Background

The developer of a large offshore wind farm had opted for a monopile foundation design requiring the use of MENCK’s MHU4400S hydraulic hammer. To ensure optimal performance, safety, and scheduling, the installation contractor needed to understand the interactions between the hammer, monopile designs, and location-specific ground conditions. 2H provided installation driving plans and risk assessments, key information for the installation contractor.

2H Project Scope

2H was tasked with performing a detailed geotechnical installation design study of over 100 monopile locations, each with a unique monopile design and soil conditions. This included an in-depth review of the soil data across the development footprint, assessment of soil resistance to driving based on reliable published industry standards and methodologies e.g. Alm & Hamre (2001) and Maynard et al (2019), blow counts derivation, recommended hammer energies incorporated into driving plans, self-weight penetration and installation hazards, which included pile refusal and pile run.

Project Execution

2H collaborated closely with the client to review the available project data and propose a methodology tailored to meet the installation contractor’s objectives. This approach allowed the client to prioritise specific monopile locations for analysis in the initial deliverables, ensuring critical areas were addressed promptly. The 2H team produced weekly reports of batched locations (10no.) detailing comprehensive installation assessments. Each report included detailed driving plans, essential for hammer operators to drive the piles efficiently, minimising driving time and mitigating potential hazards.

2H provided the installation contractor with comprehensive results including:

• Detailed assessment of soil conditions across the site, including limitations of data and sensitivity analysis

• Best estimate and upper bound soil resistance to driving using two published methods

• Most probable and highest expected self-weight penetration of the lifting tool and monopile

• Most probable and highest expected self-weight penetration of the hammer and monopile

• Blow counts per 0.25m to penetration depth including a soft start procedure, recommendation of hammer energy versus penetration depth, total number of blows and estimated driving time based on best estimate soil resistance to driving

• Static pile run risk assessment and monopile refusal risk

• Maximum monopile compression and tension stresses during driving

This is a highly cost-effective and timely method for assessing the methodology and associated risks of installing piled foundations at a fixed-bottom offshore windfarm. The installation study that 2H performed included assessment of pile refusal, pile run, pile tip damage, extrusion buckling whilst considering detailed soil behavioral characteristics such as liquefaction, cyclic degradation, partial drainage, etc.

Summary

The offshore wind industry is evolving and pushing engineering boundaries in all areas, and this project was no different. This was a large-scale development with full pile installation assessments at over 100 locations, each with unique monopile design and soil conditions.

To optimise the performance, safety and cost of procuring a hammer, it is essential to understand and model the interactions between the hammer, monopile design and the ground conditions. To ensure safety and efficient piling operations it is important to understand installation behavior and potential risks early on in a project so that they are appropriately addressed to minimise vessel delays and any subsequent costs.

More About Our Services

2H specialises in large-scale, detailed installation studies for a variety of pile designs, ground types and water depths. Our studies include a detailed review of the provided data, understanding the soil behaviour under specific loading conditions, self-weight penetration, drivability, developing robust driving plans, and identification of installation hazards, including pile refusal and pile run.

Soil resistance to driving is generally calculated using one of the CPT-based friction fatigue models, either those derived from the Alm & Hamre method or broadly based upon an Imperial College methodology. 2H has developed calibrated soil resistance to driving models (initial & residual friction, degradation function and end bearing) for site-specific conditions.

The timeline of a drivability assessment depends on the client’s specific requirements, scope of work, number of locations and the quality and quantity of the input data. Our experience indicates that many site investigations focus on the in-place (small strain) design of monopiles rather than the installation (large strain) behaviour of the soil. This approach can result in some residual risk within the geotechnical installation studies, depending on the complexity of the site conditions. The information for the installation assessments should be considered during the specification of the site investigation.