NOU 2008: 8

The Loss of the “Bourbon Dolphin” on 12 April 2007

6 The planning of the rig move

6.1 Introduction

Drilling rigs can either be positioned on the field by the use of a mooring system or else with the aid of dynamic positioning or a combination of these. The majority of the rigs operating in the North Sea and adjacent areas are moored. Many of these rigs were not designed for deep water. The existing mooring lines must then be extended with chain/wire/polyester or a combination of these, and perhaps spring buoys, so that the rig can be anchored in deep water. Some of these rigs (this does not apply to the “Transocean Rather”) will not be able to bear the weight of several hundred metres of extra chain and lighter mooring system employing wire or polyester must then be designed. For the same strength, wire weighs about one quarter as much as chain in water and polyester about 1/30th of chain’s weight in water. In the last decade the use of polyester has become more common for drillrigs in deep water, but it is not widespread in the North Sea and is regarded as relatively new technology.

When a drillrig is to be moved from one location to another, it may be a question of anything from one nautical mile to movement over wide stretches of ocean. With the exception of rigs with dynamic positioning, all rigs must be assisted by towing vessels and anchor-handling vessels in order to moor themselves.

For mooring, either the rig’s permanent system is used, where the mooring is run out from the rig, or else a pre-laid mooring system. In the first method the anchors and the mooring lines are run out and tautened from the rig. In a pre-laid system the anchors are first installed in the correct position and thereafter the mooring lines are stretched up towards the rig with the aid of an anchor-handling vessel. Use of pre-laid systems is not common in the North Sea but is employed to an increasing degree where found expedient. Using a pre-laid system can reduce the forces to which the vessels are exposed to a minimum during deployment of the anchors. During the deployment the forces are limited mostly to the weight of the mooring line between seabed and vessel. These forces are easy to control, as they are mainly vertical.

The Rosebank find was made in 2004. In April 2007 activity consisted in drilling of appraisal wells to evaluate the find.

In the rig move in question, it was the rig “Transocean Rather” that was to be moved two nautical miles from 213/26-1z Rosebank – Location “G” to Location “I”. This chapter will provide a description of the involved parties on the operator side, of the rig and the key personnel during the operation, and will discuss the procedure that was prepared for the rig move.

6.2 Brief description of Chevron, Transocean and Trident

The oil company Chevron North Sea Limited (Chevron) is a part of Chevron Upstream Europe, the strategic business unit in Chevron Corporation. Chevron Corporation has its head office in San Ramon, California, and is one of the world’s biggest integrated energy companies with operations in more than 180 countries all over the world, and involved in every sector of the oil and gas industry.

Chevron Upstream Europe has a number of exploration fields in the North Sea area, among other things in the UK Sector. There, Chevron has ownership interests in five production fields in which Chevron is the operator, one field in common operation and three fields operated by other companies.

Chevron has a comprehensive written manual for the implementation of marine operations, the “ChevronTexaco Marine Operations Manual”, see Annex 1 Section 3.1. The manual also applies to Trident, which prepared the procedure. Chevron has also prepared “Guidance to Vessel Masters” for vessels hired for Chevron by TEAM Marine, confer Section 7.1 and Annex 1, Section 3.4.

Richard Macklin was Chevron Marine & HSE Specialist for the operation.

Transocean is the world’s biggest offshore drilling contractor, and stands for the operation of around 82 mobile drilling units, which include drillships and various kinds of rig. Fourteen of the rigs are operating in the British and Norwegian sectors of Northern Europe. On a world basis Transocean has 30 rigs that can drill in waters deeper than 1,370 metres.

The company Transocean is registered in the Cayman Islands. Head office is in Houston, Texas. The business units for Europe and Africa are managed from offices in Aberdeen and Paris. Transocean has more than 12,500 employees.

For Transocean’s operation a manual for marine operations has been prepared, the “Transocean Marine Operations Manual”, which is general and is used for all Transocean’s mobile drilling units.

For the “Transocean Rather” there was also prepared a separate Operation Management Plan (OMP) for the contract with Chevron, in conformity with the requirements of UK Step Change in Safety, Health and Safety Management System Interfacing. The Emergency Response portion of the OMP satisfied the special requirements of Regulation 4 of the Offshore Installations (Prevention of Fire and Explosion, and Emergency Responses) Regulations 1995, confer Section 6.6; see Annex 1 Section 3.5.

In the Transocean data card, confer Annex 1 Section 3.6, it is stated that the vessels shall operate in conformity with the NWEA guidelines.

Key personnel on the “Transocean Rather” during the operation are discussed in Section 6.4.

Trident is a British consultancy firm that offers maritime and technical services and has moving of rigs as its speciality. The company has been in operation since 1986 and performs more than 200 rig moves per year worldwide. Trident has performed moving of semi-submersible rigs in water depths from 59 to 1,650 metres.

Trident has a marine, a technical and a positioning division, who collaborate to create an integrated solution in which all the services are delivered by Trident. The marine division is responsible for marine superintendents and towmasters. Trident does not have any employees of its own to discharge the towmaster function, but hires self-employed individuals on an ad hoc basis. The technical division has employees with naval architect qualifications, whereas the positioning division contributes hydrographical surveyors and navigation engineers.

Sean Johnson of Trident was a marine superintendent for the rig move, whereas Martin Kobiela was technical manager. During the operation, inspector Martin Troup of Trident was stationed on board the rig as survey engineer.

6.3 The “Transocean Rather” - specifications

The oil rig “Transocean” Rather has the following specifications:

Name:Transocean Rather
Type:Column Stabilized unit
Gross Tonnage:22,052
Class Notation:DNV +1A1 HELDK, DRILL, CRANE, ICE-T
Construction site:Daewoo Shipbuilding & Heavy Machinery Ltd, Okpo, South Korea
Construction year:1987 – 12, upgraded in 1995
Previous Name:Sonat Prat Rather 1988
Sonat Rather 1997
Port of Registry:Panama
Flag state:Panama
Owner:Transocean Offshore Deepwater Drilling Inc.
Operator:Transocean Offshore (UK) Inc with company address
Transocean House, Crawpeel Road, Altens, Aberdeen
Country:The United Kingdom
Accommodation section:108 bunks + 2 Transocean offices, 2 customer offices, 1 sickbay
Helideck:89ft x 89ft; Chinook 234 or Sikorsky S-61
Maximum drilling depth7,620 m
Maximum water depth1,372 m
Technical Data
Length:99 m
Breadth:87 m
Depth:44 m
Operational draught:25 m
Transit draught:9 m
Mooring equipment:
Winches:8 Hepburn w/ GE 752s
Wire/Chain8 x 6 000 ft x 3 in wire; 2,900ft x 84mm K4 chain
Anchors8 x 18 tonne Bruce anchors

6.4 The “Transocean Rather” - personnel

Offshore Installation Manager (OIM)

The OIM has the supreme authority on board the rig. He is responsible for the rig’s safety during both drilling and moving. Maritime operational authority during moving is delegated to the towmaster. The OIM has the responsibility for performing new risk assessments in the event of changes during the operation, confer Section 3.7.2.

Figure 6.1 The “Transocean Rather”

Figure 6.1 The “Transocean Rather”

The OIM on the “Transocean Rather” was Patrick O’Malley, who is trained as a drilling engineer. He has 22 years experience of drilling activities in the offshore industry, of which eight years as OIM. He is an employee of Transocean and reports to the rig manager onshore.

It is stated in Section 1.5.1 of the RMP that:

“nothing in this manual shall supersede applicable legislation covering the authority of the vessel master or Offshore Installation Manager (OIM)” (original in English)

Barge Supervisor

The Barge Supervisor is responsible for the rig’s stability, including daily ballast, unloading and loading operations, plus monitoring of environmental factors. He is the day-to-day maritime manager on the rig. During rig moves his responsibility is restricted to ballasting operations and running of the winches. The Barge Supervisor has the technical operating responsibility for the winches.

The Barge Supervisor on the “Transocean Rather” during the operation was James A. Sutherland, who is a certified deck officer. Sutherland has 23 years experience in the offshore industry, of which twelve years in the same post and the last two years on the “Transocean Rather”. He is employed by Transocean and reports to the OIM.

Transocean Towmaster /Chevron Marine Representative

The towmaster has delegated authority from the OIM to lead or monitor safety during the rig move operation. He shall, following instructions, inform the OIM and the marine representative of the operator about all operational non-conformances. The towmaster shall keep a running log of duties and incidents during the operation and perform his tasks from the pilot house.

The towmasters during the rig move were Ross Watson, John G. Sapsford and Harvey Wilks. Ross Watson came on board on 26 March and left the rig on 9 April. John Sapsford came on board on 28 March. During the first phase of the operation, Watson and Wilks functioned as towmasters, while Sapsford had the role of Chevron Marine Representative, who was to safeguard the operator’s interests during the operation. In addition to their function as towmasters, Wilks and Sapsford were the Chevron Marine Representative on board during the last phase of the operation.

The Commission has been informed by representatives of the operator, by the duty-holder and by the towmasters that this arrangement was common in the UK Sector and that it was not considered that the combination of the roles of Chevron Marine Representative and towmaster involved any safety limitations or loss of a safety barrier.

Harvey Wilks was certified as deck officer with 10 years’ offshore experience in the position as control room officer and, in unspecified periods, as reserve OIM. He was serving as towmaster for the first time on the “Transocean Rather”.

J. G. Sapsford is certified as a ship’s master with an OIM certificate in addition. He has eleven years’ practice as a barge supervisor and an OIM. From 2002 he has been marine representative in different offshore projects including rig moves.

Navigation Engineer

The navigation engineer handles the positioning equipment used during the rig move and assists the vessels with navigation-related questions. Martin Allan Troup, of Trident, was on duty when the accident occurred.

A more detailed description of work assignments and responsibilities during the rig move is provided in the RMP (Sections 2.3 and 2.4), see Annex 1, Section 3.7.

6.5 The making of the contract between Chevron and Transocean

Chevron is the operator and one of the licensees on the Rosebank field west of Shetland. Chevron prepared the programme for drilling of appraisal wells.

Figure 6.2 The Rosebank Field

Figure 6.2 The Rosebank Field

On 31 July 2005 Chevron and Transocean signed the contract for delivery of the rig in order to drill three appraisal wells in three different locations, which demanded three separate rig moves. Chevron’s maritime specialist testified that they were originally to start drilling in March 2006 but that this was delayed because the rig was still working for another customer. The contract presupposed that the rig met Chevron’s operational requirements.

Trident was hired by Chevron in order to provide the following maritime services in connection with the rig move:

  • Mooring analyses

  • Rig move procedures (RMP) and presentation of these on board the vessels

  • Navigation equipment with personnel

  • Chevron marine representative for the rig moves

Trident also acquired personnel to perform the towmaster functions.

6.6 Choice of mooring system and installation method

6.6.1 Provisional mooring analyses

According to provisional mooring analyses made on 10 May 2005, it was possible to moor the rig in March and April months with the rig’s permanent mooring system as described in Section 6.3, modified with chains inserts and wire. This system satisfied the requirements of DNV POSMOOR 1996. At that point no analyses for year-round operations had been done. The analyses were based on the “Transocean Rather West of Shetland mooring and riser analyses environmental data” (Annex 1, Section 4.2). In a meeting between the parties on 6 June 2005, it was decided that the rig should be moored with chain inserts, which is the heaviest solution, but, according to the parties, the only solution that would meet POSMOOR design requirements for year-round operations on the Rosebank felt.

Chevron, Transocean and Trident have pointed out that a chain system ensured that sufficient anchor line would be lying on the bottom under all conditions, thereby preventing the anchors being lifted up.

Polyester was considered at an early date. According to Trident’s representative, this proved unobtainable, nor could it – for technical reasons – be combined with the rig’s mooring wires.

6.6.2 Final mooring analyses

Trident prepared a new mooring analysis on 21 July 2005, see Annex 1 Section 3.2, which concluded that they needed:

  • Water depth 1,189m

    • 915m 76mm chain inserts for mooring in the months December to February

    • 381m 76mm chain inserts for mooring in the months March to May

  • Water depth 1,098m

    • 762m 76mm chain inserts or mooring in the months December to February

    • 302 m 76mm chain inserts for mooring in the months March to May

On the basis of Trident’s specifications, Chevron hired eight lengths of about 914m 76mm chain from International Mooring Systems. The same mooring system was planned for all three locations.

6.6.3 Choice of installation method

Chevron chose a conventional mooring system in preference to the possibility of pre-laying the anchor and the mooring system. The reason for this choice was first and foremost negative experiences (loss of buoyancy buoys) with a pre-lay system for the rig at a location on the margin of the Atlantic.

Figure 6.3 The mooring pattern of the  A; “Transocean Rather”

Figure 6.3 The mooring pattern of the “Transocean Rather”

6.7 The rig move to 213/26-1z Rosebank - Location “G”

The original plan was to move the rig from Shell’s Benbecula field and moor it on the first Chevron field, Location G. Due, however, to technical problems with departure from the Shell field, in August 2006 it was decided to tow the rig to Invergordon in order to perform the necessary repairs. The first rig move for Chevron was therefore the towing of the rig from Invergordon to Location G, where it was moored on 25 October 2006. The rig move from Invergordon to Location G took 14 days 15 hours (Annex 1, Section 3.7). The vessels used during this move were the “Highland Valour”, the “Highland Courage”, the “Normand Neptun” and the “Mærsk Leader”.

The procedures for this rig move were prepared by Trident.

6.8 The planning of the rig move to Location “I”

After the mooring at Location G commenced, the players began their preparations for the move to Location I. Trident’s Sean Johnson testified before the Commission that they gained some experience from the move to Location G. Among other things the chasing system did not work, so that they had to use J-chasers, see Figure 6.4. Because of the forces and weight during deployment of the anchors at Location G, there were problems with the winches on the rig. The problems that arose were not with the tension, but with the winches’ dynamic braking system, which was not dimensioned for the weight of the extra chain. On this rig move they burnt out a set of disc brakes while running out chain. More information on the winch problems during the rig move to Location G will be found in Annex 1, Section 3.7.

Figure 6.4 Lockable J-chaser

Figure 6.4 Lockable J-chaser

In order to reduce the winch loads on the rig, it was decided to use a two-boat solution to deploy each of the anchors. In addition five vessels were specified instead of four; one vessel was to be used as a pure towing and grappling vessel.

6.9 Rig Move Procedure for Location “I”

The latest revision of the rig move procedure (RMP) is dated 16 March 2007, see Annex 1 Section 3.10. Planned time for the rig move was five days eight hours.

6.9.1 The vessels

Page 5 of the RMP says that five anchor-handling vessels shall be available for the operation, four “primary” vessels plus one towing vessel.

Page 16 of the RMP shows the vessels nominated as follows;

AHV A – Primary vessel

AHV B – Primary vessel

AHV C – Assist vessel

AHV D – Assist vessel

AHV E – Towing and grappling vessel

On page 17 of the RMP it is stated that all AHVs shall have a minimum bollard pull of 180 tonnes and the towing vessel one of 150 tonnes. In addition, it is here defined what kind of equipment each of the four anchor-handling vessels shall be able to take on board.

  • All the vessels shall have gypsy for 76mm chain

  • One AHV shall have gypsy for 84mm chain

  • A J-chaser

  • A grapnel

It is thereafter described that the primary vessels must take on board the following equipment:

  • One 2300m 84mm work wire

  • Capacity for three 914m 76mm chain inserts

  • Chain gypsies for 76mm chain

  • Capacity for 900m 84mm chain

It is thereafter described that the assist vessels must take on board the following equipment:

  • One 2,300m 84mm work wire

  • Capacity for a 914m 76mm chain inserts

It follows from this that also each of the assist vessels were to recover and deploy a primary anchor and chain inserts.

The RMP did not make any explicit requirements for the vessels’ winch capacity.

6.9.2 Recovery of the secondary anchors (nos. 2, 3, 6 and 7)

Recovery of anchors is done by the anchor-handling vessel paying out work wire and going out to the anchor along the chain with the aid of a J-chaser, see Figure 6.4. The procedure states that one shall not operate with more than 150 tonnes tension on the AHV winch in “breaking” of anchors without the towmaster’s permission.

The anchor is “broken” loose from the seabed with the aid of the vessel’s winch. In cases where the chain has sunk into the seabed, the AHV lifts the chain loose from the seabed by using her winch. Thereafter the rig and the AHV winch in the wire at the same time as the AHV follows the chain until all the wire has been winched in. With the anchor under the stern roller there is only chain between the rig and the vessel, as shown in Figure 6.5.

Figure 6.5 Anchor recovery – Stage 6

Figure 6.5 Anchor recovery – Stage 6

In order to make a controlled decoupling of the chain inserts, an assistant anchor-handlings vessel is used to take the weight of the chain about 300m aft of the primary vessel with the aid of a grapnel, see Figure 6.6. This is to facilitate the anchor’s placement over the stern roller for subsequent uncoupling from the chain. The chain inserts are then stored in the primary vessel’s rig chain locker and the anchor remains lying on deck. The rig chain is thereafter winched in and the end transferred to the rig with the aid of the Permanent Chaser Pennant (PCP).

Figure 6.6 Grapnel

Figure 6.6 Grapnel

6.9.3 Recovery of primary anchors (nos. 1, 4, 5 and 8), towing to Location “G”

It was a requirement of the procedure that all the four primary anchors be broken loose at the same time. When all of the anchors have been broken loose from the seabed, but not lifted, the towing vessel shall connect herself to tow cables and assist in holding the rig in position. When the tow has been secured (i.e., the towing vessel connected to the rig), recovery of the anchors can begin. When the anchors are at the stern roller, see Figure 6.5, the tow can begin. When the rig is within 50 metres of its location, the towing vessels are uncoupled and made ready to assist as grappling vessels in the deployment of anchors.

6.9.4 Deployment of anchors

According to RMP, primary anchor no. 4 (see Figure 6.3) is the first anchor deployed. Deployment of the anchors is thereafter a pure reversal of the recovery process described above. The anchors are deployed in pairs and diagonally. During deployment of the anchors, grappling will proceed about 200 metres from the rig’s winch. After the anchors have been run out, they are pre-tautened with the aid of the rig’s winch. After all four primary anchors have been deployed, the rig is stabilised at the location.

6.9.5 Requirements for bollard pull

The calculations in the RMP focused on:

  • Tension in mooring line and workwire

  • Requirements for the anchor-handling vessel’s bollard pull

  • Vessel positions in relation to the rig

  • Anchor positions over the seabed

  • Mooring line’s angles with the fairlead, and

  • Necessary length of workwire.

In the analyses made for recovery and deployment of anchor, static forces due to the weight of chain and wire were calculated. Trident thereafter made simplified dynamic analyses exclusively in a head sea for the conditions that resulted in the highest static forces. In these analyses the movements of a typical middle-sized anchor-handling vessel was calculated in a maximum wave of 4.0 metres (Hs approximately 2.2 metres), which resulted in a 2.0-metre heave movement, according to the RMP. This heave movement was modelled as a regular movement at the end of the mooring lines and the forces were then calculated. The effect of neither the pitch nor the surge was taken into account in these dynamic force calculations.

For the recovery analyses, stage 1 (see Figure 6.7) resulted in the greatest forces. Forces due to the weight of just the mooring lines were 262 tonnes at the vessel’s stern, which resulted in a necessary bollard pull of 195.9 tonnes. In addition to these forces, calculated total external weather forces due to wind, current and wave drift were calculated at 10.9 tonnes in a head sea and 48.8 tonnes in a lateral sea. The static forces were calculated for maximum waves of 4.0m with a period of 8.5s, current speed of 1.0m/s and wind speed of 10m/s. Current against the mooring lines was not taken into account in these calculations, as confirmed in the Commission’s questioning of Martin Kobiela.

Figure 6.7 Anchor recovery – Stage 1

Figure 6.7 Anchor recovery – Stage 1

According to the simplified dynamic calculations, one would then see forces up towards 318 tonnes on the vessel’s winch.

To quote from Sean Johnson’s testimony before the Commission:

“This is an alternative task that might be relevant, but that was not necessary. The information in the curves is set up for the sake of the towmasters. The weight of the vessels was therefore much lower in recovery of anchors, because the chain could lie on the seabed and not much bollard pull was required for recovery. The vessels did not, therefore, have any high requirement for bollard pull in this stage. It was of no significance that the chain was dragged along the seabed. Johnson also explained Stage 3 of the recovery of anchors, see p. 58 of the procedures, to Reiersen, where he (Johnson) thinks the load was 136 tonnes, which was the maximum bollard pull that Johnson expected during the mooring recovery method.”

How this was communicated to Captain Frank Reiersen is discussed in more detail in Section 8.1.

For the deployment analyses, step 5 (see Figure 6.8) resulted in the greatest forces. Forces due to the weight of just the mooring lines were 243.6 tonnes at the vessel’s stern, which resulted in a necessary bollard pull of 160.2 tonnes. In addition to these forces, total external weather forces due to wind, current and wave drift were calculated at 10.9 tonnes in a head sea and 48.8 tonnes in a lateral sea. The static forces were calculated for maximum waves at 4.0m with a period of 8.5s, current speed of 1.0m/s and wind speed of 10m/s. Current against the mooring lines was not included in these calculations.

According to the simplified dynamic calculations, in Stage 5 forces approaching 292 tonnes on the vessel’s winch would then be seen.

Figure 6.8 Anchor recovery – Stage 5

Figure 6.8 Anchor recovery – Stage 5

Source copied direct from the RMP.

6.9.6 Weather criteria

Under Section 6.5.3 of the NWEA guidelines for anchor-handling, the rig move procedures shall contain weather criteria (the worst weather in which the operation can be performed) and weather window (weather forecasts that indicate that the operation can be performed within a given period), so that applied analyses and the basis for these can be handled so as to secure a controlled operation.

Chevron’s Marine Operations Manual applies to Trident, who prepared the procedure. The weather criteria in the manual were thus to be followed by Trident.

It appears from the manual’s Section 9.5.1 (Mooring and anchoring patterns) that a rig move procedure shall include the following:

“The maximum weather limits for working and stand by conditions including maximum expected anchor line tension where required.” (original in English)

Section 4.6.3 of the manual (Vessel details) states further that:

“Contractor shall as a minimum provide the following details of all vessels working in the spread when anchoring

  1. The minimum bollard pull necessary to hold the vessel at all drafts against the worst anticipated tidal stream, 40knot wind, and a 6 meters sea all acting in the same direction. The maximum and minimum bollard pull required depending on the direction of the weather relative to the vessel.

  2. Limiting environmental conditions for operating

  3. Propulsion and positioning systems” (original in English)

With reference to Table 4 on page 23 of the RMP, Martin Kobiela testified before the Commission that the indicated values are used only in the mooring analysis, but merely as a “rough guide” for maximum criteria. There are thereby no specific weather criteria for the operation. The weather conditions referred to in the RMP’s mooring analysis are:

  • Maximum waves of 4.0m, with a wave period of 8.5s

  • Current speed 1.0 m/s (1.94 knots)

  • Wind speed 10 m/s (19.4 knots)

Kobiela also testified that those performing the operation must continuously consider whether the weather conditions are satisfactory. This is evaluated there and then by the participants. There is no assessment in the RMP of how much downtime due to the weather conditions was expected during the operation.

6.9.7 Risk assessments and plans for alternative situations (contingency planning)

As described in detail in Section 3.7.2, the NWEA guidelines for anchor-handling demand that risk assessments be performed for the operation.

Section 1.2.2 (Responsibility) of Chevron’s Marine Operations Manual lays down the following regarding plans for alternative situations (contingency planning) and risk handling;

“ChevronTexaco has a duty of care to ensure that all work is carried out with minimum risk to all personnel and facilities. To that end, operational procedures shall be submitted early so that prior to approval, all parties can be consulted and as necessary a risk assessment can be undertaken to identify potential major hazards and demonstrate that adequate procedures and safe guards are in place to mitigate against potential hazards and their consequences prior to mobilisation. This shall also ensure that ChevronTexaco and the Marine Contractor Safety Management Systems complement each other” (original in English)

It is further stated in Section 1.2.3 (Clarification):

“To make clear the ChevronTexaco policy and attitude to standards and responsibilities regarding the way in which marine operations are planned and carried out at any of the ChevronTexaco sites”. (original in English)

Section 4.6.5 (Contingency plans) states that:

“The offshore unit/vessel operator shall submit a step by step contingency plan. Actions and listing showing the responsibilities in the event of an emergency which shall include but not be limited to the following: […] sudden deterioration in weather” (original in English)

Section (Information required) has the following to say:

“The Contractor will be required to submit the following information to ChevronTexaco for review:- […] HAZOP documentation and safety plan” (original in English)

Identified scenarios that according to the RMP can demand planning of alternative situations are:

  • Problems reaching the anchor by use of J-chaser

  • PCP fracture near the rig

  • Fracture of PCP during deployment of anchor wire.

The risk assessments in the RMP cover only handling of technical problems that can be expected to arise during recovery and deployment of anchors. Risk assessments related to the safety of the rigs and the vessels are not discussed.

6.10 Evaluation

In the Commission’s opinion, there exist several weaknesses in the planning of the rig move. Key elements are composition of the maritime manning on the rig, choice of mooring system and installation method, method of calculating necessary bollard pull and winch capacity plus lack of concordance between estimated and actual weather. The biggest and most serious weakness in the RMP is nevertheless the fundamental lack of risk analyses and identification (HAZID) and the handling of unforeseen events (contingency planning).

6.10.1 Maritime manning on the “Transocean Rather”

Section 6.1 of the NWEA demands that rig personnel must know the participant vessels’ operational limitations. The OIM’s formal and practical expertise in maritime operations was limited, which is why the follow-up operational authority was delegated to the towmaster hired for the rig move. The towmaster is without affiliation with the participant organisations.

The barge supervisor was the member of the permanent rig crew with the greatest maritime expertise and during the rig move provides peripheral maritime assessment support to the OIM. During the rig move he had the primary responsibility for winch and ballast operations. Overall responsibility for handling safety is vested in the OIM. The towmaster, who in practice led the maritime operation on behalf of the OIM, had not participated in the preparation of the RMP including risk assessments. This helped to remove or weaken necessary safety barriers of a human character. To the extent that the maritime expertise is not located on the highest level of command, it is essential that there be close and good communication between the possessor of that maritime expertise and the person in supreme command on board the rig throughout the rig move. In the planning phase, a high level of maritime expertise is required to handle safety.

O’Malley claimed to the Commission that he was not informed about important non-conformances during the operation – the drift from the run-out line for anchor nos. 6 and 2, grappling outside the RMP and the near-miss between the “Bourbon Dolphin” and the “Highland Valour”. The Commission cannot see that a system was established or implemented that secured an adequate flow of information. It is the duty of the OIM at all times to be informed about all matters of significance for the operation being performable in conformity with the RMP and in such a way that the safety of all involved parties is addressed.

Chevron’s Marine Representative is charged with addressing the level of operational safety enshrined in Chevron’s Marine Manual.

In addition, the doubling-up of the roles of towmaster and Chevron’s Marine Representative removed an important safety barrier for the operation.

6.10.2 Choice of mooring system and installation method

In the planning phase no qualitative analyses of alternative mooring technology (wire/polyester/spring buoys) and alternative installation methods (prelaying of anchors) were performed. Use of alternatives could have secured a more robust installation methodology, and less vulnerability to weather risk. In operational areas with great challenges as regards both weather and depths such as the area west of Shetland, it will always be a material contribution to safety management to make a thorough evaluation of all alternatives with regard to choice of materials and deployment methods.

6.10.3 The rig move procedure Requirements for bollard pull

The RMP identifies requirements for bollard pull due to the weight of the mooring lines alone as 160.2 tonnes during deployment of anchor. In addition to this, the RMP estimates 10.9 tonnes in static forces on the vessel in a head sea and 48.8 tonnes in a lateral sea with 4m max waves, 1 m/s current and 10 m/s wind.

During deployment of the anchors there will be a probability that some of the vessels will have to deploy the mooring line in a lateral sea and/or a head sea. In the case of the “Bourbon Dolphin”, we have heard testimony that the use of full side thrust reduces the bollard pull to 125 tonnes (see Section 5.5 for a more detailed explanation). This reduction will vary from vessel to vessel, but such a reduction is not unusual for a medium-sized anchor-handling vessels. In addition to this, the bollard pull will be highly reduced due to the vessel’s movement in the waves, the reduction of the bollard pull will naturally increase with deteriorating weather.

The RMP refers to the fact that the static forces on the vessel are calculated to take account of for the reduction in bollard pull. The Commission cannot see how this is taken account of in the RMP and has made repeated requests for a detailed explanation. After the hearing the Commission received two reports from Martin Kobiela. He was not successful in justifying the requirement for 180 tonnes bollard pull; see Annex 1, Sections 3.14 and 3.15.

In calculating the expected static forces in a lateral sea, the RMP does not include forces acting on the vessel due to current against the mooring lines. In a 1.0 m/s uniform current for deployment of anchor stage 5, forces acting on the AHV due to current forces against the lines have been calculated as up to 20 tonnes. Such a load means that the RMP has underestimated the static forces by around 35%. Increasing wind, waves and current will yield markedly increasing static forces (wind and current forces are proportional to the square of the speed). The weather criteria employed in the above analyses are not reflected in the RMP as operational limitations and are consequently not a reference-point for safe operations.

Given the navigational data, the Commission is able to note that the “Bourbon Dolphin” was drifting before the weather from 13:45, which indicates that she did not have sufficient thruster capacity to keep herself along the deployment line. The probable cause of the drifting around this time is thought to be increasing current forces across the mooring lines due to increasing chain lengths plus a general deterioration in the weather situation.

The Commission also finds it relevant to refer to calculations of necessary bollard pull in a head sea shown in the RMP. As mentioned above, the static forces from the weight of chain and wire alone come to 160.2 tonnes. In addition, account must be taken of 10.9 tonnes in static forces plus current forces against the mooring lines, which the Commission estimates at 3 tonnes. Exclusively static forces will thereby demand a minimum of 174 tonnes in continuous bollard pull. The static forces are calculated in a weather situation that was considerably milder than the real one at the time of the accident. As mentioned above, in addition the effective bollard pull is severely weakened due to the vessel’s movements. The 180 tonnes bollard pull standard is therefore too low.

The Commission also finds weaknesses in the calculations of tension to be found in the RMP. As mentioned above in Section 6.9.5, only simplified dynamic calculations were made. The Commission has performed its own calculations that confirm the static loads on the mooring line in the RMP. A maximum winch tension of 292 tonnes was calculated for deployment of anchors in the RMP. These analyses have been calculated with a 4m max wave (significant wave height, Hs, approx 2.15m). If we take account of the vessel’s real movement, these forces will increase by an estimated 10-15%, depending on the vessel’s movement characteristics. As Kobiela mentions in his report to the Commission (Annex 1 Section 3.15), it is normal to set Hs from 3.5 to 4.0m for anchor-handling. Corresponding analyses in a 4.0m Hs would have given an estimated increase of 25%. In normal practice for addressing safety factors one would have used a design criterion of Hs 5.0m for an operational criterion of 4.0m Hs (explained in greater detail in Section 3.8). Corresponding analyses with Hs 5.0m would have yielded an estimated increase of 30%.

Such an underestimate of forces could have caused an overload of the vessel’s winch capacity. Weather criteria

In the opinion of the Commission, it merits criticism that the RMP was not in conformity with the requirements of NWEA plus internal Chevron requirements (40 knots wind and 6 metre waves) as regards weather criteria. This is described in greater detail in Sections 3.7 and 6.9.6. Clear weather criteria shall be established in an RMP. It is expected that weather criteria are unambiguously communicated in the RMP as an important safety barrier. The forces appearing in the analyses were incomplete and defective and unsuited to visualising the forces one could expect to encounter during the operation. This had significance for both requirements for bollard pull and for winch forces.

It appeared from the RMP that, before the operation was implemented, weather forecasts would be obtained and a weather window identified in order to execute the operation in a safe manner. The Commission would question whether it is possible to identify such a weather window while lacking clear requirements for weather criteria. Both Chevron’s and Trident’s representatives have told the Commission that clear weather criteria can be unfortunate and counter-productive. Chevron’s marine manual, however, makes unambiguous requirements for clear weather criteria.

The Commission is of the opinion that maximum weather criteria can hardly be an obstacle to safe operation but on the contrary a material element of the risk analyses. The weather criteria can be handled as a reference for suspension of the operation in an unfortunate combination of waves, wind and current. All RMPs ought to define that the stated weather criterion is an upper limit for safe operation and be continuously evaluated by the person responsible for the operation. Risk assessments and plans for alternative operations (contingency planning).

Chevron lays down in its system of procedures that their operations are to be planned and executed with minimal risk. The operator has argued that the RMP is a coherent risk analysis in which the risks are continuously handled for all stages.

In the Commission’s opinion the procedure lacks an integrated and comprehensive risk analysis for the operation as a whole, despite such a requirement being enshrined in Chevron’s and Transocean’s operational manuals. In addition, the NWEA guidelines contain provisions about this. The RMP lacks formal HAZIDs and HAZOPs, and by and large lacks solutions for alternative operations (contingency planning).

As described in Section 3.6.2, it is the Commission’s opinion that under British acts and regulations the employer has the responsibility for preventing people other than his employees being exposed to risk, as long as this is “reasonably practicable”.

Witnesses from the operator and rig testified before the Commission that they did not regard it as foreseeable that an anchor-handling vessel could roll over, and consequently that no risk measures for such a scenario were prepared. Given that capsizing has for many years been a frequent cause of accidents at sea, the Commission can by no means concur in such reasoning. That there has been only a single capsize of an anchor-handling vessel does not mean that we can deduce that such vessels are incapable of capsizing. In the Commission’s opinion, therefore, “reasonably practicable” measures could in a simple manner been taken to minimise the risk, by for example an attention zone around anchor run-out lines, see Section 13.9.3.

Moreover, witnesses from the operator and rig have argued that it is the vessels themselves that must address their own safety. To this the Commission would remark that even if the vessels ought to have prepared adequate risk assessments, this does not relieve the operator of his paramount responsibility for addressing the safety of all involved parties.

It follows from Chevron’s marine manual and the NWEA guidelines that the operator must ensure that risk analyses concerning the participating vessels are prepared before the operation starts. Neither during the start-up meeting onshore or on arrival on the field did the rig ask for the vessels’ internal risk analyses. This was confirmed by Captains Reiersen and Bergtun during questioning. Reiersen also testified that normally the vessels send their risk assessments to the rig. From what the Commission has been given to understand, it is not usual for the vessels to send the risk analyses to the company.

There has been a failure to consider what might go wrong between vessel and rig and between participant vessels. In order to reduce the tension on the rig, it was decided to use a two-boat solution in the anchor-handling operation. Use of two vessels near to one another increases the risk for the involved parties. Handling of great forces between two vessels may unleash uncontrolled forces against the one vessel in the event of unforeseen events such a wire breaking or a grapnel being lost. It may also cause uncontrolled near-miss situations, as was experienced shortly before the accident. That the vessels were operating in a tough weather and current environment with great mooring weights increased the risk picture further. If, instead, a lighter mooring system or presetting of anchors had been considered, the risk could have been reduced considerably.

Transocean’s system of procedures demands that special risk analyses (HAZOP) be undertaken in multi-vessel operations. The Commission perceives this as applicable to rig moves. Such a multi-vessel risk analysis is totally absent from the RMP.

Nor had the RMP performed risk analyses for the case that a vessel did not follow the run-out line as shown on the navigation plot. At such depths and such heavy-weather areas to which the RMP applied, the risk analysis should also include the consequences of an anchor being deployed in the wrong position.

In addition the Commission notes the insufficient alternative operational solutions (contingency planning). Chevron’s marine manual contains comprehensive requirements for contingency planning. Over and above the three concrete cases discussed in Section 6.9.7, contingency planning is absent from the RMP. Requirements in the operating systems of the participant operators were not reflected in the RMP. Even if Trident had prepared a RMP, it was Chevron Marine HE&S, Richard Macklin, who was supposed to ensure that the operational requirements in the Chevron marine operations manual were conformed to in the planning and execution of the operation.