UXO Survey

This document is courtesy of Swire Blue Ocean


10 Vessel operation assessment for UXO

When the vessel is expected to operate in areas with the possibility of UXO presence the following shall be required and adhered to, enabling the full operation of the vessel in any operational condition as required by the client.

For visual clarification to the below, please see appendix I

10.1 Terms and definitions

As it has become evident that terms and definitions used within the industry is not aligned, the following shall clarify the understanding taken by Swire Blue Ocean.

10.1.1 Geophysical anomalies

The term covers any anomalies recorded during an investigation of the seabed to identify objects which could pose a threat to the installation of wind turbines in the site.

Any object observed will continue to be anomalies until further investigated and categorised.

10.1.2 Targets

A target is any anomalies that have been identified during the survey.

Any target identified shall be plotted onto the site specific chart whether it is being further investigated and categorised or not.

A full list of identified targets shall be developed and maintained and provided to the vessel operator. pUXO

Some UXO experts also name the Target as “pUXO” (Potential UXO).

pUXO’s shall be equivalent to an UXO per definition when implementing safety distances until further identification and classifications has been processed. Non-UXO

When a target is further investigated and found to be a Non-UXO it shall remain on the site chart, but clearly indicated as a Non-UXO.

10.1.3 Exclusion zone

UXO experts are using the term “Exclusion Zone” and/or “Aviodance zone” – to define the area around any anomalies which, if not disturbed in any way, is deemed the safe distance from the anomalies in which normal low energy operations can be conducted.

The UXO experts are using a standard distance of 15 meters, which has been achieved by adding the following;

  • 10 meter safety distance
  • 2,5 meter for navigational error from the survey vessel
  • 2,5 meter for errors when plotting the anomaly’s position onto the chart

No further factors are used to obtain the distance.

10.1.4 Safety Zone

The Safety Zone is normally not provided by the UXO experts unless the anomalies is identified and categorised as an UXO.

In this case the Safety zone will be individually evaluated and the safety zone set to ensure that an unexpected detonation of the UXO will not have negative influence on human or property.

10.1.5 Vessel Safety Zone (VSZ) – Used by vessel operator (Swire Blue Ocean)

The Safety Zone term, used by Swire Blue Ocean (Vessel Operator) is a zone set around the vessel. The extent of the Vessel safety Zone is from the outer shell plating of the vessel and to the “Exclusion Zone” set by the UXO experts around any anomalies identified as pUXO.

The Vessel Safety Zone is achieved by adding factors affecting the operation of the vessel in the position, allowing the vessel to operate in the conditions as per vessel specifications.

As a standard the Vessel Safety Zone is set to 50 meters.

The Vessel Safety Zone can only be less when an individual assessment has been made including the operational criteria’s as listed below.

As operational factors are many and vary from site to site (possibly also location to location) the below list indicate some of the “generic” factors used, but it shall not be seen as exhaustive;

  • Navigational errors
  • Environmental factors – including sea, swell and current
  • Soil conditions
  • Penetration
  • Leg Sway
  • ISO 19905 guidance of 1 spud diameter
  • Back-fill / soil movement

11 Mitigating Risk as vessel owner

The following cascaded procedure should be adopted to ensure that the risk is mitigated to ALARP seen from the vessel operator’s perspective and to ensure that all additional factors are included by being added on to the UXO experts ALARP which do not include these in their assessment.

  • Ensure full data coverage of each jack-up location;
  • Grade the geophysical anomalies and identify those that are Potentially UXO;
  • Assign a suitable safety distance around the vessel towards “Potential” UXO anomalies. Any anomalies that model as UXO, should be avoided by a sufficiently safe distance;
  • For due diligence purposes, an ALARP certificate should be issued to the geotechnical contractor to evidence the risk management process and highlight any operational constraints.

11.1Data Coverage

The data coverage of each individual jacking location shall consist of all anomalies plotted onto the chart, clearly identifying the classification given by the UXO expert’s i.e. pUXO, UXO, Non-UXO etc.

The data coverage shall be available and updated when changes occur, such as detonation of identified UXO’s or identification of a pUXO as Non-UXO.

The Data shall in addition to geophysical anomalies also include observations of value to the vessel operator such as;

  • Boulders
  • Wreck
  • Detonation holes

11.2 Grading of geophysical anomalies

The grading of geophysical anomalies shall be at a level of details that will enable the vessel owner to establish the Vessel Safety Zone accordingly and further ensure that non-UXO related anomalies can be assessed for potential structural hazard to spudcans.

11.3 Assigning Vessel Safety Zone (VSZ) and Safe Distance

The vessel will assign a Vessel Safety Zone towards any anomaly graded as a pUXO. The Vessel safety Zone, as described above, is set at 50 meters until individual assessment has been carried out towards the anomaly graded as a pUXO.

The Vessel Safety Zone must never overlap or conflict with the established pUXO Exclusion/avoidance zone so that a breach of the VSZ would allow the disturbance within the avoidance zone around the pUXO.

Any anomaly graded as a UXO shall be avoided with a Safety zone. This distance shall be set based on EOD expert knowledge and based on the individual anomaly.

11.4 ALARP certificate

For each individual position an ALARP certificate shall be provided to the vessel operator. The ALARP certificate shall contain sufficient data for the vessel operator to determine the VSZ, based on the identification and grading of the geophysical anomalies discovered on the position.

Even with an ALARP certificate for the individual position, it must be emphasised that;

Unless geophysical anomalies are investigated and confirmed as not UXO related, they should be considered a potential hazard.

12 Wind Park Owner / Client standard

A Zone of approximately 200m around WTG’s is typically applied within the industry where all anomalies are categorised, identified and removed as needed; in our experience this is a figure which has been widely accepted by most Wind Park Owners / Clients.

This zone provide the vessel operator(s) with sufficient room to manoeuvre both coming in and leaving the area around a WTG and provide a suitable room to adjust heading to the prevailing conditions on entering, positioning and jacking-up.

13 Industry standard

The lack of an industry standard or guidance accepted by all parties; Client, UXO experts and vessel operators makes it difficult to establish generic distances.

The vessel operator will always include several factors which are operational and differs from vessel to vessel and these cannot be known and included by the UXO experts.

It is evident that referring to an industry standard, when there is no such one will add to confusion and discussion. There is however, different views taken by different clients and UXO experts, which could form basis for an acceptable standard.

The issue has been raised within the industry buddies and it has been suggested that a draft of a standard should be produced which, if accepted, could be used to mitigate not only the RISK but also the time spend on discussion and argumentation.

Appendix I – Drawing