In April 2009 Fugro Seacore's 8 legged jack-up barge Excalibur successfully completed work on the shallow water foundation installation at the 48 turbine Gunfleet Sands offshore wind farm. Installation of 20 monopile pieces (MPs) and transition pieces (TPs) was undertaken by Excalibur on behalf of MT Hojgaard a/s, for DONG Energy Ltd. The scope of works included towing the 4.7m floating monopiles from the River Colne in the Thames Estuary, 14 miles to the Gunfleet Sandbank off the coast of Clacton-on-Sea, Essex. The operation to up-end the piles from the water was completed using the Fugro Seacore 'Sir Lancelot' Pile Handling System (PHS) designed and engineered specifically for these works. On site commissioning of the bespoke 'pile gripper can' required proving the integrity of the system to more than 25% for lifting and a 50% redundancy of friction bladders. Once vertical the PHS was used to pre-drive the piles to maximise 'self penetration' prior to installing a Menck MHU 800S hammer to complete the drive to depth.
Positioning the jack up during the foundation stage was achieved utilising the onboard GPS navigation system, which when combined with a correction service downloaded via GPRS, provided RTK positional accuracy of a matter of centimetres, even up to 10km offshore. Using Excalibur's ability to both spin around and free fall individual legs, the jack up was quickly positioned on a target pile location relative to a nominal vessel offset point, ensuring operational requirements, such as crane radii for hammer lifts and PHS movements were assured. Once positioned and preloaded, the PHS was adjusted to place the monopile within a few centimetres of the design co-ordinates.
Following the monopile lift and initial pile push into the seabed, it remained possible to adjust for verticality by moving the PHS laterally, thus enabling every MP to be installed well within the required specification and aiding subsequent TP installation.
With MP's ranging from 36-46m and weighing up to 295t, overcoming the extremely variable ground conditions proved to be a challenge. A series of 'jacking trials' were undertaken prior to commencement of operations, validating the interpretation of the 2 separate desk studies commissioned by the client.
With an air gap required below the hull of up to 20m to lift and rotate the piles and with leg penetrations ranging from 3m to over 26m, different pre-loading techniques, including the adoption of the concept of "pile set up" in clays, were utilised to ensure Excalibur would achieve the required jacking heights. Effective management of the cyclical nature of the works on such shallow water locations required the barge, pile preparation and transport, together with the installation, to be organised around each tidal cycle, with continual operational deadlines every 24 hours.
Environmental constraints concerning local fish spawning sites curtailed the available programme time for pile driving. To accommodate this particular variation the installation was achieved in two distinct phases, with placing of TP steelwork undertaken on completion of all pile driving activity. This necessitated a mid project demobilisation/re-mobilisation of equipment on Excalibur's deck but allowed for a revision to the planned method of installation of the 184 tonne x 18m high TPs.
The re-deisgn incorporated the transport of two TPs simultaneously, which together with a "beam on" approach to the pile allowed the use of Excalibur's onboard Demag crane, without the necessity of a full pre-load and jacking operation. The revised system of work saved the client approximately 5 days of programme time when compared to pile driving and also allowed the completion of additional instructed work within the same timeframe. Excluding weather downtime Fugro Seacore was able to drive the 20 MPs at an average rate of 3 per week. Operating and grouting of 17 TPs together with the grouting of a further 3 were completed in just 25 days, including weather and other delays.