Construction of Marine and Offshore Structures Notes

Construction of Marine and Offshore Structures

Preface

• The third edition has been updated to include recent advancements in the field.
• The book serves as a guide for practicing engineers and constructors in the marine environment.
• It's also a textbook for graduate engineering students.

Acknowledgments

• Thanks to members of Ben C. Gerwick, Inc. for providing information on current marine and offshore projects.
• Gratitude to Michelle Yu for word-processing the manuscript.

Author

• Ben C. Gerwick, Jr. authored books on prestressed concrete and marine/offshore structures.
• Born in Berkeley, California, in 1919.
• Received a B.S. in civil engineering from UC Berkeley in 1940.
• Served in the U.S. Navy until 1946, commanding the USS Scania (AK 40) in 1945.
• Worked in marine and offshore construction, including as President of Ben C. Gerwick, Inc.
• Professor of Civil Engineering at UC Berkeley from 1971 to 1989.
• Member of the National Academy of Engineering, National Academy of Construction, and honorary member of the American Society of Civil Engineers.
• Fellow of the International Association of Structural and Bridge Engineers and president of the International Federation of Prestressing.
• Awarded the Berkeley Fellow Medal in 1989.

Introduction

• 0.1 General: Introduces the scope and purpose of the book.
• 0.2 Geography: Discusses the geographical considerations in marine construction.
• 0.3 Ecological Environment: Covers the ecological aspects of construction.
• 0.4 Legal Jurisdiction: Addresses the legal framework governing offshore construction.
• 0.5 Offshore Construction Relationships and Sequences: Explores the relationships and sequences in offshore construction projects.
• 0.6 Typical Marine Structures and Contracts: Describes common marine structures and contract types.
• 0.7 Interaction of Design and Construction: Emphasizes the importance of the interaction between design and construction phases.

Physical Environmental Aspects of Marine and Offshore Construction

• 1.1 General: The marine environment poses unique challenges for construction.
• 1.2 Distances and Depths: Discusses the impact of distance from shore on construction, including self-sufficiency, positioning accuracy, communication, and psychological effects. Construction operations have been carried out in 1500-m water depth, exploratory oil drilling operations in 6000 m, and offshore mining tests in similar water depths.
• 1.3 Hydrostatic Pressure and Buoyancy: Pressure increases with depth according to the formula $P = V<em>w h$, where $P$ is unit pressure, $V</em>w$ is the density of seawater, and $h$ is depth. Archimedes’ principle applies to buoyancy calculations.
• 1.4 Temperature: Sea surface temperatures vary widely; the average ocean depth is 4000 m, the maximum over 10,000 m. Discusses temperature effects on marine organisms, corrosion rates, and material properties.
• 1.5 Seawater and Sea-Air Interface Chemistry: Seawater's salinity is around 3.5% by weight, including ions like sodium, magnesium, chloride, and sulfate. Discusses the chemical characteristics of seawater including effects of chloride, magnesium, and sulfates on steel and concrete. Also covers the role of oxygen, carbon dioxide, and hydrogen sulfide in corrosion and material degradation.
  • 1.5.1 Marine Organisms: Marine organisms can increase drag, erode steel (barnacles, sea urchins), and bore into concrete (mollusks). They also disturb seafloor soils and impact timber structures. Affects marine growths like algae and slime.
• 1.6 Currents: Currents influence vessel movement, wave characteristics, and soil erosion. Different types of currents include oceanic circulation, geostrophic, tidal, wind-driven, and density currents. The velocity head or pressure acting on a structure varies as the square of this current velocity ($v^2$).
• 1.7 Waves and Swells: Waves cause vessel motion (heave, pitch, roll, sway, surge, yaw) and are a primary design factor for fixed structures, $L = 3.12 T^2$, $V = L/T = 3.12 T$. Swells are waves that have traveled beyond the wind-affected zone. Discussion on calculations of wave height based on wind speed, duration, and fetch. The significant height of a wave is the average of the highest one-third of the waves.
• 1.8 Winds and Storms: Ocean winds circulate around high-pressure areas. Tropical cyclones (hurricanes, typhoons) can cause significant delays and damage. Wind velocity may reach 100 km/h (60–70 mph) or more. Discussion on storm procedures.
• 1.9 Tides and Storm Surges: Tides result from gravitational forces of the moon and sun. Storm surges are changes in sea level caused by wind and barometric pressure.
• 1.10 Rain, Snow, Fog, Spray, Atmospheric Icing, and Lightning: Discusses the effects of these atmospheric conditions on visibility, equipment operation, and safety. Significant accumulation of spray, particularly in sub-Arctic regions, can lead to atmospheric icing, or “black ice,” which poses a hazard to vessels, booms, and antennas.
• 1.11 Sea Ice and Icebergs: Unique properties of sea ice (frazil ice, sheet ice, leads, ridges). Icebergs can scour seafloors and pose collision hazards. Discusses different types of ice formations (sails, keels) and their impact on construction and operation.
• 1.12 Seismicity, Seaquakes, and Tsunamis: Discusses the impact of earthquakes on marine structures, as well as associated phenomena like seaquakes and tsunamis (very long period waves). High pore-pressure buildup and liquefaction may happen.
• 1.13 Floods: Torrential rains may create floods. Effects include heavy sediments, floating debris, scour, and erosion.
• 1.14 Scour: Scour undermines bridge piers & structures. Accelerated currents and vortices contribute. Scour depths of 2.5 time the pier may happen. It's hard to model due to scale effects.
• 1.15 Siltation and Bed Loads: Rivers carry sediments, sands, gravels, silts, and clays. It drops when hits seawaters salinity.
• 1.16 Sabotage and Terrorism: It requires additional design considerations. Passive defense includes structural redundancy, isolation, shielding, and damping.
• 1.17 Ship Traffic: It has led to structure impacts, and they have become major structural design criteria to consider statistically. The constructor needs to take proactive steps to avoid interference with navigation.
• 1.18 Fire and Smoke: It may cause explosive spalling of concretes. Fire can also cause smoke. Safe refuge should be provided to personnel.
• 1.19 Accidental Events: It may occur through collisions, dropped objects, helicopter impacts, explosions, loss of pressure, etc. Provide safety, and reduce events.
• 1.20 Global Warming: It results in rising sea levels (around 1m in the next 100 years), melted glaciers, and changed currents. Engineers need to plan accordingly.

Geotechnical Aspects: Seafloor and Marine Soils

• 2.1 General: Discusses the complexity of the seafloor due to geological history and wave action.
  • Ice ages have significant influence.
  • Rising see levels, change damages, volcanic ash, mud slides, corals extending, all sediments are examples.
• 2.2 Dense Sands: Wave action consolidates sands, resulting in high friction angles (over $40^\circ$). Disturbance happens by sampling techniques.
• 2.3 Liquefaction of Soils: Granular soils (gravel to silt) can liquefy under earthquake conditions or wave action. Underconsolidated Clays exist as well. It should be drained to prevent such events.
• 2.4 Calcareous Sands: The tiny shells crush and thus exert almost no effective pressure against the pile wall, resulting in high bearing value with deformation. Very little capacity exists in uplift.
• 2.5 Glacial Till and Boulders on Seafloor: Moraine deposits and ice rafting can deposit boulders. It has unstratified soils and is unspecific. Problems happens during the finding of samples.
• 2.6 Overconsolidated Silts: They are usually dense and resisted to penetration. They're abrasive but break under water jets. It is a problem, similar to the soft rock that degrades under waters. They are usually reported as muds.
• 2.7 Subsea Permafrost and Clathrates: relic permafrost extends under seas. They have water + ice bounds and thaw slowly. Steam jets & excavations may happen. It's mostly present in Arctic Oceans.
• 2.8 Weak Arctic Silts and Clays: They have low sheer strings after long distances from surface. Thawing release can create water. High pore pressure are causes as well.
• 2.9 Ice Scour and Pingos: Keels of seed ice ridges and icebergs scour Arctic and sub-Arctic seafloors. Pingoes are hillocks raised in silty clay soils by progressive frost heave.
• 2.10 Methane Gas: Methane gas at shallow depths in deltaic sediments may results in gas release from borings + shallow Arctic silts. This can lead to explosions/fires. Can be ascertained with drilling of holes.
• 2.11 Muds and Clays: The end weathering results into the formation of clays. High impermeable and cohesive. Thin flat plates of dynamic lubricating qualities.
  • 2.11.1 Underwater Slopes in Clays: Stepp slopes. Driven force toward failure. Increase depth increase probability of failure.
  • 2.11.2 Pile Driving Set-Up: When stopping to splice another section, the blow count suddenly jumps. Increased resistance may occur.
  • 2.11.3 Short-Term Bearing Strength: short term cohesion are present rather than longterm.
  • 2.11.4 Dredging: Clay present problem due to cohesive nature. Hard to discharge.
  • 2.11.5 Sampling: Physical disturbance and remolding result in temporary loss.
  • 2.11.6 Penetration: Combination of bearing failure under the point / edge and side shear occurs.
  • 2.11.7 Consolidation of Clays; Improvement in Strength: Clay under goes improvement upon draining, and the result in improved strength.
• 2.12 Coral and Similar Biogenic Soils; Cemented Soils: Irregular stratification. It's usually very hard. Drilling is very hard due to void and soil lost.
• 2.13 Unconsolidated Sands: It need careful survey. During the earth quake the storm makes sand to liquefy turning into a liquid.
• **2.14 Underwater Sand Dunes (