OFFSHORE GEOTECHNICAL Nardipta Pratama

GENERAL OBJECTIVES

The general objectives of a marine soil investigation are :

a. to establish the characteristics and mechanical properties of the seabed soils

b. to gather additional relevant knowledge about the site

c. including geology and stratification, as appropriate

d. This also includes, when applicable, determining the characteristics of any geohazard features that could impact upon the proposed offshore structure.

The characteristics and mechanical properties of seabed soils are determined from a combination of drilling; sampling and in situ testing; most notably the piezocone penetration test, CPTU.

APPLICATIONS

• Leg Penetration Analysis for Jackup

• Pile analysis for Platform

• Pipe line route survey

• Rig move positioning

• FPSO

• Dredging

• Windfarms

• Mudmat

OTHER APPLICATIONS

• Other structure construction • Submarine telecommunications & power cables • Seabed structures • Seabed stability studies • Environmental impact studies Non technical • seabed congestion • restricted areas • shipping movements • fishing or military activity and permitting

CONSIDERATION

• Type of offshore/subsea structure and potential foundation solutions; • The magnitude of the structural dead and live loadings; • Foundation performance, especially bearing capacity, settlement, stability, soil-

structure interaction, installation/removal aspects, etc; • Criticality of design situations and the possible need for optimization of design and

related geotechnical parameter values; • Methods to be adopted for solving/analyzing the various design situations; • Geohazards; including the presence of mass movements; pockmarks and shallow

gas; mobile sand waves; seismic activity and other related deleterious processes.

OFFSHORE ENVIRONMENTAL

• The seafloor can be divided into four major areas: the continental shelf, the continental slope, the deep ocean basin, and the mid-ocean ridge.

DEEP SEA

• The deep sea begins beyond the continental shelf break at approximately 200 m water depth.

• The main challenge for marine soil investigation specialist contractors is the drilling; sampling and in situ testing in deep waters in harsh and remote geographical environments;

• as well as meeting the Clients increasing demands for more complex investigation techniques that necessitate the development of new equipment that meets the required accuracy and operational functionality in these high pressure environments.

SEABED CHARACTERISTIC

Characterization of seabed soils following the sequence below: • Desk study

• Shallow geophysical investigations a. deployment of echo sounding and swathe bathymetry equipment

to assess bathymetric and seabed topographical conditions; b. side scan sonar and magnetometer equipment to assess seabed

features and obstructions and c. high resolution seismic reflection techniques such as sub bottom

profiling to assess seabed geology and stratigraphy.

• Marine soil investigations

DESK STUDY

Desk studies comprise the collection of information from public, in-house and commercial sources that can be evaluated to develop overviews on: 1. Confirm the presence of the anticipated ground conditions, as a result of the examination of geological maps , surface sediments, seabed mophology and previous marine investigation records 2. Bathymetric information (local topography) 3. Establish that the variability of the sub-soil is likely to be small 4. Previous experience with foundations in the area and identify any potential construction problems 5. Probable geotechnical conditions, nature seabed and rocks 6. Establish the geotechnical limit states; for example, slope instability, excessive foundation settlement, 7. Meteorological and oceanographic information (metocean) including tides, currents, wind and wave regimes; patterns, sea states 8. Investigate the likelihood of unexpected geohazards; for example, shallow gas; pock marks; submarine mass movements; hard grounds and bedforms. 9. Existing seabed structures and obstacles such as cables, pipelines etc 10. Fishing and other marine activity

GROUND INVESTIGATION

Potential limit state : • Bearing capacity failure of foundations: • Differential settlement of foundations leading to structural

damage • Instability of slopes • Damage to surrounding structures as a result of excavation • Ground collapse over pre-existing natural solution features • Collapse of excavations as a result of excessive water inflow Many investigations fail due to two main attributes: 1. Inadequate planning of the ground investigation, and; 2. Inadequate interpretation of the results of ground investigations.

SCOPE OF INVESTIGATION

OFFSHORE STRUCTURES

It is convenient to categorize oil and gas offshore structures into two broad groups: 1. Fixed and Floating structures. The main fixed and floating structures include; jack ups; gravity base and tension leg platforms as well as semi submersible platforms. 2. Subsea Structures. There are many types and form of subsea structure the most common of which include the following: subsea production templates; pre-drilling templates; wellhead protection covers; pipe bridges/crossing structures; mid-water arch anchor structures; pipeline end manifolds (PLEMs); pipeline support structures and spool pieces.

VESSEL , SURVEY PLATFORM & DEPLOYMENT SYSTEM

Investigation Platform

• Until the last decade, exploration was concentrated on continental shelves, with water depth typically less than 200 m.

• Option in the water depths include :

1. Diver operated from support vessel or barge ( water depth < 20 m)

2. Jack up rig (water depth from 20 to 120 m)

3. Remote equipment from support vessel (all water depths depending on vessel draft)

4. Specialized drilling vessel ( water depth > 20 m)

• Anchoring capabilities of vessel are generally limited to depth less than 200 m. Dynamic positioning are more appropriate for deep water.

Low Draft Barge

• Landing barge type vessel with low draft and thus ability to work in shallow water provide a reasonable working area for both equipment and diving support.

• A multi point mooring system is required to keep the vessel on station, usually with minimum of three anchors.

Support vessel

• Suitable for deploying remotely operated equipment via an A frame or deck crane.

• These vessel are limited to water depths of less than 125 m without anchor handling assistance.

• Typical cost for support vessels are low tens of thousands of US dollars per day.

Dynamic Positioned Vessels (QUEST HORIZON)

• DP vessel are ideally suited to deep water work , investigation that require large areas to be investigated and pipeline investigations.

• DP vessel usually have moon pool, so that it is possible for drilling equipment to be used in addition to remotely operated equipment.

GEOBAY HORIZON

GULF HORIZON

SELF ELEVATING

JACK UP

In favourable circumstances, drill ships can operate in water depths as shallow as 20 m. In extreme circumstances, shallow-penetration investigations may be feasible in water depths of 10 m. The primary method for drilling boreholes in water depths between 20 m and the shore, including the inter-tidal zone, is with a self elevating jack-up platform.

Self elevating jack up platforms are typically capable of supporting and deploying both rotary and percussive boring drills as well as high-quality sampling and CPTU

Tunu TOTAL INDONESIE

H27 self elevating jack up

Horizon Terra-Surf Self Elevating Platform

Top view

Wave walker 1 Self Elevating Jack up FUGRO

DRILLING BARGE

It is normal to conduct major offshore and geotechnical surveys from specialized survey vessels specifically fitted with equipment for deploying and handling geotechnical systems. Surveyors, geotechnicians and other onboard specialist personnel are provided with laboratories, workshops and processing software. Such vessels can remain at sea for many weeks.

DRILL SHIPS • Maritime vessel that has been fitted with drilling apparatus • Used for exploration drilling of new oil / gas wells in deep

water • Small drill ships used for geotechnical drilling For most marine soil investigations requiring seabed penetration greater than about 15m-20m, drilling methods will probably be required. The exceptions are the use of long piston corers, of 20 m to 30 m length, used in soft, deep water clay deposits, and the larger seabed penetration test systems, both of which require large vessels with specialist deployment equipment and sufficient deck space, and facilities, for safe operations.

MAERSK DRILLING

SEA DRILL

TRANSOCEAN

GEOTECHNICAL DRILL SHIPS

• Seabed penetration greater than about 15m-20m, drilling methods will probably be required.

• The exceptions are the use of long piston corers, of 20 m to 30 m length, used in soft, deep water clay deposits, and the larger seabed penetration test systems, both of which require large vessels with specialist deployment equipment and sufficient deck space, and facilities, for safe operations.

CALEGEO

GARDLINE

FUGRO VOYAGERS

Drilling Tools

MV VOLANS ASIAN GEOS

TEKNIK LENGKAP SAPURA

Teknik Samudra

Holland Spirit

MV VOLANS

OROGENIC

GEOBAY

GULF HORIZON

QUEST HORIZON