up installation. SHALLOW SEISMIC SURVEYS / SUB-BOTTOM PROFILING The principal aim of the seismic survey is to reveal the general near-surface geological structure and indicate reflectors which may represent a change in soil characteristics and/or stratigraphy. This requires the correlation of the seismic data with soil boring(s) (existing) in the vicinity. Shallow seismic data may also reveal any shallow gas that may be present at a particular location. The seismic acquisition equipment should be capable of identifying reflectors that are 0.5m thick and thicker to a depth greater than or equal to 30m below mudline or the anticipated footing penetration plus 1.5 times the footing diameter. Drop cores / grab samples are also required to assist in geophysical interpretation. The report should include, but not be limited to, at least two vertical cross-sections passing through the drilling location showing all relevant reflectors and allied information. Due to the qualitative nature of seismic surveys it is rarely possible to conduct analytical foundation appraisals (jack-up leg penetration analyses or well conductor / casing design) based purely on the data obtained (see ISO 19905-1, Ref. [6]). Line spacing of the survey should, typically, not be greater than 100 metres x 250 metres over the survey area. The shallow seismic survey shall be interpreted by the competent persons who were responsible for performing the work. In the interpretation every effort should be made to comment on the soil type(s) and strength(s); this will require correlation with a borehole in the vicinity of the survey and some degree of local experience. If any signs of gas seepage or new pockmarks are identified on the seabed, or if the location is in an area where a previous survey has indicated the presence of shallow gas, it is recommended that a new shallow seismic survey is performed if the existing shallow seismic survey is more than six months old. It should be noted that a high-resolution geophysical survey alone would not provide adequate information to assess jack-up mat or leg penetration and the potential for punch-through. Furthermore, exploration 3D seismic data is not a substitute for sub bottom profiler data for the identification and mapping of shallow geology and hazards in the top 100m of the seabed and should not be considered as a replacement for a site survey for bottom-founded units, e.g. jack-ups (Ref. [4]). GEOPHYSICAL DATA ACQUISITION Details of resolution and approximate penetrations of different geophysical data acquisition techniques are provided in Table 2-2.
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2.3
GEOTECHNICAL INVESTIGATIONS
2.3.1 2.3.1.1
REQUIREMENTS Site-specific geotechnical data acquisition is recommended at locations where: no relevant geotechnical data is available, or the foundation conditions are known to be potentially hazardous (possible layered strata), or the shallow seismic data cannot be interpreted with certainty.
2.3.1.2
The number of boreholes required at a site should take account of the potential lateral variability of the soil conditions, regional experience and the geophysical investigation. The number, depth and location of the boreholes required at a jack-up location should follow the recommendations provided in Clause A.6.5.1.5 and Annex D of ISO 19905-1:2012 (Ref. [6]). Guidance is provided in Clause A.7.2.3 of ISO 19901-7:2013 (Ref. [7]) for floating units. In certain limited circumstances a borehole may not be required if no potential hazards exist and there is sufficient relevant historical data and/or geophysical tie lines linking the proposed location to existing nearby geotechnical borehole locations. It is noted that such information is only be acceptable where the soil strata can be reliably correlated between the two locations. Where soil layer characteristics vary appreciably along the tie-line, or layers exist at the proposed location that were not encountered at the existing borehole, site-specific geotechnical information should be acquired. For jack-ups, the geotechnical investigation should comprise a minimum of one borehole to a depth of 30 metres below the seafloor or the anticipated penetration depth plus 1.5 times the spudcan diameter, whichever is greater (Ref. [6]). The site investigation contractor or the client representative should ensure that the borehole depth satisfies this requirement prior to termination of the borehole. GL Noble Denton can provide this assurance by performing a "real time" spudcan penetration analysis during the site investigations. Conservative assumptions must be made if the data obtained is limited at the time of the penetration analysis (i.e. limited laboratory test data) in order to ensure that the borehole is sufficiently deep and in case further data obtained from the soil samples indicate lower strengths or subsequent analysis indicate deeper predicted penetrations. Note that GL Noble Denton will only confirm that a borehole has been progressed to sufficient depth on the basis of calculations performed by one of GL Noble Denton’s geotechnical engineers for the particular jack-up unit(s) being assessed. The final borehole depth may, however, be inadequate for subsequent assessments of units that apply higher preload bearing pressures. If anchor holding is critical then consideration should be given to obtaining and testing soil samples at specific anchor locations. For drag embedment anchors the minimum survey depth should be: 2 x fluke length in sand (typically up to 8 to 10 metres) 8 x fluke length in very soft clay (typically up to 20 to 30 metres). Deeper and more comprehensive sampling may be needed for anchor piles or suction anchors.
2.3.1.3
2.3.1.4
2.3.1.5
2.3.1.6
2.3.1.7
2.3.1.8 2.3.1.9
All layers shall be adequately investigated, including any transition zones between strata, such that the geotechnical properties of all layers are known with confidence and that there are no significant gaps (usually not more than 0.5m) in the site investigation record. The site investigation report should include borehole logs, results of laboratory tests and piezocone penetrometer test (PCPT) records (when acquired), together with interpreted geotechnical soil parameters. Recommendations on the reporting of sampling, in-situ tests and laboratory tests can be found in Annex G of ISO 19901-8 (Ref. [8]). The borehole logs should include, as a minimum, the soil stratigraphy, all laboratory shear strength measurements (clays), undrained shear strength (clays) or relative density (sands) profiles inferred from PCPT, moisture content, unit weight and Atterberg Limits. Additionally the cone tip resistance corrected for pore pressure effects shall be presented on the borehole log. These should be plotted on the same page and on appropriate scales.
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2.3.1.11
The results of the cone penetration tests shall also be presented separately in terms of graphs giving the cone tip resistance (both measured values and values corrected for pore pressure effects), local sleeve friction and pore pressure versus depth. Additionally the friction ratio and the pore pressure ratio shall be presented. The electronic data should also be provided in ASCII, or similar, format. A geotechnically competent person shall be responsible for determining the appropriate geotechnical soil parameters. For spudcan penetration analyses and anchor holding assessments the site investigation report should include as a minimum: a. profiles of undrained shear strength (su) versus depth for cohesive soils (clays) and the corresponding stress-strain curve and cell pressure of the triaxial compression test; b. c. d.
e.
effective stress strength parameters () for cohesionless soils (sands); both measured and interpreted piezocone penetration test (PCPT) records (when acquired); appropriate soil classification tests including Atterberg limits (clays), water contents, particle size distributions, unit weights, relative densities (sands), sensitivity (clays), carbonate contents (carbonate soil), etc.; The overconsolidation ratio (OCR) for cohesive soils should be determined, particularly where foundation fixity is an issue.
2.3.1.12
Where more comprehensive analyses are required, additional laboratory testing to determine the cyclic/dynamic behaviour and shear moduli of the soils may be required. Such analyses are more likely for suction piles and gravity base structures or for sensitive soils where the soil strength is anticipated to reduce significantly during cyclic loading.
2.3.2 2.3.2.1
SAMPLING / TESTING The site investigation should comprise undisturbed soil sampling and/or piezocone penetrometer testing or a combination of the two methods. Table 2-3 presents general guidelines on soil surveying, sampling, and testing.
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ESTABLISH STATUTORY REQUIREMENTS
ASSIMILATE AVAILABLE GEOLOGICAL AND GEOTECHNICAL FIELD DATA AND COMPARE WITH HISTORICAL MOU PERFORMANCE DESK STUDY
GEOPHYSICAL SITE SURVEY REQUIREMENTS
JACK-UPS
MOORED VESSELS
ESSENTIAL 1 km x 1 km SURVEY (CENTRED ON PROSPECT) INCLUDING:
ESSENTIAL SURVEY OVER MOORING LINE SPREAD AREA, INCLUDING: BATHYMETRY (echo sounder, tuned transducer and swathe techniques) SIDESCAN SONAR SURVEY (conventional or swathe) SUB-BOTTOM PROFILING & SHALLOW GAS SURVEY (pinger, boomer, sparker, airgun etc.) MAGNETOMETER SURVEY (buried metallic objects) – if considered necessary in the areas where the mooring system is in, on, or adjacent to, the seabed. DROP CORES / GRAB SAMPLES (to assist in geophysical interpretation)
BATHYMETRY (echo sounder, tuned transducer and swathe techniques) SITE CLEARANCE / SIDE SCAN SONAR SURVEY (conventional or swath, ROV survey, scanning radar imagery or divers’ walk – depending on field experience) MAGNETOMETER SURVEY (buried metallic objects) SUB-BOTTOM PROFILING & SHALLOW GAS SURVEY (pinger, boomer, sparker, airgun etc.)
DROP CORES / GRAB SAMPLES (to assist in geophysical interpretation)
OPTIONAL SURVEY REQUIREMENTS 3D SUB-BOTTOM PROFILING – (reduction in resolution but increased geological interpretation) SHALLOW GAS SURVEY – DIGITAL PROCESSING (AVO / attribute analyses techniques)
WHERE SURVEYS CANNOT BE INTERPRETED WITH CERTAINTY & USED TO CONFIRM THE SAFETY OF THE SITE, OR IN GEOTECHNICALLY PROBLEMATIC OR NEW OPERATING AREAS, THE FOLLOWING IS REQUIRED JACK-UPS SITE INVESTIGATION (SI) PROGRAMME IN ADVANCE OF FIELD DEVELOPMENT FOR JACK-UP OPERATIONS AND DESIGN OF OFFSHORE FACILITIES SINGLE OR MULTIPLE BOREHOLE INVESTIGATION USING A DEDICATED GEOTECHNICAL VESSEL BOREHOLE SI FROM THE JACK-UP WHILE ON LOCATION (at reduced draft in benign conditions; if it is necessary to skid the cantilever, then some initial preload is required) NOTES: THE BOREHOLE SHOULD BE ADVANCED TO THE GREATER OF 30M AND THE ANTICIPATED SPUDCAN PENETRATION DEPTH PLUS 1.5 x THE SPUDCAN DIAMETER. THE INVESTIGATION MAY REQUIRE VANE TESTS (gas-charged materials) AND / OR PIEZOCONE TESTS (PCPTs) (layered, sensitive or cemented materials)
MOORED VESSELS
SITE INVESTIGATION (SI) PROGRAMMED IN ADVANCE OF FIELD DEVELOPMENT FOR MOORED VESSELS OPERATIONS AND DESIGN OF OFFSHORE FACILITIES BOREHOLE / PCPT / VIBROCORE INVESTIGATION USING SPECIALIST VESSEL NOTES: ADEQUATE GEOTECHNICAL DATA WILL BE REQUIRED FOR ANCHOR INSTALLATION AND PERFORMANCE ASSESSMENTS
APPROPRIATE AND ADEQUATE SITE DATA SHOULD BE ACQUIRED TO ENSURE SAFE MOU INSTALLATION AND OPERATION
Figure 2.1
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Site Investigation Requirements for Mobile Offshore Units (MOU’s)
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SEABED AND SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS Table 2-1
Foundation Risks, Methods for Evaluation and Prevention
Risk
Methods for Evaluation and Prevention
Installation problems
Bathymetric survey / Seabed surface survey
Punch-through
Gas pockets / shallow gas
Shallow seismic survey Geotechnical site investigation and soil testing Bearing capacity analyses to assess risk of punch-through Adopt a cautious approach to preloading at the site Adopt a reduced preload, if appropriate. Shallow seismic survey, geotechnical site investigation and soil testing Bearing capacity analyses to assess risk of settlement / bearing failure Ensure adequate jack-up preload capacity Shallow seismic survey Geotechnical site investigation and soil testing Increase vertical footing reaction (sand foundation) Modify the geometry of the footing(s) Bathymetric survey (identify sand waves) Sample surficial seabed soils (particle size distributions) Assess seabed currents Analysis to assess potential for scour Inspect foundations regularly and immediately after storm events or the detection of settlement Install scour protection (gravel bag/artificial seaweed) Increase spudcan penetration Seabed surface survey Shallow seismic survey Soil sampling and other geotechnical testing and analysis Geophysical survey
Faults
Geophysical survey
Metal or other object, sunken wreck, anchors, pipelines, etc.
Magnetometer and seabed survey Diver/ROV inspection Side scan sonar Diver/ROV inspection Geotechnical site investigation and soil testing Consider change in footing geometry Jetting / eductor pipe / airlifting Seabed surface survey (footprints, disturbance) Shallow seismic survey (buried channels) Seabed modification Seabed surface survey Seabed modification Evaluate location records Consider orientating jack-up to minimise spudcan-footprint interaction Consider seabed remediation: Stomping Dredging Dredging and infilling Subsea podium
Settlement under storm loading / bearing failure Sliding failure
Scour
Geohazards (turbidite flows and mud volcanoes)
Local holes (depressions) in seabed, reefs, pinnacle rocks or wooden wreck Legs extraction problems
Adverse spudcan-seabed interactions
Seabed gradient Footprints of previous jack-ups
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Table 2-2 Survey Required
Instruments
Bathymetry
Echo Sounder Side Scan Sonar (Swathe)
Morphology
Side Scan Sonar Swathe Bathymetry
High Resolution First 50-100m
Sparker Boomer Pinger Probe
High Resolution 500 – 1000m Shallow Gas
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Air Gun Water Gun Mini-Flexichoc
Details of Resolution and Approximate Penetrations of Different Geophysical Data Acquisition Methods Scale of Survey
Mesh (m)
Instrument Transmission Frequency
30 to 50 Hz
Extent of Zone Depending on Geological Structure
200 x 400 or 200 x 200 or 100 x 100
Penetration (m)
0
30 to 100 Hz
Resolution
Better than 1m
Obstacles smaller than 1m
200 to 1500 Hz
Approx. 100m
Approx. 2m
1 to 10 kHz
50 to 100
Approx. 1 to 1.5m
2.5 to 9 kHz
20 to 40
Approx. 1 to 1.5m
10 to 300 Hz
Approx. 1,000
Approx. 100m
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SEABED & SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS Table 2-3 Testing Environment
In situ
Laboratory
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In-Situ and Laboratory Soil Testing
Soil Test
Soil Property
Soil Classification Tests
Visual description Water content, w Unit weight,
Torvane, motorvane, pocket penetrometer1
Undrained shear strength, su
Unconsolidated Undrained Triaxial Test (UU)
Undrained shear strength, su
Piezocone Penetration Test (PCPT), Ball Penetrometer Test (BPT), T-Bar Test
By direct measurement: cone resistance, qc; sleeve friction, fs; and pore pressure, u Using correlations: Soil classification Relative density, DR (cohesionless soils) Undrained shear strength, su (including Nkt factors used) Effective stress friction angle, Others (e.g. overconsolidation ratio OCR, Elastic Modulus E)
Field Vane Shear Test
Undrained shear strength, su
Pressuremeter
Effective horizontal stress and stress history Shear strength parameters
Soil Classification Tests
Water content, w Atterberg limits (cohesive soils) Unit weight, Particle size distribution Relative density, DR (cohesionless soils)
Unconfined Compression Test
Undrained shear strength, su
Torvane, motorvane, pocket penetrometer
Undrained shear strength, su
Triaxial Test
Shear strength parameters, su,
Oedometer (1-D consolidation)
Stress history (OCR) Consolidation properties
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SEABED & SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS 3 km 1 km
50 m
100 m
100 m
50 m
N.B. Additional infill lines to 25 metre spacing may be required around the location
PROPOSED LOCATION
Not to Scale
Figure 2.2
Jack-Up Site Survey Line Pattern
(source: UKOOA Guidelines for conduct of mobile drilling rig site surveys, Vol. 2 Issue 1, Ref. [5])
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SEABED & SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS 150 m
500 m
NB: Area to be surveyed is dependent upon water depth. Anchor Spread requirements must be known prior to selection of area
Figure 2.3
PROPOSED LOCATION
Not to Scale
Moored Vessel Site Survey Line Pattern
(source: UKOOA Guidelines for conduct of mobile drilling rig site surveys, Vol 2 Issue 1, Ref [5])
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SEABED & SUB-SEABED DATA REQUIRED FOR APPROVALS OF MOUS
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]
GL Noble Denton 0001/ND “General Guidelines for Marine Projects” GL Noble Denton 0009/ND “Guidelines for Elevated Operations of Jack-Ups” GL Noble Denton 0032/ND “Guidelines for Moorings”. OGP Geomatics Guidance note 18-1 “Guidelines for the conduct of offshore drilling hazard site surveys”, April 2013, Version 1.2. UKOOA “Guidelines for conduct of mobile drilling rig site surveys”, Vol 2 Issue 1, April 1997 (no longer published but the source of Figure 2.2 and Figure 2.3) ISO 19905-1:2012 Petroleum and natural gas industries - Site-specific assessment of mobile offshore units Part 1: Jack-ups ISO 19901-7:2013 Petroleum and natural gas industries - Specific requirements for offshore structures – Part 7: Stationkeeping systems for floating offshore structures and mobile offshore units ISO 19901-8:2014 Petroleum and natural gas industries - Specific requirements for offshore structures – Part 8: Marine soil investigations GL Noble Denton Guidelines are available on: https://www.dnvgl.com/rules-standards/noble-denton-maa-rules-and-guidelines.html
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