Design and Installation of the Valhall Piggyback Structures
Authors: Alm Torstein , Snell Richard O. , Hampson Kevin M. , Olaussen Arne 
Published: October 2008

Abstract
This paper describes the design development and installation of a novel concept used to repair the piles at Valhall Water Injection Platform in the Southern Norwegian Sector of the North Sea. The paper describes the concept, design principles, the tolerances required to account for known and potential dimensional variations, a test programme to verify key activities and the installation.

Introduction
The Valhall Water Injection Platform jacket is a 4000 tonne steel substructure, designed to support a 16000 tonne integrated deck, situated in the North Sea in 70 meters depth of water. The platform is located in close proximity to other existing Valhall structures for drilling and topsides tie in. During the installation of the jacket in August 2002, five of the eight skirt piles experienced premature refusal in a very dense sand layer at a penetration varying between approximately 45 and 55 meters. The target penetration for these piles was 67 and 72 meters. The 67 meter target penetration was reached for the remaining three piles without abnormal behaviour.
For the refused piles, two located in two corners oppose to each other, and one in third corner, proper connection to the jacket could not be made, as there were no pile weld beads on the piles to match with the pile sleeves. In addition, since the piles had variable wall thickness make up along depths the piles bending properties in some depths were now inadequate, in particular at the mudline elevation. Finally, due to the shorter pile penetration, the pile capacities were insufficient to support the design loads, in particular on the tension side of the jacket.
A jack-up rig was chartered complete with a top-hole full-bore airlift drilling system and a geotechnical drilling system. This spread of equipment was mobilised to remove the soil column within the piles and also enable geotechnical boreholes to be drilled both inside and adjacent to the piles. Rather than piles which had plugged during driving, the investigation revealed that the toe of both the refused piles at one corner had collapsed inwards, leaving the toe almost closed.
As soon as the severity of pile damage had been revealed, the possibility for further driving was soon ruled out as an option for foundation remediation, together with a number of other remedial solutions, which would utilise the jack-up rig. As a consequence, the jack-up rig was then demobilised. Various possibilities for saving the foundation were investigated. The criticality of jacket location relative to the existing facilities constrained options to either repair or to remove and replace. Remove and replace was unattractive due to high cost and lengthy delay to the schedule.
The finally chosen option was based on a BP concept to repair using two remediation structures, each to be connected to the two refused piles at two corners. A new pile sleeve was added to this structure, allowing additional piles to be driven. These structures, named piggyback structures, were connected to the existing piles sleeves and piles already driven, by grouting, see figures 1 and 2.
Figure 1 Computer model of the lower part of jacket with Piggybacks(Available in full paper)