DNV-GL - GUIDELINE ON UWILD - Offshore Technical Guindance OTG-08 March 2014.pdf

7/17/2019 DNV-GL - GUIDELINE ON UWILD - Offshore Technical Guindance OTG-08 March 2014.pdf

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[Legal information] OTG-08 UWILD guidance May 2014 rev 1.0.docx

GUIDELINE ON UWILDOFFSHORE TECHNICAL GUIDANCE OTG-08, MARCH 2014



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TABLE OF CONTENT

Guideline on UWILD ..................................................................................................................... 1

Offshore Technical Guidance OTG-08, March 2014 ........................................................................... 1

Table of Content .......................................................................................................................... 2

1 Introduction ......................................................................................................................... 4

1.1 Introduction ................................................................................................................. 4

1.2 Objective ..................................................................................................................... 4

1.3 Scope .......................................................................................................................... 4

1.4

Structure ..................................................................................................................... 4

1.5 References ................................................................................................................... 4

1.6 Abbreviations ............................................................................................................... 5

2 Background ......................................................................................................................... 6

2.1 Regulatory Basis ........................................................................................................... 6

2.2 Position DNV GL ............................................................................................................ 6

2.3 Risk assessment ........................................................................................................... 6

3

Approach ............................................................................................................................. 8

3.1 General........................................................................................................................ 8

3.2 In-service Inspection plan (IIP)....................................................................................... 8

3.3 Flag Authorities............................................................................................................. 8

3.4 Survey Reporting .......................................................................................................... 8

3.5 Conditions .................................................................................................................... 9

4 Inspection scope ................................................................................................................ 11

4.1

General...................................................................................................................... 11

4.2 Applicable for all MOU .................................................................................................. 11

4.3 Column-stabilized units ................................................................................................ 12

4.4 Ship-shaped units ....................................................................................................... 13

4.5 Self-elevating units/Jack-ups ........................................................................................ 14

4.5.1 Introduction ........................................................................................................... 14

4.5.2 Separate surveys .................................................................................................... 14

4.5.3

Hull/Deckbox .......................................................................................................... 15

4.5.4 Leg and Spudcan ..................................................................................................... 15

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4.5.5 Long-term operation at location ................................................................................ 16



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1 INTRODUCTION

1.1 Introduction

Statutory and class regulations requires two bottom surveys in a 5 year period to inspect integrity and

water tightness of the hull. So called ‘Under water inspections in lieu of drydock’ (UWILD) survey may

replace the traditional drydocking related to these bottom surveys.

1.2 Objective

The objective of this guideline is to clarify to owners and Flag State Administrations the conditions for

which DNV GL to accept and support in-water bottom survey of Mobile Offshore Units (MOUs) as analternative to drydocking.

1.3 Scope

This guideline summarizes the scope expected to be covered by the UWILD for different types of MOUs,

including column stabilized, self-elevating and ship-shaped units.

The guideline is for the units covered by DNV GL Offshore Classification, including but not limited to

drilling units, FPSO’s, FSO and LNG production and storage units.

The guideline is line with the relevant unified requirements of the International Association of Class

Societies (IACS).

1.4 Structure

Section 2 describes the background of the UWILD approach; the regulatory basis, the position of DNV GL

and a risk assessment for acceptance of the approach for the described units.

Section 3 outlines the approach of DNV GLs UWILD based on four specific elements/conditions.

Section 4 describes the UWILD inspection scope, both generic as applicable for all hull types and as

specific for successively ship-shaped, column stabilized and self-elevating units. For the last are specific

considerations discussed, taking into account their specific fixed/floating nature and operation profile.

The document is completed with example of a UWILD planning document in Appendix A.

1.5 References

This guidance is based on the international regulations, standards and DNV-GL Offshore Service

Specifications/ Rules as listed in Table 1.



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Table 1 References

Reference1 Title

IMO MODU Code IMO Code for the Construction and Equipment of Mobile Offshore

Drilling Units

IMO SOLAS IMO International Convention for the Safety of Life at Sea

IACS UR Z15 International Association of Classification Societies’ Unified Requirement

Z15, Survey of Mobile Offshore Drilling Units

DNVGL-OSS-101 Rules for Classification of Offshore Drilling and Support Units

DNVGL-OSS-102 Rules for Classification of Floating Production, Storage and Loading

Units

DNVGL-OSS-103 Rules for Classification of Floating LNG/LPG Production, Storage and

Loading Units

DNVGL-OSS-104 Rules for Classification of Self-Elevating Units

1.6 Abbreviations

The following abbreviations are used in the document.

Table 2 Abbreviations

Abbreviation Description

CSU Colum Stabilized Unit

IACS International Association of Class Societies

IIP In-service Inspection Plan

MOU Mobile Offshore Unit

ROV Remotely Operated Vehicle

SEU Self-elevating unit

SSU Ship-shaped unit

UWILD Under Water inspection In Lieu of Drydocking

1 It is mentioned that the referred DNV GL service documents are subject to re-numbering as part of the DNV GLintegration



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2 BACKGROUND

2.1

Regulatory Basis

Bottom surveys are governed by the following regulatory basis:

According to IACS UR Z15, 4.1.1 and IMO MODU Code 1989 and 2009, 1.6.1.5, MOUs shall be

subjected two drydock surveys during any five-year period whereof the intervals between any two

such surveys should not exceed 36 months;

Further according to IACS UR Z15, 4.2 and Appendix A and IMO MODU Code 1989 and 2009,

1.6.2.5, an Administration may allow Underwater Inspections in Lieu of a Drydock survey (UWILD),

often called in-water bottom survey or bottom-survey afloat, provided that they are satisfied that

such an inspection is equivalent to a drydock survey;

The IMO SOLAS Ch. I, Reg. 10(a)(v) requires two bottom surveys in a 5 year period with the

interval period not exceeding 36 months.

The DNV GL Offshore Service Specifications governing the classification of MOUs require two

bottom surveys in the course of 5 years, and one of these is to be done together with the Renewal

Survey.

It is underlined that this regulatory basis implies acceptance by the Flag State Administration for any

UWILD approach as an alternative to drydocking for the required bottom surveys.

2.2 Position DNV GL

As a Recognized Organization (RO), DNV GL acts on behalf of the Flag State Administration in carrying

out inspections and surveys related to the statutory certification as, among others, governed by SOLAS

and MODU code as referenced above. DNV GL have long term experience with such arrangements and

consider an equivalent solution to be fully acceptable for MOUs provided that specific criteria with regard

to the condition of the unit and the alternative inspection arrangements are met, ref. Section 3 and 4 of

this guideline.

2.3

Risk assessment

The main reason to consider UWILD for Mobile Offshore Units compared to trading ships is their

exposure to a lower risk profile related to damages to the hull;

Main operations are in open sea and fixed locations, minimizing risk of grounding and fouling of

propellers and thrusters

By nature of design corrosion protection systems including anodes and / or impressed current

systems are installed to cover periods exceeding 5 years / design life. The system performance of

which may be monitored by measurement equipment / ROV surveys.

Design life of Production units stationed permanently at location in most cases exceeds the five

year certification period.



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MOUs normally operate in open waters with good visibility, making bottom survey by ROV/divers

well suitable

In addition to the lower specific risk profile as listed above, DNV GL Classification implies a tight,

controlled and transparent follow up on the conditions of the unit under class. In accordance with DNV

GL procedures, In-service Inspection Plans (IIPs) are established for all MOUs which specify the complete

inspection scope according to class rules. Subsequently, the control of the structural integrity of the

MOUs is ensured through regular class surveys according to the IIP’s.



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3 APPROACH

3.1 General

The approach of DNV GL to UWILD is based on the following elements:

1. Inspection of the MOU following the In-Service Inspection Plan

2. Acceptance of flag authorities for the UWILD approach

3. Survey report clearly stating the accreditation criteria of the bottom survey

4. The fulfillment of a set of specific conditions

The elements are further described in the following sub-sections. The requirements for the actual

execution of the UWILD scope are given in Section 4.

3.2 In-service Inspection plan (IIP)

The scope of work for inspection of MOUs is covered by the In-service Inspection Program (IIP) which

will contain the structural items to be surveyed to satisfy the requirements of main class including

watertight and structural integrity. Requirements to this can be found in DNVGL-OSS-101, OSS-102,

OSS–103 or OSS-104. The IIP constitutes the formal basis for surveying structural items under main

class and shall be completed to the satisfaction of attending surveyor before periodical survey can be

credited.

3.3 Flag Authorities

DNV GL Offshore Classification will address relevant flag authorities for their comment and general

acceptance to the practice outlined in this guideline.

As stated in Table 3, a request shall be made to the Administration when UWILD survey is intended to be

carried on an MOU as an alternative to inspection in drydock.

3.4 Survey Reporting

In connection with the survey reporting, the criteria on which the bottom survey has been credited shall

be clearly stated, referencing the acceptance from flag authorities (as applicable) and statement from

owner/master as specified in Table 3.

For non-completed scope for spudcan/mat of jack-ups, please refer to Section 4.5.4.

Note:

MODU Code 1979 : The bottom survey is not required for units

covered by the 1979 IMO MODU Code.

Norway – NMA: NMA do not specify any requirement for

intermediate bottom survey.



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3.5 Conditions

The conditions to be fulfilled for DNV GL to grant acceptance for UWILD are listed in Table 3.

Table 3 General survey scope for bottom survey of MOUs

Condition Description

Planning of in-

water survey

The owner shall submit plans and procedures for survey of the under-water hull to DNV GL

in advance of the survey and make these available on board.

Plans should include drawings or forms for identifying the areas to be surveyed, the extent

of underwater cleaning, NDT locations and methods, and provision for reporting of any

damages or deteriorations found.

See Appendix A of this document for example of survey plan table of content.

Plans should be in place for how to handle the discovery of cracks and damages which may

require repair at yard or in dock.

The owner should consider the implications of the choice of inspection method with regard

to the inherent limitations and possible need for supplementary inspection

In case of consecutive UWILD surveys the history of prior surveyed areas shall be available

in the plan.

Request to

Administration

DNV GL shall forward an application from the MOU owner/operator to the Administration

(flag state), seeking exemption from the drydocking requirements.

The application shall contain the owner’s survey plans and procedures reviewed and

accepted by DNV GL that the alternative arrangement will provide equivalent results to

drydocking.

It is assumed that the criteria set forth in the plans are covered by the owner when

preparing for the in-water survey.

Diving / ROV

company

The diving company shall be certified as a Service Supplier. Pictorial/video equipment as

described for approved diving companies shall be used to ensure that the surveyor is

satisfied with the information relayed to him.

ROV1 companies need not be approved as a service supplier, however the standard of

inspection should be equivalent to that achieved by a diver.

Diving/ROV surveys are to be witnessed and the results to be to the satisfaction of the

attending surveyor.

1 ROV means Remote Operated Vehicle, carrying camera/video equipment used for inspection purposes

as an alternative to divers.



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Condition Description

Site conditions

and visibility

The site must have sufficient water depth and clearance to all sides to allow divers to safely

survey the underwater hull.

Weather conditions at site including wave height and sea current affecting vessel motions

must be sufficiently low to allow safe inspection. If not so – the vessel needs to be taken to

a sheltered area where weather conditions are acceptable.

Water visibility must be sufficient to allow divers or ROV photo/video to get clear

pictures/recordings to the satisfaction of the surveyor.

Degree of marine growth should be limited to make a proper general visual survey possible.

Necessary cleaning is required if this is not the case.

MOU Draught Floaters shall be at minimum light draft during survey. If the vessel cannot deviate from

operational draft during survey, all parts of the hull below the operational water line shall be

covered by the bottom survey.

Condition of the

unit

The owner must give a written confirmation that the MOU has no record of damage from

dropped object, collision or grounding since the last bottom survey.

There should be no current recording of substantial corrosion, abnormal deterioration or

damage for the underwater parts or areas subject to UWILD or bottom surveys.

Units to comply with class requirements and documented through class survey work

according to the IIP.

Scope of the in-water survey to cover all relevant items of the underwater hull and

equipment

For units where the FUI > 1 special consideration will be given to an in-water survey

Reporting Upon completion of the survey, an inspection report including pictures/video recordings shall

be produced by the inspection company and presented to DNV GL for review. Reporting

should include measurements of cathodic protection system performance at renewal survey.

In case of severe

finding /

incomplete

survey

In case of findings or failure to complete in-water survey properly, drydocking may still be

required



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4 INSPECTION SCOPE

4.1 General

The UWILD survey shall provide an adequate assurance that the unit’s bottom is in a satisfactory

condition and will remain so until next scheduled bottom survey.

Each survey shall involve a general visual inspection of the unit’s bottom. The following sub-sections

summarize the expected areas covered by the alternative survey method as applicable for all MOUs

(Section 4.2) and as specific for the different structure types.

For completeness it is noted that other surveys and inspections may be carried out at the same time as

the UWILD. Examples are surveys for the Position Mooring equipment, the survey of the productionsystem, Cathodic Protection measurement etc. Additional inspections that may be included with the

UWILD scope could be:

In water inspection of risers In water inspection and dimensional control of mooring chains and wires

4.2 Applicable for all MOU

Table 4 lists the scope of inspection as applicable for all MOUs.

Table 4 General survey scope for bottom survey of MOUs

Areas covered Inspection approach

Cleaning In order to carry out a satisfactory inspection, the areas to be inspected must be sufficiently

clean to permit meaningful examination and photograph/video by diver/ROV.

Overall / spot cleaning may be required to the satisfaction of the attending surveyor.

Areas where marine growth, etc. may have to be removed are around and inside sea chests and

sea valves, on rudder nozzles with connections, guard plates for propeller shafts and thrusters.

Hull structure A representative selection of the underwater hull, as defined in the approved plan, shall be

covered by the survey.

The selection should include sufficient zones/spots to cover the hull from stern to bow, including

side and bottom plating.

Areas susceptible to damage from supply vessels, mooring equipment, dropped objects etc. are

to be inspected.

If an in-water approach is accepted for consecutive bottom surveys, the survey selection should

aim at covering different zones/spots for each survey.

The general condition of the unit, actual age, surface protection and history are importantfactors to consider when deciding the actual survey coverage.

The inspection shall cover reported suspect areas from past surveys.

The extent of inspection should be sufficient to identify damage to the hull structure, hull



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coating and general condition of critical welds.

General requirements to NDT of critical connections are covered by the IIP, whereas additional

NDT may be required for critical connections as part of the bottom survey if requested by the

Surveyor.

Seawater inlets

(sea chests and

strainers), Sea

valves

Sea suction openings are to be confirmed clear.

Sea inlets and valves are to be blanked off and dismantled for inspection during renewal

surveys (once in five year period)

Valves to be checked for tightness

Other openings

below water line

To be visually examined either from outside or inside.

Inspection to ensure that valves are functional and can be fully closed with no leakage.

Thrusters Survey the thruster for damage and visible oil leaks. Survey the connection to the hull.

Corrosion

protection

Anodes

Visual inspection of a representative amount of galvanic (sacrificial) anodes.

Potential measurements (cathodic protection readings), when required at renewal survey, shall

be performed with a bathycorrometer. Equipment should be tested and calibrated before use.

Impressed current system

Initial reading before cleaning to assess effect of installed system.

Then to be cleaned and re-inspected.

The surveyor should request readings from the permanent monitoring system and attach a

representative copy to the survey report.

4.3

Column-stabilized units

The bottom survey shall cover the submerged parts of the semi-submersible hull. More details for the

inspection scope specific for CSUs are provided in Table 5.



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Table 5 Specific additional scope for Column-stabilised units


Hull structure

Pontoon bottom plate areas

Pontoon side plate areas (both

sides)

Pontoon top plate

Columns below transit water line

Bracings1

For general scope, see Table 2

All connections to be covered by survey, i.e.

foundations fairleads, anchor rack to hull.

For cathodic protection system specific attention

should be paid to blind spots / poor coverage due

to brace arrangement / layout.

Propeller

Visual inspection of propeller blade, propeller boss,stern tube.

Check for damage, corrosion, erosion/cavitation,

oil leakage, cracks etc.

Rudder Attachments of rudder (if applicable) to be cleaned

and examined.

Visual inspection of welding, rudder blades,

attachments, paint, bearing

Units permanently on location may be exempted

from measuring the clearance of the rudder

bearing.

4.4 Ship-shaped units

Bottom survey shall cover the submerged parts of the ship-shaped hull. More details for the inspection

scope specific for SSUs are provided in Table 6.

Table 6 Specific scope for ship-shaped units


Appendages, bilge keel etc Ends of bilge keels to hull to be cleaned and examined

General inspection of bilge keels.

1 Areas above transit water line including bracings and bracing connections which are accessible for dry surveys andcovered by regular class inspections are not subject to inspection as part of bottom survey. These structural items arecovered by class scope and are included in the IIP.



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Propeller

Visual inspection of propeller blade, propeller boss, stern

tube.

Check for damage, corrosion, erosion/cavitation, oil leakage,

cracks etc.

Rudder Attachments of rudder (if applicable) to be cleaned and

examined.

Visual inspection of welding, rudder blades, attachments,

paint, bearing

Units permanently on location may be exempted from

measuring the clearance of the rudder bearing.

Moonpools and turrets The extent of the survey in the moonpool area should be

clearly identified showing any areas that are inaccessible for

survey

4.5 Self-elevating units/Jack-ups

4.5.1 Introduction

This section starts by introducing the two separate surveys as DNV GL has defined to cover the bottom

survey. Subsequently the scope of each of these is detailed further. Conclusively, the section defines

how to proceed for units for a period more than 5 years on location.

4.5.2 Separate surveys

Different from floating units, the hull (deckbox) of a jack-up structure is elevated above sea and fully

accessible for inspection during operation. At the same time the legs are available when the jack-up is in

transit mode with legs fully elevated. With due regard of this specific situation, DNV-GL has organized

the bottom survey in two parts (ref OSS-104, Pt. 7, Ch. 3, Sec. 2.3):

1) Bottom survey; covering external hull/deckbox

2) Spudcan-leg survey; covering spudcans and the underwater areas of legs, together with their

connections

The separation gives more control in planning and execution of the survey, at the same time provides a

clear overview of the unit’s (inspection) status.

It is underlined that these surveys taken together complement the statutory bottom survey as defined

by the different regulations as defined in 2.1.



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In a typical situation on location, the separate bottom survey can be completed while the spudcan/mat is

partly or fully obscured below mud line. In this situation, the separate bottom survey can be completed

(and credited) where the spudcan-leg survey remains pending.

With acceptance from the Flag, drydock in line with Section 3.3 and the referred OSS-104 paragraph,

the spudcan-leg surveys shall postponed till the next rig move1,2.

4.5.3 Hull/Deckbox

The deck box bottom and side surfaces are normally subject to general visual inspection when the jack-

up is in elevated mode. Alternatively the bottom survey may be carried out dry during transit if the jack-

up is placed on a barge or in water by divers when in floating mode, ref. requirements given in Table 7.

The following areas of the structure and the following scope shall be carried out during the survey:

Table 7 Normal Bottom survey of self-elevating units

Areas covered Inspection Approach

Visual inspection of hull, external

plate areas

Visual inspection of the deck box from boat or adjacent platform.

Alternative inspection by divers when in floating/transit mode.

Corrosion protection (CP) Visual check of the anodes, since the upper hull is usually out of the

water the CP system is usually inoperative.

Sea valves These can normally be opened from inside while the rig is in

operation.

Full closing of valves is to be checked.

Legs down to the splash zone Visual survey from the hull.

4.5.4 Leg and Spudcan

Leg and spudcan become available for inspection when the unit is in transit mode and is thereforenormally inspected separately from the hull when lifting the legs. Table 8 specifies the scope of this

survey.

1 The next due date taken from the date when the survey was due

2 For all clearness, it is noted that the interval of the spudcan-leg survey is equal to the traditional bottom survey, i.e.two times in any five (5) year period, with an interval not exceeding three (3) years between examinations.



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Table 8 Normal Survey Scope Leg and Spud Can Survey

Areas covered Inspection Approach

Legs General visual inspection of legs during jacking operation.

Connections between vertical chords and braces/diagonals are

checked together with racks and rack plate welding.

Around the pin holes in cylindrical legs

Leg-well structure

Jacking gear support structure

Spudcans ROV/diver inspection of the underside of the spudcan to look for

gross deformation.

Visual inspection around piping penetrations

Visual inspection of the spudcan from the outside, alternative check

of spudcan bottom from the inside, if access from outside is not

possible

Spudcan and mat internal structure in way of leg support

Legs – lower part

Leg / spudcan connection

The lower part of the leg, i.e. up to level 4 (in way of the

lower/upper guides in transit) and leg / spudcan connection is to be

visually surveyed.

Corrosion protection

Visual inspection of galvanic (sacrificial) anodes above sea levelwhen in transit condition

Visual inspection of a representative amount of galvanic (sacrificial)

anodes by diver or ROV when in other than transit condition

Alternative - potential measurements to evaluate efficiency of the

cathodic protection system when in elevated mode.

To be taken at different levels and sides of the leg to confirm

sufficient coverage.

4.5.5

Long-term operation at locationSelf-elevating units planning for long term operations on location beyond 5 years shall be designed as

permanently installed self-elevating units for the site specific environmental and soil conditions in

accordance to DNVGL-OSS-104, Pt. 7, Ch. 6. A separate approval process for UWILD applies for this

condition.



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APPENDIX A EXAMPLE OF PLAN DOCUMENT - TABLE OF CONTENT

The following figure represents a table of content of a plan document to be used for UWILD planning.

1. Introduction

1.1. Main Project/Vessel Particulars

Vessel Name:

Vessel Type:

Flag:

Class:

Vessel Owner / Operator:

Field:

Field Operator: Vessel Location

Vessel Summer Draft

Water Depth:

Project Launch Date:

2. Survey Procedure & Scope

2.1. General

Short description of scope, DNV GL’ s role/supervision, use of vendors

2.2. Planning

2.3. Hull Conditions

Damage/leaks/corrosion/coating

2.4. Weather/sea condition limitations

2.5. Cleaning methods hull/sea-chest

2.6. Use of Support vessel/Divers/ROV/CCTV etc.

2.7. Diving/ROV company

2.8. Items to be inspected

2.9. NDT methods to be used if required

2.10. Recording of survey and findings

2.11. Approval of vendors to be used

3. Project Team

Responsibilities

4. General

4.1. Client /OIM/etc.

Description of equipment/tools to be used

Procedures to be used to ensure safety during inspection

Risk assessment

Project Procedures & Reference Documents

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